In human anatomy, accessory usually means an extra helper. The tissue may be like the main structure. An accessory ligament is usually in addition to the primary ligament. It's not always present in every human.

A recent study from the University of Athens in Greece reported an accessory ligament. It was an accessory to the anterior inferior tibiofibular ligament (AITFL). The accessory is located just below the main ligament. It's a separate structure with a
dividing wall between the two ligaments. This partition was made of fat and fibrous tissue.

Researchers report this ligament can get pinched up against a bone in the ankle. This happens most often after an ankle sprain. Surgery may be needed to remove or repair damaged tissue from this injury before symptoms will go away. In a small study of 24 cadavers, 22 of the ankles had this extra ligament. It's not usually a problem until the ankle is injured or sprained.

Constantinos E. Nikolopoulos, MD, et al. The Accessory Anteroinferior Tibiofibular Ligament as a Cause of Talar Impingement: A Cadaveric Study. In The American Journal
of Sports Medicine. March/April 2004. Vol. 32. No. 2. Pp. 389-395.