Subtalar joint dynamics in progressive collapsing foot deformity: A cross-sectional progression model.

BACKGROUND: Peritalar subluxation (PTS) contributes to progressive collapsing foot deformity (PCFD), yet its pathophysiology remains unclear. This study analyzed subtalar joint (SJ) dynamics throughout deformity progression. METHODS: A retrospective review of 153 WBCT scans classified PCFD into four stages: (1) no PTS, (2) PTS without impingement, (3) PTS with sinus tarsi impingement (STI), and (4) PTS with subfibular impingement (SFI). Three-dimensional measurements of middle facet subluxation and incongruence, foot-ankle offset, SJ offset, and lateral process offset were compared between groups. RESULTS: Progression to PTS was characterized by internal talar rotation relative to the foot tripod. Progression to STI demonstrated medial talar translation and subtalar valgus opening. After STI onset, deformity progressed, with a shift in the center of rotation from the middle facet to the lateral talar process. CONCLUSION: Internal talar rotation initiates PTS, while STI marks a turning point where collapse shifts toward the lateral talar process. LEVEL OF EVIDENCE: Level 3 retrospective comparative study.