JBJS | Total Hip Arthroplasty with Use of the Metasul Metal-on-Metal Articulation : Four to Seven-Year Results*

The Journal of Bone & Joint Surgery, Volume 82, Issue 6

Articles | June 01, 2000

Total Hip Arthroplasty with Use of the Metasul Metal-on-Metal Articulation : Four to Seven-Year Results*

Lawrence D. Dorr, M.D.†; Zhinian Wan, M.D.†; Donald B. Longjohn, M.D.‡; Ben Dubois, M.D.†; Roger Murken, B.S.†

Investigation performed at Weber Institute, St. Gallen, Switzerland
*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other nonprofit organization with which one or more of the authors is associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was Sulzermedica Orthopedics, Austin, Texas.
†Bone and Joint Institute at Good Samaritan Hospital, 1245 Wilshire Boulevard, 2nd Floor, Los Angeles, California 90017. Please address requests for reprints to L. D. Dorr.
‡Department of Orthopedics, University of Southern California University Hospital, 1510 San Pablo Street, Los Angeles, California 90033.

The Journal of Bone & Joint Surgery. 2000; 82:789-789

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Abstract

Background: Total hip replacements with a metal-on-metal articulation were commonly used until the mid-1970s; most were then abandoned in favor of hip replacement with a metal-on-polyethylene articulation. The reason for this change was primarily early cup loosening, which was more prevalent with these metal-on-metal designs than it was with metal-on-polyethylene designs. In the late 1980s, a metal-on-metal design with improved clearance (adequate space between the femoral head and the acetabular articulation surface to allow fluid film lubrication and clearance of any debris from within this joint), metal hardness, and reproducible surfaces was introduced by Sulzer Orthopedics in Switzerland. Orthopaedic surgeons were interested in this Metasul articulation because the contribution of polyethylene wear particles to the failure of total hip replacements had become evident. This study was undertaken to review the clinical performance of this implant and to determine if early acetabular loosening or revision and wear and osteolysis were prevalent.

Methods: Between 1991 and 1994, seventy patients (seventy hips) had a total hip replacement with the Metasul metal-on-metal articulation and a cemented Weber cup. Nine patients died less than four years after the replacement; none of these deaths were related to the operation. Five patients were not available for radiographic evaluation, but they were contacted and it was known that the hip was not painful and had not been revised. Fifty-six patients (fifty-six hips) had complete clinical and radiographic data four to 6.8 years after the operation, and they made up the study group. The patients were evaluated with use of the Harris hip score, a patient-self-assessment form, and radiographs.

Results: At an average of 5.2 years (range, four to 6.8 years) after the operation, the average total Harris hip score for the fifty-three patients who did not have a revision was 89.6 points (range, 62 to 100 points). The average Harris pain score was 41.0 points (range, 30 to 44 points), and the average Harris limp score was 9.4 points (range, 5 to 11 points). One patient had revision of a loose cup, but there were no other loose acetabular components in the series. Two patients had revision of the acetabular component because of dislocation. No patient had a loose or revised femoral component. Therefore, the mechanical failure rate was one (2 percent) of fifty-six patients. Thirty-six of forty-seven patients who completed the patient-self-assessment form rated their result as excellent; seven, as very good; two, as good; one, as fair; and one, as poor. Wear could not be measured on radiographs because of the metal-on-metal articulation. No hip had radiographic evidence of acetabular osteolysis and two hips had calcar resorption, but there was no other radiographic evidence of focal osteolysis.

Conclusions: Our four to seven-year experience with this articulation surface indicates that the clinical results are similar to those of total hip replacements with a metal-on-polyethylene articulation. We believe that the Metasul articulation may have a role in reducing the wear that occurs with total hip replacement. The Metasul articulation appears to be particularly indicated for more active patients. A historical comparison with the reports in the literature of which we are aware indicated that the hips in our study had a lower rate of acetabular revision and loosening than did those with previous metal-on-metal designs and that they had no more acetabular loosening or osteolysis than did those with metal-on-polyethylene articulations followed for an average of five years.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

Wear and osteolysis have become the foremost concerns as complications of total hip replacement. Zichner and Willert²⁷ linked the occurrence of polyethylene wear, especially linear wear of 0.2 millimeter or more per year, to bone changes and loosening in patients with cemented total hip replacement. We²⁴ observed the same findings with uncemented total hip replacement, with osteolysis and loosening occurring at a greater rate when polyethylene linear wear was at least 0.2 millimeter per year or volumetric wear was at least 150 cubic millimeters per year. Therefore, the importance of a low wear rate as a factor in the durability of total hip replacement is well accepted. Jantsch et al.¹⁶ reported that the average rate of wear of three retrieved implants with a metal-on-metal articulation was 0.001 millimeter per year at an average of fourteen years after the primary operation. The possibility of making a metal-on-metal articulation that did not cause excessive loosening of the acetabular component and that was associated with a low volume of wear debris was suggested by Weber²⁵ of Switzerland. This work resulted in the development of Metasul (Sulzer Medica, Winterthur, Switzerland), a contemporary metal-on-metal articulation design. The experience of the senior author (L. D. D.) with severe wear of modular metal-on-polyethylene articulations⁹ stimulated our interest in an alternative bearing surface. We therefore began using the Weber cup (Sulzer Medica, Winterthur, Switzerland) in 1991.

