Ankle Instability, Chronic

Anatomy

Ligaments are strong bands of connective tissue made of collagen fibers. Ligaments surround joints and provide the connection from one bone to another. The lateral ligamentous structures are most commonly affected in both acute ankle sprains and chronic ankle instability. Of the lateral ankle ligaments, the weakest and most frequently injured in inversion injuries is the anterior talofibular ligament (ATFL), which runs from the anterior (front) aspect of the distal fibula distally to the anterolateral aspect of the talus. The ATFL is normally the primary restraint to internal rotation of the foot when the ankle is in plantarflexion (in which the foot is pointing downward relative to the leg).

In more severe ankle sprains, the calcaneofibular ligament (CFL), which runs from the distal tip of the fibula distally (toward the sole of the foot) and slightly posteriorly to the lateral aspect of the calcaneus, may additionally be torn or stretched. Normally, the CFL restrains the internal rotation of the foot during ankle dorsiflexion (in which the foot points upward relative to the leg). Although it is the second weakest lateral ligament of the ankle, injury to the CFL requires more than double the force required to tear the ATFL.

Causes

Severe or recurrent inversion injuries, when forced internal rotation of the foot occurs with the ankle in plantarflexion, are the most common precipitating events that lead to ankle instability. While such injuries frequently occur during athletic activity, accidents during everyday activities or a forceful step on uneven ground may also cause such a sprain.

Chronic instability has been linked to a wide range of etiological factors, which are typically divided into (a) those contributing to mechanical instability, in which the ligaments themselves remain looser than normal after injury, and (b) those [Page 75]contributing to functional instability, in which a person's control or use of the joint is abnormal and predisposes him or her to recurrent injury.

Mechanical instability of the ankle can result from physiologic ligamentous laxity or joint hypermobility, in which a person's ligaments are physiologically looser than normal. Alternatively, a suboptimal healing response in the lateral ankle ligaments may lead to scar tissue that is weaker or elongated following a sprain.

Functional instability stems from loss of proprioception or muscle weakness. Proprioception, the nervous system's ability to detect and control the position of a body part in space, is inherently compromised when the structures around a joint are injured. If there is no adequate retraining of ankle proprioception in the recovery period, this can leave a person prone to recurrent sprains and chronic instability. These proprioceptive qualities also involve the peroneal muscles, which become weaker as a result of the trauma and the immobilization period that typically follows a sprain. Peroneal weakness has been reported in almost one quarter of patients with recurrent ankle sprains.

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