This article reviews the history of the development of highly cross-linked polyethylene and provides an in-depth review of the clinical results regarding the durability of highly cross-linked polyethylene (HXLPE) used in total hip arthroplasty (THA) and total knee arthroplasty (TKA). The use of polyethylene as a bearing surface has contributed to the success of THA and TKA; however, polyethylene wear and osteolysis can lead to failure. Ongoing clinical and retrieval studies are required to analyze outcomes at longer-term follow-up.
The durability of modern highly cross-linked polyethylene in total hip arthroplasty and total knee arthroplasty has been excellent.
The manufacturing process leads to free radical generation, which leads to oxidation and reduced mechanical properties.
Postprocessing modifications such as remelting and vitamin E doping reduce the risk of oxidation.
Ten-year wear and survival data for highly cross-linked polyethylene continue to be favorable, but the long-term durability has not yet been demonstrated in vivo.
Total hip arthroplasty (THA) and total knee arthroplasty (TKA) have excellent track records with regards to pain relief and improvement of function. Over the past several decades, there have been significant improvements in the engineering and materials science of bearing surfaces. The use of highly cross-linked polyethylene (HXLPE) in large-joint arthroplasty has led to improved outcomes, particularly for THA. This article briefly reviews the history of the development of HXLPE and provides an in-depth review of the clinical results regarding the durability of HXLPE in THA and TKA.
Sir John Charnley attempted to develop a low-friction arthroplasty with the use of Teflon-bearing surfaces. The wear characteristics of this material were unsatisfactory with dramatic early failure from wear. He eventually transitioned to metal-on-polyethylene THA. Charnley’s recognition that polyethylene was a good bearing surface revolutionized hip arthroplasty.
The use of polyethylene as a bearing surface contributed to the success of THA and TKA; however, it was not without complications. Ultra-high-molecular-weight polyethylene (UHMWPE) wear can lead to a local reaction that results in bone resorption or osteolysis. Fig. 1 demonstrates a radiograph of a patient with osteolysis around the acetabular and femoral components due to polyethylene wear following THA. Polyethylene wear and osteolysis have been an important cause of long-term failure of THA. The technique of highly cross-linking polyethylene was developed to make the material more resistant to wear and reduce the incidence of this mechanism of failure in arthroplasty.
Ionizing γradiation is used to highly cross-link polyethylene by breaking hydrogen-carbon bonds, which then allows molecular cross-linking, thus improving the wear characteristics of the polymer but leading to the creation of free radicals. These free radicals can allow the polyethylene to oxidize over time and decrease the material’s fracture resistance. Irradiation with oxygen present potentiates this detrimental effect and leads to early oxidation. Fig. 2 demonstrates a thin section of a retrieved conventional polyethylene insert demonstrating white banding due to subsurface oxidation of the material. The result of oxidation is a reduction in the mechanical properties leading to a more brittle polyethylene. Delamination of the polyethylene in TKA can be seen under these circumstances. Cracking can compromise the locking mechanism. As a result, modern components are sterilized and irradiated in oxygen-free environments with the use of inert gas or gas sterilization.
Postradiation processing techniques have been developed to reduce or eliminate the number of free radicals that remain after γ irradiation. Remelting, annealing, and mechanical elimination of free radicals are 3 well-described techniques that have been used to improve the oxidative stability of HXLPE. When remelting, the polyethylene is heated past its melting point to eliminate crystals and allow the remaining free radicals to cross-link with each other. Remelting creates a material with no free radicals but slightly decreases mechanical strength and fracture toughness. During annealing, the maximum temperature is less than the melting point. Annealing results in a greater number of remaining free radicals compared with melting but may be less detrimental to the mechanical properties and J-fracture toughness.
A more recent technique for reducing oxidation in polyethylene involves manufacturing the polymer to include an antioxidant. The antioxidants can stabilize free radicals that exist as a byproduct of manufacturing and protect against future in vivo oxidation as well. Vitamin E has been the most commonly used antioxidant for this purpose. Irganox is another antioxidant that was recently introduced for TKA. Laboratory data have suggested that the use of vitamin E in polyethylene can decrease oxidation characteristics without negatively impacting the mechanical qualities. It eliminates the need for melting. There is also some evidence that it is associated with a reduction in the biologic activity of wear particle. Long-term clinical data are not yet available, but clinical studies are ongoing. Greene and colleagues presented a multicenter prospective study of 977 THA patients. The vitamin E–diffused polyethylene group had −0.04 mm/yr head penetration at 5 years and no evidence of osteolysis. An radiostereometric analysis study of vitamin E UHMWPE by the same group demonstrated 0.05-mm median head penetration in 47 patients at 5 years.
