Wear Rate of Highly Cross-Linked Polyethylene in Total Hip Arthroplasty: A Randomized Controlled Trial

McCalden, Richard W. MD, MPhil, FRCS(C); MacDonald, Steven J. MD, FRCS(C); Rorabeck, Cecil H. MD, FRCS(C); Bourne, Robert B. MD, FRCS(C); Chess, David G. MD, FRCS(C); Charron, Kory D. Dipl.MET

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.H.00244
Scientific Articles
Abstract

Background: Highly cross-linked polyethylene was introduced for clinical use in total hip arthroplasty with the expectation that it would exhibit less wear when compared with conventional polyethylene. The purpose of this study was to report the clinical and radiographic results, after a minimum of five years of follow-up, of a randomized, blinded, controlled trial comparing a conventional polyethylene with a first-generation highly cross-linked polyethylene.

Methods: One hundred patients were enrolled in a prospective, randomized controlled study comparing highly cross-linked and conventional polyethylene acetabular liners in total hip arthroplasty. Fifty patients were in each group. At the time of follow-up, clinical outcomes were assessed and steady-state femoral head penetration rates (after bedding-in) for each patient were calculated with use of a validated radiographic technique. In addition, a statistical comparison of polyethylene wear between groups was performed with use of generalized estimating equations.

Results: At a mean of 6.8 years postoperatively, there were no differences between the two polyethylene groups with regard to the Harris hip score, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), or Short Form-12 (SF-12) score. The mean femoral head penetration rate in the first through fifth years was found to be significantly lower in the group treated with the highly cross-linked polyethylene (0.003 mm/yr [95% confidence interval, ±0.027]) than it was in the group treated with conventional polyethylene (0.051 mm/yr [95% confidence interval, ±0.022]) (p = 0.006). Men treated with a conventional polyethylene liner had a significantly higher (p ≤ 0.012) femoral head penetration rate (0.081 mm/yr [95% confidence interval, ±0.065]) than both men and women with a highly cross-linked liner (−0.013 mm/yr [95% confidence interval, ±0.074] and 0.009 mm/yr [95% confidence interval, ±0.028], respectively). The general estimating equations demonstrated that the group with a highly cross-linked polyethylene liner had a significantly lower femoral head penetration rate than the group with a conventional polyethylene liner (p = 0.025), and a significantly higher femoral head penetration rate was demonstrated in men with a conventional polyethylene liner when compared with both men and women with a highly cross-linked liner (p = 0.003).

Conclusions: At a minimum of five years postoperatively, the steady-state femoral head penetration rate associated with this first-generation highly cross-linked polyethylene liner was significantly lower than that associated with a conventional polyethylene liner. Long-term follow-up is required to demonstrate the clinical benefit of this new material.

Level of Evidence: Therapeutic Level I. See Instructions to Authors for a complete description of levels of evidence.

Author Information

1Division of Orthopaedic Surgery, London Health Sciences Centre, University Campus, 339 Windermere Road, London, ON N6A 5A5, Canada. E-mail address for R.W. McCalden: richard.mccalden@lhsc.on.ca

2The Hand and Upper Limb Centre, St. Joseph's Health Centre, 268 Grosvenor Street, Suite D2-167, London, ON N6A 4L6, Canada

Article Outline

Although total hip replacements have been extremely successful, their survivorship has been limited by aseptic loosening and osteolysis secondary to wear and particulate polyethylene debris1-3. In response to the problem of polyethylene wear debris, efforts have been made to alter ultra-high molecular weight polyethylene to improve its wear performance. In the late 1990s, techniques were developed to create a more highly cross-linked polyethylene, and these resulted in substantial in vitro improvements in wear properties4-7. Soon thereafter, as a result of this laboratory evidence, highly cross-linked polyethylene was widely adopted despite very little clinical or in vivo radiographic evidence supporting its use.

In 1999, our institution began a prospective, randomized, blinded, controlled study to compare the clinical results and wear performance of highly cross-linked polyethylene acetabular liners with those of conventional polyethylene acetabular liners in total hip arthroplasty. We hypothesized that there would be no difference in clinical outcome scores between the cohorts and that a significant reduction in femoral head penetration would be seen in association with the highly cross-linked polyethylene. In addition, we sought to examine these patients at the time of midterm (five to ten-year) and long-term (more than ten-year) follow-up to demonstrate any advantages or potentially deleterious effects of highly cross-linked polyethylene.

