The CPE and XLPE were combined with 3 different femoral head bearing surfaces—ceramic, metal, and ceramicized metal—and XLPE was associated with a lower rate of revision compared with CPE in all 3 of these subgroups. XLPE was also associated with a lower revision rate in the subgroups defined according to the 3 common head sizes (<32, 32, and >32 mm; Table II) and 3 types of THA fixation (cemented, cementless, and hybrid [femur cemented]).
Six brands of acetabular prosthesis were available with both XLPE and CPE bearing options, were used in ≥800 procedures each, and were followed for ≥8 years. Five were associated with a reduced rate of revision at various time points when XLPE had been used, whereas the rate of revision of 1 prosthesis (Vitalock; Stryker) during the entire follow-up period did not differ significantly according to type of polyethylene used (Tables III and IV). Table V lists revision rates for the 10 most commonly used cementless prostheses with XLPE with a minimum 7-year follow-up (maximum, 15 years for 2 models).
According to the registry, 17,689 primary THAs using either CPE or XLPE had been performed for osteoarthritis in younger patients (<55 years of age), and the 15-year cumulative percent of revisions in this group was 17.4% (95% CI = 15.5% to 19.5%) for those treated with CPE and 6.6% (95% CI = 5.5% to 7.8%) for those who received XLPE. At 7 years, there was a 5-fold increase in the rate of revisions of procedures done with CPE compared with that of procedures performed with XLPE (HR = 5.32, p < 0.001) (Fig. 4).
To our knowledge, this study of the outcomes of the use of XLPE in THA for patients with osteoarthritis represents the longest follow-up of the largest number of procedures reported. The use of XLPE makes THA—already one of the most effective operations—even better. The data confirm the early promise shown by XLPE in RCTs—i.e., when compared with CPE, an XLPE bearing surface in THA results in a mid-term to longer-term reduction in all-cause revision. This is due to a reduced rate of revision due to loosening, lysis, and dislocation. The difference is seen both early (due to a reduced rate of revision for dislocation) and in the longer term (as a consequence of the reduction in wear-related problems).
A major strength of this study is the evaluation of the experience with XLPE in an entire national population and therefore the high external validity. Unlike the introduction of large-head metal-on-metal bearings, there was a phased introduction of XLPE, with initial laboratory testing followed by RCTs and then wider clinical use. This study completes the loop of introduction of new technology by reporting the use of XLPE recorded in a national registry with long-term follow-up. As a result, patients, surgeons, hospitals, and other health-care stakeholders can be confident that the use of XLPE will reduce the rate of revision surgery following THA.
Although our data demonstrated a significant reduction in revision overall with the use of XLPE, a study using pooled data from 6 registries, including the AOANJRR, did not demonstrate a reduced risk of revision19. That study was limited to cementless fixation with a standard 32-mm head in patients 45 to 64 years of age who had a shorter follow-up than the patients in our study. The inclusion criteria used in the previous study19 may explain the difference between its results and the data in our study, which included all patients in a national registry. The HR in the adjusted model in the previous study was in the same direction as ours, in favor of XLPE, but it did not reach significance. The rate of revision of THAs with XLPE in our study (4.4%) was higher than that in a recently published RCT by Devane et al.20 (1.9%) during an equivalent time period (10 years). However, in the RCT, the THAs were performed by experienced hip surgeons who used strict inclusion criteria, whereas our data involved all surgeons in Australia rather than just experienced hip surgeons. We therefore believe that our study has strong external validity and represents a “real world” revision rate.
We were able to adjust for sex, age, method of fixation, and femoral head size, all of which have been demonstrated to affect rates of revision of THA5,21-23. The registry showed that XLPE was associated with a lower rate of revision overall, but the observed difference could have been confounded by prosthesis type, with XLPE being used more often in well-performing prostheses. To account for known differences in prosthesis-specific revision rates, we performed a separate analysis of 6 prostheses that were available with both CPE and XLPE options for the same acetabular implant, and this analysis demonstrated lower rates of revision in association with XLPE in 5 of these models. As for the sixth model, the process for manufacturing the XLPE differs from that used by other companies, and this may account for the lack of observable difference between the XLPE and CPE. This has been previously noted with regard to the XLPE used by this manufacturer for total knee arthroplasty24.
