RESUMEN
The palmar aspect of the third metacarpal (MC3) condyle of equine athletes is known to be subjected to repetitive overloading that can lead to the accumulation of joint tissue damage, degeneration, and stress fractures, some of which result in catastrophic failure. However, there is still a need to understand at a detailed microstructural level how this damage progresses in the context of the wider joint tissue complex, i.e. the articular surface, the hyaline and calcified cartilage, and the subchondral bone. MC3 bones from non-fractured joints were obtained from the right forelimbs of 16 Thoroughbred racehorses varying in age between 3 and 8 years, with documented histories of active race training. Detailed microstructural analysis of two clinically important sites, the parasagittal grooves and the mid-condylar regions, identified extensive levels of microdamage in the calcified cartilage and subchondral bone concealed beneath outwardly intact hyaline cartilage. The study shows a progression in microdamage severity, commencing with mild hard-tissue microcracking in younger animals and escalating to severe subchondral bone collapse and lesion formation in the hyaline cartilage with increasing age and thus athletic activity. The presence of a clearly distinguishable fibrous tissue layer at the articular surface immediately above sites of severe subchondral collapse suggested a limited reparative response in the hyaline cartilage.