Wear Performance of Newest Generation of Highly Cross-Linked Antioxidant Polyethylene
Authors: Schipper Oliver , Miles Jonathan , Alvarez Luis , Hamilton Matthew 
Published: December 2024

Category:

Ankle; Basic Sciences/Biologics

Introduction/Purpose:

Highly cross-linked polyethylene (HXLPE) is the most used bearing material in orthopedic joint replacement because of its reduced wear relative to standard ultra-high molecular weight polyethylene (UHMWPE). Newer generations of HXLPE include antioxidants to improve oxidation resistance and fatigue in addition to reducing wear. There are many ways to manufacture this material, but most commercially available versions are cross-linked via high dose irradiation (>75kGy). The latest concept utilizes a chemical cross-linking process during consolidation of the material. The resultant material has higher fracture toughness than previously tested HXLPE variations. The goal of this study was to evaluate the wear performance of different polyethylene materials.

Methods:

Wear testing was performed using a total ankle system following ISO-22622 (Wear of total ankle-joint prostheses). The same size constructs were used for all tests only varying the base material used to manufacture the bearing components. There were three polyethylene versions tested: 1) direct compression molded UHMWPE (DCM), 2) gamma cross-linked Vitamin E polyethylene (GXVE), and 3) chemically cross-linked Vitamin E polyethylene (CXVE). DCM and CXVE were gamma sterilized while GXVE was ethylene oxide sterilized. All parts underwent accelerated aging per ASTM F2003 to simulate worst-case oxidation prior to testing. Mass loss measurements were taken after 0.5, 1, 2, 3, 4, and 5-million cycles. The steady-state wear rate is reported as the linear wear-rate from 0.5 million to five-million cycles. Additionally, particle analysis was performed per ASTM F1877 on particulate extracted from the worst samples after five-million cycles. The analysis quantified mean equivalent circle diameter (ECD) and aspect ratio.

Results:

The steady-state wear rate for the standard UHMWPE, GXVE, and CXVE were 3.77±0.36mg/MC, 1.44±0.22mg/MC, and -0.04±0.19mg/MC respectively. An ANOVA performed identified statistically significant differences among the datasets. A post-hoc Tukey HSD found statistically significant differences between all three datasets (p < 0.01 for all pairings). The negative values for the CXVE indicates that the mass loss was less that the method could detect. The mean ECDs for the DCM, GXVE, and CXVE materials were 180±130nm, 120±50nm, and 130±80nm, respectively. A 30% reduction in diameter occurred between DCM and GXVE, and the ECDs between the HXLPE materials were similar, indicating the cross-linked materials tended to release smaller particles relative to the DCM. The mean aspect ratio for the DCM, GXVE, and CXVE particles were 1.92±0.90, 2.11±0.87, 1.62±0.52.

Conclusion:

The reduction in wear rate between DCM and GXVE or CXVE aligns with previous studies and further supports that the highly cross-linked material is more resistant to traditional wear mechanisms. The significant improvement of the CXVE compared to GXVE suggests the chemical cross-linking process creates a more uniform and resilient material. These findings align with previously published data on the mechanical properties of the materials where the CXVE material was shown to outperform GXVE materials. The CXVE also has a smaller aspect ratio compared to the other two materials. In general, higher aspect ratio particles tend to be more bioactive.