IMAGING

Magnetic Resonance Imaging Findings in With Recent and Chronic Bilateral Forelimb Diagnosed as

Sarah N. Sampson, DVM; Robert K. Schneider, DVM, MS, Diplomate ACVS; and Patrick R. Gavin, DVM, PhD, Diplomate ACVR

High field strength MRI is an effective method for diagnosing injury to structures within the feet of horses with navicular syndrome. The prevalence of different pathologies was determined in 151 horses with normal radiographs but clinical signs of bilateral navicular syndrome. Authors’ address: Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, WA 99164; e-mail: [email protected]. © 2008 AAEP.

1. Introduction individual variation between horses as well as the Navicular syndrome is one of the most common variety of pathologic changes that can occur in each 4–10,12–17,19,21,22 causes of forelimb lameness in many types of ath- . letic horses.1–4 Before magnetic resonance imag- Navicular syndrome can now be defined as multi- ing (MRI), definitive diagnosis of pathologic change ple disease processes resulting in damage to the within the horse’s foot was determined only at post- navicular bone (NB), the collateral sesamoidean lig- mortem examination. continues to be ament (CSL), the impar ligament (IL), the navicular the first diagnostic imaging step used to try to define bursa, and/or the deep digital flexor tendon 4–6,10,13–14,17,23 pathology associated with navicular syndrome, but (DDFT). Navicular syndrome is de- it has become more apparent with the availability of scribed as heel pain that blocks to a palmar digital MRI that radiography is inadequate in this region nerve (PDN) block on both forelimbs; radiographs both for diagnosis of bone and soft-tissue patholo- may or may not have visible pathology. Further- gies. This is made clear by the reports of pathology more, it has been reported that horses with navicu- diagnosed with MRI over the last 10 yr.4–17 The lar syndrome can have a variety of other structures ability to use MRI to further define navicular syn- involved that are not directly associated with the drome in live horses has enabled a greater under- navicular region itself. Examples of these are standing of the disease processes in the foot of the pathologic changes in the distal digital annular lig- horse that contribute to heel pain. The normal ament, proximal, middle, and distal phalanges, dis- anatomy of the foot seen with MRI has been de- tal interphalangeal joint cartilage and subchondral fined,18–20 and the increasing number of horses im- bone, proximal interphalangeal joint cartilage and aged continues to improve our understanding of subchondral bone, collateral ligaments of the distal

NOTES

AAEP PROCEEDINGS ր Vol. 54 ր 2008 419 IMAGING and proximal interphalangeal joints, lamina (includ- examination findings, results of lameness examina- ing keratoma formation), and digital flexor tendon tions, previous treatments, MRI findings, treat- sheath.4,5,7,9,10,12,17 Pathologic change can occur in ments based on MRI findings, and the suggested any of these structures as the only manifestation of rehabilitation program. disease within the foot; however, multiple struc- A thorough lameness evaluation was performed in tures can be affected within the foot of one horse.4 all horses. Injection of 1.5–2 ml local anesthetic In the past, radiography and ultrasonography in over the medial and lateral PDN of the lame leg some cases have been able to define pathologic proximal to the collateral cartilage caused each change within the bone or soft tissues, respectively, horse to switch to lameness in the opposite forelimb. within the horses foot,1,24 but negative findings with Injection of local anesthetic over the medial and these modalities are not enough to diagnosis a lack lateral PDN of the opposite forelimb then eliminated of pathology.4,15,21 Furthermore, even with find- the horse’s lameness. All horses were examined at ings of pathologic change in one structure using a trot in a straight line and in small circles in each these imaging modalities, it is now readily apparent direction before and after PDN blocks. Lameness with MRI that the diagnosis of lesions within the grades were 0 (no lameness observed), 1 (mild lame- horse’s foot has been missed when only radiography ness without an observed head nod), 2 (subtle head and/or ultrasonography are used as diagnostic nod observed), 3 (obvious head nod observed), 4 (se- aids.4,10,23 To fully define navicular syndrome in vere head nod with Ͻ50% of normal weight sup- each individual horse, it is necessary to evaluate ported on the limb), and 5 (limb not bearing weight). these horses using MRI.4–7,13,14,16,17,20,25 Radiographs, including a 60° dorsopalmar projec- MRI remains a valuable diagnostic tool for orthope- tion, a palmaroproximal to palmarodistal oblique dic problems in humans for both bone and soft-tissue (skyline) projection of the distal sesamoid (navicu- injury.26–28 MRI of the live horse has only been avail- lar) bone, and a lateromedial projection of the foot able for the last 10 yr,17 but in this time, it has become were evaluated and found to be normal for the age increasingly apparent how much MRI has to offer in and use of the horse for all horses included in the the diagnosis of orthopedic injury in the horse.4–17 study. Multiple MRI studies have been performed on cadaver MRI was performed on both front feet of every specimens to evaluate horses with chronic, end-stage horse after general anesthesia. Each horse had a navicular disease.14,15,21,29–32 Many of the horses in thorough physical exam, including an electrocardio- these post-mortem studies have had flexor cortical ero- gram (ECG), complete blood count (CBC), and chem- sion or other severe bone abnormalities visible on ra- istry profile before anesthesia. Anesthesia was diographs.21 MRI has enabled evaluation of the soft induced with ketamine and diazepam and main- tissues associated with the NB in live horses for the tained with 2–5% isoflurane in oxygen. All horses first time, and, surprisingly, it has proven how little were placed in right lateral recumbency, and their information we often obtain from radiographs regard- front feet were positioned in a 1.0-Tesla magnet.a ing the changes that are occurring in the bony struc- A human knee quadrature receiver coil was posi- tures of the foot.5–7,14,16–18 tioned around the foot, and multiple image se- The objective of this study of horses with recent quences were obtained using proton density (PD), and chronic signs of navicular syndrome was to de- T2-weighted (T2W), short tau inversion recovery termine the prevalence of injury to different struc- (STIR), and gradient echo (3DGE) sequenc- tures within the horse’s foot when lameness has es. Axial, sagittal, and dorsal sections were ob- been present for variable periods of time. Addition- tained using standard MRI protocols for the foot of ally, this study will compare these findings on two the horse developed at Washington State University groups of horses based on whether or not they had (Table 1).17 All MRIs were stored on magnetic op- recent or chronic signs of navicular syndrome. tic discs for later retrieval and evaluation. All MRI observations in these two specific groups of horses were rope recovered from anesthesia in a horses restricted to those with bilateral forelimb padded recovery stall. lameness diagnosed as navicular syndrome without A standardized format was used to record MRI radiographic changes have not been reported. observations, and images were compared with im- ages previously obtained from normal horses. A 2. Materials and Methods primary finding, based on severity and lack of other The horses in this study had MRI performed on both observed abnormalities, was made in horses where front feet at Washington State University Veteri- it was clear that one principal abnormality was nary Teaching Hospital between May 1998 and No- present. In the other horses, a primary location of vember 2007, because they had developed clinical inflammation could not be identified from the mul- signs of bilateral navicular syndrome. Horses that tiple abnormal findings identified. Based on MRI had been diagnosed with clinical signs of navicular findings, the limb with the most severe findings was syndrome were accepted if they did not have abnor- recorded before the lamest limb was known, and malities on radiographs of the front feet. Informa- these results were later compared with lameness tion recorded from medical records of horses exam findings. If MRI findings were too similar included in the study consisted of history, physical between limbs, this information was recorded.

