Ultra-High Molecular Weight Polyethylene for Total Hip Acetabular Liners: A Brief Review of Current Status

This article refers to:

Cross-Linked Versus Conventional Polyethylene for Long-Term Clinical Outcomes After Total Hip Arthroplasty: A Systematic Review and Meta-Analysis

Metal-on-plastic (MoP) is one of the most common articulation couples employed for total hip arthroplasty (THA), that is, a CoCr femoral head and an ultra-high molecular weight polyethylene (UHMWPE) acetabular liner [1]. However, UHMWPE wear is a concern as the wear particles can cause a cellular response leading to bone resorption, aseptic loosening, and implant failure [1]. As such, there has been ongoing research to develop more wear-resistant versions of UHMWPE, one strategy being to crosslink the material. Highly crosslinked UHMWPE (HXLPE) has been available for THA applications since the late 1990s and has achieved widespread use [2]. While evidence for the clinical benefits of HXLPE over conventional UHMWPE (CPE) is generally robust, a meta-analysis of long-term clinical studies comparing these two materials can strengthen HXLPE’s role in THA by clearing up any controversies or uncertainties that remain.

Shi et al. performed a systematic review and meta-analysis of eight randomized controlled studies and six observational studies that included 748 THAs using HXLPE and 732 THAs using CPE [3]. The mean follow-up ranged from 10 to 15.2 years. A variety of clinical outcomes (revision rate, osteolysis), radiological outcomes (linear wear rate, femoral head penetration rate) and functional outcomes (Harris Hip Score, Western Ontario and McMaster Universities Osteoarthritis Index - WOMAC) were considered. The mathematics of meta-analysis is complex and replete with jargon that can be overwhelming to the reader not versed in its methodology [4]. Nevertheless, the authors were clear in their conclusions and provided the mathematical reasoning to support them – (1) polyethylene wear in the HXLPE group was significantly less than in the CPE group, (2) there was no significant difference in functional outcomes, and (3) HXLPE was associated with risk reduction for revision and osteolysis of 78 and 80%, respectively, compared with CPE. They ascribed some of the prior controversy in the literature to high heterogeneity, relatively short follow-up, and different software used in the radiographic wear analysis.

The authors listed one of the limitations of their meta-analysis as the nonuniformity of HXLPE types used among the examined studies, with three types identified and some studies not reporting this detail. Consequently, it is difficult to fully appreciate the effect that nuances in the HXLPEs produced from different manufacturers may have on clinical performance.

At first blush, all HXLPEs have the same basic chemistry and differ from CPEs by having crosslinks that form a three-dimensional covalent network, affording the polymer enhanced wear properties [1]. However, there are many technical aspects that differentiate the various brands of HXLPEs, including:

• Base resin. There are two base resins typically used for hip liners, that is, GUR 1020 or GUR 1050 UHMWPE resin (Ticona Inc, Florence, KY, USA) which have similar but distinct properties [2, 5].

• Crosslink density. Crosslinking is performed by gamma or electron beam (e-beam) irradiation in the range of approximately 50–105 kGy. [1] The crosslink density increases with increasing irradiation dose and the properties of the polymer are crosslink density-dependent.

• Post-irradiation thermal treatment. While the free radicals that are produced during irradiation enable covalent crosslinking to occur, residual free radicals remain that can potentiate oxidative degradation resulting in deterioration of mechanical properties. [1] This can be mitigated by post-irradiation annealing below the melt temperature (130–140 °C) to increase chain mobility, allowing residual free radicals to combine and quench. However, the mobility of the chains in the crystalline regions is not increased so residual free radicals therein remain. Consequently, annealing is not 100% efficient and leaves the polymer still susceptible to some degree of oxidative degradation. Another method is to melt the polymer (∼150 °C). This allows all of the residual free radicals throughout to quench. The downside is that upon cooling there is a reduction in the degree of crystallinity relative to the pre-melt condition that affects the mechanical properties.

• Sterilization. The acetabular liners are sterilized by gamma irradiation, gas plasma exposure, or ethylene oxide exposure [1]. Terminal gamma sterilization has the potential to induce additional crosslinking and increase residual free radical content.

While the bullets above illustrate some of the major differences that can exist from one HXLPE to another, there are many other “shades of gray” among the various manufacturing methods that further differentiate these materials, for example, the temperature at which irradiation is performed, whether irradiation is performed in air or under an inert atmosphere (argon), the inclusion of mechanical deformation during post-irradiation annealing, whether the sterile packaging contains air or argon, etc. Thus, HXLPEs not only differ among manufacturers but can differ among various brands within the same manufacturer. Manufacturers often include details of the processes they use to produce HXLPE as information posted on their websites, for example, brochures and specifications, and some publications detail the procedures as well [6]. It would behoove investigators to include such information in their publications to unequivocally characterize the material under study.

A more recent development in the evolution of UHMWPE material science has been the inclusion of vitamin E (α-tocopherol) into UHMWPE to produce vitamin E-enhanced highly crosslinked UHMWPE (VEHXLPE). Vitamin E is a free radical scavenger and serves to stabilize the polymer against in vivo oxidative degradation. This is important because free radical formation and subsequent oxidative degradation of UHMPWE can be triggered by lipid adsorption [1]. As with HXLPE, irradiation is employed to crosslink the polymer. However, the presence of vitamin E can interfere with this process by reducing the crosslinking efficiency of the irradiation. There are two methods of incorporating vitamin E into the polymer to minimize this effect, that is, blending and by diffusion [6]. Blending involves adding vitamin E to the resin at a concentration of 0.3 wt%, followed by consolidation and irradiation after which the implant geometry is machined. The relatively low concentration of vitamin E helps to minimize its negative impact on radiation-induced crosslinking. By contrast, the diffusion method requires the UHMWPE resin to first be consolidated, then the gross implant geometry is machined followed by irradiation and then soaking in vitamin E to a body temperature equilibrium value of 0.7 wt% followed by a thermal homogenization step to create a more uniform distribution throughout the part. The final implant geometry is then machined to account for any dimensional changes that may have occurred during the previous steps. VEHXLPE has been commercially available for THA applications since 2007 [6]. Currently, most published studies are short-term with a limited number being mid-term [7–10]. While most results appear promising, it is too early to reach definitive conclusions [1].

In summary, Shi et al. have performed an excellent meta-analysis showing the superiority of HXLPE over CPE acetabular liners for THA applications [3]. The field, however, has continued to progress and now the question is “Are VEHXLPE liners superior to HXLPE liners?.” It would be expected that over the next few years long-term studies comparing the clinical, radiological, and functional outcomes with these two materials for this application will be published and will hopefully include high-quality randomized controlled studies. Then a meta-analysis will help answer this new question.

Declaration of interest

The author reports no conflict of interest.