Radiographic Predictors of Middle Facet Subluxation in Progressive Collapsing Foot Deformity

Research Type: Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies Introduction/Purpose: Middle facet subluxation (MFS) of the subtalar joint is a key indicator of Peritalar subluxation (PTS) in progressive collapsing foot deformity (PCFD). While its assessment relies on weight-bearing computed tomography (WBCT); it remains unclear which weight-bearing radiographic (WBR) parameters predict or correlate with MFS. This study aimed to identified the radiographic parameters that are correlated with and predict MFS. Methods: Seventy-three feet diagnosed with PCFD were evaluated using WBR and WBCT. Five parameters were measured in both modalities: talonavicular coverage angle (TNC), hindfoot moment arm (HMA), talo-first metatarsal angle and calcaneal pitch angle. Sinus tarsi impingement was also included as binary variable in both modalities. Subsequently, MFS was measured on WBCT. Multivariate regression analysis was performed to identify radiographic predictors for MFS, and bivariate analysis was conducted for each radiographic predictor to determine its individual correlation with MFS. Results: Multivariate regression analysis identified TNC and HMA measured on WBR as predictors of MFS, explaining 52.4% of its variability (R2=0.5241, F-ratio = 38.2, P < .0001). Aditionally, Pearson Correlation showed both predictors were significantly correlated with MFS. TNC exhibited a moderate correlation strength (r = 0.68, 95% CI 0.54-0.79, P < .0001), while HMA demonstrated a weak correlation strength (r = 0.39, 95% CI 0.17-0.56, P = .0006). Conclusion: This study demonstrates that TNC and HMA explain about half of the variability in MFS. While conventional radiographic assessment provides valuable insights, it has limitations in fully evaluating MFS.