University of Florida Thesis Or Dissertation Formatting

CONTACT MECHANICS AND THREE-DIMENSIONAL ALIGNMENT OF NORMAL DOG ELBOWS

LAURA C. CUDDY

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2011

To my family

ACKNOWLEDGMENTS

To the UF Surgery Service, my surrogate family, I am eternally grateful for the opportunity to train in one of the best residency programs worldwide. I hope I make you proud.

To my mentors, Dr. Dan Lewis and Dr. Antonio Pozzi for their intellectual support, dedication and hard work.

To Dr. Bryan Conrad and Dr. Scott Banks, for the many long and frustrating hours spent analyzing kinematic data.

To Dr. MaryBeth Horodyski for her consistent and cheerful guidance with statistical analysis.

To my fellow Irishman Dr. Noel Fitzpatrick for his intellectual and financial contributions, without which this study would not have been possible.

To the UF College of Veterinary Medicine Graduate Office for their financial support and guidance.

To my resident mates, past and present, thank you for all your support and guidance during our journey.

To my family, for their continuing support and encouragement.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...... 4

LIST OF TABLES ...... 7

LIST OF FIGURES ...... 8

LIST OF ABBREVIATIONS ...... 10

ABSTRACT ...... 11

CHAPTER

1 INTRODUCTION ...... 13

History of Elbow Research ...... 13 Elbow ...... 14 Anatomy ...... 14 Ultrastructure and Composition of the Normal MCP ...... 15 Medial Coronoid Pathology ...... 17 Epidemiology ...... 17 Etiopathogenesis ...... 19 Genetics ...... 19 Abnormal endochondral ossification ...... 20 Abnormal bone structure ...... 20 Abnormal elbow biomechanics ...... 21 Histopathology ...... 25 Diagnosis ...... 28 Treatment for Medial Coronoid Disease ...... 29 Conservative management ...... 29 Removal of pathologic cartilage and bone...... 31 Arthroscopy ...... 31 Sub-total coronoidectomy ...... 32 Biceps Ulnar Release Procedure ...... 33 Corrective Osteotomies ...... 34 Ulnar osteotomy ...... 34 Humeral osteotomy ...... 35 Elbow Denervation ...... 37 Salvage Procedures ...... 37 Medial compartment resurfacing ...... 37 Total elbow arthroplasty ...... 38

2 CONTACT MECHANICS AND 3D ALIGNMENT OF NORMAL DOG ELBOWS .... 39

Background ...... 39

Materials and Methods...... 41 Specimen Preparation ...... 41 Testing Protocol ...... 46 Testing Sequence ...... 47 Data Analysis ...... 47 Statistical Analysis ...... 50 Results ...... 50 Contact Mechanics ...... 50 Anatomic Location of Peak Contact Pressure ...... 54 3D Alignment ...... 55 Humeroradial articulation ...... 55 Humeroulnar articulation ...... 56 Proximal radioulnar articulation ...... 57 Discussion ...... 64

3 EFFECT OF PROXIMAL ULNAR ROTATIONAL OSTEOTOMY ON CONTACT MECHANICS AND 3D ALIGNMENT OF NORMAL DOG ELBOWS ...... 68

Background ...... 68 Materials and Methods...... 70 Specimen Preparation ...... 70 Testing Protocol ...... 70 Testing Sequence ...... 70 Data Analysis ...... 72 Statistical Analysis ...... 74 Results ...... 74 Contact Mechanics ...... 74 3D Alignment ...... 76 Humeroradial articulation ...... 76 Humeroulnar articulation ...... 76 Proximal radioulnar articulation ...... 76 Proximal ulna-distal ulna ...... 80 Discussion ...... 81

4 CONCLUSION ...... 86

LIST OF REFERENCES ...... 91

BIOGRAPHICAL SKETCH ...... 103

LIST OF TABLES

Table page

1-1 Modified Outerbridge Scoring System for arthroscopic evaluation of cartilage pathology ...... 31

2-1 Contact mechanical data acquired from the medial and lateral elbow compartments at each of the nine elbow poses tested ...... 52

2-2 Static 3D alignment of the humeroradial articulation for the nine elbow poses tested ...... 58

2-3 Static 3D alignment of the humeroulnar articulation for the nine elbow poses tested ...... 60

2-4 Static 3D alignment of the proximal radioulnar articulation for the nine elbow poses tested ...... 62

3-1 Contact mechanical parameters obtained from the medial and lateral elbow compartments pre- and post- osteotomy for each of the nine elbow poses tested ...... 75

3-2 Static 3D alignment of the humeroradial articulation pre- and post-osteotomy for the nine elbow poses tested ...... 77

3-3 Static 3D alignment of the humeroulnar articulation pre- and post-osteotomy for the nine elbow poses tested ...... 78

3-4 Static 3D alignment of the proximal radioulnar articulation pre- and post- osteotomy for the nine elbow poses tested ...... 79

3-5 3D orientation of the distal ulna relative to the proximal ulna following proximal ulnar rotational osteotomy ...... 81

LIST OF FIGURES

Figure page

1-1 Anatomy of the normal elbow ...... 14

1-2 Cranial view of a normal left forelimb demonstrating the mechanical axis (red) and anatomic axis (black) ...... 15

1-3 Primary and secondary trabecular orientation in the MCP ...... 16

1-4 Normal MCP subchondral bone densities in juvenile and geriatric dogs ...... 17

1-5 FMCP configurations: tip and radial incisure ...... 18

1-6 Characteristic stance associated with MCD...... 19

1-7 Proximal radioulnar step incongruency ...... 22

1-8 Physiologic joint incongruency...... 23

1-9 Elliptical ulnar trochlear notch ...... 24

1-10 Biceps brachii/brachialis common tendon of insertion and the MCP ...... 25

1-11 Subchondral microcrack formation in FMCP ...... 26

1-12 Regional bone mineral density in dogs with FMCP ...... 27

1-13 Axial CT slice demonstrating a fragment in the apex of the MCP ...... 28

1-14 Standard arthroscopic portals for the dog elbow and arthroscopic image demonstrating a cartilage flap at the apex of the MCP ...... 32

1-15 Biceps ulnar release procedure ...... 34

1-16 Pfeil ulnar osteotomy ...... 35

1-17 Canine unicompartmental elbow resurfacing ...... 38

2-1 Contact areas (A) and force distribution (B) in the normal elbow ...... 40

2-2 Limb loaded in the custom jig in the materials testing machine ...... 45

2-3 I-scan sensor and representative pressure map obtained ...... 47

2-4 Objective contact mechanical outcome measurements ...... 48

2-5 Joint co-ordinate system ...... 49

2-6 Displacement of the MCP and corresponding superimposed contact maps in neutral, pronation and supination ...... 51

2-7 PCP location ...... 54

3-1 Location and orientation of the osteotomy ...... 72

3-2 A. Plate luting and stabilization and B. caudal displacement of the apex of the MCP ...... 72

3-3 Contact parameters and 3D alignment A. pre- and B. post-osteotomy ...... 73

3-4 3D alignment of the proximal radioulnar joint post-osteotomy ...... 80

LIST OF ABBREVIATIONS

CA Contact area

CP Mean contact pressure

CT Computed tomography

EI Elbow incongruency

FMCP Fragmented medial coronoid process

MCD Medial compartment disease

MCP Medial coronoid process

MRI Magnetic resonance imaging

OCD Osteochondritis dissecans

PCP Peak contact pressure

UAP Ununited anconeal process

2D Two-dimensional

3D Three-dimensional

Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science

CONTACT MECHANICS AND THREE-DIMENSIONAL ALIGNMENT OF NORMAL DOG ELBOWS

Laura Cuddy

December 2011

Chair: Daniel D. Lewis Major: Veterinary Medical Sciences

Fragmented medial coronoid process (FMCP) is the most common cause of forelimb lameness in dogs. FMCP refers to a spectrum of pathology ranging from cartilage malacia, fibrillation or fissuring, to subchondral bone erosion or eburnation affecting the medial coronoid process (MCP) of the elbow of dogs. The exact cause of

FMCP has not yet been elucidated, but histopathological studies have suggested that repetitive excessive loading may lead to microfracture formation in the trabecular subchondral bone of the craniodistal apex or the radial incisure of the MCP.

Arthroscopic assessment and debridement of pathologic cartilage and subchondral bone is considered the preferred treatment by many surgeons due to the decreased morbidity and increased magnification compared with arthrotomy. Adjunctive treatments to unload the MCP have been investigated, although none have gained widespread popularity. Irrespective of the method of treatment elected, osteoarthritis of the elbow inevitably progresses, and the prognosis for complete return to normal function is guarded.

There is a paucity of studies investigating the biomechanics of normal dog elbows.

Contact areas have been assessed using joint casting techniques, and more recently

tactile array pressure sensors have been employed to estimate joint forces transmitted across the proximal radioulnar joint. These techniques, however, provide mostly qualitative data and were performed at one static elbow pose. Kinematic studies of the forelimb of dogs at a walk and a trot have been investigated in two and three dimensions, although kinematics of each the humeroradial, humeroulnar and proximal radioulnar joint during different elbow poses has not yet been investigated. Chapter 2 describes the effect of three elbow flexion angles, spanning the range reported during the stance phase at a walk, and antebrachial rotation angles on the contact mechanics and 3D alignment of normal dog elbows. We found that altering elbow flexion and antebrachial rotation angles significantly altered the contact mechanics and alignment of

normal dog elbows. This study provided a model for future investigation of pathologic

states, as well as surgical procedures advocated for the treatment of FMCP.

Chapter 3 describes the effect of a novel surgical procedure, proximal ulnar

rotational osteotomy, on the magnitude and distribution of contact pressures on the

articular surface of the medial and lateral elbow compartments and on 3D limb

alignment in the normal dog elbow. We found that this procedure rotates the apex of the

MCP caudal and abaxial to the radial head and decreases contact pressures in the

medial compartment, likely through alteration in the mechanical axis through the

humeroradial articulation. This procedure may therefore unload the medial compartment

of the elbow and ameliorate pain in dogs with medial compartment disease.

CHAPTER 1 INTRODUCTION

History of Elbow Research

Until the mid 1970s, most cases of osteoarthrosis of the elbow joint in the dog were considered to be primary in nature, with secondary osteoarthritis occurring due to

UAP.1 FMCP of the ulna, along with OCD of the humeral trochlea, was first identified in dogs in 1974, and was initially classified as ununited coronoid process.2,3 The term

‘ununited’ implied that nonunion resulted from failure of normal ossification of a physis,

however, as the MCP is not a secondary center of ossification, this was determined to

be a misnomer and replaced with ‘fragmented’ by Olsson in 1976.4-6 It was at first

believed that FMCP and OCD were both manifestations of osteochondrosis, the

retention of cartilage due to a disturbance in the process of endochondral ossification,

with the subchondral bone only secondarily affected.1,6-8 It was proposed that the

thickened cartilage originating from osteochondrosis of the MCP easily ossified due to

its vascular supply from the annular ligament, forming the osteochondral fragment

commonly identified.1

Histopathologic studies subsequently substantiated that FMCP is not a

manifestation of osteochondrosis, but is instead most likely a result of supraphysiologic

loading, potentially of abnormal subchondral bone, cumulating microfracture formation

in fatigue failure of the trabecular subchondral bone of the craniodistal apex or the radial

incisure of the MCP.9-11 Despite extensive research over the past three decades, the

exact etiopathogenesis of FCP has yet to be elucidated and remains controversial.

Elbow

Anatomy

The elbow of the dog is a compound joint consisting of the humeroulnar, humeroradial and proximal radioulnar articulations.12,13 The elbow is most commonly

described as a hinge joint, with motion primarily occurring in a sagittal plane as the

smooth, concave trochlear notch of the ulna glides along the humeral trochlea. The

radial head and lateral coronoid process of the ulna articulate with the capitulum of the

humerus, whereas the MCP of the ulna articulates with the trochlea of the humerus. The

ulna has a concave surface between the medial and lateral coronoid processes, the

radial notch, in which the radial head pivots to allow pronation and supination of the

antebrachium.12 The annular ligament attaches to the apices of the medial and lateral

coronoid processes, encircling but not attached to the enclosed radial head to enable

motion during antebrachial rotation.12 The elbow may therefore be referred to more

correctly as a trochoginglymus joint, although these hinge and pivot motions are

considered to occur independently.13,14

Figure 1-1. Anatomy of the normal elbow15

Normal, full-thickness, cartilage-free areas are present on the ulnar trochlear notch and on the humeral condyle.16-18 Initial studies purported that the lateral and medial

coronoid processes constituted only 20-25% of the total articular and weight-bearing

surface of the proximal radioulnar articulation, and that the remaining 75-70% was

contributed by the radial head.19 However, more recent studies have suggested that

force transmission through the radial head and MCP is more evenly distributed between

the medial and lateral compartments.20,21 This may be attributable to the inherent varus

alignment of normal dog elbows.22

Figure 1-2. Cranial view of a normal left forelimb demonstrating the mechanical axis (red) and anatomic axis (black)

Ultrastructure and Composition of the Normal MCP

The MCP is not a separate center of ossification, and growth is considered to occur

by interstitial growth of the cartilaginous anlage.23,24 Endochondral ossification of the

cartilaginous anlage of the MCP, starting at the apex and the junction with the trochlear

notch, gradually covers the MCP with cortical bone between 8-18 weeks after birth.25

The trabecular structure in the MCP displays early formation at the caudomedial edge

and distinct alignment towards the humeral side at an unusually early age compared to

other bones, likely an adaptation to significant mechanical loading at an early age

according to Wolff’s law.25,26 The orientation of the trabeculae indicate that the MCP is

loaded primarily perpendicular to the humeroulnar surface during normal weight

bearing, with secondary orientation towards the annular ligament due to tensile stress.26

The alignment of the trabecular bone in the radial notch is not consistent with

compressive loading occurring across the proximal radioulnar joint.26 The overlying

articular cartilage is formed early in development and is not renewed, compared with

subchondral bone which demonstrates regular turnover.27

Figure 1-3. Primary and secondary trabecular orientation in the MCP26

The histologic appearance of normal MCPs is consistent with normal hyaline

cartilage and bone, with subjective articular cartilage thinning occurring with age.28 CT

osteoabsorptiometry of normal dog elbows has demonstrated increases in the subchondral bone density with increasing age, with highest subchondral bone densities at the MCP and the central aspect of the humeral trochlea.20 Increased subchondral

bone density is correlated to previous load history, and it can be therefore inferred that

higher force transmission occurs through the MCP than the radial head, in direct

contradiction to the conventional belief of the radial head bearing the majority of load.19

The increased force transmission may be due to the lower surface area of the medial coronoid process, or a direct reflection of physiologic humeroulnar incongrency.29

Figure 1-4. Normal MCP subchondral bone densities in juvenile and geriatric dogs20

Medial Coronoid Pathology

Epidemiology

Elbow dysplasia is an umbrella term grouping four developmental disease

processes of the elbow joints of dogs: Fragmented Medial Coronoid Process (FMCP),

Ununited Anconeal Process (UAP), Osteochondritis Dissecans (OCD) of the humeral

trochlea and elbow incongruency (EI). FMCP is the most common of these conditions,

encompassing a spectrum of pathology, ranging from cartilage malacia, fibrillation or

fissuring, to subchondral bone erosion, eburnation or fracture affecting the MCP of the

ulna.30 Fragmentation of the MCP may either occur at the apex of the MCP or in the

region of the radial incisure at the lateral aspect of the MCP.22

Figure 1-5. FMCP configurations: tip and radial incisure22

FMCP is most commonly recognized in growing, large and giant breed dogs between four to nine months of age.1,31 Bernese Mountain dogs, Labrador retrievers,

Golden retrievers and Rottweilers are breeds that are commonly affected, with Bernese

Mountain Dogs 140 times and Rottweilers 36 times more likely to develop FMCP than

comparable mixed-breed dogs.32 Males are 1.6 times more likely to be affected than

females, although this may be a reflection of differential rate of growth.33

Clinical signs associated with MCD include a stiff or a stilted forelimb gait, most

obvious when first rising or after prolonged rest or vigorous exercise.1,34 Bilateral

involvement is common, in which case lameness may be difficult to detect.1 Anecdotally,

dogs affected with FMCP often stand with the elbow adducted, carpus abducted and the

antebrachium supinated in a presumed attempt to shift weight away from the affected

medial compartment to the lateral compartment of the elbow joint, although this theory

has not been substantiated.1,35

Figure 1-6. Characteristic stance associated with MCD. Photograph courtesy of Antonio Pozzi.

