Association Between Middle Facet Subluxation and Foot and Ankle Offset in Progressive Collapsing Foot Deformity

Abstract

Background: Subtalar middle facet (MF) subluxation was recognized as a reliable marker for progressive collapsing foot deformity (PCFD) diagnosis. Foot and Ankle Offset (FAO) is an established measurement, predictive of malalignment severity. The objective of this study was to assess the potential association between MF subluxation and FAO in PCFD patients. Methods: 56 individuals with flexible PCFD (74 feet) were assessed. Two blinded foot and ankle surgeons calculated MF uncoverage, MF incongruence, and FAO. Agreement was quantified using intraclass correlation coefficient (ICC). A multivariate regression analysis and partition prediction models were applied to assess relationship between values. Results: All ICCs were >0.80. MF subluxation and FAO were found to be correlated (r(s) = 0.56; P < .0001). Changes in the MF subluxation were noticeably explained by FAO and BMI (R-2 = 0.33). MF incongruence was not correlated with the assessed variables (P = .10). In this cohort, an MF subluxation of 27.5% was a threshold for increased FAO (FAO of 3.4%+/- 2.4% when below; FAO of 8.0% +/- 3.5% when above). Conclusion: We found a correlation between MF subluxation and FAO. An MF subluxation of 27.5% was found to be a threshold for higher FAO, which corresponded to a greater malalignment. These data may help surgeons optimize treatment decisions in PCFD patients.