Sesamoid Bone in the Tendon of the Supinator Muscle of Dogs: Incidence and Comparison of Radiographic and Computed Tomographic Features

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Sesamoid bone in the tendon of the supinator muscle of dogs: incidence and comparison of radiographic and computed tomographic features

Word count: 8473

Manon Dorny Student number: 01609678

Supervisor: Dr. Ingrid Gielen Supervisor: Prof. dr. Wim Van Den Broeck Supervisor: Dr. Aquilino Villamonte Chevalier

A dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master of Veterinary Medicine

Academic year: 2018 - 2019

Ghent University, its employees and/or students, give no warranty that the information provided in this thesis is accurate or exhaustive, nor that the content of this thesis will not constitute or result in any infringement of third-party rights. Ghent University, its employees and/or students do not accept any liability or responsibility for any use which may be made of the content or information given in the thesis, nor for any reliance which may be placed on any advice or information provided in this thesis.

ACKNOWLEDGEMENTS

I would like to thank the people that helped me accomplish this thesis and helped me achieve my degree in veterinary science.

First of all I would like to thank Dr. Ingrid Gielen, Dr. Aquilino Villamonte Chevalier and Prof. Dr. Wim Van Den Broeck. I thank them all for their time spend in helping me with my research, their useful advice and their endless patience. Without their help, I wouldn’t have been able to accomplish this thesis.

Next I would like to thank my family and friends for their continuing support and motivation during the last years of vet school. My parents and partner especially, for all the mental breakdowns they had to endure in periods of exams and deadlines. Thank you to my dad and father in law for proofreading this study and helping me improve my English writing skills.

Last I would like to thank my sister who was studying veterinary science with me. It was a tough few years, we laughed together, cried together, but in the end we made it together.

TABLE OF CONTENT

FRONT PAGE

CLAUSE

ACKNOWLEDGEMENTS

TABLE OF CONTENT

SUMMARY...... 5

1. Introduction...... 7 1.1. Anatomy of the elbow...... 7 1.2. Elbow dysplasia...... 10 Medial coronoid disease...... 10 Osteochondrosis of the medial humeral condyle...... 10 Ununited anconeal process...... 11

Elbow incongruity...... 12 Incomplete ossification of the humeral condyles...... 12 1.3. Sesamoid bones...... 12 2. Problem and aims...... 14 3. Material and method...... 14 4. Results...... 15 5. Discussion...... 16 6. Conclusion...... 19

APPENDIX...... 20

REFERENCES...... 21

Summary

Sesamoid bones are present in different tendons and muscles. They develop in spots with pressure or friction. This study focuses on the sesamoid bone in the supinator muscle of the dog. The supinator muscle originates on the lateral collateral ligament of the elbow and the lateral epicondyle of the humerus. Sometimes this muscle contains a sesamoid bone in its origin. This sesamoid bone is located craniolateral of the head of the radius and occasionally they articulate. Sesamoid bones can be important in diagnosing elbow dysplasia, as they can be confused for a fragment of, for example, the medial coronoid process.

In this study X-rays and CT scans of 100 dogs were scored by 3 observers with different degrees of experience. A scoring sheet was made to assess the elbows for a sesamoid bone and elbow dysplasia. The incidence of the presence of a sesamoid bone was evaluated on X-ray and CT. Interobserver statistics were performed using the Cohen’s Kappa statistics and the association between the sesamoid bone and elbow dysplasia was assessed using Chi square statistics.

On X-ray the incidence of a sesamoid was an average of 8,33% of the dogs. In 43,52% of these dogs a bilateral sesamoid bone was observed. On CT a sesamoid was found in 26% of the dogs. In 76,92% of these dogs a bilateral sesamoid bone was observed. An average of 72% of the sesamoid bones were missed on X-ray. The sesamoid bone was round to oval shaped with a diameter of 0,5-6,56 mm x 0,5- 6,2 mm. The Kappa value for X-ray was 0,6910 (substantial agreement) and for CT 1,00 (perfect agreement). There was a weak positive association between elbow dysplasia and appearance of a sesamoid bone in the supinator muscle (χ2 = 9,1474; p = 0,002491).

CT is clearly superior to X-ray for assessing this sesamoid bone. For assessing the X-rays experience is important, but for CT it made no difference in this study. There is an association between elbow dysplasia and the appearance of a sesamoid bone, but further research is needed to assess this correlation.

Samenvatting

Sesambeenderen komen voor op verschillende plaatsen in het lichaam. Ze ontstaan op plaatsen waar er druk of wrijving wordt uitgeoefend. In dit onderzoek wordt de focus gelegd op het sesambeen in de musculus supinator ter hoogte van het ellebooggewricht van de hond. In de aanhechting van deze spier op de laterale collateraalband van de elleboog en de laterale epicondyl van de humerus bevindt zich bij sommige honden een sesambeen. Dit sesambeen bevindt zich craniolateraal van het hoofd van de radius en articuleert hier soms mee. Sesamsbeenderen kunnen belangrijk zijn, aangezien ze verward kunnen worden met een fragment van bijvoorbeeld de mediale processus coronoideus.

