History of Hip Resurfacing Harlan C. Amstutz, M.D. Introduction The THARIES Experience Cementless Surface Replacements Hemisurface Experience The New Era of Full Surface Replacement References Introduction Surface arthroplasty represents a significant development in the evolution of hip arthroplasty. It is a direct descendant of the cup arthroplasty originally conceived by Smith-Petersen (1948). Surface replacement is a bone conserving alternative to total hip replacement that restores normal joint biomechanics and load transfer and ensures joint stability. Historically, these appealing characteristics have been recognized by several investigators and various designs and biomaterials have been used. In the early 1950s, Charnley experimented with a cementless all Teflon double cup arthroplasty (Charnley 1961, 1963). Loosening of both components due to rapid wear and an intense tissue reaction resulted in clinical failure and abandonment of the procedure. In the mid 1960s, Maurice M・ler used a metal-on-metal (Co-Cr-Mo) resurfacing system which was a press-fit (M・ler and Boitzy 1968). Despite satisfactory early results, this system was abandoned because of loosening of the components. In 1970, Gerard in France also implanted metal-on-metal resurfacing prostheses with motion occurring not only between the components but between the components and bone (Gerard 1978; Gerard et al. 1974). Since total hip systems using fixation with cement were very successful in the short term, it was only after the exploration of revision of the stems that resurfacing became refined with acrylic fixation in the mid 1970's in five different countries. Cemented surface replacement systems using a high density polyethylene acetabular component and a metal femoral cup were first implanted in Italy by Paltrinieri and Trentani (1971), followed by Furuya in Japan (Furuya 1976; Furuya et al. 1978), Freeman in England (Freeman et al. 1975, 1978) Eicher and Capello in the United States (Capello et al. 1978), Amstutz in the United States (Amstutz et al. 1977, 1984b, 1986), Wagner in Germany (1978) and Tanaka in Japan (1978). Wagner in 1982 used a ceramic femoral component but no results were published. Resurfacing procedures addressed the problem of preserving femoral bone stock at the initial operation and also had the potential of easy revision since the femoral canal was not violated. Most resurfacing systems were challenged by selecting for surgery young and/or high activity level patients who often had acetabular (developmental dysplasia) or femoral head (advanced osteonecrosis) bone stock deficiencies. The early results were encouraging but the longer follow-up was often disappointing (Ahnfelt et al. 1990; Bell et al. 1985; Head 1981; Howie et al. 1990a, 1990b). Because of unpredictable results, resurfacing procedures were largely abandoned in the 1980s except at the University of California, Los Angeles (UCLA), where it was used on a highly selective bases with porous acetabular components for younger patients who would require at least two replacements in their lives. Although failure is multifactorial, it is now understood that bearing wear debris induced osteolysis is the main problem (Amstutz et al. 1994; Bell et al. 1985; Howie et al. 1990a, 1990b, 1993; Mai et al. 1996; Nasser et al 1990; Schmalzried et al. 1992, 1994). Surface replacement failure was primarily due to the high volumetric polyethylene wear, secondary to a large prosthetic head and not due to high frictional torque, neck fractures or osteonecrosis of the femoral head. The large diameter of surface replacement components results in polyethylene wear rates which are 4-10 times higher than that of conventional total hip arthroplasties with femoral diameters of 28 mm (Kabo et al. 1993). Howie et al. (1993) reported on the histologic analysis of 72 retrieved Wagner resurfacing specimens. They demonstrated the presence of polyethylene wear debris and macrophage activation at the cement-bone interface even in solidly fixed prostheses. Our own studies of retrieved specimens have also shown the presence of fixed prostheses and the presence of variable amounts and sizes of polyethylene wear debris provoking a histiocytic response along the cement interfaces (Amstutz et al. 1991). The role of polyethylene debris in the failure of surface replacements is further amplified by our results of surface hemiarthroplasty for Ficat Stage III or early stage IV osteonecrosis of the femoral head (Amstutz et al. 1994; Grecula et al. 1995) where cemented femoral components of similar design were implanted articulating against the host acetabulum. In the absence of polyethylene, no loosening or osteolysis was observed after more than 15 years postoperatively (Amstutz 1997). Those hips which have required reoperation were revised for groin pain associated with deterioration of the acetabular cartilage. The retrieved femoral head specimens showed no evidence of osteolysis, were viable and the cement-bone interface remained intact. The concern that resurfacing of the arthritic femoral head would cause osteonecrosis has not been substantiated by our human retrieval