Dinesh Nathwani, Alister Hart, Ravi Shenoy, Herb Schwartz and Matthew Hill review the mechanical properties and advances in the development of polyethylene bearing surfaces for use in knees, discussing a new material with potentially improved mechanical and wear properties
Highly cross-linked polyethylene (HXLPE) has been and continues to be commonly used for liners in total hip arthroplasty (THA) due to its known ability to reduce wear during articulation. Its use in the tibial tray for total knee arthroplasty (TKA), however, has been much more controversial. Specifically, decreased mechanical properties and larger wear particles are concerning. A new micro-composite with hydrophilic properties, known as BioPoly®, may be able to offer an alternative material to offset some of the current concerns regarding HXLPE in total knee replacements (TKR).
History and properties of HXLPE
Before examining the use of HXLPE in the knee, it is important to understand the history and properties of the material as used for hip liners. Ultra-high molecular weight polyethylene (UHMWPE) was first introduced in the 1970s by John Charnley as a bearing material for THA. Although the material was successful, concerns eventually grew in regard to implant loosening due to osteolysis (bone loss surrounding an implant) in the 1990s. Polyethylene wear particles are the primary cause of osteolysis as they induce an autoimmune response that results in bone resorption. Osteolysis has now been shown to be one of the most common causes of revision for primary THA . In response to this problem, HXLPE was developed.
HXLPE has a proven capability both in vitro and in vivo to reduce polyethylene wear in hip liners. A recent literature-based systematic review found weighted average wear rates of 0.042mm/year (range: 0.002–0.08mm/year) and 0.137mm/year for HXPLE and conventional UHMWPE respectively, based on radiographic analysis . For comparison, it has been shown that osteolysis is unlikely to occur at wear rates below 0.1mm/year . Patients with these lower wearing HXLPE hip liners have an 87 per cent less likely incidence of osteolysis when compared with conventional UHMWPE liners . Despite the decreased wear and decreased incidence of osteolysis from HXLPE liners, there are still some questions regarding the clinical benefit of the technology. A recent randomised clinical trial  for THA (mean follow-up = 6.8 years) found significantly lower wear for HXLPE when compared with conventional UHMWPE (0.003mm/year vs 0.051mm/year), but clinical outcomes were not significantly different between the two different types of liners. Nevertheless, HXLPE has been able to consistently demonstrate decreased wear when compared with conventional UHMWPE, leading to its widespread use in hip liners for THA.
Although wear is reduced, the processing required for HXLPE components has significant deteriorative effects on the oxidative and mechanical properties of the material, potentially increasing the risk of fracture during articulation. Cross-linking of polyethylene requires the generation of free radicals (via gamma or e-beam irradiation, or chemically), which cause polymer chains to bond together in the amorphous phase of the material. Depending on the amount of cross-linking required, doses between 75 and 100kGy are typically used, with cross-linking density increasing with higher dosages. Free radicals, however, are also produced in the crystalline phase of the material, and these free radicals are unable to react. Therefore, these remaining free radicals are susceptible to oxidation in vivo, resulting in chain breakage and decreased mechanical properties  due to premature aging of the material. In order to minimise this effect, HXLPE is often thermally processed by either annealing (Stryker X3®) or re-melting (DePuy Marathon®, Zimmer Durasul®) after irradiation, to melt the crystalline phase and allow the previously trapped free radicals to react. The crystallinity of the material, however, is altered by this thermal treatment. Specifically, re-crystallisation of the material is hindered after thermal processing because of interference from the cross-links. This results in decreased mechanical properties for the material; however, the good news is that the negative effect on mechanical properties from thermal processing is not as large as the negative effect premature oxidation would have caused if the free radicals had been left unreacted. Even though these materials can be balanced for mechanical properties and oxidative resistance, they do not have the mechanical strength of conventional UHMWPE.
