Functional location of subject-specific joint axes of the human ankle complex

Two non-invasive methods for locating the functional joint axes of the ankle complex were assessed. There is believed to be considerable variation in joint axis location among individuals, making he accurate subject-specific location of these axes important for improved orthopaedic treatment of musculoskeletal disorders of the foot and ankle. The first method, based on the work of previous investigators, uses optimization. An assessment of the accuracy of this method, not performed to date, was carried out on an anthropomorphic mechanical linkage and three cadaver specimens. This method was substantially more accurate when tested on the linkage than when tested on the cadavers, suggesting that its application is limited in vivo. The second method uses a novel approach to move the foot primarily about the subtalar joint, therefore allowing the subtalar joint axis to be located using helical axis decomposition. The accuracy and repeatability of this method were also assessed on six cadaver specimens, with generally good results. In three specimens, subtalar axis location errors were typically less than 10° and 2 mm.