The purposes of this study of patients who were managed with the Metasul articulation were to determine the clinical performance of this implant; to determine if early acetabular loosening, wear, and osteolysis had occurred; and to compare these results with those in reports in the literature concerning other metal-on-metal designs and metal-on-polyethylene articulations.

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Table I: Harris Hip Scores at Three Follow-up Intervals*

*The values are given as the average and the standard deviation.

Harris Hip Score	Score at 6 Months(points)	Score at 1 Year(points)	Final Score(points)
Pain	42.9 ± 1.8	42.4 ± 3.3	41.0 ± 3.6
Limp	9.5 ± 2.1	9.3 ± 2.3	9.4 ± 2.1
Total	92.8 ± 7.2	90.6 ± 12.5	89.6 ± 9.7

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Table I: Harris Hip Scores at Three Follow-up Intervals*

*The values are given as the average and the standard deviation.

Harris Hip Score	Score at 6 Months(points)	Score at 1 Year(points)	Final Score(points)
Pain	42.9 ± 1.8	42.4 ± 3.3	41.0 ± 3.6
Limp	9.5 ± 2.1	9.3 ± 2.3	9.4 ± 2.1
Total	92.8 ± 7.2	90.6 ± 12.5	89.6 ± 9.7

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Fig. 1:: Photograph of the Weber cup, which has a metal articulation surface, a polyethylene backing, and a Sulzmesh fixation surface.

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Fig. 2:: Anteroposterior radiograph of an Anatomic Porous Replacement stem and a cemented Weber cup, implanted five years previously, in a sixty-year-old man who was an unlimited community walker.

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Fig. 3-A:: Figs. 3-A and 3-B: Radiographs of the patient who had loosening of the cup.
Fig. 3-A: Immediate postoperative radiograph of the cup that eventually loosened, which was in a seventy-five-year-old woman who was a limited community walker. There is no cement in zone⁵ 1 and a large cement column in zone 3 (arrows).

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Fig. 3-B:: Radiograph made four years postoperatively, just prior to revision of the loose cup in this elderly woman, showing varus tilt and rotational migration of the cup and a zone-3 gap (arrows).

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Fig. 4-A:: Figs. 4-A and 4-B: Radiographs of one of the patients who had recurrent dislocations.
Fig. 4-A: Immediate postoperative radiograph of the hip that eventually had recurrent dislocations, which was in a sixty-nine-year-old woman who was an active community walker.

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Fig. 4-B: Radiograph showing dislocation of the hip. The cup was revised after three dislocations over three years. Note the radiographic appearance of the metal articulation of the cup, which demonstrates why it is not possible to differentiate the metal head from the metal articulation of the cup in an attempt to measure wear.

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Fig. 5-A:: Survivorship curve, with radiographic loosening and revision of the cup because of loosening as the end point, for the fifty-six hips with complete follow-up. As there was only one loose cup that was revised and there were no other loose cups, 95 percent confidence intervals are not given.

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Fig. 5-B:: Survivorship curve, with revision because of either loosening or dislocation of the cup as the end point, for all seventy hips. The 95 percent confidence intervals are included.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

From 1991 through 1994, the senior author performed 395 primary total hip replacements, seventy of which were done with a metal-on-metal articulation. These seventy operations were not performed in a consecutive fashion because the metal-on-metal acetabular component could be implanted only when it was available from Sulzer Orthopedics in Switzerland.

Of the seventy patients (seventy hips) who initially had a primary total hip arthroplasty with the Metasul metal-on-metal articulation, nine died less than four years after the operation, but none of the deaths were a result of the operation. All of these patients had been followed postoperatively, and all had well functioning hips at the time of death. Five patients (or their families) were contacted, but these patients would not return for follow-up or have radiographs made. None of these patients had pain in the hip or had had a revision. Fifty-six patients (fifty-six hips) were available for both clinical and radiographic evaluation four to 6.8 years (average, 5.2 years) after the operation, and these patients made up the study group. Twenty patients were followed for four to less than five years; twenty-four, for five to less than six years; and twelve, for six to seven years. The average age of the fifty-six patients was seventy years (range, thirty-five to eighty-five years) at the time of the index arthroplasty. There were thirty-two men and twenty-four women. There were more men in the study group because the only acetabular cup sizes available were fifty-two millimeters and larger. Forty-nine hips had primary osteoarthritis, three had congenital dysplasia, two had posttraumatic arthritis, and two had osteonecrosis.

