Posterior vs. Middle Facet of the Subtalar Joint: The Search for an Early Sign of Peritalar Subluxation and Progressive Flatfoot Deformity

CATEGORY: Hindfoot; Ankle; Other INTRODUCTION/PURPOSE: Adult acquired flatfoot deformity (AAFD) is a three-dimensional (3D) and complex pathology that is characterized by peritalar subluxation (PTS) of the hindfoot. Measuring the amount of subluxation of the subtalar joint at the posterior facet was described and utilized for many years as a marker of PTS. The subluxation of the middle facet was recently proposed as possibly a more accurate and reliable early marker of pronounced deformity, with almost 100% sensitivity and specificity for symptomatic AAFD. The objective of this study was to compare the amount of subluxation of both the medial and posterior facets in patients with increased hindfoot valgus and controls. METHODS: In this IRB-approved retrospective comparative study, we included a total of 91 patients with AAFD (108 feet) who underwent standing weightbearing CT (WBCT) as a standard baseline assessment of their foot deformity. One blinded Fellowship- Trained Orthopedic Foot and Ankle Surgeon with more than 10 years of experience performed WBCT measurements regarding the amount of subluxation of the subtalar joint (percentage of uncoverage) at the posterior and middle facet, as previously described in the literature (Please see attached Figure). Measurements were performed at the sagittal midpoint of the articular facets using Coronal Plane WBCT images. The inter-method agreement between the posterior and middle facet subluxation was assessed using Spearman’s Correlation and Bivariate Analysis. A paired comparison of the measurements was performed using Wilcoxon. P-values of <0.05 were considered significant. RESULTS: The inter-method Spearman’s correlation between the subluxation of posterior and middle facets was measured 0.69. In a bivariate analysis, both measurements were found to be significantly and linearly correlated (P<0.0001), (R2=0.5). The mean/median value and 95% Confidence Interval (CI) for subluxation of the subtalar joint facets were found to be more pronounced in the middle facet (29.8%/26.8%, CI 25.7% to 33.9%) when compared to the posterior facet (12.5%/15.1%, CI 8.4% to 16.6%), p-value <0.0001. The median difference between the measurements (Hodges-Lehman factor) was found to be 12.8% higher subluxation in the middle facet (CI 6.6 to 21.2%).We also found that for each 1% increase in the amount of posterior facet subluxation, a 1.7 times higher subluxation could be expected in the middle facet. CONCLUSION: This study is the first to compare measurements of the amount of subluxation of the posterior and middle facets of the subtalar joints as markers of peritalar subluxation in patients with AAFD. We found a positive and linear correlation between the measurements, with the middle facet subluxation being significantly more pronounced than the posterior facet by an average of almost 13%. Our results are consistent with the idea that the middle facet subluxation might represent an earlier sign of pronounced and progressing peritalar subluxation in patients with adult acquired flatfoot deformity.