Etiopathogenesis

While the etiopathogenesis of other components of elbow dysplasia such as UAP or OCD of the humeral trochlea are well understood, there is no uniform agreement regarding the etiopathogenesis of FMCP. Abnormal endochondral ossification of the

MCP, abnormal bone structure or abnormal biomechanics of the elbow joint have been proposed as potential etiologies.1,6,11,16,17

Genetics

FMCP was identified as an inherited trait in the early 1990s.36-39 The genes

responsible have yet to be identified and phenotypically-normal animals can produce

offspring affected with FMCP, making identification of parent animals with low risk of

transmission difficult.39,40 In a study of Bernese Mountain Dogs in The Netherlands, the

associated common ancestors of FMCP and EI were different, indicating that these may

be genetically separate diseases in these populations.39 Despite extensive breeding

programs, the incidence of FMCP can only be reduced and not eliminated.41 Further

research including identification of genetic markers, such as bi-allelic single nucleotide polymorphisms in dogs with FMCP, is required to further reduce the incidence.33

Abnormal endochondral ossification

Early researchers suggested that FMCP was a form of osteochondrosis, a disturbance in endochondral ossification, supported by the clinical observation that

FMCP and OCD are commonly identified in the same elbow and that chondromalacia in the region of the MCP is commonly identified.2,3,30,42 However, histopathological studies

of FMCPs have demonstrated that these lesions are non-healing fractures, likely a

result of repetitive excessive loading cumulating in fatigue failure and microfracture

formation originating in the trabecular subchondral bone of the craniodistal tip or the radial incisure of the MCP.9,10 Furthermore, diffuse microcracks have been identified in

the subchondral bone of the MCP, extending beyond the grossly diseased bone.9

Abnormal bone structure

Subtrochlear sclerosis is one of the first radiographic signs of FMCP in dogs.43

Dogs with FMCP have a higher proportion of trabecular bone in the MCP, an adaptive

response to increased loading consistent with Wolff’s law.26 However CT

osteoabsorptiometric studies have identified that subchondral bone density also

increases with increasing age in normal dogs.20 Increased stiffness may make this

region less deformable, making it prone to fatigue fracture as well as transmitting

increased force to the overlying cartilage.44 It has also been suggested that the ability of

the subchondral bone in the region of the MCP to repair microdamage may be

abnormal, resulting in accumulation of microdamage and subsequent fatigue fracture.9

Abnormal elbow biomechanics

Currently abnormal elbow biomechanics, specifically humeroulnar incongruency or

conflict, resulting in supra-physiologic loading of the MCP is considered the most likely

etiology for FMCP.45 Incongruency refers to the malalignment of articular surfaces.46

The main current biomechanical theories proposed to cause humeroulnar conflict and resultant supraphysiologic loading of the MCP include static elbow incongruency

(radioulnar length disparity), dynamic elbow incongruency (radioulnar longitudinal incongruence), ulnar trochlear notch geometric incongruency, primary rotational instability of the radius and ulna relative to the distal humerus, and musculotendinous mismatch.16,17,45-48 Some authors have implicated tensile forces originating from the

annular ligament resulting in an avulsion of the MCP, although this theory has not been

substantiated.26

Static or dynamic proximal radioulnar ‘step’ incongruency. Disparate or

asynchronous growth of the paired radius and ulna may occur during skeletal

development, resulting in a short radius. This phenomenon has been identified in

puppies in which the ulna was transiently as much as 3 mm longer than the radius at 4-

6 months of age.17 A radiographic study revealed that Swiss mountain dogs with FMCP

had a proportionally shorter radius compared with the ulna, a longer proximal ulna and

an increased distance between the trochlear notch and proximal radius compared with

unaffected dogs of the same breed.49 Experimental shortening of the radius increases

contact area of the MCP in cadaveric limbs and FMCP has been induced by traumatic

premature closure of the distal radial physis.50,51

It is, however, uncommon to diagnose radioulnar incongruency at the time of

diagnosis of FMCP and it is postulated that this may therefore be a temporary state,

only occurring at certain joint positions or during certain phases of development, and

inapparent at the time of diagnosis of FMCP. The maturing intraosseous ligament has

been postulated to begin to resist longitudinal movement between the radius and ulna

around 6 months of age, and in an unpublished longitudinal radiographic study,

incongruency through the proximal radioulnar joint could be induced up to five months

of age by manual manipulation.15,52 Furthermore, lesions associated with FMCP appeared at a time when the MCP was elevated more proximal to the radial head, and following fragmentation a period of longitudinal growth of the radius re-established normal congruency of the proximal radioulnar joint.52

CT studies have demonstrated increased humeroradial joint space and significant radioulnar incongruency in dogs with MCD compared with normal control dogs.53 3D

image rendering of CT images has been found to be 82% sensitive and 100% specific

for detecting radioulnar step lesions induced in cadaveric specimens.54 Arthroscopic

estimation has been described to be 86% sensitive and 100% specific for positive

radioulnar incongruency (short radius)55 and has a higher diagnostic value compared

with both radiography and CT.56

Figure 1-7. Proximal radioulnar step incongruency46

Incongruency between humeral trochlea and ulnar trochlear notch. The

trochlear notch of the elbow has a small concave incongruence, which allows for more

equal stress distribution under high load and ensures better nutrition of the articular

cartilage.46,57 Such physiologic incongruence has also been noted in humans but is not

thought to be involved in the development of FMCP in dogs.46,57

Figure 1-8. Physiologic joint incongruency46,57

The trochlear notch reaches its maximum diameter at the apex of the MCP.47

Trochlear notch dysplasia as a cause for FMCP was first proposed in 1986.16,17 In

radiographic and cadaveric studies, a more elliptical trochlear notch with decreased

diameter at the apex of the MCP and at the anconeal process was identified in dogs

affected with elbow arthrosis and in medium- and large-breed dogs compared with

normal dogs.16,17 It was subsequently postulated that the abnormally small curvature of

the trochlear notch would lead to overloading of the underdeveloped MCP in immature

dogs, resulting in fragmentation.16,17 However, if the trochlear notch is more elliptical, the humeral condyle would exert pressure on both the MCP and the anconeal process and it is uncommon to identify concurrent UAP and FMCP in the same joint.58

Subsequent studies have not identified a significant difference in the radius of curvature

of the ulnar trochlear notch between breeds predisposed to developing FMCP and those rarely affected.47,59

Figure 1-9. Elliptical ulnar trochlear notch46

Primary rotational instability. The articular circumference of the radial head is greater than the corresponding circumference of the radial notch of the ulna, allowing

axial rotation during pronation and supination of the antebrachium. FMCP occurring in the region of the radial incisure, may be related to primary rotational instability occurring during pronation or supination, resulting in lateral shear and excessive loading of the

MCP between the radial head and the medial humeral condyle.33,45

Musculotendinous mismatch. This refers to a disparity between the muscle

tensions generated during pronation or supination of the antebrachium relative to the

humerus. Supra-physiologic loading of the MCP purportedly occurs during maximal

flexion of the elbow as a result of the forces generated by the medially-located combined tendon of insertion of the biceps brachii and brachialis muscles.45,60 These forces cause supination of the antebrachium and a resultant shearing force between the radial head and the radial incisure at the level of the proximal radioulnar joint.45

Supination of the antebrachium actually results in a pivoting of the radial head externally

with minimal motion of the ulna. The pull of the biceps brachii and brachialis tendon of insertion would instead draw the MCP cranially and shear it against the radial head, reflecting what occurs during pronation at the joint surface and consistent with trabecular orientation demonstrated in lesions associated with the radial incisure.61

Figure 1-10. Biceps brachii/brachialis common tendon of insertion and the MCP60

Histopathology

Although FMCP was initially considered to occur due to cartilage retention related

to osteochondrosis, a landmark study comparing humeral condylar OCD flaps and

FMCPs removed from elbows at surgery using light and transmission electron

microscopy substantiated a defect in endochondral ossification associated with the

OCD flaps.10 In contrast, pathology of FMCP lesions was more consistent with

subchondral bone fracture and fibrous repair.10 However, a more recent case study

identified incidental histologic lesions consistent with OCD in a case of FMCP in an

asymptomatic 20-week old Golden Retriever.62 To substantiate the theory that FMCP is

due to subchondral bone fracture rather than osteochondrosis, a more recent

histomorphometric study of FMCPs excised at surgery revealed diffuse damage, osteocyte loss and increased porosity in the subchondral bone of the FMCP, particularly in the radial incisure, the region of the MCP that commonly fragments.9 FMCP

specimens demonstrated higher fatigue microdamage compared with normal MCPs,

with fatigue microdamage positively correlating with the severity of FMCP.9 It was

concluded that fatigue fracture of the subchondral trabecular bone is therefore important

in the pathogenesis of FMCP.9

FMCPs have a wide variety of histological and immunohistochemical characteristics compared with normal MCPs, with lower collagen type X in young dogs with FMCP.28 Cartilage necrosis has been induced in the stifles of pigs by selectively

impairing the cartilage canal blood supply.63 Investigation into the anatomic sites of

cartilage canals in young golden retriever dogs has not revealed any association

between their location and those in which FMCP occurs commonly.64

Figure 1-11. Subchondral microcrack formation in FMCP9

A recent microCT study noted that trabecular orientation on the MCP was significantly different between dogs with no evidence of FMCP (control) and dogs with transverse tip fragments involving the radial incisure on horizontal plane imaging and on

sagittal plane imaging, although there was no difference between controls and dogs

with transverse tip fragments solely.61 It is therefore unlikely that a single unified theory

can explain medial coronoid pathology occurring at the apex of MCP which is consistent with acute overloading, and that occurring at the radial incisure which is more indicative or chronic abnormal torsional loading.61,65

Bone mineral density patterns reveal that the MCP is eccentrically loaded in dogs

with and without FMCP, with 50% higher bone mineral density at the abaxial half versus

the axial half of the MCP.66 This suggests that load transmission occurs primarily across

the abaxial surface of the MCP, and may result in microcrack formation and

fragmentation at the junction between these two regions.66 In dogs with FMCP, bone

mineral density was actually highest in the centroabaxial portion of the MCP than near

the apex as would be expected if supraphysiologic loading of the apex was responsible

for associated pathology.66

Figure 1-12. Regional bone mineral density in dogs with FMCP66

Diagnosis

Radiography is used as a routine screening procedure for elbow disease. Findings associated with MCD are usually related to secondary osteoarthritic changes, and include osteophyte formation on the proximal anconeus and proximal radius, as well as subchondral sclerosis in the region of the semi-lunar notch and medial coronoid process.67-69 Radiographic assessment of osteophytosis of the elbow correlates poorly

with arthroscopic findings, and MCD is often diagnosed in dogs with few or no

radiographic changes.70

The gold standard for diagnosis of pathology referable to the medial coronoid

process is a combination of CT and arthroscopic examination. CT allows evaluation of

subchondral bone surfaces and elbow congruency, but does not yield information on

status of the overlying cartilage.69 Arthroscopy on the other hand is very useful to

assess surface cartilage, but cannot identify isolated subchondral bone lesions that are

not associated with surface cartilage lesions. These two modalities are therefore

considered complimentary for the diagnosis of MCD.71

Figure 1-13. Axial CT slice demonstrating a fragment in the apex of the MCP71

Although useful for examining soft tissue structures around the elbow joint,

ultrasonography is of limited diagnostic value for diagnosis of MCD, with poor

agreement between ultrasonographic and surgical findings.72 Bone scintigraphy has

been purported as a useful diagnostic tool for MCD in dogs, especially where some

ambiguity remains regarding the source of lameness after standard diagnostic

techniques.73,74 MRI has been utilized to characterize the anatomy of normal dog

elbows and showed good correlation with surgical findings.75,76 MRI is more useful to

define the soft tissue structures around the elbow, and its use in the diagnosis of FMCP

is limited due to the small size of the dog elbow, the complex articulations and the thin

cartilage surface.77 Magnetic resonance arthrography, using gadolinium-DTPA, has

been recommended for imaging the canine elbow.78 Delayed gadolinium enhanced MRI

of cartilage has recently been investigated as a pre-operative tool to predict outcome in

dogs with FMCP.79 The decrease in glycosaminoglycans associated with cartilage

damage should result in concentration of gadolinium on T1-weighted images.

Biomarkers have been investigated as an additional diagnostic tool for elbow

osteoarthritis.80,81 Collagenase-generated cleavage neoepitope of type II collagen is

increased in dogs with MCD compared with dogs with unaffected elbows, and there is

moderate correlation between the amount and the severity of cartilage lesions.81

Decreased serum tissue inhibitor of metalloproteinases-1 has been detected in dogs with osteoarthritis.80 These tests have not been implemented clinically.

Treatment for Medial Coronoid Disease

Conservative management

Conservative management consists of weight management, consistent, non- concussive exercise, and medications to treat inflammation and pain.35 Medical

management results in a more rapid return to weight-bearing than surgical intervention, and in the longer term similar degrees of osteoarthritis have been noted in both groups.82,83 Although weight loss is one of the tenets of treatment for osteoarthritis, a

longitudinal study investigating the effect of feeding on the development of elbow

osteoarthritis also found a significant benefit to lifelong feeding of a diet 25% less dense

in calories compared with a control diet.84 Despite widespread use, electrostimulated acupuncture has not been demonstrated to have any effect in objective or subjective outcome measures in dogs with elbow osteoarthritis secondary to elbow dysplasia.85

Intra-articular administration of various substances has been reported to

ameliorate pain and mitigate progression of osteoarthritis of the elbow; however, none

of these substances have gained widespread clinical acceptance. Intra-articular

autologous adipose-derived mesenchymal stem cells have been advocated for the

treatment of elbow osteoarthritis, however definitive randomized, controlled, double-

blinded studies are required to definitively demonstrate their effectiveness.86 Intra-

articular autologous conditioned plasma appears superior to intra-articular cortisone and

hyaluronic acid in a prospective, randomized, double-blinded study.87 Although canine

platelets cannot be concentrated to the same degree as in humans or horses, increased

concentration of growth factors may result in anti-inflammatory, anti-degradative and

analgesic effects.87 Intra-articular botulinum toxin type A shows promise as any agent to

reduce pain and lameness associated with osteoarthritis, although further investigation

with objective outcome measurement is warranted.88 Analgesia is attributed to localized

muscular paralysis and inhibition of release of neurotransmitters than facilitate

nociception.88

Removal of pathologic cartilage and bone

Surgical intervention is often advised for dogs up to 24 months of age with clinical or radiographic signs. Dogs with significant osteoarthritic changes are usually considered poor surgical candidates and conservative treatment is recommended.34

Gross pathology of the cartilage of the MCP is commonly characterized arthroscopically

using the Modified Outerbridge Scoring System and may range from cartilage malacia,

fibrillation or fissuring to subchondral bone erosion or eburnation.89

Table 1-1. Modified Outerbridge Scoring System for arthroscopic evaluation of cartilage pathology89 Modified Outerbridge Score Gross Cartilage Quality 0 Normal 1 Chondromalacia 2 Partial thickness fibrillation 3 Deep fibrillation 4 Full thickness cartilage loss 5 Subchondral bone eburnation

Erosive cartilage lesions, often referred to as ‘kissing lesions’, frequently develop

on the articulating surface of the humeral trochlea in association with MCP pathology.

‘Kissing lesions’ are much more likely to occur when there are displaced fragments of

the medial coronoid process rather than in dogs with non-displaced fissures.