All patients were treated with a Weber cup, which has a stainless-steel outer mesh, a polyethylene substratum, and a three-millimeter-thick cobalt-chromium metal articulation surface molded into the polyethylene (Fig. 1). This cup is a single-piece (monoblock) component that is molded together as a single unit at the factory. The cup was available in four-millimeter increments with four sizes between fifty-two and sixty-four millimeters. The femoral head is made of cobalt-chromium alloy and has a twenty-eight-millimeter diameter for cup sizes of fifty-two and fifty-six millimeters (used in sixty-six of the seventy replacements) and a thirty-two-millimeter diameter for cup sizes of sixty and sixty-four millimeters (used in four of the seventy replacements). The cobalt-chromium alloy is Protasul-21 WF (Sulzer Medica, Winterthur, Switzerland). The amount of clearance between the femoral head and the acetabular surface was proprietary knowledge, but it averaged 115 micrometers in the implants that we retrieved. The frictional moment of this metal-on-metal design with a twenty-eight-millimeter head was the same as that of a metal-on-polyethylene prosthesis with a thirty-two-millimeter head²², and therefore it was much reduced compared with that of earlier metal-on-metal designs.

Thirty-one femoral stems were uncemented. These femoral components included six Anatomic Porous Replacement-II stems (Sulzer Medica, Austin, Texas) and twenty-five Anatomic Porous Replacement-II-T stems (Sulzer Medica, Austin, Texas). The uncemented Anatomic Porous Replacement femoral components were titanium alloy with titanium porous coating⁴. The Anatomic Porous Replacement-II component did not have proximal circumferential porous coating and was replaced by the Anatomic Porous Replacement-II-T component, which had circumferential porous coating and a hollowed stem for stiffness relief when the stem size was fifteen millimeters or greater. Twenty-five stems were cemented. Fifteen of these stems were Anatomic Porous Replacement titanium stems (Sulzer Medica, Austin, Texas), and ten were Anatomic Porous Replacement cobalt-chromium-alloy stems (Sulzer Medica, Austin, Texas). At first, the Anatomic Porous Replacement titanium stem was used with cement because the taper allowed use of the Metasul femoral head. When the cobalt-chromium stem became available with the taper required for the Metasul head it was used instead. Nineteen patients who had bilateral total hip replacement each had the same type of fixation of the stem in both hips but had a metal-on-metal articulation in one hip and a metal-on-polyethylene articulation in the other. Seventeen of these patients had the two hips operated on during separate surgical procedures, and two had both operations on the same day. Eleven of the seventeen patients who had a staged total hip replacement had the metal-on-metal replacement after the metal-on-polyethylene replacement, and the other six had the metal-on-metal replacement performed first.

Epidural anesthesia was augmented with general anesthetic agents so that the average arterial blood pressure was maintained between sixty and eighty millimeters of mercury. A posterolateral incision was used³. Bone preparation of the acetabulum was accomplished by reaming to the cortical bone of the cotyloid notch. Eight six-millimeter drill-holes were placed in the acetabulum, with two rows of three holes in the ilium, one hole in the pubis, and one in the ischium. Simplex cement (Howmedica, Rutherford, New Jersey) was inserted at three to four minutes after mixing and was pressurized for one to two minutes with the use of an Asepto syringe (Davol, Cranston, Rhode Island). At five to six minutes after mixing of the cement, the acetabular component was implanted and pressurized into the doughy cement. The cement-setting time was twelve to fifteen minutes. The desired position of the acetabular cup was 35 to 40 degrees of lateral opening and 20 degrees of anteversion. Because of the thick plastic rim on this cup (Fig. 1), the acetabular cavity had to be reamed until it was at least two millimeters larger than the cup size to avoid impingement of the rim on the acetabular bone. The need to ream the acetabular cavity to a size that was larger than the cup, in combination with the absence of spacer pegs on the fixation surface of the cup, and the availability of cup sizes in intervals of four millimeters caused the cup to be inserted so that the cement mantle typically was uneven - that is, thinner in zone⁵ 1 and thicker in zone 3 (Fig. 2).

The uncemented femoral components were inserted with a mallet into the femur after reaming and broaching. The cemented femoral components were inserted without reaming, and a distal plastic cement plug was used. Two packages of bone cement were mixed, and the cement was inserted with a cement gun at four minutes after mixing and was manually pressurized with use of additional cement for two minutes. The femoral component was implanted and was impacted the final two centimeters into a position of 10 degrees of anteversion with a mallet.

