From Asymptomatic Flatfoot to Progressive Collapsing Foot Deformity: Peritalar Subluxation Is the Main Driver of Symptoms.

Abstract

BACKGROUND Flatfoot, or pes planus, is a common anatomical variation marked by a reduced or absent longitudinal arch. Although it is often considered benign, the condition can progress to progressive collapsing foot deformity (PCFD), a debilitating pathology. This study aimed to identify imaging biomarkers that distinguish asymptomatic flatfoot from PCFD by comparing 3D measurements among normally aligned feet, asymptomatic flatfeet, and feet with PCFD. METHODS A prospective, comparative, and controlled study was conducted on 561 feet in 475 subjects: 88 control subjects with 98 normally aligned feet, 66 control subjects with 132 asymptomatic flatfeet, and 321 patients with 331 feet with symptomatic PCFD. Bilateral weight-bearing computed tomography (WBCT) scans were performed, and various 3D measurements were analyzed, focusing on hindfoot valgus (Class A deformity), midfoot and/or forefoot abduction (Class B), arch collapse (Class C), and peritalar subluxation (PTS) (Class D). Statistical analysis, including multivariable nominal regression, was used to identify significant predictors of symptoms. RESULTS Progressive increases in Class A, B, and C deformity parameters were observed from normally aligned feet to asymptomatic flatfeet and finally to feet with PCFD. Significant differences in PTS (Class D deformity) were found only in the comparison between the feet with PCFD and the control groups, with reduced joint coverage and increased sinus tarsi coverage, indicating extra-articular impingement. Multivariable analysis identified the minimum sinus tarsi distance as the strongest predictor of symptoms, with a threshold of 1.9 mm best distinguishing symptomatic from asymptomatic feet. CONCLUSIONS This study demonstrated that although hindfoot valgus, midfoot and/or forefoot abduction, and arch collapse deformities progressively increase from normal alignment to PCFD, significant subtalar joint subluxation and sinus tarsi impingement were unique to symptomatic PCFD. These findings suggest that PTS can serve as a crucial biomarker for diagnosing pathologic flatfoot (PCFD) and differentiating it from asymptomatic flatfoot. Future research should explore the predictive value of PTS biomarkers in identifying flatfoot at high risk for collapse and their impact on clinical management and surgical decision-making. LEVEL OF EVIDENCE Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.