OXIDATIVE STABILITY OF FIRST AND SECOND GENERATION HIGHLY CROSS-LINKED UHMWPE RETRIEVALS

Introduction Radiation cross-linked UHMWPEs were developed to address osteolysis-induced joint arthroplasty failure by improving wear resistance and reducing associated particulate debris. Introduced clinically fifteen years ago, they are the primary bearing surface in use with excellent clinical outcomes and wear resistance. First generation materials sought to maintain oxidative stability by reducing or eliminating free radicals through thermal treatments, while second generation aimed to further balance oxidation resistance and improve mechanical properties through sequential irradiation and annealing or the incorporation of an antioxidant. Recent reports have identified lipid absorption and cyclic loading as potential in vivo oxidation-inducing mechanisms. In this on-going retrieval study, we report on the current status of oxidative stability in these two generations of UHMWPE bearings. Materials & Methods Six types of highly cross-linked UHMWPE hip and knee bearings (Table 1) were surgically-retrieved and collected under IRB approval. Standard material analysis was performed on cross-sections of loaded and unloaded bearing surfaces of the components. Thin sections (150 µm thickness) were extracted in boiling hexanes under reflux for 16 hours followed by vacuum drying for 24 hours. FTIR was used to evaluate oxidation and calculated from post-hexane absorbance spectra by normalizing the area under 1740 cm −1 (1680–1780 cm −1 ) to the area under 1370 cm −1 (1330–1390 cm −1 ), per ASTM F2102-13. Gravimetric swelling of regional cross-sectional blocks (1–2 mm 3 ) for 2 hours in 130°C boiling xylenes was used to assess cross-link density, per ASTM 2214. Results Irradiated and melted retrievals all showed detectable (OI>0.1) subsurface oxidation in the articular surface of retrievals (Fig 1). Behavior between materials types differed: 47% of Longevity acetabular liners (MOI=0.14±0.19; Table 2) showed detectable oxidation as opposed to 19% in Marathon retrievals (MOI=0.07±0.08), both with comparable sample sizes and in vivo durations. We saw no concomitant change in the cross-link density, except in one case where OI>1.0. Sequentially irradiated and annealed (X3) retrievals showed the highest incidence of detectable oxidation (76%), highest average maximum oxidation (0.35±0.39), signs of oxidative embrittlement and a loss of cross-link density which correlated with decreasing oxidation (R 2 =0.30; p-value=0.000016). Oxidation was in both loading regions of X3 knees, while Prolong knees were observed to have oxidation solely at the articular surface. Antioxidant-stabilized E1 retrievals showed low detectable oxidation values (MOI=0.11±0.03) in both regions without change in cross-link density. Discussion Throughout the first decade of service, irradiated and melted UHMWPE retrievals showed subsurface oxidation, but with little to no impact on material properties. Detectable oxidation and embrittlement were identified in sequentially irradiated and annealed retrievals at shorter time points. Residual free radicals and pre-implantation shelf oxidation, as a result of air permeable packaging, are potential factors behind the higher oxidation at earlier time points. Antioxidant-stabilized retrievals showed no change in their oxidative behavior with the lowest oxidation and variability in this very short 0–3 year follow-up. Continued analysis is needed to understand the second decade of behavior along with longer-term follow-up with patients to understand if these changes could affect clinical outcomes through oxidation-induced changes in material or mechanical properties.