Three-dimensional subtalar joint space mapping identifies sinus tarsi–predominant structural compromise in early- versus end-stage varus ankle osteoarthritis

Subtalar compensation is increasingly recognized in varus ankle osteoarthritis (OA), yet the breakdown of this compensatory mechanism in advanced disease and the regional degenerative signature within the subtalar joint remain poorly characterized. We applied simulated weight-bearing computed tomography (sWBCT)–based three-dimensional (3D) subtalar joint space width (JSW) mapping to quantify regional structural differences between early- and end-stage disease. Patients with varus ankle OA were stratified by the Takakura–Tanaka classification into early-stage (stage 2) and end-stage (stage 4) groups and underwent bilateral sWBCT. Conventional radiographic and sWBCT-derived alignment parameters were measured. Subtalar bone-to-bone distance mapping was performed and partitioned into four regions of interest (ROIs): the anterior, middle, and posterior articular facets, and the sinus tarsi interosseous region. For each ROI, distance distribution metrics and surface-burden metrics—defined as the proportion of ROI surface with bone-to-bone distance < 3.0, < 2.0, < 1.0, and < 0.5 mm—were extracted. Associations between alignment parameters and JSW indices were assessed using partial Spearman correlations controlling for disease stage. End-stage OA showed a global reduction in subtalar complex JSW and a greater surface burden of severe narrowing than early-stage OA. Between-group separation was most pronounced and consistent in the sinus tarsi compartment: all eight C-SIN metrics differed significantly between stages with uniformly large effect sizes (η² = 0.257–0.562; all linear mixed model [LMM] q ≤ 0.024). After adjustment for disease stage, the tibiotalar angle on sWBCT (TTA-sWBCT) was independently associated with posterior-facet JSW indices (partial | r p | = 0.44–0.62; all q ≤ 0.036) and with the sinus tarsi severe-narrowing burden (partial | r p | = 0.49–0.58; all q ≤ 0.027). sWBCT-based 3D distance mapping identified sinus tarsi interosseous-space compromise as the most prominent structural difference between early- and end-stage varus ankle OA, whereas posterior-facet articular JSW was more closely linked to coronal malalignment after adjustment for disease stage.