Arthroscopy

Arthroscopic examination of the elbow joint of the dog was first reported in 1993

and, in combination with CT, is considered the gold standard for diagnosis of medial

coronoid pathology due to the complementary nature of these modalities.30,71

Arthroscopy has been demonstrated to be superior to both arthrotomy and medical

management for the treatment of medial coronoid disease due to increased

magnification and capacity for more thorough examination of the elbow joint.90 The

establishment of arthroscopic portals for examination of elbow is considered safe as long as standard portals are employed.91

Figure 1-14. Standard arthroscopic portals for the dog elbow and arthroscopic image demonstrating a cartilage flap at the apex of the MCP92

Sub-total coronoidectomy

Removal of a larger portion of the MCP of the ulna, regardless of the extent of

gross pathology, has been advocated as an alternative to focal treatment or osteotomy

of pathologic cartilage or bone.93 The rationale for this treatment stems from

histopathologic studies which have demonstrated microscopic subchondral bone

pathology extending further than that grossly visible to encompass the entire cranial

portion of the MCP, which may be a source of residual pain and inflammation.9

Subjective lameness scoring has indicated a favorable outcome following this

procedure, although a randomized, blinded prospective clinical trial with objective

outcome measures is required to further substantiate the purported benefits compared

with focal treatment.93

Biceps Ulnar Release Procedure

A large portion of the combined tendon of insertion of the biceps brachii and brachialis muscles inserts on the abaxial portion of the MCP, with the remainder attaching to the proximal radius.60 It has been proposed that contraction of this biceps

brachii-brachialis myotendinous unit may rotate the MCP against the radial head,

resulting in supraphysiologic loading and subsequent fatigue fracture of the MCP in the

region of the radial incisure.45,60 Some authors therefore advocate a tenotomy of the

distal insertion of the combined tendon of insertion onto the ridge, immediately caudal to

the abaxial part of the MCP.22,45 Dogs are considered candidates for this procedure

when rotational instability with excessive force of supination is suspected as a cause for

medial coronoid disease, or when a focal subchondral lesion in the region of the radial

incisure is present.45 Biceps ulnar release procedure, therefore, is considered primarily

in juvenile dogs with clinical forelimb lameness attributable to the elbow but with minimal

arthroscopic or CT changes to mitigate progression of disease instead of dogs with end-

stage coronoid disease.45 A study of 39 dogs with 49 biceps ulnar release procedures

alone or in combination with SCO demonstrated significant improvement in lameness

based on visual analogue scores performed by the owners, and no significant difference

was noted between operated limbs and the contralateral ‘normal’ limb on force plate

analysis.94 More recently, arthroscopic-assisted bicep ulnar release procedure with a

custom-designed guarded cutting knife has been described with good subjective

outcome.22 However, there are as yet no biomechanical studies or randomized,

controlled clinical studies to substantiate these findings.

Figure 1-15. Biceps ulnar release procedure45

Corrective Osteotomies

Ulnar osteotomy

Static or dynamic proximal ulnar osteotomy. Ulnar osteotomy has resulted in radiographic resolution of elbow osteoarthritis in a case report in one dog following bilateral FMCP removal and unilateral ulnar osteotomy.95 The main complication associated with fixation of osteotomized segments is implant failure. Dynamic proximal ulnar osteotomy was first described in 1998 to displace the MCP in a caudomedial direction and reduce force transmission through the medial compartment.96 A proximal ulnar osteotomy obliqued in two planes (caudoproximal-to-craniodistal and proximolateral-to-distomedial) has since been recommended to reduce the incidence of varus associated with an osteotomy obliqued solely in the sagittal plane.45

Pfeil proximal ulnar osteotomy. A transverse ulnar osteotomy has been designed to realign the mechanical axis of the limb and unload the medial compartment of the elbow by re-aligning the mechanical axis of the forelimb, similar to sliding humeral

osteotomy and high tibial wedge osteotomy in people. This procedure has been utilized clinically with initial promising results, although randomized, controlled clinical trial with objective outcome measures. (Vezzoni, personal communication).

Figure 1-16. Pfeil ulnar osteotomy. Image courtesy of Aldo Vezzoni.

Distal ulnar osteotomy. Distal ulnar osteotomy is less complex to perform and theoretically associated with lower morbidity due to excessive translation of the proximal segment. However, distal ulnar ostectomy does not restore normal proximal radioulnar

joint congruency, most likely because of the interrosseous ligament preventing proximal

movement of the proximal ulnar segment, although ex-vivo studies have demonstrated

no difference in proximal displacement of the ulna with release of the interosseous

muscle.50,97

Humeral osteotomy

Sliding humeral osteotomy. Sliding humeral osteotomy is a surgical procedure

designed to unload the medial compartment of the elbow through realignment of the

mechanical axis of the limb.98,99 The elbow inherently has a degree of varus angulation,

and therefore the medial compartment experiences significant compressive load.98,100 A

mid-diaphyseal transverse osteotomy is performed and the proximal segment is

translated lateral to the distal segment and stabilized with a custom, locking, stepped

plate applied to the medial aspect of the humerus.101 By translating the distal segment

medially, alignment is restored between the mechanical axis of the limb and the center

of the elbow.98,99 Ex-vivo biomechanical studies have demonstrated that contact area

and pressures transmitted across the humeroulnar joint decrease following sliding

humeral osteotomy.98,99 A case series of 49 dogs with 59 limbs in which SHO had been

performed with or without focal treatment of the diseased MCP revealed a subjective

decrease in lameness and pain scores three and six months post-operatively, although

no control group or objective outcome measures were available.101 Histopathology

performed in one dog 17 months post-operatively revealed resurfacing of the humeral

trochlea with fibrocartilage.101 Major complication rates associated with this procedure, primarily implant failure and humeral fracture, were initially unacceptable, although more recent generations of the procedure have been associated with a reduced severe

complication rate in the region of 5% which is deemed clinically acceptable.101

Wedge humeral osteotomy. In biomechanical studies, a 10° opening medial

wedge osteotomy reduced contact area in the medial elbow compartment, but did not

change contact pressures across the humeroulnar and humeroradial joints.98,99 This procedure has not been utilized clinically.

Rotational humeral osteotomy. An ex-vivo cadaveric study investigating a

transverse osteotomy of the distal humerus with 15° external rotation noted lateral shifts

in peak pressure location and center of pressure, although no significant differences in

contact area, peak or mean contact pressure occurred.102 Further investigation of this

procedure is required before clinical implementation.

Elbow Denervation

Denervation of the sensory supply to the joint capsule of dog elbows via combined

medial and lateral approaches may be a feasible treatment option to reduce pain and potentially mitigate the progression of elbow osteoarthritis in dogs.103 Research in cadaveric specimens and in four clinically-normal dogs demonstrated appropriate removal or nerve tissue with no reported adverse affects to forelimb function or cutaneous sensation.103 This procedure will likely only benefit dogs in which soft tissue

structures, such as the joint capsule or synovium, are significant contributors to pain, as

the subchondral bone itself is a significant source of pain in osteoarthritis.103,104 This procedure has not yet been implemented clinically.

Salvage Procedures

Medial compartment resurfacing

The Canine Uni-compartmental Elbow system (CUE) (Arthrex, Inc., Naples, FL) has been developed to enable re-surfacing of the medial compartment of the elbow as a salvage technique to reduce pain associated with eburnation of the subchondral bone of the medial compartment. The technique is considered reasonably low morbidity and less invasive than total elbow arthroplasty as luxation of the elbow is not required and

minimal bone stock is removed. A cobalt-chrome humeral implant with a bone in-growth

surface (Biosync) is implanted into the humeral trochlea, with a corresponding ultra

heavy molecular weight polyethylene cylinder implanted in the opposing medial

coronoid process. Preliminary results in clinical cases are promising, with 80% of cases

returning to acceptable or full limb function, with an average six month recovery period.105

Figure 1-17. Canine unicompartmental elbow resurfacing105

Total elbow arthroplasty

Total elbow replacement or arthroplasty is warranted when end-stage disease of

both medial and lateral elbow compartments is present. The system most commonly

used currently is the TATE Elbow™ system (BioMedtrix, LLC., Boonton, NJ). When

compared to the earlier IOWA system, the TATE Elbow™ offers several advancements.

The TATE Elbow™ is considered more of a re-surfacing implant and as such requires a

less invasive approach, reducing the incidence of post-operative luxation and fracture

and theoretically hastening post-operative recovery. The IOWA system is a cemented

system and may undergo aseptic loosening whereas the TATE Elbow™ is cementless

with a porous surface that relies on bony ingrowth for stability, and therefore should

have a lower risk of aseptic loosening, and a longer lifespan. Complications associated

with total elbow arthroplasty include fracture of the humerus, ulna or medial epicondyle,

infection and implant failure and may be catastrophic, potentially necessitating

amputation.

CHAPTER 2 CONTACT MECHANICS AND 3D ALIGNMENT OF NORMAL DOG ELBOWS

Background

3D kinematic analysis in humans has revealed that supination and pronation of the antebrachium is complex and may significantly alter joint contact pressures at the elbow.106,107 In-vivo 2D and 3D kinematic studies and inverse dynamics analysis of

normal and diseased forelimbs of dogs at a walk and trot have been reported.108-111

These modalities address movement of the elbow only in the sagittal plane, and cannot

assess motion between the three individual articulations of the elbow during motor

activity.

Previous cadaveric studies have evaluated relative articular contact areas in the

humeroulnar and humeroradial articulations using joint casting techniques in axially-

loaded normal elbows and following surgical procedures designed to unload the medial

compartment of the elbow.50,98,112 Solid casting techniques allow quantification of

contact area, but may underestimate or fail to adequately evaluate some contact

regions and cannot assess joint contact pressures. Trans-articular force maps of normal

elbows and elbows following sliding and wedge humeral osteotomies have been

generated using tactile array pressure sensors; however, the exact contact area, peak

and mean contact pressure and peak pressure location could not be obtained from the

data supplied by these sensors.21,99 In addition, these studies have been performed only

at one static pose, with the elbow at 135° flexion and with the antebrachium in a neutral

position.21,50,98,99,112

Figure 2-1. Contact areas (A)112 and force distribution (B) in the normal elbow21

The most salient limitation of all previous studies investigating elbow biomechanics in dogs is that the 3D joint alignment and contact mechanics have not been evaluated simultaneously. An integrated approach to investigation of joint pathology has been advocated in human orthopedic research and a similar approach may help to elucidate the etiopathogenesis of elbow dysplasia in dogs.113 Some authors have implicated

proximal radioulnar joint instability or incongruency in the pathogenesis of FMCP.45,60

Understanding the effects of antebrachial rotation and elbow flexion and extension on

elbow contact mechanics and alignment would aid our understanding of normal elbow

function and develop a methodology to study the biomechanical basis of elbow

dysplasia. Surgical techniques involving the elbow could subsequently be evaluated

using a model that allows acquisition of both 3D alignment and contact mechanical

parameters.

The objectives of this cadaveric study were to: 1) evaluate the effect of antebrachial rotation (neutral, 16° pronation and 28° supination) and elbow flexion angle

(135, 115 and 155°) on humeroulnar and humeroradial contact area and pressures and

2) simultaneously determine the 3D alignment of each the humeroradial, humeroulnar and radioulnar joints at each elbow pose. We hypothesized that 1) in pronation mean

(CP) and peak contact pressure (PCP) would increase in the medial compartment and decrease in the lateral compartment, with a cranial shift of anatomic location of PCP in the medial compartment, and 2) in supination CP and PCP would decrease in the medial compartment and increase in the lateral compartment, with a caudal shift of the anatomic location of PCP in the medial compartment. We further hypothesized that contact pressures would increase in flexion and decrease in extension. Minimal axial rotation of the ulna was expected during pronation and supination of the antebrachium, with the majority of motion occurring due to gliding of the radial head relative to the radial incisure of the ulna. We also hypothesized that in pronation the apex of MCP would rotate internally and translate proximally relative to the center of the radial head, and conversely in supination the apex of MCP would rotate externally and translate distally relative to the center of the radial head.

Materials and Methods

Specimen Preparation

Thoracic limbs (n=18) were collected by disarticulation at the glenohumeral joint in

18 adult dogs (mean 27 ± 4 kg body weight) that were euthanized for reasons unrelated

to this study. An equal number of unpaired right (n=9) and left (n=9) thoracic limbs were

selected for this study. Orthogonal radiographic projections were obtained of each

elbow and specimens were excluded if there was any evidence of elbow pathology. The

specimens were wrapped in saline (0.9% NaCl) solution soaked towels, sealed in

double plastic bags and stored at -20°C.

The limbs were thawed to room temperature the day prior to testing. Tissues were kept moist during testing by intermittently spraying the specimens with room temperature isotonic saline (0.9% NaCl). Soft tissue structures proximal to the elbow including skin, fascia, muscle and neurovascular tissue were excised, excluding the origins of the antebrachial muscles and the medial and lateral collateral ligaments on

the medial and lateral epicondyles. The extensor carpi radialis, common digital extensor, and supinator muscle bellies were excised to expose the cranial aspect of the elbow and radial diaphysis. The anconeus muscle, anconeal ligament and the joint capsule were completely excised. The annular ligament was preserved.

Fiduciary arrays were implanted in the humerus, radius and ulna to determine the

3D, static pose of the elbow during testing. Three 25 mm long M3 nylon phillips machine screws (McMaster-Carr Supply Company, Cleveland, OH) were implanted in both the humerus and the radius forming the fiduciary array. Fiduciary arrays custom-made with

3.175 mm diameter fiberglass rod (McMaster-Carr Supply Company, Cleveland, OH) were implanted into each the proximal ulna, distal ulna and the third metacarpal bone.

0.0625 mm diameter stainless steel (type 302) balls (McMaster-Carr Supply Company,

Cleveland, OH) were adhered with tape to the center of each screw head and into three

depressions drilled in each fiberglass array as markers for CT identification. A CT scan

was performed with the specimen in a supine position for identification of the metal ball

markers and defined anatomic landmarks (Aquilion 8-slice Multi-Detector Row

Computed Tomography Unit, Toshiba, Tustin, CA.). Images were reconstructed by use of bone and soft tissue algorithms with a slice thickness of 1 mm. Specimens were excluded if there was evidence of joint pathology on CT examination. Following CT, the

stainless steel ball markers were carefully removed from the screw heads and fiberglass arrays to minimize displacement of the fiduciary arrays.

A 4.0 mm diameter hole was drilled transversely through the humeral condyle, from the medial to the lateral humeral epicondyle. A medial epicondylar osteotomy, circumscribing the transcondylar hole, was performed using an oscillating bone saw, while preserving the origin of the medial collateral ligament and the antebrachial flexor musculotendinous units on the medial epicondyle. The medial epicondyle was reflected distally, which allowed sensors to be placed in both the medial and lateral elbow compartments. The joint was inspected grossly following the osteotomy and elbows were subsequently excluded if there was evidence of cartilage damage in the medial aspect of the humeral condyle subsequent to the osteotomy or gross evidence of elbow pathology including cartilage malacia, fibrillation or eburnation, subchondral bone exposure or fragmentation in the region of the medial coronoid process, ununited anconeal process or cartilage lesions on the humeral trochlea or osteoarthritis. One strip of a custom I-scan sensor (Tekscan, Inc. Boston, MA) was placed in medial compartment and the other in the lateral compartment of the elbow. The sensors were secured cranially by placing a 2.7 mm cortical bone screw (Synthes, Inc., West Chester,

PA) through the reinforced peripheral tabs devoid of sensing elements into the proximal radial diaphysis. The medial epicondyle was anatomically repositioned and secured by insertion of a 3.8 mm transcondylar screw through the hole previously drilled. A nut placed on the screw was used to compress the osteotomy.

Braided steel cable (1.6 mm diameter) was passed through a 2.5 mm diameter hole drilled transversely through the caudal aspect of the olecranon process and a

crimp (McMaster-Carr Supply Company, Cleveland, OH) was used to secure the two ends of the cable to form a loop 20 mm in diameter. A hole was drilled from caudodistal-

to-cranioproximal centrally through the proximal humerus. The hole was initiated at the insertion of the teres major muscle and emerged distal to the insertion of the teres minor muscle. A 4.8 mm diameter steel J-hook (McMaster-Carr Supply Company, Cleveland,

OH) was placed through this hole and secured with a hex nut. A turnbuckle link (Stanley

Hardware, New Britain, CT) was attached between the hook in the proximal humerus and the cable loop in the caudal olecranon. This mechanism simulated overall pull of the triceps muscles and allowed for alteration of the length of the simulated triceps

mechanism.