The patients were followed prospectively, with clinical evaluation performed preoperatively and postoperatively at six months, at one year, and at the time of the present study (or before death). The clinical evaluation was performed with the Harris hip score¹² preoperatively and at each clinic visit by the senior author (L. D. D.) or the fellows. The patient-self-assessment form HKB-21 (Ortho-Graphics, Salt Lake City, Utah), which is a modification of the Short Form-36 questionnaire for pain and functional outcome¹⁷, was mailed to each patient for completion of this study. Forty-seven of the fifty-three patients who had not had a revision completed and returned the questionnaire. Three patients who had been followed for five to six years had died prior to the mailing of the questionnaires, two patients could not complete the questionnaire because of their mental status, and one patient did not return the questionnaire but graded his result as very good on the telephone. For the forty-seven patients who returned the questionnaire, Harris hip scores for pain and limp were determined from this form. These scores, combined with the range-of-motion score determined by the physician, were used to ascertain the total Harris hip score for this group. For the six patients who did not return the self-assessment questionnaire, the Harris hip score was determined from the medical records as well as from information from a telephone conversation.

An anteroposterior pelvic radiograph that included the proximal part of the femur and the entire stem as well as a seventeen-inch (forty-three-centimeter) lateral radiograph of the involved hip were made at the time of the clinical evaluation. The radiographs were analyzed by two authors (B. D. and Z. W.) who were not involved in the care of the patients. Interobserver variability was less than 1 percent. Radiolucent lines and osteolysis around the femoral stem were measured according to the zones described by Gruen et al.¹¹ on the anteroposterior and lateral radiographs (a total of fourteen zones). Acetabular radiolucent lines were measured with use of the zones described by DeLee and Charnley⁵ on the anteroposterior and lateral radiographs. To be counted, a radiolucent line adjacent to either the femoral or the acetabular component needed to occupy at least 50 percent of the zone. The thickness of the acetabular cement mantle was measured in each of the DeLee and Charnley zones on the anteroposterior pelvic radiograph. Osteolysis of the femur or pelvis was measured with the zonal technique used for the radiolucent lines. Measurements of wear were not successful because it was not possible to distinguish between the edge of the femoral head and the metal articulation surface of the acetabular component on the radiographs (Fig. 2 and Fig. 4-B). Migration of the acetabular cup was measured with use of a line connecting the teardrops as a reference line⁹.

A survivorship analysis was performed with use of SPSS statistical software (SPSS, Chicago, Illinois). All seventy hips were used to determine the survival rate with revision for any reason as the end point; the revisions included one for a loose cup and two for dislocation. The fifty-six hips with complete follow-up were used for analysis of survival with mechanical failure, defined as either radiographic loosening or revision because of loosening, as the end point.

Results

Introduction | Materials and Methods | Results | Discussion | References

Three patients had revision of the acetabular component; one revision was performed because of loosening of the cup, and two were performed because of recurrent dislocations. There were no revisions of the femoral component. Fifty-three patients (fifty-three hips) did not have a revision, and their total Harris hip scores, pain scores, and limp scores did not differ notably among the three follow-up intervals (at an average of six months, one year, and 5.2 years) (Table I). At the time of the final follow-up, twenty-six patients were unlimited community walkers, twelve were active community walkers, twelve were limited community walkers, three were household walkers, and three were confined to a wheelchair¹⁰. Of the forty-seven patients (forty-seven hips) without revision who returned the self-assessment questionnaire, thirty-six rated the result as excellent; seven, as very good; two, as good; one, as fair; and one, as poor. The patient who rated the result as fair needed a total hip replacement and a total knee replacement on the contralateral side. The patient who rated the result as poor had substantially reduced function due to aging. The nineteen patients who had bilateral total hip replacement could not discern any difference between the two hips regarding pain, function, motion, or stability. They did not favor one hip over the other.

Complications from the operation included a deep infection in one patient (2 percent), a peroneal nerve palsy in one patient (2 percent), and dislocation in two patients (4 percent).

One patient had progressive loosening of the socket over the first four years postoperatively (Fig. 3-A and Fig. 3-B). The socket had no apparent cement in zone⁵ 1 and failed by progressive varus tilt and rotational migration with a progressively widening radiolucent gap in zones 2 and 3. This hip was revised with a constrained acetabular cup because of concern about potential dislocation in an elderly patient with an impaired mental status. The outcome of the revision was satisfactory at the time of the latest follow-up.

Two patients had recurrent dislocations. At the time of the reoperation, one patient was found to have impingement of the metal femoral neck on the metal edge of the socket, which had made a notch in the femoral neck. Extensive titanium debris from the neck of the prosthesis was seen throughout the capsule. A short modular femoral head had been placed on this femoral neck, which had contributed to the impingement. There was less than twelve micrometers of wear of the metal articulation surface of the cup, and the metal head had a stripe where it had crossed the edge of the metal cup; only in this area of the femoral head was there wear of a depth of more than twelve micrometers⁷. The second patient with recurrent dislocations had the reoperation elsewhere (Fig. 4-A and FIg. 4-B). The dislocations recurred following the revision operation, but the hip otherwise functioned satisfactorily and the stem had not been revised as of the latest follow-up evaluation. All three of the acetabular components were evaluated after they were retrieved, and the results of two of these evaluations are included in the report by Doorn et al.⁷.

