Factors Influencing Different Classes in Progressive Collapsing Foot Deformity
Authors: Schmidt Eli , Carvalho Kepler , Kim Ki Chun , Ehret Amanda , Rojas Edward O. , Lintz Francois , Ellis Scott J. , Mansur Nacime SB , Lalevée Matthieu , Netto Cesar de Cesar 
Published: November 2022

Category: Midfoot/Forefoot; Basic Sciences/Biologics; Hindfoot Introduction/Purpose: The current classification system of progressive collapsing foot deformity (PCFD) is comprised of 5 possible classes that describe different deformity components. Each class is defined by clinical and radiographic findings. These components are ostensibly independent from one another during evaluation and treatment. However, 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: In this IRB-approved retrospective case-control study, we assessed 32 feet diagnosed with PCFD and 28 controls matched on gender, BMI and age. All measurements were performed using weight-bearing CT (WBCT) scans and completed by two foot and ankle surgeons. 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 the talus-first metatarsal (Meary) angle, class D (peritalar subluxation) 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. Data were checked for multicollinearity with the Belsley-Kuh-Welsch technique. Heteroskedasticity and normality of residuals were assessed respectively by the Breusch-Pagan test and the Shapiro-Wilk test. A p-value <0.05 was considered significant. Results: After removing confounding variables, each class was separately evaluated. In Class A, Meary was positively correlated (rs=0.46; p=0.009) with HMA, explaining 21% of changes in this angle (R2=0.21). Class B evaluation showed that MFU was correlated with TNCA (rs=0.76; p=0.001), explaining 63% of TNCA variations (R2=0.63). In Class C, HMA (rs=0.71; p=0.001) and MFU (rs =0.75; p=0.001) were correlated to Meary's angle and both measures explained 58% of changes in this angle (R2=0.58). When assessing Class D, TNCA (rs =0.76; p=0.001) and Meary (rs=0.75; p=0.001) correlated with MFU and were responsible for 63% of variations on this angle. Finally, Class E deformity, determined by TTA, was not correlated with any other measurement. 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 found to be 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. Further, these results support the concept that a specific component correction may impact other misalignments, decreasing the necessity for adjuvant procedures. This could have a direct effect in clinical practice, changing how providers assess PCFD and plan treatments.