Bi-unicondylar knee prosthesis functional assessment utilizing force-control wear testing

Recent in vivo studies have identified variations in knee prosthesis function depending on prosthesis geometry, kinematic conditions, and the absence/presence of soft-tissue constraints after knee replacement surgery. In particular, unicondylar knee replacements (UKR) are highly sensitive to such variations. However, rigorous descriptions of UKR function through experimental simulation studies, performed under physiological force-controlled conditions, are lacking. The current study evaluated the long-term functional performance of a widely used fixed-bearing unicompartmental knee replacement, mounted in a bi-unicondylar configuration (Bi-UKR), utilizing a force-controlled knee simulator during a simulated (ISO 14243) walking cycle. The wear behaviour, the femoral-tibial kinematics, and the incurred damage scars were analysed. The wear rates for the medial and the lateral compartments were 10.27 +/- 1.83 mg/million cycles and 4.49 +/- 0.53 mg/million cycles, respectively. Although constant-input force-controlled loading conditions were maintained throughout the simulation, femoral-tibial contact point kinematics decreased by 65 to 68 per cent for average anterior/posterior travel and by 58 to 74 per cent for average medial/lateral travel with increasing cycling time up to 2 million cycles. There were no significant differences in damage area or damage extent between the medial and the lateral compartments. Focal damage scars representing the working region of the femoral component on the articular surface extended over a range of 16-21 mm in the anterior-posterior direction. Kinematics on the shear plane showed slight variations with increasing cycling time, and the platform exhibited medial pivoting over the entire test. These measures provide valuable experimental insight into the effect of the prosthesis design on wear, kinematics, and working area. These functional assessments of Bi-UKR under force-controlled knee joint wear simulation show that accumulated changes in the UKR articular conformity manifested as altered kinematics both for anterior/posterior translations and internal/external rotations.