Nonlinear finite element studies of cementless knee implants

Total joint replacement has been significantly developed in the last two decades to provide a solution to the arthritic joint. Cemented and cementless implants are two types that are currently used in the joint arthroplasty. This work deals with the initial fixation and stability of cementless implants. Due to the important role of friction and screws/posts in the initial fixation, friction characteristics at the bone-implant interface and pull-out test behavior under inclined load were determined. Finally, a 3-D finite element model of the knee-implant system was developed to investigate the influence of different friction models and fixation design configurations on fixation stability under static loading. The micromotion at the knee-implant interface and stress distribution within the bone and polyethylene were studied, as well.

In the first part due to the importance of friction role in the fixation stability of implant, bi-directional friction tests between cancellous bone/polyurethane cubes and a porous-coated metal plate were performed to determine the mechanical properties of the interface required in 3-D finite element model studies of cementless implants. Measured results showed that the interface load-displacement curve was highly nonlinear with significant coupling between two perpendicular directions. The influence of the coupling terms on results was investigated by comparison of predictions with measurement results. The equations developed here were used in the subsequent 3-D finite element analyses, i.e., the pull-out tests and the knee-implant system.

Second part included the study undertaken to investigate the effect of combined loading on pull-out fixation response of bone screws, porous coated posts, and smooth-surfaced posts inserted in polyurethane material. Finite element models of the screws/posts were proposed and validated by experimental tests. The developed models were used to investigate the post-operative short term fixation stability of various implant designs.

In the last part, a three dimensional nonlinear finite element model was developed to investigate tibial fixation designs and friction models in total knee arthroplasty in the immediate postoperative period without biological attachment. Coulomb's friction (μ = 0.045) was considered to model the polyethylene-femoral component interface. Tension tests were performed to obtain the mechanical behavior of the polyethylene required for the simulation. It was found that Coulomb's friction significantly underestimates the relative micromotion at the bone-implant interface. Maximum Mises stress in polyethylene exceeded the yield stress and was found 1–2 mm below the contact surface for all designs.

The finite element analyses revealed the significant dependency of relative micromotion and stress transfer at the bone-implant interface on the friction model and on the baseplate anchorage configuration. However, stresses within the polyethylene were found to be independent of the baseplate fixation. The study also confirmed the superior performance of screws in preventing the micromotion and the lift-off. (Abstract shortened by UMI.)