Due to relatively low volumetric wear rates and the ability to use large heads, metal-on-metal bearing total hip and resurfacing arthroplasty are gaining in popularity. Despite generally favorable short- and intermediate-term clinical results, there is concern that adverse local and remote tissue responses will occur in some individuals as a consequence of the host response to the metallic degradation products of these bearings. It has been well documented that the serum and urine metal concentrations in patients with these implants are substantially higher than those seen in patients with conventional metal-on-polyethylene bearings and that these elevated levels may persist for the duration of the implant's lifetime. This is of particular concern in the younger and more active patient where life expectancy after implantation may exceed two or three decades. Even though volumetric wear rates are substantially reduced compared to conventional bearing couples, metal-on-metal bearings actually produce a greater number of particles since the debris generated is in the nanometer size range. Nanometer debris possesses a high specific surface area, likely accounting for relatively high rates of metal release into the surrounding tissues. In addition, multiple investigators have reported a distinct histopathological pattern associated with failed metal on metal bearings with and without osteolysis that is suggestive of a delayed-type hypersensitivity response. Over the last two years, there have been multiple published clinical reports of early failures of contemporary metal on metal total hip replacements that have implicated metal hypersensitivity as a factor contributing to osteolysis and/or pain. At this time, the association of metal release from orthopedic implants with any metabolic, bacteriologic, immunologic, or carcinogenic toxicity remains uncertain since cause and effect have not been definitively established in patients with these devices. Nonetheless, continued surveillance of patient populations with metal-metal bearings is warranted to define the nature and magnitude of the risk of adverse biological responses, particularly those purportedly due to metal hypersensitivity.

The major advantages of alumina ceramic bearings include their extreme hardness and scratch resistance, low coefficient of friction, their hydrophilic nature with improved lubrication, and their superior wear resistance compared to other bearing surfaces.  The major disadvantage to their use is the risk of fracture.  In the 1970’s and 1980’s, reports of ceramic fractures ranged from zero to seven percent.  These early failures of alumina bearings were related to suboptimal material properties and to uncontrolled variables such as implants that were poorly designed for fixation as well as poor taper designs for mating of the ceramic to the implant.  The combination of significantly improved alumina ceramic material (Biolox forte) with implants that have high fixation track records and metal tapers designed to work with alumina implants for proper transfer of loads have led to a dramatic decline in the risk of ceramic fracture.  The risk today is estimated to be 0.02% for the femoral head and 0.0075% for the acetabular component.   

In a prospective IDE study 1382 alumina ceramic implants were implanted without a ceramic fracture out to ten years.  Since FDA approval that same ceramic implant has been used in approximately 52,000 hips and there has been recorded four acetabular fractures (0.008% risk) and nine alumina head fractures (0.017% risk).  A major supplier of alumina ceramic bearings (CeramTec) has distributed over two million Biolox forte alumina ceramic total hips between 2000 and 2005.  A total of 314 fractures and/or intraoperative chips have been reported (0.016%) and 20% of the fractures were related to a specific adverse event including such issues as severe patient trauma, autoclaving followed by cold immersion of the alumina head, mismatch in diameter of head and acetabular insert, and recurrent instability.  While we can assume that the manufacturer’s data may not include all fractures, certain trends have been identified.  Fifty percent of ceramic fractures occurred within twelve months following implantation, 70% within twenty-four months, and 83% within thirty-six months.  Femoral head fractures were higher for the 28mm compared to 32mm size, and higher for the longer and shorter head sizes compared to the standard zero neck length.  Fractures of the femoral head occur more frequently than the acetabular alumina insert. 

Reports of higher fracture rates since 1995 have involved design or technical issues: a ceramic sandwich design which interposed polyethylene between the alumina ceramic and metal shell; a femoral head fracture rate of 1.9% where the femoral stem was manufactured in-house without specific attention to the design of the femoral trunion; zirconia femoral head fractures as a result of a change in manufacturing technique that led to a defective batch of implants leading to a recall. 

