AJVR, Vol 72, No. 1, January 2011 33 O steoarthritis is a common cause of distal tarsal pain in horses, 1 but there is limited information on the etiopathogenesis of this condition. Although degen- erative lesions on all aspects of the joints have been reported, standard imaging has revealed these lesions are most commonly in the dorsomedial aspect of the CD joint and the dorsolateral aspect of the TMT joint. 2 Whether these are true sites of predisposition in horses with early stages of pathological change and the reason Histopathologic features of distal tarsal joint cartilage and subchondral bone in ridden and pasture-exercised horses Carolyne A. Tranquille, BSc; Sue J. Dyson, PhD; Anthony S. Blunden, PhD; Simon N. Collins, PhD; Tim D. H. Parkin, PhD; Allen E. Goodship, PhD; Rachel C. Murray, PhD Objective—To determine whether histopathologic characteristics of the osteochondral units of equine distal tarsal joints were associated with exercise history in horses without lameness. Sample Population—30 cadaver tarsi from horses without lameness and with known exercise history were separated into 3 groups: nonridden, pasture exercise (group P); low- intensity, ridden exercise (group L); and high-intensity, elite competition exercise (group E). Procedures—Standardized sites from the centrodistal and tarsometatarsal joints underwent histologic preparation. A grading system was adapted to describe location, depth, and shape of lesions; cellular arrangement; organization at cartilage and subchondral bone (SCB) junctions; and organization of SCB. A high score signified a more severe pathological change than a low score. Exercise groups were compared by calculation of Spearman rank correlations. Results—In the centrodistal joint, lesions were present in groups L and E but only medially. Cellular arrangement scores were higher at the dorsomedial location in group P than in groups L and E. Groups L and E had higher scores than group P for the organization of the cartilage, SCB junctions, and SCB, with higher scores at the dorsomedial location. In the tarsometatarsal joint, lesions were evident across the whole joint surface, with more severe lesions located laterally in all 3 groups. Overall, group E had higher scores for cellular arrangement and SCB organization than groups P and L. Conclusions and Clinical Relevance—Ridden exercise may increase the risk of osteochondral lesions at distal tarsal sites predisposed to osteoarthritis relative to the risk with nonridden exercise. (Am J Vet Res 2011;72:33–41) for these apparent predisposition sites have not been investigated to our knowledge. Overloading of the osteochondral tissues is con- sidered a likely factor in osteoarthritis development, as is exercise-associated osteochondral damage. 3 Elite show jumpers reportedly have a higher prevalence of distal tarsal osteoarthritis than horses undergoing gen- eral purpose exercise, which may reflect a response to overloading. 4 However, there has been little evaluation of relationships between exercise type and tarsal osteo- arthritis location and type. At the level of the TMT joint, compressive strains are highest dorsally and laterally as suggested by strain-gauge evidence. 5 This finding is also supported by nuclear scinti- graphic evidence of radiopharmaceutical uptake patterns Received September 14, 2009. Accepted December 20, 2009. From the Centre for Equine Studies, The Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk, CB8 7UU, England (Tranquille, Dyson, Blunden, Murray); School of Veterinary Science, The University of Queensland, Gatton Campus, Gatton, QLD 4343, Australia (Collins); Institute of Comparative Medicine, Faculty of Veterinary Medicine, University of Glasgow, Glasgow, G61 1QH, Scotland (Parkin); and The Royal Veterinary College and Institute of Orthopaedics and Musculoskeletal Science, University College London, North Mymms, Hatfield, Hertfordshire, AL9 7TA, England (Goodship). Supported by The Horse Trust. The authors thank Ray W. Wright for histologic processing and Marion V. Branch for technical assistance. Address correspondence to Ms. Tranquille (carolyne.tranquille@aht. org.uk). ABBREVIATIONS AC Articular cartilage CC Calcified cartilage CD Centrodistal HC Hyaline cartilage SCB Subchondral bone TMT Tarsometatarsal