Finite Element Modelling For Stability of a Total Knee Replacement.

The use of, and interest in, total knee replacements (TKR) has been growing over the last few decades. Loosening and migration of tibial components have been identified as one of the primary causes of failure in the proximal tibia. Clinical studies show the use of metal implants as one of the primary methods for the treatment of knee joints and associated bone defects. Alignment and fixation techniques play an important role in achieving high success rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these cases, current clinical practice is to use an extended implant stem to ensure stability. The problem with this is that it reduces the potential for future knee revision In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo assessment of various configurations of augmented and non-augmented TKR that can be used for the treatment of tibial defects. These configurations are based on a standard tibial insert, namely a fixed bearing revision tibial tray. This has provided insight and information that can be used to improve surgical decision making when dealing with defective bone stock. The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable construct with stresses lying within an allowable threshold. The use of a stem extension generally showed a reduction in stress levels in the cancellous bone contributing to an increase in stress shielding and thus it is recommended that these are not used unless there is some other overriding clinical requirement. Further, the analysis demonstrated that, contrary to some clinical opinion, wedge augmentation (rather than block augmentation) may provide a better approach to treat the defect. This was largely due to improved cement stress distribution caused by a mechanism termed “reverse-shear”. The use of a cement augment was found to provide a more favourable stress distribution in the cancellous bone. However, metal augments have been recommended as the cement augment was shown to operate too close to its fatigue endurance limits. Future work should focus on further enhancements of the bio-fldelity of the FE model particularly in the material distribution. The idealisation of the cancellous bone as a uniform isotropic material can be improved to provide a spatially varying distribution of material properties, reflecting the natural variation in bone density. Another aspect to further enhance this work would be to extend the applied loads to reflect other lower body movements and to consider the effect of friction at the condyles on the anterior-posterior load applied