In dit onderzoek werden radiografieën en CT scans van de ellebogen van 100 honden bekeken door 3 onderzoekers met verschillend niveau van expertise. Een scoreblad werd ingevuld m.b.t. aanwezigheid van het sesambeen en elleboog dysplasie. Er werd nagegaan in hoeveel percent van de honden een dergelijk sesambeen voorkwam op RX en CT. Interobserver statistiek werd toegepast a.d.h.v. Cohen’s Kappa statistiek. Daarnaast heeft men ook gekeken naar aanwezigheid van elleboog dysplasie en of er een verband was met aanwezigheid van het sesambeen (χ2 statistiek).

Bij gemiddeld 8,33% van de honden werd een sesambeen gevonden op RX. Gemiddeld 43,52% van deze honden had bilateraal een sesambeen. Op CT werd in 26% van de honden een sesambeen gevonden. Daar hadden 76,92% van de honden bilateraal een sesambeen. Er werden gemiddeld 72% van de sesambeenderen gemist op RX. Het sesambeen was rond tot ovaal van vorm met een diameter van 0,5-6,56mm x 0,5-6,2mm. De Kappa waarde voor RX was 0,6910 (voldoende tot goede

overeenstemming) en voor CT 1,00 (perfecte overeenstemming). Er bleek een zwakke positieve associatie tussen aanwezigheid van het sesambeen en aanwezigheid van elleboog dysplasie (χ2 = 9,1474; p = 0,002491).

CT is dus duidelijk superieur t.o.v. RX om dit sesambeen te beoordelen. Op RX is ervaring een pluspunt om dit sesambeen te kunnen onderscheiden, maar op CT is er geen onderscheid op vlak van ervaring. Het sesambeen is gecorreleerd met aanwezigheid van elleboog dysplasie, maar verder onderzoek is nodig om deze correlatie te beoordelen.

1. Introduction

1.1. Anatomy of the elbow

The elbow is a joint consisting of 3 bones, the humerus, the radius and the ulna (Fig 1). The humerus is a long bone that ends distally in the humeral condyle. The humeral condyle consists of two parts, the medial trochlea humeri and the lateral capitulum humeri. The capitulum humeri articulates with the head of the radius. The trochlea humeri articulates with the trochlear notch of the ulna and a portion of the fovea of the radius. Caudally on the humeral condyle the olecranon fossa is located. This is the fossa fitted for the olecranon of the ulna when the leg is in extension. On the cranial side of the humeral condyle is the radial fossa. The head of the radius fits in that fossa when the elbow is in flexion. Both of these fossae communicate with each other through the foramen supratrochleare. On the medial and the lateral side of the humeral condyles are the epicondyles. These structures are mainly insertion points for different muscles and ligaments. The radius is also a long bone that lies parallel to the ulna. It articulates proximally with the humerus, but also with the ulna, forming the humeroradial and the proximal radioulnar joint. The radius is shorter than the ulna and its most important function is attachment of muscles. The most proximal part of the radius is called the head of the radius. On the radial head there is an indentation, the articular fovea. This part articulates with the capitulum humeri. The radius articulates with the radial notch of the ulna through the articular circumference, an osseous band on the radial head. The ulna is also a long bone. Its trochlear notch articulates proximally with the humeral trochlea forming the humeroulnar joint. The trochlear notch is a round shaped indentation on the cranial side of the olecranon. Proximally the trochlear notch ends in the anconeal process. The ulna articulates with the proximal radius through the radial notch. This radial notch is formed by the processus coronoideus medialis and lateralis that are located at the distal part of the trochlear notch. The medial coronoid process is larger than the lateral one (Evans and de Lahunta, 2013; Trostel et al., 2003).

Fig. 1: A: Lateral view right distal humerus and proximal radius and ulna. B: Cranial view right distal humerus and proximal ulna and radius. (From: Trostel et al., 2003)

The elbow joint consists of 3 joints, the humeroradial joint, the humeroulnar joint and the proximal radioulnar joint (Fig 2). The humeroradial joint is formed by the humeral trochlea and the fovea of the radius. The humeroradial joint consists of the trochlea humeri and the trochlear notch of the ulna. The incisura radii and the articular circumference of the radius form the proximal radioulnar joint. There is one capsule surrounding the three joints. The collateral ligaments and the anconeal process restrict the movement of the elbow joint. The lateral collateral ligament inserts on the lateral epicondyle of the humerus and divides in two ligaments that end on the neck of the radius and the annular ligament. The medial collateral ligament attaches to the medial epicondyle of the humerus, divides in two ligaments and ends on the proximal radius and ulna. The annular ligament of the radius goes around the radius and inserts on the medial and lateral coronoid processes of the ulna. This ligament holds the radius close to the ulna (Evans and de Lahunta, 2013; Trostel et al., 2003).

Fig. 2: A: Cranial view of the left elbow joint. B: Sagittal view of the proximal radius and ulna. (From: Trostel et al., 2003)

Like every joint the elbow has extensor and flexor muscles (Fig 3). The extensor muscles are the musculus triceps brachii, the anconeal muscle and the musculus tensor fasciae antebrachii. The triceps brachii muscle is the biggest muscle consisting of four heads, the caput longum, the caput laterale, the caput mediale and the caput accesorium. The four heads all insert on the tuber olecrani. The long head attaches on the distolateral border of the scapula. The lateral head attaches on the tricipital line between the tuberosita deltoideus and the tuberositas teres minor. The medial head attaches on the tuberculum minor and the accessory head attaches on the proximocaudal part of the neck of the humerus. The anconeal muscle is a small muscle that attaches on the lateral epicondylar crest, the lateral epicondyle and part of the medial epicondyle. It inserts on the lateral surface of the proximal end of the ulna. The musculus tensor fasciae antebrachii also inserts on the tuber olecrani, but also on the antebrachial fascia, and it arises from the caudal border of the scapula (Liebich et al., 2009; Evans and de Lahunta, 2013).

