DISCUSSION ON METAL-ON-METAL BEARINGS
Metal-on-metal (MoM), ceramic-on-ceramic (CoC), and ceramicized metal-on-polyethylene (CMoP) bearings all were developed as a reaction to the well recognized mediocre long-term results with metal-on-polyethylene hips in younger, more active patients.
Polyethylene wear increases with increased activity and load, and often elicits an aggressive response from the body where loosening and bone loss occurs (osteolysis), making subsequent revision more difficult.
Alternate bearings (MoM, CMoP, and CoC) are more resistant to wear, and their wear debris does not usually appear to stimulate near as much osteolytic bone loss response as does polyethylene wear. Metal-on-metal bearings have the added benefit that they can withstand high impact without shattering, as do ceramic heads, and the larger metal head sizes render them statistically about 10 times more stable to dislocation than traditional small femoral head on poly and ceramic-on-poly hips from a posterior approach.
Resurfacing metal-on-metal hip arthroplasty, in addition, offers much less bone sacrifice than a stemmed prosthesis, since most of the femoral head and neck are retained. Resurfacing is also associated with the attainment of a greater activity level as compared to a traditional hip replacement after the initial healing period.
Metal-on-metal big femoral head total hip arthroplasty (THA), which is a variety of a stemmed prosthesis, has been utilized in patients with avascular necrosis or too much deformity of the femoral head. It has also been used in older patients with decreased bone density and decreased activity demands, where the increased stability of the femoral head is desired. It offers the greatest theoretical stability of all the common total hip arthroplasty options because the larger head relative to the small neck design gives the least impingement.
Recently, however, concerns have been increasingly raised about the body’s reaction to the wear products (metal ions) of metal-on-metal bearings and the early failure of some metal-on-metal products. This discussion paper addresses recent issues regarding bearing choices and presents some education and perspective in making decisions as to bearing preferences.
Each bearing option – metal-on-polyethylene (MoP), ceramic-on-ceramic (CoC), ceramicized metal-on-polyethylene (CMoP), and metal-on-metal (MoM) – has pros and cons and risks and gains that need to be considered in selecting a type of replacement that is appropriate for each individual patient.
Metal-on-polyethylene total hip arthroplasty (aka “traditional total hip arthroplasty”) was developed by Sir John Charnley in England and has been successfully used since the ’60s in older patients. Polyethylene wear, osteolysis, loosening, and recurrent dislocation have been the most common problems with traditional total hip arthroplasty. Activity restrictions are necessary and common to prolong the life of a metal-on-polyethylene prosthesis. Non-cemented cups and stems and cross-linking of the polyethylene are now commonly utilized to try to increase prosthesis longevity by decreasing wear. Cross-linking definitely makes the polyethylene more resistant to wear, but also more brittle, and as femoral head size has increased in an effort to reduce dislocation, fracture of the polyethylene is being increasingly reported. Long-term results on cross-linking are not yet available. Few orthopaedists generally recommend metal-on-polyethylene hips in young, active patients, but if utilized, significant lifelong restrictions of high-impact activities are generally indicated.
Because of the recognized polyethylene wear problems, several alternate bearings, as mentioned above, have been developed. Ceramic-on-ceramic bearings have the least tested wear of the bearings available, but have several drawbacks. Fractures of the ceramic head continue to be reported, and high-impact activities are not recommended with these replacements. They are more susceptible to squeaking, with some series reporting up to 17% frequency. If problems such as squeaking or instability arise, they cannot be readily revised to a metal-on-polyethylene replacement later because the minute ceramic wear particles left in the soft tissues at revision rapidly degrade the softer polyethylene in a metal-on-polyethylene revision THA.
Ceramicized metal (Oxinium-Smith & Nephew) on polyethylene bearings resolve the fracture and impact problems for the femoral head and seem to give excellent wear data in simulators. The surface of zirconium metal is oxidized to a thin ceramic layer and conveys excellent wear characteristics. This is presently my prosthesis of choice in patients allergic to metal. Mid- and long-term data is not available yet in the hip, but results in total knee arthroplasty are promising. Whether it holds up to high-impact over time remains to be seen.
Metal-on-metal bearings, which have been a big part of my practice over the last seven to eight years, have been in use in England since Charnley popularized the metal-on-polyethylene total hip arthroplasty. When fabricated and performed correctly, many of the early prostheses have lasted 30 to 40 years, with little or no osteolysis seen on follow-up. Because of the large size ball and the small neck, they are extremely stable. They are, however, more sensitive to the position of implantation, particularly pertaining to the cup. If the cup is too vertical or horizontal or facing too forward (anteversion) or backward (retroversion), then edge loading and impingement can occur and increased metal wear is generated.
