M u s c u l o s k e l e t a l I m a g i n g • R ev i ew Ostlere Metal-on-Metal Prostheses

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Musculoskeletal Imaging Review

Simon Ostlere1 Ostlere S

How to Image Metal-on-Metal Prostheses and Their Complications OBJECTIVE. Metal-on-metal arthroplasty is a durable alternative to traditional metalon-polyethylene total hip replacement for young active patients. Although midterm results for resurfacing arthroplasty are reasonable, there is increasing recognition of the problem of metal-induced periprosthetic reactive masses. CONCLUSION. Imaging plays an important role in the investigation of symptomatic metal-on-metal arthroplasty. Radiographs will identify fracture and loosening, but cross-­ sectional imaging is required to diagnose and stage periprosthetic reactive masses.

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Keywords: arthroplasty, complications, hip, imaging, metal-on-metal prostheses, prostheses, resurfacing, total hip replacement DOI:10.2214/AJR.11.6840 Received March 8, 2011; accepted after revision May 5, 2011. 1 Department of Radiology, Nuffield Orthopaedic Centre, Windmill Rd, Oxford OX3 7LD, United Kingdom. Address correspondence to S. Ostlere ([email protected]).

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he first documented metal-onmetal arthroplasty was performed in the 1930s. The stainless steel prosthesis used in a small number of patients failed because of poor fixation and wear [1]. Twenty years later, reasonable results were reported with the McKee-Farrar cobaltchromium-molybdenum metal-on-metal total hip replacement (THR), with loosening of the acetabular component being the main cause of failure [2, 3]. The successful introduction of a metal-on-polyethylene prosthesis by Charnley in the 1960s resulted in a temporary lull in interest in metal-on-metal arthroplasty. However, it became clear that the long-term results of metal-on-polyethylene THR in young patients were unsatisfactory [4], thus leading to a resurgence of research activity in metal-on-metal technology over the past 20 years and the development of a successful resurfacing design. Metal-on-metal arthroplasty is an attractive alternative to conventional metal-onpolyethylene THR for young active patients. The primary problem with metal-on-polyethylene THR in this age group is the high rate of revision as a result of bone lysis secondary to excessive wear of the polyethylene liner [5, 6]. This high rate of revision is not only because these patients have a longer life expectancy but also because they are more active, resulting in higher rates of polyethylene wear. Metal-on-metal designs were developed to overcome the problem of excessive polyethylene wear. The metal bearings are durable and produce a relatively low level of wear particles that, in theory, should re-

duce the risk of foreign-body reaction, thus increasing the chances of long-term survival of the prosthesis. Metal-on-metal THR using a conventional femoral stem has been available for many years, but the resurfacing design has more recently garnered substantial market share. Resurfacing has the major advantage of sparing the native femoral neck. The initial resurfacing designs in the 1970s using either Teflonon-Teflon (fluorine-containing resins, DuPont) or metal-on-polyethylene did not fare well [7, 8]. In the 1990s metal-on-metal designs were developed, but failure rates were initially unacceptable [9, 10]. Further refinements in the manufacturing process and instrumentation have led to the current generation of resurfacing devices, which have become a popular choice for young patients. The results of the initially published outcome studies for modern resurfacing arthroplasty were encouraging, but recently concerns over the incidence of periprosthetic metal-induced reactive masses have emerged. Aseptic loosening, infection, and femoral neck fracture are other complications that have been recorded. This article reviews the role of imaging in the diagnosis and management of complications related to metal-on-metal arthroplasty. Types of Prostheses Metal-on-metal prosthesis may have a conventional THR or resurfacing design (Fig. 1). Metal-on-metal THR has a standard femoral stem and a range of femoral head sizes. Some

