Aseptic tenosynovitis of the carpal flexor sheath caused by rupture of the accessory ligament of the deep digital flexor tendon.

A mare was evaluated for acute left forelimb lameness with effusion of the carpal flexor sheath. No osseous abnormalities were noted during radiographic examination. Significant disruption of the accessory ligament of the deep digital flexor tendon was seen during ultrasonographic examination. Carpal sheath effusion and lameness resolved after medical treatment.