420 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING

Table 1. Magnetic Resonance Imaging Sequences Used for Evaluation of the Foot and of the Horse Using a 1.0-T Gyroscan Magnet

Image TR TE FOV/ Matrix Slice #/ Gap Time Orientation Sequence (ms) (ms) FA FOV Size Width(mm) (mm) (min)

Axial DE TSE T2 2116 100 90 15/15 512 ϫ 512 30/4 mm 0.5 2:34 TSE PD 2116 10.5 90 15/10 512 ϫ 512 30/4 mm 0.5 2:34 Axial STIR 1725 35 90 15/15 256 ϫ 256 30/3.5 mm 1.0 4:42 Axial 3D GE 47 9 25 10/10 256 ϫ 256 30/3.0 mm Ϫ1.5 3:13 Navicular Sagittal DE TSE T2 3395 110 90 14/10 256 ϫ 512 22/4 mm 0.5 2:21 TSE PD 3395 13.8 90 14/10 256 ϫ 512 22/4 mm 0.5 2:21 Sagittal STIR 1500 35 90 14/10 256 ϫ 256 22/3.5 mm 0.5 5:48 Coronal 3D GE 47 9 25 10/10 256 ϫ 256 30/3.0 mm Ϫ1.5 3:13 Navicular Coronal DE TSE PD 3395 13.8 90 13/13 512 ϫ 512 16/3.5 mm 0.3 2:21 DIP CL TSE T2 3395 110 90 13/13 512 ϫ 512 22/3.5 mm 0.3 2:21

TSE, turbo spin echo; T2, T2-weighted; PD, proton density; STIR, short tau inversion recovery; 3D GE, three dimensional gradient echo; TR, repetition time; TE, echo time; FA, flip angle; rFOV, relative field of view; DIP, distal interphalangeal joint; CL, collateral ligament; DE, dual echo (PD and T2).

3. Results directions. All other horses had a unilateral lame- One hundred fifty-one horses were accepted into the ness observed before the PDN block on the lame leg. study between May 1998 and November 2007. Seventy-two horses had recent onset of signs of na- MRI Observations vicular syndrome (lameness developed within the Many different abnormalities were observed on last 6 mo); 90 horses had chronic signs of navicular MRIs of the front feet of the horses in this study. syndrome (lameness had been present for Ͼ6 mo). The majority in the recent onset group had a pri- Of the horses with recent onset, there were 51 mary abnormality present, and the majority in the geldings, 17 mares, and 2 stallions, and the average chronic group had multiple abnormalities present age of 8.2 yr (median age ϭ 8 yr, range ϭ 3–17 yr). surrounding the navicular region. The prevalence Forty-two were Quarter Horses, 17 were Warm- of different pathologic changes and the primary ab- bloods, 7 were , 4 were Paint Horses, normalities are listed in Table 2. 1 was an Appaloosa, and 1 was an Arabian. Thirty- four were used in Western performance events, 15 Recent Navicular Syndrome Horses were used for eventing or jumping, 10 were used as pleasure or show horses, and 9 were used for dres- NB sage. The use was unknown in four horses. The Abnormal high-signal intensity (compatible with mean lameness grade was 3 when the horses were fluid) was observed in the NB on STIR sequences in trotted in a straight line over a smooth hard surface 61 horses (108 limbs), and this was the most fre- (range ϭ 1–4). Lameness was observed on both quently observed abnormality based on the number forelimbs in 76% of horses when the horse was trot- of horses it affected. Twenty-three of these horses ted in small circles in both directions. All other (30 limbs) also had abnormal low-signal intensity horses had a unilateral lameness observed before (consistent with sclerosis) in the medullary cavity of the PDN block on the lame leg. the NB. One of these horses had a 3-mm NB flexor Of the horses with chronic signs of navicular syn- cortex erosion just medial to the sagittal ridge that drome, there were 51 geldings, 25 mares, and 3 was not visible on radiographs. Fluid was seen stallions with a mean age of 9.4 yr (median age ϭ 9 either as focal high-signal intensity within the prox- yr, range ϭ 3–18 yr). There were 46 Quarter imal, middle, or distal aspect of the NB (Fig. 1A) on Horses, 22 Warmbloods, 8 Thoroughbreds, 7 Paint the sagittal STIR sequences or as diffuse fluid Horses, 2 Arabians, 1 Andalusian, 1 Appaloosa, 1 throughout the medullary cavity (Fig. 1B). Both Lippizaner, 1 Paso Fino, and 1 Morgan. Thirty-two types were present within the bone in many horses. were used in Western performance events, 15 were The amount and strength of increased signal inten- used for dressage, 14 were used as pleasure or show sity on STIR sequences was graded as mild, moder- horses, 6 were used for jumping, 6 were used for ate, severe, or very severe (Fig. 2). If horses had eventing, 1 was used for racing, and 1 had not yet abnormal high-signal intensity in the NB, it always been trained. The mean lameness grade was 2 affected the distal one-third of the bone, either as when the horses were trotted in a straight line over focal or diffuse high signal. In many horses, it also a smooth hard surface (range ϭ 1–4). Lameness extended diffusely into the more proximal aspect of was observed on both forelimbs in 52% of horses the NB. This is seen most clearly on the sagittal when the horse was trotted in small circles in both STIR sequences. Increased signal intensity in the

