Synovial osteochondromatosis in the antebrachiocarpal joint of a pony

Synovial chondromatosis is the formation of multiple benign cartilaginous nodules within a synovial lined structure. When the abnormal cartilage nodules undergo endochondral ossification to form ossified bodies, the condition is termed synovial osteochondromatosis. This case report documents an instance of synovial osteochondromatosis in the left antebrachiocarpal joint of a 12-year-old Connemara pony gelding following trauma to the area. The pony presented with a mild left forelimb lameness, positive to carpal flexion and a severe antebrachiocarpal joint effusion with a firm swelling on the dorsomedial aspect of this joint. Radiographs, ultrasonography, and magnetic resonance imaging (MRI) revealed multiple irregularly shaped but very smoothly marginated mineral opacities within the antebrachiocarpal joint. Arthroscopic removal of the nodules and subsequent histopathology confirmed the diagnosis. At 10 months, the pony was sound in all gaits and had returned to its previous level of work, with no effusion of the left antebrachiocarpal joint.