Annular ligament (AL) injury, desmitis, and constriction is characterised by a thickened convex contour around the back and sometimes the front aspect of the fetlock caused by a thickening of the AL. The flexor digital synovial sheath is distended above the annular ligament and when viewed from the side, a distinct and characteristic notch in the palmar or plantar outline of the fetlock caused by the annular ligament constriction of the swollen flexor sheath is visible. Lameness is caused by restriction of the normal free movement of the digital flexor tendons that run through the tunnel. However thickening of the ligament and a flexor sheath notch, common in older horses and ponys, does not always cause lameness and other lameness conditions may clinically look like AL constriction. This makes a definitive diagnosis hard without advanced diagnostics in some cases. This article discusses the anatomy, clinical signs, diagnosis, and treatment of this condition and the related condition of annular ligament insertion injury.

The annular ligament attaches to the outside (abaxial) surfaces of both proximal sesamoid bones and extends as a band of tough fibrous tissue transversely across the front and back (palmar/plantar) aspect of the fetlock creating a ring around the fetlock. In the back of the fetlock it creates a canal or tunnel that together with the palmar ligament and the proximal sesamoid bones themselves. Through this tunnel the superficial and deep digital flexor tendons pass. This tunnel is lined by the digital synovial sheath that runs from just above the fetlock to the DDF insertion in the foot. On the back midline of the tunnel there is a connection between the annular ligament and the flexor sheath and the superficial digital flexor tendonmore on normal anatomy and illustration.