Survivorship and Retrieval Analysis of Sikomet Metal-on-Metal Total Hip Replacements at a Mean of Seven Years

Milošev, Ingrid PhD; Trebše, Rihard MD; Kovač, Simon MD; Cör, Andrej PhD; Pišot, Venčeslav MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.E.00604
Articles
Abstract

Background: Second-generation metal-on-metal total hip replacements were introduced in the early 1990s with the aim of eliminating polyethylene wear and the resulting complications of osteolysis and aseptic loosening. The goal of the present study was to evaluate the intermediate-term results in a series of patients who were managed with one of these implants.

Methods: Between 1994 and 2002, we performed 640 total hip replacements in 591 patients with use of a Bicon-Plus cementless threaded cup with a polyethylene liner housing a metal inlay made of Sikomet low-carbon cobalt-chromium-molybdenum alloy that articulates with a Sikomet metal femoral head. Clinical and radiographic evaluation was performed retrospectively at a mean of 7.1 years postoperatively. Histologic analysis was performed on specimens retrieved from seventeen hips that were revised, and wear measurements were made for six hips that were revised.

Results: Thirty-four hips (thirty-four patients) were revised because of infection (six hips), aseptic loosening (twenty-three hips), pain without loosening (two hips), or other reasons (three hips). The survival rate of the prosthesis as a whole at ten years, with revision for any reason as the end point, was 0.91 (95% confidence interval, 0.88 to 0.95). The survival rate of the cup was 0.94 (95% confidence interval, 0.90 to 0.97), and that of the stem was 0.96 (95% confidence interval, 0.94 to 0.98). Linear or expansile osteolysis, or both, was observed on the radiographs of sixteen (64%) of the twenty-five hips that were revised because of aseptic loosening and/or pain. Histological analysis of pericapsular tissue was performed for seventeen of the twenty-five hips that were revised because of aseptic loosening and/or pain. Thirteen of these seventeen hips demonstrated a hypersensitivity-like reaction with aseptic inflammatory changes accompanied by moderate to extensive diffuse and perivascular infiltration of lymphocytes. In the six retrieved specimens that were subjected to wear analysis, the main wear mode was abrasive wear. The mean cumulative linear wear for the bearing was 31.3 μm, and the mean annual wear rate was 6.3 μm/yr. The mean clearance was 87.6 μm.

Conclusions: After a mean duration of follow-up of seven years, aseptic loosening was the major reason for failure of Sikomet metal-on-metal prostheses. The histological findings and the prevalence of osteolysis suggest the possibility of a hypersensitivity-like immunological response to wear particles.

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

Author Information

1 Orthopaedic Hospital Valdoltra, Jadranska c. 31, 6280 Ankaran, Slovenia. E-mail address for I. Milošev: ingrid.milosev@ijs.si. E-mail address for R. Trebše: rihard.trebse@ob-valdoltra.si. E-mail address for S. Kovač: simon.kovac@ob-valdoltra.si. E-mail address for V. Pišot: venceslav.pisot@ob-valdoltra.si

2 Faculty of Medicine, Institute of Histology and Embryology, University of Ljubljana, Korytkova 2, 1000 Ljubljana, Slovenia. E-mail address: andrej.coer@mf.uni-lj.si

Article Outline

Second-generation metal-on-metal total hip replacements made of cobalt-chromium-molybdenum alloy were introduced in the early 1990s. By eliminating polyethylene wear as a cause of osteolysis and aseptic loosening, it was postulated that this type of articulation might achieve better long-term clinical results than conventional total hip replacements1.

The longest clinical follow-up studies of second-generation metal-on-metal total hip replacements have demonstrated survival rates ranging from 94.1% to 100% after a mean duration of follow-up of five to seven years2-4. Most published studies have involved one particular bearing (Metasul; Zimmer, Winterthur, Switzerland) combined with different cups, and the majority of those series were relatively small and investigated only early2-8 and intermediate-term9 follow-up. Of concern, there have been reports of increased metal serum levels in patients with Metasul10,11 and Sikomet12 total hip replacements, and histological examination of retrieved tissue specimens has shown the possibility of a hypersensitivity reaction to metal debris in patients with metal-on-metal total hip replacements13-15.

In the present study, we report on a series of 640 metal-on-metal bearings with low-carbon Sikomet SM21 cobalt-chromium-molybdenum alloy after a mean duration of follow-up of 7.1 years. Revised hips were investigated in detail on the basis of radiographic, wear, and histological analyses.

