The ankle is held in place by fibrous straps of tissue called ligaments. Ligaments connect bone to bone in joints and serve as struts that lock the joint into position so that the joint moves only in the direction its role in body movement requires. In the ankle, there are two sets of ligaments that keep the ankle moving only up and down, and not side to side. The ligaments on the inner side of the ankle form a continuous sheet that is roughly triangular-shaped and is very strong. Injury and weakening of this ligament are uncommon. The ligaments on the outer side of the ankle are three in number and are frequently sprained, torn, or ruptured during common ankle sprain injuries. With either one serious sprain or each subsequent mild sprain to follow, the ligaments gradually stretch and weaken. If they are torn or ruptured during a serious sprain injury and not repaired, scar tissue fills in the longer space and the joint weakens immediately as a result.

Chronic ankle instability occurs when these ligaments are weak or absent. The ankle is more likely to turn inward, or less commonly outward, on uneven surfaces or with increased activity. This leads to a never-ending cycle of injury and further weakening. Walking and running can become difficult, and the ability to play sports becomes harder and harder as the instability continues. Simple activities can cause sprains, and walking on rocky or other uneven surfaces can become impossible.

Treatment involves stabilizing the ankle using either external or surgical means to reduce the excessive and abnormal motion. External braces in mild cases can be used to prevent the cycle of sprains during activity, and physical therapy can retrain muscles to respond to the instability. Unfortunately, ligament damage cannot be repaired or strengthened through these means. Surgery is necessary to provide this anatomic stabilization and repair the damage next to the joint. Surgery for ankle instability essentially artificially recreates the ligament structure. This is done through a number of different techniques, including tightening of tissue that surrounds the damaged ligament and reforming a ligament-like structure using a tendon found in the area. The end result is a stronger ankle joint that does not abnormally move inward. If the ligament is damaged on the inner side of the ankle, leading to the less common excessive outward motion of the foot, more surgery may be necessary to stabilize the foot on the ankle. This may include fusion of the joint below the ankle that facilitates the outward motion, as well as tendon transfers to tighten up the resulting flattening of the foot.