Ankle injuries are among the most common injuries that present to physician offices and emergency departments (EDs) because the ankle is the most frequently injured joint in the body. [1, 2, 3, 4, 5, 6, 7, 8] Ankle injuries are a major cause of time loss from work or other daily activities and constitute up to 25% of all time-loss injuries from running and jumping sports. [9, 10] Sprains account for 85% of ankle injuries and, of these sprains, 85% are caused by inversion injuries. An inversion sprain results in an injury to the lateral ligaments, one of which is the calcaneofibular ligament (CFL). Most ankle sprains can be managed with a short period of immobilization followed by rehabilitation therapy, but chronic instability is best treated surgically. 
An estimated 1 ankle inversion injury occurs per 10,000 people per day, or 23,000 ankle inversion injuries per day. Of these ankle inversion injuries, the CFL is the second most common ligament injured after the anterior talofibular ligament (ATFL).
The CFL courses from the distal fibula to the calcaneus by extending from the distal anterior margin of the lateral malleolus to insert onto the posterior lateral tubercle of the lateral wall of the calcaneus. [8, 12, 13] The CFL lies deep to the peroneal tendons, is cylindrical in shape, and, because it crosses 2 joints, it acts as a subtalar joint stabilizer.
The CFL is 20-30 mm long, 3-5 mm thick, and 4-8 mm wide, and the angle of the CFL from the fibula to the calcaneus is 10 º -45 º posterior to the axis of the fibula. Except in the extremes of inversion, the CFL is in a lax position. With an inverted ankle, strain on the CFL is highest in dorsiflexion; thus, when the ankle is dorsiflexed or in a neutral position, the CFL is the lateral ligament that is most often injured in inversion sprains. Although isolated CFL tears are uncommon, CFL tears in combination with ATFL tears are the second most common injury pattern (20% of injuries). Midsubstance rupture of the CFL remains the most common injury pattern, although a number of fibula or calcaneus avulsion-type injury patterns exist. 
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