The flexor muscles are the musculus brachialis, the musculus extensor carpi radialis and the musculus biceps brachii. The musculus biceps brachii attaches on the tuberculum supraglenoidales. When this muscle crosses over the elbow joint the muscle splits in two tendons. One of these tendons inserts

on the ulnar tuberosity and the other on the radial tuberosity. The musculus brachialis attaches on the proximal part of the sulcus musculus brachialis. It ends on the tendon of the musculus biceps brachii that inserts on the radial tuberosity and it also ends on the ulnar tuberosity. The musculus extensor carpi radialis attaches on the lateral epicondylar crest of the humerus. At the distal third of the radius it splits in two tendons. One of these tendons inserts on a small tuberosity on metacarpal II and the other on metacarpal III.

Fig. 3: Muscles of the brachium and antebrachium. A: Lateral aspect.; B: Lateral aspect with lateral head of the triceps removed.; C: Medial aspect with musculus biceps brachii removed.; D: Caudolateral aspect.; E: Antebrachial muscles, craniolateral aspect (From: Evans and de Lahunta, 2013).

Apart from extension and flexion of the elbow it is also possible to supinate and pronate the elbow. The most important muscles for these functions are the supinator muscle and the brachioradialis muscle for supination and the pronator teres muscle for pronation (Fig. 3; Trostel et al., 2003; Evans and de Lahunta, 2013). The supinator muscle originates from the lateral collateral ligament of the elbow and the base of the lateral epicondyle of the humerus (Barone, 2000). It ends on the proximal quarter of the craniomedial part of the radius (Barone, 2000; Evans and de Lahunta, 2013). The musculus brachioradialis is also called the musculus supinator longus. It attaches on the proximal end

of the lateral epicondylar crest of the humerus and ends on the radius between the third and the distal fourth of the radius. The musculus pronator teres attaches on the medial epicondyle of the humerus and ends on the medial border of the radius (Evans and de Lahunta, 2013).

1.2. Elbow dysplasia

Elbow diseases are often seen in rapidly growing large breed dogs but also in other less specific breeds (LaFond et al., 2002; Samoy et al., 2011). Common elbow diseases are medial coronoid disease, osteochondrosis of the elbow, ununited anconeal process, articular cartilage injury and elbow incongruity (Kirberger and Fourie, 1998; Samoy et al., 2011). These diseases are commonly referred to as elbow dysplasia. Other less common diseases like incomplete ossification of the humeral condyles are sometimes included in the elbow dysplasia complex (Dallago et al., 2015). The incidence of elbow dysplasia ranges from 0% to 70% depending on the breed (Hazewinkel et al., 1995; Flückiger, 2002).

Medial coronoid disease

Medial coronoid disease (MCD) is a common disease in young, large breed dogs (Villamonte- Chevalier et al., 2015; Dallago et al., 2015). It describes the changes that can occur in the cartilage and subchondral bone of the medial coronoid process. Lesions associated with medial coronoid disease are chondromalacia, fissures, non-displaced fragments, displaced fragments and erosion (Dallago et al., 2015). The exact cause of medial coronoid disease has not been determined yet. There are genes coding for elbow dysplasia, but also other factors like obesity, trauma and other causes are considered (Morgan et al., 2000; Temwichitr et al., 2010; Guthrie and Pidduck, 1990). Another possible cause for medial coronoid disease is elbow incongruity. Due to the malalignment of the joint surfaces, the weight bearing through the medial side of the joint can be altered. Elevated loading of the medial coronoid process can then lead to lesions and fractures of the medial coronoid process (Gemmill and Clements, 2007; Danielson et al., 2006).

Most dogs with medial coronoid disease are lame. Other symptoms are exorotation of the affected legs, moderate joint distension, crepitation during movement and decreased range of motion of the elbow joint. The first symptoms usually occur at 4 to 6 months old. In some cases they occur between 6 to 8 months or even after 6 years (Temwichitr et al., 2010). Traditionally, a diagnosis is made with radiographs. The best views for visualizing the medial coronoid process are the mediolateral and the craniocaudal medial oblique (Cr15L-CdMO) radiographs (Hazewinkel and Voorhout, 1986; Wosar et al., 1999). Another diagnostic tool that can be used for medial coronoid disease is computed tomography. Based on the high sensitivity and specificity it has been considered that CT is the preferred non-invasive technique to assess medial coronoid disease lesions of the canine elbow joint (Villamonte-Chevalier et al., 2015).

Osteochondrosis of the medial humeral condyle

Osteochondrosis develops most likely due to a problem in the endochondral ossification (Demko and McLaughlin, 2005; Samoy et al., 2011). During the endochondral ossification the cartilage is gradually replaced by bone. Sometimes this process is interrupted on a focal area on the bone. In the elbow joint this usually occurs on the medial humeral condyle. This results in a weaker degenerative cartilaginous spot that is more prone to lesions during normal weight bearing. If the cartilaginous area is damaged, a flap is formed and degradation products will go into the joint causing inflammation. The formation of the flap is called osteochondritis dissecans. The flap can stay attached to the bone or can detach and form a joint mice. Usually this flap does not mineralize and stays cartilaginous (Demko and McLaughlin, 2005).

