The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 90, Issue Supplement 3

Clinical Issues | August 01, 2008

Modern Metal-on-Metal Hip Resurfacing: Important Observations from the First Ten Years

Michael A. Mont, MD¹; Thomas P. Schmalzried, MD²

¹ Center for Joint Preservation and Replacement, Rubin Institute for Advanced Orthopaedics, Sinai Hospital of Baltimore, 2401 West Belvedere Avenue, Baltimore, MD 21215. E-mail address: mmont@lifebridgehealth.org
² Joint Replacement Institute, 2200 West Third Street, Suite 400, Los Angeles, CA 90057

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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Wright Medical and DePuy). Also, a commercial entity (Wright Medical) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

The Journal of Bone and Joint Surgery, Inc.

J Bone Joint Surg Am, 2008 Aug 01;90(Supplement 3):3-11. doi: 10.2106/JBJS.H.00750

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Total hip resurfacing, in which a cementless acetabular component is combined with a metal-on-metal (cobalt-chromium alloy) bearing, was introduced approximately ten years ago. Previous resurfacing devices had failed because of insufficient initial fixation or as a result of progressive inflammatory bone resorption (osteolysis) due to high volumetric wear of the large-diameter acetabular bearing, which is composed of conventional polyethylene^1,2. Extensive pelvic bone loss often made for a challenging revision.

Figures in this Article

Introduction

Current-generation cementless monoblock metal-on-metal bearing acetabular components are thin shelled (<5 mm). The amount of acetabular bone removed is no greater than that which would be removed during standard total hip arthroplasty. The cementless fixation principles that apply to the placement of these components are the same as those established over the past twenty years for the placement of standard total hip prostheses. Extensive wear simulation testing by numerous investigators has proven that the combination of high carbon content, low diametrical clearance, and low surface roughness produces the lowest wear of metal-on-metal bearings^3-6. Improvements in manufacturing allow the current generation of these bearings to be more consistently machined to tight tolerances. Given that the alloy has a high carbon content and the manufacturing parameters are held constant, heat-treating should have no appreciable effect on wear resistance⁷.

The clinical results from the first ten years come primarily from England, Australia, and Western Europe. Due to U.S. Food and Drug Administration (FDA) regulations, the U.S. experience is comparatively limited, although the devices from several American orthopaedic manufacturing companies are available in the non-U.S. market. Currently, there are two devices that have full FDA approval and several others that are still in the clinical-trial stage and awaiting approval. The following is an overview of modern metal-on-metal total hip resurfacing, highlighting the lessons learned over the past ten years.

Patient Selection: The Key to Success

Resurfacing represents approximately 10% of all total hip arthroplasties in countries where both technologies have been widely available now for ten years. In Australia, resurfacing arthroplasty comprised 7.9% of the hip arthroplasty procedures performed in 2007, and, in various European countries, the rate has ranged between 6% and 9%⁸. The data from the Australian registry, which includes more than 10,000 patients, have been useful in comparing the survivorship associated with resurfacing arthroplasty to that associated with standard total hip arthroplasty⁸. Men who are younger than sixty-five years of age have similar or slightly superior survival curves after resurfacing than they do after standard total hip arthroplasty (approximately 98% survival for both procedures at a mean follow-up of approximately five years)⁸. A higher risk of short-term failure has been demonstrated in men who are sixty-five years of age or older and in all women. In the Australian registry, hips that underwent resurfacing as treatment for osteoarthritis have had better survival than those that underwent resurfacing for the treatment of developmental dysplasia, inflammatory arthritis, and osteonecrosis.

The main cause of resurfacing failure is femoral neck fracture, with the greatest number of fractures occurring within six months of surgery. The Australian experience, as reported by Shimmin et al., revealed a femoral neck fracture rate of approximately 1% in men (0.98) and nearly 2% in women (1.91)^9,10. Awareness of this risk has led to a decreased utilization of resurfacing in women. Recent studies from Australia suggest that it is not male or female sex per se that predisposes to fracture, but the femoral neck diameter (a surrogate for bone mass, or bending strength), as small-boned men have a fracture risk equal to that of women¹¹.

In the U.S. FDA trial of the Conserve Plus prosthesis (Wright Medical, Arlington, Tennessee), tightening the inclusion criteria (no osteopenia, no body mass index of >35, and no femoral cysts >1 cm) significantly reduced the occurrence of short-term complications (p < 0.001)¹². At a minimum follow-up of twenty-four months (mean, thirty-three months; range, twenty-four to sixty months), the overall complication rate decreased from 13.4% to 2.1% and the femoral neck fracture rate decreased from 7.2% to 0.8%¹².

