Simulator testing of total knee replacements

Abstract

The human knee joint is a highly complex system making both the implantation and testing of total knee replacements (TKRs) a very demanding task. The first hydraulic knee simulator was introduced in 1977 [1] followed closely by an ingenious gear-driven mechanical system [2]. Despite the development of TKRs throughout this period, it took another 20 years until the first multi-station wear simulator became available [3]. It is this development that can be regarded as the trigger for standardization and commercialization of TKR wear testing. Unfortunately, there remains an ongoing debate about how best to conduct wear testing of new devices. As a result, there exist two different ISO standards for carrying out wear testing: ISO 14243-1 describes a force-controlled test method, whereas ISO 14243-3 describes a displacement-controlled test method. While both methods accord that the flexion-extension motion of the joint needs to be displacement controlled, the philosophy of anterior-posterior and torsional loads is different: Part 1 of the standard understands anterior-posterior motion of the knee joint as a result of the anterior-posterior load applied, whereas Part 3 describes a predefined anterior-posterior displacement that needs to be generated by the simulator. The same difference between test methods applies to the simulation of torsional motion. Direct comparison to clinical data has revealed an accurate reproduction of the in vivo motions when using the force-controlled test method [4, 5], whereas the displacement-controlled method has yielded certain limitations for constrained implant designs (i.e., tibial post or high conforming designs). However, it is important to note that force-controlled test frames are more complex to design and operate.