Ankle sprains are some of the most common injuries seen by physicians. An estimated 2 million acute ankle sprains occur each year in the United States. These injuries comprise nearly 1 in every 10 emergency room visits with an incidence rate of 2.15 per 1,000 person-years, resulting in an annual health care cost of 2 billion dollars. The peak incidence of ankle sprains occurs between 15 and 19 years of age. Athletic activities account for up to nearly half of all athletic injuries, with basketball (41.1%), football (9.3%), and soccer (7.9%) being associated with the highest percentage of ankle sprains during athletics.¹

With the overwhelming prevalence of ankle sprains, it is imperative that the treating clinician be well acquainted with the presentation, diagnosis, and treatment of acute ankle instability. The importance of a systematic approach to these injuries cannot be stressed enough, as long-term sequelae from lateral ankle sprains have been estimated to occur in up to 50% of patients. Furthermore, the clinician should always be wary for possible associated injuries of ankle sprains, for example, fifth metatarsal fractures, tendon injuries, and numerous others. For the purposes of this chapter, ankle sprains are separated into three distinct entities: lateral sprains, medial sprains, and syndesmotic injuries, also known as high ankle sprains.

An understanding of the anatomy of the ankle joint and foot is crucial to evaluating and treating ankle sprains. The lateral ankle ligaments comprise the anterior talofibular ligament (ATFL), the calcaneofibular ligament (CFL), and the posterior talofibular ligament (PTFL) (Fig. 23-1). The ATFL functions mainly to restrict internal rotation of the talus underneath the bony mortise, which refers to the arch of bone formed by the lateral malleolus, the distal tibial plafond, and medial malleolus. The CFL primarily resists adduction of the talus and also plays a major role in stability of the subtalar joint. The PTFL, the strongest of the lateral ankle ligaments, resists posterior and rotatory instability of the talus.

The vast majority of ankle sprains involve the lateral side of the ankle and its ligamentous complex. The typical mechanism involves inversion, plantar flexion, and internal rotation of the talus and other structures of the foot relative to the ankle mortise. This scenario usually leads to injury to the ATFL and/or CFL. Given the fact that the ATFL is the weakest of the three lateral ligaments, the ATFL is the most commonly torn ligament in lateral ankle sprains.

Medial ankle sprains of the ankle involve injury to the deltoid ligament. The deltoid ligament has both superficial and deep components (Fig. 23-2). The superficial component of the ligament primarily resists eversion of the foot. The deep deltoid ligament functions with the medial malleolus to prevent lateral displacement of the talus and also resists external rotation of the talus. These injuries are rare, and usually never isolated. Rupture of the deltoid ligament typically occurs in conjunction with a lateral ligamentous injury or fracture of the lateral malleolus.

High ankle sprains comprise the spectrum of injuries to the syndesmotic ligaments. The syndesmotic ligaments are a highly complex group of ligaments that stabilize the mortise, keeping the fibula articulating with the distal tibia (Fig. 23-3). The syndesmosis is made up of the anteroinferior tibiofibular ligament, interosseous ligament, the posteroinferior tibiofibular ligament, and the inferior transverse ligament. Injury to the syndesmotic ligaments can vary greatly, from a minor sprain to complete disruption, resulting in diastasis, or gross separation of the fibula and tibia. While they mostly exist in the setting of medial/lateral or bony injury, they can present in isolation and are generally thought to result from an external rotation injury.

Understanding the wide range of severity of ankle sprains will benefit the clinician encountering these extremely common injuries. Ankle sprains are traditionally classified according to grades I to III. Grade I sprains consist of intraligamentous tears with pain but no instability. Grade II sprains are incomplete tears with pain and mild to moderate instability. Grade III sprains are the most severe and disabling. Complete ligamentous rupture occurs. Patients often cannot tolerate weight bearing on the injured extremity and display gross instability and pain on physical examination. Patients sustaining a grade III sprain usually describe hearing an audible “pop.” Whether or not a tearing sensation and audible noise can be appreciated at the time of injury, most patients present with pain, loss of support, and difficulty with weight bearing.²