The purpose of this study was to evaluate the clinical, radiographic, and wear-performance results at the time of midterm follow-up (at a minimum of five years) in a randomized, blinded, controlled trial comparing conventional with highly cross-linked polyethylene. In particular, we report the steady-state femoral head penetration rates (after bedding-in) in these two groups of patients.

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Materials and Methods

Between November 1999 and October 2001, 100 consecutive patients were enrolled, after providing written consent, in a randomized controlled trial that had been approved by the hospital ethics board. To be included in the study, a patient had to have degenerative arthritis of one hip requiring total hip arthroplasty, a designation of A or B according to the Charnley hip classification8, and an age between forty and seventy-nine years. Exclusion criteria were preexisting bone disease (such as severe osteoporosis or osteomalacia), systemic conditions affecting bone density (such as inflammatory arthritis or renal disease), and a contralateral revision or poorly functioning total hip replacement. Patient demographics and follow-up data are summarized in Table I. There were no apparent differences between the groups with regard to those parameters.

All patients received the identical hybrid total hip replacement consisting of a cemented collared femoral stem (VerSys; Zimmer, Warsaw, Indiana) and cementless trispiked acetabular component (Trilogy; Zimmer). At the time of surgery, the patients were randomized to receive either a conventional polyethylene liner (Trilogy; Zimmer) or a highly cross-linked polyethylene liner (Longevity; Zimmer). We defined conventional polyethylene as a polyethylene for which no formal cross-linking or free-radical-reduction process had been used, although there may be some degree of cross-linking in the material as a result of the sterilization process. The conventional polyethylene liners used in this study were made of calcium-stearate-free GUR 1050 resin machined from compression-molded sheet polyethylene. The final implant was then sterilized with gamma radiation (25 kGy) in an inert nitrogen environment. To manufacture the highly cross-linked polyethylene liners, calcium-stearate-free GUR 1050 resin was also utilized to create compression-molded sheets, which were then machined into the final implant geometry. Prior to machining, the compression-molded sheet was subjected to a process consisting of e-beam irradiation of 10 Mrad (100 kGy) to achieve the desired level of cross-linking with melt-annealing for free-radical reduction. The final liners were then sterilized with use of a gas-plasma technique. All liners had a 28-mm inner diameter and a 10° lip. The acetabular components had an outer diameter ranging from 48 to 58 mm. The surgery was performed, through a modified lateral approach, at a teaching institution by five experienced hip surgeons, each of whom had performed more than 100 total hip replacements per year.

The patients were followed preoperatively and postoperatively by a blinded study nurse who measured clinical performance with a number of different validated clinical outcome tools, including the Harris hip score9, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)10, and Short Form-12 (SF-12)11. The patients were seen at six weeks, at three months, and then at yearly intervals. Radiographs were made at six weeks postoperatively and yearly thereafter. Two patients, one from each group, were lost to follow-up at approximately three years postoperatively. Both patients had been placed in a nursing home and their families refused to have them return for follow-up. At the time of writing, nine study patients had died of causes unrelated to the index procedure. One revision, due to late deep infection, was done at 5.5 years postoperatively. There were no other reoperations, complications, or failures. Therefore, the study outcomes were known for ninety-eight of the original 100 patients.

Polyethylene wear rates were measured on anteroposterior pelvic radiographs at six weeks postoperatively and at yearly intervals with use of a validated computer-assisted edge-detection method (Hip Analysis Suite, version 8.0.3.0; University of Chicago, Chicago, Illinois)12. Radiographic analysis was performed by a single individual with considerable expertise with the technique who was blinded to the type of polyethylene that had been used for each patient. Postoperative radiographs were excluded from the analysis if they did not meet the strict criteria previously reported in the literature12-15. Specifically, radiographs on which the metallic shell obscured the femoral head such that a clear delineation between the femoral head and the acetabular cup was not visible could not be used for the computer analysis16. A six-week postoperative radiograph as well as one made at a minimum of two years postoperatively were deemed necessary for inclusion of a patient into the study in order to properly calculate a regression line representing the penetration rate.