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Materials and Methods

Patient Demographics

Between December 1994 and December 2002, 4772 primary total hip replacements were performed at our institution. Of these, 591 patients (640 hips) underwent total hip replacement with metal-on-metal bearings. Selection of the type of implant was at the discretion of the surgeon. There were no specific inclusion or exclusion criteria. This study received institutional review board approval. Data on the patients, including age, gender, initial diagnosis, details regarding the implant, the reason and date of revision, and complications, were retrospectively retrieved from the hospital computer database. Our standard postoperative regimen included patient assessment at three, six, and twelve months after the index procedure and at one or two-year intervals thereafter. Pain, range of motion, and function were recorded and standard anteroposterior and lateral radiographs were made at each follow-up visit. The analysis of the clinical and radiographic data from the database was beyond the scope of the present study.

The mean patient age at the time of the metal-on-metal total hip replacement was fifty-seven years (range, eighteen to eighty years). The study group included 364 women (62%) and 227 men (38%). The mean duration of follow-up was 7.1 years (range, 2.3 to 10.5 years).

The diagnosis was osteoarthritis for 487 hips (76%). The remaining major diagnoses are listed in Table I. Forty-eight patients (fifty-three hips) had had a previous hip operation, including femoral osteotomy (thirty hips), open reduction and internal fixation of a fracture around the hip joint (eleven hips), pelvic osteotomy or shelfplasty (eleven hips), and vascularized fibular bone-grafting (one hip).

Of the 591 patients (640 hips), twenty-eight patients (twenty-nine hips; 4.5%) died for reasons unrelated to the hip surgery, leaving 563 patients (611 hips) available for analysis.

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Implant

The acetabular component is a Bicon-Plus metal-on-metal cup (Plus Orthopedics, Rotkreuz, Switzerland). It consists of a cementless threaded shell made of commercially pure titanium (ASTM [American Society for Testing and Materials] F67, ISO [International Organization for Standardization] 5832-2). An RCH-1000 Chirulen ultra-high molecular weight polyethylene liner is interposed between the shell and the Sikomet SM21 Co-28Cr-6Mo low-carbon-alloy articulating surface (ASTM F799 and 1537, ISO 5832-12). A 28-mm-diameter metal femoral head manufactured from Sikomet SM21 was used in all patients. Sikomet SM21 is a low-carbon, forged, vacuum-melted Co-28Cr-6Mo alloy exhibiting fine grain structure with an almost carbide-free surface16.

The Bicon-Plus cups were used with various femoral stems, including 423 SL-Plus stems (Plus Orthopedics), 109 Ribbed hip stems (W. Link, Hamburg, Germany), sixty Thrust plate prostheses (Allopro, a Zimmer Company, Warsaw, Indiana), twenty-one Anca stems (Wright Cremascoli Ortho, Toulon, France), six VerSys stems (Zimmer), six Copf/Holz stems (Chendo, Saalstadt, Germany), five SLR-Plus stems (Plus Orthopedics), and ten other stems.

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Operative Technique

All operative procedures were performed through a direct lateral17 or anterolateral surgical approach18. Templating was used preoperatively to determine the sizes of the components to be implanted. Antibiotic prophylaxis with a first-generation cephalosporin was given during the first twenty-four hours postoperatively. Low-molecular-weight heparin or warfarin was given routinely for prophylaxis against deep-vein thrombosis for as long as three months postoperatively.

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Wear and Roughness Analysis

Retrieved components were examined with a Zoom Research Stereo Microscope, (SZH10; Olympus, Hamburg, Germany). A scanning electron microscope (JSM-5800; JEOL, Tokyo, Japan) combined with energy-dispersive x-ray analysis (Link ISIS 300; Oxford Instruments, Oxford, United Kingdom) was used to inspect the wear damage on the surface of the femoral head.

Wear was measured with use of a coordinate-measuring machine (CMM5; SIP, Geneva, Switzerland) having a spatial resolution of <1 μm19. Wear was defined as the maximum deviation from the original spherical profile, thus giving the deepest local wear instead of the mean wear of the component. Clearance was defined as the difference between the measured diameters of the head and cup. The precision of the wear measurements was estimated to be ±2 μm.

