The results of the radiographic analysis of all patients in the three studies revealed no indication of periprosthetic osteolysis around the acetabular or femoral components of any patient. None of the HXLPE components showed radiographic loosening, failure, or fracture. In addition, there are no reports from any of the centers of cases in which revisions were performed as a result of polyethylene wear in this study population. In contrast, in radiographic analysis of the 10-year followup films of the noncrosslinked polyethylene control group, the overall incidence of periprosthetic osteolysis was 13.5%, acetabular osteolysis was 3.75%, femoral osteolysis 11.90%, and osteolysis on both sides 2.50%. Furthermore, 22 of these patients (13.75%) have had revision surgery for reasons related to polyethylene wear and osteolysis after this 10-year radiographic evaluation.
The primary limitation of this study, like with any long-term observational study, is in getting an increasingly aging group of patients to return for routine office visits. This raises concerns of study bias and calls into question whether the results are generalizable and indicative of what can be expected in the larger population of patients undergoing THA. These concerns are especially relevant in single-surgeon or single-center studies such as Study 1 of this article. We have attempted to minimize such concerns by collecting data from a large number of medical centers across the United States and Europe with a large number of surgeons contributing patient radiographs for analysis. Another limitation of this study relates to the difficulty in accurately measuring small displacements of the femoral head into the acetabular component from plain radiographs taken over a number of years. There is an inherent uncertainty in such measurements, which is compounded by variability in image quality. This limitation was addressed by assembling a large number of patient radiographs having multiple observations over time. It was further minimized by having a control cohort with polyethylene known to have a well-established average wear rate for comparison. Another limitation of this study is the lack of contemporary controls or randomization between conventional and HXLPE. However, Weinstein et al. , in a Spine Patient Outcomes Research Trial (SPORT) study, showed no result difference in cohorts randomized to different treatment modalities compared with the observational cohort exposed to the same treatment panorama.
The single-center prospective registry series (Study 1) address the issues of long-term wear performance and the possible effect of using a new technology on the long-term outcome of patients undergoing THA as judged by patient-reported outcome measures. We could not demonstrate an increase in the femoral head penetration over time in patients having conventional sized femoral heads (28-32 mm) in either the minimum 7-year followup group or the smaller minimum 10-year group. This is the largest and longest term single-center report on the clinical performance of this form of HXLPE. The patient-reported outcomes are in line with what one would expect of patients undergoing contemporary primary THA and may indicate that this patient group is representative of the current state of the art.
The multicenter study of patients undergoing primary THA, Study 2, of patients with conventional femoral head sizes of 28 and 32 mm, was stimulated by a preliminary report from a radiostereometric analysis center in Sweden, which indicated that there may be an increase in the rate of femoral head penetration after 5 years of in vivo use . If true, this could suggest some type of change had occurred in the polyethylene. The study design of this multicenter study allowed for the assessment of wear rates at an early time period, up to 5 years from surgery, and a late time period after 5 years from surgery. This detailed evaluation of polyethylene performance is the first of its kind. This study provides the strongest data available indicating that the extremely low in vivo wear rates observed with this type of HXLPE remains unchanged over time.
The second multicenter study, Study 3, focused on the radiographic outcome and wear analysis of primary THA using femoral head diameters greater than 32 mm to determine if the wear of the HXLPE was affected with the use of larger diameter femoral heads. Unlike in the first two studies, the results from the three statistical methods used were not in agreement (Tables 5,6). Although no differences could be demonstrated relating to femoral head diameter in Studies 1 and 2, which compared 28- and 32-mm heads, in Study 3, which combined 28- and 32-mm heads against 36-mm heads, one technique of data analysis, namely the first to last method, indicated an increase in both linear and volumetric wear among the 36-mm head size group. However, the other two data analysis techniques did not demonstrate a difference in either linear or volumetric wear based on femoral head diameter. This points out the strengths and weaknesses of each method. The strength of the first to last method is that it uses the maximum number of patients in the statistical analysis, whereas its weakness is that it does not use any radiographic measurement between the first year and latest followup. The strength of the group regressions method is that it uses all of the radiographic measurements available and its weakness is that the data cannot be used in multivariate analysis and it violates the rule that each wear observation should be treated independently. The strength of the individual regression method is that it treats each patient as an independent variable and the data can be used in a multivariate analysis, whereas its weakness is that many patients are excluded from the analysis as a result of the fact that each patient needs at least three radiographic measurements to create a slope of the linear regression line. Although the individual regression method is the most valid statistical approach, its use is best suited for very large populations with multiple intermediate observations. Therefore, in assessing radiographic wear data, because of these differing strengths and weaknesses, we continue to advocate the use of all three analytical methods. This approach allows other researchers to compare their data with this report in various ways depending on the nature of their followup. Although this study cannot conclusively determine if the wear rate of this material is affected by the use of large-diameter head sizes, the differences reported here are small and the wear is below the generally accepted threshold for producing periprosthetic osteolysis [22, 24].
The studies presented here indicate that the use of the formulation of HXLPE analyzed in these studies has resulted in a dramatic decrease in the rate of polyethylene wear in patients followed as long as 13 years after primary THA. In comparison with historical controls using conventional UHMWPE, the incidence of periprosthetic osteolysis resulting from polyethylene wear has been eliminated. Furthermore, although there are a few reports of using CT scans to identify extremely small lytic areas adjacent to the implants that cannot be seen on a plain radiographs , there have been no reports of clinical importance of periprosthetic osteolysis with the use of any form of HXLPE worldwide. Although other innovations that have been introduced to address periprosthetic osteolysis resulting from wear debris such as metal-on-metal and ceramic-on-ceramic bearings have resulted in the appearance of new complications such as pseudotumors and squeaking [12, 34, 48-50], the modification to the existing polyethylene bearing material by adding crosslinks has not resulted in any unforeseen deleterious effects.
The stepwise approach to introducing new medical technology outlined by Malchau  was modified for the introduction of HXLPE and has been discussed by Malchau et al. . Several additional outcome studies including randomized radiostereometric analysis evaluation were also started when electron beam and melted HXLPE was first made available for clinical use. These studies were conducted in coordination with our laboratory and others to provide early clinical performance data on this new bearing material [4, 7, 19, 20]. The data from these early studies indicated no adverse effects with the use of this new material and confirmed the preclinical in vitro wear data. These early reports were presented in major national and international orthopaedic scientific venues before their publication. As these early positive scientific findings were made public, larger diameter femoral heads greater than 32 mm were introduced for clinical use, which triggered other early clinical studies of patients receiving these larger diameter femoral heads . This modified stepwise approach balanced the caution of using new technology with the desire to use a material that had the potential of drastically reducing the incidence of the major cause of late-term failure of THA, particle-induced periprosthetic osteolysis.
This series of clinical and radiographic studies of 768 patients is the largest study of patients undergoing primary THA having one formulation of HXLPE. We found an increase in linear and volumetric wear with femoral head diameters of 36 mm by using one statistical method; however, the magnitude of this increase in wear was small and to date not clinically important because no osteolysis was found in this cohort. This finding was not confirmed using the other two statistical methods. Although continued longer-term studies are warranted, the successful application of crosslinking technology has all but eliminated the incidence of polyethylene particle-induced periprosthetic osteolysis in THA.
We thank Christopher Barr for his assistance in data analysis and data presentation assistance.
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