There were no other loose cups. The thickness of the cement mantle was an average (and standard deviation) of 0.8 ± 1.3 millimeter in zone 1, 4.5 ± 3.5 millimeters in zone 2, and 6.5 ± 2.9 millimeters in zone 3 (p = 0.001 for zones 1 and 2 compared with zone 3). Of the fifty-three hips that had not been revised, eleven had radiolucent lines at the bone-cement interface of the acetabular component; all of the lines were present on the immediate postoperative radiographs. Of these eleven hips, seven had a radiolucent line in one zone; three, in two zones; and one, in three zones. The radiolucent lines in the three zones were not continuous but occupied at least 50 percent of each zone. No hip had progressive radiolucent lines at the time of the latest follow-up. No hip had radiographic evidence of focal pelvic osteolysis.

A radiolucent line was seen in at least one zone of either the bone-cement or the bone-prosthesis interface of the femoral component in thirteen hips. Twelve of these femoral components were uncemented, and one was cemented. Of these thirteen hips, seven (all with an uncemented component) had the radiolucent line only in zone 4. Of the remaining six hips, one had a radiolucent line in eight zones; one, in five zones; one, in four zones; two, in two zones; and one, in one zone. None of these radiolucent lines were progressive, and none of the femoral stems were loose. Twenty-three hips had rounding of the cut edge of the femoral neck. Eight hips had so-called cortical cancellization, two (one of which had the loose cup) had calcar resorption, and none had calcar hypertrophy. No hip had evidence of focal femoral osteolysis.

A survivorship curve with mechanical failure (aseptic loosening) of the cup or of the cup and stem as the end point showed a survival rate of 98.2 percent at seven years (Fig. 5-A). A survivorship curve with revision of the cup for any reason (loosening or dislocation) as the end point showed a survival rate of 94.1 percent (95 percent confidence interval, 88.9 to 99.3 percent) at seven years (Fig. 5-B).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

The rationale for the use of a metal-on-metal articulation is that it produces fewer wear particles than a metal-on-polyethylene articulation does. Schmalzried et al.²⁰ estimated that the combined linear wear of the acetabular and femoral components of McKee-Farrar (metal-on-metal) implants retrieved at twenty years was 4.2 micrometers per year. Schulte et al.²¹ measured wear of Charnley metal-on-polyethylene prostheses on radiographs at twenty years and reported an average rate of 0.10 millimeter per year, which is twenty-five times greater than the 0.0042 millimeter per year reported for the retrieved McKee-Farrar implants. The McKee-Farrar implants failed because of design deficiencies rather than accelerated wear. Measurements of wear of retrieved implants with a Metasul metal-on-metal articulation have continued to show combined acetabular and femoral linear wear of three to five micrometers per year⁸. This low production of wear particles seems to have prevented osteolysis as a cause of failure of the so-called first-generation metal-on-metal total hip replacements, which were characterized by a thin all-metal cup and a thirty-eight to forty-two-millimeter femoral head that were machined together. We have yet to observe osteolysis as a complication in our study group of Metasul metal-on-metal total hip replacements, which are characterized by a twenty-eight-millimeter femoral head articulating with a metal surface secured into a polyethylene modular insert. Two patients had calcar resorption, which was caused by a loose cup in one of them and by proximal stress-shielding in the other. Our patients had an average age of seventy years, but most were either active community walkers or unlimited community walkers.

The Metasul metal-on-metal articulation has a polyethylene backing for the acetabular metal surface. The reason for this design was the ease of machining the metal surface into the polyethylene. These cups were factory assembled (not modular) so that backside wear, which has been observed on the convex side of the modular polyethylene insert⁹, would not occur. We are not aware of any reports of disassembly of the metal surface from the polyethylene. In the study of retrieved implants by Doorn et al.⁷, which included two of the failed components from our series, a small number of polyethylene particles was seen in most of the specimens. In an analysis of joint fluid from the hips of patients with metal-on-metal total hip replacement, we⁸ found metal and polyethylene particles in the fluid from some patients at one year after the operation. We subsequently evaluated the fluid from more patients and found no polyethylene in seven hips at two years, nine hips at three years, and five hips at four years. We believe that an unexpected benefit of the polyethylene backing was the dampening of the metal-on-metal articulation when it was loaded because none of our patients with bilateral total hip replacement could differentiate the metal-on-metal hip replacement from the metal-on-polyethylene hip replacement.