The limbs were loaded in a material testing machine (MTS 858 MiniBionix®, MTS,

Eden Prairie, MN) and secured to a custom jig proximally by passing a 6 mm diameter

and 76 mm overall length adjustable clevis pin (McMaster-Carr Supply Company,

Cleveland, OH) from the jig transversely through a hole drilled through the humeral

head at a level just distal to the lesser tubercle and securing with a cotter pin (Figure 2-

1). The distal part of the jig consisted of a 66 mm internal diameter, 3.2 mm thick D2 ring

(IMEX TM Veterinary Inc. Longview, TX) secured 50 mm proximal to a wooden base with

100 mm long, 6 mm diameter threaded rods (IMEX TM Veterinary Inc. Longview, TX).

The paw was placed through the D2 ring. Pronation and supination of the antebrachium

was performed by rotating a 2.8 mm diameter centrally threaded full splintage pin (IMEX

TM Veterinary Inc. Longview, TX) placed transversely through the metacarpal bones

about the circumference of the D2 ring. The position of the pin was maintained using 6.3

mm Titanium hybrid rods with 6 mm thread mounted to the D2 ring with 6 mm Hex nuts

(IMEX TM Veterinary Inc. Longview, TX). The base was covered in coarse sandpaper

with a digit block at the cranial aspect of the nails. The paw was not rigidly fixed to the

base and the metacarpal pad was skewered with a 1.6 mm Kirschner wire protruding 5

mm above the surface of the base to prevent excessive cranial translation of the paw

during loading.

Figure 2-2. Limb loaded in the custom jig in the materials testing machine

Testing Protocol

The turnbuckle was adjusted to maintain an initial elbow flexion angle of 135º ± 5º, corresponding to the mean elbow flexion angle during the stance phase of trotting in healthy Greyhounds and walking and trotting in mixed-breed dogs.108,111,114,115 An axial

load of 50 N was applied with the abduction/adduction component of the jig

unconstrained. With real-time monitoring of the contact patterns, the jig was locked in a

position that maintained approximately 50:50 medial-to-lateral force distribution. A probe was used to apply gentle pressure to the sensing elements overlying the most cranial aspect of the radial head and the apex of the medial coronoid process, creating points of reference to enable future determination of relative anatomic locations of peak contact pressure on the contact map. A static axial load of 200 N was applied by the material testing machine, corresponding to the peak vertical ground reaction force of dogs at a walk (5.0 - 7.5 N/kg).116 The elbow flexion angle was measured during loading

with one arm of the goniometer aligned along the shaft of the humerus to the center of

the humeral head, and the other arm along the radial diaphysis to the ulnar styloid to

ensure that elbow flexion remained within 5° of the desired angle.

Instantaneous intra-articular contact area and pressure measurements were

acquired after maintaining peak force for 5 seconds using the I-scan system (Tekscan

Inc., Boston, MA), consisting of a custom-designed, plastic laminated, thin-film (0.1 mm)

electronic pressure sensor, sensor handle, and data acquisition and analysis software.

The sensor consists of two 31 x 12 mm sensing pads with 0.01 MPa sensitivity and a

measurement range of 0.5-30 MPa. Each new sensor was conditioned and calibrated

according to the manufacturer’s guidelines immediately before testing of the specimen.

With the specimen loaded at 200 N, the 3D static pose of the fiduciary markers on each

the humerus, radius, ulna and third metacarpal were digitized using a Microscribe 3DX digitizing arm (Immersion Corp., San Jose, CA) possessing an accuracy of 0.23 mm.117

Figure 2-3. I-scan sensor and representative pressure map obtained

Testing Sequence

Each specimen was tested with the antebrachium in a neutral position, in 16º

pronation and in 28º supination, approximating the reported midpoint of the linear region

of load-deformation curves generated from antebrachii rotated to failure.118 These

values corresponded with the subjective normal degrees of pronation and supination

obtained during pilot studies. This testing protocol was repeated with the elbow in 115º

and 155º of flexion ± 5° by adjusting the turnbuckle, approximating the extremes of

reported angles of the elbow during weight bearing at a walk.

Data Analysis

The digital pressure sensing data acquisition software was used to generate a

contact map. The parameters measured using the I-scan system included the CA, CP,

PCP and anatomic location of PCP in each compartment. CA was defined as the area

in contact between the capitulum of the humeral condyle and the radial head in the lateral compartment and between the trochlea of the humeral condyle and the medial coronoid process of the ulna in the medial compartment. The CP was defined as the average of the pressures across each contact area. The PCP was defined as the highest pressure measured in each contact area. The relative anatomic location of PCP for each pose was defined as the distance from the cranial tip of the radial head in the lateral compartment and from the apex of the MCP in the medial compartment caudal to

the peak pressure sensel.

Figure 2-4. Objective contact mechanical outcome measurements

Joint co-ordinate systems have been well-established in human kinematic studes in order to ensure repeatability of reference points for each joint.119,120 Such co-ordinate

systemics have previously been described for the dog.121 The locations of the stainless

steel balls attached to the fiduciary arrays and the defined anatomic landmarks for the

humerus (center of the humeral head, medial and lateral epicondyles), radius (center of

radial head, radial and ulnar styloid processes) and ulna (apex of MCP, apex of the

olecranon, radial and ulna styloid processes) were identified on the CT scans using

ImageJ software (ImageJ 1.42q, NIH, USA). Therefore the orientation of the anatomic landmarks relative to the fiduciary arrays with respect to the CT volume was known.

Rotations (flexion/extension, varus/valgus, internal/external) and translations (cranial- caudal, medial-lateral, proximal-distal) of the center of the radial head relative to the origin of the humerus (a point midway on a line between the medial and lateral epicondyles), the apex of the MCP relative to the humeral origin and of the center of the radial head to the apex of the MCP were calculated using body-fixed axes. Calculations were performed using a custom-written computer program (Matlab, The MathWorks

Inc., Natick, MA).

Figure 2-5. Joint co-ordinate system

Statistical Analysis

Separate 1-way repeated measures ANOVAs were used to evaluate differences in dependent variables of contact mechanics (CA, CP, PCP and anatomic location of PCP in the medial and lateral compartments) at the three elbow flexion angles (115º, 135º and 155º) and the 3 elbow positions (neutral, supination, pronation). Separate 1-way

repeated measures ANOVAs was used to evaluate differences in rotation (internal-

external rotation, varus-valgus, flexion-extension), and translation (cranial-caudal,

proximal-distal and medial-lateral) for each the humeroradial, humeroulnar and

radioulnar joints for each of the nine elbow poses. Where significant differences were

detected, post-hoc pairwise comparisons with a Bonferroni adjustment were calculated.

All statistical analyses were performed using SPSS statistical software (version 17.0;

SPSS Inc, Chicago, IL), with the level of significance for all statistical tests set at α

≤.0167.

Results

Contact Mechanics

CA was significantly greater in the lateral (58-64% total CA) compartment than the

medial compartment at all flexion angles and antebrachial rotations (Table 2-1, Figure

2-2). There was no significant difference in contact pressure distribution between medial

and lateral compartments at any pose. At all three flexion angles, pronation decreased

CA in both medial and lateral compartments, with CA ranging from 77-92% and 90-96%

of neutral, respectively. No consistent alteration in CP was noted in either compartment.

Pronation significantly increased PCP in the lateral compartment (110% of neutral) at

155°. At all three elbow flexion angles, supination decreased CA in both medial and

lateral compartments, with CA ranging from 81-85% and 90-91% of neutral respectively.

Supination significantly increased PCP in the lateral compartment at all flexion angles

(ranging from 106-122% of neutral) and in the medial compartment at 155° (115% of neutral). When the elbow was flexed to 115°, CA decreased 14% in the medial compartment at a neutral rotation, and CP and PCP increased in both medial and lateral compartments. When the elbow was extended to 155º, CA, CP and PCP decreased in

the medial and lateral compartments.

Figure 2-6. Displacement of the MCP and corresponding superimposed contact maps in neutral, pronation and supination

Table 2-1. Contact mechanical data acquired from the medial and lateral elbow compartments at each of the nine elbow poses tested 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Contact area (mm2) Total 209.2 ± 179.5 ± 27.8 181.2 ± 22.9 198.2 ± 22.0 187.2 ± 25.7 175.7 ± 20.2 184.7 ± 24.5 164.6 ± 32.1 158.6 ± 23.6 31.3

Medial 87.0 ± 13.9 67.3 ± 12.7 70.8 ± 14.9 74.7 ± 14.5 69.3 ± 15.4 63.3 ± 15.4 74.9 ± 15.8 63.1 ± 20.4 62.4 ± 13.2 P1-2 = 0.001 P1-3 ≤ 0.001 P1-4 = 0.002 P4-5 = 0.407 P4-6 ≤ 0.001 P1-7 = 0.023 P7-8 = 0.122 P7-9 = 0.03 P2-3 = 1.0 P2-5 = 1.0 P5-6 = 0.41 P4-7 = 1.0 P2-8 = 0.768 P8-9 = 1.0 P3-6 = 0.07 P5-8 = 0.325 P3-9 = 0.08 P6-9 = 1.0

Lateral 122.2 ± 112.2 ± 24.4 110.3 ± 16.0 123.5 ± 17.5 117.9 ± 24.2 112.4 ± 14.3 109.8 ± 18.2 101.5 ± 22.5 98.2 ± 17.9 24.6 P1-2 = 0.013 P1-3 = 0.007 P1-4 = 1.0 P4-5 = 0.369 P4-6 = 0.03 P1-7 = 0.02 P7-8 = 0.048 P7-9 =0.004 P2-3 = 1.0 P2-5 = 0.157 P5-6 = 0.66 P4-7 = 0.28 P2-8 = 0.056 P8-9 = 1.0 P3-6 = 1.0 P5-8 = 0.002 P3-9 = 0.01 P6-9 = 0.06 Peak pressure magnitude (MPa) Medial 8.4 ± 1.9 9.1 ± 2.3 8.8 ± 2.1 9.0 ± 2.3 9.5 ± 2.5 8.7 ± 1.9 7.4 ± 1.7 7.3 ± 2.7 8.5 ± 1.8 P1-2 = 0.098 P1-3 = 0.33 P1-4 = 0.016 P4-5 = 0.286 P4-6 = 0.48 P1-7 ≤ 0.001 P7-8 = 1.0 P7-9 ≤ 0.001 P2-3 = 1.0 P2-5 =0.04 P5-6 = 0.07 P4-7 ≤ 0.001 P2-8 = 0.001 P8-9 = 0.07 P3-6= 1.0 P5-8 ≤ 0.001 P3-9 = 0.34 P6-9 = 0.95

Lateral 8.2 ± 1.7 7.9 ± 1.6 8.7 ± 1.6 8.0 ± 1.5 8.8 ± 1.7 8.7 ± 2.0 6.6 ± 1.4 7.0 ± 1.4 8.1 ± 1.7 P1-2 = 1.0 P1-3 = 0.005 P1-4 = 1.0 P4-5 = 0.008 P4-6 = 0.004 P1-7 ≤ 0.001 P7-8 = 0.223 P7-9 ≤ 0.001 P2-3 = 0.01 P2-5 = 0.014 P5-6 = 1.0 P4-7 ≤ 0.001 P2-8 ≤ 0.001 P8-9 = 0.001 P3-6 = 1.0 P5-8 ≤ 0.001 P3-9 = 0.007 P6-9 = 0.04

Table 2-1. Continued 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Mean pressure (MPa) Medial 3.6 ± 0.7 3.7 ± 1.0 4.0 ± 0.9 3.9 ± 0.9 4.0 ± 1.0 4.0 ± 1.1 3.4 ± 0.8 3.0 ± 1.2 4.0 ± 0.9 P1-2 = 1.0 P1-3 = 0.02 P1-4 = 0.006 P4-5 = 1.0 P4-6 = 1.0 P1-7 = 0.642 P7-8 = 0.273 P7-9 ≤ 0.001 P2-3 = 0.193 P2-5 = 0.03 P5-6 = 1.0 P4-7 = 0.002 P2-8 = 0.01 P8-9 = 0.002 P3-6= 1.0 P5-8 = 0.01 P3-9 = 1.0 P6-9 = 1.0

Lateral 3.3 ± 0.8 3.7 ± 0.9 3.6 ± 0.9 3.7 ± 0.7 3.9 ± 0.8 3.8 ± 0.7 3.1 ± 0.7 3.1 ± 0.9 3.3 ± 0.7 P1-2 = 0.02 P1-3 = 0.07 P1-4 = 0.005 P4-5 = 0.092 P4-6 = 0.99 P1-7 = 0.12 P7-8 = 1.0 P7-9 = 0.23 P2-3 = 1.0 P2-5 = 0.095 P5-6 = 1.0 P4-7 ≤ 0.001 P2-8 = 0.001 P8-9 = 0.47 P3-6= 0.05 P5-8 ≤ 0.001 P3-9 = 0.008 P6-9 ≤ 0.001 Peak pressure location (mm)* Medial 6.4 ± 2.0 4.6 ± 1.8 8.5 ± 2.3 6.8 ± 2.3 4.8 ± 2.3 8.6 ± 2.3 7.7 ± 1.9 4.5 ± 1.7 9.9 ± 2.6 P1-2 = 0.17 P1-3 = 0.11 P1-4 = 1.0 P4-5 =0.15 P4-6 = 0.157 P1-7 = 0.32 P7-8 = 0.02 P7-9 = 0.08 P2-3 ≤ 0.001 P2-5 = 1.0 P5-6 = 0.07 P4-7 = 0.77 P2-8 = 1.0 P8-9 ≤ 0.001 P3-6= 1.0 P5-8 = 1.0 P3-9 = 0.24 P6-9 = 0.5

Lateral 7.4 ± 1.7 8.8 ± 3.0 6.9 ± 1.5 8.1 ± 3.7 8.3 ± 3.4 7.1 ± 3.9 7.3 ± 2.5 8.6 ± 2.5 6.7 ± 2.1 P1-2 = 0.55 P1-3 = 1.0 P1-4 = 1.0 P4-5 = 1.0 P4-6 = 0.56 P1-7 = 1.0 P7-8 = 0.541 P7-9 = 1.0 P2-3 = 0.2 P2-5 = 1.0 P5-6 = 1.0 P4-7 = 1.0 P2-8 = 1.0 P8-9 = 0.14 P3-6=1.0 P5-8 = 1.0 P3-9 = 1.0 P6-9 = 1.0 *Peak pressure location is measured in millimeters caudal to the apex of the medial coronoid process in the medial compartment and from the most cranial extent of the radial head in the lateral compartment.

Anatomic Location of Peak Contact Pressure

Flexion or extension of the elbow did not significantly alter the anatomic location of

PCP in the medial compartment relative to the apex of the medial coronoid process, or in the lateral compartment relative to the most cranial aspect of the radial head at any antebrachial rotation (Table 2-1, Figure 2-1). At all elbow flexion angles, the anatomic location of PCP in the medial compartment was on average 4.4 mm (range 3.9-5.4 mm) closer to the apex of the medial coronoid process in pronation than in supination.

Although the anatomic location of PCP moved on average 1.6 mm (range 1.2-2.2 mm) further away from the cranial aspect of the radial head in the lateral compartment during pronation and 0.7 mm (range 0.5-1.0 mm) closer during supination, these differences were not statistically significant.

Figure 2-7. PCP location

3D Alignment

Humeroradial articulation

The humeral origin was defined as the midpoint of a line drawn between the medial and lateral epicondyles. At an elbow flexion angle of 135° and a neutral antebrachial rotation, the center of the radial head was located 10.2 ± 2.3 mm caudal,

3.0 ± 1.9 mm lateral and 14.4 ± 1.1 mm distal to the humeral origin (Table 2-2). The

center of the radial head was externally rotated 33.6 ± 4.2° and was 29.2 ± 4.7° varus

relative to the humeral origin. The humeroradial flexion angle was significantly more

extended during pronation and more flexed during supination than the elbow flexion

angle measured during mechanical testing.

During pronation and supination, no significant translations in either the cranial- caudal, medial-lateral or proximal-distal planes occurred. During pronation, the center of the radial head rotated a mean of 5.0° (range 3.5-6.2°) internally relative to the humeral origin and a further 4° (range 2.4-4.5°) varus was induced compared with a neutral antebrachial rotation. During supination, the center of the radial head rotated an average of 6.8° (range 4.8-7.2°) externally relative to the humeral origin and varus was reduced by a mean of 9.0° (range 6.6-12.4°). When the elbow was flexed to 115°, the center of the radial head displaced 2.4 mm (range 2.1-2.7 mm) more cranial and 3.4 mm (range 3.2-3.5 mm) more proximal on the axial plane towards the humeral origin.

