The effect of bone relaxation on the simulated pull-off force of a cementless femoral knee implant

Abstract

Aseptic loosening is the primary cause of revision in cementless total knee arthroplasty (TKA), emphasizing the importance of strong initial stability for long-term implant success. Pre-clinical evaluations are crucial for understanding implant fixation mechanics and improving implant designs. Finite element (FE) analysis models often use linear elastic bone material models, which do not accurately reflect bone’s mechanical behavior. Incorporating a von Mises yield model to simulate bone’s plastic behavior improved predictions of primary stability but tends to overestimate fixation, potentially due to neglecting bone viscoelasticity. Stress relaxation in bone can affect primary stability by reducing press-fit forces on implants. This study aimed to include bone relaxation into FE models of femoral TKA reconstructions to investigate the impact of bone material models on primary fixation. Simulated pull-off tests were conducted using three material models: elastic, plastic, and plastic-viscoelastic. Six femoral reconstructions, previously used in another study, were included. The average pull-off force decreased (about 79%) from 31 kN with the elastic model to 6.3 kN when bone plasticity was included. Introducing stress relaxation showed a minimal effect, leading to an additional reduction in pull-off force of 0.8%. A significant positive correlation was found between bone mineral density and pull-off force across the three material models. Additionally, elastic strain energy within the femur correlated strongly with pull-off force, suggesting higher strain energy increases pull-off force. This study is the first to integrate plastic and viscoelastic bone behavior in FE simulations, offering insights into cementless implant fixation within context of realistic bone mechanics.