The exact etiology for this interruption is unknown. It is most likely a multifactorial complex that includes genetics, rapid growth, overnutrition, trauma, ischemia, hormonal influences and so on. This could explain why it is seen more frequently in young, rapidly growing, large breed, male dogs with a high nutrition intake (Demko and McLaughlin, 2005; Samoy et al., 2011). Osteochondrosis dissecans is often present with a fragmented coronoid process. In at least 12% of the cases osteochondrosis of the medial humeral condyle occurs together with a fragmented coronoid process (Burton and Owen, 2008).

Clinical signs are lameness, uni- or bilateral, pain and stiffness. The lameness gets worse through exercise and gets better with rest. The range of motion can be reduced due to pain (Demko and McLaughlin, 2005).

Again, radiographs are a good diagnostic tool. Osteochondrosis of the medial humeral condyle is visible on the extended and flexed mediolateral view and the craniocaudal view from 5 to 6 months of age (Kirberger and Fourie, 1998; Demko and McLaughlin, 2005). It is seen as flattening of the subchondral bone of the humeral condyle. Sometimes a small concave indentation is also visible (Kirberger and Fourie, 1998; Burton and Owen, 2008; Samoy et al., 2011). Sometimes the lesion is surrounded by sclerosis and osteoarthritic changes can be present in the joint. The arthrosis lesions are less severe than in cases with medial coronoid disease, but a combination of these two pathologies results in the most severe arthrosis (Kirberger and Fourie, 1998). Osteochondrosis dissecans is also visible on computed tomography. It is visualized as a sclerotic rim around an area with lesser opacity on the medial humeral condyle (Samoy et al., 2011).

Ununited anconeal process

Ununited anconeal process is a condition where there is no radiographic fusion between the anconeal process and the proximal ulna after 20 weeks of age. The anconeal process can grow as an extension of the proximal ulnar growth centre or it can have its own ossification centre (Lenehan and Van Sickle, 1985; Samoy et al., 2011). A separate ossification centre is described in some breeds like the German Shepherd, Grey Hounds, Saint Bernard, Weimeraner, Vizsla, Afgan, English pointer, Bassett, Dachshund, Golden Retriever, Labrador retriever, Doberman Pinscher and Pit Bull (Lenehan and Van Sickle, 1985; Frazho et al, 2010). Normally, fusion with the ulna must be complete at the age of 20 to 24 weeks (Lenehan and Van Sickle, 1985; Sjöström et al., 1995; Samoy et al., 2011). If the union is not complete at that age, it will not occur spontaneously. This doesn’t mean that the anconeal process is completely loose. It can be attached to the ulna by a bridge of fibrocartilage or fibrous tissue that isn’t visible on radiographs (Sjöström et al., 1995).

Dogs are usually presented with front limb lameness at 5 to 9 months of age (Lenehan and Van Sickle, 1985; Sjöström et al., 1995). Although, there are cases with clinical signs outside of that age range (Samoy et al., 2011). Lameness can be uni- or bilateral and usually increases with exercise (Lenehan and Van Sickle, 1985; Samoy et al., 2011). Swelling and pain of the joint can occur due to inflammation (Sjöström et al., 1995). With flexion or extension crepitation is present and the range of motion is decreased due to pain and osteoarthritis (Sjöström et al., 1995; Samoy et al., 2011). Osteoarthritis of the joint is caused by instability of the joint due to the ununited anconeal process (Sjöström et al., 1995).

This condition can be diagnosed on a mediolateral radiograph of the flexed elbow. There is a fracture line visible between the anconeal process and the proximal ulna or the fragment can be displaced proximally. CT can be used to get more insight on the displacement of the fragments and to assess other pathological changes of the joint (Samoy et al., 2011).

Elbow incongruity

Elbow incongruity means that the alignment of the radius, the ulna and/or the humerus is disturbed (Burton and Owen, 2008). There are two types of elbow incongruity described. The first type is caused by an underdeveloped radius or ulna. If one of these bones is too short, the joint surface will not be smooth and aligned, but there will be a step between the lateral coronoid process and the proximal radius. The second type is an incongruity between the trochlear notch of the ulna and the humeral trochlea. If the trochlear notch grows at a slower rate than the humeral trochlea, the trochlear notch will be too small to cover the humeral trochlea resulting in incongruity (Kirberger and Fourie, 1998; Samoy et al., 2011).

Elbow incongruity is often seen with other pathology, for example medial coronoid disease, ununited anconeal process or osteochondrosis dissecans (Kirberger and Fourie, 1998; Samoy et al., 2011). This can be due to the fact that the loading of the bones is changed due to the incongruity. This theory is supported by data from studies of Bernese mountain dogs (Ubbink et al., 1999). This means it is difficult to link elbow incongruity to lameness, because the lameness could also be explained by other pathologies (Samoy et al., 2011). Some studies also indicate that some degree of elbow incongruity is normal in dogs (Burton and Owen, 2008).

Elbow incongruity can be diagnosed using the extended mediolateral and craniocaudal radiographs. Radiographical changes are a step between de lateral coronoid process of the ulna and the proximal radius, the humerus can be displaced cranially, the humeroulnar and the humeroradial joint space can be increased and the trochlear notch can be more elliptic of shape (Kirberger and Fourie, 1998; Samoy et al., 2011). Computed tomography can also be a tool in diagnosis of elbow incongruity (Gemmill and Clements, 2007).

