Four Surgeons Assess MoM Hip

Metal on metal (MoM) replacement hip systems, first introduced almost four decades ago, have come under new scrutiny because of reports of adverse local tissue reactions (ALTR). The United Kingdom Medicines and Healthcare Products Regulatory Agency recently issued warning about the use of MoM articulations. The FDA is reviewing and DePuy recalled its Articular Surface Replacement (ASR) system. What is the future of MoM as a bearing couple in total hip and surface replacement designs?

Dr. A. Seth Greenwald, of Orthopaedic Research Laboratories, Cleveland, brought four senior orthopaedic surgeons, Robert Barrack of Washington University School of Medicine, John Cuckler of the Alabama Spine and Joint Center, Adolph Lombardi of the Mt. Carmel, New Albany Surgical Hospital and David Murray of the Nuffield Orthopaedic Centre, Oxford, to share their insights into the pros and cons of MoM bearings.

First the Pros

MoM bearings in hip arthroplasty, in Dr. Barrack’s view, is more compelling in the case of surface replacement. “There is retention of more of the patient’s own bone and less loss of bone density over time in the femoral neck, ” he noted. A phone survey of about 1, 000 patients from five major arthroplasty centers indicated that patients with surface replacement had less thigh pain and functioned at a higher level when compared to patients of a similar age, gender and activity level with modern total hip arthroplasty (THA). The survey included those devices with large metal heads.

Dr. Cuckler added, “Metal-on-metal bearings are simply the only choice for resurfacing designs, given the constraints of polyethylene as a bearing surface in this type of implant. Enhanced stability—resistance to dislocation—is a theoretical advantage of large femoral bearings, which in turn require a large inside diameter acetabular component, again leaving MoM as the only bearing choice.”

Source:  RRY Publications / Photo by Andrew HuthDr. Lombardi noted that the re-introduction of MoM articulations for total hip and surface replacement was initially met by the orthopedic community with enthusiasm. Reviews of the first generation replacements revealed that flaws were related to manufacturing techniques. The significant advantage of MoM was the ability to employ a large femoral head and enhance stability and almost eliminate postoperative dislocation.

Mr. Murray said that the main advantage of a MoM bearing is that it is the only one that has been shown, over an extended period, to be reliable for hip resurfacing. The option to use large heads, he believes, can be achieved with other bearing couples with a lower risk of metal debris complications.

Then the Cons

The contraindications for the use of MoM bearings include known or suspected metal sensitivity and renal impairment, said Dr. Barrack. Dr. Cuckler added that there is fairly convincing anecdotal evidence that women are at greater risk for hypersensitivity reactions to the MoM bearing couple. Cobalt and chromium ions cross the placenta and concerns about possible adverse effects on a fetus make this bearing unattractive for women of childbearing age. Dr. Cuckler prefers the MoM bearing for active, heavy men as the bearing surface has no risk for fracture and a large diameter femoral head can be used.

Dr. Lombardi went on to say that the majority of adverse tissue reactions to metal debris had occurred in females leading him to use the MoM bearings on the high-demand male patient. Mr. Murray agreed. He would use the MoM bearing for hip resurfacing in a young, fit, active man with normal hip anatomy. “I would not use a MoM bearing for a conventional hip replacement, ” he said

Dr. Cuckler pointed out that implant position, particularly the proper version of the acetabular component and avoidance of a vertical cup position in excess of 45 degrees is critical to the avoidance of accelerated wear. Impingement of the femoral neck is also a probable source of failure.

Finally, in Mr Murray’s view, with MoM devices even minor design or manufacturing or metallurgy changes can cause major problems. In addition designs that work well with one philosophy, such as resurfacing, do not work well in other situations. It is therefore difficult to generalize about design caveats. Furthermore problems with the articulation often do not appear until after five years. “I would, therefore, recommend that surgeons only use designs that have been shown to work well for at least five or ideally ten years, ” he said. “My usage of MoM bearings has decreased because of concern about reactions to metal debris.”

Finally, the Registry Data

Both the Australia and UK/Wales large joint registries included outcome data on the use of MoM bearings. Starting with Australia, here is the conclusion and the data (n=17, 808 MoM).

“It is now clear that primary total conventional hip replacement using metal on metal bearing surface and head sizes over 28mm have a higher risk of revision compared to all other bearing surfaces. The impact of head size is more apparent in head sizes greater than 32mm. The increased risk of revision of metal on metal bearing surface is due to higher rates of loosening and metal sensitivity. It is not age related. There is however an interaction between age and head size. The risk of revision for head sizes larger than 32mm is higher regardless of age and this risk is greater the younger the patient. There is gender variation in outcome, with females having a higher risk of revision when metal on metal bearing surfaces are used. This gender difference is only evident when head size is greater than 32mm. The higher risk of revision with metal on metal bearing surfaces is not isolated to a small number of prostheses. Of those with head sizes greater than 32mm and over 200 procedures recorded, almost all have a cumulative percent revision that is higher than the entire group of primary total conventional hip replacement.” – 2010 Annual Report National Joint Replacement Registry Australian Orthopaedic Association

The United Kingdom/Wales National Joint Registry had the following comment and data regarding MoM implants (N= 8, 882 large head metal on metal prostheses).

“Revision rates varied according to type of prosthesis (P<0.0001). The five year revision rate was lowest in patients who received a cemented prosthesis (2.0%; 1.8% to 2.1%) and highest after Large Head Metal on Metal (LHMoM) Total Hip Replacement (THR) (7.8%; 6.6% to 9.3%). The five year revision rate was 3.4% (3.2% to 3.7%) in patients who received a cementless prosthesis and 2.7% (2.4% to 3.0%) in patients who received a hybrid prosthesis. In patients who received a resurfacing prosthesis the five year revision rate was 6.3% (5.7% to 7.0%).”

“In men younger than 55 years, the five year revision rates were lowest in those who had a hybrid prosthesis (2.6%) and highest in those with hip resurfacing (5.6%) and with a LHMoM THR (6.4%). In women younger than 55 years, the lowest five year rates were seen in those who had a cemented prosthesis (3.6%) and the highest in those with resurfacing (8.3%) and a LHMoM THR (9.2%). In men aged 55 to 64 years, the lowest five year revision rates were seen in patients with a cemented prosthesis (2.6%) or a hybrid prosthesis (2.7%) and the highest after resurfacing (4.5%) and a LHMoM THR (5.5%).”

“In women aged 55-64 years, five year revision rates were lowest in those with a cemented prosthesis (2.5%) and highest in those with resurfacing (7.9%) and a LHMoM THR (9.5%). In men of 65 years and above, the five year revision rates were lowest in those who had a cemented prosthesis (2.0%) and highest in those who had a resurfacing prosthesis (6.0%) or a LHMoM THR (6.4%). In women in the corresponding age group, the lowest five year revision rate was 1.6% with a cemented prosthesis and the highest 8.8% with resurfacing and 10.5% with a LHMoM THR.” – National Joint Registry for England and Wales. 7^th Annual Report 2010

Finally, the carefully considered conclusions of Seth Greenwald’s panel combined with the most recent registry data is clearly pointing toward a decidedly conservative approach to MoM, especially with larger head sizes.