In response to these limitations, second-generation HXLPEs have incorporated antioxidants in an attempt to limit oxidation without the need for thermal processing – thus allowing similar mechanical properties to conventional UHMWPE. Specifically, current second-generation HXLPEs incorporate vitamin E by either diffusing (Biomet E1®) or blending (Zimmer Vivacit-E®) vitamin E with polyethylene. While these materials have been able to demonstrate reduced wear when compared with conventional UHMWPE, HXLPE blended with vitamin E (VitE poly) has also shown slightly inferior wear properties when compared with HXLPE without vitamin E . Generally, VitE poly has been shown to decrease cross-link density (resulting in increasing wear) with increasing concentration in blended vitamin E materials – to the point that concentrations above 0.3wt% are not able to achieve high levels of cross-linking, even at a gamma dose of 200kGy . VitE poly, however, has been shown to be effective at reducing the oxidative index and free radical concentration of irradiated polyethylene, even at concentrations as low as 0.05 per cent . This decrease in oxidation from VitE poly prevents the need for irradiated polyethylene to be thermally treated, resulting in significantly stronger mechanical properties [9,10]. The fatigue strength of the material is still less than conventional UHMWPE , but it has been shown that the fatigue strength of VitE poly is maintained after accelerated aging, unlike gamma-sterilised conventional UHMWPE . Overall, VitE poly demonstrates much better mechanical properties when compared with first-generation HXLPE, and although the wear rate is slightly worse – due to decreasing cross-link density – the material still wears much less than conventional UHMWPE.
HXLPE for TKA
While the improved wear characteristics of HXLPE and VitE poly are attractive for use in TKA, the decrease in mechanical properties when compared with conventional UHMWPE is concerning due to the complex and elevated stresses seen in knee replacements. Similar to THA, osteolysis is one of the main complications for TKA , and it is often the culprit for long-term failure of the device. Additionally, wear is even more of a problem for younger TKA patients who are prone to experiencing early onset osteolysis . HXLPE and VitE poly have consistently demonstrated reduced wear in knee simulator testing compared with conventional UHMWPE [14–16], but in vivo analysis of polyethylene wear for TKA is limited when compared with THA and has seen mixed results [17,18]. The magnitude and complexity of stresses seen in TKA also bring into question the use of HXLPE or VitE poly for use as a tibial tray. THA articulation occurs mostly as sliding motion in a ball-and-socket joint, while TKA articulation can occur as rolling, sliding, and rotating. The mechanism of wear between these two joints is different in that THA wear is mostly due to micro-adhesion and abrasion, while TKA wear can be due to delamination and pitting along with micro-adhesion and abrasion. This is confirmed by analysis of the wear particulates from THA and TKA, which has shown that THA wear particulates are submicron in size (<1µm) while TKA wear particulates can range from 2 to 20µm . It is not surprising then that the contact stress for tibial trays has been shown to be much larger than for hip liners , and it has even been seen in some studies that the contact stress during activities of daily living for TKA exceeds the yield strength of conventional UHMWPE . These higher forces combined with the reduced fatigue strength of HXLPE and VitE poly could potentially bring risks of tibial tray fracture for patients.
Despite the concerns regarding decreased mechanical properties, mid-term clinical outcomes of HXLPE bearings for TKA have been mostly encouraging, but a consistent clinical benefit compared with conventional UHMWPE has not been demonstrated. A recent analysis of the Kaiser Permanente Total Joint Replacement Registry found a minimal increase in revision incidence between HXLPE and conventional UHMWPE (3.1% vs 2.7%), and the authors concluded that the risks of revision between the two bearings were similar . This analysis, however, is in contrast with the most recent report from the Australian National Joint Replacement Registry, which found that HXLPE has a lower cumulative percentage revision than conventional polyethylene at five years (4.0% vs 2.6%) and ten years (5.8% vs 3.6%) . Three recent mid-term, randomised clinical trials (mean follow-up range: 2–5.9 years) comparing HXLPE and conventional UHMWPE bearings have been conducted, and they have all found no significant difference in clinical or radiological outcomes between the two bearings [24–26]. One recent, non-randomised clinical trial (minimum follow-up 4–5 years) found a significantly greater improvement in short-form 6D health-related quality of life for HXLPE bearings; however, other clinical outcomes were strongly trending but not significantly different between the two types of bearing . A partial reason for the lack of significantly different clinical outcomes for HXLPE is that there has yet to be long-term (>10 years follow-up) data compared with conventional UHMWPE; however, long-term outcomes could also reveal problems related to the decreased mechanical properties of HXLPE. Unfortunately, there have been very few, if any, documented clinical trials of VitE poly for TKA at this time. Overall, the current clinical outcomes of HXLPE are positive, especially in regards to a possible decrease in revision rate, but significant differences in clinical outcomes have not yet been found when compared with conventional UHMWPE bearings.
