The standard treatment of arthrosis of the ankle joint is arthrodesis while new prosthesis leed to good clinical results. Currently there is much controversial discussion, and knowledge of the fundamentals of biomechanics is becoming ever more important.

Ten macroscopically and roentgenographically normal foot specimens were tested comparing tibio-talar arthodesis vs. nativ situation on a kinematic gait simulator. The stance- phase of normal walking gait was simulated from heel-contact to toe-off. Ground reaction forces were simulated by a tilting angle- and force-controlled translation stage upon which a pressure measuring platform was mounted. Force was applied to the tendons of the foot flexor and extensor muscle groups by cables attached to an additional set of six force-controlled hydraulic cylinders. Tibial rotation was produced by an electrical servo motor.

The change after arthrodesis was a varying degree of relocation of average force and maximum pressure from the lateral onto the medial column of the foot; the increase force on talonavicular joint and decrease on calcaneocuboid joint is statistically significant. The average force increased from native 66.7N to 80.8N upon arthrodesis in the talonavicular joint and decreased in the calcaneocuboid joint from 71.9N to 58.5N. Peak pressure increased from 3728kPa to 4552kPa in talonavicular joint and decreased in calcaneocuboid joint from 3809kPa to 3627kPa. After arthrodesis, we measured inconsistent changes in Chopart joint. On some feet, the changes in stress were slight, but on majority, relocation of force and peak pressure was significant. The result was a change in the function of Chopart joint with increased extension load on talonavicular joint at time of highest joint load during push-off.

These in vitro observations explain the clinical observations that have followed ankle arthrodesis. For one, there are reports on tibiotalar arthrodesis patients who are largely mobile and free of complaints, which correlates with the observation that not all preparations indicate a clear relocation of force and intraarticular peak pressure onto the talonavicular joint. In these cases, the ability of strong muscular plantar flexion could explain a good functional result. In contrast, and in addition to subtalar joint degeneration, arthroses in the talonavicular joint have been frequently observed following tibiotalar arthrodeses. The relocation of both force and intraarticular peak pressure onto the medial column of the foot in the majority of preparations explain the degeneration on the extensor side of the joint with osteophyte formation impressively.