Exploratory Analysis of the Effect of Foot Deformities on Hindfoot Alignment Measurement: A Comparison of Second Ray, Ankle Mortise, and Forefoot Alignment on Weight-bearing CT

Background: Accurate hindfoot alignment (HA) assessment is essential for surgical planning of foot and ankle procedures, as measurement errors may lead to inappropriate indication. Weight-bearing computed tomography (WBCT) enables HA assessment through coronal images, typically aligned perpendicular to the second ray. However, this forefoot-based reference may not be appropriate in all clinical scenarios, particularly when isolated hindfoot evaluation is required. This exploratory study sought to evaluate how 3 anatomical reference axes affect HA measurements in patients with different foot deformities. Methods: This retrospective cohort study examined 136 WBCT and standard foot radiographs of patients older than 18 years. HA was measured as hindfoot alignment angle (HAA) in coronal images perpendicular to (1) second ray, (2) ankle mortise (bisection of medial and lateral malleoli), and (3) forefoot midpoint (bisection of first and fifth metatarsal heads). Meary, Sgarlato, hallux valgus, and intermetatarsal I-II and I-V angles assessed foot morphology. Spearman correlation analysis was performed to assess relationships between 2-dimensional (2D) parameters and HAA measurements. Results: Median age was 50.3 years and 64% were female. HAA showed significant variations across different reference systems (second ray: 10.0 degrees, ankle mortise: 15.2 degrees, forefoot midpoint: 12.7 degrees; P < .01), with 20.6% of feet showing a discrepancy exceeding 10 degrees between the second ray and ankle mortise. Spearman correlation analysis showed correlation between HAA and Meary angle across all reference axes (ρ = −0.58 to −0.49, P < .01), and between HAA and Sgarlato angle with second ray referencing (ρ = −0.41, P < .01). Conclusions: Reference axis selection substantially influences 2D HA measurements on WBCT. The ankle mortise reference could provide isolated measurement of HA regardless of concurrent forefoot deformities. When using 2D measurement methods, reference system selection could be tailored to whether surgical planning requires comprehensive foot alignment assessment or isolated hindfoot evaluation. Level of Evidence: Level III, retrospective cohort study.