Biomechanical testing of the calcified metacarpal articular surface and its association with subchondral bone microstructure in Thoroughbred racehorses.

Abstract

Palmar/plantar osteochondral disease (POD) and third metacarpal/-tarsal condylar fractures are considered fatigue injuries of subchondral bone (SCB) and calcified cartilage due to repetitive high loads in racehorses. In combination with adaptive changes in SCB in response to race training, the accumulation of SCB fatigue is likely to result in changes of joint surface mechanical properties. To determine the spatial relationship and correlation of calcified articular surface biomechanical properties with SCB microstructure and training history in the distal palmar metacarpal condyle of Thoroughbred racehorses. Cross-sectional study. Third metacarpal condyles were examined from 31 Thoroughbred horses with micro-computed tomography (microCT). Hyaline cartilage was removed and reference point indentation (RPI) mechanical testing of the calcified articular surface was performed. Training histories were obtained from trainers. The association among indentation distance increase (IDI, an inverse RPI measure of toughness), and microCT and training variables was assessed using a mixed-effects generalised linear model. Untrained horses had higher IDI than horses that had commenced training (P<0.001). Death as a result of musculoskeletal bone fatigue injury (P=0.044) and presence of POD (P=0.05) were associated with higher IDI. The microCT variables connectivity density and trabecular pattern factor were positively (P=0.002) and negatively (P<0.001) correlated with IDI respectively. The application of RPI to the calcified articular surface is novel and there is a potential for measurement variability with surface unevenness. Commencement of race training is associated with altered material properties of the calcified articular surface in horses. Reduced articular surface material properties can also be detected in horses that have fatigue injuries of the distal metacarpus and at other sites in the skeleton. Measures of SCB connectivity and trabecular surface shape may be more important determinants of resistance to failure of the calcified articular surface than traditional measures such as SCB volume and density.