The roughness of the femoral head was measured at five sites on the surface with a diamond stylus profilometer (Talysurf Hobson Form Talysurf Series 2; Leicester, United Kingdom) with use of a 2-mm evaluation length and a 0.25-mm cut-off length. The measurement sites were at the dome, at 45° and 90° from the dome, and at representative sites within the worn zone and within the unworn zone below the equator. The results were expressed as the average surface roughness (Ra) and the maximum peak-to-valley height (Rmax).

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Histological Analysis

Periprosthetic tissue samples for histological analysis were fixed in 10% buffered formalin; embedded in paraffin; sectioned into 5-μm-thick serial sections; stained with use of hematoxylin and eosin, Perls, PAS, and Giemsa histochemical staining methods; and examined with a light and polarizing microscope (Eclipse 80i; Nikon, Tokyo, Japan) equipped with a digital camera (DXM1200-F; Nikon). The amounts of polyethylene particles, metal particles, and diffuse and perivascular lymphocytes were graded according to the method of Willert et al.14.

In addition, we analyzed histological sections from two groups of patients who had undergone revision total hip replacement with use of other articulations. Control group I included seven uncemented stems made of a titanium-based alloy, an Al2O3 femoral head, and a polyethylene liner in a metal cup. These prostheses had been revised because of aseptic loosening after an average of 14.7 years (range, 5.4 to 19.2 years) in situ. Control group II included eight all-cemented Charnley-Müller prostheses (Alivium; Zimmer) consisting of a curved monobloc femoral component made of cobalt-chromium-molybdenum alloy and a polyethylene cup. These stems had been revised because of aseptic loosening after an average of 21.9 years (range, 10.9 to 27.4 years) in situ.

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Radiographic Analysis of Retrieved Implants

Anteroposterior radiographs of the pelvis, centered over the pubic symphysis, were analyzed according to the method of DeLee and Charnley20 on the acetabular side and according to the method of Gruen et al.21 on the femoral side. Acetabular loosening was defined radiographically as the presence of a complete radiolucent line measuring >1 mm in all three zones, cup migration of >3 mm, or >5° of change in cup inclination22. The stability of femoral component fixation was assessed with use of the classification system described by Engh et al.23,24. The femoral component was considered to be loose if serial radiographs demonstrated a change in the position of the femoral component (that is, subsidence of ≥2 mm or varus or valgus tilting).

Osteolysis was classified as linear or expansile and as periarticular or remote from the joint according to the criteria proposed by Zicat et al.25.

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Statistical Analysis

Implant survival, with 95% confidence intervals, was estimated according to the Kaplan-Meier method26 with use of statistical software (SPSS 10.0; SPSS, Chicago, Illinois). Failure was defined as revision for any reason or revision because of aseptic loosening. The time to revision was calculated as the time between the date of implantation and the date of revision. Patients without revision were censored at the date of death. In addition to the survival of the prosthesis as a whole, survival rates were calculated separately for the femoral and acetabular components with revision for any reason and with revision because of aseptic revision as the end points.

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Results

Survivorship Analysis

The survival rate of both components at ten years was 0.91 (95% confidence interval, 0.88 to 0.95) with revision for any reason as the end point. The survival rate of the cup alone was 0.94 (95% confidence interval, 0.90 to 0.97), and the survival rate of the stem alone was 0.96 (95% confidence interval, 0.94 to 0.98) (Fig. 1, a and b). The survival rate of both components at ten years was 0.93 (95% confidence interval, 0.90 to 0.96) with revision because of aseptic loosening as the end point. The survival rate of the cup alone was 0.95 (95% confidence interval, 0.91 to 0.98), and the survival rate of the stem alone was 0.98 (95% confidence interval, 0.96 to 0.99) (Fig. 1, c and d).

The survival rate was calculated separately for 402 patients (423 hips) with an SL-Plus stem. The survival rate of both components at ten years was 0.92 (95% confidence interval, 0.88 to 0.96) with revision for any reason as the end point. The survival rate of the cup was 0.93 (95% confidence interval, 0.88 to 0.97), and that of the stem was 0.98 (95% confidence interval, 0.96 to 0.99). The survival rate of both components at ten years was 0.93 (95% confidence interval, 0.88 to 0.97) with aseptic revision as the end point. The survival rate of the cup was 0.94 (95% confidence interval, 0.89 to 0.98), and that of the stem was 0.98 (95% confidence interval, 0.97 to 1.00).