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Metal-on-Metal Prostheses newer designs of THRs and all resurfacing prostheses use a larger femoral head that articulates with a thin acetabular metal shell. Prostheses with a larger head are more stable, almost eliminating the risk of dislocation. Despite the increased surface area, metal wear is no greater than with prostheses with smaller heads, reportedly because the precision engineering allows formation of a lubricating film of fluid that protects the surfaces during rapid movement of the joint [11]. With the resurfacing design, the acetabular component is uncemented and the femoral component is cemented. The femoral component caps the prepared femoral head, thus conserving the native femoral neck. If the prosthesis fails, then revision is relatively straightforward because femoral bone stock is preserved and a primary femoral component can be used. Improved stability and mobility are cited as additional advantages, but there is only weak evidence that resurfacing arthroplasty is associated with higher postoperative activity than conventional THR [12]. The main disadvantages of resurfacing arthroplasty are the lack of long-term outcome data, the risk of femoral neck fracture, and the potential adverse effects of metal wear particles and metal ions. Compared with conventional THR surgery, the surgery is more challenging and the component position is more critical. Outcomes Conventional metal-on-metal THR has a long track record, but results are mixed [13, 14]. Acceptable serum ion levels [15] and good survivorship results at 10 years have been seen with well-established designs [16, 17], whereas newer models of metal-on-metal THR using large heads have a higher failure rate [18]. With modular metal-on-metal THR designs, local corrosion at the headneck junction is a well-recognized problem [19, 20]. In one design, high failure rates were caused by corrosion of the cemented cobalt-chromium femoral stem at the interface, with the cement resulting in large, cystic metal-induced reactive masses [14]. Long-term outcomes for the current generation of resurfacing devices are not available, but short- and medium-term results are encouraging [21–23]. However, recent data indicate that the overall revision rate for resurfacing may exceed that of conventional THR and that the revision rate in women is nearly twice that in men, raising concerns regarding the safety of resurfacing arthroplas-

ty in women [18, 24]. The difference in revision rates between the sexes is likely to be because females are more susceptible to periarticular metal-induced reactive masses. The susceptibility of females for this complication is thought to be primarily because of excess wear related to the smaller cup size used in females, but metal hypersensitivity may also be a factor [25]. An additional concern is that initial data suggest that the outcome of revision arthroplasty performed for failed resurfacing is disappointing and the need for rerevision is surprisingly high [26, 27]. There is considerable variation in outcome between designs and some of the poor-­ performing devices have been withdrawn from the market. Recently the United Kingdom Medicines and Healthcare Products Regulatory Agency issued a report recommending close clinical and imaging monitoring of all patients with metal-on-metal prostheses [28]. The resulting interest from the media and lawyers has somewhat tempered the enthusiasm for metal-on-metal arthroplasty, particularly for use in women [29–32]. The outcome data support the use of resurfacing devices in young men but surgeons should be cautious in recommending the prosthesis to women. In addition to the local effect of metal particles, there has been some concern regarding the safety of persistently high systemic levels of metal ions and metal particles. However, there is no evidence to date that the levels

of ions or particles generated by a good-functioning metal-on-metal prosthesis is associated with any adverse systemic effects [33].

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Imaging Radiography is routinely used to check the postoperative state of prostheses. In the symptomatic hip, complications may be identified on radiography but often cross-sectional imaging is required because radiography frequently shows normal findings in cases of a reactive mass. Normal Radiographic Appearances of Metalon-Metal Hip Prostheses The position of the prosthesis components on postoperative radiographs has a bearing on the risk of developing complications, particularly for designs with large-diameter femoral heads. The recommended range of cup inclination is 35–55° and of cup anteversion, 10–30° [34]. The femoral component should be placed in an anatomic position [35]. A varus position should be avoided. The position of the resurfacing femoral stem should be central in the femoral neck on the frontal view, but the position seen on the lateral view is thought to be less critical (Fig. 2). The pin should not impinge on the cortex. A surrounding thin lucency and focal sclerosis at the tip of the stem are features seen in asymptomatic patients [22] (Fig. 3). Occasionally there is notching of the superior

Fig. 1—Normal radiographic appearances of metal-on-metal prostheses. A and B, Radiographs show resurfacing arthroplasty (A) and hip replacement with large-diameter femoral head (B).

