Assessing Middle Facet Subluxation in PCFD: The Impact of Plane Orientation on Measurements

Category: Hindfoot; Other Introduction/Purpose: Middle facet subluxation (MFS) and middle facet incongruence angle (MFIA) are established indicators of peritalar subluxation (PTS) in evaluating Progressive Collapsing Foot Deformity (PCFD) using Weight-Bearing CT (WBCT). Traditionally, MFS and MFIA are assessed in the coronal plane, which is not perpendicular to the plane of the middle facet (MF). However, the angle between the plane of the medial facet and the plane of measurement changes with plantar flexion of the talus. This approach may lead over- or underestimating the MFS and MFIA as well as a over-dysplastic appearance of the middle facet. We hypothesized that measuring these parameters in a plane perpendicular to the MF would result in changes in MFS, MFIA, and the incidence of diagnosed dysplastic joints. Methods: In this retrospective case-control study, 89 patients with PCFD and 11 controls without PCFD who underwent WBCT were evaluated. Measurements of MFS, MFIA, and middle facet dysplasia were conducted using both the classical method and the new method. The classical method consisted of identifying the center point of the middle facet on sagittal view and taking measurements in the coronal plane as described in the literature. The new method comprised of taking measurements after rotation of the coronal plane around the axis defined by the center of the middle facet on sagittal view until the coronal plane was perpendicular on sagittal view. The inclination of the MF in the sagittal plane was also recorded. Additional WBCT parameters (Foot and Ankle Offset (FAO), Talo-navicular coverage angle (TNCA), Talus-first metatarsal angle axial) were also measured. The Shapiro-Wilk test assessed data normality. A p-value of < 0.05 was considered statistically significant. Results: Significant differences were observed between the two methods across all parameters for PCFD. For controls, only the MFIA was significantly different (p:0.0045) between the two methods. Regarding the PCFD the new method demonstrated lower MFS (25.4% vs. 40.3%, p< 0.0001), lower MFIA (4.7 degrees vs. 13.1 degrees, p< 0.0001), and fewer dysplastic joints (1% vs. 37%, p< 0.0001) when compared to the classical method. However, multivariate analysis did not show inclination angle as predictive of MFS. TNCA (p < 0.05) talus-first metatarsal angle axial (p < 0.05) and FAO (p < 0.05) were stronger predictors of MFS than the inclination of the talus. Conclusion: This study confirms that MFS is a complex deformity influenced by multiple parameters but not talar inclination. The new method showed lower MFS, MFIA, and dysplasia measurements compared to current classical method. Future research should compare these two measuring techniques to more comprehensive modalities (coverage mapping). When assessing MFS, surgeons should be mindful that the plane of measurement relative to the structure is critical, and any change in the inclination of the talus may influence the appearance of MFS. We suggest using a plane of measurement perpendicular to the plane of the middle facet when assessing its subluxation.