There were fourteen surgical complications, including one dislocation, eight femoral nerve palsies (seven of which were transitory), two sciatic nerve palsies that did not resolve, one transitory femoral sensory lesion27, and two instances of nonlethal pulmonary embolization.

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Analysis of Revisions

There were thirty-four revisions. The reasons for revision included deep infection (six), aseptic loosening (twenty-three), pain without loosening (two), recurrent dislocations (one), cup fracture (one), and metal inlay dissociation (one) (see Appendix).

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Details on Revision Operations

Of the six deep infections, three were due to Staphylococcus aureus and three were due to other organisms (Propionibacterium acnes, Staphylococcus capitis, and Staphylococcus epidermidis). The mean time in situ for the hips with infection was thirty-two months (range, four to forty-six months). Four of the infected hips had a two-stage revision of both components, one had a one-stage revision of the socket, and one had a two-stage revision of the socket. At the time of review, all hips were infection-free.

The mean time in situ for the twenty-three hips that were revised because of symptomatic aseptic loosening was fifty-eight months (range, ten to 114 months). The reason for revision was loosening of the cup only for eleven hips, loosening of both the acetabular and femoral components for four, and loosening of the femoral component only for eight. An extensive joint effusion, as described by Willert et al.14 and Bösch and Legenstein28, was present in seventeen of the twenty-three hips that were revised because of aseptic loosening.

In two hips that were revised because of aseptic loosening (Cases 26 and 27), unnoticed liner malpositioning occurred during the index procedure, and in one hip (Case 16) the aseptic failure occurred in association with impingement between the metal inlay and the femoral neck (see Appendix).

Two hips (Cases 30 and 31) were revised because of pain that began five and 2.5 years postoperatively, respectively. No visible signs of loosening were noted, but there was a small osteolytic zone at the dome of the cup in one of them. In one hip (Case 32) a Bicon-Plus elevated-rim metal-on-metal cup was implanted after recurrent dislocations, and in another hip (Case 33) the cup fractured after a fall and subsequently was exchanged to a metal-on-polyethylene articulation. One patient (Case 34) started to experience pain four years postoperatively; the pain progressed substantially after slight trauma, and, at the time of revision, five years postoperatively, the patient had dissociation of the metal inlay from the polyethylene liner29.

Whenever the bearings were exchanged, the metal-on-metal combination was replaced with a metal-on-polyethylene, ceramic-on-polyethylene, or ceramic-on-ceramic bearing.

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Radiographic Findings

The preoperative radiographs of the twenty-five hips that were revised because of symptomatic aseptic loosening and/or pain showed cup migration in twelve hips and cup tilting in ten hips. Four hips had stem subsidence, and five had stem tilting.

Osteolysis was observed in sixteen (64%) of the twenty-five hips. Linear osteolysis was observed in eight hips, expansile osteolysis was observed in two, and both linear and expansile osteolysis were observed in six. Expansile osteolysis was most often noted in zones 1 and 7 on the femoral side and in zone 1 on the acetabular side.

No osteolysis was observed in the other nine hips that were revised because of aseptic loosening and/or pain.

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Wear and Roughness Analysis of Revised Hips

The maximum linear wear, the wear volume of the articulating bearing, the cumulative and annual wear rates, and the derived clearances are shown in a table in the Appendix for the six specimens that were subjected to wear analysis. With the exclusion of one hip (Case 27), which was revised because of excessive wear due to liner malposition, the mean cumulative linear wear of the femoral heads was 7.42 μm (range, 1.9 to 16.4 μm) and the mean value for the inserts was 23.9 μm (range, 5.6 to 42.1 μm). Thus, the mean cumulative linear wear for the bearing was 31.3 μm (range, 12.8 to 52.7 μm). With the mean time in situ of 5.1 years, the annual wear rate was 6.3 μm/yr (range, 3.12 to 9.17 μm/yr). The measured linear wear values for the head, cup, and bearing are given in Figure 2 and show a linear increase in total wear with time.

With the exclusion of one hip (Case 27), the average volumetric wear of the bearing was 1.43 mm3 (range, 0.13 to 3.91 mm3). The mean annual volumetric wear rate was 0.43 mm3/yr (range, 0.02 to 1.63 mm3/yr). Given a density of the alloy of 8.38 g/cm3, the mean gravimetric wear rate was 3.6 mg/yr (range, 0.17 to 13.66 mg/yr). The mean clearance was 87.6 μm (range, 85.5 to 95.2 μm). There was no apparent relationship between clearance and time in situ (Fig. 2).