Incomplete ossification of the humeral condyles

Incomplete ossification of the humeral condyles (IOHC) is uncommon in dogs (Robin and Marcellin- Little, 2001). It is mostly seen in growing, large breed dogs and spaniels, but it can also be seen it other breeds like Pugs, Rottweilers and Labrador Retrievers (Robin and Marcellin-Little, 2001; Gielen et al. 2018). It is caused by the non-fusion of the medial and the lateral humeral condyle. This results in a lesion like a fissure or a fracture between the humeral condyles. Sometimes these lesions can reach up to the supratrochlear foramen (Gielen et al., 2018). Normally the fusion is complete at the age of 8 to 12 weeks. Dogs with IOHC are predisposed for condylar fractures (Robin and Marcellin- Little, 2001; Moores, 2006). These fractures can occur without any trauma. It can happen during normal movement, but since the lesion between the condyles weakens the bone, it fractures more easily. This condition can be diagnosed on radiographs or CT. CT is preferred because on radiographs IOHC can be missed due to superposition of bony structures (Gielen et al., 2018). The preferred radiograph for this condition is the Cr15M-CdLO view, but IOHC lesions can easily be missed if the beam is 5 degrees offset (Rovesti et al., 1998).

1.3. Sesamoid bones

Sesamoid bones are small nodules located in certain tendons or joint capsules. They develop where there is friction but they can also be formed prenatally (Evans and de Lahunta, 2013). According to Evans and de Lahunta (2013), sesamoid bones have 3 important functions: protect tendons from bony prominences, increase surface for attachment of tendons and change the direction of pull. Up to 44 sesamoid bones can be shown in the appendicular skeleton of a dog (Evans and de Lahunta, 2013). They are located in the elbow, the stifle, the tarsus, the carpus, the metacarpophalangeal joints, the metatarsophalangeal joints and interphalangeal joints (Allan and Davies, 2018; Evans and

de Lahunta, 2013). Not all sesamoid bones can be identified on radiographs, as sometimes sesamoid bones are still cartilaginous at the age of the dog when radiographs are taken. In some dogs sesamoid bones are unilaterally present or absent. For example, the intraarticular tarsometatarsal sesamoid has an incidence of 27%, whereas the lateral plantar tarsometatarsal sesamoid has an incidence of 50% (Allan and Davies, 2018). The biggest sesamoid bone is the patella in the m. quadriceps femoris, which is present in all dogs (Frandson et al., 2009). Sesamoid bones are also present in other species and in humans. Depending on the species a larger or lower number of sesamoid bones are present in comparison with dogs.

In some dogs the elbow contains one sesamoid bone, the sesamoid bone in the origin of the supinator muscle. The musculus supinator is a small muscle that is located on the craniolateral side of the elbow (Fig. 3; Evans and de Lahunta, 2013). It originates from the lateral collateral ligament of the Fig 4. Localisation of the sesamoid bone in the tendon of origin elbow and the base of the lateral epicondyle of the supinator muscle (From: Evans and de Lahunta, 2013). of the humerus (Barone, 2000). From its origin the m. supinator crosses over the joint capsule of the elbow and the radius to insert on the proximal quarter of the craniomedial part of the radius (Barone, 2000; Evans and de Lahunta, 2013). The musculus supinator is covered by the musculus extensor carpi radialis and the common digital extensor muscle (Liebich et al, 2009). Its function is, like its name suggests, the supination of the paw (Evans and de Lahunta, 2013).

In some dogs a sesamoid bone can be found in the tendon of origin of the supinator muscle (Fig 4). Radiological surveys of the elbow found a sesamoid bone associated with the elbow in 9,4%, to 31% of the examined dogs (Wood et al., 1985). This small bone articulates with the lateral aspect of the head of the radius (Evans and de Lahunta, 2013). If it is present, it is usually bilateral (Wood et al., 1985). Why this sesamoid bone is only present in 9,4% to 31% of the dogs and not in others is unknown.

On radiographs and CT scans the sesamoid bone can be seen as a small circular to oval, well defined bone with a dense uniform mineralization. The size can vary from 0,5 mm to 5,0 mm in diameter (Wood et al., 1985). To visualize the sesamoid bone it is recommended to use a craniocaudal medial- to-lateral oblique radiograph or CT (Wood et al., 1985). The main advantage of CT is avoiding superposition of overlying structures (Buzug, 2008).

The sesamoid bone in the supinator muscle is important since it can be mistaken for other pathologies, like medial coronoid disease. Fragmented medial coronoid process is the most diagnosed form of elbow dysplasia in growing dogs (Temwichitr et al., 2010).

2. Problem and aims

In this study data were collected from 100 dogs retrospectively from the patient database of the Department of Veterinary Medical Imaging and Small Animal Orthopaedics of Ghent University. Inclusion criteria were bilateral radiographic and CT assessment of the elbow joints of 50 dogs without and 50 dogs with elbow disease. The incidence of the presence of the sesamoid bone will be evaluated both on radiographs and on CT images. The hypothesis is that a larger number of sesamoid bones will be identified on CT images in comparison with radiographs. Evaluating the radiographs and CT images will give an insight in when and why this bone will easily be mistaken for other pathologies. A second aim is to evaluate if the presence of the sesamoid bone can be correlated with elbow pathology like medial coronoid disease. Additionally, the detection of a sesamoid bone on radiographs and the sensitivity of this finding is most probable dependent on the training and experience level of the observer. The hypothesis is that the detection of the sesamoid bone and the sensitivity would increase with the level of practice.

