Role of subchondral bone remodelling in collapse of the articular surface of Thoroughbred racehorses with palmar osteochondral disease.

REASONS FOR PERFORMING STUDY: To gain a better understanding of the aetiology of articular surface collapse in horses with palmar osteochondral disease. OBJECTIVES: To determine whether acceleration of focal bone resorption associated with reduced physical activity contributes to articular surface collapse in racehorses with palmar osteochondral disease. STUDY DESIGN: Cross-sectional study comparing metacarpal bones from horses at varying stages of race training. METHODS: Metacarpal bones from 36 racing Thoroughbred horses were examined with high-resolution peripheral quantitative computed tomography to determine the proportion of the articular surface that had collapsed and with backscattered scanning electron microscopy to quantify porosity and eroded bone surface. Racing and training histories were obtained for comparison with imaging data. RESULTS: In 21 cases, inward collapse of the calcified cartilage layer was observed on backscattered scanning electron microscopy. An increased extent of articular surface collapse was associated with greater numbers of microfractures in the calcified cartilage and superficial subchondral bone (Spearman's correlation [rs ] = 0.62, P<0.001). In the deeper bone (6-10 mm), porosity was lower with a greater extent of articular surface collapse (rs = -0.38, P = 0.023), whereas in the superficial bone (0-4 mm) there was no association between articular surface collapse and porosity (rs = 0.19, P = 0.26). Both porosity (median 14, range 3.8-26 vs. 3.8, 1.6-17%, P = 0.008) and eroded surface (1.1, 0.74-4.5 vs. 0.64, 0.11-4.7 mm(-1) , P = 0.016) of the superficial subchondral bone were higher in resting than in training horses, and in some resting horses subchondral bone voids were highly concentrated, resulting in an apparent loss of support for the overlying calcified cartilage layer. CONCLUSIONS: Articular surface collapse is common in cases of palmar osteochondral disease and is likely to be a sequel to fatigue injury of subchondral bone. Focal subchondral bone resorption appears to contribute to collapse of the calcified cartilage and is potentiated by a reduced-intensity exercise regimen.