studies. More recently it has been verified that the femoral head remains viable in the vast majority of cases (Howie et al. 1993). The THARIES Experience The THARIES(tm)1 (Total Hip Articular Replacement using Internal Eccentric Shells) was developed at UCLA Medical Center in 1973 (Amstutz et al. 1977, 1984b, 1986, 1991). The prosthesis was cemented and consisted of a Co-Cr-Mo femoral component, and an all-polyethylene acetabular component. Both components were eccentric with polyethylene maximum wall thickness of 3.5 to 5.5mm (in small, medium and large components) in the weight bearing areas. The technique was designed on the principle of resecting all non-viable femoral head bone but also preserving as much of the head and neck as possible, to allow fixation of the prosthesis. There were six femoral shell sizes with diameters ranging from 36 to 54mm at 3 and 4mm increments. One of the unique features of the THARIES(tm) development was the design and evolution of specialized instruments to obtain a consistently reproducible reamed femoral head. A special femoral neck pin-centering guide was crucial in avoiding violation of cortical bone during femoral preparation. Although the procedure was intended to preserve maximum bone stock, acrylic fixation of the acetabular component entailed in some cases the removal of a larger than desired amount of pelvic bone to allow for polyethylene and acrylic cement. 322 of 586 THARIES(tm) were implanted at UCLA between June 1975 and November 1984, by the senior author. The mean patient age at the time of surgery was 51 years (range 20-67 years). The primary diagnoses were consistent with this young population and included osteoarthritis 53%, osteonecrosis 16%, developmental dysplasia 10%, rheumatoid arthritis 7%, post-trauma 5%, SCFE 4% and other diagnoses 5%. Fifty-five percent were male with an average weight of 81 kg whereas females had an average weight of 62 kg. The average follow-up was 117 months with 172 patients over 10 years at last review (Fig. 1, A-C). There have been 189 revisions of which only 4 were due to femoral neck fractures. This suggests that this complication can be avoided with proper surgical technique and adequate instrumentation and implant design. Figure 1: A) Preoperative radiographic of a 46 year old male with osteoarthritis. Figure1: B) Postoperative radiograph following a cemented THARIES(tm) resurfacing using a transtrochanteric approach. Figure1: C) The 21 year postoperative radiograph illustrates approximately 1.5 mm of wear. There is widening of the bone-cement interface in zone 3 to 1.5 mm and marginal osteolysis in zone 1 of the acetabulum. There is suggestion of a lateral neck osteolytic foci, but he remains asymptomatic. Aseptic loosening of one or both components was responsible for 97% of the failures. There were 56% acetabular failures alone, 31% femoral failures and in 10% loosening of both components was present at revision surgery. With revision as the endpoint, the 5, 10, and 16 year survivorship for the entire group was 88, 48, and 26% respectively (Fig. 2). The best survivorship of hips was in osteoarthritis males who had larger components than women. Survivorship was 91% at 5 years, 66% at 10 years and 43% at 15 years (Fig. 3). Figure 2: Kaplan Meier survivorship of all cemented THARIES. Table 1 contains the results by etiology for survivorship based on any revision, femoral failure, or acetabular failure. The best survivorship was in the OA/PT group which contains older men. For all groups, except osteonecrosis, the survivorship at 12 years was significantly better on the femoral side than on the acetabular side; there was very little difference between the femoral and acetabular survivorship for osteonecrosis patients. Therefore, the femoral failure rate was higher and durability less in patients with osteonecrosis after surface replacement than for other etiologies. Table 1. Percent survival based on the Kaplan Meier Curves for various etiologies. Etiology Any Revision Years After Surgery Femoral Failure Years After Surgery Acetabular Failure Years After Surgery 5 10 12 5 10 12 5 10 12 OA/PT 94 60 41 97 81 70 97 70 55 AVN 82 37 14 86 60 44 93 61 32 DDH 80 31 21 92 77 66 87 41 32 RA 90 49 31 90 71 61 95 62 45 OTHER 74 15 4 91 60 45 80 24 8 Since the OA patients tended to be older and the other patients tended to be younger a comparison was made between the different etiologies based on patients younger than 50 years at surgery. There was a significant difference between the etiologies (p=0.011 Logrank test) and the 10 year estimated survivorships were: osteoarthritis 51%, osteonecrosis 33%, developmental dysplasia 28%, rheumatoid disease 50%, and other 11% (which included the most difficult diagnostic category failure of surface replacements). Figure 3: THARIES survivorship curve of osteoarthritic males. Our experience has been that although most patients' THARIES(tm) prosthesis failed, they had gained an advantage over other comparable patients who had initially had conventional replacements when revision was required because they had a virgin femoral canal. The concept of resurfacing