Contribution of bone cement volume with increased bone loss in the fixation of a patellar implant

Purpose: Complications related to the patellofemoral joint after total knee arthroplasty (TKA) represent up to 50% of TKA re-operations. Shear forces at the bone-cement-implant interface produce wear and occasionally result in failure of fixation of the patellar implant. During revision surgery, variable amounts of patellar bone loss are observed from implant removal. Different volumes of bone cement are thus used to fill the remaining bone cavity and to fix the patellar implant. This study aims to computationally simulate and evaluate the fixation mechanics of the patellar implant-cement-bone interface with increasing amounts of bone loss. Materials and Methods: Patellar implant fixation to bone using bone cement in revision TKA was simulated using the finite element method. To study the fixation mechanics with increasing bone loss, the diameter of the bone cavity prepared around each of the three implant pegs (used as implant anchors into the bone) was uniformly increased by 0.5 mm. This resulted in a total of 5 bone-cement-implant models, with a bone cement thickness around the pegs ranging from 0.5 mm to 2.5 mm, loaded in shear to test fixation success. Results: Findings from the study indicate that increasing the cement thickness (bone hole diameter) is beneficial in increasing stiffness of the construct, reducing mean implant and cement stresses, reducing interface contact pressure between bone-and-cement and between cement-and-implant, and also reducing the maximum relative motion between these structures at these interfaces. Conclusions: Data collected from this finite element analysis support using an increased bone hole diameter, and thus bone cement filling around the implant pegs, for patellar implant fixation in revision TKA.