THE EFFECT OF VITAMIN E ON THE WEAR RESISTANCE OF HIGHLY CROSSLINKED POLYETHYLENE

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint arthroplasty. However, longevity of these implants has been compromised by wear and fatigue damage of the polyethylene. The addition of vitamin E to the polyethylene is a process recently introduced in the market to stabilize free radicals produced during radiation crosslinking. The objective of the present study is to investigate the effect of the addition of vitamin E on the wear characteristics of UHMWPE. Sequentially cross-linked and annealed UHMWPE material (X3™, Stryker Orthopaedics, Mahwah, NJ) was used as a control.

Trident™ acetabular cups (Stryker Orthopaedics, Mahwah, NJ) with inner diameters of 36 mm and 44 mm and a wall thickness of 3.8 mm were tested on a 12 station MTS hip joint simulator. The simulator used a physiologic loading pattern with a maximum load of 2450N. The test was conducted under standard clean conditions with alpha calf fraction serum diluted to a protein concentration of 20 g/l for a total of three million cycles. All cups ran against CoCr femoral heads, and gravimetric measurements were taken every half-million cycles.

Results show that sequentially crosslinked components, size 3 6mm, had an average volume loss of 9.4 ± 2.5 mm3, while vitamin E components of the same size had an average of 16.5 ± 3.1 mm3. This represents a 75% increase for vitamin E components that is statistically significant (p = 0.039). Size 44 mm sequentially crosslinked components had an average volume loss of 6.8 ± 3.7 mm3, while vitamin E components had an average of 19.7 ± 3.2 mm3. This denotes a statistically significant increase of 192% for material with vitamin E (p = 0.011). Linear regression analysis yielded wear rates of 4.1 ± 0.9 mm3/mc and 6.1 ± 1.3 mm3/mc for size 36 mm sequentially crosslinked and vitamin E components, respectively, which represents a non-significant increase of 49% for vitamin E components. Size 44 mm sequentially crosslinked components had a wear rate of 3.8 ± 1.3mm3/mc, while vitamin E components had a wear rate of 8.1 ± 0.7 mm3/mc. This represents a statistically significant increase of 117% in wear rate for vitamin E components (p = 0.013).

The results of this testing indicate that the addition of vitamin E degrades wear performance relative to sequentially crosslinked material. Research shows that the introduction of Vitamin E affects the ability to create crosslinks during irradiation by reacting with some of the free radicals. Oral et al have shown that the crosslink density decreases when Vitamin E is blended into UHMWPE. Their research has also shown that a decrease in crosslink density causes an increase in wear rate. The results of the current testing show that the addition of vitamin E to polyethylene reduces the wear resistance of highly crosslinked polyethylene.