We found that the prevalence of early loosening was very low with the Metasul metal-on-metal total hip replacement and was similar to the rates of loosening of metal-on-polyethylene total hip replacements reported in the literature^2,18,21. The rate of revision because of mechanical failure in this study was one (1.4 percent) of seventy, and the combined rate of loosening and revision for loosening in patients who had a complete follow-up at an average of 5.2 years was one (1.8 percent) of fifty-six. At seven years, the survival rate was 98.2 percent for the cups and 100 percent for the stems. Schulte et al.²¹ reported similar rates at five to six years after Charnley total hip replacement (Thackray, Leeds, England). None of 330 cups were revised, and four (1.2 percent) were loose. Beckenbaugh and Ilstrup² reported that three (1.2 percent) of 255 hips had a cup revision and sixteen (6.3 percent) had a loose cup at four to seven years after replacement with a metal-on-polyethylene articulation. More recently, Ritter et al.¹⁹, whose patients were followed for the same duration as ours (an average of 5.2 years), reported that two (2 percent) of 100 all-polyethylene cups and eight (6 percent) of 138 metal-backed cups were revised. Twenty-three (23 percent) of the all-polyethylene cups and fifty-four (39 percent) of the metal-backed cups loosened, compared with none of the fifty-five cups that were not revised because of loosening in our study group. Although there are important limitations in comparing disparate groups, the mechanical failure rate (the combined rates of revision and loosening) in our study group of hips treated with the Metasul metal-on-metal articulation was as good as or better than those of previous studies of metal-on-polyethylene articulations.

Early loosening of cups that had the McKee-Farrar metal-on-metal articulation (Norwich, England) was the primary reason that this design was abandoned. The McKee-Farrar cup was a thin all-metal cobalt-chromium cup into which a thirty-eight-millimeter femoral head articulated with an equatorial contact area (at the rim of the cup). This cup and femoral head were machined together so the implants were not interchangeable. The clearance between the femoral head and the cup was almost zero, so there was often high friction between the two components. The Stanmore metal-on-metal hip prosthesis (Stanmore, England) was very similar to the McKee-Farrar replacement. Comparison of the Stanmore metal-on-metal hip prosthesis and the Charnley cup by Dobbs⁶ contributed to the popularity of the Charnley concept of metal on polyethylene because ninety-seven Stanmore prostheses had a survival rate (with removal of the implant as the end point) of 75.5 percent at five to six years while 174 Charnley prostheses had a survival rate of 94.9 percent at five to six years. Jacobsson et al.¹⁵ followed fifty-five McKee-Farrar and forty-one Charnley total hip replacements for eleven to twelve years and reported a cumulative survival rate (with reoperation as the end point) of 89.2 percent for the Charnley implants and 82 percent for the McKee-Farrar implants. Jantsch et al.¹⁶ reported on 136 McKee-Farrar hip replacements followed for fourteen years; 100 hips did not have a reoperation, and thirty-six did. Of the thirty-six reoperations, twenty-nine involved revision of the cup at an average of six years; twenty-two of the twenty-nine cups were loose. McKee and Chen¹⁸ operated on 300 hips between 1965 and 1969 and revised twelve (4 percent) of them at three to seven years. After the same duration of follow-up, our rate of revision because of loosening was 1.4 percent (one of seventy hips) and our annual removal rate according to survivorship analysis (the number of implants removed, on the average, each year) was 0.43 percent, with three of seventy cups removed (one because of loosening and two because of dislocation). In a survivorship analysis, with removal as the end point, of 230 McKee-Farrar replacements followed for an average of 13.9 years, August et al.¹ reported a 94 percent survival rate for the cups at five to six years and an annual removal rate of 1 percent. The annual removal rate for the McKee-Farrar implant was 1.61 percent in the study by Jacobsson et al.¹⁵.

Despite the limitations of historical comparisons with the literature, our experience with the Metasul metal-on-metal articulation appears to have included fewer revisions and a lower prevalence of loosening of the cup compared with other metal-on-metal designs. We cannot determine whether this is due to less impaction of the stem on the cup, better clearance and thus less jamming (temporary catching or locking of the head in the cup), or better fixation of the cup. We did note impaction of the femoral neck on the cup with the resultant need for revision because of recurrent dislocation in one of our patients, but the cup had not loosened in the three-year postoperative period prior to this revision.

Two concerns regarding metal-on-metal particulate debris have been the size of the particles and increased systemic levels of cobalt and chromium. Willert et al.²⁶ reported a mild histological reaction to metal particles from first-generation metal-on-metal total hip replacements, with 33 percent of the particles smaller than fifty nanometers. Doorn et al.⁷ examined tissue from patients with McKee-Farrar or Metasul metal-on-metal total hip replacements and observed that the extent of granulomatous inflammatory reaction and the presence of foreign-body-type giant cells was less intense than has been reported with metal-on-polyethylene total hip replacements. This less intense tissue reaction was probably due to the lower numbers of particles. However, the smaller size of metal particles can cause a greater transport of the particles from the joint tissues. To our knowledge, no increase in the rate of pulmonary or cardiac disease or in cancer risk has been identified in association with metal-on-metal articulations, although this possibility remains a concern to some²³.

