Biomechanical analysis of TKA alignment and design

Total knee arthroplasty with total hip arthroplasty nowadays represents around 90% of the entire joint reconstruction market. They are required mainly as a result of osteoarthritis. Knee prosthesis can be implanted following two main philosophies: mechanical alignment and kinematic alignment. The mechanical alignment consists in implanting the knee prosthesis so that the interface between bone and prosthesis is perpendicular to the tibial axis. Since each component is perpendicular to the mechanical axis, this allows, once the load is applied, to have a load distribution in which the compression component prevails. Nevertheless, the components of the human knee do not lie perpendicular to each other. In fact, they have a physiological joint line of about 3° of varus. For this reason, many surgeons prefer to follow the philosophy of kinematic alignment. This philosophy consists in implanting the knee prosthesis by making a cut parallel to the joint line. However, this cut parallel to the joint line can result in shear and compression stresses at the prosthetic bone interface as opposed to the compression only that comes with mechanical cutting. Furthermore, if standard tibial components are used, the stem of the tibial component of the prosthesis will not be parallel to the mechanical axis and therefore in the diaphysis of the tibial bone but will also be inclined. As both approaches have examples of success and failure, the aim of this work is, through the use of finite elements, to compare these two different alignment philosophies in different TKA designs of knee implant. Specifically, the purely mechanical configuration and the purely kinematic configuration have been studied, with cut at 3°, respecting the joint line, and 5° to simulate a varus condition or mal alignment. In addition to these two standard configurations, used today, two new configurations have been designed: a first configuration having a kinematic approach for the cut and a mechanical approach for the stem. The cut is obtained by cutting parallel to the joint line. The stem, this time, is modeled to be parallel to the tibial axis, and so conforms to the mechanical approach. The last configuration studied consists in the insertion of the tibial component in a mechanical way, following a perpendicular cut, but this time modifying the shape of the insert. In particular, the proximal part was modified in order to follow the joint line of the human knee. In this way we obtained an asymmetric insert that allowed a kinematic alignment for the femoral component and a mechanical alignment for the tibial component. These different configurations were studied by applying a static load in full extension. From the results obtained, we can see that the peak of Von Mises stress on the cortical bone of the tibia in the mechanical configuration is around 8 MPa in both the medial and lateral parts. Regarding the kinematic configuration we note that in the 3° cut we have a lateral unloading while medially we have a peak around 8 MPa. In the 5° cut of the kinematic configuration, on the other hand, there is a lateral and medial peak 45-50% higher than in the mechanical configuration. This indicates how fundamental the precision in the cut becomes in case of kinematic approach. In the last two configurations, both at 3° and 5° the stress results on the cortical bone of the tibia are comparable with those of the mechanical configuration.