AAEP PROCEEDINGS ր Vol. 54 ր 2008 421 IMAGING

Table 2. Prevalence of Types of Pathologic Change and Primary Abnormality in the Recent and Chronic Navicular Syndrome Groups

Recent Group Chronic Group

Structure Horses Limbs Primary Abnormality Horses Limbs Primary Abnormality

Multiple 18% NA NA 44% NA NA NB 85% 75% 33% 92% 84% 16% NBF 13% 10% NA 17% 11% NA FCE 1% 0.7% NA 10% 6% NA CSL 75% 60% 15% 91% 87% 24% DDFT 44% 34% 14% 49% 37% 6% PASTERN 31% NA 47% NA PTNB 92% NA 97% NA ANB 35% NA 41% NA DTNB 39% NA 21% NA IL 36% 28% 10% 63% 60% 8% Bursa 44% 40% 0 61% 71% 1% DIP Effusion 50% 49% 0 46% 39% 0 DDAL 8% 4% 1% 10% 7% 0 Bone Bruises 6% 3% 0 4% 6% 0 DIP CL 1% 0.7% 0 1% 1% 0 DIP OA 3% 1% 0 4% 3% 0 Laminar defects 4% 2% 0 4% 2% 0

NA, not applicable.

NB was determined to be the primary abnormality DDFT lesions also had enlargement of the CSL. in 24 horses. DDFT injury was determined to be the primary ab- normality in 13 horses. NB Fragments Thirty horses had the NB sectioned coronally, and CSL nine of these horses (15 limbs) had NB fragments Thickening, with or without abnormal hyperintensity, (NBF) off the distal NB margin. Six horses had of the CSL was observed in 54 horses (86 limbs; Fig. 4). these bilaterally, and five of those horses had lateral Thirty-nine (45%) limbs had damage to the deep flexor fragments on each foot. The other one horse had tendon at the level of the CSL. Damage to the CSL medial and lateral fragments on each foot. Three was the primary abnormality in 11 horses. horses had these unilaterally: one horse had a me- dial fragment, and two horses had lateral frag- IL ments. Two of the unilateral fragments were on Abnormal high-signal intensity and/or thickening of the most lame limb, and one was on the least lame the IL were observed in 26 horses (40 limbs). Ten limb. horses (19 limbs) also had abnormal signal intensity on PD and T2W sequences at the IL insertion onto DDFT the NB; this is compatible with sclerosis in this area Thirty-two horses (49 limbs) had thickening, irreg- (Fig. 5). IL damage was determined to be the pri- ularity, and/or abnormal high-signal intensity mary abnormality in seven horses. within the DDFT (Fig. 3) that most commonly lo- cated in the tendon proximal to the NB. In some Other Findings horses, abnormalities extended proximally as far as Six limbs had damage to the distal digital annular the horse was imaged (midway in the proximal pha- ligament. Four limbs had abnormal hyperintensity lanx). Lesions within the DDFT were divided into within the third phalanx. Two horses had osteoar- four locations: distal to the NB (DTNB; 39% of thritis of the distal interphalangeal joint. Three limbs), at the NB (ANB; 35% of limbs), proximal to horses had keratomas within the medial or lateral the NB (PTNB; 92% of limbs), and between the one quarter of the area, and all were found on the pastern and fetlock joint (31% of limbs). Twenty- most lame limb. These were not visible on dorso- four (49%) limbs had lesions in more than one re- palmar oblique projections of the distal phalanx. gion, and 25 (51%) limbs had lesions in only one One horse had desmitis of one CSL of the distal region. Of the 25 limbs that had lesions in only one interphalangeal joint. region, 84% had lesions PTNB, 8% had lesions DTNB, and 4% had lesions ANB. No horses were Synovial Structures seen that had a DDFT lesion within the pastern Thirty-six horses (71 limbs) had increased synovial region that did not have lesions more distally in the fluid observed in the distal interphalangeal joint tendon as well. Twenty-seven (84%) horses with (Fig. 6). All of these horses also had other abnor-

422 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING mined. Abnormalities were not observed in the NB or its supporting soft tissues in four horses.

Correlation of MRI Findings and the Lamest Limb Fifty-six of the 72 horses had obvious differences in severity of pathology between limbs. In 93% (52 of 56) of horses, the most severe abnormalities oc- curred in the limb on which the horse was most lame. In 7% (4 of 56) of horses, the worst findings were in the opposite limb. Sixteen of the 72 horses had similar findings on both limbs.

Lameness Follow-Up Follow-up information was obtained for 76% (55 of 72) of horses by reevaluation of horses or by tele- phone conversations with trainers or owners. This information is shown in Table 3.

Chronic Navicular Syndrome Horses

CSL Thickening, with or without increased abnormal sig- nal intensity, of the CSL was observed in 72 horses (137 limbs; Fig. 4). This was the most frequent abnormality observed based on the number of limbs it affected, although less total horses were affected compared with those with increased signal intensity in the NB. Ligament thickening was slightly more likely to be seen bilaterally than increased signal intensity within the medullary cavity of the NB, which is described below. Thirty-six horses (65 limbs) had adhesions present between the CSL and the DDFT. Of these limbs, 38 had damage to the tendon at the level of the CSL. One horse had damage to the tendon distal to this level (at the level Fig. 1. These are sagittal STIR images showing (A) abnormal of the NB). Damage to the CSL was determined to high-signal intensity (fluid) in the distal one-third of the NB and be the primary abnormality in 19 horses. (B) spread diffusely throughout the medullary cavity of the NB. NB malities observed in the NB or supporting soft tissue Abnormal high-signal intensity was observed in the structures, although they may not have been the NB on STIR sequences in 73 horses (133 limbs), and primary abnormality noted. Thirty-two horses (58 this was the most frequently observed abnormality limbs) had increased synovial fluid observed in the based on the number of horses it affected; however, navicular bursa. In most horses, however, a rela- slightly fewer limbs were affected than with thicken- tively small increase in fluid was observed. In 17 ing of the CSL. Eighteen horses (26 limbs) had ab- limbs (29%), there was severe bursitis, and 12 of normal decreased signal intensity on PD and T2W these (71%) had deep flexor tendonitis within the sequences that was located within the central medul- navicular bursa. Although a severe increase in sy- lary cavity of the NB, which is compatible with sclero- novial fluid was observed in the bursa in 17 horses, sis (Fig. 7). In 8 horses, this sclerosis was associated, a large amount of fluid consistent with bursitis was at least partially, with a flexor cortical erosion (Fig. 8). not considered the primary abnormality in any of Pathologic change within the NB was determined to be these horses; however, one horse with multiple other the primary abnormality in 13 horses. chronic lesions and an acute lameness history did have severe fluid in the bursa. Six limbs had effu- NBF sion of the proximal interphalangeal joint. Forty-two horses had their NBs imaged in a coronal plane. Of these horses, 15 (36%; 19 limbs) had Multiple Abnormalities NBF (Fig. 9). Four horses had distal margin frag- Multiple abnormalities were observed in 13 horses ments on both limbs. Of these four, one horse had in which a primary abnormality could not be deter- medial and lateral fragments on each limb, one