Brodner et al.³ determined that serum cobalt levels in patients with a Metasul metal-on-metal articulation were elevated compared with the levels in patients with a ceramic-on-polyethylene hip replacement. Jacobs et al.¹³ found elevated serum cobalt and chromium concentrations in eight patients who had had a metal-on-metal total hip replacement for more than twenty years, but no systemic diseases were associated with this long-term ion elevation. Recently, Jacobs et al.¹⁴ reported increased chromium ion levels in patients with a so-called hybrid hip replacement consisting of a cemented cobalt-chromium stem, a modular cobalt-chromium head, and a titanium acetabular component. They concluded that the systemic release of ions originated from the modular femoral head on a Morse taper neck. No clinical complications from the increased systemic ion levels were observed, but the authors expressed concern about metal-on-metal articulations. We did not measure serum ion levels in our patients, but there was no lymphatic cancer or unexpected clinical disease in this small sample. Certainly, these concerns about smaller particle size and systemic ion release must continue to be monitored.

Despite the limitations of historical controls, our results demonstrate that the rate of acetabular loosening with Metasul metal-on-metal total hip replacement is lower than that reported with McKee-Farrar total hip replacement and is not different from that reported with metal-on-polyethylene total hip replacement. One loose acetabular cup had been removed since our first report on these patients after an average of 2.7 years of follow-up⁸, but there were no other loose cups after an average of 5.2 years. The only evidence of osteolysis was in two hips with calcar resorption, and one of these was in the patient with the loose cup. The follow-up of these patients was not long enough for us to recommend the use of the Metasul metal-on-metal prosthesis without reservation or to support the contention that it is superior to any other articulation surface, and we will continue to study the results associated with the articulation surface in our patients.

References

Introduction | Materials and Methods | Results | Discussion | References

August, A. C.; Aldam, C. H.; and Pynsent, P. B.: The McKee-Farrar hip arthroplasty. A long-term study. J. Bone and Joint Surg.,68-B(4): 520-527, 1986.68-B(4)520 1986

Beckenbaugh, R. D., and Ilstrup, D. M.: Total hip arthroplasty. A review of three hundred and thirty-three cases with long follow-up. J. Bone and Joint Surg.,60-A: 306-313, April 1978.60-A306 1978

Brodner, W.; Bitzan, P.; Meisinger, V.; Kaider, A.; Gottsauner-Wolf, F.; and Kotz, R.: Elevated serum cobalt with metal-on-metal articulating surfaces. J. Bone and Joint Surg.,79-B(2): 316-321, 1997.79-B(2)316 1997

Cohen, J. L.; Bindelglass, D. F.; and Dorr, L. D.: Total hip replacement using the APR II system. Tech. Orthop.,6: 40-58, 1991.640 1991

DeLee, J. G., and Charnley, J.: Radiological demarcation of cemented sockets in total hip replacement. Clin. Orthop.,121: 20-32, 1976.12120 1976 [PubMed]

Dobbs, H. S.: Survivorship of total hip replacements. J. Bone and Joint Surg.,62-B(2): 168-173, 1980.62-B(2)168 1980

Doorn, P. F.; Mirra, J. M.; Campbell, P. A.; and Amstutz, H. C.: Tissue reaction to metal on metal total hip prostheses. Clin. Orthop.,329S: 187-S205, 1996.329S187 1996

Dorr, L. D.; Hilton, K. R.; Wan, Z.; Markovich, G. D.; and Bloebaum, R.: Modern metal on metal articulation for total hip replacement. Clin. Orthop.,333: 108-117, 1996.333108 1996 [PubMed]

Dorr, L. D.; Lewonowski, K.; Lucero, M.; Harris, M.; and Wan, Z.: Failure mechanisms of anatomic porous replacement I cementless total hip replacement. Clin. Orthop.,334: 157-167, 1997.334157 1997 [PubMed]

Dorr, L. D.; Wan, Z.; and Gruen, T.: Functional results in total hip replacement in patients 65 years and older. Clin. Orthop.,336: 143-151, 1997.336143 1997 [PubMed]

Gruen, T. A.; McNeice, G. M.; and Amstutz, H. C.: Modes of failure" of cemented stem-type femoral components. A radiographic analysis of loosening. Clin. Orthop.,141: 17-27, 1979.14117 1979 [PubMed]

Harris, W. H.: Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J. Bone and Joint Surg.,51-A: 737-755, June 1969.51-A737 1969

Jacobs, J. J.; Skipor, A. K.; Doorn, P. F.; Campbell, P.; Schmalzried, T. P.; Black, J.; and Amstutz, H. C.: Cobalt and chromium concentrations in patients with metal on metal total hip replacements. Clin. Orthop.,329S: 256-S263, 1996.329S256 1996

Jacobs, J. J.; Skipor, A. K.; Patterson, L. M.; Hallab, N. J.; Paprosky, W. G.; Black, J.; and Galante, J. O.: Metal release in patients who have had a primary total hip arthroplasty. A prospective, controlled, longitudinal study. J. Bone and Joint Surg.,80-A: 1447-1458, Oct 1998.80-A1447 1998