This can cause a local reaction to the metal wear in the form of a fluid-filled sac or soft tissue mass, often referred to as a “pseudotumor”, which can injure surrounding structures. In addition, significantly increased metal ions in the blood from increased wear may produce systemic injury to the liver or heart.
Recent evidence suggests that the larger femoral head size may produce increased wear at the head-neck junction of the stem. The United Kingdom equivalent of the FDA recently published a recommendation to continue yearly monitoring of metal-on-metal big femoral head total hip arthroplasty (not for hip resurfacing) for metal ion levels.
There are also occasional patients that develop actual allergic responses to metal-on-metal bearings that may require revision. These true allergic reactions to metal are rare, with most adverse response to metal-on-metal bearings usually being secondary to increased local metal wear often associated with less than optimal position of the cups.
I have been using the Wright Medical Conserve Plus metal-on-metal big femoral head arthroplasty since 1999 and have had generally very good results. We have seen, however, six modular neck metal fractures in very heavy, or very active patients and now use a one-piece non-modular stem. We have also seen two metal allergies, with subsequent cup loosening and two pseudotumors (in patients who had the components in good position) that have required re-operation. Although our results with metal-on-metal big femoral head arthroplasty have been very good, the recent findings of increased neck trunnion wear with larger femoral head sizes is concerning and has not been previously recognized or reported. Corroborative studies are presently needed, but because of this concern, I am now beginning to look at alternatives to the metal-on-metal big femoral head arthroplasty. The Smith & Nephew ceramicized Oxinium head on cross-linked polyethylene alternative is what I use in older, less active patients who need stability.
In contrast, in younger, more active patients the metal-on-metal hip resurfacing performed with the Smith & Nephew Birmingham prosthesis has been extremely successful in our practice. Sixteen year data in England and Australia and Scandinavia shows excellent mid-term results in a patient population that had previously performed poorly with metal-on-polyethylene components. Our results with the BHR have been excellent, with over 765 implanted since 2006.
In our series to date, we have a 98.6% success rate. Because of our experience and Dr. McMinn’s experience in females with hip dysplasia, I now lean to metal on poly hip replacement from an anterior approach for this small subgroup.
Although the Birmingham hip resurfacing has been performing very well when implanted in the correct alignment, some other resurfacing products have performed dismally. Both the DePuy ASR and Zimmer Durom prostheses were released to the market in the U.S. without FDA approval. Because of design and metallurgy issues, they have performed poorly and had a very high early revision rate. The FDA recently recalled these two prostheses from distribution, but because they were implanted without FDA approval, much legal activity has been directed toward the companies and the physicians that implanted them. Numerous TV ads by lawyers seeking clients with complications related to the DePuy or Zimmer metal-on-metal bearings are inundating the airwaves.
These concerns have subsequently exploded in the lay press. When statistics for longevity for the DePuy ASR and Zimmer Durom and others are combined with the BHR results, the overall success rate doesn’t look as good as conventional metal-on-polyethylene. Careful analysis, however, shows that the BHR has continued to perform extremely well when implanted in the correct position.
One needs to be careful to compare apples to apples and not “throw out the baby with the bath water” regarding metal-on-metal resurfacing. When it comes to young, very active individuals, which is a new demographic that we are seeing more commonly over the last 15 years, what other viable options are available? Metal-on-polyethylene doesn’t hold up well to high-impact and excess activity, and instability is still a concern, even with an anterior approach to the hip. Ceramic is fragile to high-impact and very hard to revise if complications occur. Ceramicized metal-on-polyethylene may be a good option for patients who are metal allergic or have significant avascular necrosis, but this operation still sacrifices good bone of the femoral head, neck, and shaft, and makes later revision much more difficult.
The anterior approach to total hip arthroplasty also has pros and cons. It may make posterior dislocation less common, but can be associated with anterior instability. Technically, it is very difficult to perform this procedure in heavy or well-muscled patients, especially for resurfacing. Commonly, the femoral stem used with the anterior approach has been modified to allow easier insertion, and this may potentially compromise later stem stability. At present, I generally recommend the anterior approach for thin, metal allergic female patients. This is a metal-on-poly component that limits future high-impact activities.
Overall, for the physiologically young and very active individuals with good bone density, I still firmly believe that the Smith & Nephew Birmingham hip resurfacing is a superior choice and continue to use it in this demographic population with excellent results.
We are happy to address any concerns you may have regarding implant options, choices, and techniques, and will continue to strive for the optimum outcome for each of our individual patients.
-John S. Rogerson, M.D. September 2014