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Ostlere femoral neck because of either inadvertent extension of the reaming required to prepare the femoral head or impingement against the acetabular rim [36] (Fig. 4). Notching is usually an insignificant finding, although some investigators have suggested that notching may increase the risk of fracture [35]. A minor degree of femoral neck thinning is a common finding on follow-up radiographs of asymptomatic hips, but this finding is also associated with a reactive mass [37–39].

Fig. 2—Lateral radiograph shows acceptable angulation of femoral pin in patient who underwent resurfacing hip replacement.

Fig. 3—Resurfacing hip replacement. Radiograph shows minor lucency and sclerosis around tip of femoral stem.

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Fig. 4—Notching of superolateral aspect of femoral neck in 54-year-old woman who underwent resurfacing hip replacement. A, Early postoperative radiograph shows defect (arrow) in superior cortex of femoral neck. B, Follow-up radiograph shows defect has healed.

A Fig. 5—64-year-old woman with resurfacing arthroplasty. A, Standard T1-weighted MR image of resurfacing replacement shows troublesome metal artifact. B, T1-weighted metal artifact–reducing sequence shows reduction in extent of artifact at expense of some blurring of image.

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Reducing Metal Artifact on MRI When MRI is performed in this setting, optimum parameters should be used to reduce the effect of metal artifact. Artifact-reducing sequences will vary somewhat among MRI scanner manufacturers. The artifacts from embedded metal devices are caused by metal-­ induced local inhomogeneity of the magnet field. Gradient-echo imaging and spectral fat-suppression techniques should be avoided because they are susceptible to artifacts related to field inhomogeneity. MRI parameters should be optimized to reduce metal artifact as much as possible while maintaining adequate image quality. Reducing the time of dephasing improves the artifact and can be achieved by using fast spin echo with a long echo-train and short TE, increasing the readout bandwidth and image matrix, and reducing slice thickness. Most of the artifact reduction may be achieved with bandwidth values and matrix sizes in the mid range. Further increases in these values will result in an increase in noise for little reward [40]. The other popular technique in clinical use is view angle tilting; this technique uses a gradient in the slice select direction during readout, resulting in the affected signals being projected into the correct pixel. However, this advantage is at the expense of image blurring [41] (Fig. 5). Complications of Metal-on-Metal Arthroplasty Proper positioning of the components is important in minimizing the degree of metal wear particularly with prostheses with largediameter heads. A vertical cup may result in edge loading in which the area that is normally subjected to the highest loads extends beyond the contour of the acetabular cup resulting in excessive loading at the edge of the cup [42] (Fig. 6). In the case of resurfacing arthroplasty, an excessively horizontal cup or insufficient anteversion can result in impingement causing subluxation of the femoral head, leading to edge loading

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Metal-on-Metal Prostheses

Fig. 6—Resurfacing hip replacement. Radiograph shows excessive abduction of acetabular cup.

and increased wear [43]. Poor positioning of the femoral component is unusual and rarely contributes to failure of the prosthesis. If components are in an adverse position, the patient warrants close observation. In these cases, serum cobalt and chrome ion measurements may be helpful to predict which prostheses are likely to fail [44]. Loosening—Loosening of the components in the absence of a reactive mass is unusual [31]. The uncemented acetabular component may rarely suffer mechanical loosening that is sometimes associated with debonding of the porous coating [45]. Lucency around and migration of the component are signs of loosening [31] (Fig. 7). Review of old radiographs is important to detect minor degrees of migration and progressive lucency. Infection—As with any joint prosthesis, infection is a recognized complication of metal-on-metal prostheses. Imaging features are those of any THR and include a joint effusion, periarticular collections, and sinus formation. Differentiating infection from a reactive mass may be difficult on the basis of imaging criteria alone (Fig. 8). Fracture—A well-recognized early complication of resurfacing arthroplasty is fracture of the femoral neck, which is usually associated with osteonecrosis of the femoral head [46] (Fig. 9). The incidence is quoted as being around 1.5%. Notching of the superior femoral neck (Fig. 4) and varus position of the femoral component have been implicated as risk factors [8, 47]. Improvements in surgical technique to minimize any disruption of the capsular vessels may lead to

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Fig. 7—Loosening of resurfacing components. A, Radiograph shows that loose acetabular cup has migrated into vertical position. B, Radiograph shows that loose femoral component with excessive lucency around femoral stem in 43-year-old man.

