Weight-bearing CT Scans to Evaluate Lateral Ankle Instability
Ankle sprains are commonly encountered in foot and ankle orthopaedic practices and are typically managed nonoperatively. However, a subset of patients develop chronic lateral ankle instability, which, if left untreated, can cause recurrent osteochondral injury and ultimately tibiotalar arthritis. WBCT scans have been used to study the alignment of the foot and ankle in patients with chronic lateral ankle instability to determine if certain risk factors can be elucidated.
In an early study, van Bergeyk et al used simulated WBCT scans to study hindfoot alignment in 12 patients with three or more episodes of lateral ankle sprains or instability with ongoing symptoms for at least 6 months. They compared this cohort of patients with 12 control patients. The study demonstrated that patients with chronic lateral ankle instability had measurements of hindfoot alignment including calcaneal metatarsal angle and medial calcaneal varus angle that were, on average, in approximately 3° to 4° more varus than the control patients. The authors concluded that hindfoot varus was an important factor correlated with recurrent ankle instability.
More recently, Lintz et al used measurements on WBCT scans to examine hindfoot alignment in patients with chronic lateral ankle instability. They compared FAO, calcaneal offset, and the hindfoot angle in 34 patients with chronic lateral ankle instability with 155 patients without ankle instability. Calcaneal offset represents the difference (in millimeters) between a neutral position of the calcaneus and the actual position of the calcaneus. The hindfoot angle is formed between the weight-bearing axis of the tibia at the apex of the center of the talar dome and a line draw through the long axis of the calcaneus. Patients with chronic lateral ankle instability had approximately 4% more varus in their FAO, 9 mm more of varus in their calcaneal offset, and 16° more of varus in their hindfoot angle. For every 1% change in the FAO toward a varus alignment, there was a 35% increased odds ratio of developing chronic lateral ankle instability.
J Am Acad Orthop Surg. 2020;28(14):e595-e603. © 2020 American Academy of Orthopaedic Surgeons