Chopart Peritalar Subluxation in Progressive Collapsing Foot Deformity Assessed by Three- Dimensional Coverage Maps
Authors: Behrens Andrew , Dibbern Kevin N. , Mansur Nacime SB , Ehret Amanda , Chen Nathan , Stebral Hannah J. , Lintz Francois , Bernasconi Alessio , Anderson Donald D. , Lalevée Matthieu , Netto Cesar de Cesar 
Published: November 2022

Category: Hindfoot; Midfoot/Forefoot; Other Introduction/Purpose: Progressive collapsing foot deformity (PCFD) is a complex three-dimensional (3D) deformity characterized by peritalar subluxation (PTS) of the hindfoot through the triple joint complex. In this context, adjacent structures adopt different positions, and bone relations change, producing areas of higher or less contact and stress. The objective of this study was to use 3D distance maps (DMs) and coverage maps (CMs) from weightbearing CT (WBCT) images to assess subluxation across the Chopart joint in PCFD patients. We hypothesized that CMs would show decreased coverage indicative of subluxation through regions of the Chopart joint in PCFD patients when compared to controls. Methods: In this IRB-approved, retrospective case-control study, we analyzed WBCT data of 20 consecutive patients with flexible PCFD and 20 controls. Using principle component analysis, coverage area was divided into six regions on the talar head and 4 on the calcaneal-cuboid articular surface. Novel 3D distance mapping (DM) technique was used to objectively characterize joint coverage across the entire Chopart surface on both talus and calcaneus. Distance maps were measured in millimeters and colored to highlight covered areas. Distances less than 4mm were defined to be covered, while areas with distances greater than 4mm uncovered. Joint coverage was defined as percentage of articular area with DMs lower than 4 mm. Coverage Maps (CM) were built highlighting areas of coverage (teal) versus non-coverage (pink). The Foot and Ankle Offset (FAO) was used as a reference semi-automatic three-dimensional measurement. Comparisons were performed with independent t-tests, assuming unequal variances and p-values <.05 considered significant. Results: Changes in coverage percentages of PCFD cases relative to controls are indicated in the attached figure. The superomedial and the inferomedial regions of the talar head were found to have a 75% and 79% decrease in coverage in PCFD cases relative to the controls (ps<0.003). The medial side of the head had an overall increase in coverage (ps<0.002). On the calcaneus, the calcaneal-cuboid joint plantar region was found to have a significant coverage decrease of 12% relative to the controls (p=0.037), while the lateral quadrant observed a 13% increase (p=0.002). DMs evaluation showed no significant differences in the mean distances for either the calcaneocuboid or the talonavicular (ps>0.05). The FAO was found to be correlated with changes in talar and calcaneal coverages (rs=0.90; R2=0.81). Conclusion: Our results support the hypothesis that significant changes occur at the Chopart joint in early flexible PCFD cases. Increased coverage in the lateral region of the talar head and decreased coverage in the medial and plantar regions point to internal rotation of the talus indicating subluxation through the entire joint. This behavior was also translated in the calcaneal- cuboid, where plantar and medial areas had a decrease in coverage. Coverage 3D mapping enabled objective subluxation quantification through the Chopart joint in early-stage PCFD. These findings may assist clinical assessment of normal joint alignment restoration during PCFD corrections.