BioPoly for TKA
BioPoly offers itself as an alternative orthopaedic material with large potential due to its unique properties. A micro-composite of UHMWPE and hyaluronic acid (HyA), BioPoly utilises the unique properties of the HyA molecule already found in natural cartilage and synovial fluid to articulate with cartilage and reduce wear. Currently, BioPoly is used clinically in Europe as the articulating surface for focal partial resurfacing implants of the knee, patella and shoulder1. These implants are not currently FDA approved for use in the USA.
The BioPoly material is a hydrophilic polymer, allowing it to smoothly articulate with native cartilage without damaging itself or the opposing cartilage surface. The HyA in the material acts as a lubricating molecule in the UHMWPE backbone, reducing the friction and wear during articulation. BioPoly is manufactured by an ISO-13485 certified company (BioPoly LLC of Fort Wayne, Indiana, USA), and the company has shown through exhaustive biocompatibility testing that the material is safe for human use and does not have concerns of degradation or leaching of HyA in vivo.
For use in a tibial bearing application in TKR, BioPoly is a very interesting material alternative and may be able to change the amount and mechanism of polyethylene wear. Specifically, with HyA, the hydrophilic property of BioPoly may allow for nearly frictionless articulation of hydrophilic BioPoly with hydrophobic metal in a hydrophilic (synovial fluid) environment. This is unique in that a BioPoly tibial bearing would attract synovial fluid to its surface and allow it to act as a fluid ‘cushion’ of sorts to protect the tibial bearing during articulation. This extra protection with a fluid layer could help to significantly reduce the wear of the BioPoly surface, and the company claims that preliminary hip simulator testing with a cross-linked BioPoly material incorporating an antioxidant was able to reduce wear by 40 per cent when compared with HXLPE. While more testing is needed, this significant decrease in wear may be able to help reduce the incidence of osteolysis in THA and TKA patients – and further prolong the life of total joint replacements, which would be especially beneficial for younger patients.
BioPoly LLC is currently in the process of optimising its material for usage in total joint applications. They have already found that antioxidant integration is possible with the material (which could help with oxidative protection), and they are working to engineer the material for ultra-low wear but with strong mechanical properties. It is possible that a BioPoly total joint material could offer itself as an alternative to HXLPE in TKA and help reduce risks of fracture, improve implant lifespan and, ultimately, improve clinical outcome for patients.
For more information regarding BioPoly RS Partial Resurfacing Implants please visit the BioPoly website www.biopolyortho.com or the website of its global distributor OrthoD www.orthod.com.
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Dinesh Nathwani is a consultant orthopaedic surgeon and honorary senior clinical lecturer at Imperial College Healthcare NHS trust, London. He is involved in a phase II trial investigating the five-year outcome of BioPoly RS implant.
Alister Hart is a consultant orthopaedic surgeon specialising in hip and knee problems, and director of research at the Royal National Orthopaedic Hospital (RNOH) in Stanmore, London.
Ravi Shenoy is an SpR on the North East London Training Programme.
Herb Schwartz is the founder, president and CEO of BioPoly LLC.
Matthew Hill is a product development engineer at BioPoly LLC.