Middle Facet Subluxation Correlation with Foot and Ankle Offset in the Assessment of Progressive Collapsing Foot Deformity
Authors: Maly Connor , Mansur Nacime S. , Lalevee Matthieu , VandeLune Christian , Cychosz Chris , Rojas Edward O. , Dibbern Kevin N. , Ellis Scott J. , Lintz Francois , Bernasconi Alessio , Netto Cesar de Cesar 
Published: January 2022

Category: Other; Hindfoot; Midfoot/Forefoot Introduction/Purpose: Identifying markers of severity and progression in Progressive Collapsing Foot Deformity (PCFD) provides surgeons with critical information, possibly aiding in the decision making along the treatment algorithm. Subtalar middle facet subluxation (MFS), the percent undercoverage of the talus in relation to its calcaneus counterpart, was recognized as a reliable marker in weight-bearing computerized tomography (WBCT) for PCFD diagnosis. The Foot and Ankle Offset (FAO), the relative position between the center of the ankle joint and the foot tripod, is a three-dimensional WBCT tool predictive of disease severity. Our objective is to assess the relationship between the amount of MFS and FAO in flexible PCFD patients. We hypothesize that MFS is a reliable assessment of disease severity and correlates with FAO. Methods: In this retrospective IRB-approved comparative study, a total of 56 individuals with PCFD (74 feet) who underwent WBCT for baseline assessment were analyzed. Two blinded fellowship-trained foot and ankle surgeons performed the measurements. MFS was executed in the coronal-plane, at the midpoint (on sagittal-plane) of the middle facet (see attached figure). Dedicated software was utilized to perform the FAO, using the most plantar voxels of the first metatarsal, fifth metatarsal, calcaneal tuberosity and centre of the ankle. Interobserver agreement was quantified for MFS and FAO using intraclass correlation coefficient (ICC). Intermethod agreement between MFS and FAO was assessed by Spearman's correlation. Bivariate linear regression analysis was used to assess the relationship between MFS and FAO. A partition prediction model and multivariate analysis were utilized to assess influence of MFS measurements on FAO values and vice versa. Results: A total of 56 patients (74 feet) were included in the study. The ICCs for interobserver reliability was 0.87 for MFS and 0.95 for FAO. In a bivariate analysis, MFS and FAO were found to be significantly and linearly correlated (P< 0.0001, R2 0.26). Foot Angle Offset = 2.22 + 0.12*Medial Facet Subluxation (%). In multivariate analysis, FAO and body mass index (BMI) were significantly correlated with MFS (<0.001 and 0.02, respectively). The partition prediction model demonstrated that an MFS of 27.5% was an important threshold for increased FAO, with FAO of 3.4% +-2.4% when MFS was below threshold and 8.0% +-3.5% when above threshold. Conclusion: We found a positive linear correlation between MFS and FAO measurements. An MFS of 27.5% was an important threshold for higher FAO values, which corresponded to a worst overall alignment. Our results are consistent with the idea that MFS is a reliable marker for PCFD diagnoses and severity, correlating well with the FAO. This data may support clinical decisions in PCFD patients. Also, BMI was found to be positively correlated with MFS. Future prospective and longitudinal studies are needed to confirm the findings of this study.