Finite element analysis of primary stability in cementless tibial components with varying interference fits

Cementless knee implants achieve initial fixation through an interference fit, where the tibial implant is press-fitted into an undersized bone cavity. The dimensions between the implant and bone cuts must be carefully balanced to achieve an optimal interference fit, ensuring good primary stability, which is crucial for long-term fixation and successful osseointegration. However, the ideal interference fit remains uncertain. Excessive interference fit may lead to bone plastic deformation, while insufficient fit can result in large micromotions, small movements at the bone-implant interface, that compromise stability. This study evaluates how interference fit affects bone plasticity and micromotions, and how different loading conditions influence primary stability using finite element analysis.

Finite element models, based on experimentally implanted components, simulated interference fits of 350 microm and 700 microm. Micromotions, gap dynamics, and bone deformation were assessed during gait and squat activities under both simplified and complex loading conditions.

Higher interference fits increased bone plastic deformation, limiting elastic energy accumulation, whereas lower interference fits exhibited a reduced effect. Micromotions and gaps were consistently larger in lower interference fit implants. Furthermore, simplified loading underestimated micromotions and gaps compared to the complex loading.

These findings help explain why higher interference fits provided limited improvements in primary stability during experimental tests, despite differing predictions from simulations. This study enhances our understanding of bone-implant interactions and suggests that increasing interference fit does not necessarily improve implant stability. It also highlights the importance of incorporating complex loading conditions for more accurate primary stability assessment.