Abstract
Ultra-high molecular weight polyethylene (UHMWPE) has been the gold standard material of choice for the load-bearing articulating surface in knee joint prostheses. However, the application of joint replacements to younger (aged < 64 years) and more active people plus the general increase in life expectancy results in an urgent need for a longer lasting material with better in-use performance.
There are three major material related causes that can lead to joint failure in UHMWPE knee joint replacements: free radical induced chemical degradation; mechanical degradation through wear and delamination; and UHMWPE micron and submicron wear debris induced osteolysis. As a potential solution to these problems, highly crosslinked UHMWPE stabilised with infused antioxidant vitamin E (α-Tocopherol), which is abbreviated as E-Poly, has been of great interest.
In the current work, the wear performance and mechanical properties of Vanguard cruciate retaining (CR) E-Poly tibial inserts were assessed and compared with Vanguard CR Arcom tibial inserts. Also E-Poly plates were compared with direct compression moulded UHMWPE wear plates. Both a multi-directional pin-on-plate tester and a six-station Prosim (Manchester, UK) knee wear simulator were used to assess wear properties of E-Poly plates and E-Poly tibial inserts respectively. All E-Poly plates and tibial inserts were sterilised and vacuum packed in the same way as Vanguard implants before wear testing. The wear knee simulator test was conducted in accordance with ISO 14243-3:2004 with the exception that a more aggressive Tibial Rotation and Anterior/Posterior displacement profiles, based on the kinematics of the natural knee were incorporated.
Under the same aggressive pre-clinical wear testing condition, compared with Vanguard Arcom CR tibial inserts, Vanguard E-Poly CR tibial inserts experienced an 85% reduction in the mean wear rate. The former had a mean wear rate of 6.51±1.75 mm3 per million cycles (MC) and the latter had a mean wear rate of 0.96±0.11 mm3/MC over the 7 million cycle testing period. A similar reduction (80%±8.5) in the mean wear factor was also observed on E-Poly plates compared with a series of direct compression moulded GUR1050 UHMWPE plates processed under a range of manufacturing processing conditions. Wear testing was conducted with a configuration of flat-ended stainless steel indenters multi-directionally sliding against the UHMWPE plates.
Mechanical properties on Vanguard Arcom UHMWPE and E-Polys were evaluated using the small punch test. All tests were carried out using an Instron 5565 Universal Testing System at a constant crosshead speed of 0.5mm/min. With regard to work-to-failure, no statistical difference was observed, with the former being 254.2±4.1 mJ and the latter 255.6±28.2 mJ. However, all E-Polys exhibited strain stiffening due to the stretch of crosslinks. This resulted in a ca 12% reduction in elongation to break observed for E-Polys compared with that of Arcom UHMWPE. The former had an elongation to break of 4.1±0.2 mm and the latter of 4.7±0.3 mm.
In conclusion, we have found that Vitamin E Stabilised UHMWPE tibial inserts are promising for knee joint prostheses. However, further investigations are needed to address potential issues such as the particle size and size distribution of E-Poly wear debris and the associated reactivity.
