The Effect of the Mobile Subtalar Joint on Calcaneal and Supramalleolar Osteotomies.

BACKGROUND: The mobile subtalar joint (STJ) may compensate for supra- and inframalleolar deformities and counteract the effect of realigning calcaneal or distal tibial osteotomies. The purpose of this study was to evaluate the compensatory effect of the mobile STJ after supramalleolar osteotomy (SMOT) and calcaneus osteotomy (COT) and whether the extent of the compensation correlates with STJ shape and orientation.

METHODS: In 10 human lower leg cadavers without evidence of deformity or prior trauma 700 Newton load were applied as a simulated standing pose. The center of force (COF) migration, maximum pressure (Pmax), and the area loaded were measured with high-resolution sensors in the ankle before and after 10-mm varus/valgus sliding COT and 10-degree varus / valgus SMOT. A computed tomographic evaluation of subtalar anatomy was conducted to correlate posterior facet curvature, its varus/valgus orientation in the coronal plane, and the effect on COF, Pmax, and area loaded.

RESULTS: The COF migration was significant for both varus and valgus SMOTs (varus SMOT: 1.78 mm, P = .0029; valgus SMOT: 1.85 mm, P = .0018) but not for COT (varus COT: 0.45 mm, P = .85; valgus COT: 1.15 mm, P = .11). Pmax and area loaded changed but not significantly. The radius of the posterior STJ surface showed a moderate correlation (varus SMOT: r = 0.61, P = .063; valgus SMOT: r = 0.28, P = .43, varus COT: r = -0.61, P = .063; valgus COT: r = 0.13, P = .38) and the axis a weak inverse correlation (varus SMOT: r = -0.51, P = .013; valgus SMOT: r = 0.58, P = .079; varus COT: r = -0.51, P = .14; valgus COT: r = 0.38, P = .28) with the COF migration after the osteotomies.

CONCLUSION: The compensatory capacity of a mobile STJ is relatively small but appears to limit the effect of COT more than SMOT. The COT is less effective in influencing ankle joint pressure for realignment purposes than SMOT in mobile STJ and clinically more consistent in stiff STJ. Correlations are moderate to weak, whereas the curvature more than orientation of posterior facet inversely correlates with osteotomy's effects.

LEVEL OF EVIDENCE: IV (biomechanical cadaver study).