We collected 94% (520) of the 556 possible radiographs (i.e., excluding those not obtained because of the patient's death or loss to follow-up). Twenty-six additional radiographs were eliminated from the analysis because of poor radiographic quality or failure to meet the selection criteria; this left 494 radiographs to be analyzed with the software. Evaluation with Hip Analysis Suite12,17 resulted in elimination of additional radiographs from the final analysis, so that 78% (385) of the 494 analyzed radiographs were included in the final analysis. Every patient included in the analysis had a minimum of three postoperative radiographs available. The average completeness of the radiographic collection (including the postoperative radiograph and those made at each of the five years of follow-up) per patient included in the final analysis was 81% (range, 67% to 100%).

A linear regression analysis was performed with use of the linear penetration measurements in each hip over time to yield a specific steady-state penetration rate for each hip18-20. The steady-state penetration rate for each polyethylene group (highly cross-linked and conventional polyethylene) was calculated as the average of the individual patients' penetration rates within that group. Analysis was performed by using the six-week postoperative radiograph as the baseline and fitting regression lines through the one through five-year follow-up points (the steady-state wear rate beginning at one year) and through the two through five-year follow-up points (the steady-state wear rate beginning at two years). In addition, an analysis based on the sex of the patient was performed to determine if there were any sex-related differences between the cohorts.

We attempted to identify femoral and acetabular osteolytic lesions by comparing the most recent follow-up anteroposterior pelvic radiograph with the perioperative views. The location of any femoral osteolytic lesion was classified according to the zones of Gruen et al.21 and the location of any osteolytic lesion in the pelvis was classified according to the zones defined by DeLee and Charnley22. Osteolytic lesions were noted to be present or absent, but their size was not measured.

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Statistical Methods

The sample size was determined on the basis of an overall alpha error (two-sided) of 0.05 and a statistical power of 80% (beta error = 0.20). As our primary outcome measure was polyethylene wear, we assumed a conservative decrease of 20% in the femoral head penetration rate in the group with the highly cross-linked polyethylene compared with that in the control group. This gave us a sample size of thirty-six patients per group, which was increased to fifty patients to allow for withdrawal and losses that would occur over the period of follow-up.

The demographic variables and clinical outcome measures were all continuous variables, and the statistical comparison of the polyethylene groups was carried out with the one-sample Kolmogorov-Smirnov test for normal distribution and subsequently with the independent-samples t test. Comparisons between preoperative and latest outcome measures for each group were performed with the paired-samples t test. A p value of ≤0.05 was considered to be significant for all statistical tests.

To compare the steady-state femoral head penetration rate between groups, the linear regressions of the patients within each polyethylene group or each group categorized according to both the type of polyethylene and the sex of the patient (polyethylene/sex group) were tested with use of the one-sample Kolmogorov-Smirnov test for normal distribution. Subsequently, the comparison of the polyethylene groups was carried out with the independent-samples t test and the comparison of the polyethylene/sex groups was carried out with one-way analysis of variance with least significant difference as the post hoc method. In addition, we employed general linear regression with the generalized estimating equation approach to better analyze longitudinal data that consist of repeated observations23.

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Source of Funding

Financial support for this study was provided by Zimmer, Inc. (Warsaw, Indiana). The funding was used to support the salary of a research nurse who enrolled patients and gathered preoperative and postoperative clinical outcome data. In addition, the funding supported the salary of a research technician who performed the radiographic image analysis.

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Results

A significant clinical improvement was found in both groups when preoperative outcome scores were compared with postoperative scores (Table II). However, there was no difference in any of the clinical outcome scores between the groups, which had virtually identical Harris hip, WOMAC, and SF-12 scores (Table II).

On radiographic analysis, there was a significant difference in the steady-state femoral head penetration rate between the group treated with the highly cross-linked polyethylene and that treated with the conventional polyethylene (p = 0.006) (Figs. 1 and 2 and Table III). The steady-state penetration rate was found to be extremely low in the highly cross-linked polyethylene group on the basis of both one through five-year and two through five-year baseline linear regression data (mean, 0.003 mm/yr [95% confidence interval, ±0.027] and −0.029 mm/yr [95% confidence interval, ±0.032], respectively) (Table III). The sex-based analysis, with use of both one through five-year and two through five-year baseline regression data, showed a higher femoral head penetration rate in the men treated with the conventional polyethylene (mean, 0.081 mm/yr [95% confidence interval, ±0.065]) compared with both the men and the women treated with the highly cross-linked polyethylene. A summary of these sex-related results is provided in Table III and Figure 3. Box plots of head penetration data, based on the polyethylene type and the sex of the patient, are provided to illustrate any differences in the distribution and the number of outliers within the groups (Figs. 2 and 3). There was no evidence of osteolysis on radiographic follow-up.

