Abstract
In 2002 and 2006, we reported the long-term results of 208 total hip replacements performed with the Zweymüller stem and a threaded cup in 200 patients. The present study gives an update on this patient cohort. At a minimum of twenty years postoperatively, seventy-three patients (seventy-five hips) were available for follow-up; twelve patients were lost to follow-up. The key findings of our previous reports were the absence of aseptic femoral stem loosening and a poor rate of survival of the threaded cup. Since then, two revisions have been performed because of aseptic stem loosening. We observed osteolytic lesions around the proximal part of the femoral component on twenty-four (47%) of fifty-one radiographs, but no stem was deemed at risk for loosening. The probability of survival of the stem at twenty years was 0.96 (95% confidence interval, 0.91 to 0.99), and the probability of survival of the cup at twenty years was 0.67 (95% confidence interval, 0.57 to 0.75). The Zweymüller femoral stem, a tapered, rectangular implant, continues to give excellent long-term results.
Level of Evidence:
Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.
Two hundred and eight total hip arthroplasties were performed in 200 consecutive, unselected patients with an average age of sixty-one years (range, twenty-two to eighty-four years) with use of a tapered, rectangular titanium stem (Alloclassic Zweymüller; Zimmer, Winterthur, Switzerland) and a conical screw cup (CSF; Zimmer) between October 1986 and November 19871.
The metal-backed articulating surface of the cup was made of ultra-high molecular weight polyethylene, gamma sterilized in an inert gas atmosphere. All femoral heads were made of alumina-oxide (Sulox; Feldmuehle Nobel, Stuttgart, Germany).
Primary stability of the femoral component is achieved by precision rasping and press-fit implantation. This implant continues to be used, which allows for a meaningful long-term follow-up.
At the fifteen-year follow-up, 102 of the original 200 patients were available for follow-up and eighty-seven patients (ninety-two hips) were examined clinically. Eighty-four patients (eighty-eight hips) had radiographs available. The main finding in the original report was the low rate of femoral stem revision. Eighty-three of the 102 patients (eighty-nine of 108 hips) had the primary joint replacement still intact. In the time period between the beginning of the original study and the fifteen-year follow-up, twenty-two hips were revised for any reason. None of the revisions were done because of aseptic stem loosening, but one stem was loose according to radiographic criteria. The probability of survival of the stem with revision for any reason as the end point was 0.98 at fifteen years. The probability of survival of the threaded cup was 0.85. The single most important reason for revision was excessive polyethylene wear, followed by aseptic cup loosening2.
In the present report, we extend the follow-up period to a minimum of twenty years.
Institutional review board approval was obtained for the ongoing follow-up evaluation of Zweymüller total hip arthroplasties at our institution. Of the 200 patients (208 hips) in the original report, 115 patients (120 hips) died less than twenty years postoperatively and twelve patients (thirteen hips) were lost to follow-up, leaving seventy-three patients (seventy-five hips) available for follow-up.
Twenty-four patients (twenty-four hips) who were unable to attend our outpatient clinic completed a standardized telephone interview. The remaining forty-nine patients (fifty-one hips) were available for clinical and radiographic follow-up.
Clinical and Radiographic Assessment
The clinical and radiographic results were evaluated according to the methods used at the time of the ten and fifteen-year follow-up visits. Specifically, the clinical results were assessed with use of the Harris hip score3, and anteroposterior and lateral hip radiographs were classified with use of the Gruen and DeLee and Charnley systems4-6. The Zicat method was used to determine the sizes of osteolytic lesions7.
Statistical Methods
The Kaplan-Meier method was used to estimate the probabilities of survival of the femoral stem, the acetabular cup, and both components together8. For calculation of the probability of implant survival with failure defined as revision for any reason, patients without revision were censored at the date of the latest follow-up or death. For estimation of the probability of implant survival with failure defined as revision because of aseptic loosening, patients with revision for any other reason were also censored at the date of revision.
Source of Funding
There was no external funding source for this project, although one or more of the authors received support for travel to meetings associated with the study from a commercial entity (Zimmer, Winterthur, Switzerland).
The average Harris hip score for the forty-nine patients (fifty-one hips) for whom the score was available decreased from 85.4 points (range, 46 to 100 points) at ten years to 83.4 points (range, 44.4 to 100 points) at twenty years.
Sixty-four patients (87.7%) were still very satisfied or satisfied with the outcome of the arthroplasty, one patient (1.4%) was rather satisfied, and one patient was dissatisfied. Seven patients (9.6%) did not answer the question.