The Nordic Arthroplasty Register Association (NARA) reported on design-specific differences between XLPE and CPE in THA25. With regard to cemented designs, XLPE versions of the ZCA (Zimmer) and Reflection (Smith & Nephew) all-polyethylene cups were found to have better survival than the CPE versions. This correlated with our analysis of the Reflection cup, but there were not enough XLPE ZCA cups for us to compare them with the CPE ZCA cups. With regard to all cementless cup designs, the XLPE shells in the NARA study had better overall survival (with all-cause revisions as the end point) than the CPE shells. Only 1 cementless cup, Trilogy (Zimmer), fulfilled the inclusion criteria for both types of polyethylene in the NARA study, and the revision rates did not differ between the XLPE and CPE versions of that cup. We could not analyze the Trilogy cup because there were not enough of them in the CPE group. The NARA database confirmed design-specific differences in polyethylene and suggested that their results should be confirmed in larger studies with longer follow-up.
The rate of revision of THAs with XLPE was lower than that of THAs with CPE from 6 months onward. This early benefit is not directly due to the better wear-related characteristics of XLPE, but it is an indirect consequence of them. The reason for this is that the use of larger femoral heads reduces the risk of revision due to dislocation, which is one of the most common reasons for early revision26. While the use of a larger femoral head with a CPE cup is known to increase the risk of long-term wear, this risk is not evident with XLPE. Thus, the introduction of XLPE enabled surgeons to selectively use larger femoral heads, thereby reducing revision for dislocation while avoiding the long-term wear problems associated with CPE.
The most common reason for long-term failure of THA requiring revision is loosening and osteolysis, and periprosthetic osteolysis is largely due to an inflammatory process caused by polyethylene wear27,28. While it has been suggested that particle-induced osteolysis is not a problem with XLPE29, there remains some concern about wear-related issues in the longer term, especially with the use of larger femoral heads. However, there have been few reports of osteolysis with the use of XLPE30-32. A systematic review demonstrated an 87% lower risk of osteolysis with XPLE than with CPE33. In our study, only 80 THAs (0.01%) with XLPE were revised because of lysis and 22 (0.004%) were revised because of wear of the acetabular insert, suggesting that these are not common medium to longer-term clinical problems with XLPE. There was also no evidence of increased mechanical failure with XLPE liners, with only 9 revisions for breakage of the acetabular insert.
Wear-related and implant-longevity issues are particularly important in younger patients, who are generally more active and have a longer life span than their older counterparts. There are now some longer-term reports of the clinical results of THA with XLPE in younger patients34-36, and these studies demonstrated no revisions for polyethylene wear or osteolysis. In light of these findings and our own analysis of 15,502 THAs with XLPE in younger patients, we believe that the evidence of reduced long-term wear with XLPE is now so strong that, when a polyethylene bearing surface is used in THA, it should be XLPE, particularly in younger patients.
There are some limitations with this analysis of registry data. The utilization of XLPE and CPE has changed over time. There is the potential for surgeon indication bias, but we believe that this would favor XLPE in younger patients. When the analysis was adjusted for age, particularly <55 years, the effect of XLPE was more marked. We do not believe that trends in surgical techniques, perioperative care, or rehabilitation protocols were significant confounding variables in either group. We also do not believe that newer implant design changes over this time period would have favored either group37.
While we adjusted for known risk factors that may have influenced the rate of wear-related revision of THA, there may be other such factors. We did not have information on body mass index (BMI) from the commencement of data collection, although the registry now collects those data. However, there is no evidence that BMI would have a more or less detrimental effect on the outcome of THA with XLPE or CPE. We also did not have information on activity levels, which have been shown to influence polyethylene wear rates38. Age is often regarded as a surrogate for activity, and we demonstrated the most reduction in revision rates in patients who were <55 years old. We believe that the XLPE is better than CPE in younger patients and those most likely to return to more active pursuits.
The use of XLPE has improved the outcomes of THA at 16 years with no observed untoward effects. The benefit is evident both early and late, with a reduced rate of revisions due to dislocation (because XLPE allows the increased use of larger femoral heads) and to wear-related issues. The evidence derived from our study suggests that the longevity of THA with XLPE is likely to be improved, compared with that of THA with CPE, beyond 16 years and may enable younger patients to undergo THA confident of a reduced need for revision in the long term.
Details of the statistical analysis are available with the online version of this article as a data supplement at jbjs.org (http://links.lww.com/JBJS/E819).
Note: The authors thank the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) and the hospitals, orthopaedic surgeons, and patients whose data made this work possible. The Australian Government funds the AOANJRR through the Department of Health and Ageing.
Investigation performed at the Australian Orthopaedic Association National Joint Replacement Registry, Adelaide, South Australia, Australia
A commentary by Robert S. Namba, MD, is linked to the online version of this article at jbjs.org.
Disclosure: The authors indicated that no external funding was received for any aspect of this work. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJS/E817).
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