The generalized estimating equations revealed a significant interaction between the effects of the polyethylene type and the sex of the patient on femoral head penetration. Generalized estimating equations demonstrated that cross-linked polyethylene had a significantly lower wear rate than conventional polyethylene (p = 0.025). The effect of time (a reflection of the wear rate) was significant. Thus, at the initial (one-year) follow-up evaluation, the polyethylene groups were not significantly different (p = 0.219), but there was a highly significant difference between the groups at the later follow-up points (p = 0.009).

Figure. No caption a...
Figure. No caption a...

The generalized estimating equation analysis of the effect of the sex of the patient on femoral head penetration rates revealed a significant difference among the four polyethylene groups (men and women treated with conventional polyethylene and men and women treated with highly cross-linked polyethylene). In particular, a significant difference was found between men and women treated with conventional polyethylene (p = 0.014) but not between men and women treated with cross-linked polyethylene (p = 0.604). In addition, the polyethylene type had a strong effect in the male cohort but not in the female cohort—that is, the femoral head penetration rate in the male group treated with the highly cross-linked polyethylene was significantly lower than that in the male group treated with the conventional polyethylene (p = 0.003), but no significant difference was found between the female group treated with the conventional polyethylene and the female group treated with the highly cross-linked polyethylene (p = 0.091).

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Discussion

This randomized controlled trial demonstrated that, after a minimum of five years of follow-up, there were no clinical differences between patients treated with an acetabular liner made of conventional polyethylene and those treated with a cross-linked polyethylene liner. This finding was not unexpected as it is likely that the clinical benefits of highly cross-linked polyethylene, if they exist, would not be apparent until ten years postoperatively and beyond. Our findings are similar to those of other studies comparing the early and midterm clinical results between patients treated with highly cross-linked polyethylene and those treated with conventional polyethylene18,24-29. We did not find any significant difference between cohorts with regard to radiographic changes or the presence of osteolysis. Once again, it is unlikely that any difference, if one exists, would be seen at the time of midterm follow-up, especially since both cohorts had average wear rates that were well below the so-called “osteolysis threshold” of 0.1 mm/yr as proposed by Dumbleton et al.30. Interestingly, Leung et al.31 recently demonstrated that patients treated with a highly cross-linked polyethylene acetabular liner had a decreased incidence and volume of pelvic osteolysis, as shown by computed tomography, compared with patients treated with a conventional liner.

In our clinical trial, steady-state femoral head penetration rates were significantly lower in the group treated with the highly cross-linked polyethylene than they were in the group treated with the conventional polyethylene. The steady-state head penetration rate in the highly cross-linked polyethylene group was 0.003 mm/yr (95% confidence interval, −0.024 to 0.030), representing a reduction in head penetration of >95% as compared with the conventional polyethylene group. This very low wear rate is likely a result of the substantial number of negative wear rates seen in the highly cross-linked polyethylene group, as indicated by the negative lower bound of the 95% confidence interval. The low wear rate and the prevalence of negative wear rates compare well with the findings in a number of recent series in which the steady-state wear of highly cross-linked polyethylene was examined13,14,18,24,32-34. It is important to note that while the present study showed a trend suggesting decreased wear of highly cross-linked polyethylene at two to three years postoperatively, it was only after five years of follow-up, when steady-state rates could be calculated, that we were able to detect a difference between the cohorts at a significance level below 0.05. This finding is consistent with the fact that a number of earlier investigators who reported on total head penetration, and therefore did not take into account the bedding-in process, generally observed little improvement with highly cross-linked polyethylene as compared with conventional polyethylene25-28,35-38. Our study supports the concept that more than five years in vivo is required to show the true difference between the mean wear rates of these materials39.

