Ultra-High Molecular Weight Polyethylene: Influence of the Chemical, Physical and Mechanical Properties on the Wear Behavior. A Review

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) is the most common bearing material in total joint arthroplasty due to its unique combination of superior mechanical properties and wear resistance over other polymers. A great deal of research in recent decades has focused on further improving its performances, in order to provide durable implants in young and active patients. From "historical", gamma-air sterilized polyethylenes, to the so-called first and second generation of highly crosslinked materials, a variety of different formulations have progressively appeared in the market. This paper reviews the structure-properties relationship of these materials, with a particular emphasis on the in vitro and in vivo wear performances, through an analysis of the existing literature.

Keywords: UHMWPE; Ultra-high molecular weight polyethylene; Vitamin E; crosslinking; degradation; gamma radiation; mechanical properties; oxidation; wear.

Figure 1

Cross-section of a gamma-air sterilized tibial insert, exhibiting a characteristic “crown-effect” (subsurface white band), along with its Fourier Transform InfraRed (FTIR) spectra, showing the presence of abundant oxidation products. Adapted from [17], with permission.

Figure 2

Small punch load displacement curves for gamma-air sterilized ultra-high molecular weight polyethylene (UHMWPE) tibial inserts at surface and subsurface locations: (a) control, unaged; (b) shelf-aged for 5 years; (c) shelf aged for 10 years. Adapted from [27], with permission.

Figure 3

Retrieved polyethylene tibial insert showing severe wear damage, including severe delamination and wear (10 years in vivo).

Figure 4

(a) Crosslink density and (b) pin-on-disk (POD) wear rate of irradiated UHMWPE as a function of increasing dose. Adapted from [42] with permission.

Figure 5

Oxidation profiles of conventional UHMWPE (gamma-inert sterilized, 25–40 kGy), irradiated and once-annealed UHMWPE (CrossfireTM Stryker-Howmedica-Osteonics; Rutherford, NJ, USA), and irradiated and melted UHMWPE (DurasulTM Zimmer, formerly Centerpulse; Austin, TX, USA) after 128 weeks of real-time aqueous aging at 40 °C. Reproduced with permission from [71].

Figure 6

(a) Small punch curves of virgin UHMWPE irradiated at 75 kGy in air: control and aged 2/4 weeks (ASTM F2003-00); (b) Small punch curves of UHMWPE blended with increasing concentrations (0–500 ppm) of vitamin E, irradiated at 75 kGy in air and aged 4 weeks; (c) Oxidation indexes of UHMWPE blended with increasing concentrations of vitamin E (0–500 ppm), irradiated at 75 kGy in air and aged 2/4 weeks. Adapted from [133], with permission.