evolved into one of a conservative time buying procedure that allowed the prospect of possibly increased durability after revision. It was anticipated that the surface replacement patients could benefit from newer and hopefully more advanced implants and simpler revisions. For example, beginning in 1983 cementless acetabular components were used either with a cemented or cementless stem implanted into a virgin femoral canal secured long-term function in these patients some of whom were still young and active. Cementless Surface Arthroplasties Because acrylic cement was thought to be the "weak link", cementless fixation was introduced into our surface replacement components in 1983 just as it was in conventional stem type devices. While acetabular durability improved considerably with the chamfered cylinder design (CCD), femoral osteolysis and/or fracture occurred with much greater frequency than in our cemented THARIES(tm) experience. Those results are consistent with our current hypothesis which states that the debris travels a path of least resistance and because the CCD socket was securely interference fitted at surgery into the acetabulum and subsequently circumferentially secured by bone ingrowth, debris penetration into the pelvis was prevented and as a result the femoral head and neck or in a few cases iliopsoas bursa became the most vulnerable site. The subsequent osteolysis often caused femoral loosening or fracture due to loss of structural integrity. When we changed to a hemispherical socket design to facilitate insertion, the acetabulum became more vulnerable to debris due to the penetration of screws through the subchondral plate into the pelvis. The dome of the acetabulum often is relatively osteopenic and again according to our hypothesis became susceptible to debris penetration under the fluid pressures of the joint. This mechanism became significantly obtunded when the components were press-fit with an interference fit into an under-reamed cavity and there was no penetration of the subchondral bone by screws. On the femoral side the cementless components almost without exception became fixed (whether TiAlloy/CP Ti mesh or the later beaded cobalt chrome) but the components often tilted during insertion creating gaps between the bone and the component in some areas which apparently added to the vulnerability to debris penetration. The resultant effect of a very good acetabular component with a good seal and a poor one on the femoral side shortened the durability of the femoral components in all etiologies which was contrary to our experience with the all cemented THARIES(tm) series (Fig. 4). We now believe that the cemented acetabular bone cement interface membrane was the path of least resistance and became a storage reservoir for debris. Ultimately this linear osteolysis did lead to loosening of the acetabular component while the acrylic cement provided better protection for the femoral component bone interface. However, the reactive sclerotic bone at the interface on the acetabulum side acted as a debris barrier so that acetabular balloon osteolysis was rarely observed (only 1 patient at 14 years). On the other hand, when cementless components become osseointegrated in the acetabulum, there is no sclerotic line (condensed bone) and the cancellous bone becomes penetrable by the debris under pressure. All of the lessons of this early era were useful in the evolution of our current metal on metal surface arthroplasty design but long-term durability is unlikely for young and active patients because of the limitation of polyethylene especially since wear is increased due to the large ball size. Figure 4: Forty-six year old developer 12 years following cementless surface arthroplasty with a chamfered cylinder design socket, ti-alloy bearing and a C.P. titanium mesh. Note 1.5 mm of wear. Marginal osteolytic erosion superiorly. He remains asymptomatic despite sustaining traumatic dislocation of both hips, normal and prosthetic hips at 8 years post-op. Hemisurface Experience The initiation of our precision-fit surface hemiarthroplasty experience began in 1980 because we were disappointed with the early results of full surface arthroplasty (THARIES(tm)) as well as total hip replacement in young patients with osteonecrosis. Because the acetabulum is relatively normal in stage III osteonecrosis, the concept of hemiarthroplasty was appealing in order to defer total hip arthroplasty. Conventional stemmed hemiarthroplasty may fulfill this goal but this procedure resects the femoral head and part of the neck and violates the femoral canal, and revision may be completed by removing the stem whereas precision fit surface hemiarthroplasty maximizes tissue conservation. By preserving proximal femoral bone, revision surgery is facilitated. We reported the medium term results using custom titanium-alloy shells to precision-fit against the most preserved articular cartilage in ten patients whose average age was 32 years in 1987 (Tooke, 1987) and subsequently the longer-term results in 1994 (Amstutz, 1994). Further analysis has refined indications in relation with long-term results (Beaul・/span> et al, 2001) and two patients have now reached 22 years without being revised. Figure 5: A) Thirty-five year old man who had bilateral ON caused by alcoholism - right, stage III; left, stage II. Figure 5: B) Fifteen years after Ti Alloy custom hemi-surface. Note the new bone that has filled in the acetabular fossa (arrow). There is no evidence of femoral neck narrowing or erosion. The morbidity has been minimal with no sepsis, thromboembolic nerve palsy or other complications. The quality of pain relief and range of motion has been good to excellent in all patients initially and results, although dependent on quality of acetabular cartilage, surprisingly durable. There have been no cases of prosthetic loosening, and the proximal bone is preserved and maintained (Fig. 6). The absence of osteolysis and loosening is due to the absence of a polyethylene bearing. The survivorship of the series performed in patients whose average age was 32 was 85% at 5 years, 67% at 10 years and 42% are still functioning at 15 years (Tooke et al. 1987, Amstutz et al. 1994). Figure 6: Retrieved hemisurface specimen from a 34 year old male with steroid induced ON revised after 10 years due to pain associated with acetabular cartilage wear. Note the preservation of bone on the 3 mm slab sections. There is a slight artifact at the bone/cement interface. Surface or cup hemiarthroplasty is appealing and Hazelwood et al. (1996) have confirmed that surface arthroplastys provide a more normal transfer of stress to the proximal femur and may be expected to prevent proximal bone loss as a result of stress shielding. Sedel et al. (1987) reported good results in 82% of hips at an average 7 year follow-up after Luck cup arthroplasty for ON. Meulemeester and Rosing (1989) reported only 8% failure at an average of 8 years using the Thomine cup. Scott et al. (1987) reported a 13% revision rate at average of 3 years in a series of TARA surface hemiarthroplasties performed for stage III and IV ON. Krackow reported 84% good or excellent results in 9 hips with the TARA with an average age of 41 years at an average of 3 years postoperatively (Krackow KA, et al., 1993). Wagner (1978) reported no revisions at 4 years maximum follow-up in a series of ceramic surface hemiarthroplasties. Nelson using a hemispherical titanium alloy component for femoral head resurfacing presented survivorship of 82% at five years in patients whose osteonecrosis was either idiopathic or caused by etiologies other than sickle cell disease (Nelson, et al., 1997, In Press). Wear of the acetabular cartilage was the cause for revision in all cases. While the histological response was very benign with a few macrophages and some metallic debris scattered throughout a predominantly loose connective tissue despite significant burnishing of the soft titanium alloy components. It is our belief that a harder bearing surface such as cobalt chromium or aluminum might produce even longer durability by minimizing the friction and metallic debris due to the soft titanium alloy component (Fig. 7, A-B). However, it is also surprising how well the articular cartilage space has been preserved in some patients for over 15 years, even though soft titanium alloy femoral components were used and the presence of rather advanced articular cartilage fibrillations in some areas of the acetabulum at surgery (Fig. 5B) We believe that the favorable survival was due to the "precision" fitting of these custom implants to the remaining normal acetabular cartilage. Figure 7: A) Thirty-five year old woman with SLE and Ficat stage III ON. Figure 7: B) 22 years post-operatively cobalt chromium alloy hemisurface with preservation of thin articular cartilage. Note new bone in the acetabular fossa and preservation of proximal bone. Patient is essentially asymptomatic. For those requiring reoperation, revision to either full surface arthroplasty or total hip replacement was easy and much like a primary replacement because of bone stock preservation and intact intra-medullary canals and because there was no debris incited granuloma. Since the quality of the articular cartilage once collapse has occurred is related to long-term durability, we recommend non-weight bearing when a stage III osteonecrosis has been identified in order to minimize secondary acetabular articular cartilage changes. Surgical delay caused by the need for custom components is now overcome by their routine availability in millimeter increments. The new femoral component (Conserve(tm)2), available in one-millimeter increments, has greater than hemispherical coverage and incorporates a short stem to ensure accurate reaming and alignment of the component (Fig. 8, 9, A-B). Figure 8: Wright Conserve(tm) - A new hemisurface femoral component has greater hemispherical coverage and incorporates a short-stem to ensure accurate reaming and alignment with a uniform cement mantle. Figure 9: A) Thirty-one year old cable pole climber with left Ficat stage III ON. Risk factors included steroids for ulcerative colitis, mild alcohol consumption and trauma. Figure 9: B) Post operative hemisurface arthroplasty. The goal of