Abstract
Introduction:
Moderately crosslinked polyethylene maintains a balance of wear resistance and mechanical properties. The GVF poly was manufactured from GUR1020 UHMWPE bars, sealed in vacuumed foil package, and gamma sterilized at 4 Mrads. The MARATHON® polyethylene inserts were manufactured from GUR1050 UHMWPE bars, crosslinked by gamma irradiation at 5 Mrad, and followed by a remelting process that eliminates free radicals. The final sterilization method is gas plasma (GP) or ethylene oxide (EtO). Both methods will not introduce free radicals. Previous studies have shown MARATHON polyethylene (GP sterilized) with 83% lower wear than conventional polyethylene in a simulation test [1], compared to a 10-year clinical study that showed 77% wear reduction [2]. There is no study to compare the wear performance of MARATHON (EtO sterilized) and conventional poly.
Materials and Methods:
Four groups of polyethylene inserts (Table 1) were paired with matching femoral heads that were manufactured from CoCrMo (ASTM F1537) with diameters of 28, 32, and 36 mm. The inserts were chosen to have similar thickness at the dome for MARATHON, while for GVF it was the largest head size available.
Wear testing was performed on an AMTI Hip Simulator per the ISO 14242-1 standard [3] at 1 Hz using the described inputs (Table 2), which provide a larger range of motion than the ISO standard. The cups were mounted in accordance with ISO 14242-1 using custom fixturing and secured with cement while the femoral heads were mounted on a vertical taper support. Testing was performed in 25% bovine calf serum at 37 ± 2°C. Wear of the inserts was determined gravimetrically. Finally, wear rates were calculated by linear regression and then compared between the groups using ANOVA analysis (α = 0.05).
Results:
The total wear over 5-million cycles and the wear rate of each group is in Figure 1. The wear of GVF (Group A) was the highest over that of the MARATHON (Groups B, C and D), with a wear reduction of 53%, 61%, and 74%, respectively. For MARATHON, there was a 7% wear increase per mm increase in head size (p < 0.01).
Discussion:
The fact that the wear of MARATHON was at least 53% lower than that of GVF suggested that the poly wear was primarily affected by gamma-irradiation dose and process used for crosslinking of the polyethylene. In addition, the lower wear of MARATHON provides an option of using large head size which has the dual benefits of increased joint stability and improved head/shell ratio for better range of motion.