The center of the radial rotated 19.0° (range 17.6-19.6°) more external relative to the humeral origin and varus increased by 11.0° (range 8.1-13.0°). When the elbow was extended to 155°, the center of the radial head displaced 3 mm (range 2.8-3.3 mm) more caudal and 2.0 mm (range 1.9-2.1 mm) further distal from the humeral origin. The

center of the radial head rotated 13.0° (range 10.9-15.2°) more internal and varus decreased 14.8° (range 14-16.1°) compared with 135° elbow flexion.

Humeroulnar articulation

At an elbow flexion angle of 135° and a neutral antebrachial rotation, the apex of the MCP was located 12.9 ± 1.9 mm caudal, 6.3 ± 2.6 mm medial and 10.4 ± 3.0 mm distal to the origin of the humerus (Table 2-3). The apex of the MCP was externally rotated 32.1 ± 5.8° and 30.5 ± 6.2° varus relative to the humeral origin. When compared with the elbow flexion angle measured during testing, the calculated humeroulnar flexion angle was ≤ 5° difference for 135° and 115°, and a mean of 12° lower for 155°.

During both pronation and supination, no significant translations in either cranial-

caudal, medial-lateral planes or proximal-distal planes occurred. During pronation, the apex of the MCP rotated an average of 1.2° (range 0.7-1.9°) internal relative to the

humeral origin and 1.5° (range 1.1-1.9°) further varus was induced. During supination,

the apex of the MCP rotated an average of 1.9° (range 0.7-1.9°) more external relative

to the humeral origin and no significant alteration in varus-valgus angulation occurred.

When the elbow was flexed to 115°, the apex of the MCP displaced 2.2 mm (range 1.8-

2.7 mm) more caudal and 4.0 mm (range 3.5-4.8 mm) more proximal on an axial plane

toward the humeral origin. The apex of the MCP was rotated 21.0° (18.6-23.7°) more

external and 10° (range 9.3-11.1°) more varus was induced compared with an elbow

flexion angle of 135°. When the elbow was extended to 155°, the apex of the MCP

displaced 2.8 mm (range 2.8-2.9 mm) more cranial and 3.4 mm (range 2.9-3.8 mm)

more distal on an axial plane away from the humeral origin. The apex of the MCP was

rotated 14.0° (range 13.7-14.9°) more internal and 12.8° (range 12.4-13.2°) less varus

was induced compared with an elbow flexion angle of 135° and a neutral antebrachial rotation.

Proximal radioulnar articulation

At an elbow flexion angle of 135° and a neutral antebrachial rotation, the apex of the MCP was 1.5 ± 4.5 mm caudal, 10.3 ± 2.6 mm medial 3.9 ± 2.2° internally rotated relative to the center of the radial head (Table 2-4).

During pronation, the apex of the MCP moved 1.0 mm (range 0.7-1.2 mm) more

cranial relative to the center of the radial head at 115° and 155° elbow flexion, however

no other significant rotation or translation was noted. During supination, the apex of the

MCP translated 2.8 mm (range 2.2-3.3 mm) more caudal and 1.6 mm (range 1.1-1.7 mm) more medial relative to the center of the radial head. No significant alteration in alignment was noted when the elbow was flexed to 115° or extended to 155°.

Table 2-2. Static 3D alignment of the humeroradial articulation for the nine elbow poses tested

135° 115° 155°

Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Translations (mm) Cranial- 10.2 ± 2.3 10.6 ± 2.4 11.1 ± 1.6 12.9 ± 2.5 13.1 ± 2.4 13.2 ± 1.9 7.1 ± 2.6 7.6 ± 2.1 8.1 ± 1.8 caudal P1-2 = 0.44 P1-3 = 0.24 P1-4 ≤ 0.001 P4-5 = 1.0 P4-6 = 0.7 P4-7 ≤ 0.001 P7-8 = 0.63 P7-9 = 0.19 P2-3 = 0.76 P2-5 ≤ 0.001 P5-6 = 1.0 P1-7 ≤ 0.001 P2-8 ≤ 0.001 P8-9 = 0.1 P3-6 ≤ 0.001 P5-8 ≤ 0.001 P3-9 ≤ 0.001 P6-9 ≤ 0.001

Medial- 3.0 ± 1.9 3.8 ± 1.1 4.1 ± 0.8 4.2 ± 1.5 3.6 ± 1.7 4.4 ± 1.0 3.2 ± 1.4 2.8 ± 1.7 3.2 ± 1.5 lateral P1-2 = 0.09 P1-3 = 0.248 P1-4 = 0.02 P4-5 = 0.06 P4-6 = 1.0 P4-7 = 0.02 P7-8 = 0.62 P7-9 = 1.0 P2-3 = 1.0 P2-5 = 1.0 P5-6 = 0.09 P1-7 = 1.0 P2-8 = 0.11 P8-9 = 0.85 P3-6 = 0.65 P5-8 = 0.06 P3-9 = 0.26 P6-9 = 0.03

Proximal- -14.4 ± 1.1 -14.1 ± 1.4 -13.9 ± 1.9 -11.0 ± 1.6 -10.6 ± 1.7 -10.7 ± 2.0 -16.3 ± 1.9 -16.1 ± 1.8 -16.0 ± 1.6 distal P1-2 = 0.18 P1-3 = 0.42 P1-4 ≤ 0.001 P4-5 = 0.78 P4-6 = 0.71 P4-7 ≤ 0.001 P7-8 = 1.0 P7-9 = 0.2 P2-3 = 1.0 P2-5 ≤ 0.001 P5-6 = 1.0 P1-7 ≤ 0.001 P2-8 ≤0.001 P8-9 = 1.0 P3-6 ≤ 0.001 P5-8 ≤ 0.001 P3-9 ≤ 0.001 P6-9 ≤ 0.001

Table 2-2. Continued

135° 115° 155°

Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Rotations (°) Flexion- 138.2 ± 5.6 148.2 ± 6.6 121.4 ± 7.7 123.2 ± 7.3 132.4 ± 8.7 110.0 ± 7.5 146.1 ± 6.9 157.1 ± 6.8 125.6 ± 5.9 extension P1-2 ≤ 0.001 P1-3 ≤ 0.001 P1-4 ≤ 0.001 P4-5 ≤ 0.001 P4-6 ≤ 0.001 P4-7 ≤ 0.001 P7-8 ≤ 0.001 P7-9 ≤ 0.001 P2-3 ≤ 0.001 P2-5 ≤ 0.001 P5-6 ≤ 0.001 P1-7 ≤ 0.001 P2-8 ≤ 0.001 P8-9 ≤ 0.001 P3-6 ≤ 0.001 P5-8 ≤ 0.001 P3-9 = 0.03 P6-9 ≤ 0.001

Internal- 33.6 ± 4.2 28.2 ± 3.4 40.8 ± 4.1 52.0 ± 5.5 45.8 ± 5.9 60.4 ± 4.4 20.8 ± 3.6 17.3 ± 3.5 25.6 ± 4.3 external P1-2 ≤ 0.001 P1-3 ≤ 0.001 P1-4 ≤ 0.001 P4-5 ≤ 0.001 P4-6 ≤ 0.001 P4-7 ≤ 0.001 P7-8 ≤ 0.001 P7-9 ≤ 0.001 P2-3 ≤ 0.001 P2-5 ≤ 0.001 P5-6 ≤ 0.001 P1-7 ≤ 0.001 P2-8 ≤ 0.001 P8-9 ≤ 0.001 P3-6 ≤ 0.001 P5-8 ≤ 0.001 P3-9 ≤ 0.001 P6-9 ≤ 0.001

Varus- 29.2 ± 4.7 33.7 ± 4.5 20.8 ± 5.6 41.3 ± 4.5 46.7 ± 3.7 28.9 ± 5.2 15.2 ± 4.9 17.6 ± 4.7 8.6 ± 4.1 valgus P1-2 ≤ 0.001 P1-3 ≤ 0.001 P1-4 ≤ 0.001 P4-5 ≤ 0.001 P4-6 ≤ 0.001 P4-7 ≤ 0.001 P7-8 ≤ 0.001 P7-9 ≤ 0.001 P2-3 ≤ 0.001 P2-5 ≤0.001 P5-6 ≤ 0.001 P1-7 ≤ 0.001 P2-8 ≤ 0.001 P8-9 ≤ 0.001 P3-6 ≤ 0.001 P5-8 ≤ 0.001 P3-9 ≤ 0.001 P6-9 ≤ 0.001 For translational variables, positive values indicate cranial, distal and medial positions of the radius relative to the humerus. For the rotational variables, positive values indicate greater external radial rotation and varus. P-values for post-hoc pairwise comparisons are given where significant differences were detected by ANOVA.

Table 2-3. Static 3D alignment of the humeroulnar articulation for the nine elbow poses tested 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Translations (mm) Cranial- 12.9 ± 1.9 12.6 ± 1.8 13.5 ± 2.5 15.1 ± 1.7 15.3 ± 2.3 15.3 ± 1.8 10.1 ± 2.9 9.8 ± 3.3 10.6 ± 2.1 caudal P1-2 = 0.73 P1-3 = 0.2 P1-4 ≤ 0.001 P4-5 = 1.0 P4-6 = 1.0 P4-7 ≤ 0.001 P7-8 = 0.86 P7-9 = 0.76 P2-3 = 0.04 P2-5 ≤ 0.001 P4-5 = 1.0 P1-7 ≤ 0.001 P2-8 = 0.003 P8-9 = 0.45 P3-6 = 0.04 P5-8 ≤ 0.001 P3-9 ≤ 0.001 P6-9 ≤ 0.001

Medial- -6.3 ± 2.6 -6.3 ± 2.6 -5.0 ± 2.4 -5.1 ± 2.8 -5.5 ± 2.7 -4.8 ± 2.4 -6.6 ± 2.2 -7.3 ± 2.5 -6.0 ± 2.7 lateral P1-2 =1.0 P1-3 = 0.004 P1-4 = 0.09 P4-5= 0.39 P4-6 = 0.35 P4-7 =0.01 P7-8 = 0.02 P7-9 = 0.08 P2-3 ≤ 0.001 P2-5 = 0.42 P5-6 = 0.03 P1-7 = 0.72 P2-8 = 0.03 P8-9 ≤ 0.001 P3-6 = 1.0 P5-8 = 0.05 P3-9 = 0.07 P6-9 = 0.04

Proximal- -10.4 ± 3.0 -11.4 ± 3.0 -10.2 ± 3.5 -6.9 ± 3.1 -6.6 ± 3.5 -6.3 ± 3.7 -14.2 ± 2.5 -14.3 ± 3.3 -13.6 ± 3.5 distal P1-2 = 0.2 P1-3 = 1.0 P1-4 ≤ 0.001 P4-5= 0.76 P4-6 = 0.24 P4-7 ≤ 0.001 P7-8 = 1.0 P7-9 = 0.53 P2-3= 0.09 P2-5 ≤ 0.001 P5-6 = 0.62 P1-7 ≤ 0.001 P2-8 ≤ 0.001 P8-9 = 0.12 P3-6 < 0.001 P5-8 ≤ 0.001 P3-9 ≤ 0.001 P6-9 ≤ 0.001

Table 2-3. Continued 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Rotations (°) 133.5 ± 7.7 136.3 ± 7.7 129.8 ± 7.7 118.6 ± 8.0 120.3 ± 9.3 114.9 ± 7.9 142.6 ± 7.3 146.0 ± 7.1 139.5 ± 7.3 Flexion- P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P = 0.14 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 extension 1-2 1-3 1-4 4-5 4-6 4-7 7-8 7-9 P ≤ 0.001 P < 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 2-3 2-5 5-6 1-7 2-8 8-9 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 3-6 5-8 3-9 P ≤ 0.001 6-9

32.1 ± 5.8 30.2 ± 5.4 34.0 ± 6.4 50.7 ± 6.3 50.0 ± 6.8 53.9 ± 6.4 17.6 ± 3.8 16.5 ± 4.5 19.1 ± 4.7 Internal- P = 0.04 P = 0.02 P ≤ 0.001 P = 1.0 P ≤ 0.001 P ≤ 0.001 P = 0.06 P = 0.02 external 1-2 1-3 1-4 4-5 4-6 4-7 7-8 7-9 P = 0.001 P ≤ 0.001 P = 0.006 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 2-3 2-5 5-6 1-7 2-8 8-9 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 3-6 5-8 3-9 P ≤ 0.001 6-9

30.5 ± 6.2 32.4 ± 6.7 30.9 ± 6.6 41.6 ± 5.4 43.0 ± 4.8 40.2 ± 5.1 18.1 ± 5.9 19.2 ± 6.4 18.0 ± 6.2 Varus- P ≤ 0.001 P = 1.0 P ≤ 0.001 P = 0.09 P ≤ 0.001 P ≤ 0.001 P = 0.07 P = 1.0 valgus 1-2 1-3 1-4 4-5 4-6 4-7 7-8 7-9 P = 0.1 P ≤ 0.001 P = 0.002 P < 0.001 P ≤ 0.001 P = 0.04 2-3 2-5 5-6 1-7 2-8 8-9 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 3-6 5-8 3-9 P6-9 ≤ 0.001 For translational variables, positive values indicate cranial, distal and lateral positions of the proximal ulna relative to the humerus. For the rotational variables, positive values indicate greater humeroulnar flexion, external ulnar rotation and varus. P-values for post-hoc pairwise comparisons are given where significant differences were detected by ANOVA.

Table 2-4. Static 3D alignment of the proximal radioulnar articulation for the nine elbow poses tested 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Translations (mm) Cranial- -1.5 ± 4.5 -1.5 ± 4.3 -4.3 ± 5.0 -2.3 ± 4.6 -0.7 ± 4.8 -4.5 ± 5.1 -2.3 ± 5.3 -1.2 ± 5.1 -5.6 ± 2.8 caudal P1-2 = 1.0 P1-3 ≤ 0.001 P1-4 = 0.24 P4-5 ≤ 0.001 P4-6 ≤ 0.001 P4-7 = 1.0 P7-8 = 0.02 P7-9 = 0.03 P2-3 ≤ 0.001 P2-5 = 0.3 P5-6 ≤ 0.001 P1-7 = 0.3 P2-8 = 1.0 P8-9 = 0.003 P3-6 = 0.82 P5-8 = 0.56 P3-9 = 0.48 P6-9 = 0.79

Medial- -10.3 ± 2.6 -10.5 ± 2.2 -8.6 ± 4.4 -10.1 ± 2.3 -10.2 ± 1.8 -9.0 ± 4.3 -10.0 ± 3.1 -10.2 ± 2.4 -8.1 ± 3.7 lateral P1-2 = 1.0 P1-3 = 0.13 P1-4 = 1.0 P4-5 = 1.0 P4-6 =0.34 P4-7 = 1.0 P7-8 = 1.0 P7-9 = 0.007 P2-3 = 0.18 P2-5 = 0.97 P5-6 = 0.70 P1-7 = 1.0 P2-8 = 0.90 P8-9 = 0.02 P3-6 = 0.70 P5-8 = 1.0 P3-9 = 0.15 P6-9 = 0.08

Proximal- -0.3 ± 2.3 -0.9 ± 2.9 -0.4 ± 2.2 -0.9 ± 2.6 -1.1 ± 3.1 -0.7 ± 2.4 -0.9 ± 2.3 -0.9 ± 2.5 -0.2 ± 2.8 distal P1-2 = 0.62 P1-3 = 1.0 P1-4 = 0.27 P4-5 = 1.0 P4-6 = 1.0 P4-7 = 1.0 P7-8 = 1.0 (P7-9 = 0.98 P2-3 = 0.73 P2-5 = 1.0 P5-6 = 1.0 P1-7 = 0.76 P2-8 = 1.0 P8-9 = 0.91 P3-6 = 0.44 P5-8 = 1.0 P3-9 = 1.0 P6-9 = 1.0

Table 2-4. Continued 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination

