Assessing Subtalar Joint Alignment in Progressive Collapsing Foot Deformity: Axial Rotation Matters

Research Type: Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies Introduction/Purpose: Significant differences in talar shape between progressive collapsing foot deformity (PCFD) patients and controls have been described based on coronal plane measurements. However, digitally reconstructed radiographs suggest that subtalar alignment changes significantly with axial rotation of the lower limb. This raises the question of whether axial plane differences contribute to previously observed coronal subtalar joint changes in PCFD. We hypothesize that PCFD and control patients will exhibit significant differences in the bimalleolar axis (Goutallier method: the bimalleolar line passing through the middle of the dense surfaces of the malleoli) relative to the coronal plane while standing with the second metatarsal aligned with the sagittal plane. Additionally, we hypothesize that changes in the axial plane will result in significant differences in subtalar alignment in PCFD. Methods: A total of 42 PCFD patients and 27 controls (asymptomatic contralateral feet) underwent weight-bearing CT (WBCT). The following angles were measured: • Inferior facet of the talus vs. horizontal (inftal-hor) • Inferior vs. superior facets of the talus (inftal-suptal) • Bimalleolar axis (Goutallier method) vs. coronal plane • Talar inclination angle • Meary’s angle • Foot and Ankle Offset (FAO) Two trained observers performed all measurements. Intraclass correlation coefficients (ICC) were calculated to assess inter- and intraobserver reliability. Linear regression analysis was conducted to determine the contributions of Meary’s angle, FAO, bimalleolar axis vs. coronal plane, and talar inclination to inftal-suptal and inftal-hor variance. Inftal-hor and inftal-suptal angles were measured after 10° of internal (+) and external (-) foot rotation in the axial plane and analyzed using ANOVA. Results: All angles differed significantly between PCFD and controls (p < .001), including the mean bimalleolar axis (PCFD: 7.76 ± 5.85; controls: 14.62 ± 4.66) (p < .001). ICC values were almost perfect to substantial but moderate for talar inclination. Internal (+) and external (-) rotation significantly altered (p < .001) mean inftal-hor ((+): 0.0 ± 7.51; (-): 21.02 ± 8.13) and inftal-suptal ((+): 2.93 ± 6.8; (-): 16.95 ± 7.37) angles. The regression model explained 64.93% of inftal-suptal variance, with bimalleolar axis (p < .001) and talar inclination (p < .001) as significant predictors. Meary’s angle (p = .498) and FAO (p = .613) were not. A 1-degree change in the bimalleolar axis and talar inclination explained s1.08 and 0.97 degrees of inftal-suptal variance, respectively. Conclusion: PCFD and controls exhibit significant differences in the bimalleolar axis while standing in WBCT with the second metatarsal aligned with the sagittal plane. Internal or external rotation of the foot by 10 degrees leads to significant changes in inftal-hor and inftal-suptal angle measurements. Regression analysis confirmed that axial rotation (measured through the bimalleolar axis) and talar inclination are significant predictors of inftal-hor and inftal-suptal angles, raising the question of whether talar shape can be accurately assessed without a 3D evaluation.