Early intervention with resurfacing is associated with better outcomes. Hips with a greater degree of arthritis radiographically had a significantly lower mean preoperative Harris hip score (p = 0.004) and significantly lower mean preoperative range of motion (p = 0.0004). With a minimum two-year follow-up, hips with a lesser degree of arthritic changes radiographically had significantly better pain relief (p = 0.03) and a trend toward a higher activity level (p = 0.1)¹³.

A number of studies have looked at the best candidates for resurfacing^14-18. Exclusions should include patients with known metal allergies and/or hypersensitivities or those who have altered renal function. However, on the basis of multiple studies, various patients have been deemed less than ideal candidates for the procedure. Other relative contraindications for resurfacing include inflammatory arthritis, developmental hip dysplasia, women of child-bearing age, and a limb-length discrepancy of >2 cm. Radiographic factors that can make resurfacing challenging include alterations in proximal femoral geometry, such as a short femoral neck, a high-riding greater trochanter, or an increased neck width-to-head ratio¹³.

Patients with osteonecrosis have had disparate results. Some reports have suggested that resurfacing might not be appropriate for osteonecrotic hips because osteonecrosis is often associated with dead bone and is sometimes associated with complete femoral head involvement. However, most cases of osteonecrosis do not involve the entire head and several recent reports showed favorable results in this diagnostic group^19-21. Revell et al. reported on seventy-three resurfacing procedures performed in sixty patients with osteonecrosis¹⁹. They found an overall survival rate of 93.2% at a mean of 6.1 years. They concluded that resurfacing should be considered a safe and effective form of surgery for this patient group. Mont et al. also reported similar successful results in patients who underwent resurfacing for the treatment of osteonecrosis of the femoral head; in that study, successful clinical results were seen in forty-one of forty-two osteonecrotic hips at a mean follow-up time of five years²⁰. Fitts et al. have recently reported similar successful results with use of the Cormet device in this challenging patient population²¹.

On the basis of such survival and outcomes data, the utilization profile for resurfacing has evolved such that the procedure is now used predominantly in younger men and less so in older patients, especially women. Recent studies have indicated that more patients in their forties through sixties are candidates for total hip arthroplasty. Because resurfacing is viewed by many as a "conservative" bone-sparing alternative to standard total hip arthroplasty, there may be a commensurate expansion of the utilization of this procedure. Many of these younger patients who would normally wait for a standard total hip arthroplasty now seek earlier intervention because they view resurfacing as a way to maintain an active lifestyle. They elect earlier intervention so that they may return to the job, activity, or sports-related level of function that they enjoyed before the development of arthritis²².

Experience Counts

Marker et al. analyzed 550 resurfacing procedures performed by a single surgeon²³. The influence of the learning curve was apparent, as twelve of fourteen fractures occurred in the first sixty-nine resurfacings. After this, the fracture rate was 0.4% (two of 481). Women and obese patients had a higher prevalence of fractures. These authors emphasize the need for careful patient selection and surgical technique.

High success rates have been reported by high-volume resurfacing surgeons. Daniel et al., who studied the results of hip resurfacing in a group of 446 hips in patients who were younger than fifty-five years and had a diagnosis of primary osteoarthritis, reported a 99.8% survivorship at a mean follow-up of 3.3 years²⁴. Treacy et al. described a 98% survival rate in 4,000 osteoarthritic hips²⁵. Failure was associated with female sex, previous surgery, and osteonecrosis. Of forty-one revisions, eighteen were for a femoral neck fracture. DeSmet reported his experience with 2,474 resurfacings with an approximately 3.8% failure rate²⁵. Of forty-eight revisions, 65% were for cup malposition.

Following formal industry-sponsored training with the Birmingham Hip Resurfacing prosthesis (Smith and Nephew, Memphis, Tennessee), safety surveys were completed on the first 540 cases performed by eighty-nine American surgeons²⁶. Adverse events were documented in forty cases (7.4%) and included nine nerve injuries (1.7%) and eight dislocations (1.5%). There were fourteen early revisions (2.6%), including ten for a femoral neck fracture (1.9%), two for dislocations, and two for acetabular component loosening. Complications were frequently seen in patients older than fifty-five years and in women, emphasizing the importance of appropriate patient selection.