(1) (2) (3) (4) (5) (6) (7) (8) (9) Rotations (°) Flexion- -2.6 ± 4.1 -9.1 ± 2.9 12.4 ± 6.1 -1.9 ± 3.0 -8.4 ± 3.1 11.7 ± 6.6 -2.1 ± 3.3 -9.5 ± 3.2 16.0 ± 3.1 extension P1-2 ≤ 0.001 P1-3 ≤ 0.001 P1-4 = 0.94 P4-5 ≤ 0.001 P4-6 ≤ 0.001 P4-7 = 1.0 P7-8 ≤ 0.001 P7-9 ≤ 0.001 P2-3 ≤ 0.001 P2-5 = 0.61 P5-6 ≤ 0.001 P1-7 = 1.0 P2-8 = 1.0 P8-9 ≤ 0.001 P3-6 = 0.81 P5-8 = 0.46 P3-9 = 0.13 P6-9 = 0.07

Internal- -3.9 ± 2.2 -4.2 ± 2.5 -3.8 ± 1.7 -4.4 ± 2.2 -4.1 ± 2.0 -4.2 ± 2.0 -4.1 ± 2.3 -3.8 ± 2.3 -3.8 ± 2.3 external P1-2 ≤ 1.0 P1-3 = 1.0 P1-4 = 0.26 P4-5 = 0.25 P4-6 = 1.0 P4-7 = 0.34 P7-8 = 0.67 P7-9 = 1.0 P2-3 = 1.0 P2-5 = 1.0 P5-6 = 1.0 P1-7 = 1.0 P2-8 = 0.73 P8-9 = 1.0 P3-6 = 0.44 P5-8 = 0.86 P3-9 = 1.0 P6-9 = 0.80

Varus- 3.6 ± 5.3 4.2 ± 5.7 3.2 ± 6.0 3.3 ± 5.9 4.4 ± 5.9 2.7 ± 6.0 4.1 ± 5.7 5.0 ± 5.8 3.0 ± 5.9 valgus P1-2 = 0.18 P1-3 = 1.0 P1-4 = 1.0 P4-5 ≤ 0.001 P4-6 = 0.28 P4-7 = 0.17 P7-8 = 0.08 P7-9 = 0.04 P2-3 = 0.24 P2-5 = 1.0 P5-6 = 0.001 P1-7 = 1.0 P2-8 = 0.99 P8-9 = 0.008 P3-6 = 0.82 P5-8 = 0.50 P3-9 = 0.48 P6-9 =0.79 For translational variables, positive values indicate cranial, distal and lateral positions of the distal ulna relative to the radius. For the rotational variables, positive values indicate greater radioulnar flexion, external ulnar rotation and varus. P-values for post-hoc pairwise comparisons are given where significant differences were detected by ANOVA.

Discussion

Our results substantiate that both antebrachial rotation and elbow flexion and extension caused alterations in elbow contact mechanical parameters and alignment. In contrast to our first proposed hypothesis, both pronation and supination increased PCP

and decreased CA in both medial and lateral elbow compartments. We suspect that this

increase in PCP associated with antebrachial rotation occurs due to relative tightening

of the joint as the medial and lateral collateral ligaments become progressively more

taut in order to restrict motion to the sagittal plane.118 Consistent with our hypothesis,

when the elbow was flexed to 115°, PCP increased in the medial compartment due to

proximal displacement in the axial plane of the MCP and radial head toward the distal

humerus, as well as the increased varus angulation induced with elbow flexion. Similarly

when the elbow was extended to 155°, PCP decreased in both compartments due to

distal displacement in the axial plane of the MCP and radial head relative to the origin of

the humerus and decreased varus angulation. Alterations of the mechanical axis

relative to the anatomic axis at different elbow flexion angles may also contribute to the

changes in PCP.

This study established three distinct areas of contact in the elbow (radial head,

medial coronoid process and craniolateral aspect of the anconeus) consistent with

previous humeroradial and humeroulnar joint contact area mapping performed using

joint casting techniques, and subchondral bone densities measured with CT

osteoabsorptiometry.20,112 The articular surface of the ulna contributed substantially to

load transfer through the elbow, consistent with previous studies utilizing tactile array

pressure sensors, and with the higher subchondral bone densities noted in the ulna

compared to the radial head in dogs of all ages.20,21 Our study is the first to report the

effect of pronation and supination on the topography of the contact points in the

humeroradial and humeroulnar joints. Pressure mapping is an accepted method for

evaluating the interaction of the articular surfaces during motion.113,122 Both in-vivo and

ex-vivo studies have correlated change in location of contact points with abnormal kinematics and development of osteoarthritis.113 In accordance with our hypothesis, in

this static, cadaveric model the anatomic location of PCP in the medial compartment

shifted closer to the apex of the MCP in pronation and shifted further away from the

apex in supination. Interpretation of these findings with respect to a biomechanical basis

for elbow dysplasia is difficult because the limbs tested in this study were normal.

However, the observation that supination shifts the point of PCP away from the apex of

the MCP suggests that dogs with MCD that stand with the elbow adducted and

antebrachium supinated may be an attempt to unload the MCP. Similar compensatory

postural adaptations have been reported in people with unilateral knee osteoarthritis.123

Simultaneous in-vivo joint contact mapping and 3D alignment of human or dog elbows has not previously been reported. Correlations between elbow biomechanics in

people with those of dogs must be interpreted with caution. Although similar

anatomically, there are notable functional differences, the most obvious being the weight-bearing function in dogs and the necessity for antebrachial rotation for everyday activity in people.14 The varus deviation noted in the normal dog elbows in this study is

consistent with the relative pronation of the dog antebrachium, compared with humans

who usually hold the antebrachium in supination, resulting in a carrying angle or valgus

deviation of 10-15°.14 3D kinematic studies of the superior radioulnar joint in people

have revealed that during pronation and supination the radius glides around a relatively

immovable ulna, although slight radial deviation of the ulna may occur during rotation

with a fixed manus.14,124 Although both situations may occur in quadrupeds, depending on the phase of gait occurring during the antebrachial rotation, the latter combined with

axial load transmission is considered the dominant pose and was simulated in this

study. Consistent with the results reported in people, minimal axial rotation of the ulna

occurred during antebrachial rotation, with motion primarily occurring due to gliding of

the radial head within the radial incisure and pronation caused increasing varus of the

elbow, with decreasing varus in supination.124 In contrast with our hypothesis, proximal-

distal displacement of the radial head and apex of the medial coronoid process relative

to the humeral origin or each other did not occur with pronation or supination, although

this may be a reflection of the minute displacements detected and the small sample

size.

Histopathological studies have demonstrated that medial coronoid disease likely

occurs as a result of humero-ulnar incongruency, resulting in repetitive excessive

loading, fatigue failure and microfracture formation in the trabecular subchondral bone

of the medial coronoid process.9,10 We evaluated only normal elbows and therefore cannot draw any conclusion regarding the mechanism of elbow dysplasia. However, the significant effect of antebrachial rotation on contact pressures would suggest that rotational alignment of the humeroradial and humeroulnar joints may be considered an important mechanical variable in elbow biomechanics, as suggested by other authors.45,60

This is the first quantitative descriptive analysis of the ex-vivo contact mechanics

and 3D alignment of normal dog elbows. The results of previous biomechanical studies

investigating the normal dog elbows and surgical interventions should be re-interpreted in light of the results of our study. Future investigations are warranted to evaluate the contact mechanics and 3D alignment in pathologic elbows, in particular elbows from dogs affected with fragmented medial coronoid process, and the biomechanical effects of surgical procedures advocated for the treatment of medial coronoid disease.

CHAPTER 3 EFFECT OF PROXIMAL ULNAR ROTATIONAL OSTEOTOMY ON CONTACT MECHANICS AND 3D ALIGNMENT OF NORMAL DOG ELBOWS

Background

In people affected with unicompartmental knee osteoarthritis, collapse of the medial compartment may occur due to a pre-existing varus deformity or progressive narrowing of the medial joint space secondary to osteoarthritis. Progressive cartilage loss and narrowing of the medial compartment joint space results in increased load transmission in the medial compartment which contributes to the degenerative process.125-128 High tibial osteotomies are often performed to unload the medial compartment in people with medial gonarthrosis.129-131 These osteotomies shift the mechanical axis of the limb through the lateral compartment of the knee, unloading the affected medial compartment.125,132 Cartilage resurfacing of the medial compartment has been observed in response to the improved mechanical environment.133,134

Many dogs affected with MCD develop progressive collapse of the medial joint space, suggesting that a similar mechanism may occur in these dogs as in humans with unicompartmental knee osteoarthritis.135 Thus, humeral or ulnar osteotomies designed to unload the medial compartment have been developed to manage dogs with

MCD.45,98,99,101 Sliding humeral osteotomy has been demonstrated to shift the mechanical axis of the humerus away from the medial compartment of the elbow98,99, with fibrocartilage being reported to resurface medial compartment full cartilage defects following this procedure.101

Gait adaptations have also been shown to effectively unload the medial compartment of the knee in people with medial compartment osteoarthritis.136-138

Walking with a toe-out gait reduces knee adduction moment, particularly in the second

phase of stance, shifting the ground reaction force more posterior and less medial, and mitigates the progression of medial knee osteoarthritis.136 Dogs with FMCP and MCD of

the elbow often stand with the affected forelimb supinated, elbow adducted, carpus

abducted and paw externally rotated.34 Whether this posture is involved in the

pathogenesis of this condition or an adaptive mechanism to unload the medial

compartment remains unclear. If this posture is an adaptive mechanism to ameliorate

pain, altering the mechanical axis through the elbow by simulating this posture might

unload the medial elbow compartment. Our previous study examining the joint

mechanics of normal dog elbows at three different elbow flexion and antebrachial

rotation angles demonstrated a significant caudal shift of peak contact pressure on the

medial coronoid process during supination compared with pronation. This pressure shift

could potentially unload the pathologic MCP. Based on these observations, we wanted

to evaluate an ulnar osteotomy procedure that could mimic supination of the

antebrachium to shift the mechanical axis of the forelimb and load transmission from the medial to the lateral compartment of the elbow.

The purpose of the study was to investigate the effects of this proximal ulnar rotational osteotomy on the contact mechanics and 3D alignment of normal dog elbows.

We hypothesized that proximal ulnar rotational osteotomy would decrease the contact pressures in the medial compartment with corresponding increases in the lateral compartment. We further hypothesized that proximal ulnar rotational osteotomy would

displace the apex of the MCP caudal and distal relative to the radial head, and reduce

varus angulation through the humeroradial articulation, with minimal changes in

alignment through the humeroulnar articulation.

Materials and Methods

Specimen Preparation

Thoracic limbs (n=12) were collected by disarticulation at the glenohumeral joint in

12 adult dogs weighing 26 ± 4 kg that were euthanatized for reasons unrelated to this

study. An equal number of unpaired right (n=6) and left (n=6) thoracic limbs were

selected for this study. Orthogonal radiographic projections were obtained of each

elbow and specimens were excluded if there was evidence of elbow pathology. The

specimens were wrapped in saline (0.9% NaCl) solution soaked towels, sealed in

double plastic bags and stored at -20°C. The limbs were prepared and loaded in the

custom jig in the material testing machine exactly as described in Chapter 2.

Testing Protocol

A static axial load of 200 N was applied by the material testing machine.116

Instantaneous intra-articular contact area and pressure measurements were acquired after maintaining peak force for 5 seconds using the I-scan system (Tekscan Inc.,

Boston, MA). Each sensor was conditioned and calibrated according to the manufacturer’s guidelines immediately before testing of the specimen. With the specimen loaded at 200 N, the 3D static pose of the fiduciary markers on each the humerus, radius, ulna and third metacarpal were digitized using a Microscribe 3DX digitizing arm (Immersion Corp., San Jose, CA).

Testing Sequence

Each specimen was tested with the antebrachium positioned in a neutral position,

in 16º pronation and in 28º supination.118 This testing protocol was repeated with the

elbow in 115º and 155º of flexion ± 5° by adjusting the turnbuckle. Following conclusion

of acquisition of biomechanical parameters in normal elbows, the transcondylar humeral

screw was removed and the medial epicondyle reflected distally to allow access to the proximal radioulnar joint surface and removal and examination of the sensing strips. A caudolateral approach was made to the proximal ulna between ulnaris lateralis and the ulnar and humeral heads of the deep digital flexor muscles which were reflected from the ulnar surface using Freer periosteal elevator. An incomplete transverse osteotomy was made in the proximal ulna using an oscillating bone saw, 10 mm distal to the radial head. A custom 2.7 mm dynamic compression bone plate with a 30° central pivot was applied to the proximal ulnar segment with two 10 mm long 2.7 mm cortical bone screws. The osteotomy was completed and a Kern bone-holding forceps applied to the olecranon was used to pivot the proximal ulnar segment externally around the radial head. The joint surface was inspected to eliminate gap formation at the between the radial head and the radial incisure of the ulna. The plate was loosely secured to the distal segment with an 18 mm long 2.7 mm cortical bone screw. Plate luting was performed using polymethylmethacrylate (Technovit Liquid/Powder Kit, Jorgensen

Laboratories, Inc., Loveland, Co) and the distal screw tightened while the joint surface was examined. When the acrylic was dry, a 10 mm long 2.7 mm cortical bone screw was placed in the distal segment. The sensor was re-calibrated according to the

manufacturer’s recommendations and replaced in the elbow. The specimen was loaded

in the MTS machine and the testing sequence was repeated.

Figure 3-1. Location and orientation of the osteotomy

Figure 3-2. A. Plate luting and stabilization and B. caudal displacement of the apex of the MCP

Data Analysis

The digital pressure sensing data acquisition software was used to generate a

contact map and measure CA, CP and PCP in the medial, lateral and total (medial +

lateral) elbow compartments. Rotations (flexion/extension, varus/valgus,

internal/external) and translations (cranial-caudal, medial-lateral, proximal-distal) of the center of the radial head relative to the origin of the humerus (a point midway on a line between the medial and lateral epicondyles), the apex of the MCP relative to the humeral origin and of the center of the radial head to the apex of the MCP were

calculated using body-fixed axes as previously described. Calculations were performed

using a custom-written computer program (Matlab, The MathWorks Inc., Natick, MA).

The alignment of the humeroradial articulation is described as the displacement of the

center of the radial head relative to the humeral origin, the humeroulnar articulation as

the displacement of the apex of the medial coronoid process of the ulna relative to the

humeral origin, and the proximal radioulnar articulation as the displacement of the apex

of the medial coronoid process relative to the center of the radial head.

Figure 3-3. Contact parameters and 3D alignment A. pre- and B. post-osteotomy

Statistical Analysis

Separate 1-way repeated measures ANOVAs were used to evaluate differences in dependent variables of contact mechanics (CA, CP and PCP in the medial and lateral

elbow compartments) at the three elbow flexion angles (115º, 135º and 155º) and the 3

elbow positions (neutral, supination, pronation) prior to and post-osteotomy. Separate 1-

way repeated measures ANOVAs was used to evaluate differences in rotation (internal- external rotation, varus-valgus, flexion-extension), and translation (cranial-caudal,

proximal-distal and medial-lateral) for each the humeroradial, humeroulnar and

radioulnar articulations for each of the nine elbow poses prior to and post-osteotomy.

Where significant differences were detected, post-hoc pairwise comparisons with a

Bonferroni adjustment were calculated. All statistical analyses were performed using

SPSS statistical software (version 17.0; SPSS Inc., Chicago, IL), with the level of

significance for all statistical tests set at α ≤.05.

Results

Contact Mechanics

Following proximal ulnar rotational osteotomy, CP decreased a mean of 10%

(range 5-18%) in the medial compartment and increased a mean of 17% (range 5-30%) in the lateral compartment. PCP decreased a mean of 9% (range 4-15%) in the medial compartment and increased a mean of 13% (range 9-27%) in the lateral compartment.

CA did not change significantly in the medial compartment; however CA decreased a mean of 19% (range 10-33%) in the lateral compartment.

