Cross-Linked Polyethylene for Total Hip Arthroplasty: A Model for Evaluation of a Medical Device: Commentary on an article by R. de Steiger, MBBS, FRACS, FAOrthA, et al.: "Cross-Linked Polyethylene for Total Hip Arthroplasty Markedly Reduces Revision Surgery at 16 Years".

Abstract

Commentary The article by Professor de Steiger and associates describes an important study providing clinical confirmation of the promise of improved survivorship with highly cross-linked polyethylene (XLPE) liners in total hip arthroplasty (THA). The authors report the results of their survivorship analysis comparing 41,171 THAs performed with ultra-high molecular weight conventional polyethylene (CPE) and 199,131 performed with gamma, or electron beam, irradiated XLPE between 1999 and 2016. The 16-year cumulative percentage of revisions was 11.7% (95% confidence interval [CI] = 11.1% to 12.3%) for CPE compared with 6.2% (95% CI = 5.7% to 6.7%) for XLPE. The hazard ratio at 9 years was 3.02. Reduced in vitro wear of XLPE liners had previously been reported with preclinical testing1, but laboratory testing of earlier polyethylene enhancements, such as Hylamer, did not accurately predict clinical performance2. Early clinical experience with XLPE was encouraging as the devices were not identified as outliers in large national registries such as the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR). Reduced wear of XLPE liners in vivo was reported in radiostereometric analysis (RSA) studies3, resulting in further optimism for this bearing surface. Improved survivorship with XLPE liners was recently reported in a randomized controlled trial (RCT) with a 10-year follow-up that demonstrated a revision rate of 1.9% with Marathon liners compared with 14.6% with Enduron liners4. This was a single-center study of 122 THAs performed with cemented stems and cementless Duraloc cups by 2 experienced surgeons in 1 medical center. The Kaiser Permanente Total Joint Replacement Registry previously reported improved survivorship of XLPE liners compared with CPE liners5. The cumulative incidence of revision was 5.4% of 1,815 THAs with a CPE liner compared with 2.8% of 25,008 with an XLPE liner at 7 years; the hazard ratio for aseptic failure was 1.9. The study by de Steiger et al. utilized cases from the AOANJRR and expanded on this earlier report with longer follow-up, larger sample sizes, and a greater variety of hip implants. The AOANJRR was initiated in 1999, about the time that several highly cross-linked irradiated polyethylene products became commercially available. This timely coincidence allowed for a longitudinal clinical study of a large number of THA cases representing the gradual transition from CPE to XLPE. The registry quickly collected a large volume of cases performed by many surgeons, in many different hospitals, with a variety of implants. However, unlike prospective trials, in which interventions are performed concurrently, there are temporal differences between cohorts in registry analyses. At the beginning of the study period examined by de Steiger et al., most THAs were done with a CPE liner, whereas at the end of the study period most were performed with XLPE. During the 17-year study period, there may have been differences in patient selection, perioperative care protocols, and possibly surgeon experience. Although the authors speculated that any patient selection likely favored the use of XLPE in younger individuals, the opposite may be true because the study samples represented only 74% of the THAs performed for osteoarthritis. Until the AOANJRR formally recalled the ASR (Articular Surface Replacement) in 2009, surgeons may have chosen metal-on-metal or ceramic-on-ceramic bearings—not necessarily XLPE liners—for younger and more active patients. Despite this limitation, the subanalysis performed by de Steiger et al. showed a reduced risk of revision with XLPE cups even in patients under the age of 55 years. The strengths of this observational registry study include data that are generalizable to the “real world,” unlike the findings from an RCT, which may involve limited numbers of patients, few implant types, and limited numbers of surgeons in a single center. In the community, THA procedures are performed by a variety of surgeons with varying skills and specializations. Additionally, in the community, multiple implant combinations are utilized and procedures are performed in a variety of hospital settings. The story of XLPE liners is a good example of how new implants should be introduced and monitored: with careful preclinical testing, rigorous early clinical studies, widespread surveillance, and carefully performed mid-term clinical investigations. The current study is an important, and reassuring, contribution to the orthopaedic literature as XLPE has been widely used throughout the world in millions of patients.