Orientation of the Subtalar Joint: Measurement and Reliability Using Weightbearing CT Scans

Background: Up to 60% of patients with an osteoarthritic ankle joint develop talar tilt with progression of the osteoarthritic process. The configuration of the subtalar joint, in particular the posterior facet, may contribute to the development of this wear pattern. Recently, the subtalar vertical angle (SVA) was used to describe the posterior facet of the subtalar joint in the frontal plane. The aim of this work was to analyze if the orientation of the subtalar joint may influence the type of asymmetric ankle osteoarthritis. Methods: In total, 60 ankles were retrospectively analyzed including 40 osteoarthritic patients and 20 healthy controls. The osteoarthritic ankles were divided into 4 groups: varus ankle joints with (incongruent) or without (congruent) a tilted talus and valgus ankle joints with (incongruent) or without (congruent) a tilted talus. The orientation of the subtalar joint was described using the SVA. The SVA was determined for every patient using weightbearing CT scans. Additionally, the inter- and intraobserver reliability was assessed using intraclass correlation coefficients (ICCs). Results: The inter- and intraobserver reliability was excellent (ICC > 0.989 and >0.975, respectively). The varus groups (incongruent and congruent) had significantly lower SVA values, that is, more varus orientation of the subtalar joint than the valgus groups (P < .05). The SVA of the control group was between the values of the varus and valgus ankles. Conclusion: The SVA provided a reliable and consistent method to assess the varus/valgus configuration of the posterior facet of the subtalar joint. In our cohort, varus osteoarthritis of the ankle joint occurred with varus orientation of the subtalar joint whereas in patients with valgus osteoarthritis, valgus orientation of the subtalar joint was found. Our data suggest that the subtalar joint orientation may be a risk factor for the development of ankle joint osteoarthritis. Level of Evidence: Level III, retrospective case control study.