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Fig. 8—Infected metal-on-metal total hip replacement in 75-year-old woman. A and B, T1-weighted (A) and STIR (B) coronal MR images show large periprosthetic collection (arrows). It is impossible to differentiate infection from reactive lesion on basis of imaging criteria alone, although lack of any substantial solid component and absence of low signal intensity on STIR sequence favor infection.

a fall in the incidence of this complication [31]. Occasionally a fracture of the femoral stem accompanies a loose or fractured femoral component [39, 48–50] (Fig. 10). Metal-induced reactive mass—The histology findings of periprosthetic tissue obtained

at the time of revision of metal-on-metal implants have been described and labeled “aseptic lymphocyte-dominated vascular-associated lesions” and are compatible with a delayed hypersensitivity to the metal particles [51]. In patients with periarticular masses, marked

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Fig. 9—Fracture of femoral neck in 64-year-old woman after resurfacing arthroplasty. A, Radiograph shows femoral component has migrated proximally from femoral stem. B, Subsequent follow-up radiograph shows displaced fracture.

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Fig. 10—Fracture of femoral stem in patient who underwent resurfacing arthroplasty. Radiograph shows abnormal alignment of femoral stem (arrow); this finding indicates there is fracture within cup.

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Fig. 11—Proven reactive mass and bone involvement. A, Radiograph of 57-year-old man who underwent resurfacing arthroplasty shows focal thinning of femoral neck proximally (arrows). B, Radiograph of 67-year-old woman who underwent resurfacing arthroplasty shows lytic lesion in femoral neck with sclerotic border (arrows). C, Radiograph of 55-year-old man who underwent resurfacing arthroplasty shows wide lucency around femoral stem.

necrosis is an additional consistent feature [52]. Necrotic masses related to the use of a cobalt-chromium alloy in joint replacements, including metal-on-metal designs, were reported in the early 1970s [53]. In a number of reports, investigators have described similar lesions related to the current generation of metal-on-metal prostheses [25, 54–56]. The formation of a periprosthetic mass is now recognized as being a major cause of revision and fuels concern regarding the long-term safety of metal-on-metal prostheses. These reactive lesions have been termed “pseudotumors” because they can be predominantly solid lesions with necrosis that macroscopically resemble a malignant neoplasm [25]. The biologic pathway leading to the formation

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of these masses is complex. The nanometersized particles are cytotoxic to the macrophages once ingested by phagocytosis, accounting for the necrosis seen within the lesions. There are also histologic features compatible with a type IV hypersensitivity allergic reaction [57, 58]. The risk factors for developing a reactive mass are female sex, small prosthetic cup size, poor positioning of the components, and inadvertent downsizing of the femoral head in women with high preoperative head-neck diameter ratios [38, 43, 59]. The incidence of this complication varies depending on the type of prosthesis used. Large femoral head metal-on-metal THRs are associated with up to an 8% failure rate at 5 years [60], whereas more traditional designs with small femo-

ral heads have low levels of wear particles and low incidence of reactive masses [58]. For resurfacing arthroplasty the incidence of revision performed because of a reactive mass increases with time from surgery and is 4% by 8 years. Women outnumber men by 8 to 1 [59]. Patients with bilateral resurfacing hip replacements who develop a reactive mass in one hip have a 33% chance of having a lesion on the other side. Reactive masses are undoubtedly underreported because abnormalities are seen on ultrasound in 4% of women who have been discharged from the care of surgeons but, on questioning, still have some symptoms related to their prosthesis [44]. Reactive masses are related to high serum and joint fluid ion levels, indicating that excessive

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Fig. 12—Bilateral resurfacing arthroplasty complicated by reactive mass in 59-year-old woman. A and B, T1-weighted (A) and STIR (B) axial MR images. On right, typical anterior solid reactive mass (arrows) shows low signal intensity on STIR image and intermediate to high signal on T1-weighted image. Smaller lesion (arrowheads) on left side with signal intensities similar to muscle was initially overlooked. C, On ultrasound, mass (arrows) on left side is clearly identified lying deep in relation to femoral artery.