3. Materials and method

Study design

Data of 100 dogs were collected retrospectively from the patient database of the department of Veterinary Medical Imaging and Small Animal Orthopaedics of Ghent University, Belgium. Inclusion criteria were bilateral radiographic and CT assessment of the elbow joints of 50 dogs without and 50 dogs with elbow disease. The cases date from 2007 to 2018. All the cases were assessed and scored by 3 observers, 2 radiologists experienced in scoring radiographs and CT images of elbows and 1 non- experienced veterinary student. The scoring sheet included presence of a sesamoid bone, dimensions of the sesamoid bone, arthrosis, ununited processus anconeus, medial coronoideus disease, osteochondrosis and incomplete ossification of the humerus condyles. To assess the images properly a DICOM viewer software was used (Radiant DICOM viewer, OsiriX).

Radiographic technique

Dogs were sedated using dexmedetomidine (0,005mg/kg of body weight). Three standard radiographs were taken of each elbow, a lateral extension, a lateral flexion and a craniocaudal radiograph, using a digital radiography system, EDR6 (digital radiographic system) EKLIN device from Canon (Canon medical systems). For the lateral views dogs were placed in lateral position, on the side of the leg that needed to be X-rayed. The upper limb was pulled caudally and the head was pulled back to prevent superposition. Ideally the angle between humerus and ulna is 120° for the extended view and less than 45° for the flexed view. The X-ray beam needs to be centered on the medial epicondyle. This was done for both legs. For the craniocaudal view the dogs were placed in sternal position with its legs extended cranially. The head was pulled back to prevent superposition. The X-ray beam was centered on the joint-space distal to the medial epicondyle of the humerus. Both legs were X-rayed in this position.

Computed tomographic technique

Dogs were sedated using dexmedetomidine (0,005mg/kg of body weight) and intubated. Anesthesia was induced using Propofol (bolus of 2mg/kg of body weight) and maintained using Isoflurane and 100% oxygen. The dogs were positioned in left lateral decubitus with the thoracic legs extended cranially. The head of the dog was pulled back and laterally to prevent superposition and artefacts. CT images were obtained with a four-slice scanner (LightSpeed, GE medical systems) using 120kVp, 140mA and 25cm field of view parameters. Adjacent transverse views of 1.3mm thickness were

made from the proximal aspect of the olecranon to 2cm distal of the elbow joint using a bone algorithm (De Rycke et al., 2002).

Statistical analysis

Statistical analysis was used to calculate if there is a significant association between having a sesamoid bone and having elbow dysplasia. The Chi square (χ2) test was used with a 2 x 2 contingency table of occurrence of sesamoid bone and occurrence of elbow dysplasia. This test was performed with and without the Yates correction. Also the Phi and the Cramer’s V value were calculated. For interobserver variability of the parameter for presence of a sesamoid bone the Kappa test was used. The unweighted Kappa test was used since there is no need for a weighted Kappa test. The weighted Kappa test distinguishes between the magnitudes of disagreement. The presence of a sesamoid bone is not in grades, it’s present or it’s not, meaning that the magnitude of disagreement is not applicaple on this study. The values of the Kappa test were: 0,01-0,20: slight agreement; 0,21- 0,40: Fair agreement; 0,41-0,60: Moderate agreement; 0,61-0,80: Substantial agreement; 0,81-1,00: Almost perfect agreement (Landis and Koch, 1977). Statistical analyses were performed with help of a statistical program (SPSS).

4. Results

There were 100 dogs included in this study. The majority of these dogs were Golden Retrievers (n = 42) and Labrador Retrievers (n = 34). Apart from those breeds Breed Number there were 16 other breeds included and one dog of unknown Golden retriever 42 breed (Table 1). There were 40 female and 60 male dogs. The Labrador Retriever 34 average age was 1 year and 8 months with the youngest being 4 Cavalier King Charles Spaniel 4 months and the oldest 7 years and 8 months. Bernese Mountain Dog 3 Interobserver variability was calculated for presence of the German Shepherd 2 sesamoid bone. The Kappa value for sesamoid bones on X-ray Malinois 2 was 0,6910 meaning that there was a substantial agreement Cane Corso 1 between the 3 observers. The Kappa value for sesamoid bones White Swiss Shepherd 1 on CT was 1,00, meaning that there was a perfect agreement American Staffordshire Terrier 1 between the 3 observers. This means that the averages of the Bordeauxdog 1 percentages of the 3 observers can be used without Boxer 1 compromising the outcome of this study. Border Collie 1 Australian Shepherd 1 For the percentages of sesamoid bones on X-ray the averages of the 3 observers were calculated. For CT no averages were English Springer Spaniel 1 needed since the 3 observers found the same sesamoid bones. English Cocker Spaniel 1 On X-ray a sesamoid bone craniolateral of the head of the radius Labione 1 was observed in an average of 8,33% of the dogs. 43,52% of Stabyhoun 1 these dogs was observed as having bilateral sesamoid bones. On Brittany Dog 1 CT the 3 observers found a sesamoid bone in 26% of the dogs. Unknown 1 76,92% of these dogs were observed as having a bilateral TOTAL 100 sesamoid bone. An average of 72% of the sesamoid bones were Table 1. List of breeds and number of missed on X-ray. It was assumed that no sesamoid bones were dogs in each breed included in this study. missed on CT. Still, it might be possible that the 3 observers missed the same sesamoid bone. The exact results of every observer are found in the appendix.

