The accessory ligament of the deep digital flexor tendon (inferior check ligament [ICL]) is a strong fibrous band that is the direct continuation of the common palmar ligament of the carpus. Desmitis of the ICL may be seen alone or develop secondarily to injury to the superficial digital flexor tendon. In horses with severe ICL damage, the deep flexor tendon may also be affected. Injury to the ICL appears to be more common in adult or aged horses, affirming that degenerative aging changes may predispose the ligament to injury. Injury is relatively uncommon in racehorses and is common in ponies and Warmbloods (show jumpers and dressage). Injury of the ICL is usually unilateral. Diagnosis is typically from clinical examination, with swelling in the proximal one-third of the metacarpus dorsal to the superficial digital flexor tendon. Ultrasonographic examination is typically confirmatory, with areas of enlargement, fiber pattern disruption, and loss of the normal border of the ligament. Injury to the ICL is usually treated with rest and controlled exercise, along with possible shockwave and/or intralesional injection of platelet-rich plasma or stem cells. Desmotomy of the ICL has also been performed in horses that have not responded to conservative therapy.