Abstract
Abstract:
Between 1994 and 1999, 217 metal-on-metal total hip arthroplasties with a low-carbide bearing surface were performed with use of the cementless Zweymüller SL-Plus stem and the Bicon-Plus threaded cup in 194 consecutive patients. After a minimum follow-up of ten years, 181 living patients (203 hips) were available for evaluation. The revision rate after an average of twelve years was 18% (thirty-six hips in thirty-six patients were revised). The main reason for revision was aseptic loosening of one or both components. The probability of survival of the stem at fifteen years was 77% (95% confidence interval [CI], 65% to 86%). The probability of survival of the cup was 80% (95% CI, 62% to 90%). These high failure rates at mid-term follow-up led us to abandon the use of low-carbide metal-on-metal total hip arthroplasty components.
Level of Evidence:
Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.
Summary of Original Publication1
The Zweymüller-Plus total hip arthroplasty system (Smith & Nephew Orthopaedics, Rotkreuz, Switzerland) was introduced in 1993, as a successor of the Alloclassic total hip arthroplasty. It consists of the SL-Plus stem (a cementless, grit-blasted, rectangular tapered femoral component with three modifications in the conical stem shape) and a new biconical threaded cup with a spherical shape (Bicon-Plus cup) in contrast to the conical shape of the Alloclassic cup. The Zweymüller-Plus system can be implanted with ceramic-on-ceramic, ceramic-on-polyethylene, metal-on-polyethylene, or metal-on-metal (Sikomet) bearings. Sikomet SM21 is a low-carbide, wrought cobalt-chromium-molybdenum alloy.
Two hundred and seventeen Zweymüller-Plus total hip arthroplasties with a Sikomet SM21 metal-on-metal articulation and a 28-mm-diameter head were implanted, between 1994 and 1999, in 194 consecutive patients, and the results were retrospectively reviewed at a mean of seventy-seven months postoperatively. All procedures were performed by one of the authors (P.K.), and the mean age (and standard deviation) of the patients at the time of the index surgery was 55 ± 9 years (range, twenty-five to seventy years). All 194 patients (217 hips) were available for follow-up at a minimum of five years.
The mean Harris hip score2 improved from 45 points preoperatively to 88 points at the final evaluation. Periprosthetic osteolytic lesions around the stem were confined to the proximal part of the femur. There were nine stems with aseptic loosening, and all nine were associated with an osteolytic lesion of ≥2 mm. Two cups had aseptic loosening but were not associated with an osteolytic lesion. With the term aseptic loosening, we refer to the aseptic loss of biological fixation of the implants, as it is manifested both by pain clinically and on radiographs.
Fourteen hips (6.5%) were revised at an average of thirty-six months postoperatively. Nine were revised because of aseptic loosening; two, because of technical failure (malposition of an implant); and three, because of deep infection. Histologic examination of the retrieved periprosthetic tissues from the eleven patients who had undergone revision because of aseptic loosening or technical failure showed metallosis and extensive lymphocytic and plasma-cell infiltration around the metal debris. With removal of the component because of aseptic loosening as the end point, survivorship was 93% for the stem and 98% for the cup.
These findings supported the hypothesis that periprosthetic osteolysis and aseptic loosening in hips with a low-carbide metal-on-metal articulation are possibly associated with hypersensitivity to metal debris. The purpose of the present retrospective follow-up report was to extend the results of that series to a mean of twelve years (range, ten to sixteen years) postoperatively.
We received institutional review board approval for this ongoing follow-up evaluation of the results of Zweymüller-Plus total hip arthroplasty. Of the 194 patients (217 hips) in the original study, seven patients (eight hips) died less than ten years postoperatively with their prosthesis still in place. Six patients (six hips) were lost to follow-up, leaving 181 living patients (203 hips). Information on forty-eight patients (fifty-three hips) who were unable to attend our outpatient clinic was gathered with telephone interviews. The remaining 133 patients (150 hips) were available for clinical follow-up at a minimum of ten years. Eighty-five patients (109 hips) had radiographs made at least ten years postoperatively that were available for evaluation.
Clinical and Laboratory Assessment
The Harris hip score2 was used to clinically evaluate the 133 available patients (150 hips). Special emphasis was given to symptoms in the thigh or groin pain. Serum metallic ion (cobalt-chromium) levels were not determined, and a detailed analysis of volumetric and linear wear of articulating metal surfaces was not done.
