The Effect of Subtalar Motion on Calcaneal Osteotomies

CATEGORY: Ankle Arthritis, Hindfoot INTRODUCTION/PURPOSE: Background There is evidence that the subtalar joint may compensate for supramalleolar deformities and thereby slower or halt progression of ankle arthritis. The mobile subtalar joint may also compensate for the effect of realigning osteotomies of the calcaneus and the tibia. The compensation is limited by the joint orientation and limited ROM (arthritic joint, tarsal coalitions, previous fusion). OBJECTIVES: - the curvature of posterior facet influences subtalar ROM; - subtalar ROM influences subtalar compensatory capacity; - there is compensation of realigning osteotomies through a mobile subtalar joint, resulting in a less effective correction of weightbearing axis; - the osteotomy of the calcaneus (COT) is more affected by the compensation than the supramalleolar tibia osteotomy (SMOT). METHODS: 11 fresh-frozen human lower leg cadaver were mounted into a carbon frame. High resolution ankle TekScan sensors were fixed in the ankle joint via anterior arthrotomy. 300 N load were applied according to half body weight and Achilles tendon pull was simulated. The center of force (COF) migration, max. pressure (Pmax), and the area loaded were measured in the ankle with 10 mm varus / valgus sliding calcaneus osteotomy and 10° varus / valgus SMOT. A CT evaluation of subtalar anatomy (curvature of posterior facet) was conducted and the correlation of posterior facet curvature and subtalar motion and the correlation of subtalar motion and SMOT/COT effect calculated. RESULTS: The COF migration was significant for valgus COT and for both SMOT versus the initial position, while the varus SMOT versus the varus COT. Pmax and area loaded changed but not significantly. There was a significant correlation of posterior facet curvature and subtalar motion (r = 0.87), a moderate inverse correlation of subtalar motion and COT effect (r = -0.52), and a poor inverse correlation of subtalar motion and SMOT effect (r = -0.27). In contrast, in a previous study with a stiff cavovarus model significant COF migration and peak pressure changes were found in the ankle joint for all lateral closing SMOT and valgus COT. CONCLUSION: The compensatory capacity of mobile subtalar joint limits effect of COT more than SMOT, likely because the subtalar joint is closer to COT than to SMOT. Biomechanically, the COT is therefore less effective in influencing ankle joint pressure than SMOT. The effect of the COT is more reliable in stiff subtalar joints. The curvature of posterior facet correlates with subtalar ROM.