Jacobsson, S. A.; Djerf, K.; and Wahlstrom, O.: A comparative study between McKee-Farrar and Charnley arthroplasty with long-term follow-up periods. J. Arthroplasty,5: 9-14, 1990.59 1990 [PubMed]

Jantsch, S.; Schwagerl, W.; Zenz, P.; Semlitsch, M.; and Fertschak, W.: Long-term results after implantation of McKee-Farrar total hip prostheses. Arch. Orthop. and Trauma Surg.,110: 230-237, 1991.110230 1991

McHorney, C. A.; Ware, J. E., Jr.; Rogers, W.; Raczek, A. E.; and Lu, J. F.: The validity and relative precision of MOS short- and long-form health status scales and Dartmouth COOP charts. Results from the Medical Outcomes Study. Med. Care,30 (Supplement 5): 253-MS265, 1992.30 (Supplement 5)253 1992

McKee, G. K., and Chen, S. C.: The statistics of the McKee-Farrar method of total hip replacement. Clin. Orthop.,95: 26-33, 1973.9526 1973 [PubMed]

Ritter, M. A.; Keating, E. M.; Faris, P. M.; and Brugo, G.: Metal-backed acetabular cups in total hip arthroplasty. J. Bone and Joint Surg.,72-A: 672-677, June 1990.72-A672 1990

Schmalzried, T. P.; Peters, P. C.; Maurer, B. T.; Bragdon, C. R.; and Harris, W. H.: Long-duration metal-on-metal total hip arthroplasties with low wear of the articulating surfaces. J. Arthroplasty,11: 322-331, 1996.11322 1996 [PubMed]

Schulte, K. R.; Callaghan, J. J.; Kelley, S. S.; and Johnston, R. C.: The outcome of Charnley total hip arthroplasty with cement after a minimum twenty-year follow-up. The results of one surgeon. J. Bone and Joint Surg.,75-A: 961-975, July 1993.75-A961 1993

Streicher, R. M.; Schön, R.; and Semlitsch, M. F.: Untersuchung des tribologischen Verhaltens von Metall/Metall-Kombinationen für künstliche Hüftgelenke. Biomedizinische Technik,35: 107-111, 1990.35107 1990

Visuri, T.; Pukkala, E.; Paavolainen, P.; Pulkkinen, P.; and Riska, E. B.: Cancer risk after metal on metal and polyethylene on metal total hip arthroplasty. Clin. Orthop.,329S: 280-S289, 1996.329S280 1996

Wan, Z., and Dorr, L. D.: Natural history of femoral focal osteolysis with proximal ingrowth smooth stem implant. J. Arthroplasty,11: 718-725, 1996.11718 1996 [PubMed]

Weber, B. G.: Experience with the Metasul total hip bearing system. Clin. Orthop.,329S: 69-S77, 1996.329S69 1996

Willert, H.-G.; Bertram, H.; and Buchhorn, G. H.: Osteolysis in alloarthroplasty of the hip. The role of ultra-high molecular weight polyethylene particles. Clin. Orthop.,258: 95-107, 1990.25895 1990 [PubMed]

Zichner, L. P., and Willert, H.-G.: Comparison of alumina-polyethylene and metal-polyethylene in clinical trials. Clin. Orthop.,282: 86-94, 1992.28286 1992 [PubMed]

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Fig. 1:: Photograph of the Weber cup, which has a metal articulation surface, a polyethylene backing, and a Sulzmesh fixation surface.

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Fig. 2:: Anteroposterior radiograph of an Anatomic Porous Replacement stem and a cemented Weber cup, implanted five years previously, in a sixty-year-old man who was an unlimited community walker.

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Fig. 3-B:: Radiograph made four years postoperatively, just prior to revision of the loose cup in this elderly woman, showing varus tilt and rotational migration of the cup and a zone-3 gap (arrows).

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Fig. 5-B:: Survivorship curve, with revision because of either loosening or dislocation of the cup as the end point, for all seventy hips. The 95 percent confidence intervals are included.

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Table I: Harris Hip Scores at Three Follow-up Intervals*

*The values are given as the average and the standard deviation.

Harris Hip Score	Score at 6 Months(points)	Score at 1 Year(points)	Final Score(points)
Pain	42.9 ± 1.8	42.4 ± 3.3	41.0 ± 3.6
Limp	9.5 ± 2.1	9.3 ± 2.3	9.4 ± 2.1
Total	92.8 ± 7.2	90.6 ± 12.5	89.6 ± 9.7

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Table I: Harris Hip Scores at Three Follow-up Intervals*

*The values are given as the average and the standard deviation.

Harris Hip Score	Score at 6 Months(points)	Score at 1 Year(points)	Final Score(points)
Pain	42.9 ± 1.8	42.4 ± 3.3	41.0 ± 3.6
Limp	9.5 ± 2.1	9.3 ± 2.3	9.4 ± 2.1
Total	92.8 ± 7.2	90.6 ± 12.5	89.6 ± 9.7