Influence of soft tissue artifacts on the calculated kinematics and kinetics of total knee replacements during sit-to-stand

The current study aimed to quantify the soft tissue artifacts of selected markers on the thigh and shank, and their effects on the calculated joint center translations, angles and moments of the knee during sit-to-stand. Ten patients with total knee replacements rose from a chair under simultaneous surveillance of a motion capture system, a force-plate and a fluoroscopy system. The “true” poses of the thigh and shank were defined by those of the femoral and tibial components obtained using a three-dimensional fluoroscopy method. The soft tissue artifacts of the skin markers were calculated as their movement relative to the underlying prosthesis components. The joint center translations, angles and moments at the knee were also calculated separately using skin markers and the registered prosthesis poses. Considerable soft tissue artifacts were found, leading to significantly underestimated flexion and internal rotation angles, and extensor moments, but overestimated joint center translations and adduction. The current study provides accurate data of the kinematics and kinetics of total knee replacements during sit-to-stand. The effects of soft tissue artifacts on the calculated joint center translations, angles and moments were also quantified for the first time in the literature. The results may help in developing guidelines for using skin markers and in establishing databases in the biomechanical assessment of sit-to-stand in patients with total knee replacements.