Background: Middle facet subluxation (MFS) of the subtalar joint is a key indicator of peritalar subluxation in progressive collapsing foot deformity (PCFD). Although its assessment relies on weightbearing computed tomography (WBCT), it remains unclear which weightbearing radiographic (WBR) measurements correlate with MFS. We hypothesize that radiographic parameters assessing hindfoot alignment are correlated with MFS. Methods: Seventy-three feet diagnosed with PCFD were evaluated using WBR and WBCT. Five parameters were measured in both modalities: talonavicular coverage (TNC) angle, hindfoot moment arm (HMA), talo–first metatarsal angle, calcaneal pitch angle, and sinus tarsi impingement. MFS was then measured on WBCT. Multivariate and bivariate regression analyses were used to identify which WBR measurements predict or correlate with MFS. Partition analysis was used to identify cutoff values for WBR measurements linked with MFS. Results: Multivariate regression identified TNC angle and HMA as combined MFS predictors ( R ² = 0.524, P < .0001). In the bivariate analysis, TNC angle demonstrated a moderate individual correlation with MFS ( r = 0.68), whereas HMA showed a weak individual correlation ( r = 0.39). Partition analysis identified practical radiographic thresholds (TNC angle ≥ 38 degrees, HMA ≥ 11.4 mm) for MFS ≥18% yet these thresholds explained only 27.3% of variability ( R ² = 0.273). Conclusion: TNC angle and HMA are the most useful WBR measurements for identifying MFS in PCFD. In settings without WBCT access, these offer a practical assessment tool, associated with MFS ≥18%; however, these cutoffs accounted for only 27.3% of variance ( R ² = 0.273), underscoring residual unexplained factors.
Casas et al. (Fri,) studied this question.