Table 3-1. Contact mechanical parameters obtained from the medial and lateral elbow compartments pre- and post- osteotomy for each of the nine elbow poses tested 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Contact Pressure (MPa) Medial Pre 3.4 ± 0.5 3.6 ± 0.6 3.9 ± 0.6 3.8 ± 0.6 3.9 ± 0.7 3.8 ± 0.7 3.3 ± 0.6 3.0 ± 1.1 3.9 ± 0.8 Post 3.1 ± 0.5 3.1 ± 0.8 3.4 ± 0.6 3.5 ± 0.6 3.4 ± 0.8 3.6 ± 0.7 3.1 ± 0.7 2.8 ± 0.9 3.2 ± 0.8 P = 0.024 P = 0.052 P = 0.243 P = 0.093 P ≤ 0.001 P = 0.006 P = 0.33 P = 0.627 P = 0.882 Lateral Pre 3.3 ± 0.8 3.5 ± 0.9 3.5 ± 0.7 3.6 ± 0.7 3.8 ± 0.7 3.7 ± 0.5 3.0 ± 0.6 2.9 ± 0.9 3.2 ± 0.7 Post 4.2 ± 0.7 4.2 ± 1.0 4 ± 0.8 3.9 ± 0.6 4.3 ± 1.0 3.9 ± 0.8 3.7 ± 0.9 3.6 ± 0.8 3.8 ± 0.8 P ≤ 0.001 P = 0.009 P = 0.027 P = 0.014 P = 0.058 P = 0.195 P = 0.005 P = 0.006 P = 0.003

Peak Contact Pressure (MPa) Medial Pre 8.1 ± 1.5 8.5 ± 1.8 8.6 ± 1.9 8.6 ± 1.8 9.0 ± 1.9 8.3 ± 1.4 7.1 ± 1.3 7.0 ± 2.7 8.3 ± 1.4 Post 7.3 ± 1.2 7.4 ± 1.7 8.0 ± 1.3 7.9 ± 1.6 7.8 ± 1.7 8.0 ± 1.3 6.5 ± 1.4 6.5 ± 2.1 7.6 ± 1.1 P = 0.004 P = 0.072 P = 0.125 P = 0.075 P = 0.028 P = 0.125 P = 0.116 P = 0.479 P = 0.125

Lateral 7.9 ± 1.6 Pre 7.9 ± 1.4 7.9 ± 1.7 8.4 ± 1.5 7.8 ± 1.4 8.5 ± 1.8 8.3 ± 1.5 6.3 ± 1.0 7.0 ± 1.4 8.4 ± 1.5 Post 9.7 ± 1.7 8.6 ± 1.7 9.3 ± 1.5 9.1 ± 1.6 9.4 ± 1.7 9.1 ± 1.6 8 ± 1.4 7.7 ± 1.5 P = 0.081 P ≤ 0.001 P = 0.03 P = 0.002 P ≤ 0.001 P = 0.015 P = 0.013 P ≤ 0.001 P = 0.025

Contact Area (mm2) Medial Pre 85.2 ± 14.8 67.3 ± 14.7 68.7 ± 15.2 72.5 ± 12.9 70.7 ± 13.8 60.5 ± 15.5 74.6 ± 16.8 63 ± 18.6 59.6 ± 12.3 Post 83.8 ± 18.8 64.7 ± 19.5 78 ± 24.2 76.3 ± 12.9 62.7 ± 14.4 77.2 ± 17.7 69.2 ± 22.4 56.6 ± 19.8 60.3 ± 15.5 P = 0.796 P = 0.317 P = 0.243 P = 0.796 P = 0.317 P = 0.006 P = 0.796 P = 0.317 P = 0.882 Lateral Pre 118.8 ± 23.6 110.1 ± 24.5 110.8 ± 18.5 122.7 ± 20.0 115.3 ± 25.4 111.7 ± 16.4 109.1 ± 18.2 98.4 ± 24.4 96.6 ± 19.3 Post 79.1 ± 14.7 86.4 ± 17.4 92.3 ± 22.5 100 ± 23.0 93.7 ± 23.3 100.7 ± 19.1 85.2 ± 16.1 85.3 ± 14.1 83.2 ± 17.6 P ≤ 0.001 P = 0.01 P = 0.02 P ≤ 0.001 P = 0.017 P = 0.057 P ≤ 0.001 P = 0.033 P = 0.071

3D Alignment

Humeroradial articulation

Following proximal ulnar rotational osteotomy, no significant alterations in alignment of the center of the radial head relative to the humeral origin occurred in the cranial-caudal or medial-lateral planes. The center of the radial head moved a mean of

0.8 mm (range 0 - 1.5 mm) closer to the humeral origin, although this was only statistically significant at four of the nine elbow poses tested, and rotated externally a mean of 5° (range 1.5 – 7.7°) relative to the humeral origin following the osteotomy. The

calculated humeroradial flexion angle was significantly lower following the osteotomy at

all nine elbow poses. Varus angulation of the center of the radial head relative to the

humeral origin reduced a mean of 6.5° (range 3.3–10.0°) following the osteotomy.

Humeroulnar articulation

Following proximal ulnar rotational osteotomy, the apex of the medial coronoid

process did not translate or rotate significantly in any plane relative to the humeral

origin. There was no significant difference in humeroulnar flexion angle pre- and post-

osteotomy.

Proximal radioulnar articulation

Following proximal ulnar rotational osteotomy, the apex of the MCP translated a

mean of 2.7 mm (range 1.7 - 3.4 mm) caudal and1.2 mm (range 0.4 – 2.0 mm) abaxial

and rotated a mean of 1.2° (range 0.2 – 2.1°) externally relative to the center of the

radial head.

Table 3-2. Static 3D alignment of the humeroradial articulation pre- and post-osteotomy for the nine elbow poses tested 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Translations (mm) Cranial-caudal

Pre 10.2 ± 2.3 10.6 ± 2.4 11.1 ± 1.6 12.9 ± 2.5 13.1 ± 2.4 13.2 ± 1.9 7.1 ± 2.6 7.6 ± 2.1 8.1 ± 1.8 Post 11.0 ± 2.3 10.5 ± 2.1 11.8 ± 1.8 13.0 ± 2.4 12.8 ± 2.4 13.5 ± 2.1 7.9 ± 1.8 7.5 ± 1.8 8.8 ± 1.5

P = 0.159 P = 0.73 P = 0.09 P = 0.66 P = 0.26 P = 0.12 P = 0.04 P = 0.51 P = 0.06 Medial-lateral

Pre 3.0 ± 1.9 3.8 ± 1.1 4.1 ± 0.8 4.2 ± 1.5 3.6 ± 1.7 4.4 ± 1.0 3.2 ± 1.4 2.8 ± 1.7 3.2 ± 1.5 Post 3.2 ± 1.2 3.1 ± 1.1 3.9 ± 1.2 4.1 ± 1.1 3.1 ± 1.5 4.8 ± 1.1 3.1 ± 1.0 2.2 ± 2.2 4.1 ± 0.9

P = 0.67 P = 0.026 P = 0.68 P = 0.78 P = 0.07 P = 0.18 P = 0.83 P = 0.06 P = 0.09 Proximal-

distal -14.4 ± 1.1 -14.1 ± 1.4 -13.9 ± 1.9 -11.0 ± 1.6 -10.6 ± 1.7 -10.7 ± 2.0 -16.3 ± 1.9 -16.1 ± 1.8 -16.0 ± 1.6 Pre -13.6 ± 2.1 -13.7 ± 1.8 -13.0 ± 2.2 -10.1 ± 1.6 -10.6 ± 1.7 -9.4 ± 2.0 -15.8 ± 2.3 -15.7 ± 1.8 -15.0 ± 1.7 Post P = 0.06 P = 0.32 P = 0.04 P = 0.02 P = 0.93 P ≤ 0.001 P = 0.20 P = 0.18 P = 0.002

Rotations (°) Flexion- extension Pre 138.2 ± 5.6 148.2 ± 6.6 121.4 ± 7.7 123.2 ± 7.3 132.4 ± 8.7 110.0 ± 7.5 146.1 ± 6.9 157.1 ± 6.8 125.6 ± 5.9 Post 122.4 ± 9.7 136.2 ± 9.6 108.9 ± 6.5 110.5 ± 6.2 120.3 ± 8.3 102.2 ± 5.8 131.1 ± 8.5 143.8 ± 9.4 113.6 ± 7.7 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 Axial rotation Pre 33.6 ± 4.2 28.2 ± 3.4 40.8 ± 4.1 52.0 ± 5.5 45.8 ± 5.9 60.4 ± 4.4 20.8 ± 3.6 17.3 ± 3.5 25.6 ± 4.3 Post 40.1 ± 7.1 33.6 ± 5.8 45.1 ± 6.4 59.6 ± 2.7 53.5 ± 5.6 65.2 ± 3.7 24.0 ± 5.4 21.1 ± 6.0 27.1 ± 6.5 P = 0.14 P = 0.004 P = 0.07 P ≤ 0.001 P ≤ 0.001 P = 0.003 P = 0.007 P = 0.003 P = 0.15 Varus-valgus Pre 29.2 ± 4.7 33.7 ± 4.5 20.8 ± 5.6 41.3 ± 4.5 46.7 ± 3.7 28.9 ± 5.2 15.2 ± 4.9 17.6 ± 4.7 8.6 ± 4.1 Post 22.5 ± 4.1 28.7 ± 4.0 12.3 ± 3.1 32.2 ± 4.3 40.3 ± 3.9 18.9 ± 5.6 11.4 ± 4.9 14.3 ± 4.7 2.8 ± 3.1 P = 0.004 P = 0.01 P ≤ 0.001 P ≤ 0.001 P = 0.001 P ≤ 0.001 P = 0.013 P = 0.004 P ≤ 0.001 For translational variables, positive values indicate cranial, distal and medial positions of the radius relative to the humerus. For the rotational variables, positive values indicate greater external radial rotation and varus. P-values for post-hoc pairwise comparisons are given where significant differences were detected by ANOVA.

Table 3-3. Static 3D alignment of the humeroulnar articulation pre- and post-osteotomy for the nine elbow poses tested 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Translations (mm) Cranial-caudal

Pre 12.9 ± 1.9 12.6 ± 1.8 13.5 ± 2.5 15.1 ± 1.7 15.3 ± 2.3 15.3 ± 1.8 10.1 ± 2.9 9.8 ± 3.3 10.6 ± 2.1 Post 13.2 ± 1.5 12.5 ± 2.2 13.5 ± 2.2 14.7 ± 1.4 14.9 ± 1.5 14.9 ± 1.5 10.3 ± 2.9 9.6 ± 3.3 10.5 ± 3.5

P = 0.36 P = 0.85 P = 0.79 P = 0.27 P = 0.53 P = 0.12 P = 0.60 P = 0.66 P = 0.96 Medial-lateral

Pre -6.3 ± 2.6 -6.3 ± 2.6 -5.0 ± 2.4 -5.1 ± 2.8 -5.5 ± 2.7 -4.8 ± 2.4 -6.6 ± 2.2 -7.3 ± 2.5 -6.0 ± 2.7 Post -6.4 ± 2.9 -7.0 ± 2.9 -5.9 ± 2.9 -5.3 ± 3.0 -6.4 ± 3.0 -5.2 ± 3.1 -7.1 ± 2.8 -7.8 ± 2.1 -6.4 ± 2.7

P = 0.81 P = 0.22 P = 0.051 P = 0.62 P = 0.04 P = 0.37 P = 0.23 P = 0.32 P = 0.48 Proximal-

distal -10.4 ± 3.0 -11.4 ± 3.0 -10.2 ± 3.5 -6.9 ± 3.1 -6.6 ± 3.5 -6.3 ± 3.7 -14.2 ± 2.5 -14.3 ± 3.3 -13.6 ± 3.5 Pre -10.8 ± 3.3 -10.6 ± 4.5 -10.0 ± 4.0 -6.5 ± 3.6 -6.9 ± 4.1 -5.8 ± 3.6 -13.7 ± 2.7 -14.1 ± 2.6 -13.6 ± 2.9 Post P = 0.53 P = 0.42 P = 0.71 P = 0.40 P = 0.61 P = 0.42 P = 0.40 P = 0.62 P = 0.96

Rotations (°) Flexion- extension Pre 133.5 ± 7.7 136.3 ± 7.7 129.8 ± 7.7 118.6 ± 8.0 120.3 ± 9.3 114.9 ± 7.9 142.6 ± 7.3 146.0 ± 7.1 139.5 ± 7.3 Post 133.5 ± 8.8 137.0 ± 8.7 130.2 ± 6.8 119.1 ± 8.4 121.6 ± 8.8 115.4 ± 7.9 143.6 ± 8.6 147.1 ± 9.1 140.8 ± 8.8 P = 0.98 P = 0.70 P = 0.82 P = 0.69 P = 0.86 P = 0.62 P = 0.26 P = 0.25 P = 0.17 Axial rotation Pre 32.1 ± 5.8 30.2 ± 5.4 34.0 ± 6.4 50.7 ± 6.3 50.0 ± 6.8 53.9 ± 6.4 17.6 ± 3.8 16.5 ± 4.5 19.1 ± 4.7 Post 33.1 ± 7.0 31.1 ± 6.3 35.1 ± 7.4 51.3 ± 3.2 49.8 ± 5.6 55.0 ± 4.4 18.4 ± 3.7 17.0 ± 4.1 19.1 ± 3.8 P = 0.64 P = 0.66 P = 0.68 P = 0.68 P = 0.94 P = 0.46 P = 0.43 P = 0.59 P = 0.99 Varus-valgus Pre 30.5 ± 6.2 32.4 ± 6.7 30.9 ± 6.6 41.6 ± 5.4 43.0 ± 4.8 40.2 ± 5.1 18.1 ± 5.9 19.2 ± 6.4 18.0 ± 6.2 Post 31.4 ± 5.1 32.4 ± 4.6 31.2 ± 5.1 43.1 ± 4.6 43.7 ± 4.4 41.6 ± 4.5 18.3 ± 5.3 18.3 ± 5.4 17.6 ± 5.9 P = 0.53 P = 0.98 P = 0.87 P = 0.027 P = 0.16 P = 0.05 P = 0.81 P = 0.48 P = 0.76 For translational variables, positive values indicate cranial, distal and lateral positions of the proximal ulna relative to the humerus. For the rotational variables, positive values indicate greater humeroulnar flexion, external ulnar rotation and varus. P-values for post-hoc pairwise comparisons are given where significant differences were detected by ANOVA.

