Influence of selective arthrodesis on the movement transfer between calcaneus and tibia in vitro

Arthrodeses of foot and ankle are well established, accepted, and practical methods for treatment of painful joint degeneration, foot deformity, and instability. Consecutive changes in gait and over-use injuries have been explained by the created lever arms and the overall compensatory motion in the neighbouring joints, rather than by changes in the mechanical coupling of foot and tibia. Thus the purpose of this study was to quantify the change of movement transferred from calcaneus to tibia, and vice versa, for selective joint fusions (ankle, subtalar, and talonavicular joints) under different flexion and loading conditions. In six fresh cadaveric foot-leg specimens, transfer of rotational movement between calcaneus and tibia occurred in all arthrodesis conditions. Fusion of the subtalar joint, which is commonly believed to be crucial in the transfer of rotational movement in the ankle joint complex, decreased the movement transfer from calcaneal inversion to external tibial rotation about 71.8% and, vice versa, from external tibial rotation to calcaneal inversion about 35.8%. However, the movement transfer did not change when calcaneal eversion and internal tibial rotation were the input movements. The ankle (talocrural) joint must have more than 1 degree of freedom, since significant movement transfer still occurred when the subtalar joint was fused. It could be that other structures such as ligaments also play an important role in transferring movement. Consequently it may be difficult to predict the effect of a planned arthrodesis since the resulting restriction of motion and movement transfer may be substantially determined by the integrity of the surrounding soft tissue, especially the ligaments.