Native varus angle does not affect the primary fixation of cementless TKA using kinematic alignment - A population-based FE study

Proper primary fixation in cementless total knee arthroplasty (TKA) is essential for long-term survival of the implant. It is, however, poorly understood how different surgical implant alignment techniques and implant materials impact implant fixation. The aim of this computational study was therefore to investigate the effect of variations in coronal knee alignment on the primary fixation of femoral cementless polyetheretherketone (PEEK-OPTIMA™) and Cobalt-Chromium (CoCr) TKA in a patient population. Patient-specific finite element (FE) models of kinematically aligned femoral reconstructions of 179 varus patients (0-9 degrees) were created, varying the implant material (CoCr and PEEK) and applying patient-scaled loading conditions determined from musculoskeletal models. These variations resulted in 716 FE simulations. The primary fixation of the implant was quantified by the median and maximum values of the resulting micromotions on the bone-implant surface. Micromotions remained stable across different varus angles for both PEEK and CoCr models. Overall, PEEK models generated larger micromotions than CoCr models. However, both CoCr (3.6 µm) and PEEK (19.1 µm) displayed median micromotions below the threshold range of 40 to 112 µm for osseointegration. This study presents the results of a novel workflow investigating the effect of implant material and kinematic alignment on primary fixation of cementless TKA reconstructions in Dutch varus patients. The current results do not suggest an effect of varus alignment on primary fixation and micromotion distribution. Future studies will compare these outcomes with mechanical alignment applied to the same cohort, this may present more pronounced differences in fixation at larger native varus angles.