In summary, good judgment may come from bad experiences: surgeons who adopt this procedure in the future may not need to repeat the learning curve with regard to patient selection for resurfacing.

Bone Necrosis and Surgical Approach

Concerns persist that the surgical dislocation of the hip for resurfacing causes bone necrosis and clinical failure^27-31. Most commonly, the procedure is performed through a posterior approach, although the advantage of an anterolateral or direct anterior approach for preservation of the blood supply to the femoral head and neck is recognized. Steffen et al. found that the anterolateral approach led to less disruption of the blood supply to the proximal part of the femur than the posterior approach did³¹. In another study, Khan et al. measured the concentration of infused tagged cefuroxime in bone samples from the hips of twenty patients who underwent either an anterolateral or a posterior approach³⁰. The posterior approach was associated with a significant reduction in blood supply to the femoral head as compared with the anterolateral approach. With use of laser Doppler flowmetry and a vascular-preserving surgical approach, Beaulé et al. studied blood-flow measurements in the femoral heads of ten hips in which advanced osteoarthritis was treated with metal-on-metal hip resurfacing²⁸. Nine hips had a mean decrease of 70% in femoral head blood flow after standard reaming and preparation. These data suggest that femoral head reaming substantially impacts blood flow to the femoral head. The investigators inferred that the extraosseous blood supply is still important to the arthritic femoral head.

The reductions in the measured blood supply associated with the posterior approach have rarely resulted in clinical failure, however, and the approach of choice is posterior for many high-volume resurfacing surgeons. In the experience of one of us (T.P.S.), the prevalence of bone necrosis-associated failure that occurred after use of the posterior approach was approximately one in 500 (0.2%)³².

No superiority of approach has been demonstrated clinically. McBryde et al. compared the posterolateral and direct lateral approaches for two surgeons in a consecutive series of 790 patients (909 hips)³³. There were no differences found between the two approaches with regard to complications, additional surgery, implant survival, or Oxford hip scores. The eight-year survival rate was 97.9% for the direct lateral approach and 97.2% for the posterolateral approach. In another comparison study of twenty-three resurfacings performed through a direct lateral approach and forty-one through a posterior approach, no differences were found in various clinical and radiographic indices³⁴.

In summary, it appears that both the anterolateral and posterior approaches can be utilized for resurfacing with no effect on six to ten-year survival. At this time, there is limited experience with the direct anterior approach for resurfacing. It is recommended that when a surgeon adds resurfacing to his practice, he or she use the same "familiar" approach used for standard total hip arthroplasty. Adequate exposure must be obtained to facilitate proper bone preparation and component insertion, which may necessitate a larger incision than the one that would be made for a total hip arthroplasty in the same patient. Finally, efforts should be made to preserve the soft tissue on the femoral neck, regardless of approach.

Surgical Techniques to Avoid Early Fracture of the Femoral Neck

After patient selection, proper femoral bone preparation is the most important factor in avoiding early fracture of the femoral neck. Placement of the femoral component in slight valgus is desirable, while relative varus is associated with a higher risk of fracture. Varus placement of the femoral component, notching of the femoral neck, and other technical problems were found in 85% of early fractures¹⁰. Shimmin and Back reported that varus placement of the femoral component of >5° compared with the preoperative femoral neck-shaft angle was seen in 71% of the hips that fractured⁹. Beaulé et al. found that the mean femoral stem-shaft angle in the coronal plane in hips that had an adverse outcome was significantly lower than that seen in the rest of the cohort (133° compared with 139°, p = 0.03)¹⁵. Hips with a stem-shaft angle of =130° had an increase in the risk of an adverse outcome by a factor of 6.1.

In a finite-element analysis, Radcliffe and Taylor demonstrated that valgus alignment of the femoral component produced strain patterns more similar to those seen in the intact femur³⁵. As the implant angle increases from varus to valgus, the strains in the superior femoral neck are reduced while those in the inferior neck are increased. The authors concluded that valgus alignment of the resurfacing prosthesis is preferable to varus alignment.

Anglin et al. loaded ten notched cadaver femoral pairs to failure; one side was implanted at 0° relative to the femoral neck, the other at 10° of valgus³⁶. Across all specimens, failure load correlated strongly with bone mineral density (r = 0.89, p < 0.001). Valgus placement increased the fracture load by an average of 28% over neutral in specimens with normal bone mineral density; it had no significant effect on fracture load in specimens with low bone mineral density. For specimens with normal bone mineral density, component position had a greater effect than bone mineral density, explaining 54% of the fracture load variance. Thus, valgus femoral component positioning is preferred, and low bone density is a relative contraindication to resurfacing.