The sesamoid bone was round to oval shaped with a diameter of 0,5-6,56 mm x 0,5-6,2 mm. On X-ray the sesamoid bones had a uniform mineralization and the borders were smooth and well defined. A medulla and cortex were visible on CT. Figure 5 shows an X-ray and CT scan of a dog with a sesamoid bone in the supinator muscle.

Fig. 5. X-ray and CT scan of the right elbow of the same dog.

An average of 57% of the dogs had elbow dysplasia in one or both elbows. An average of 29,67% of these affected elbows contained a sesamoid bone. After statistical analysis there was a 44% chance to have elbow dysplasia without a sesamoid bone and a 70% chance to have elbow dysplasia with a sesamoid bone. Statistical analysis, using a chi-square test, showed that there was a significant correlation between elbow dysplasia and a sesamoid bone in the supinator muscle. The chi square value was 9,1474 with a p-value of 0,002491 (significant at p<0,05). The Phi and Cramer’s V values were 0,214.

5. Discussion

In this research a sesamoid bone in the supinator muscle of the elbow was detected in 8,33% of the dogs on X-ray and 26% of the dogs on CT. Other surveys which only studied radiographs found a sesamoid bone in 9,4%, 11% and 31% of the dogs (Grondalen, 1982a; Grondalen, 1982b; Wood, 1985). The latter radiographed the intact elbows first, dissected the supinator muscle from the bony structures and radiographed the dissected supinator muscle again. This can explain why the study by Wood (1985) found a percentage of sesamoid bones closer to the percentage this study detected on CT scan. By dissecting the supinator muscle from the bony structure, superposition is bypassed. The other 2 surveys only assessed intact elbows on X-ray, which can explain why the percentages of sesamoid bones detected in those surveys are closer to the percentage of sesamoid bones found on radiographs in this study.

The hypothesis that a higher number of sesamoid bones would be detected on CT scans appears to be confirmed. The most probable reason is superposition of overlying structures on radiographs. This is visualized on figure 6. The sesamoid bone is located craniolateral to the head of the radius. To properly see the sesamoid bone the radiographs should be craniomedial to caudolateral oblique, because this view reduces superposition of the radial head on the sesamoid bone (Boroffka and

Kirberger, 2015). In a standard radiographic survey of the elbow of a dog this radiograph isn’t included. It is even advised to pronate the elbow 15° to take the craniocaudal radiograph (Flückiger, 2010). By pronating the elbow, it becomes a craniolateral 15° caudomedial oblique view. This view is better to assess the elbow for medial coronoid disease, since the craniomedial structures are projected on the cassette (Rau et al., 2011). The sesamoid bone is located on the craniolateral side of the elbow, intending that on this radiograph it is difficult to visualize the sesamoid bone in the supinator muscle. On the pronated craniocaudal radiograph the sesamoid bone of the supinator muscle is often superposed on the radial head and difficult to notice. On CT images there is no superposition, which makes it easier to visualize the sesamoid bone. Still it is important to look closely as the sesamoid bone can be very small and is sometimes only detectable on 1 CT image. This way it can easily be missed if a CT scan is assessed too quickly.

Fig. 6. X-ray and CT scan of the right elbow of the same dog. On X-ray there is no sesamoid bone visible craniolateral of the head of the radius (black circle), yet CT shows that there is a sesamoid bone present.

Wood et al. (1985) found that in most cases the sesamoid bone is bilateral. In this study an average of 43,52% of the dogs were observed with a bilateral sesamoid bone on X-ray. On CT this percentage increased to 76,92%. This again can be explained by superposition. Sometimes the sesamoid bones are bilateral but only one of the two is visible on X-ray due to small differences in angle of the X-ray beam with the consequent superposition. This means that the percentages of uni- and bilateral sesamoid bones on CT are more representative than the percentages found on X-ray. Concluding as Wood et al. (1985) stated, most dogs that have a sesamoid bone in the supinator muscle have them bilaterally.

There were some differences between the 3 observers. The experienced radiologists found 13 and 14 sesamoid bones on X-ray. The non-experienced observer found 11 sesamoid bones. This can be explained by experience. Rau et al. (2011) studied the differences between experienced and inexperienced observers in diagnosing medial coronoid disease on X-ray, CT and arthroscopy. Radiography is considered inferior to CT for diagnosing medial coronoid disease. Rau et al. (2011) concluded that experienced observers could have a specificity on X-ray almost as high as on CT for diagnosing medial coronoid disease. Inexperienced observers miss a lot on X-ray because a certain

amount of experience is needed to properly assess X-rays of the elbow (Rau et al. 2011). The elbow is a complex joint with multiple joints in different planes and a lot of bony protrusions, which makes it a difficult joint to evaluate (Mason et al., 2002). Another study by Burton et al. (2008) assessed the reliability of radiological assessment of ulnar trochlear notch sclerosis in dysplastic canine elbows. The radiographs were scored by orthopeadic surgeons and diagnostic imaging specialists. Interobserver agreement was assessed using the Kappa statistic test and resulted in fair agreement. This indicates again that interpreting X-rays of the elbow is not easy and can be discussible. These results can explain the lower amount of sesamoid bones observed by the non-experienced observer in this study. On the CT scans the 3 observers saw the same number of sesamoid bones. CT scans are easier to assess because there is no superposition of overlying structures. Still basic knowledge of anatomy and interpreting CT scans is needed to properly assess CT studies.