management in patients with osteonecrosis is to preserve the femoral head rather than replace it because the patient population tends to be younger. Precision fit surface hemiarthroplasty offers an attractive, bone preserving and "time buying" alternative to selected patients with stage III or early stage IV osteonecrosis of the femoral head. This method should be considered as part of a lifetime treatment plan in the young patient despite the potentially better initial performance of conventional total hip arthroplasties that will eventually require revision. Unfortunately, if the osteonecrosis is far advanced and where both sides of the joint are arthritic, replacement or resurfacing of both sites of bearing surfaces are necessary. Since conventional joint arthroplasty with polyethylene is unlikely to produce lifetime durability for the young, alternate bearing materials with less wear and tissue reaction must be considered. The New Era of Full Surface Arthroplasty The unsolved problem of treating young arthritic hips and the desire to increase the longevity of prosthetic reconstructions prompted a world-wide renewed interest in metal-on-metal articulations. The above studies and our accumulated experience suggest that in young and active patients who are most likely to outlive any type of hip reconstruction with polyethylene, a surface arthroplasty with a low wear bearing material is a prosthetic solution worth pursuing. It is probable that a reduction in volumetric wear debris will reduce the local inflammatory response to a level insufficient to cause interface destruction and prosthetic loosening. Several types of metal-on-metal prostheses were developed in the 1960s but by the mid 1970s they were completely displaced by polyethylene bearings. These metal systems fell out of favor as a result of the early reports of seizing ("cone clutch" effect) and loosening associated with metal staining which was attributed solely to wear of the bearing surfaces (Amstutz and Grigoris 1996). Some of these joints are believed to have been prone to early failure because of bearing design flaws and manufacturing limitations on bearing surface quality. Another likely cause was the poor head-neck diameter ratio leading to prosthetic impingement and adverse stem features, such as a curvaceous design with sharp edges introducing stress concentrations in the cement. Limitations of recommended implantation techniques at that time, resulted in unsupported cups and early cementing methods, also contributed to premature failures. Most of these factors have now either been improved or can be reinvestigated. However, despite the limitations, a significant number of these hips have survived for 25 to 30 years because of low wear rates and minimal osteolysis. The survivors had, often by chance, the necessary polar bearing, component orientation to avoid impingement and good cementation, suggesting that with these systems it could be extremely durable despite poor stem design and technique (Figure 10). Figure 10: A) Forty-seven year old woman with bilateral OA secondary to DDH. Figure 10: B) Post Bilateral THR, Right Charnley Low-friction arthroplasty (Polyethylene bearing) and Left McKee-Farrar (Metal-on-Metal bearing). Figure 10: C) 32 years post initial THR. The Charnley prosthesis was revised at 13 years. McKee-Farrar still in place. Review of the metal-on-metal literature reveals no bearing seizures, and no significant bearing wear or metallosis in the post 1967 era when most of the gross bearing design flaws had been eliminated (Amstutz and Grigoris 1996). Recent research suggests that the volumetric wear of cast cobalt chrome alloy bearings is 40 to 100 times less than that of the metal-on-polyethylene combination (McKellop et al. 1996; Semlitsch et al. 1989). Studies of McKee-Farrar prostheses after more than 20 years of function indicate a volumetric wear at least 25 times less than that with polyethylene over comparable time and a volume of reactive periprosthetic tissue significantly less than with polyethylene (Doorn et al. 1996; Schmalzried et al. 1996b). Furthermore, in pendulum tests the 28 mm all-metal (CoCrMo forged alloy Protasul 21WF) bearings exhibits the same frictional torque values as the 32mm metal-on-polyethylene counterparts (Streicher et al. 1990). All types of metal-on-metal total hip arthroplasties (McKee-Farrar, Ring, M・ler and Huggler) widely used in the past had large diameter heads similar to the sizes used in surface arthroplasties. Three metal-on-metal surface arthroplasties have been developed, two in Europe by Heinz Wagner in Germany (Wagner and Wagner 1996) and Derek McMinn in England (McMinn et al. 1996). Both systems were initially all-cementless. The Wagner design has a bearing surface of forged cobalt-chrome alloy (F799 with high carbon content) and has a grit blasted, titanium alloy carrier with macro features for fixation to bone (Fig. 11). Figure 11: The Wagner resurfacing components. The initial McMinn surface arthroplasty was cast cobalt-chrome alloy, uncoated press-fit both on the femoral and acetabular side. The acetabular component was 160o in profile and