Thigh-calf contact force measurements in deep knee flexion.

BACKGROUND Knee models often do not contain thigh-calf contact which occurs in deep knee flexion. Thigh-calf contact is expected to reduce muscle forces and thereby affects internal stresses in the knee joint. The purpose of this study was to measure thigh-calf contact forces. Two deep knee flexion activities were selected: squatting and kneeling. METHODS Ten healthy subjects participated in the experiment. Contact pressures between the thigh and calf were measured using the Tekscan Conformat pressure mapping sensor. Knee flexion angles were measured unilaterally using an infrared motion capture system. Contact forces were averaged in terms of means and standard deviations. The magnitude and location of the resultant contact force were calculated. Correlations between anthropometric subject parameters and experimental outcome were studied. FINDINGS In general, thigh-calf contact did not take place below 130 degrees knee flexion. The average maximal contact forces for each leg were 34.2% bodyweight during squatting and 30.9% bodyweight during kneeling. Corresponding average maximal knee flexion angles were 151.8 degrees during squatting and 156.4 degrees during kneeling. Thigh and calf circumferences were correlated with the contact force measurements. INTERPRETATION The current study shows that thigh-calf contact is substantial (>30% bodyweight on one leg) and likely reduces the forces inside the knee during deep knee flexion. Subsequently, total knee replacements may be subjected to lower loads than assumed before, which reduces the risk of implant failure at large flexion angles. Results presented in this study can be utilized in knee models that focus on deep knee flexion.