Subtalar Joint Offset: A 3D WBCT Measure to Distinguish Global Hindfoot Malalignment From Isolated Subtalar Alignment

Foot and ankle offset (FAO) is a semiautomatic weightbearing computed tomography (WBCT) measurement that assesses the 3-dimensional (3D) relationship between the ankle joint center and the foot tripod, offering a global view of hindfoot alignment (HA). Because FAO does not isolate subtalar alignment, we introduce and assess the reliability of subtalar joint offset (SJO), a 3D WBCT measure of SJ alignment relative to the foot tripod.

We conducted a retrospective study on 204 feet categorized as neutral (n = 30), valgus (n = 107), or varus (n = 67) based on FAO values (-0.6% < neutral < 5.2%). SJO was calculated by identifying the position of the SJ calcaneal middle facet in relation to the foot tripod using WBCT software (Talas, Cubevue; CurvebeamAI). Intra- and interobserver intraclass correlation coefficients (ICCs) were calculated. We calculated the SJO-FAO difference in millimeters on the same reference plane and analyzed correlations by group (Bonferroni-adjusted α = 0.006).

ICC values were excellent (intraobserver: 0.98; interobserver: 0.92). FAO significantly differed across groups: neutral (2.52 ± 2.38), valgus (10.68 ± 4.03), and varus (-8.9 ± 6.92) (P < .001). SJO also varied significantly: neutral (15.3 ± 2.07), valgus (20.85 ± 2.78), and varus (10.32 ± 3.08) (P < .001). The SJO-FAO difference was smallest in valgus (10.16 ± 2.01), intermediate in neutral (12.88 ± 2.67), and largest in varus (19.08 ± 5.12) (P < .001). A moderate correlation was found between FAO and SJO in the varus and valgus groups (ρ = 0.68, r² = 0.51; and ρ = 0.68, r² = 0.57), and low in the neutral group (ρ = 0.47, r² = 0.36) (all P < .001).

SJO is a reliable WBCT metric for distinguishing subtalar alignment across valgus, neutral, and varus HA. Prospective, multicenter studies should test whether the SJO-FAO difference quantifies subtalar contribution and informs realignment surgery.