AAEP PROCEEDINGS ր Vol. 54 ր 2008 423 IMAGING

Fig. 2. These are sagittal (left) and axial (right) STIR images showing (A) mild, (B) moderate, (C) severe, and (D) very severe increased signal intensity, which indicates inflammation of the NB.

horse had medial and lateral fragments on one limb sion fragment on the proximo-lateral aspect of the and a medial fragment on the other limb, and two NB within the CSL. Of the horses with unilateral horses had lateral fragments. Ten horses had dis- fragments, seven were found on the lamest leg, and tal margin fragments on one limb only: six were four were found on the least lame leg. medial, and 4 were lateral. One horse had an avul-

424 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING

Fig. 3. These are axial PD images of the pastern and foot arranged (top) proximally to (bottom) distally that show (A) mild, (B) moderate, and (C) severe lesions within the DDFT at different levels of the limb.

NB Flexor Cortex Eight horses in the study were found to have flexor cortical erosions, and these were not visible on ra- diographs. All erosions involved the flexor surface in the distal one-third of the bone over or in associ- ation with the mid-sagittal ridge. One of these horses had defects in both NBs, and the remaining seven horses had a single defect in only one NB— four in the left front, and 3 in the right front. The horse with bilateral lesions had a small erosion on either side of the sagittal ridge on one limb. Eros- ions measured 3–6 mm, depending on the horse, in a medial to lateral dimension. Of the horses with unilateral erosions, five were found on the lamest leg, and two were found on the least lame leg. IL Abnormal high-signal intensity and/or thickening of the IL were observed in 50 horses (95 limbs).

Fig. 5. These are sagittal PD images showing diffuse thickening Fig. 4. These are (left) sagittal and (right) axial PD images of the IL (A) with and (B) without abnormal low-signal intensity showing (A) mild, (B) moderate, and (C) severe enlargement of at the insertion of the ligament on the distal navicular bone the CSL. (arrow).

AAEP PROCEEDINGS ր Vol. 54 ր 2008 425 IMAGING

Fig. 7. This is a sagittal proton density image from a horse with abnormal low signal intensity centrally located within the navic- ular bone, indicating remodeling of the trabeculae resulting in sclerosis.

DDFT Thirty-nine horses (58 limbs) had thickening, irreg- ularity, and/or abnormal high-signal intensity within the DDFT, and it was graded as mild, mod- erate, or severe at four separate levels of the tendon within the horse’s distal limb (Fig. 3). Twelve horses (13 limbs) had damage DTNB, 19 horses (24 limbs) had damage ANB, 38 horses (56 limbs) had damage PTNB, and 21 horses (27 limbs) had dam- age within the pastern region. Tendon damage was most commonly located in the tendon just prox- imal to the NB, and it was least likely to be located distal to the NB. Tendon damage DTNB was not seen bilaterally in any horse, and it was found in the lamest limb in 8 of 12 horses (67%). Tendon dam- age in the pastern region was never seen without Fig. 6. These are (A) sagittal and (B) axial STIR images that show tendon damage also occurring in a more distal por- (arrows) increased fluid in the distal interphalangeal joint. There is also effusion of the DDFT sheath visible on the sagittal image. tion of the tendon in the same limb. In five horses,

Twenty-five horses (36 limbs) also had increased low-signal intensity on PD and T2W sequences at the IL insertion onto the NB, which is compatible with sclerosis in this area (Fig. 5). IL damage was determined to be the primary abnormality in six horses.

Table 3. Comparison of Follow-Up Lameness Information Between the Recent and Chronic Navicular Syndrome Groups

Chronic Recent Group Group Fig. 8. These are axial (A) 3DGE and (B) PD images of a horse Performance Level 2yr 3yr 2yr 3yr with abnormal high-signal intensity in the (arrows) flexor cortex of the NB at the mid-sagittal ridge. This horse also has an area Same level of work 51% 33% 19% 13% of abnormal low-signal intensity within the medullary cavity Lower level of work 13% 11% 12% 12% associated with the flexor cortex lesion visible on the (arrowhead) Total 64% 44% 31% 25% PD image in B.

426 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING

Fig. 11. This is an (arrows) axial STIR image proximal to the Fig. 9. This is a coronal 3DGE image showing a (arrow) distal NB showing abnormal low-signal intensity tissue within the na- margin fragment off the lateral angle located within the IL. vicular bursa at the dorsal aspect of the DDFT lobes.

Distal Interphalangeal Joint tendonitis of the DDFT was the primary finding Thirty-six horses (62 limbs) had increased synovial observed. fluid observed in the distal interphalangeal joint. All horses also had other abnormalities observed in Navicular Bursa the NB or its supporting soft-tissue structures. Fifty-six horses (96 limbs) had increased synovial One horse (1 limb) had damage to one CL of the fluid observed in the navicular bursa; this was distal interphalangeal joint. This horse had dam- graded as mild, moderate, or severe (Fig. age at the insertion of the medial CL on the left front 10). Twenty-eight horses (49 limbs) also had ab- leg. There was associated abnormal high-signal in- normal tissue present in the proximal aspect of the tensity on STIR images, which indicates inflamma- navicular bursa that appeared to be fibrous tissue tion, and abnormal low-signal intensity on PD and between the DDFT and the proximal suspensory T2 images, which indicates increased bone density ligament of the NB (Fig. 11). In these limbs, the (sclerosis; Fig. 12). Three horses (4 limbs) had os- bursitis was often less pronounced because of oblit- teoarthritis of the distal interphalangeal joint. eration of bursal space by scar tissue. A large This horse had cartilage and subchondral bone dam- amount of fluid consistent with bursitis was consid- age that was not visible on radiographs as well as ered the primary abnormality in one horse. some changes in the navicular region (Fig. 13).