The purely mechanical effect of notching of the superolateral aspect of the femoral neck is the subject of some debate. In a cadaver study of nonarthritic femora, Mardones et al. found that resection of up to 30% of the anterolateral quadrant of the head-neck junction did not significantly alter the load-bearing capacity of the proximal part of the femur³⁷. However, a 30% resection significantly decreased the amount of energy required to produce a fracture (p = 0.0015). These results appear to suggest that the proximal aspect of the femur is relatively tolerant of neck notching. However, the resection was not intended to simulate the discrete cortical disruption of a resurfacing neck notch. The loading condition utilized was highly compressive and not representative of more severe in vivo conditions. Furthermore, the femora tested did not have remodeling of the head-neck junction that is typical of arthritic hips. Thus, it may not be appropriate to extrapolate these results to resurfaced hips.

While slight valgus is desired, excessive valgus and other aberrations of surgical technique can result in superolateral notching of the femoral neck. With use of laser Doppler flowmetry, blood flow was measured in fourteen osteoarthritic femoral heads during routine total hip arthroplasty, before and after notching of the femoral neck²⁷. In ten hips, there was a reduction in blood flow of more than 50% from the baseline value after simulated notching of the femoral neck. These results suggest that damage to the extraosseous vessels can predispose to osteonecrosis, and excessive dissection around the femoral neck and/or notching should be avoided. Beyond the mechanical weakening caused by a notch, loss of blood supply probably contributes to in vivo failure. Thus, it is not surprising that some element of necrosis is present in resurfaced heads that were retrieved following early fracture of the femoral neck.

Another important identified surgical technique variable is femoral cementing. In two recent retrieval studies, aberrancies in cementing technique have been implicated in early femoral failures. Morlock et al. found that 63% of fifty-five retrieved specimens demonstrated undesirable cement distribution³⁸. In general, the cement thickness was highest at the pole and lowest on the sides, with large variations between specimens. The polar cement mantle sometimes exceeded 5 mm (37%). Since the polymerization temperature of the cement increases with its volume, bone necrosis can be expected to rise as the volume of cement increases. Evidence of trauma, including fractures within the femoral neck, was observed, as could occur with vigorous impaction to overcome the resistance to seating of the component during implantation. Some failures had evidence of a pseudarthrosis beneath the implant.

Very similar observations were made by Campbell et al. In another retrieval analysis of ninety-eight specimens³⁹, the amount of cement in the retrieved femoral specimens varied considerably and was actually greater in loose components. Histology typically verified necrosis of interfacial bone. Most of the short-term fractures had regions of new bone at the edges consistent with repair of a previous stress fracture or similar damage through which the fracture had occurred. A finite element model indicated that the peak temperatures within the femoral head interfacial bone were high enough, and the durations were long enough, to cause thermal bone damage. The model actually underestimated the amount of cement seen in many of the retrievals. These observations should be a cause for concern for cementing techniques in which controlling mantle thickness and the extent of penetration may be difficult.

The available resurfacing systems differ in the tolerances (amount of space) between the component and the prepared bone, as some are tight press-fit designs while others are intended to have a cement mantle. There are also variables in bone preparation, such as pulsed lavage, bone drying, and femoral suction^40,41. Thus, it is important to understand system-specific differences as well as the effects of femoral preparation and specific cementing techniques.

Radcliffe and Taylor analyzed two cementing techniques that produce varying cement-mantle thicknesses⁴². They used computed tomography-based finite element analysis to determine the effect of cement-mantle thickness on strain in the resurfaced femoral head. They concluded that a thicker cement mantle increases strain-shielding within the superior resurfaced femoral head.

Bitsch et al. developed a laboratory model in which open-cell foam specimens were used to simulate bone⁴³. Analyses of two cement viscosities, two foam porosities, and six cementing techniques were performed. Manual application of medium-viscosity cement consistently gave even, 3-mm-deep, cement penetration over the available fixation surface. Sixteen of seventy-two specimens showed incomplete seating, which occurred with all techniques except the manual technique. Pouring any cement into the shell resulted in variable degrees of deeper penetration and a risk of incomplete seating, which has been associated with bone necrosis and early fracture.

