Does cyclic stress play a role in highly crosslinked polyethylene oxidation?

Area of Science:

• Biomaterials Science
• Polymer Chemistry
• Orthopedic Implant Technology

Background:

• Oxidation compromises ultrahigh-molecular-weight polyethylene (UHMWPE) mechanical integrity in vivo.
• Post-irradiation remelting was thought to enhance stability in highly crosslinked polyethylene (HXLPE).
• Recent findings indicate oxidation in retrieved remelted HXLPE, with biologic prooxidants and physiologic loading as potential causes.

Purpose of the Study:

• To investigate if cyclic stress from wear or compression increases oxidation and crystallinity in remelted HXLPE.
• To determine if oxidative aging reduces wear resistance in remelted HXLPE.

Main Methods:

• Remelted and annealed HXLPE prisms underwent wear simulation (500,000 cycles).
• Samples were subjected to accelerated aging and re-tested for wear.
• Fourier transform infrared spectroscopy analyzed oxidation and crystallinity in microtomed thin films.
• Remelted HXLPE compression cylinders were tested using fatigue experiments.

Main Results:

• Remelted HXLPE showed low oxidation (index ≤ 1) under cyclic loading or aging alone.
• Consecutive cyclic loading, aging, and loading steps nearly doubled oxidation in near-surface regions.
• Loading type (wear vs. compression fatigue) did not significantly alter oxidation behavior.
• Annealed HXLPE exhibited higher oxidation (index > 3) and delamination wear compared to remelted HXLPE (index < 3).

Conclusions:

• Cyclic stress from wear or compression may initiate loss of oxidative stability in remelted HXLPE.
• This stress may synergistically accelerate oxidation progression in HXLPE components.
• Further research on cyclic stress effects and retrieval studies are needed to understand in vivo behavior.