Degeneration and necrosis of the subchondral bone is common in racehorses and may precede slab fractures of the third carpal bone. The condition was initially identified in the proximal articular surface of the third carpal bone and is considered to be a consequence of cyclic trauma. Clinical signs include lameness, reduced performance, and effusion of the middle carpal joint. Horses typically improve with intra-articular analgesia of the middle carpal joint. A skyline view radiograph is critical for diagnosis and typically shows lysis and sclerosis in the third carpal bone (typically radial facet). Arthroscopic examination is the treatment of choice, with removal of the abnormal cartilage and subchondral bone.