The risk of alumina ceramic fracture has been significantly reduced by the development of Biolox forte.  This material undergoes hot isostatic pressing resulting in marked reduction in grain size and impurities and a significantly higher burst strength.  The implants undergo proof testing prior to distribution. 

Alumina ceramic bearings offer a significant advantage as a bearing surface producing low wear for the young and most active patients.  The risk of fracture is extremely low when properly used and less than the risk of fracture of a metal femoral component and/or failure of a polyethylene insert.     

The goal in the development of future bearing materials is to continue to reduce the particle load and thus the biological response to wear debris while at the same time improving on the mechanical properties of the bearing material to reduce fracture risks and to optimize interoperative flexibility.  The current bearing surfaces have in part achieved those goals with good follow-up at 5-6 years. 

There remains room for improvement however.  Fracture risk remains a concern with highly-crosslinked polyethylenes especially with elevated rim liners and thin polyethylene.  Second generation highly-crosslinked materials have been developed to maintain the improved wear characteristics of the polyethylene and reduce the risk for in vivo oxidation while at the same time improving on the mechanical properties of the existing materials.  Similarly, newer ceramic materials currently not approved by the FDA for use in the United States have been used outside this country to improve on the mechanical properties of ceramics.  This should allow the use of larger head sizes in some cases.  It still however will not give us the flexibility as it relates to interoperative sizing when compared with metal on polyethylene.  New bearing combinations such as ceramic on metal are currently being investigated and we can anticipate their introduction into the North American market in the relatively near future.

Continued research in bearing materials for total hip replacement has lead to a marked improvement in wear performance.  Ongoing work is likely to make further improvements in the excellent existing bearing options.  It’s important to remember however that our patients represent the final testing grounds for these new bearings.  As a result, follow-up is imperative.

Hip resurfacing is an option for a selected group of primary hip arthroplasty candidates who could benefit from bone conservation and a large diameter hard-on-hard bearing.  Recently introduced in the United States, the number devices implanted in Europe has more than doubled from 2004 to 2006, comprising approximately 10% of the total volume of hip implants.  The device is distinct from a total hip replacement, which can also provide a large diameter metal bearing, in that it does not disrupt metaphyseal or diaphyseal bone, and both spares and loads the femoral neck bone.  The indication is limited to end-stage destruction of the articular surface with symptoms resistant to non-surgical management.  The ideal candidate is one for whom the benefits of bone conservation are maximal, and the negative consequences of a complication are minimal.  Thus, the ideal candidate is generally younger, desires a high activity lifestyle, and possesses good proximal femoral bone quality.  While reports indicate that resurfacing patients can achieve high functional levels, it is not known whether the activity level after resurfacing represents an intrinsic advantage or selection bias.  The procedure lacks a long term track record.  Hip resurfacing introduces the risks of exposure to metal ions and post-operative femoral neck fracture.  Thus, patients with large bone cysts extending below the implant coverage area, osteoporosis, a broad femoral neck relative to the femoral head, distorted mechanics (coxa vara, coxa breva), compromised renal clearance, or metal hypersensitivity are not candidates.  Retrieval analysis correlates complications with technical factors such as implant sizing, incomplete seating, cement technique, and bone quality.  In conclusion, metal-on-metal resurfacing fits the needs of a selected group relative to total hip replacement, seems to have surpassed the outcome of prior resurfacing designs that failed early, but has not displaced total hip replacement as the procedure of choice for most hip arthroplasty patients.