had two fins for iliac fixation and a central stud for axial stability. The reaming for the femoral component was similar to that of the THARIES(tm) with a chamfered cylinder. The femoral component had peripheral antirotation ridges and a short central stem to assist in alignment and initial stability. Subsequently the femoral and socket components were altered for use with acrylic bone cement. The socket component had macro circular recessions and beads. McMinn then modified the components further for cementless fixation on the acetabular side using a series of sharp fins and reintroduced HA coating (McMinn, 1996). We began a pilot surface arthroplasty program using the Wagner and McMinn components. The socket was customized by shortening the central stud and adding grooves and rounded depressions for acrylic keying (Fig. 12A-B, 13, A-B) (Schmalzried et al. 1996a). Figure 12: A) The original McMinn resurfacing component. Figure 12: B) Original McMinn design on the left and modified design for cementation on the right. Figure 13: A) Preoperative radiograph of a 44 year old male with Ficat stage IV osteonecrosis of the femoral head. Figure 13: B) The three year postoperative radiograph. There are no cement-bone radiolucencies and the patient has UCLA ratings of pain 10, walking 10, function 10, and activity 8 We reported on the assessment of technique, initial fixation and early results of 21 hips (Schmalzried et al. 1996b). In our experience, a total of 42 McMinn design Metal on Metal Surface Arthroplasties of the hip were implanted in 23 males and 16 females, average age 47.5 years. The femoral component was cemented in all, the socket cemented in 19, and 23 had a cementless socket. At a mean follow-up of 7.8 years, UCLA hip scores improved for Pain (3.9 to 9.3), walking (6.2 to 9.3), function (5.8 to 8.5) and activity (4.6 to 6.6). 12 hips failed secondary to aseptic loosening at a mean time of 50.3 months: 9 on the socket side, and 3 on the femoral side. There was 1 femoral neck fracture at 9.7 months. The overall survivorship at 7 years was 78.6% with any re-operation surgery as end point which was somewhat disappointing but , as anticipated, the results were not as good for the socket with 18.6% aseptic loosening as for the femoral component with 6.7% aseptic failure. There were many problems with these early designs due to sub-optimal fixation and poor bearing preparation as determined by a sophisticated analysis of roundness and clearance. In 1996, we began custom implantation with new devices designed to minimize wear and optimize fixation. The acetabular component is one piece and has sintered beads (102 ( avg., 50-200 ( - 38% porosity) on the outer dimension designed for interference fitting to obtain initial stability even in the smaller diameter dysplastic acetabulum while bone ingrowth occurs (Fig. 14). The femoral component is patterned after the THARIES(tm) chamfered cylindrical design but with a short stem to ensure precision reaming and a fixation with a uniform cement mantle. The component is identical to that used for hemisurface arthroplasty but with improved sphericity and surface finish bearing tolerances to minimize friction and wear. The short-term results are promising with osseointegration (absence of radiolucencies) in all acetabulae (Fig. 15A-B). Figure 14: Metal/metal surface arthroplasty. Figure 15: A) Fifty-nine year old male with progressive left hip pain due to osteoarthritis. Note subluxation. Figure 15: B) Postoperative metal/metal surface arthroplasty. Patient has returned to golf and bowling four months post-operatively. The major advantage of surface arthroplasty is its conservative nature. No "bridges are burned" now that the acetabular reconstruction is also very conservative and removed very little bone. If a revision is ever required, it would be a relatively simple surgery to perform. We believe that the lessons learned about design and technique of implantation of resurfacing components, combined with modern precision manufacturing of metal-on-metal bearing surfaces, have ushered in a new era of surface arthroplasty of the hip. Since the volumetric wear reduction is substantial, we anticipate greatly improved durability. It is inconceivable that metal-on-metal devices will wear out! Further, it is our hypothesis that the metallic wear debris is much better tolerated by the tissues around the implant than is the polyethylene. We have histological data to support this premise although we need more information with respect to the overall body response. In addition, there have been improvements to the technique to minimize the risk of nerve injury and an anti-inflammatory (Indomethacin) has been effective in reducing heterotopic ossification (bone formation) where it is not wanted. Although the technique modifications have enabled this technically demanding surgery to be performed more easily, it remains more difficult than that of stem-type devices. References 1. Ahnfelt L, Herberts P, Malchau H, et al: Prognosis of total hip replacement.Acta Orthop Scand, Suppl 238 61:1-26, 1990. 2. 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