Lipid-induced degradation of UHMWPE: Clinical assessment of retrieved orthopedic implants, analysis of synovial fluids, and in vitro validation.

Abstract

The stability and durability of ultra-high molecular weight polyethylene (UHMWPE) components are critical to the long-term success of total arthroplasty. Although radiation-induced degradation has been extensively studied and is recognized as a major issue, UHMWPE degradation by synovial lipids has garnered limited attention. Specifically, clinical evidence for lipid-induced degradation is lacking, and the validity of squalene as a model lipid remains unestablished. Therefore, this study was aimed at assessing UHMWPE degradation caused by lipids in clinical practice and developing a clinically relevant model. UHMWPE components and synovial fluid retrieved from revision surgeries were analyzed for lipid content. An in vitro lipid-induced degradation model was developed by introducing these lipids into UHMWPE. Degradation was evaluated using high-spatial-resolution mechanical testing and oxidation index (OI) measurements. Limited squalene was detected in either retrieved components or synovial fluid, whereas cholesterol esters and triglycerides were abundant. In vitro, all tested lipids elevated the OI of UHMWPE, but only lipids containing unsaturated bonds caused minor reductions in mechanical properties. Detailed analysis of retrieved components failed to detect evidence of lipid-induced degradation. These findings indicate that synovial lipids can increase OI without necessarily impairing mechanical properties. Consequently, OI alone is insufficient for evaluating degradation, and mechanical testing is essential. The extent of synovial-lipid-induced degradation was subtle compared with other degradation mechanisms. Squalene-based models do not quantitatively replicate clinical conditions, and their results must be interpreted with caution. These findings can guide the accurate assessment and prevention of lipid-induced degradation of UHMWPE in clinical settings and implant development.