It was also suggested that the sesamoid bone could be mistaken for medial coronoid disease. An example of a patient of this study is visualized in Figure 7. On the lateromedial radiograph there is a small bony opacity located cranial to the medial coronoid process and superposed on the radial head (black arrow). Inexperienced observers assessing these radiographs might think that this is a fragment of the medial coronoid process. While looking at the CT scans there is no fragment of the medial coronoid process (white arrow). The coronoid process is intact, but at the lateral side of the radial head a sesamoid bone is present (black arrowhead). The bony opacity on the radiograph is most likely that same sesamoid bone.

Fig 7. A: Lateromedial radiograph of the left elbow. The black arrow marks a bony opacity; B: CT scan of left elbow of same dog. The white arrow marks the intact medial coronoid process; C: CT scan of the left elbow of the same dog. The black arrowhead marks the sesamoid bone in the supinator muscle.

The chi square test showed that there is a significant correlation between presence of a sesamoid bone and elbow dysplasia (χ2 = 9,1474; p = 0,002491). The Phi and Cramer’s V values were 0,214. This suggests that the correlation between the two variables is weak. Further research might be interesting to assess how both variables are correlated. Important to notice is that correlation doesn’t necessarily means that there is causation as other variables possibly influence this matter. It

was also not taken into account that two sesamoid bones could be from the same dog. The statistical test was performed on individual elbows and not per dog. Since most dogs have bilateral sesamoid bones, this can also influence the results of these statistics. Although a possible explanation for this correlation can be as Wilson et al. (2014) described for the biceps brachii and medial coronoid disease. A possible underlying cause for medial coronoid disease involves transverse-plane radioulnar incongruity. This theory states that the incongruity is amplified by the contraction of the biceps brachii. Contraction of the biceps brachii results in elbow supination and flexion. This rotational movement can induce damage to the axial margin of the medial coronoid process when it collides with the radial head. This can eventually lead to medial coronoid disease (Hulse et al., 2010; Wilson et al., 2014). It might be possible that the sesamoid bone in the supinator muscle has the same kind of effect on the structures of the elbow joint. If the sesamoid bone collides with a structure of the elbow during certain movements, it can make the elbow more prone to arthrosis and other elbow dysplasia lesions.

In another study the proximal epiphyseal plate of the radius was closed in all dogs with a sesamoid bone (Wood et al., 1985). This epiphyseal plate closes around 8 to 12 months of age (Kealy et al., 2011). Wood (1985) suggested that there might be a correlation between appearance of a sesamoid bone and age, but statistical analysis of the data showed that there was no significant increase in frequency of sesamoid bones in mature dogs (Wood et al., 1985). In this recent study there were 4 dogs with a sesamoid bone but with the epiphyseal plates not fully closed. This shows that a sesamoid bone can be present even if the proximal epiphyseal plate of the radius is not closed yet. A possible explanation for the lack of sesamoid bones in young dogs might be that the sesamoid bone is not fully ossified yet. Sesamoid bones can still be cartilaginous at the age of the dog at which the radiographs are taken (Allan and Davies, 2018). Cartilage is not visible on X-ray, so cartilaginous sesamoid bones are not yet visible on radiographs. It could be interesting to take radiographs of young dogs to assess them for a sesamoid bone in the supinator muscle and re-evaluate these dogs after a certain age to see if more dogs have sesamoid bones at a higher age.

6. Conclusion

This study is a retrospective study and therefore has its limitations. It may not be applicable to the entire population of dogs, since the cases were not selected entirely randomly. Inclusion criteria were X-rays and CT scans of both elbows meaning that not all healthy dogs and dogs with elbow disease were included in this study. Still some valuable results were found. CT is the superior technique to assess the presence of this sesamoid bone. 26% of the dogs in this study have a sesamoid bone in one or both elbows and most of them have them bilateral. For radiographical interpretation of elbows experience is important, unlike for CT. For CT the 3 observers found the same amount of sesamoid bones, meaning that experience is less important for interpretation of CT. Still knowledge of elbow anatomy is needed. A positive correlation was found between elbow dysplasia and presence of the sesamoid bone, but this correlation was a weak correlation. Further research might be interesting to see how both variables are correlated and if other variables might play a role in this.

Appendix

Table 1. Difference between X-ray and CT. Observer 1 and 2 are experienced in radiographical examination of elbows, observer 3 is non-experienced.

Obs. 1 Obs. 2 Obs. 3 Average

Total dogs with sesamoid on X-ray (100 9% 8% 8% 8,33% dogs)

- unilateral - 44,44% - 62,5% - 62,5% - 56,48%

- bilateral - 55,56% - 37,5% - 37,5% - 43,52%

Total amount of sesamoid bones on X-ray 14/200 13/200 11/200 12,67/200 (200 elbows) (7%) (6,5%) (5,5%) (6,3%)

Total dogs with sesamoid on CT (100 26% 26% 26% 26% dogs)

- unilateral - 23,08% - 23,08% - 23,08% - 23,08%

- bilateral - 76,92% - 76,92% - 76,92% - 76,92%

Total amount of sesamoid bones on CT 46/200 46/200 46/200 46/200 (23%) (200 elbows) (23%) (23%) (23%)

Missed sesamoids on X-ray 32/46 33/46 (72%) 35/46 (76%) 33,33/46 (70%) (72%)

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