We believe that the use of generalized estimating equations is the most appropriate method of analysis for this type of study23. We contend that it may be inappropriate to represent the wear rate by creating a regression line for the entire polyethylene group (containing all data points at various time intervals for all patients), as was done in previous studies13,14,16,17,40, because all data points within the group are not independent of one another23,41-44. Similarly, although the method of averaging individual patient regressions (i.e., the regression lines representing the wear rates for each patient)18,24 is a reasonable way to provide a visual comparison between groups, the method may be problematic statistically as it does not account for each patient's five data points (i.e., one for each year of follow-up) but instead applies statistical testing of the interpolated line of best fit41,43. In comparison, statistical analysis with generalized estimating equations accounts for the potential dependence between the related observations23,42,44. This issue is applicable to our data since repeated measurements were performed for each patient at various time intervals. A major strength of generalized estimating equations as compared with other repeated-measures approaches, such as analysis of variance, is that the generalized estimating equation model allows inclusion of several covariates (in our case, sex, polyethylene group, and time) whereas analysis of variance analysis allows only one explanatory variable. Therefore, the generalized estimating equation approach was used to determine if there were statistical differences between polyethylene groups and between polyethylene/sex groups.

To our knowledge, we are the first to examine the effect of the sex of the patient on the steady-state rate of femoral head penetration into highly cross-linked polyethylene. All analysis methods demonstrated significantly higher wear rates in male patients who had conventional polyethylene than in either male or female patients treated with highly cross-linked polyethylene. These findings are in agreement with the previous observations by Schmalzried et al.45, who demonstrated that polyethylene wear was related to use and was higher in male patients. While our data support this concept, with the relatively small numbers and the limitations of the edge-detection technique, one might still question the validity of this finding. A larger study with utilization of more precise tools, such as radiostereometric analysis, is probably required to confirm the effect of the patient's sex on wear of these new highly cross-linked polymers.

While this prospective, randomized, controlled trial provides Level-I evidence to support the concept that first-generation highly cross-linked polyethylene is associated with decreased femoral head penetration at the time of midterm follow-up, the study had several potential limitations. First, the numbers of patients were not large, and, although they were sufficient to demonstrate a clear difference in head penetration rates, the study was likely underpowered to show more subtle differences in clinical performance or the prevalence of osteolysis. Also, the addition of more sophisticated tools, such as computed tomography, is probably required to accurately assess osteolysis. Of note, the decision to use a cemented stem for the femoral reconstruction led to a tendency for older patients (those with Dorr type-B or C femoral bone46) to be enrolled in the study, which limited our follow-up (nine deaths occurred prior to the five-year follow-up point). In addition, we had no detailed activity scores with which to demonstrate the potential lower activity of these elderly patients, which may in turn have influenced their wear rates45.

Our decision to use edge-detection techniques as our primary wear-measurement tool is potentially problematic. There remains little doubt that the steady-state wear rate of highly cross-linked polyethylene is below the limit of detection of this technique. This is evidenced by the substantial number of patients who demonstrated negative wear values. This phenomenon of negative wear has been noted by several authors and there is no clear consensus on how to deal with it13,14,18,39. Lastly, because of image-quality issues, only approximately 78% of all available radiographs could be used for the analysis. While this is potentially worrisome, it is typical of the inclusion and exclusion rates in other investigations involving use of this technique12-15. Future work involving the prospective evaluation of these low-wear materials probably requires the use of radiostereometric analysis, the accuracy and precision of which exceed those of conventional edge-detection techniques32,39.

In summary, this prospective, randomized, controlled trial, which had a 98% rate of clinical follow-up and was performed at a single center, demonstrated, after the longest average duration of follow-up reported in this type of study in the literature, a clear decrease in femoral head penetration with the use of this first-generation highly cross-linked polyethylene. Our data confirm our initial hypothesis and strongly support the earlier laboratory and in vitro work4-7 that predicted a marked improvement in wear with this highly cross-linked polymer. Longer follow-up is required to determine if this will translate into improved clinical performance and longevity of these implants. To confound the issue further, because of concerns about ongoing oxidation47-50 and/or mechanical changes51-54, several manufacturers have now produced second-generation highly cross-linked polyethylene, which is already in clinical use55,56. These new materials should undergo the same rigorous in vivo testing to confirm their benefit.

NOTE: The authors thank Dr. John Martell for his help and guidance in the analysis of the head penetration data and Dr. Jan Brandt for his guidance regarding the interpretation of the individual patient regression analyses and correlated statistics. They also acknowledge the Biostatistics Consulting Unit at the University of Western Ontario for conducting the advanced statistical analyses utilized in this study.

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Zimmer, Inc., Warsaw, Indiana. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Zimmer, Inc.). Also, a commercial entity (Zimmer, Inc.) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our quarterly CD-ROM/DVD (call our subscription department, at 781-449-9780, to order the CD-ROM or DVD).

Investigation performed at University Hospital, University of Western Ontario, London, Ontario, Canada

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