Acetabular Issues

Acetabular bone loss was an issue with the prior generation of resurfacing components that utilized a cemented polyethylene socket. Current-generation components are press-fit into the acetabulum and subsequently achieve biological fixation. If the thickness of the component is <5 mm, then the acetabular reaming for the resurfacing should be no greater than that for a standard total hip arthroplasty⁴⁴.

The standard cobalt-chromium-alloy resurfacing acetabular component achieves initial stability from a mechanical press-fit. Some manufacturers also offer a component with lateral screws for dysplastic hips or for revisions. For standard total hip arthroplasty, many surgeons utilize a modular titanium-alloy acetabular component that generally has at least one hole through the component, which allows for confirmation of complete prosthetic seating. There are differences in the shapes of the available resurfacing sockets (such as true hemisphere, subhemisphere, and peripheral rim flare or expansion). There is also variability in the sizing nomenclature; for some components the actual diameter is 1 to 2 mm greater than the nominal diameter. These differences have implications for the acetabular reaming technique, so it is critical for the surgeon to know the details of the component he or she intends to implant. The goal for all components is the same: full seating with sufficient peripheral press-fit (low interfacial motion) to allow bone ongrowth or ingrowth.

Due to differences in the stiffness of the cobalt-chromium-alloy resurfacing components compared with the more flexible titanium-alloy standard total hip arthroplasty shells as well as the shape and sizing issues outlined above, obtaining a consistent press-fit with a monoblock resurfacing shell is more challenging. This may, at least in part, account for the higher prevalence of loose resurfacing acetabular components, especially early in a surgeon's experience^{12,23,26,38,39}. It is prudent to slightly underream the acetabulum, with the degree of (nominal) underreaming being dependent on the specific component to be implanted. If full seating cannot be achieved, and there is no soft-tissue impediment, the surgeon should extract the component, ream the periphery slightly larger, and then reinsert it. The stability of the component is enhanced by full seating. Until the surgeon is comfortable with the feel of the press-fit component against the resurfacing socket, it may be helpful to obtain an intraoperative radiograph. One of us (M.A.M.) routinely uses fluoroscopic guidance and confirmation of seating on all cases.

Some surgeons believe that the large-diameter bearings have a broader range of acceptable positioning because of their inherent stability. However, proper component position is necessary to achieve the low wear potential of the metal-on-metal bearings. The lateral opening angle (abduction angle) of the acetabular component influences the wear of the bearing. Similar to bearings made of other materials, metal bearings with higher (>55°) acetabular abduction angles are at increased risk for higher wear. This has been demonstrated both in wear simulator studies⁴⁵ and in retrieval analyses^38,39. High wear of a metal-on-metal bearing can cause metallosis and adverse tissue responses of an inflammatory and/or immune type. The bending moment placed on the component from the lateral head contact may also challenge bone ingrowth, hence compromising fixation.

Higher acetabular abduction angles (>55°) are associated with higher ion levels in the blood^46,47. This is a direct result of the limited contact area of the bearing in that orientation and the consequent high and localized contact stresses. A high contact area is necessary to minimize contact stress, improve lubrication, and achieve the extremely low wear of a metal-on-metal bearing. For this reason, a lower lateral opening angle is preferred (<50°). As of this writing, we are not aware of the existence of any formal report on the effect of socket anteversion on wear rates or ion levels. However, it is likely that both the lateral opening angle and the amount of anteversion influence wear in vivo.

Tribology

The effect of metallurgy on the clinical performance of resurfacing components has been the topic of some controversy. While the cobalt-chromium alloys all have similar chemical compositions, their microstructures can be different. The "as-cast" material microstructure has large grains with blocky carbides evenly distributed throughout the structure. Wrought material has a small grain size with a fine, homogeneous distribution of small carbides. Heat treatment results in a more uniform distribution of carbides along grain boundaries and in the matrix. Published hip-simulator studies indicate that only carbon content appreciably affects wear: high carbon content (greater than 0.20%) demonstrates consistently lower wear than low carbon content (less than 0.05%)³. When implants of similar geometrical form are tested in a hip-joint simulator, alternative combinations (such as cast; cast:wrought; heat-treated) of the currently adopted high carbon cobalt-chromium alloys do not exert a major influence on the magnitude of the wear volume⁴.

