Defining Radiographic Threshold Values for the Progressive Collapsing Foot Deformity Classification System: A Large-Cohort Diagnostic Study

BACKGROUND: The Progressive Collapsing Foot Deformity (PCFD) classification categorizes deformity patterns into 5 classes (A, hindfoot valgus; B, midfoot/forefoot abduction; C, medial column collapse; D, peritalar subluxation; E, ankle valgus). Currently, the classifying process relies on surgeons' experience and interpretation of clinical and radiographic measurements. The goal of the present study was to establish cut-off threshold values for commonly used PCFD measurements assessing class A to D, using 2 large cohorts of PCFD patients and controls. METHODS: This prospective comparative diagnostic study included 154 neutrally aligned or asymptomatic flatfoot volunteers (103 females, mean age 41.7 years, body mass index [BMI] 28.9) and 321 PCFD patients (136 females, mean age 50.7 years, BMI 29.8). Participants underwent weight-bearing computed tomography (WBCT). Measurements were obtained after blinded segmentation and included hindfoot moment arm (HMA, class A), talonavicular coverage angle (TNCA) and talus-first metatarsal angle axial (TFMA-A, class B), talus-first metatarsal angle sagittal (TFMA-S) and forefoot arch angle (FAA, class C), and coverage maps (middle facet [MF], posterior facet [PF], sinus tarsi [ST], class D). Receiver operating characteristic curves, Youden indexes, and areas under the curves (AUCs) identified optimal cut-offs. A partition prediction model refined those values, and a multivariate analysis identified independent predictors of symptomatic PCFD. HMA, TNCA, MF, and ST coverages were identified as independent predictors of PCFD. CONCLUSION: This study established robust cut-off values for PCFD measurements across classes A-D using large comparative cohorts, significantly enhancing diagnostic performance (AUC ≥ 80% for most). These thresholds improve the clinical applicability and reliability of the PCFD classification, supporting clinical and surgical decision making and facilitating future comparative research. LEVEL OF EVIDENCE: Level II, prospective comparative diagnostic study.