Deformities Influencing Different Classes in Progressive Collapsing Foot.

Background: The current classification system of progressive collapsing foot deformity (PCFD) is comprised of 5 possible classes. PCFD is understood to be a complex, three-dimensional deformity occurring in many regions along the foot and ankle. The question remains whether a deformity in one area impacts other areas. The objective of this study is to assess how each one of the classes is influenced by other classes by evaluating each associated angular measurement. We hypothesized that positive and linear correlations would occur for each class with at least one other class and that this influence would be high.

Methods: We retrospectively assessed weight bearing CT (WBCT) measurements of 32 feet with PCFD diagnosis. The classes and their associated radiographic measurements were defined as follows: class A (hindfoot valgus) measured by the hindfoot moment arm (HMA), class B (midfoot abduction) measured by the talonavicular coverage angle (TNCA), class C (medial column instability) measured by Meary's angle, class D (peritalar sub-luxation) measured by the medial facet uncoverage (MFU), and class E (ankle valgus) measured using the talar tilt angle (TTA). Multivariate analyses were completed comparing each class measurement to the other classes. A p-value <0.05 was considered significant.

Results: Class A showed substantial positive correlation with class C (ρ=0.71; R²=0.576; p=0.001). Class B was substantially correlated with class D (ρ=0.74; R²=0.613; p=0.001). Class C showed a substantial positive correlation with class A (ρ=0.71; R²=0.576; p=0.001) and class D (ρ=0.75; R²=0.559; p=0.001). Class D showed substantial positive correlation with class B and class C (ρ=0.74; R²=0.613; p=0.001), (ρ=0.75; R²=0.559; p=0.001) respectively. Class E did not show correlation with class B, C or D (ρ=0.24; R²=0.074; p=0.059), (ρ=0.17; R²=0.071; p=0.179), and (ρ=0.22; R²=0.022; p=0.082) respectively.

Conclusion: This study was able to find relations between components of PCFD deformity with exception of ankle valgus (Class E). Measurements associated with each class were influenced by others, and in some instances with pronounced strength. The presented data may support the notion that PCFD is a three-dimensional complex deformity and suggests a possible relation among its ostensibly independent features. Level of Evidence: III.