Effect of Gamma Radiation Level on the Wear of Antioxidant UHMWPE in a Hip Simulation Test (original) (raw)
Introduction: Crosslinked-remelted polyethylenes were introduced into clinical use in 1997. Clinical data on crosslinkedremelted polyethylenes indicate substantially lower wear [1,2] Moreover, using an antioxidant (AO) to stabilize the free radicals (thus eliminating the need for remelting) in the polyethylene will achieve the same goal without sacrificing mechanical properties [2]. However the presence of an AO reduces crosslinking during gamma irradiation via a free radical scavenging process. Therefore, the gamma irradiation dose needs to be increased to achieve the same wear reduction as the polyethylene without AO additives. This study evaluates the wear of an AO containing polyethylene (0.075% w/w pentaerythritol tetrakis [3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], known as PBHP in GUR 1020) acetabular liners under three different levels of gamma radiation doses. The control group is a 7.5 Mrad, remelted polyethylene and all polyethylene groups articulate against 36mm cobalt chrome femoral heads. Methods: Four groups of polyethylene liners (Table 1) were paired with matching femoral heads that were manufactured from CoCrMo (ASTM F1537) with a diameter of 36 mm. The control polyethylene was fabricated using extruded bars of GUR 1020 UHMWPE (MediTECH, Ft. Wayne, IN) that were placed in foil bags, crosslinked using 75 kGy of gamma radiation, remelted at 155 ºC to extinguish free radicals, and then machined into acetabular liners (AltrX™, DePuy Orthopaedics, Inc., Warsaw, IN). For the anti-oxidant polyethylene, UHMWPE powder (GUR 1020 resin, MediTECH, Fort Wayne, IN) was compounded with 0.075% PBHP, consolidated by compression molding, machined into liners, sealed in vacuum packages and gamma sterilized/crosslinked at nominal doses of 115 kGy (85 + 30), 125kGy (95 +30), and 135 kGy (105 + 30) for groups A, B, and C respectively. All three AO groups had a 2 step gamma irradiation dose. Wear testing was performed on a 12-station hip simulator (AMTI, Watertown, MA) per ISO 14242-1 standard [4] at 1Hz. The cups were mounted at 30 degree of inclination and secured with cement while the femoral heads were mounted on a vertical taper support. Testing was performed in 25% bovine calf serum (Hyclone Laboratories, Logan UT) at 37±2°C with sodium azide (0.2%wt) and EDTA (20mM). At 0.5 Mcyc intervals, the components were cleaned for analysis and the serum was replaced. Wear of the liners was determined gravimetrically at 0 Mcyc and then after each 0.5 Mcyc interval using a digital balance (XP250, Mettler-Toledo). A corrected wear value was computed by incorporating the apparent weight gain of non-articulating loaded soak controls. Finally, a student's t-test was used to compare wear rates for statistical significance (two-sided test p < 0.05). Results: The wear rates of the AO poly decreased as the irradiation level was increased (Fig 1). The wear rates for Groups A, B, and C were 7.7±0.8, 7.0±1.4, and 6.6±1.0 mg/Mcyc, respectively. The wear rate for AltrX was 7.3±1.0 mg/Mcyc. All 3 AO poly groups were statistically similar when compared to AltrX Group D (p>0.05). Discussion: By increasing the amount of irradiation, a general but not significant trend in reduced wear was observed. The mechanical properties of the AO poly as a function of radiation dose is being reviewed separately [5]. The lowest irradiation dose (Group A) demonstrated comparable wear and mechanical properties to that of clinically successful AltrX. Significance: Wear reduction and oxidation stability are key factors affecting polyethylene acetabular component survivorship. The AO poly material maintained a significant wear rate reduction similar to AltrX. Acknowledgments: The authors would like to thank Mark Hanes for his contribution to the testing involved and reviewing this abstract.