The Influence of Body Weight Index on Initial Stability of Uncemented Femoral Knee Protheses: A Finite Element Study.

Background and objective: Obesity is one of the risk factors for osteoarthritis. The end-stage treatment for osteoarthritis is total knee arthroplasty (TKA). However, it remains controversial whether a high body mass index (BMI) affects the initial stability of the femoral prosthesis after TKA. Finite element analysis (FEA) was used to investigate this question in this study.Methods: Four femur models that assembled with TKA femoral components were reconstructed and divided into high BMI group and normal BMI group. The three-dimensional femurs were modeled and assigned inhomogeneous materials based on computed tomography (CT) images. Then each FEA model was applied with gait and deep bend loading conditions to evaluate the maximum principal strain on the distal femur and the relative micromotion between the femur and prosthesis.Results: The mean strain of the high BMI group increased by 32.7% (936.9 mu epsilon versus 706.1 mu epsilon) and 50.9% (2064.5 mu epsilon versus 1368.2 mu epsilon) under gait and deep bend loading conditions, respectively, compared to the normal BMI group. Meanwhile, the mean micromotion of the high BMI group increased by 41.6% (2.77 mu m versus 1.96 mu m) and 58.5% (62.1 mu m versus 39.2 mu m), respectively. Under gait condition, the maximum micromotion for high BMI group was 33.8 mu m and would compromise the initial stability. Under deep bend condition, the maximum strain and micro-motion exceeded -7300 mu epsilon and 28 mu m for both groups.Conclusion: High BMI caused higher strain on the bone and higher micromotion between the prosthesis and the femur. Gait activities could be risky for prosthesis stability in high BMI group while be safe in normal group. Deep bend activities were highly dangerous for both groups with high BMI and normal BMI and should be avoided.