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

Roughness was measured on thirteen retrieved femoral heads. The mean values of average surface roughness (Ra) and maximum peak-to-valley height (Rmax) measured at various positions on the head are given in Table II. The value measured at the dome was 0.10 μm (range, 0.08 to 0.15 μm), similar to values measured in the nonbearing zone and lower than those measured at 45° and especially at 90° from the dome in the wear zone. The average Ra value of all of the measurements in the wear zone was 0.12 μm (range, 0.08 to 0.31 μm), compared with 0.10 μm (range, 0.08 to 0.14 μm) in the nonbearing zone. The Rmax values exhibited a much larger span than did the Ra values, with a mean Rmax value of 1.32 μm (range, 0.14 to 5.80 μm) in the wear zone and of 0.85 μm (range, 0.14 to 1.76 μm) in the nonbearing zone.

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Scanning Electron Microscopic Analysis

Retrieved specimens showed abrasive, adhesive, and third-body wear, the former being the main wear mode and characterized by shallow scratches in the most heavily loaded zone (Fig. 3). A self-polishing effect, characteristic of metal-on-metal articulations because of the ductility of the cobalt alloy, often was noted. An example is given in Figure 3, where large scratches of about 2 μm in width are progressively polished into narrower scratches. Organic deposits, which are produced by oxidation during tribology, often were found among scratches around the border of the load-bearing zone (Fig. 4). Several larger wear defects were observed on retrieved femoral heads.

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Histological Analysis

Histological samples were collected for seventeen of the twenty-five hips that were revised because of aseptic loosening and/or pain. In thirteen (76%) of these seventeen specimens, we observed the hypersensitivity-like reaction described by Willert et al.14, with aseptic inflammatory changes, including infiltrates of lymphocytes and sometimes accompanied by plasma cells (Fig. 5) (see Appendix). In the inner layer of the neocapsule the lymphocytes were diffusely distributed, whereas in the intermediate vascular layer the lymphocytic infiltrates mostly surrounded postcapillary venules. In the majority of hips, perivascular lymphocytic infiltrates were rated as many (2+). Plasma cells also were noted in association with the lymphocytic infiltration. In twelve of the seventeen hips, the tissues showed an extensive fibrin exudation. Necrosis was not a permanent histological feature. Polyethylene particles were few and were observed only in two hips. The foreign-body reaction to metal wear particles in these seventeen hips was rated as few (1+) in four hips, many (2+) in eleven hips, abundant (3+) in one hip, and excessive (4+) in one hip. In some cases, yellow-golden inclusions were seen within macrophages (Perls test negative) (Fig. 5).

The average time in situ for the group of revised hips in which no lymphocytic infiltration was observed (sixty-three months; range, forty-nine to seventy months) was not significantly different from that for the group of revised hips in which lymphocytic infiltrations were observed (sixty-five months; range, ten to 114 months) (p = 0.79). Furthermore, there was no apparent correlation with patient age or gender.

Control group I comprised seven hips with an uncemented stem made of a titanium-based alloy, an Al2O3 femoral head, and a polyethylene liner in a metal cup, and control group II comprised eight hips with an all-cemented Charnley-Müller prosthesis made of cobalt-chromium-molybdenum alloy and a polyethylene cup. In contrast to the tissue obtained from the hips that had revision of a metal-on-metal implant, the tissue obtained from the two control groups showed distinctly smaller numbers of lymphocytes. Analysis of the periprosthetic tissue samples from control group I revealed that two hips had few (1+) and one hip had abundant (3+) diffuse lymphocytes. One hip had few (1+) perivascular lymphocytes. Analysis of periprosthetic tissue samples from control group II revealed that three hips had few (1+) and one hip had many (2+) diffuse lymphocytes. No perivascular lymphocytes were noted in the latter group. Polyethylene particles were detected in four of the seven hips in control group I and in seven of the eight hips in control group II. The foreign-body reaction, including monocytes and giant cells filled with polyethylene particles, was much more extensive in the two control groups than in the group with revision of a metal-on-metal implant. In control group I, the reaction to metal particles was rated as few (1+) in four hips and as many (2+) in three hips. In control group II, the reaction was described as few (1+) in six hips and as many (2+) in two hips.

