Abstract
The addition of vitamin E has been shown to improve wear performance in highly crosslinked (HXL) ultra high molecular weight polyethylene (UHMWPE) total knee replacements (TKR) [1]. We set-out to verify if a new type of vitamin E stabilized HXL UHMWPE would substantially improve wear performance, and we present our new results together with our previous ones to tell a fuller story. This paper therefore reports in vitro wear of tibial bearings of both conventional and HXL UHMWPE (with vitamin E) for a total of 16 specimens covering both ends of the TKR size spectrum, very large and very small.
Different designs, sizes and four material types/processes of UHMWPE were tested. In material type 1, tested previously, the polyethylene was machined from isostatic molded GUR1020 bar stock, crosslinked with 10 Mrad, and then doped with vitamin E. From this material, 4 samples of large posterior stabilized (LPS1) TKRs were tested. Material type 2 was HXL where vitamin E was blended into the polyethylene (GUR1020) at the powder stage and the final irradiation was to 9 Mrad. From this material, 2 large cruciate retaining (LCR2) samples and 2 small cruciate retaining (SCR2) samples were tested. The above sample groups from both material types 1 and 2 were compared in the same simulator testing to corresponding identical design, size and sample numbers of conventional UHMWPE not highly crosslinked and with no vitamin E (material types 3 & 4 respectively).
Each test was run on a significantly upgraded (in house) 4-station Instron-Stanmore force-controlled knee simulator. The machine simulated flexion with anatomically realistic joint reaction forces and torques between tibia and femur, and included a spring-based system to simulate soft-tissue restraining forces and torques. The force-control waveforms of the walking cycle specified in ISO-14243-1 were applied for 5 million cycles (Mc) at 1Hz, with bovine serum lubrication with 20g/l protein concentration at 37°C). The tibial bearing inserts were weighed at various intervals standardized between all tests.
No gross delamination or fracture of the tibial inserts was observed in any tests, but all inserts showed measurable wear. The vitamin E stabilized material exhibited an 85% reduction in wear for the LPS1 designs (p < 0.05, ANOVA) compared to its corresponding conventional poly control material. The LCR2 and SCR2 designs with the new vitamin E material exhibited wear reductions of 61% and 77%, respectively when compared to their corresponding conventional bearings (p < 0.05, ANOVA).
The vitamin E highly crosslinked UHMWPE tibial bearings significantly reduced overall wear when compared to conventional tibial bearings of the same design. Such level of wear reduction should translate to worthy clinical significance in preventing osteolysis. Highly crosslinked UHMWPE stabilized with vitamin E appears to be promising for use as a bearing surface in TKR, from at least two different technologies/processes.