The frequency of isolated thigh pain did not change: the two patients who had reported thigh pain at the time of the fifteen-year follow-up evaluation continued to have thigh pain. No other patient developed thigh pain.
Fifty-one radiographs were reviewed at the time of the twenty-year follow-up. We found various degrees of osteolysis around the acetabular and femoral components in twenty-six cases (51%).
We observed osteolytic lesions only in the proximal zones of the femoral bone-implant-interface on twenty-four (47%) of the reviewed radiographs. Osteolysis was seen in Gruen Zone 1 in eight hips, Zone 2 in one, Zone 7 in eight, Zone 8 in nineteen, and Zone 14 in fourteen. The average size of the lesions was 1.36 cm2 (range, 0.27 to 8.15 cm2). There were no signs of osteolysis in the distal femoral zones. No stem was deemed at risk for loosening at the time of the twenty-year follow-up according to the criteria of Engh et al.9
Radiolucent lines of >1 mm in width were found adjacent to the femoral component in seventeen cases (33%). Radiolucent lines were seen in Gruen Zone 1 in eight hips, Zone 7 in three, Zone 8 in five, Zone 13 in one, and Zone 14 in four.
Stress shielding was observed in Gruen Zones 1, 7, 8, and 14. Acetabular osteolysis was noted on nine (18%) of the fifty-one reviewed radiographs.
The average wear rate was 0.06 mm/yr (range, 0.01 to 0.13 mm/yr) according to the system of Charnley and Halley6.
At the time of the twenty-year follow-up, a total of forty-five hips had been revised for any reason.
Since the time of our last report, there were two femoral stem revisions. In total, five stems were revised for different reasons. In one patient (Case 1), the stem was revised because of malpositioning. In another patient (Case 3), the stem was replaced along with the cup after multiple failed acetabular reconstructions. In a third patient (Case 9), the stem was explanted because of infection. In two patients (Cases 31 and 36), the stems were revised because of aseptic loosening.
A total of fifteen cups were revised: nine cups were revised because of aseptic loosening, three cups were replaced because of progressive wear, and three cups were replaced for different reasons.
Twenty-seven polyethylene liners were revised because of progressive wear.
At twenty years, the probability of survival of the stem was 0.96 (95% confidence interval, 0.91 to 0.99) with revision for any reason as the end point (Fig. 1) and 0.98 (95% confidence interval, 0.92 to 0.99) with aseptic loosening as the end point (see Appendix).
The probability of survival of the acetabular cup, including the polyethylene liner, was 0.67 (95% confidence interval, 0.57 to 0.75) with revision for any reason as the end point (Fig. 2).
The probability of survival of the cup (excluding revisions of the polyethylene liner alone) was 0.88 (95% confidence interval, 0.81 to 0.93) with revision for any reason as the end point (see Appendix).
The probability of survival of the total hip replacement was 0.65 (95% confidence interval, 0.55 to 0.73) at twenty years (see Appendix).
The core finding of our twenty-year follow-up study is the low rate of revisions of the femoral component. The radiographic follow-up shows a low rate of osteolysis in the meta-diaphyseal femoral zones and demonstrates the stability of the osseous integration of the stem.
As expected, excessive polyethylene wear was the most frequent cause for revision surgery, accounting for 67% (thirty) of all forty-five revisions. In three of those cases, the intraoperative situation required an additional exchange of the acetabular component (leaving twenty-seven revisions of the polyethylene liner alone). The second most common cause for revision surgery was aseptic loosening of the conical screw cup (20%; nine of forty-five).
It is our understanding that the large number of small acetabular components with conventional polyethylene inserts paired with 32-mm femoral heads in the cohort is responsible for many revisions because of polyethylene wear and cup loosening.
Only a few authors have reported the long-term results of cementless hip replacements after more than fifteen years of follow-up. The results of the twenty-year follow-up of the Zweymüller stem are at least comparable with the long-term results reported for other cementless stem designs10-12.
The authors prefer the Zweymüller stem because of its excellent primary stability and forgiving nature during implantation. If there is a disadvantage of this stem, it is the proximal shoulder. The shoulder adds stability, but it complicates bone-sparing implantation.
A table showing the revisions in chronological order, a radiograph showing stress shielding, a radiograph showing aseptic loosening of the stem, and Kaplan-Meier curves showing survival of the stem with aseptic loosening as the end point, survival of the acetabular component (excluding polyethylene inlay), and survival of both the stem and the acetabular component together are available with the online version of this article as a data supplement at jbjs.org.
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Disclosure: One or more of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of an aspect of this work. In addition, one or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.