Fig. 10. These are (top) axial and (bottom) sagittal STIR images showing (A) mild, (B) moderate, and (C) severe amounts of increased fluid within the navicular bursa, which is indicated by increased signal intensity within the bursa.

AAEP PROCEEDINGS ր Vol. 54 ր 2008 427 IMAGING

Fig. 12. The top two images are (A) axial PD and (B) T2W images showing (arrows) abnormal increased signal intensity within the medial collateral ligament of the distal interphalangeal joint at its insertion onto the distal phalanx. The (C) STIR image shows increased signal intensity within the distal phalanx at the CSL insertion, which is indicative of inflammation within the bone. The (D) coronal T1-weighted image shows the (arrow) abnormal decreased signal intensity within the distal phalanx at the insertion of the CSL, which indicates sclerosis in this area.

Distal Digital Annular Ligament horse had similar injuries on both limbs, one had a Eight horses (11 limbs) had thickening, with or worse injury on the lamest leg, and one had a worse without increased signal intensity, in the distal dig- injury on the least lame leg. The DDAL was not ital annular ligament (DDAL; Fig. 14). Of those determined to be the primary abnormality in any horses, 5 had unilateral injury, and four had bilat- horse. eral injury. Of the unilateral injuries, 3 were on the lamest leg, one of which had adhesions to the Bone Bruises of Phalanges digital flexor tendon sheath and DDFT. One was Five horses (6 limbs) had increased signal intensity in on the least lame leg. Of the bilateral injuries, one one or more of the phalanges seen on STIR sequences

Fig. 13. These are (A) axial STIR, (B) PD, and (C) 3DGE images showing a cartilage and subchondral bone defect in the medial aspect of the distal phalanx (arrows).

428 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING 66 horses (84%) had obvious differences in severity of pathologic change between limbs. In the horses with obvious differences between limbs, 90% (59 of 66) had the most severe and highest number of findings on the most lame leg, and 11% (7 of 66) of the horses had the most severe and highest number of findings on the least lame leg. Lameness Follow-Up Fifty-six horses (71%) were available for follow-up evaluation by phone calls with the owner or trainer. Twenty-five (45%) of these had remained lame and out of work after the diagnosis and MRI. Twenty- nine (52%) had remained in the same or lower level of work for the next 6–12 mo after diagnosis. Two horses (4%) had died of an unrelated cause shortly after diagnosis. Within 1 yr of diagnosis, eight horses died of unrelated causes or were euthanized Fig. 14. This is an axial PD image at the level of the proximal for their lameness: four were euthanized for ongo- middle phalanx showing (arrow) diffuse thickening of the ing lameness, two sustained a broken leg in pasture, DDAL. There is also an area of abnormal increased signal inten- one was attacked by a cougar, and one was eutha- sity present in the medial lobe of the DDFT visible in this horse. nized because of a tumor. Follow-up lameness in- formation is available in Table 3.

(Fig. 15). Two horses (2 limbs) had abnormalities in 4. Discussion the second phalanx, and 3 horses (4 limbs) had abnor- This is the first report of MRI observations in a malities in the third phalanx. All these horses had group of horses with clinical signs of navicular syn- obvious pathologic change in the navicular region of drome without radiographic changes that correlates both front feet, and it was suspected that the inflam- MRI findings with lameness and follow-up informa- matory change (bone bruise) within bones other than tion. It is clear that clinical signs of navicular syn- the NB was a result of a traumatic incident subse- drome can result from a variety of abnormalities, quent to the development of navicular syndrome; how- which has been shown previously.4–17 Addition- ever, this cannot be verified. Two horses had ally, abnormalities like NB edema/sclerosis, CSL unilateral inflammation in the third phalanx at the desmitis, IL desmitis, navicular bursitis, and deep insertion of the IL, which is most likely a result of IL digital flexor tendonitis may be related and may injury at the bone/ligament interface on the distal occur together in the same horse. phalanx. Both of these horses had IL thickening and It is clear in this study that horses with a chronic abnormal increased signal intensity on the same foot. history of navicular syndrome have a higher number The other horse with inflammatory changes in the of structures undergoing pathologic change than distal phalanx had changes along the dorsal surface of those with recent signs of navicular syndrome. the bone, but this was not associated with laminar This is evident by the large number of chronic horses thickening or inflammation. (35 of 79) in which a primary abnormality could not be determined based on which structure was af- Lamina fected more severely than the others. These horses Three horses (three limbs) had irregularity of the grouped in the multiple abnormality category had lamina on one foot near the toe. None of these moderate to severe changes in at least two struc- horses had rotation of the distal phalanx within the tures, which included at least two of the following hoof capsule, but they did have focal thickening of structures: CSL, IL, NB, DDFT, navicular bursa, lamina in areas near the toe that may represent distal interphalangeal joint, distal interphalangeal unhealthy lamina as a result of an old abscess or joint collateral ligament, DDAL, second phalanx, or seedy toe. One of these horses had the laminar third phalanx. The majority of horses in this cate- irregularity in the least lame leg, and the other two gory had more than two structures moderately to horses had the laminar irregularity in the lamest severely involved, and it was not possible to deter- leg. These changes were not determined to be a mine which of the pathologic changes initiated the primary abnormality in any horse, and these horses subsequent disease process. Aging these lesions were not sensitive to hoof testers over the toe in any and determining the chronological order of different case. pathologic changes remains impossible at this time. It is known that some horses may develop inflam- Correlation of MRI Findings and Lamest Limb mation of the NB as an early onset of disease, Of the 79 horses evaluated with MRI, 13 horses whereas other horses may develop desmitis of the (16%) had very similar findings on both limbs, and CSL as the initial onset of the disease.4 We have

