Abstract
Introduction
When evaluating the biomechanical performance of a total knee arthroplasty (TKA) implant design, device companies are usually required to select the “worst case scenario” for testing by the regulatory bodies. However, most test standards (e.g., ASTM, ISO) do not explicitly specify how the “worst case” should be determined. It is quite often that an extreme size (the smallest or the largest) in a system is taken as the “worst case” size. The smallest size is sometimes selected under the rationale that it has the smallest geometry thus the weakest mechanical structure. While the largest size is sometimes selected under the rationale that it is used on the biggest patients associated with the highest loads. However, implant geometry and in vivo load are two compounding factors that together determine the implant's biomechanical challenge. As the result, the true “worst case” must be determined considering both factors, and the choice could be design-specific. This study evaluated the femorotibial contact stress of a TKA implant system, and demonstrated that the extreme sizes may not simply be the “worst case”.
Methods
The femorotibial contact stress of a posterior-stabilizing TKA implant system was assessed using finite element analysis. Multiple sizes ranging from size 0 to 6 were analyzed. For each size, the CAD models were assembled at knee extension. A load equivalent to 4 times of patient body weight was applied. Average patient body weights were calculated based on the company's clinical database: 72.5, 76.0, 80.0, 87.4, 95.2, 103.4, and 111.0 kg for sizes 0, 1, 2, 3, 4, 5 and 6, respectively. Von Mises stresses in the polyethylene tibial insert were examined and compared among different sizes.
Results
The peak femorotibial contact stress was found to be 20.0, 19.4, 19.2, 20.6, 20.4, 18.9 and 20.0 MPa for sizes 0, 1, 2, 3, 4, 5 and 6, respectively. The difference between the highest (size 3) and the lowest (size 5) was less than 9%. Neither size 0 nor size 6 represented the “worst case” in the scope.
Discussion
Due to the cross-influence of two compounding factors (geometry and load), the determination of the “worst case” in an implant system is not as simple as how the question is often handled. For the implant system analyzed in this study, from the smallest to the largest size, the dimension of the tibial insert grew by 49% while the average patient body weight also grew by about 53%. Under the influencing of both factors, the outcome contact stress showed little variation across sizes. Neither the smallest nor the largest size demonstrated the “worst case”. It should be noted that the finding in this study may not directly apply to other TKA implant systems of different designs. The implant system analyzed in this study features a matched femorotibial geometry on every size, which is able to produce a “proportional” mechanical response. This study highlighted the importance of a more thorough assessment when selecting the clinical worst case for implant testing.