There is theoretical and experimental evidence that design characteristics (bearing geometry) can significantly lower wear. Wear volumes are reduced with larger-diameter heads and smaller clearance⁵. To achieve this benefit in vivo, the components must be appropriately oriented⁴⁵. The clinical benefit of reducing wear is a reduction in ion production. A resurfacing prosthesis with a diametrical clearance of 100 µm demonstrated significantly lower urine cobalt excretion and whole-blood ion levels compared with a resurfacing prosthesis with a diametrical clearance of 250 µm⁴⁸.

Activity Level and Function

Three published reports of small patient groups have directly compared resurfacing and conventional total hip arthroplasties^49-51. Vail et al. compared the two-year follow-up results of fifty-seven resurfaced hips with those of ninety-three hips that received conventional total hip arthroplasty and reported that the functional scores of the two groups were similar but that the activity and range-of-motion scores were greater in the patients who had resurfacing⁴⁹. Similarly, Pollard et al. compared two matched groups of fifty-four patients who had a mean time of follow-up of five to seven years and found that, although the patients had similar hip function scores, the patients who underwent resurfacing had higher activity and quality-of-life scores⁵⁰. These studies had potential confounding factors, including the learning curves associated with the procedures, varied follow-up times, diverse indications for the procedure as well as medical comorbidities, and unreported preoperative hip scores.

Mont et al. compared the clinical and radiographic outcomes of two closely matched groups of fifty-four patients who underwent resurfacing and conventional total hip arthroplasty, respectively⁵¹. At a mean time of follow-up of forty months, the mean Harris hip scores increased similarly in both groups. Radiographic outcomes, revision rates, complications, and pain and satisfaction scores of the two groups were similar. The patients who underwent resurfacing had significantly higher postoperative weighted activity scores than the conventional arthroplasty patients did, although they had higher preoperative weighted activity scores as well. The outcomes of both procedures were similar.

In a study by Naal et al., 112 patients who underwent hip resurfacing were analyzed, at a mean of twenty-four months after surgery, with a postal questionnaire concerning sport disciplines⁵². Approximately 26% of the patients performed sports four times per week or more for longer than sixty minutes at each session. These patients, on the average, returned to a higher level of sports participation after surgery. In a study by Narvani et al. of fifty-one resurfacing procedures, thirty-three hips (65%) belonged to patients who participated in sports before surgery⁵³. This proportion increased to forty-seven hips (92%) following surgery. There were also marked increases in intensity and frequency of sports participation following surgery.

At this point, it is too early to determine whether sporting activities have any influence on the outcome of hip resurfacing. Recently, a randomized prospective study comparing resurfacing (n = 107 patients) to standard total hip arthroplasty (n = 103 patients) was reported⁵⁴. Patients in both groups demonstrated high satisfaction rates with similar functional results and complication rates at two to five years after implantation. However, in a study of a subgroup of these patients (n = 165 patients), these same authors found higher activity levels in the patients who had resurfacing⁵⁵.

The experience to date suggests that patients who have undergone resurfacing are, on the average, more active than patients who have undergone a standard total hip arthroplasty. However, there are limitations in the design of most of the studies, with the most notable limitations being the lack of randomization and blinding. On this basis, it would be inaccurate to conclude that a resurfacing prosthesis provides any higher functional capacity than that provided by a modern standard total hip prosthesis. The real difference may be in the attitude of some resurfacing patients, who simply choose to pursue more vigorous activities. At this time, there are no data on the effect of any level of activity on the survival of a contemporary total hip or resurfacing prosthesis.

Gait studies of patients who have had resurfacing compared with patients who have had total hip arthroplasty have been equivocal. Mont et al. found superior gait parameters (walking speed, abduction moments) in patients who had resurfacing as compared with patients with total hip arthroplasties and osteoarthritic hips⁵⁶. The gait indices were similar to those of matched control patients without hip disorders. However, Shimmin et al. recently found no differences in the gait of twelve patients with a well-functioning total hip prosthesis as compared with the gait in fourteen resurfacing patients⁵⁷. Both of these groups were also not different from a control group of age-matched subjects with bilaterally asymptomatic hip-joint function. In another study, gait analysis was performed on patients after three types of arthroplasties: conventional total hip arthroplasty, large diameter arthroplasty, and resurfacing arthroplasty⁵⁸. When eight patients in each of these groups were compared with eight control patients, the authors found that the large head and resurfacing groups had normal gait patterns that were superior (in terms of recovery of function) to those of the patients who had conventional hip arthroplasty.