Radiographic Assessment
Anteroposterior and frog-leg lateral radiographs of the hip were compared with previous radiographs and were analyzed for radiolucent lines, osteolysis, and migration of the components. The locations of radiolucent lines and osteolytic lesions around the femoral stem were reported in relation to the zones described by Gruen et al.3 and those adjacent to the cup, in relation to the zones described by DeLee and Charnley4. We estimated the size of the osteolytic lesion with use of the method of Zicat et al.5. Radiographic criteria for cup loosening included radiolucent lines extending around the entire cup as well as a progressive change in the cup position. We radiographically classified femoral implant stability of the nonrevised stems, according to the criteria of Engh et al.6, as stable through osseointegration, stable through fibrous tissue ingrowth, or as unstable. The acetabular component stability was radiographically assessed on the basis of the criteria of Hodgkinson et al.7 (i.e., migration or >1 mm of radiolucency in all DeLee and Charnley zones).
Statistical Analysis
Kaplan-Meier survivorship analysis8 was used to determine the survival rate (and 95% confidence interval [CI]) for each component of the total hip arthroplasty system, with revision for any cause or revision for aseptic loosening as the end points. The worst-case scenario for survival of this metal-on-metal total hip arthroplasty was also calculated, taking into account hips that had been revised as well as hips with a pending revision.
Survivorship was calculated at fifteen years postoperatively.
Source of Funding
The authors of the present study received no external funding resources.
The average follow-up time of all hips that were available for evaluation was twelve years (range, ten to sixteen years). The average Harris hip score (and standard deviation) at a mean follow-up of twelve years was 83 ± 6 points, slightly but not significantly lower than the average of 88 ± 4 points at seventy-seven months postoperatively.
Since the start of the study, a total of thirty-six hips (18%; thirty-six patients) were revised for any reason (see Appendix). The stem alone had been revised in thirteen hips; the cup, in five; and both components, in eighteen hips. The reason for revision was aseptic loosening in twenty-six hips, septic loosening in seven hips, and technical errors (malposition or dislocation) in three hips. Revision surgery was performed at an average of seven years (range, one week to fourteen years) postoperatively.
In comparison with our previous report on these patients1, the number of revised hips more than doubled. An additional twenty-two hips (twenty-two patients) had a revision: ten were revised because of aseptic loosening of both components; four, because of aseptic loosening of the SL-Plus stem; three, because of aseptic loosening of the Bicon-Plus cup; four, because of septic loosening of one or both components; and one, because of technical error. Moreover, thirteen hips (thirteen patients) had a pending revision, so the number of Zweymüller total hip arthroplasties that had failed (the number of revised hips and the number with a pending revision) since the start of the study increased to forty-nine (24%) of 203 hips. No difference regarding failure rate was found between male and female patients.
In all new revision cases, loose implants were easily removed and black-stained periprosthetic tissues, due to metallosis, were found. In stem revisions, the femoral cavity was sclerotic with bone defects and osteolysis (in one hip, the proximal part of the femur was absent because of severe osteolysis). We addressed these difficulties by using a combination of a long cementless revision stem (in seven hips), cerclage wires, and structural allograft (in ten hips). The removal of the acetabular cup was also associated with cavitary bone defects due to osteolysis, which necessitated structural bone allografts combined with a cemented roof reinforcement ring in ten hips to reinforce the acetabulum. In two hips with revision of both components, black-stained pseudotumors were noted intraoperatively and were removed. Conventional histologic study of these specimens showed extensive necrosis, fibrin exudation in the newly formed hip capsule, and a high degree of diffuse and perivascular lymphocytic infiltration.
The survivorship rate at fifteen years, with removal of any component for any cause as the end point, was 68% (95% CI, 50% to 80%) (see Appendix). When the end point was removal of any component for aseptic loosening (loss of biological fixation), the survivorship rate was 77% (95% CI, 59% to 88%) (see Appendix). The probability of survival of the stem, with revision for any reason as the end point, was 77% (95% CI, 65% to 86%) at fifteen years (see Appendix). The probability of survival of the acetabular component, with revision for any reason as the end point, was 80% (95% CI, 62% to 90%) (see Appendix). Kaplan-Meier curves with aseptic loosening of the stem or cup as the end points are shown in the Appendix.
According to the radiographic stability criteria of Engh et al.6 and Hodgkinson et al.7, eighty-eight (81%) of the 109 unrevised hips with available radiographs had radiographic evidence of bone ingrowth, and eight hips (7%) had stable fibrous fixation of one or both components; thus, a total of ninety-six total hip replacements (88%) were radiographically stable at the last evaluation. At an average of 11.68 years after surgery, thirteen hips (12%) were recorded as radiographically unstable because of evidence of aseptic loosening of the stem (nine hips), the cup (three hips), or both components (one hip). These hips make up our list of pending revision cases (see Appendix). When the pending revision cases were included in the analysis, the survivorship rate at fifteen years, with removal of any component for any cause as the end point, was 57% (95% CI, 41% to 69%) (Fig. 1).