AAEP PROCEEDINGS ր Vol. 54 ր 2008 429 IMAGING

Fig. 15. These are (left) sagittal and (right) axial STIR images showing different areas of abnormal increased signal intensity seen in horses in this study. (A) Focal increased signal intensity at the (arrows) insertion of the IL on the distal phalanx. (B) Diffuse increased signal intensity in the NB and medial one-half of the middle phalanx. (C) Focal and slightly diffuse abnormal increased signal intensity within the dorsodistal middle phalanx. also seen that horses with changes to the NB and its present to as a unilateral lameness. supporting structures can go on to develop DDFT It is only after the lame limb is blocked successfully injuries that seem to be a progression of a disease that it becomes apparent that there is an obvious process centered around the NB. In other horses, lameness in the contralateral limb, which was seen primary deep flexor tendonitis occurs without any in 50–76% of horses in this study. Because of the pathologic change to the NB or its ligamentous sup- bilateral nature of navicular syndrome, the majority porting structures. of horses are seen to be moderately lame on one limb In this study, the majority of horses diagnosed before nerve blocks. This does not always indicate with navicular syndrome were Quarter Horses, and the actual degree of lameness in these horses, be- there were also many Warmbloods and Thorough- cause they are protecting both feet to some extent, breds. Because these breeds of horses perform in a even when circling in small circles. To truly deter- wide variety of events, it suggests that the activity mine the lameness grade of the lamest limb, lame- itself may not be an important factor in the disease ness would have to be eliminated from the least process. The horses in this study often had one lame limb and vice versa. This is impractical in a limb that was much more lame than the other limb, clinical setting and was not done with the horses in which explains the reason that some of these horses this study. Distal limb flexion does exacerbate

430 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING lameness in some of these horses, and this would had a difference in the amount of increased abnor- suggest that compression-type pressure over the NB mal signal intensity in the NB between limbs, the and its supporting structures was painful. It has more severe changes were most commonly seen in also been shown that extreme extension of the distal the lamest leg (78% of horses). In contrast, of 13 interphalangeal joint can exacerbate lameness in horses with abnormal decreased signal intensity in navicular syndrome horses.3 More than 50% of the the NB (sclerosis) separate from the IL insertion horses in this study had sensitivity to hoof testers that was worse in one limb than the other, only 54% over the middle of the frog on at least one limb, and (7 of 13) of horses had the most severe findings in the this correlates with the location of the pathologic lame leg. Therefore, in determining the limb that change in many of these horses. This area is di- may be more painful, it is possible that increased rectly under the NB and its supporting structures. signal intensity on STIR sequences is more impor- The area of the DDFT within the foot was also most tant than decreased signal intensity on PD se- 4 commonly affected with pathologic change. quences. This is in contrast to horses with small In horses with a chronic history of navicular flexor cortical erosions, in which 71% (5 of 7) were syndrome, a higher percentage (91%) of horses found on the most lame leg if they were present were found to have pathologic change within the unilaterally. Obviously, the size of these lesions, CSL than in horses with recent onset of navicular 4 being only 3–7 mm in width, may result in them syndrome (75%). In chronic navicular syndrome causing less pain than larger lesions visible on ra- horses, there is a high likelihood of adhesion (46%) diographs, and larger lesions may correlate even between the CSL and the DDFT, even if there is better with lameness. not pathologic change within the DDFT itself. The prevalence of distal margin NBFs in horses Only 56% of horses with adhesions between the with chronic navicular syndrome (36%) was compa- CSL and the DDFT had lesions within the DDFT. rable with findings in horses with recent onset of Adhesions in this region are associated in many navicular syndrome (37.5%). Of the 11 chronic horses with scar-tissue formation within the na- horses with fragments in 1 limb only, 64% were vicular bursa in the proximal aspect. This adds found in the lamest limb. It remains unknown how to the complexity of determining a primary abnor- important these fragments are in the pain caused by mality in these horses and makes it difficult to pathologic change in the navicular region and how determine the inciting cause. It would make they affect progression of the lesions, if at all. All sense that the CSL becomes inflamed and thick- horses with distal margin NBFs had IL desmitis ened in horses in which there is not damage to the and/or NB inflammation in the distal one-third of DDFT, but subsequently forms adhesions to the the bone centered over the IL insertion. This sug- DDFT because of proximity. The inflammation in this area could then result in bursal inflamma- gests that there is a pathologic process resulting in tion and secondary scar-tissue formation in the fragmentation of the NB distal margin in these proximal bursa. These horses can also have in- horses. flammation of the NB, but there are horses that Deep digital flexor tendonitis was the primary only have inflammation of the NB without sur- abnormality in only five horses (6%) in the chronic rounding pathologic change to the associated lig- group, which is different than the prevalence in aments. Therefore, a cause and effect is not the recent group where 13 horses (18%) had deep necessarily occurring here, whereas adhesions be- flexor tendonitis as the primary abnormality. tween the CSL and DDFT were only seen to occur The overall prevalence of horses with DDF ten- in horses with pathologic change in the CSL. donitis in the chronic group compared with the In chronic navicular syndrome horses, 44% of recent group is actually not that different (46% horses had multiple abnormalities that precluded versus 44%, respectively), but the horses in the the determination of a primary abnormality; this chronic group have more separate structures in- is in contrast to the horses with recent onset of volved in the navicular region. This precludes navicular syndrome in which only 18% of horses determination of deep digital flexor tendonitis as a were grouped into the multiple abnormality cate- primary abnormality in many of the chronic gory. The most common primary abnormality in horses because the severity of changes in the NB the chronic horses was pathologic change within and supporting ligaments are equal or worse than the CSL (24% of horses). Horses with recent on- the DDFT lesions. Of the limbs that had lesions set of navicular syndrome had pathologic change in the DDFT at the level of the CSL in the chronic within the NB (33% of horses) as the most common group, 62% (36 of 58 limbs) had adhesions to this primary abnormality compared with the CSL (15% ligament. This indicates that horses with ten- of horses). donitis in this region may be pre-disposed to form- NB damage was the second most common finding ing adhesions to the CSL, which in turn may lead (16% of horses) in the chronic group of horses and to inflammation within this ligament. Of the 56 was present in almost as many limbs as pathologic chronic limbs with DDFT lesions just proximal to change in the CSL (133 versus 137 limbs in 73 and the NB, 43 (73%) of these limbs had increased 72 horses, respectively). In this study, if horses fluid within the navicular bursa, and 26 (44%) of