Cobalt and Chromium Ion Exposure

There are concerns regarding increased exposure to metal particles and ions from all metal-on-metal bearings. A patient with a metal-on-metal resurfacing prosthesis will have a measurable increase in cobalt and chromium ion levels (serum, whole blood, and urine). There is tremendous variability in the measured elevations from patient to patient. To date, an absolute safe level has not been determined. Conversely, an absolute unsafe level has also not been determined. The urine and blood metal ion levels from patients who underwent resurfacing arthroplasty have been analyzed at various intervals after implantation⁵⁹. The mean levels increase after implantation and reach a peak at six to twelve months postoperatively. A steady decrease occurs over the next year, such that mean blood chromium levels at two years were significantly lower than they were at one year (p = 0.0068). In another study, age and activity level did not correlate with the whole-blood concentration of metal ions in sixty-four patients who had undergone resurfacing⁶⁰. In one case report, serum metal ion levels in a patient who had just completed a triathlon were elevated right after the event but returned to pre-race levels six days later⁶¹. Increased ion concentration following prolonged exercise is due, at least in part, to decreased urinary ion excretion (decreased urine volume), and not necessarily due to increased production⁶².

Vertical acetabular components (a high lateral opening angle or abduction angle) are associated with higher ion levels^46,63. Hart et al. reported that patients with exceptionally high ion levels (outliers) had a mean cup inclination of 56° compared with a mean of 42° for non-outliers⁴⁶. These outliers represented 4% of the patients with blood levels of metal that were tenfold greater than the mean. A similar observation has been made by Isaac et al.⁴⁷: hips with a lateral opening angle of >55° were substantially more likely to have high ion levels.

Adverse local immune responses, such as aseptic lymphocytic vasculitic-associated lesions, have been reported in association with metal-on-metal bearings⁶⁴. A series of twenty patients who developed a mass after resurfacing has been presented⁶⁵. All were women. Histology showed lymphocyte aggregates with absence of polymorphonuclear leukocytes. Patients with persistent pain, a mass, or a fluid collection following total hip arthroplasty should be carefully evaluated, especially for the presence of infection. Aspiration studies may not be conclusive. However, the histology of an infection is distinctly different from that of an adverse immune reaction or an inflammatory foreign-body response. A biopsy and tissue analysis facilitates the differential diagnosis. At this time, there is no reliable blood or other screening test for metal sensitivity.

Narrowing of the femoral neck is a common phenomenon after hip resurfacing arthroplasty^66,67. A retrospective review of the results of 163 hip resurfacing arthroplasties at a maximum of six years revealed some degree of narrowing of the femoral neck in 70% of the patients; in 28% of the patients, the narrowing was >10% as compared with the original postoperative width, but it was not associated with any adverse clinical outcome⁶⁶. It has been suggested that narrowing of the femoral neck occurs in the first three years and then stabilizes. Progressive narrowing after three years or rapid extensive narrowing warrants close observation. The etiology of femoral neck narrowing is unknown. Ball and Schmalzried demonstrated scalloped bone loss in the posterior neck adjacent to the femoral component and acetabular rim remodeling in fourteen hips (20%)⁶⁷. They did not find any adverse effect on clinical outcome, but they did believe that it occurred more often in active patients, and they hypothesized that it was due to neck-socket impingement.

Revision of Resurfacing

In the previous generation of resurfacings (metal-on-polyethylene), revision procedures were not always straightforward. This is because high volumetric wear of the large-diameter polyethylene components led to massive acetabular osteolysis. Revisions often required extensive bone-grafting (bulk allografts) of large defects. Conversion of a current-generation resurfacing prosthesis to a standard total hip prosthesis appears to be easier. Ball et al. reported on twenty-one resurfacing prostheses (in twenty patients) that were converted to a conventional-stem total hip prosthesis because of femoral component failure⁶⁸. In eighteen of these hips, the acetabular component was retained, whereas, in three hips, both components were revised. These hips were compared with a group of sixty-four hips that underwent primary total hip arthroplasty during the same time period. At a mean follow-up of approximately four years, there were few differences between the conversion arthroplasty group and the conventional total hip arthroplasty group with regard to operative time, blood loss, or complication rates. They were also similar in terms of clinical scores, with no aseptic loosening of femoral or acetabular components in either group. Another report by McGrath et al. compared the outcomes of thirty-nine resurfaced hips that underwent conversion to a conventional total hip prosthesis and compared them with a matching group of primary total hip prostheses⁶⁹. The mean perioperative time was longer (by nineteen minutes) for the conversions, but other perioperative parameters were similar. Thirty-eight of thirty-nine stems were well-aligned and appeared to have achieved osseointegration. The authors concluded that when a resurfaced hip fails, conversion to conventional total hip arthroplasty is associated with early clinical and radiographic outcomes similar to those seen after primary conventional total hip arthroplasty.