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Discussion

The majority of published results for metal-on-metal bearings pertain to the Metasul total hip replacement after a mean duration of follow-up of 2.2 to seven years (see Appendix). The most common reasons for revision of the Metasul total hip replacement have been recurrent dislocation, infection, early cement debonding, technical errors, high serum cobalt levels secondary to impingement3, and unexplained pain3,9,14. The published results for metal-on-metal bearings14,30-33 other than Metasul are not substantially different, except for those described by Bösch and Legenstein28, who reported catastrophic results in association with low-carbon Biomet metal-on-metal total hip replacements, with a 23% revision rate at 8.2 years. Those studies generally had a shorter duration of follow-up and included a smaller number of hips than the current series does.

In the present series, the ten-year survival rate was 0.91 with revision for any reason as the end point and 0.93 with revision for aseptic loosening as the end point. Ceramic-on-polyethylene bearings that have been implanted in patients of similar age to those in our series have shown results comparable with those for our Sikomet metal-on-metal bearings34-36. Grübl et el.34, in a study of 208 tapered rectangular Ti-Al-Nb stems combined with Al2O3 femoral heads and CSF (cementless self-fixation) cups, reported a ten-year survival rate of 0.92 for the prosthesis as a whole, 0.93 for the cup only, and 0.99 for the stem only, with revision for any reason as the end point. These results are similar to ours for SL-Plus stems and Bicon-Plus cups. Reports on systems involving metal-on-polyethylene bearings37-39 have described similar survival rates, ranging from 0.90 and 0.92 for the prosthesis as a whole, with revision for any reason as the end point.

The histological findings that we observed in the tissues of thirteen of seventeen revised hips included diffuse and perivascular lymphocytic aggregates, plasma cells, massive extensive fibrin exudation, the presence of moderate metal wear debris, and a lack of foreign-body giant cells and polyethylene wear debris. Although the lymphocytic aggregates also were observed in the metal-on-polyethylene control groups, they were less frequent and of much lower intensity than in the metal-on-metal group and we regard them to be a specific feature of the metal-on-metal systems. These results are in agreement with data reported by other authors14,28,40 and support the idea that a relationship exists between wear debris from metal-on-metal bearings and a specific immunological reaction similar to type-IV delayed hypersensitivity.

The size of wear particles from metal-on-metal total hip replacements has been reported to range from 25 to 36 nm41. Because of their small size, the number of particles generated per year for metal-on-metal total hip replacements in vitro has been estimated to be as much as 100 times greater than the number of polyethylene particles generated per year in vivo41. Willert et al.14 suggested that low but continuous release of metal ions from metal-on-metal bearings and their reactions with the surrounding tissues, which start immediately after implantation, accelerate or at least facilitate sensitization and a consequent immunological reaction. We could not quantify an association between the magnitude of the metal wear observed in the tissues and the number of lymphocytic aggregates. Furthermore, we could not correlate the presence of lymphocytic aggregates and the appearance of osteolysis.

Data regarding the in vivo wear rate of second-generation metal-on-metal prostheses are relatively scarce. A steady-state wear rate of 2 to 5 μm/yr after a maximum of 5.5 years in situ was reported in a study of forty-four retrieved Metasul implants42. A wear rate of 4 μm/yr after a maximum of five years in situ was reported for eight retrieved Metasul implants43. The largest series, which included 608 individual retrieved Metasul implants, demonstrated a steady-state wear rate of 6.2 μm/yr44. A similar rate of 7.6 μm/yr was reported in a study of twenty-two low-carbon Sikomet implants that were retrieved after a mean of thirty-two months in situ45. These data on retrieved metal-on-metal implants confirm that the wear rate is much lower than that associated with conventional metal-on-polyethylene prostheses, which ranges from 50 to 250 μm/yr44. In the present study, only six retrieved implants were analyzed for wear. Nevertheless, low values for both linear and volumetric wear rates were found, yielding a linear wear rate of 6.3 μm/yr and a volumetric wear rate of 0.43 mm3/yr. Our value for linear wear lies between the value reported for Metasul implants (6.2 μm/yr)44 and that reported for Sikomet implants (7.6 μm/yr)45.

In the specimens that were examined, numerous shallow scratches were noted in the worn areas, thus confirming abrasive wear as the main wear mode. Third-body wear caused by intrinsic carbide from the alloy46 was not observed, probably because of the low carbide content in the alloy. Other types of wear defects were observed, including large wear defects of delamination and deep scratches as well as smaller wear defects due to Al2O3 particles29.

