Volume 9, Issue 4 (April 2012)
Laboratory Evaluation Method for the Functional Performance of Total Knee Replacements
In activities of daily living, the normal anatomic knee moves through a range of flexion of up to 155°, and at all angles there are ranges of laxity in both the anterior-posterior direction and internal-external rotation. The ideal situation is that after a total knee replacement (TKR), the knee moves in a similar way, providing normal kinematics, stability, and freedom of motion. Our goal was to formulate a simple standard testing method for evaluating proposed new TKR designs or existing designs that would relate to clinical performance. We developed a Desktop Knee Machine with which combinations of forces and moments were applied to knee models at a range of flexion angles while the neutral path of motion and the laxities about the neutral path were measured. In order to develop and demonstrate the method, three representative TKR types were modeled using computer-assisted design software, and then stereolithographic (SLA) models were made for testing. Different combinations of loads and flexion angles were applied so as to represent a full spectrum of activities and positions. The magnitudes of the forces were less than what would occur in vivo due to the limitations of SLA models, but they were chosen to reach the laxity limits of the TKR models tested. It was concluded that the testing method was a valid and efficient means of comparing the kinematic characteristics of different TKR designs and providing possible indicators of clinical performance.