Start with pre-operative planning.  The femoral neck determines the smallest possible prosthetic size while the acetabulum determines the largest possible size.  Proper patient positioning and exposure are fundamental to success.  Even with a posterior approach, the majority of the quadratus femoris can be preserved.  A complete capsulotomy may be necessary.  Preparing the femur first can facilitate acetabular exposure.  Osetophytes and other bone that overhangs or obscures the head-neck junction posteriorly or inferiorly can be judiciously removed.  A valgus axis (135-140°) for femoral reaming is preferred as early failure has been associated with varus component position.  The guide pin insertion point is commonly superior and anterior to the ligamentum teres and the fovea centralis.  Assure that there is adequate clearance of the superior and anterior neck in order to avoid creating a stress-riser (notch) on the tension side of the bone.  Acetabular preparation and component insertion is essentially identical to that of a total hip.  The author prefers only a slight press-fit with complete seating of the component.  Over-penetration of cement, incomplete seating of the femoral component, and insertional trauma have been associated with early femoral side failure.  On this basis, the author favors manual application of viscous cement directly onto the bone, as is commonly done with total knee replacement.

Introduction:  Metal-on-metal resurfacing was first introduced in the mid-1960’s. The early models of metal-on-metal designs were abandoned because of high rates of component loosening, biological incompatibility of the alloy constituents, and high complication rates. More recently, there has been an advent in the use of metal-on-metal resurfacing with the development of new technology. Wear particle generation, osteolysis, and subsequent aseptic implant loosening is reduced by the advances in both metal-on-metal bearing surfaces and cemented fixation of femoral components. To date, there are a limited number of prospective studies evaluating the results of current metal-on-metal designs. The authors will review the current literature, outline the history of metal-on-metal resurfacing, and report the results of an ongoing metal-on-metal FDA-IDE study.

Methods: Between August 2000 and December 2006, 1892 metal-on-metal hip resurfacings (in 1629 patients) were performed by ten orthopaedic surgeons in an FDA-IDE study.  The metal-on-metal prosthesis was implanted in 1176 men (1375 hips) and 453 women (517 hips). The most common indications for surgery were primary osteoarthritis in 1508 hips (79.8%), osteonecrosis in 177 hips (9.4%), hip dysplasia in 131 hips (6.9%), trauma in 53 hips (3.0%), and inflammatory arthritis in 20 (1%). The mean patient age was 51 years (range, 15-82 years). The mean body mass index was 27.5 (range, 16.3 to 48.2). All patients were evaluated using Harris Hip Scores and SF-12 Health surveys. Patients were radiographically assessed for alignment, loosening and radiolucencies. After an investigator meeting in October 2002, the prosthetic design was slightly altered (thin acetabular shells), the indications modified (excluded BMI >35, osteopenia), and the surgical technique changed (cementing, no notching allowed) to improve outcome.

Results:  At latest follow-up, the implant freedom from revision survival rate was 96.25% and 1468 hips out of 1640 hips (89.5%) were considered to have successful clinical outcome based on a Harris Hip Score greater than 80 points. The mean postoperative Harris Hip score was 92.5 points (range, 27 to 100 points). The mean latest follow-up SF-12 mental and physical component scores were 55.1 points (range, 17.3-68.5 points) and 49.8 points (range, 15.9 to 64.4 points), respectively. Radiolucencies were documented in 181 hips (9.6%), although none were progressive. Common postoperative complications included heterotopic ossification in 188 hips (9.9%), nerve problems in 39 hips (2.1%), hematoma in 36 hips (1.9%), trochanteric bursitis in 29 hips (1.5%), component loosening in 25 hips (1.32%), dislocations in 22 hips (1.1%), and femoral neck fractures in 21 hips (1.1%). Notable, only eight hips (0.4%) experienced protrusio acetabuli and only four hips (0.2%) showed osteolysis.

Conclusion:  The results of this prospective FDA-IDE study illustrate the durability and effectiveness of a modern metal-on-metal resurfacing design across patient populations and multiple surgeons.  In addition, the results indicate that patients should be carefully selected in order to reduce the incidence of complications and further reflects that there is a significant learning curve associated with this procedure. The authors await long-term follow-up to see if these promising results will be maintained.

10:27 a.m.

Paul F. Lachiewi