Numerical and experimental studies on the mechanical behaviour of the distal femur following total knee arthroplasty
The history of total knee arthroplasty stretches back over 70 years. Many studies have shown that TKA is, in general, a successful operation for the relief of joint pain, with patient satisfaction rates of 90-95% and implant survival rates at 10-15 years of greater than 90%. However, a number of studies have also shown the potential for failures or complications arising post-implantation leading to revision surgery. This thesis presents finite element (FE) models of the distal femur following primary and revision total knee arthroplasty. Pre-arthroplasty models are also developed for comparison. Particular attention is given to how femoral component design and method of fixation impacts the mechanical environment of the distal femur and stability of the prosthesis. FE analyses with fully bonded interfaces indicate that femoral components are subject to areas of low stress (stress shielding) immediately under the anterior flange and chamfer regardless of internal implant features. However, internal implant features were found to play a role in the pattern and magnitude of stress concentrations. Both stresses and motions were observed to increase with increasing flexion angle, indicating the importance of testing at multiple angles. The initial models of the distal femur were extended to incorporate the effects of ageing and endosteal thinning of the femoral cortex, through novel application of pre-existing FE modelling techniques, specifically the ability to assign variable material properties corresponding to the nodal temperatures output from a heat transfer analysis. The findings from this study indicate that older patients with osteoporosis may be at increased risk of periprosthetic fracture compared to younger healthy patients. The use of a revision femoral component with a cemented stem as a means to mitigate this fracture risk was also investigated. FE analyses using frictional interfaces were employed to determine the influence of femoral component design on micromotion at the interface. These models showed that all primary implants were subject to similar magnitudes of relative motion at the interface, however, the distinct internal implant features led to very different regional variations. Furthermore, certain internal implant features (i.e. femoral box) were found to be highly sensitive to errors in surgical bone cuts. This aspect of the thesis also concluded that the addition of a stem served to significantly reduce motions at the interface in comparison to primary stemless implants. Long stemmed prostheses were found to result in the smallest levels of interface motion. This study also detailed the design and creation of an in vitro test setup for the purposes of determining the influence of stem length and fixation on the stability of revision prostheses. Experimental results using this test rig showed that a cemented short stem provides as much initial stability as the uncemented long stem, and is easier to fit surgically. Corresponding FE models incorporating a virtual representation of the test rig and in vitro loading conditions revealed that the relative motion at the multi-planar bone-prosthesis interface cannot be adequately described using a single reference point. However, in vitro setups may be used to predict a general measure of implant stability and to provide a source of calibration for FE. The distal femur models were further modified to investigate how the presence of condylar defects as classified by AORI defect classification system (Engh 2006) and weak osseous support due to osteoporosis may adversely affect the survival of the prosthesis. These investigations revealed that fixation of the femoral component, the presence of a large condylar defect and the level of osseous support all had an impact on stress in the implant, it is concluded that a non-modular approach should be adopted in older patient groups with severe osteoporosis to mitigate the risk of component junction failure and distal femoral fracture.