Future

Further research is being done to address less invasive approaches for the performance of resurfacing. Various surgeons have attempted anterior approaches, but these approaches are still technically difficult and should not be considered as standard technique. A number of studies have looked at minimally invasive approaches for hip resurfacing. Chana used a new posterior gluteus maximus muscle-splitting approach with an average incision length of 7 cm⁷⁰. At a mean follow-up of two years, the author had a success rate of 97.8%. Hip resurfacing by a mini-anterior approach has also been reported⁷¹. The authors in that study had only one femoral neck fracture in their first 100 procedures, with a median follow-up of three years. Other studies have reported on the safety of these less invasive approaches^72,73, although these approaches are not presently used routinely. We believe that, in the future, navigation will be used in resurfacing, as it will be a common tool for arthroplasty surgery.

To date, navigation has been applied specifically to help aid in the placement of the femoral component. Various computed tomography navigation-based systems have obtained improved accuracy when compared with manual systems, without substantially increasing operative time. Schnurr et al. found an error of 23° with use of conventional methods compared with 7° with navigation⁷⁴. Hodgson et al. analyzed the variability of surgical technique and experience when computer-assisted or free-hand techniques were used⁷⁵. Better repeatability in varus or valgus placement (1.2°) was found in the computer-assisted group when compared with the free-hand (2.8°) group. Cobb et al. analyzed the accuracy of femoral component placement with use of three different methods (digital planning software, computed tomography-based computer-assisted surgery, and conventional instrumentation)⁷⁶. Among a group of medical student subjects, the error was 23° when conventional methods to insert a guidewire were used, and it was 22° when the digital planning method was used. When the computed tomography-based navigation method was used, the error was only 7° (p < 0.002). They concluded that surgical navigation may play a major role in reducing the length of the learning curve. These studies appear promising, although it remains to be determined if the potential improved optimization of component alignment outweighs the potential increased costs, learning curve, and possible additional operating time over manual methods.

In the future, new designs may utilize different interfaces and materials to reduce or eliminate metal components and thus any potential adverse effects of metal ions. The designs might include ceramic-on-ceramic articulations, a ceramic head against a metal acetabulum, or metallic surfaces that are coated with a thin ceramic layer. It is also theoretically possible that, because of improvements in polyethylene and other polymers, there may be technology in the future that allows resurfacings to be made with larger hard-on-soft bearings or even soft-on-soft bearings.

Other potential design-related factors for future implant generations include the amount and extent of cement fixation, implant positioning, minimization of stress-shielding, improvement in modularity and/or geometry options, metallurgy, and acetabular fixation (such as with ancillary screws or spikes). Another possibility is the use of cementless femoral-side fixation. This has been reported by Lilikakis et al. in seventy hips with no femoral side failures at short-term follow-up (mean follow-up time, approximately two years)⁷⁷.

Conclusions

In summary, resurfacing hip arthroplasty has had a tremendous increase in popularity because of the potential benefits of femoral bone conservation, possible increased functional ability, and ease of potential revision procedures. In young men, the survivorship has been similar to that of standard total hip arthroplasty. Most authors recommend meticulous attention to indications and techniques. There are studies that have compared resurfacing to standard total hip arthroplasty to try to show the superiority of resurfacing in terms of gait, activity level, and various other functional parameters. These studies have been difficult to analyze and perform, and we believe that any currently reported results are preliminary at best and could be misleading. We do not believe that this should be a competition; another perspective is to regard resurfacing as simply an alternative to standard total hip arthroplasty. Resurfacing might be appropriate for a group of premenopausal women. However, resurfacing appears to have the most favorable benefit-to-risk ratio for young men (younger than sixty years) who need a total hip arthroplasty and would like to remain active. Image Not Available

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Topics

cobalt ; fracture ; femur ; neck of femur ; hip region ; hip joint ; metals ; chromium ; hip replacement arthroplasty ; osteonecrosis

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Michael A. Mont, Thomas P. Schmalzried; Modern Metal-on-Metal Hip Resurfacing: Important Observations from the First Ten Years. The Journal of Bone & Joint Surgery. 2008 Aug;90(Supplement_3):3-11.

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