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Fig. 13—CT scan of 55-year-old woman who underwent resurfacing arthroplasty shows two predominately cystic masses (arrows) lying anterior to hip joint.

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Fig. 14—61-year-old woman who underwent metal-on-metal total hip replacement with large-diameter head. A and B, T1-weighted (A) and T2-weighted (B) axial MR images show anterior (arrows) and lateral (arrowheads), mainly solid, reactive mass with minor central cystic component. Low signal intensity on T2-weighted image is typical of metal-induced reactive mass.

A Fig. 15—55-year-old woman who underwent resurfacing arthroplasty. A and B, T1-weighted (A) and T2-weighted (B) axial MR images show intrapelvic reactive mass (arrows) lying within psoas muscle.

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Fig. 16—Axial STIR MR image of 68-year-old woman who underwent resurfacing arthroplasty shows posterior thin-walled cyst (arrow). Lesion has neck (arrowhead) that is seen pointing toward posterior joint space.

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Fig. 17—67-year-old woman who underwent resurfacing arthroplasty. A–C, T1-weighted coronal MR image (A), STIR coronal MR image (B), and ultrasound image (C) show anterior thick-walled cystic reactive mass (arrows). Fig. 18—67-year-old man who underwent resurfacing arthroplasty. A and B, T1-weighted axial MR image (A) and ultrasound image (B) of posterior hip show small posterior thin-walled cyst (arrows) typical of reactive lesion. GT = greater trochanter.

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Fig. 19—Reactive mass that developed after resurfacing arthroplasty in 55-year-old man. A and B, T1-weighted (A) and STIR (B) coronal MR images show thick-walled cyst (stars) and discrete masses (arrowheads) with very low signal intensity on STIR image and intermediate to high signal on T1-weighted image. Lesion involves bone (arrows). C, Extended FOV ultrasound image shows extensive hypoechoic lesion (arrows) lying along lateral border of proximal femur. GT = greater trochanter.

shedding of metal particles is the initiating process [44]. The frequent finding of bilateral reactive masses in patients with bilateral resurfacings implies that these patients have an increased susceptibility to hypersensitivity independent of the degree of wear. How-

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ever there is no evidence to show that a preoperative history of metal allergy is a risk factor. Indeed, in one study the measurement of lymphocyte proliferation responses to metals was not raised in patients with a reactive mass [61].

Patients may present with symptoms of a reactive mass as early as a few months after surgery, but with most patients there is a delay of several years. Presenting symptoms include pain, a palpable mass, and femoral neuropathy [62, 63]. The severity of symp-

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Metal-on-Metal Prostheses

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Fig. 20—Reactive mass with femoral neck bone lysis in 55-year-old woman who underwent resurfacing arthroplasty. A, Ultrasound image shows hypoechoic lesion (star) arising from anterior joint space and erosion of femoral neck (arrows). FH = femoral head. B, Lateral radiograph confirms erosion on anterior femoral neck (arrow). C, Erosion is not evident on early postoperative lateral radiograph.

