FRANK J. MILNE STATE-OF-THE-ART LECTURE


Fig. 15. A, Transverse section of a deep digital flexor tendon showing early core necrosis. Some fascicles undergoing degeneration contain spindle-shaped and rounded cells, resembling fibroblasts and tenochondrocytes, which are believed to have migrated into the fascicle from the surrounding interstitium. At the bottom of the image centrally there is a focal area of collagen dissolution. Haema- toxylin and eosin. Magnification  100. B, Transverse section of a deep digital flexor tendon showing core necrosis. There is a central area with loss of normal collagen structure, basophilia and increased cellularity, comprising numerous fibroblasts and differentiating chondrocytes. The striated appearance of the surrounding tendon tissue is a processing artefact, the result of separation of collagen fibres. Haematoxylin and eosin. Magnification  100. (Reproduced from Equine Vet J 2009;41:25–33, with permission).


lesions of both the navicular bone and the DDFT often had multifocal lesions involving the medial and lateral lobes of the DDFT, especially from the level of the proximal aspect of the navicular bursa distally. Lesions of the navicular bone were often in the same sagittal plane as the DDFT lesions. Defects in the palmar compact bone of the navicular bone often had focal adhesions to the DDFT. Small focal adhesions may be more difficult to identify in low-field images because of the thicker slice thick- ness compared with high-field images. These DDFT lesions appear to be degenerative with vascular compromise, especially in the septae, and matrix changes, characterized by increased pro- teoglycan deposition and changes in tenocyte mor- phology, and fibrocartilaginous metaplasia.51,82,83 There was no evidence of any acute inflammatory response.


Clinical Features of Primary Injuries of the DDFT


A previous study demonstrated that horses that jump had a higher frequency of occurrence of pri- mary DDFT injuries than horses from other disci- plines.72 A more recent study indicated that elite show jumpers were particularly at risk.84 Horses 10 to 15 years of age had an increased risk of pri- mary DDFT lesions than horsessix years old (odds ratio, 3.30).85 A theoretical model indicated that a 1° decrease in the angle of the solar surface of the distal phalanx would result in a 4% increase in force of the DDFT on the navicular bone at the end of stance.86 However, in a recent study of 300 horses with foot pain, there was no significant association between injury type and angles of the distal pha- lanx, although there was a trend for the angle of the dorsal aspect of the distal phalanx with the horizon- tal to be smaller in horses with injuries of the podo- trochlear apparatus or the DDFT compared with other groups.87 In a small radiological study com-


114 2013  Vol. 59  AAEP PROCEEDINGS


paring 20 horses with DDFT lesions and 20 control horses, the angle at which the DDFT passed over the palmar aspect of the navicular bone was more acute than in the control horses.88 Primary lesions of the DDFT as the principal


cause of lameness were seen in 80 horses between January 2001 and March 2005.89 These horses had closed core lesions or dorsal or less commonly pal- mar abrasions or long full-thickness splits; horses with short isolated parasagittal plane splits were not included. Horses with increased signal inten- sity in the palmar third of the navicular bone in fat-suppressed images were included, but horses with other forms of navicular pathology or other lesions potentially contributing to pain and lame- ness were excluded. Horses presented with either unilateral (n  48) or bilateral (n  30) forelimb lameness or unilateral hindlimb lameness (n  2). Lameness often improved with rest but was exacer- bated by work. Lameness varied considerably in degree (3 to 7 of 8; the most frequent lameness score was 4) but was usually worse on a circle on a hard surface. There were usually no localizing clinical signs, although some horses pointed at rest. Pal- mar digital analgesia rendered 25 horses (31%) sound; improved lameness 50% in a further 33 (41%), but produced no change in 22 (27%). Lame- ness was abolished in all horses by palmar nerve blocks at the base of the proximal sesamoid bones. Intra-articular analgesia of the DIP joint (5 mL mepivacaine; lameness assessed at 5 minutes after injection) improved or abolished lameness in 41 of 75 horses (55%) but produced little change in 34 (45%). Intrathecal analgesia of the navicular bursa resulted in improvement in lameness in 25 of 37 horses (68%). Intrathecal analgesia of the DFTS improved lameness in three of 12 horses (25%). Nineteen of 76 horses (25%) had IRU in the region of the DDFT in lateral pool phase scintigraphic im-


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