AAEP PROCEEDINGS ր Vol. 54 ր 2008 431 IMAGING these had obvious scar tissue within the proximal present. This could potentially give the horse a aspect of the bursa. better chance to return to work in the future com- Comparison of the locations of deep digital flexor pared with the horse that is continually treated as a tendonitis in the chronic group and the recent group chronic navicular syndrome horse until lameness shows that 50% more of the chronic horses had ex- forces the horse to stop work. Identification of CL tension of the deep flexor tendonitis into the pastern injury also provides an argument for a significant region. This would make sense with the chronicity rest and rehabilitation program to be instituted as a of the lameness in that the tendon lesions would be means to allow the ligament to heal and the horse to more likely to worsen and enlarge over time. Also return to its previous work. It is unlikely that of interest is the fact that none of the horses in the many of these bone, cartilage, and ligament lesions recent or chronic group had a DDFT lesion in the would heal if the horse continued in treatment for pastern without having a tendon lesion at some navicular syndrome, which supports the use of MRI point more distally in the same limb. It may be to provide a definitive diagnosis. possible to extrapolate from this that, most com- In the recent and chronic groups, the severity of monly, the tendon lesion begins at or near the NB MRI findings had a high correlation with the lam- and over time extends proximally in some horses. est leg of the horse (93% and 90%, respectively). It may also be useful information when using ultra- The multitude of structures affected and the se- sound on the pastern region in horses, because it is verity of pathology in many of the chronic horses likely that a horse with a DDFT lesion in the pastern may account for the slight decrease in accuracy in visible with ultrasound will have equal or more se- picking the limb with the most severe findings. vere lesions within the foot on the same leg. Fur- These horses, in general, had more variation in thermore, extension of the tendonitis proximally the number and types of pathologic change in each into the area covered by the digital flexor tendon limb which added greater complexity in determin- sheath makes injection of the sheath a possible di- ing the changes that accounted for the greatest agnostic and therapeutic technique for horses with pain. With more cases, we may eventually be this diagnosis.b It could also be considered that able to determine the lesions that result in lame- horses with tendonitis in the pastern may be candi- ness more often than other lesions or the specific dates for navicular bursa injection based on the high groups of lesions that cause lameness more fre- likelihood that there are DDFT lesions within the quently. At this point in time, all lesions are foot, which is most commonly seen just above the NB given weight in determining the severity of the in an area covered by the bursal lining. The obser- disease process, although the clinical examination vation that deep digital flexor tendonitis occurs as a is also taken into account when deciding the rele- primary problem in some horses with clinical signs vance of some lesions in a clinical setting. of navicular syndrome is an important observation. When comparing prognosis for return to work in The observation that injury extends proximally in the recent and chronic groups, it is clear that horses the tendon above the level of the proximal interpha- are much less likely to be able to remain in work as langeal joint affects treatment for some horses. they become more chronically affected (Table 3). Rest, rehabilitation, and physical therapy to stretch Four horses with chronic navicular syndrome were the DDFT have not been routinely used to treat euthanized for ongoing lameness within 1 yr of their horses with navicular disease, but they may offer diagnosis, and this may provide insight into another another option for treatment of some of these horses reason that some people chose to have their horses in the future. diagnosed using MRI. This modality can provide a Multiple other abnormalities were also found in very specific diagnosis, and it can give owners the the horses in this study, including DDAL desmitis, information they need to make difficult decisions distal interphalangeal joint , bone regarding further treatment of horses or humane bruises of the first, second, and third phalanges, euthanasia. It can be very difficult for some owners distal interphalangeal collateral ligament desmitis, to stop treatment on a horse when they do not have laminar abnormalities, and adhesions of the DDFT a specific diagnosis to confirm a poor prognosis. to the DDFT sheath. It is difficult to relate these As was shown in this study, the causes of navic- possible findings to the changes in the heel region of ular syndrome and more generally, of pain that is many of these horses that have multiple abnormal- eliminated with a PDN block are numerous. We ities. The presence of hyperintensity on STIR im- no longer believe that there is one pathologic pro- ages, indicative of a bone bruise, within the cess that leads to the myriad of pathologic changes phalanges does suggest that a rest and rehabilita- present within the foot of the horse that causes tion program could be helpful in these specific heel pain. This study of navicular syndrome pro- horses, whereas without an MRI, these horses would vides information that leads to the conclusion that most likely be kept in work with anti-inflammatory heel pain can be caused by pathologic change medication and not given a chance for bone healing within many separate structures in the foot, and to occur. The identification of osteoarthritis with therefore, inflammation of a single structure is cartilage damage visible with MRI provides an ar- enough to cause lameness in the horse. When gument for arthroscopy to debride lesions that are comparing horses with recent onset of navicular

