Ultra-high molecular weight polyethylene (UHMWPE) for hip and knee arthroplasty: The present and the future

Abstract

Purpose: to review advances and clinical performance of polyethylene in total joint arthroplasty, summing up historical problems and focusing on the latest innovations.

Methods: search for medical grade Ultra-High-Molecular-Weight-Polyethylene (UHMWPE); Data Sources: PubMed, Scopus, Cochrane Library.

Results: the increasing number of joint arthroplasties and high-activity patients led to progressive developments of bearing surfaces to improve performance and durability. Different strategies such as crosslinking UHMWPE (HXLPE) and the addition of vitamin-E (HXLPE) have been tested to improve wear and oxidation resistance.

Conclusion: Recent innovations about UHMWPE showed improvements either for hip and knee, with the potential of long-term survivorship.

Keywords: Arthroplasty; Crosslinking; Hip; Knee; Mechanical properties; Polyethylene; UHMWPE; Vitamin E; Wear.

Fig. 1

Low voltage scanning electron microscopy LVSEM (JEOL 6320FV low-voltage microscope operating at 1 kV) images of permanganic acid etched freeze fracture surfaces coated with a gold–palladium layer (10 nm thickness) of four polyethylenes obtained with different pressure and thermal treatments. The following polyethyelenes were studied: (1) untreated control (PE) (2) high-pressure crystallized, uncrosslinked polyethylene (HP-PE), (3) 50 kGy gamma radiation crosslinked polyethylene (XPE), and (4) high-pressure crystallized, 50 kGy gamma radiation crosslinked polyethylene (HP-XPE).

Fig. 2

Pictures of retrieved tibial plateau, after revision for aseptic loosening. The figure reports the year of service in vivo for each implant. Macroscopic damages, as delamination, fractures, pitting and signs of oxidative degradation are visible on the implants. All implants were gamma sterilized.

Fig. 3

Oxidation levels within the UHMWPE components characterized using FTIR microspectroscopy (MicroFTIR) (AutoImage FTIR Microscope System, Perkin–Elmer). Spectra were run in transmission mode, with a 4 cm1 resolution and 16 scans per spectrum. In preparation for FTIR analysis, the UHMWPE components were sectioned using a microtome into 180-m-thick specimens. FTIR microscopy spectra of EtO sterilized acetabular liner after 11 years in vivo service and removed for aseptic loosening, shows the presence of esters and acids at a minimum oxidation level (0.02 A) and after NO treatment, any nitrate absorption at 1650–1620 cm–1 is visible (means no oxidation) (Fig. 3).

Fig. 4

Oxidation levels within the UHMWPE components characterized using FTIR microspectroscopy (MicroFTIR) (AutoImage FTIR Microscope System, Perkin–Elmer). Spectra were run in transmission mode, with a 4 cm1 resolution and 16 scans per spectrum. In preparation for FTIR analysis, the UHMWPE components were sectioned using a microtome into 180-m-thick specimens. FTIR microscopy spectra of gamma-sterilized acetabular liner after 9 years in vivo service and removed for aseptic loosening shows two absorptions at 1740 (0.03 A) and at 1717 cm–1 (0.04 A) due to ester and ketone groups; after NO treatment, a secondary nitrate absorption appears at 1633 cm–1, which increases inside the sample up to 0.13 A (means oxidation).

Fig. 5

Wear of polyethylene (PE) and crosslinked polyethylene (XPE) respectively on ceramic (Al2O3) and metal (CoCr) surfaces. A 5 × 5 mm square articulation pattern was performed into a multidirectional wear tester OrthoPOD™ (Advanced Mechanical Technology Inc.). The wear tests revealed a linear increase in wear with number of cycles for all four groups; nevertheless, the wear factor of PE–CoCr was the highest and was twice as high as that of PE–Al2O3, while the wear factor of XPE–CoCr was 2.8 times higher than that of XPE–Al2O3.

Fig. 6

Oxidation (Ketone formation) profiles as a function of time at 90 C for the specimens irradiated to ta 100 kGy radiation dose and doped with different doses of Vitamin E and for an additive-free, not-irradiated UHMWPE for control. The sample with the highest tocopherol concentration (0.5%) did not show oxidation, on the contrary the control material showed high oxidation, while a stabilizing effect can be observed in all the tocopherol-containing samples proportionally to the initial tocopherol percentage.

Fig. 7

Oxidation (Hydroperoxides) as a function of the amount of Vitamin E and of the time of stocking at environmental temperature in irradiated UHMWPE at 60 kGy in air. The samples with the hicghest initial concentration of Vitamin E showd the lower formation of Hydroperoxides as index of oxidation. An increased formation of Hydroperoxides can be observed in the others tocopherol-containing samples inversely proportional to the initial tocopherol percentage, with the highest values for the material without Vitamin E.^,

Fig. 8

Histological analysis of peri-prosthetich membranes after aseptic loosened total hip arthroplasties, with respectively Ethylene Oxide (EtO) sterilized acetabular liner (a1 and a2) and gamma-sterilized acetabular liner (b1 and b2). Tissues are prepared with Hematoxylin and eosin stain, optical microscope, magnification (figure a1 and b1). Figures a2 and b2 have an optical filter to increase the fluency of the polyethylene debris. This Figure shows how the debris from EtO sterilized implants are smaller compared to those from gamma-sterilized implants and the consequent less aggressive inflammatory reaction.

Fig. 9

Volumetric wear rate for HUMWPE (Ultra High Molecular Weight Polyethylene) according to five different rectangular motion path patterns and linear tracking. The wear tests showed that the motion path pattern had a significant impact upon the wear rate: the volumetric wear rate was found to decrease significantly from the more square motion path (5 mm × 5 mm) to the more elongated path. Since the first is more likely hip situation, while the second would better reproduces a knee-like motion, this finding has a number of clinically relevant implications [36, Turell et al. Quantification of the effect of cross-path motion on the wear. Wear 2003; 255 (7–12):1034-1039.