Dynamic Ball & Socket Joint Force Simulator

Abstract

The stability of an implant in the bone, one factor in joint replacement survival, is usually tested using biaxial fatigue loading. These loading protocols do not replicate physiological loading conditions. The Dynamic Ball and Socket Joint Force Simulator (DBSJFS) was designed to apply physiological loads through an articular surface to study implant stability. The DBSJFS was custom built to integrate with a MTS 858 Mini Bionix machine. The design allows femoral components to be tested dynamically in three dimensions with four degrees of freedom. Two rotary actuators turn the distal end of the femur around perpendicular axes while the linear actuator applies a downward force through the centroid of the femoral component head. The rotations of the femur and the dynamic loading patterns are correlated together to replicate the resultant joint force in the hip. Procedures replicating the gait cycles of a person walking normally, going up and down stairs, and sitting down and standing up were developed using component hip force data taken from HIP98, a study performed in Berlin, Germany by Bergmann that recorded hip joint forces in vivo using instrumented femoral stems implanted into patients. The DBSJFS can operate at 1/3rd physiological speed under loads representing a 900 N individual. At 1/5th physiological speed and 700 N loads, the average and maximum absolute error produced in each channel for the normal walking gait are as follows: Y' axis motor 0.63 deg, 1.49 deg; X' axis motor 0.45, 1.39 deg; linear actuator 10.78, 27.04 N.