Combined effect of the body mass index and implant size on the wear of retrieved total knee prostheses

Total Knee Arthroplasty is a well-established surgical procedure performed to relieve pain and to restore function in knee osteoarthritis. A proper choice of the implant size is mandatory in order to guarantee the success of the implant and to respect the bone stock of tibial plateau and femoral condyles. The aim of this study was to investigate the combined effect of Body Mass Index and implant size on the wear damage of retrieved knee implants, a still debated subject. To this purpose, twelve total knee prostheses of the same design (NexGen, Zimmer, Warsaw, Indiana) but with different sizes were investigated. These prostheses were all cemented fixed bearing inserts and posterior stabilised. Roughness analyses were performed on femoral components to assess surface modifications. Micro-Raman spectroscopic characterization was performed on polyethylene inserts to evaluate crystallinity changes. A worsening of the surface and a nonhomogeneous aspect was observed in the prostheses characterized by a high ratio between preoperative Body Mass Index and implant size, which we called Phi factor. Delamination, a good indicator for polyethylene osteolysis, was more pronounced in the inserts having a high Phi than in those with a low Phi. Micro-Raman analyses showed more significant crystallinity changes in the prostheses of lower size, in agreement with the results previously obtained in vitro. The findings of the present study suggested that the Phi factor can be considered a predictor factor of the Total Knee Arthroplasty overall performance. Potentially, it could be used as a discriminating factor to redact Total Knee Arthroplasty outcome and prevent complications.