432 2008 ր Vol. 54 ր AAEP PROCEEDINGS IMAGING syndrome to chronic cases, it is clear that the 9. Dyson SJ, Murray RC, Schramme MC, et al. Collateral inflammation spreads in many horses within the desmitis of the distal interphalangeal joint in 18 horses (2001–2002). Equine Vet J 2004;36:160–166. relatively small space within the heel and affects 10. Zubrod CJ, Farnsworth KD, Tucker RL, et al. Injury of the more structures over time, because the pathologic collateral ligaments of the distal interphalangeal joint diag- process is specific to each individual horse. We nosed by magnetic resonance. Vet Radiol Ultrasound 2005; also believe that trauma to one or both front feet 46:11–16. that may go unnoticed by the owner may lead to 11. Zubrod CJ, Schneider RK, Tucker RL. Use of magnetic res- onance imaging to identify suspensory desmitis and adhe- some horses that have significantly worse findings sions between exostoses of the second metacarpal bone and in one limb, although abnormalities are found in the suspensory ligament in four horses. J Am Vet Med Assoc both feet. Horses that have pathologic changes 2004;224:1815–1820. in the navicular region may be more affected by a 12. Zubrod CJ, Schneider RK, Tucker RL, et al. Use of magnetic specific traumatic incident to one foot than horses resonance imaging for identifying subchondral bone damage in horses: 11 cases (1999–2003). J Am Vet Med Assoc with healthy, normal feet, or they may be more 2004;224:411–418. likely to sustain permanent damage compared 13. Dyson S, Murray R, Schramme M, et al. Magnetic reso- with normal, sound horses. This could be a result nance imaging of the equine foot: 15 horses. Equine Vet J of a loss of elasticity to inflamed ligaments or 2003;35:18–26. tendons within the foot that are already present or 14. Dyson S, Murray R, Schramme M, et al. Lameness in 46 a NB that has remodeled to a point where it is not horses associated with deep digital flexor tendinitis in the digit: diagnosis confirmed with magnetic resonance imag- as resilient as a normal bone. ing. Equine Vet J 2003;35:681–690. Inflammation of several different tissues in the 15. Whitton RC, Buckley C, Donovan T, et al. The diagnosis of heel of the horse’s foot can cause similar clinical lameness associated with distal limb pathology in a horse: signs that are currently recognized as one syndrome. a comparison of radiography, computed tomography and For this reason, MRI is a valuable technique for magnetic resonance imaging. Vet J 1998;155:223–229. 16. Widmer WR, Buckwalter KA, Fessler JF, et al. Use of radi- evaluating horses with navicular syndrome, because ography, computed tomography and magnetic resonance im- a specific diagnosis can be made that effects new or aging for evaluation of navicular syndrome in the horse. Vet further treatment. We are still in the learning Radiol Ultrasound 2000;41:108–116. stages with MRI, and more horses need to be eval- 17. Sampson SN, Schneider RK, Tucker RL. Magnetic reso- uated before onset of disease and earlier in the dis- nance imaging of the equine distal limb. In: Auer JA, Stick JA, eds. Equine surgery, 3rd ed. Philadelphia: W.B. ease process to learn more about navicular Saunders Co., 2005;946–963. syndrome. This may enable us to correlate findings 18. Busoni V, Snaps F, Trenteseaux J, et al. Magnetic resonance with the onset of the specific disease process occur- imaging of the palmar aspect of the equine podotrochlear appa- ring in different horses, which in turn will enable ratus: normal appearance. Vet Radiol Ultrasound 2004;45: the evaluation of new diagnostic and treatment 198–204. 19. Murray RC, Schramme MC, Dyson SJ, et al. Magnetic res- methods. onance imaging characteristics of the foot in horses with palmar foot pain and control horses. Vet Radiol Ultrasound This study was partially funded by a grant from 2006;47:1–16. the American Quarter Horse Foundation and Victo- 20. Dyson SJ, Murray R. Magnetic resonance imaging of the ria Cavallero. equine foot. Clin Tech Equine Pract 2007;6:46–61. 21. Schramme MC, Murray RC, Blunden TS, et al. A compari- References and Footnotes son between magnetic resonance imaging, pathology, and radiology in 34 limbs with navicular syndrome and 25 control 1. Ackerman N, Johnson JH, Porn CR. Navicular disease in limbs, in Proceedings. 51st Annual American Association of the horse: risk factors, radiographic changes, and response Equine Practitioners Convention 2005;348–358. to therapy. J Am Vet Med Assoc 1977;170:183–187. 22. Busoni V, Heimann M, Trenteseaux J, et al. Magnetic res- 2. Lowe JE. Sex, breed and age incidence of navicular disease and its treatment. In Practice 1982;4:29. onance imaging findings in the equine deep digital flexor 3. Stashak TS. Lameness. In: Stashak TS, ed. Adams’ tendon and distal sesamoid bone in advanced navicular dis- lameness in horses. Philadelphia: Lippincott, Williams, & ease: an ex vivo study. Vet Radiol Ultrasound 2005;46: Wilkins, 2002:664–680. 279–286. 4. Sampson SN, Schneider RK, Gavin PR, et al. Magnetic res- 23. Barber M, Sampson SN, Schneider RK, et al. Unilateral onance imaging of the front feet in 72 horses with recent navicular bone injury diagnosed with magnetic resonance onset of signs of navicular syndrome without radiographic imaging in a horse. J Am Vet Med Assoc 2006;229:717–720. abnormalities. Vet Radiol Ultrasound 2008. In Review. 24. Jacquet S, Coudry V, Denoix JM. Severe tear of the collat- 5. Schneider RK, Sampson SN, Gavin PR. Magnetic resonance eral sesamoidean ligament in a horse. Vet Rec 2006;159: imaging evaluation of horses with lameness problems, in 818–820. Proceedings. 51st Annual American Association of Equine 25. Dyson SJ, Murray R. Magnetic resonance imaging evalua- Practitioners Convention 2005;21–34. tion of 264 horses with foot pain: the podotrochlear appara- 6. Dyson SJ, Murray RC, Schramme MC. Lameness associ- tus, deep digital flexor tendon and collateral ligaments of the ated with foot pain: results of magnetic resonance imaging distal interphalangeal joint. Equine Vet J 2007;39:340–343. in 199 horses (January 2001-December 2003) and response to 26. Moon KL, Genant HK, Helms CA, et al. Musculoskeletal treatment. Equine Vet J 2005;37:113–121. applications of nuclear magnetic resonance. Radiology 7. Sanz M, Sampson SN, Schneider RK, et al. Epidermoid cyst 1983;147:161–171. in the foot of a horse diagnosed with magnetic resonance 27. Sabiston CP, Adams ME, Li DKB. Magnetic resonance im- imaging. J Am Vet Med Assoc 2006;228:1918–1921. aging of osteoarthritis: correlation with gross pathology us- 8. Murray RC, Dyson SJ, Schramme MC, et al. Magnetic res- ing an experimental model. J Orthop Res 1987;5:164–172. onance imaging of the equine digit with chronic . 28. Polly DW, Callaghan JJ, Sikes RA, et al. The accuracy of Vet Radiol Ultrasound 2003;44:609–617. selective magnetic resonance imaging compared with the

AAEP PROCEEDINGS ր Vol. 54 ր 2008 433 IMAGING

findings of arthroscopy of the knee. J Bone Joint Surgery 32. Svalastoga E, Reimann I, Nielsen K. Changes in the fibro- (US edition) 1988;70:192–198. cartilage in navicular disease—a histological and histochem- 29. Murray RC, Blunden TS, Schramme MC, et al. How does ical investigation of navicular bursa. Nord Vet Med 1983; magnetic resonance imaging represent histologic findings in 35:372–378. the equine digit? Vet Radiol Ultrasound 2006;47:17–31. 30. Blunden A, Dyson S, Murray R, et al. Histopathology in horses with chronic palmar foot pain and age-matched con- aPhilips Gyroscan, Medical Systems, 5680 DA Best, The Neth- trols. Part 1: navicular bone and related structures. E- erlands. quine Vet J 2006;38:15–22. b 31. Blunden A, Dyson S, Murray R, et al. Histopathology in Schneider RK. Injection of the digital flexor tendon sheath as horses with chronic palmar foot pain and age-matched con- a diagnostic and treatment technique for horses with deep digital trols. Part 2: the deep digital flexor tendon. Equine Vet J flexor tendonitis. Unpublished cases, 2001. 2006;38:23–27.

434 2008 ր Vol. 54 ր AAEP PROCEEDINGS