A key finding of the previous report was that osteolytic areas were confined to the proximal part of the SL-Plus stem with an absence of osteolytic lesions around the Bicon-Plus cup. At a mean follow-up period of twelve years, the pattern of osteolytic areas around the stem continued to be the same. Thirty-two (29.4%) of 109 hips that had completed ten-year radiographic follow-up had focal osteolytic areas around the SL-Plus stem. The majority (75.9%) were located at proximal Gruen zones 1, 7, 8, and 14. Radiolucent lines of >2 mm were detected around the stem in thirty-five (32.1%) of 109 hips, and the distribution was also proximal (89% were in zone 1 and 68%, in zones 7 and 8).
In contrast to the findings in our previous report, twenty-nine (26.6%) of 109 stable hips had osteolytic lesions around the Bicon-Plus cup with no particular pattern of distribution. Three cups showed evidence of migration and are included in our pending revision cases. In two hips, the cup was not properly placed (one had an increased valgus position and one, suboptimal fixation), and as a result, these hips were revised. Apart from that, neither cup position nor their outside diameter made any difference with regard to failure rate. No impingement between the SL-Plus stem neck and the peripheral rim of the Bicon-Plus cup was observed intraoperatively.
Heterotopic ossification was observed in thirteen patients (fifteen hips) and was classified in ten of them as grade 3 or 4, according to the Brooker classification system9.
The mid-term revision and survival rates of the low-carbide (Sikomet) metal-on-metal Zweymüller-Plus total hip arthroplasty system are poor.
The survivorship rates, with removal of the components for any cause as the end point, were much lower at fifteen years (77% for the stem and 80% for the cup) than the corresponding survivorship rates at nine years (93% for the stem and 98% for the cup) and were worse than that expected on the basis of the data in our previous study1.
These findings must be viewed in light of the limitations inherent in our study design, since it was not a prospective randomized study and there was no control group with the same total hip arthroplasty components and different bearing surfaces. Moreover, we did not have the opportunity to measure serum metallic ion (cobalt-chromium) levels, nor volumetric and linear wear of articulating metal surfaces. Thus, no comparison regarding these parameters can be done with other studies on metal-on-metal total hip arthroplasties.
Recent studies10,11 that have described a high rate of survivorship and durability for the Zweymüller-Plus total hip arthroplasty components, when used with other than metal-on-metal bearing surfaces, support our suspicions that the low-carbide metal-on-metal surface—and not a design feature of this prosthesis— could be responsible for these poor results.
The mid-term survivorship rates of the Zweymüller-Plus components in our study are also substantially lower than those in studies of cementless total hip arthroplasties not involving the use of a low-carbide metal-on-metal bearing surface. Grübl et al.12, in a report on the Zweymüller Alloclassic system at fifteen years, noted a survivorship rate of 98% for the stem and 85% for the cup, with revision for any reason as the end point. Good results have also been reported for other cementless designs followed for a minimum of fifteen years13-15. Streit et al.16 reported a survival rate of 94% at twelve years, with revision for any reason as the end point, for a cementless total hip arthroplasty system with high-carbide (Metasul) metal-on-metal articulation. The same survival rate at a mean follow-up of twelve years was reported for Metasul metal-on-metal total hip arthroplasty by Saito et al.17.
In contrast to these good outcomes13-17, studies of low-carbide metal-on-metal articulation have described disappointing long-term results18,19. Increased revision rates and aseptic component loosening were also identified among patients who received modern metal-on-metal hip resurfacing in two review articles comparing modern metal-on-metal total hip resurfacing and standard total hip arthroplasty20,21.
On the basis of the evidence derived from our study, we speculate that the low-carbide metal alloy (Sikomet SM21) that was used in the metal-on-metal articulation of the Zweymüller-Plus total hip system with a 28-mm-diameter head plays a key role in the amount and type of metallosis produced, which then activates a delayed type of immunological hypersensitivity reaction, resulting in periprosthetic focal osteolysis, aseptic loosening, and final failure22,23. As a result, we suggest that physicians using this type of articulation should reevaluate their patients often to ensure an early diagnosis and treatment of any possible failure. We abandoned the use of the metal-on-metal Zweymüller-Plus total hip arthroplasty system in 2001.
Tables showing data on the revisions and pending revisions and figures demonstrating Kaplan-Meier curves are available with the online version of this article as a data supplement at jbjs.org.
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