In our series, sixteen of the twenty-five hips that were revised because of aseptic loosening and/or pain had osteolysis. Park et al.15 reported that ten of 169 hips with another metal-on-metal total hip replacement (Ultima) had an osteolytic lesion that was localized to the greater trochanter after a minimum of twenty-four months of follow-up. In contrast, Dorr et al.47 found no osteolysis in ninety-six hips with well-fixed Metasul implants after five to eleven years of follow-up. Similar results have been reported by other authors3,9. Compared with those results for Metasul total hip replacements, we speculate that the Sikomet low-carbon bearings are somehow implicated in the development of osteolysis as the discrepancy in the carbon content in the bearing is the only substantial difference between the devices. Cobalt-chromium-molybdenum alloys used for the manufacture of metal-on-metal total hip replacements differ in the content of carbon; high-carbon alloys contain >0.2% carbon (by weight) and low-carbon alloys contain <0.07% carbon (by weight). Metasul is a high-carbon cobalt-chromium-molybdenum alloy, Sikomet SM21 is a low-carbon alloy, and Ultima is a combination of a low-carbon head and a high-carbon cup. The volumetric wear rates of cobalt-chromium-molybdenum alloy are affected by the carbon content and the material combinations used, with the wear rate decreasing from a low-carbon head/low-carbon cup to a low-carbon head/high-carbon cup to a high-carbon head/high-carbon cup41,48. Mean particle size and morphology are not affected by carbon content41, which implies that the increased wear rate of low-carbon and low-carbon/high-carbon combinations is related to a higher number of particles released in the periprosthetic tissue, thus contributing to the development of osteolysis. Therefore, it seems that the slight difference among the wear rates of these three metal-on-metal articulations affects the threshold for osteolysis formation.

Another possible explanation for the failures of some metal-on-metal total hip arthroplasties is a patient's predisposition to metal hypersensitivity. Whether the predisposition to metal sensitivity is the cause of implant loosening or a consequence of the interaction between metal debris and periprosthetic tissue is still not clear. Lintner et al.40 suggested that, in a selected, albeit small, number of patients with a specific predisposition, agents derived from the metal constituents of the cobalt-chromium-molybdenum alloy initiate a process that triggers the immune mechanism, eventually leading to pain and implant loosening. The most likely candidate appears to be cobalt. Hallab et al.49 used lymphocyte transformation testing and noted a change in the prevalence of metal-specific sensitivity from predominantly nickel to predominantly chromium that would suggest that implant-related metal sensitivity can develop in subjects with implants. The development of hypersensitivity, however, cannot be explained by the difference in carbon content as Willert et al. observed a high prevalence of hypersensitivity-like reactions in a study of predominantly Metasul high-carbon metal-on-metal retrieved implants14.

Second-generation metal-on-metal bearings were introduced with great optimism as an improvement over other types of bearings. Our results to date, however, for Bicon-Plus low-carbon metal-on-metal bearings are similar to those reported for ceramic-on-polyethylene and metal-on-polyethylene systems after comparable periods of follow-up. Thus, despite the extremely low wear rate and the avoidance of polyethylene wear, the survival probability of metal-on-metal bearings has not improved significantly after early or intermediate-term follow-up. Osteolysis remained a common feature observed at the time of revision. Moreover, several new problems specific to metal-on-metal arthroplasty, such as increased metal serum levels50, a hypersensitivity-like immunological reaction, and joint effusions, have been observed and are presently not well understood. Whether the long-term results associated with metal-on-metal bearings will achieve the promise that was postulated remains to be seen.

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Appendix

Tables showing specific details of the revision cases, wear parameters, and histological findings as well as a summary of the literature to date on metal-on-metal bearings is available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on “Supplementary Material”) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ▪

NOTE: The work was supported by the Slovenian Research Agency through grants No. L3-6023 and P2-0148, and by Unior d.d. Zrece. The authors thank Mr. J. Fiser for roughness measurements and Dr. C. Rieker for CMM measurements.

In support of their research for or preparation of this manuscript, one or more of the authors received grants or outside funding from Plus Orthopedics AG, Rotkreuz, Switzerland. The work was also supported by the Slovenian Research Agency through grants No. L3-6023 and P2-0148, and by Unior d.d. Zreče. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Orthopaedic Hospital Valdoltra, Ankaran, Slovenia

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