Category: Hindfoot; Ankle; Ankle Arthritis; Midfoot/Forefoot Introduction/Purpose: Markers used for diagnosis and severity grading are well studied in patients with Progressive Collapsing Foot Deformity (PCFD). Medial facet subluxation (MFS) in weight-bearing computerized tomography (WBCT) has been established as an early indicator of peritalar subluxation (PTS). When the disease affects the ankle leading to a valgus talar tilt (class E), structures distal to this topography may behave differently, trying to compensate for proximal deformity. The aim of this study is to assess predictors of deformity in PCFD patients with and without valgus of the ankle. Our hypothesis is that MFS cannot be used in Class E patients as an accurate marker for the evaluation and staging of PCFD. Methods: In this IRB-approved retrospective case-control study, we analyzed WBCT imaging of 21 consecutive patients with PCFD with valgus of the ankle and 64 controls (flexible PCFD without ankle involvement). MFS (defined by the percentage of uncoverage), middle facet incongruence angle, middle cuneiform-to-floor distance, forefoot arch angle, talonavicular uncoverage angle, hindfoot moment arm (HMA), Foot and Ankle Offset (FAO), and talar tilt angle (TTA) were obtained and compared between groups using one-way ANOVA. A multivariate regression analysis was performed to evaluate which of the measurements influenced the alignment. A partition prediction model was constructed to assess how the variables contributed to the deformity. P values <0.05 indicated statistical significance. Results: Differences between groups were found for MFS, HMA, FAO, and TTA, with a lower mean value of MFS in PCFD patients with valgus of the ankle. An inverse relation between MFS and TTA was found, which was demonstrated by an increase in the talar tilt and a decrease in middle facet uncoverage. FAO values were affected by MFS in the control group (R2: 0.25) but not in the ankle valgus group (R2: 0.001), which was influenced mainly by the TTA (R2: 0.53). Additionally, a FAO value higher than 12.14 was found to be a strong predictor of deformity at the ankle. Conclusion: Medial facet subluxation was lower in patients with PCFD and valgus of the ankle (Class E), which demonstrates that MFS is a weak predictor of deformity severity. Therefore, this suggests that talar tilt angle and Foot and Ankle Offset should be used as disease markers in this group of patients. Furthermore, a FAO value above 12.14 may be an indicator of ankle involvement in PCFD patients.