The types of wear that most frequently impact implanted polyethylene in THA and TKA are adhesive, abrasive, and third-body wear. Adhesive wear is an expected wear pattern in arthroplasty that is due to the friction between the polyethylene and the metal or ceramic-bearing surface with which it articulates. Abrasive wear occurs because of a harder surface sliding against a softer surface. In this case, polyethylene is the softer surface. Fig. 3 demonstrates the articular surface of a highly cross-linked and melted polyethylene insert retrieved because of sepsis at 5 years showing retention of the original machine marks, indicating extremely low wear. Fig. 4 shows a retrieved conventional polyethylene tibial insert demonstrating delamination and pitting due to subsurface oxidation as well as loss of material due to adhesive and abrasive wear. Third-body wear is caused by some substance other than the prosthesis causing abrasion of the polyethylene. This substance is most often a piece of bone, cement, or metal in arthroplasty. HXLPE retrievals can sometimes show surface changes that appear to be wear but are just scratches. The material restores its form because of shape memory.
Highly cross-linked polyethylene in total hip arthroplasty
Both biomechanical and clinical studies have shown an advantage to the use of HXLPE over non-cross-linked polyethylene in the hip. In a retrieval study by Muratoglu and colleagues, conventional polyethylene showed more scratching and increased loss of machining marks when compared with HXLPE. Minoda and colleagues reported on wear particle analysis of HXLPE from a failed THA and found that not only were fewer wear particles generated than with conventional UHMWPE but also the particles from HXLPE were smaller and rounder. They hypothesized that this may be associated with a less vigorous macrophage response to wear in HXLPE. Although simulator studies showed marked improvements in wear with HXLPE via adhesive and abrasive wear mechanisms, it should be noted that third-body wear was not significantly improved with the use of HXLPE in a simulator study.
Clinical studies have shown improved wear rates at short-term, mid-term, and now long-term follow-up. Digas and colleagues showed 62% lower proximal penetration with HXLPE over conventional polyethylene at 2 years after surgery using radiostereometry. The HXLPE in that study was compression-molded, γ-irradiated in nitrogen and remelted. Rohrl and colleagues also showed a marked reduction in wear at 2 years with the use of HXLPE that was irradiated in an inert environment and heat annealed with no evidence of any clinical disadvantage. The use of HXLPE with large head arthroplasty also showed excellent wear rates (0.06 mm/yr) at 3 years. Bragdon and colleagues used radiostereometry and found that HXLPE shows acceptable wear rates with large head arthroplasty in ram-extruded, irradiated, and remelted HXLPE used with 36- and 40-mm heads. These results have also been shown to be applicable to younger populations. Ayers and colleagues showed improved wear rates with irradiated and melted HXLPE over conventional UHMWPE in a cohort with mean age of 58.
Dorr and colleagues published 5-year data showing a reduced linear wear rate in ram-extruded electron-beam-irradiated and melted HXLPE with an annual linear wear rate that was only 45% of the rate seen with conventional polyethylene in their radiograph-based clinical study. Engh and colleagues showed a 95% reduction in wear at 5 years in a randomized controlled study comparing γ-irradiated and heat-treated HXLPE with conventional UHMWPE. That study also revealed decreased osteolysis in the HXLPE group compared with the standard UHMWPE group. Multiple other studies have demonstrated an advantage of HXLPE at this time point in both cemented and uncemented components.
Meta-analyses and systematic reviews have supported the evidence presented by smaller trials demonstrating that wear rates are reduced with HXLPE over conventional UHMWPE. These results have been seen among various manufacturers with differing component designs and locking mechanisms and in many different countries across a variety of hip arthroplasty patient populations.
As expected, the improvements in wear rates have been seen in association with a reduction in osteolysis. A systematic review looking at 9 such studies found an odds ratio of 0.13 for risk of osteolysis in HXLPE compared with conventional UHMWPE. Osteolysis, which was one of the largest challenges facing arthroplasty surgeons, has not been reported at a clinically significant level with the use of HXLPE.
Longer-term data are now becoming available on HXLPE in THA. Bragdon and colleagues published a multicenter study including a cohort with minimum follow-up of 10 years. Again, HXLPE showed wear rates less than the threshold for clinical osteolysis. The authors also found that patient-reported outcomes were comparable with previously published standards for THA.
The data have clearly shown improved wear results with HXLPE over conventional UHMWPE. Manufacturing methods differ for HXLPE, but there are minimal differences in the clinical results for the currently used manufacturing processes of HXLPE up to 10 years out. At longer-term follow-up of 15 and 20 years, the manufacturing differences outlined in the introduction may lead to clinical differences in the device performance. Only time and well-designed outcome studies will provide clinicians, patients, and device manufacturers with this information.