toms is very variable. Radiographs usually show normal findings, but in advanced cases there may be evidence of bone lysis and signs of component loosening or femoral neck narrowing [38, 39] (Fig. 11). Masses can be detected on MRI, CT, and ultrasound. Ultrasound is a useful screening tool because it is quick, cheap, and is not affected by artifacts from the metal components. The anterior, lateral, and posterior aspects of the hip can be rapidly assessed. In large patients, sensitivity of ultrasound for reactive masses is reduced, but with the use of low-frequency probes, a satisfactory examination is possible in nearly all cases. Doppler ultrasound usually shows no or minor intralesional vascularity. On MRI, the extent of the disease and relationship of the abnormality to normal structures may be better appreciated. The solid components will usually show elements of low signal on T2weighted images reflecting the metal deposition. The fluid contained in cystic lesions may also show similar signal characteristics [39]. Small lesions lying close to the prosthesis may be overlooked on MRI but are clearly visible on ultrasound [39] (Fig. 12). Masses can be identified on CT (Fig. 13), but MRI is preferred on account of its superior soft-tissue contrast. Our policy is to use ultrasound as the initial investigation followed by MRI if staging is required. Lesions may have a variety of appearances. They may be located anteriorly, posteriorly, or laterally or in a combination of these positions (Figs. 12–19). The lesions may be solid or cystic. Occasionally it is difficult to differentiate a solid lesion from a cystic lesion when the fluid has low signal intensity on fluid-sensitive sequences. There is no evidence to suggest gadolinium is useful when no lesion is seen on the unenhanced scan. The pau-

city of Doppler signal on ultrasound suggests that any enhancement would be unimpressive. Predominantly solid lesions tend to be located anteriorly, usually within the psoas muscle (Fig. 14). Anterior masses may extend proximally into the pelvis (Fig. 15) and may involve the femoral nerve or, exceptionally, the external iliac vessels [62, 63]. Predominantly cystic lesions tend to arise from the posterior joint space and may have thin or thick walls (Figs. 16–18). Laterally placed lesions usually involve the trochanteric bursa, which is seen to communicate directly with the posterior, or less commonly, the anterior joint space (Fig. 19). Careful inspection of the MR images and thorough ultrasound technique are required to identify any communication of the bursa with the joint because simple thickening of a noncommunicating trochanteric bursa should not be labeled as a “reactive mass.” Occasionally bone involvement is apparent on ultrasound or MRI (Figs. 19 and 20). There is no established pathway for the management of patients with reactive masses. Patients with troublesome symptoms or larger lesions are candidates for revision. Unfortunately results are poor in the advanced cases with objective measurements of function being comparable with those recorded before the primary surgery [26]. There is uncertainty regarding the correct management of patients with smaller lesions and minor symptoms because it is not known whether these lesions are likely to progress. Conclusion Metal-on-metal hip replacement is a durable alternative to traditional metal-on-polyethylene THR for young active patients. The

resurfacing design is a relatively recent advance that has the advantage of sparing the native femoral neck. Although midterm results for resurfacing arthroplasty are reasonable, there is increasing recognition of the problem of metal-induced periprosthetic reactive masses, particularly in women, raising concerns that long-term performance may be less favorable. Imaging plays an important role in the investigation of the symptomatic metal-on-metal hip replacement. Radiographs will identify fracture and loosening, but cross-sectional imaging is usually required to diagnose and stage periprosthetic reactive masses. References 1. Wiles P. The surgery of the osteoarthritic hip. Br J Surg 1958; 45:488–497 2. McKee GK, Watson-Farrar J. Replacement of arthritic hips by the McKee-Farrar prosthesis. J Bone Joint Surg Br 1966; 48:245–259 3. August AC, Aldam CH, Pynsent PB. The McKeeFarrar hip arthroplasty: a long-term study. J Bone Joint Surg Br 1986; 68:520–527 4. Sochart DH, Porter ML. The long-term results of Charnley low-friction arthroplasty in young patients who have congenital dislocation, degenerative osteoarthrosis, or rheumatoid arthritis. J Bone Joint Surg Am 1997; 79:1599–1617 5. Amstutz HC, Campbell P, Kossovsky N, Clarke IC. Mechanism and clinical significance of wear debris-induced osteolysis. Clin Orthop Relat Res 1992; 276:7–18 6. Harris WH. Wear and periprosthetic osteolysis: the problem. Clin Orthop Relat Res 2001; 393: 66–70 7. Herberts P, Lansinger O, Romanus B. Surface replacement arthroplasty of the hip: experience with the ICLH method. Acta Orthop Scand 1983;

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F O R YO U R I N F O R M AT I O N

The reader’s attention is directed to the commentary on this article, which appears on the preceding pages.

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