Table 3-4. Static 3D alignment of the proximal radioulnar articulation pre- and post-osteotomy for the nine elbow poses tested 135° 115° 155° Neutral Pronation Supination Neutral Pronation Supination Neutral Pronation Supination Translations (mm) Cranial-caudal

Pre -1.6 ± 4.5 -1.5 ± 4.3 -4.3 ± 5.0 -2.3 ± 4.6 -0.7 ± 4.8 -4.5 ± 5.1 -2.3 ± 5.3 -1.2 ± 5.1 -5.6 ± 2.8 Post -5.0 ± 4.4 -3.2 ± 4.4 -7.7 ± 3.5 -4.9 ± 4.2 -2.9 ± 4.9 -7.4 ± 3.5 -5.2 ± 4.7 -3.5 ± 4.7 -8.7 ± 2.8

P ≤ 0.001 P = 0.024 P = 0.001 P = 0.001 P = 0.001 P = 0.007 P = 0.001 P = 0.005 P ≤ 0.001 Medial-lateral

Pre -10.3 ± 2.6 -10.5 ± 2.2 -8.6 ± 4.4 -10.1 ± 2.3 -10.2 ± 1.8 -9.0 ± 4.3 -10.0 ± 3.1 -10.2 ± 2.4 -8.1 ± 3.7 Post -8.6 ± 3.9 -10.1 ± 3.6 -6.8 ± 5.0 -8.9 ± 3.7 - 9.7 ± 3.3 -7.1 ± 5.4 -8.9 ± 4.8 -9.6 ± 2.9 -6.1 ± 4.7

P = 0.048 P = 0.58 P = 0.002 P = 0.06 P = 0.54 P = 0.005 P = 0.17 P = 0.17 P = 0.001 Proximal-

distal -0.3 ± 2.3 -0.9 ± 2.9 -0.4 ± 2.2 -0.9 ± 2.6 -1.1 ± 3.1 -0.7 ± 2.4 -0.9 ± 2.3 -0.9 ± 2.5 -0.2 ± 2.8 Pre -0.5 ± 2.4 -0.5 ± 2.8 -0.1 ± 2.4 -0.8 ± 2.5 -0.9 ± 2.6 -0.6 ± 2.5 -0.6 ± 2.3 -0.9 ± 2.5 -1.0 ± 2.1 Post P = 0.72 P = 0.40 P = 0.50 P = 0.69 P = 0.75 P = 0.82 P = 0.59 P = 0.95 P = 0.22

Rotations (°) Flexion- extension Pre -2.6 ± 4.1 -9.1 ± 2.9 12.4 ± 6.1 -1.9 ± 3.0 -8.4 ± 3.1 11.7 ± 6.6 -2.1 ± 3.3 -9.5 ± 3.2 16.0 ± 3.1 Post 13.0 ± 6.2 2.6 ± 5.0 27.1 ± 4.7 12.8 ± 4.9 3.4 ± 4.4 25.5 ± 6.4 13.7 ± 5.8 4.2 ± 4.8 30.0 ± 4.5 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 P ≤ 0.001 Axial rotation Pre -3.9 ± 2.2 -4.2 ± 2.5 -3.8 ± 1.7 -4.4 ± 2.2 -4.1 ± 2.0 -4.2 ± 2.0 -4.1 ± 2.3 -3.8 ± 2.3 -3.8 ± 2.3 Post -2.1 ± 2.8 -2.1 ± 3.1 -2.3 ± 2.6 -3.2 ± 2.8 -2.9 ± 3.1 -3.3 ± 3.0 -2.7 ± 2.4 -3.0 ± 2.9 -3.6 ± 2.4 P = 0.052 P = 0.009 P = 0.08 P = 0.054 P = 0.04 P = 0.18 P = 0.07 P = 0.25 P = 0.83 Varus-valgus Pre 3.6 ± 5.3 4.2 ± 5.7 3.2 ± 6.0 3.3 ± 5.9 4.4 ± 5.9 2.7 ± 6.0 4.1 ± 5.7 5.0 ± 5.8 3.0 ± 5.9 Post 1.1 ± 4.8 2.7 ± 5.6 1.2 ± 5.0 0.6 ± 5.0 1.4 ± 5.1 0.5 ± 5.1 1.9 ± 4.2 2.6 ± 4.2 2.0 ± 4.8 P = 0.013 P = 0.24 P = 0.06 P = 0.03 P = 0.012 P = 0.046 P = 0.06 P = 0.04 P = 0.43 For translational variables, positive values indicate cranial, distal and lateral positions of the distal ulna relative to the radius. For rotational variables, positive values indicate greater radioulnar flexion, external ulnar rotation and varus. P-values for post-hoc pairwise comparisons are given where significant differences were detected by ANOVA.

Figure 3-4. 3D alignment of the proximal radioulnar joint post-osteotomy

Proximal ulna-distal ulna

Following proximal ulnar rotational osteotomy, the distal ulnar segment translated a mean of 1.3 ± 2.5 mm cranial and 0.8 ± 1.6 mm axial, with no translation in the proximal-distal plane relative to the proximal ulnar segment. The distal segment was flexed a mean of 15.7 ± 1.4° in the sagittal plane, rotated 2.0 ± 3.8° internally and was

1.5 ± 3.8° less varus relative to the proximal ulnar segment post-osteotomy.

Table 3-5. 3D orientation of the distal ulna relative to the proximal ulna following proximal ulnar rotational osteotomy 135° Neutral Translations (mm) Cranial-caudal Pre 1.4 ± 5.7 Post 2.7 ± 4.9 P = 0.547 Medial-lateral Pre 2.6 ± 4.3

Post 1.8 ± 4.6 P = 0.1 Proximal-distal Pre -2.2 ± 5.8 Post -2.2 ± 7.3 P = 0.998 Rotations (°) Flexion-Extension Pre 0.1 ± 2.3 Post -15.5 ± 2.5 P ≤ 0.001 Axial rotation Pre 1.4 ± 3.3

Post -0.6 ± 4.5 P = 0.226 Varus-valgus Pre -2.7 ± 5.3 Post -1.2 ± 4.8 P = 0.197 For translational variables, positive values indicate cranial, distal and lateral positions of the proximal ulna relative to the radius. For rotational variables, positive values indicate greater radioulnar flexion, external ulnar rotation and varus. P-values for post-hoc pairwise comparisons are given where significant differences were detected by ANOVA. Discussion

In this cadaveric biomechanical study, proximal ulnar rotational osteotomy shifted

contact pressures from the medial to the lateral elbow compartment, and externally

rotated and displaced the apex of the MCP caudally and abaxially relative to the radial

head. Although the alignment of the humeroulnar articulation was within normal

parameters, we found that the varus angulation of the normal humeroradial articulation

was reduced following the osteotomy. These changes would appear to shift load

transmission through the elbow from medial to the lateral compartment, supporting our hypothesis.

The concept of altering the mechanical axis to redistribute articular force through

the joint was introduced in 1958 when the high tibial osteotomy was first described.130

Since then, this technique has gained widespread use, particularly in younger, more

active patients.129 Overall, high tibial osteotomies are an effective procedure to improve

knee function and reduce pain, although there is no specific evidence that this

procedure is superior to conservative management.139 Its use, therefore, still remains

controversial and the results are reported to deteriorate over time.131,140 Following

proximal ulnar rotational osteotomy, the magnitude of humeroradial varus reduction was

comparable to that following medial opening wedge high tibial osteotomy in humans,

which has been reported to reduce maximum varus from 13.5° to 5.4°.141

We were able to produce alterations in contact mechanics with minimal alterations

in elbow alignment, and the 3D alignment of the elbow post-osteotomy was similar to

that observed in normal elbows positioned in supination, suggesting that elbow

incongruency is not induced. The proximal-distal displacement of the apex of the MCP relative to the center of the radial head following the osteotomy was negligible. In both humans and dogs, the articulation between the anconeal process and the olecranon fossa is the primary stabilizer of the elbow joint in rotation, allowing minimal alteration in the alignment of the humeroulnar joint.14,118 As the anconeal process locks in the

olecranon fossa of the distal humerus, external rotation of the proximal ulna re-orients

the distal antebrachium laterally with external rotation centered through the

humeroradial articulation. This alteration in the mechanical axis of the limb re-distributes

the load from the medial compartment to the lateral compartment during weight-bearing, as demonstrated by the shift in CP and PCP from medial to lateral elbow compartments.

The shift in mechanical axis is further substantiated by the gross observation that the paw was further lateralized and externally rotated following the osteotomy. A toe-out gait has been shown to decrease the adductor moment and lower medial compartment pressure in people with knee osteoarthritis.136 The effects of our osteotomy are

therefore most likely attributable to alteration in the mechanical axis of the humeroradial

articulation and decreased adductor moment of the elbow joint, rather than alteration of

congruency at the proximal radioulnar articulation.

The reduction in medial compartment CP and PCP of 10% and 9% respectively

obtained following proximal ulnar rotational osteotomy in this study is lower than that

measured in human cadaveric knees following medial opening wedge high tibial

osteotomy, where CP decreased 21% and PCP decreased 17% in the medial

compartment with concurrent medial collateral transection.125 Following the reduction in

mechanical load, cartilage regeneration in the previously damaged medial compartment

has been reported following both high tibial wedge osteotomy and sliding humeral

osteotomy.101,133,134 Given the shift in load from medial to lateral compartments

secondary to alteration of the mechanical axis of the forelimb in this study, we would

expect proximal ulnar rotational osteotomy to demonstrate similar effects.

The effect of the increased pressure shift to the lateral compartment is as yet unknown. It has been shown that healthy cartilage has the capacity to withstand and thicken in response to increased load in the medial compartment of the knee in people, in contrast to osteoarthritic cartilage which thins in response.113 Even in dogs with end-

stage MCD, the lateral compartment is often grossly without pathologic change, and the increased pressure in the lateral compartment may be tolerated if within physiologic range.113 Second-look arthroscopic examination of the medial and lateral elbow

compartments in dogs 12 months following sliding humeral osteotomy demonstrated

cartilage resurfacing of previously-exposed subchondral bone on the humeral condyle,

with no visible disease in the lateral compartment.101 Using delayed gadolinium- enhanced magnetic resonance imaging methods, no significant change in the cartilage thickness in the lateral compartment was noted over a two year study period following high tibial osteotomy in people.134 As normal cartilage can adapt to increases in

pressure without initiating degeneration, the proximal ulnar rotational osteotomy may be

most beneficial in dogs with early medial coronoid or medial compartment disease

instead of cases in which end-stage medial compartment disease is present.113

Furthermore, the ideal candidate for high tibial osteotomy is considered to be young (<

60 years of age) with increasing age identified as a negative prognostic factor.131,142,143

Knee adduction moment is considered by many to be highly correlated with force

transmission through the medial compartment of the knee in people, although a more

recent study utilizing a force transducer embedded in a total knee arthroplasty implant

disputes this assumption.144,145 While high tibial osteotomy results in normalization of

several dynamic knee function parameters, including knee adduction moment, the effect

of proximal ulnar rotational osteotomy on the gait of dogs is unknown.141 Conversely,

simple gait modification, such as walking with medialized knees, toe-out gait or walking poles, or using lateral wedge shoe insoles, are attractive, non-invasive interventions to

reduce knee adduction moment and consequently overall medial contact force in

people.137,138 It must be considered that inducing gait changes may actually produce

adaptive muscle co-contraction that could actually increase the medial compartment

load in-vivo.146

CHAPTER 4 CONCLUSION

The purpose of the first part of the study was to address the fundamental biomechanics of normal dog elbows and establish a model for future investigation of pathologic states and surgical procedures to address elbow pathology. Previous studies have examined forelimb kinematics in two dimensions, whereas others have investigated contact mechanics of normal dog elbows at one static elbow pose.21,110,112 However,

simultaneously evaluation of 3D kinematics and contact mechanics of normal dog

elbows had not previously been performed. We found that the biomechanics of normal

dog elbows are complex and alter significantly with changes in antebrachial and elbow

flexion angle; changes in antebrachial rotation significantly increased contact pressures

and decreased contact areas in both medial and lateral elbow compartments and contact pressures were increased with increasing flexion of the elbow joint, with corresponding decreases noted in extension. We concluded that studies evaluating disease processes and interventional surgical procedures to treat pathology referable to the elbow should therefore evaluate the elbow in several different poses.

The second part of this study investigated a novel surgical procedure using the

refined thoracic limb model. Dogs with MCD stand with a characteristic stance with the

elbow adducted, carpus abducted and paw externally rotated in a presumed attempt to

shift load transmission away from the diseased medial compartment. Similarly humans

walking with a toe-out gait or with their knees medialized have been found to unload the

medial compartment of the knee and mitigate the progression of medial compartmental

osteoarthritis. While gait adaptation can be performed in humans to unload the medial

compartment of the knee, this is impractical in dogs, and therefore we came up with the

concept of a proximal ulnar rotational osteotomy to externally rotate and abduct the forelimb distal to the elbow, unloading the medial compartment. We found that proximal ulnar rotational osteotomy shifts contact pressure from the medial to the lateral elbow compartment by decreasing varus angulation through the humeroradial articulation and displacing the MCP caudally and abaxially. Proximal ulnar rotational osteotomy may be performed in both dogs with early medial coronoid disease in which there is potential for cartilage resurfacing of the medial compartment, as well as those with end-stage MCD

in which unloading of eburnated subchondral bone may ameliorate associated pain. The

effect of pressure shifting to the lateral compartment is unknown, however the cartilage

of the lateral compartment is often grossly normal when examined arthroscopically,

even in dogs with end-stage MCD, and normal cartilage has been shown to adapt

favorably to increased pressure transmission.

Proximal ulnar rotational osteotomy was designed to simulate supination in the

normal forelimb, and the calculated 3D alignment of the humeroradial and humeroulnar

joints post-osteotomy matched closely that of pre-osteotomy supination. However,

during supination in normal elbows pre-osteotomy, contact pressures increased in both medial and lateral compartments, whereas post-osteotomy, contact pressures decreased in the medial and increased in the lateral compartment. We suspect that the uniform bi-compartmental increase in pressure observed in the normal joint in supination may occur due to tensioning of the collateral ligaments rather than a change in joint alignment. A similar phenomenon was reported in a biomechanical analysis of high tibial osteotomy, where over-tensioning and constraint of the medial collateral ligament was found to increase load transmission through the medial compartment.125

Increased tension in both the medial and lateral collateral ligaments resulting from torqueing of the pre-osteotomy elbow during pronation and supination may have artificially elevated contact pressures in both medial and lateral compartments.125 Post-

osteotomy, tension on the collateral ligaments was not induced as the supination was

achieved by uncoupling the radius from the ulna rather than by torqueing the elbow.

Elbow biomechanics are complex and difficult to replicate ex-vivo. In this study, we

adapted a testing model previously described112 which is a gross simplification of the

dog’s thoracic limb. Simulation of physiologic loading conditions through cadaveric

forelimbs is difficult, as musculotendinous forces cannot be simulated accurately. Our

model enabled us to determine the motion of the antebrachium relative to the humerus

in all 6 degrees of freedom. Therefore, we believe that elbow biomechanics were

sufficiently replicated to allow relevant study of the effects of elbow flexion and

antebrachial rotation on humeroradial and humeroulnar contact mechanics and 3D alignment. The medial epicondylar osteotomy allowed us to insert the sensors into each of the elbow compartments while maintaining the integrity of the medial collateral

ligament and the origins of the flexors of the antebrachium. The joint capsule may assist

in load transmission, and was significantly disrupted to enable sensor placement,

although the anconeal process and lateral collateral ligament, the primary stabilizers of

the elbow during pronation and supination respectively, were preserved.63-67 Many of

the normal in-vivo muscular forces involved in weight bearing were not simulated in our

model. The biceps musculotendinous unit has been considered an important stabilizer

of the elbow during stance phase, however, inverse dynamic analysis of the forelimb at

a walk illustrates a large extensor moment at the elbow throughout stance until just

before termination when a small flexor moment occurs.60,147 Therefore the triceps muscle group which was simulated in this model would appear to be the dominant muscular group supporting the elbow throughout stance. Evolution of a more complex cadaveric thoracic limb model incorporating muscular forces or in-vivo studies may be

beneficial for future investigations.

Our analysis was restricted to flexion angles spanning the range reported during

the stance phase of walking and antebrachial rotation angles approximating the

reported midpoint of the linear region of load-deformation curves generated from

antebrachii rotated to failure.108-111,118 These antebrachial rotational angles were

consistent with measurements obtained during pilot studies of rotations at which

pronation and supination occurs without undue strain on the elbow.118 The normal

degree of pronation and supination in the dog at a walk and a trot has not yet been

reported and these angles may not reflect antebrachial rotation incurred while walking in

a straight line and may be more representative of complex activities during stance,

including turning with the forelimb planted.

The limitations of the digital pressure sensing system and 3D joint kinematic

analysis have been thoroughly discussed.148-152 The I- Scan sensors have improved

accuracy and repeatability compared with Fuji Film pressure sensitive film.153 The

accuracy of the digitizing arm has been validated and demonstrates high precision.117

The inter-marker distance averaged 1%, indicating minimal discrepancies in actual

accuracy and precision of marker position measurements. The difference noted

between elbow flexion angles measured by goniometry during testing and our kinematic

analysis is most likely attributable to operator error.

Cadaver studies are considered level V evidence in evidence-based medicine criteria, and therefore we cannot make any clinical recommendations based on our results.154 However these results can be used as a basis for further investigation,

including in-vivo 3D kinematic analysis of normal dog elbows and in dogs with MCD to

try to elucidate the biomechanical factors associated with MCD. Future investigation of

the proximal rotational ulnar osteotomy requires development of custom locking plate

and in-vivo clinical analysis with objective kinematic and force-plate analysis measures.

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BIOGRAPHICAL SKETCH

Laura C. Cuddy was born in Cork, Ireland. She is the youngest of three daughters.

She attended St. John the Baptist National School and later Midleton College where she

completed her Leaving Certificate in 2003. She attended University College Dublin,

Ireland where she earned a degree of Bachelor of Veterinary Medicine in June 2008.

She completed a rotating internship at the University of Florida College of Veterinary

Medicine in June 2009, and remained at the University of Florida College of Veterinary

Medicine to complete a combined master’s degree program in Small Animal Clinical

Sciences and a residency in Small Animal Surgery.

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