Hindered amine light stabilizers: An alternative for radiation cross-linked UHMwPE implants

To reduce wear UHMwPE implants are commonly cross-linked by use of gamma or e-beam irradiation. After irradiation however, radicals are still present that may cause oxidative degradation of the polymer. A way to reduce oxidative degradation could be to add a stabilizer to the polymer. Currently Vitamin E is the state of the art in stabilizing irradiation cross-linked UHMwPE implants. However, this stabilizer has some drawbacks. It interferes with the cross-link chemistry, leading to a lower cross-link density, it will be consumed and it results in the discolouration of the UHMwPE compound due to Vitamin E conversion products. It has been shown that all these drawbacks can be overcome by using HALS stabilizers. Due to their different mechanism of action, HALS stabilizers do not interfere with the cross-link chemistry. As part of the stabilization mechanism the radical scavenging molecules are actually regenerated, which allows for using a lower total concentration. The HALS conversion products do not result in discoloration of the polymer compound. Although their biocompatibilities still have to be evaluated, HALS stabilizers are potential alternative stabilizers for UHMwPE implants.