Large scale, population-based finite element analysis of cementless tibial tray fixation

Abstract

Joint replacements are a common treatment of osteoarthritis, rheumatoid arthritis, or fractures of both the hip and knee. The rising number of procedures being performed each year means that there is a need to assess the performance of an implant design in the general population. The majority of computational studies assessing implants do not take into account inter-patient variability and only use a single patient model. More often than not, it is then assumed that the results can be extrapolated to the general population. This thesis describes a method allowing population-based assessment of joint replacements, focussing on the tibial tray component of a total knee replacement. To generate a large population of models for finite element analysis, two statistical models were used. One was of the tibia, capturing both the variability of the morphology and bone quality, and the other was of the internal knee loads during a gait cycle. Assessment of the statistical models showed that they could adequately generate representative tibiae and gait cycle loads. An automated method was then developed to size, position, and implant the tibial tray in the generated population of tibiae in preparation for finite element analysis. The use of a population-based study, a unique approach compared to current studies, was demonstrated using three case studies assessing the performance of the tibial tray. The first case study examined the factors which might increase the risk of failure of the tibial tray and the effect of under sizing the tibial tray on primary stability. Factors such as bone quality and patient weight were seen to increase the risk of failure. It was found that under sizing the tibial tray did not significantly affect the primary stability of the tibial tray. It was also observed that the peak strain occurred during swing phase of the gait cycle, whereas peak micromotion occurred at the beginning of stance phase of the gait. The second case study investigated the effect of tibia resection depth on primary stability of the tibial tray. A more distal resection was found to increase the peak strain and micromotion of the bone-tray interface. The worsening primary stability with a more distal resection, suggest that to obtain optimal primary stability of the tibial tray it is necessary to resect as little bone as possible. The third case study compared three tibial tray designs. It was found that the trays with pegs or flanges surrounding the stem tended to perform better, reducing the strain and the micromotion at the bone-tray interface. It was noted that the performance of the trays predicted by the analysis was similar to that observed clinically. This shows the potential use of population-based studies to help predict the clinical outcome of joint replacements.

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