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Metal-on-metal bearings: A gathering storm?
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Introduction
One step forward, two steps back is a concerning assessment of the currency of contemporary metal-on-metal (MoM) total and surface replacement hip systems employed as a remedy in the treatment of degenerative arthritis. First introduced almost 4 decades ago, this bearing couple currently constitutes 25% of the primary hip systems utilized in the United States. Recently they have received excessive press coverage, most of it of a decidedly negative slant, attributed to adverse local tissue reactions (ALTR). There are more substantive peer-reviewed reports in the clinical literature which describe these phenomena resulting in the revision of some of these devices. This has triggered a recent United Kingdom Medicines and Healthcare Products Regulatory Agency (MHRA) warning on the use of MoM articulations, United States FDA review of the bearings and recall of DePuy’s Articular Surface Replacement (ASR) system. It seems the world of MoM articulations is at a watershed in their use as a bearing couple with total hip and surface replacement designs.
In this first part of a two-part Round Table discussion, we bring together experienced orthopedic surgeons whose collective background is intended to provide readers insight into the pros and cons of MoM bearings selected for use in hip arthroplasty.
– A. Seth Greenwald, DPhil (Oxon)
Moderator
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A. Seth Greenwald, DPhil
(Oxon)
Robert L. Barrack, MD
Adolph V. Lombardi Jr., MD,
FACS
John M. Cuckler, MD
David W. Murray, MD, FRCSA. Seth Greenwald, DPhil (Oxon): What are the advantages of MoM bearing use for both total hip and surface replacement procedures?
Robert L. Barrack, MD: The case for an advantage for MoM bearing use in hip arthroplasty in my 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. We have data from a multicenter phone survey of almost 1,000 patients from five major arthroplasty centers that also indicate that patients with surface replacement have less thigh pain and function at a higher level compared to patients that are similar age, gender and activity level with modern total hip arthroplasty (THA), including those with large metal heads. The advantages in THA are less striking and probably restricted to a higher degree of stability when components are reasonably well positioned.
John M. Cuckler, MD: Metal-on-metal bearings are simply the only choice for resurfacing designs, given the constraints of thin polyethylene as a bearing surface in this type of implant. Enhanced stability — resistance to dislocation — is a theoretical advantage of large femoral heads, which in turn require a large inside diameter acetabular component, again leaving MoM as the only bearing choice.
Adolph V. Lombardi Jr., MD, FACS: The re-introduction of MoM articulations for total hip and surface replacement was met by the orthopedic community with great enthusiasm. Review of first-generation MoM replacements revealed that flaws were related to manufacturing techniques. With significant advances in technology, strict tolerances could be placed on manufacturing issues such as surface finish, sphericity and clearance. Perhaps, the most significant advantage of MoM THA was the ability to utilize large femoral heads. These large femoral heads afford enhanced stability based on a combination of greater range of motion prior to impingement and greater jump distance. Multiple reports have documented that these large femoral heads have virtually eliminated postoperative dislocation.
David W. Murray, MD, FRCS: The main advantage of a MoM bearing is that it is the only bearing couple that has been shown over an extended period to be reliable for hip resurfacing. In my opinion its primary role should therefore be for resurfacing. Other potential advantages such as low wear and the option to use large heads to decrease the risk of dislocation can be achieved with other bearing couples with a lower risk of metal debris related complications.
Greenwald: What are your patient indications and contraindications in considering the use of MoM bearings?
Barrack: The contraindications for use of MoM bearings are well described and include known or suspected metal sensitivity, women of childbearing age and significant renal impairment among others. There are a number of other relative contraindications for hip resurfacing including significant limb length discrepancy, osteoporosis and large cysts. The contraindications are listed on the FDA website for each device.
Cuckler: There is fairly convincing anecdotal evidence that women are at greater risk for hypersensitivity reactions to the MoM bearing couple, thus representing a concern in the selection of this bearing for women. In addition, the data regarding the passage of cobalt and chromium ions across the placenta, and the concerns over possible adverse effects on the fetus, make this bearing less attractive for women of childbearing age. I also consider those patients with increased risk for renal failure, those with severe diabetes, lupus, or known renal function impairment, contraindicated, as the kidneys are the primary pathway for ion excretion.
I prefer the MoM bearing for active, heavy men, as this bearing surface has no risk for fracture and the advantage of enhanced stability with the use of a large diameter femoral head.
Lombardi: With several recent publications on adverse tissue reactions to metal debris, the indications for proceeding with MoM arthroplasty have narrowed. At the time of the reintroduction of the MoM bearings, it was advised that these bearings should not be utilized in patients with renal failure or in women of child-bearing age. Recent studies by Glyn-Jones and colleagues and Ollivere and colleagues have documented that the majority of adverse tissue reactions to metal debris have occurred in females. This may be related to smaller sized components or excessive anteversion, because there tends to be a higher percent of developmental dysplasia of the hip in women. Therefore, my current indication for the utilization of MoM bearings is the high-demand male patient.
Murray: My main indication for a MoM bearing is for hip resurfacing in a young fit active man with relatively normal hip anatomy. I would not use a MoM bearing for a conventional hip replacement
Greenwald: Are there particular design caveats which the orthopedic surgeon should be aware of?
Barrack: There are nuances of metallurgy and component geometry including coverage angle, which cause components of different design to perform dramatically differently. Suffice it to say that these MoM devices cannot be generically considered together based on striking differences in clinical performance documented in a number of total joint registries and abundantly apparent in the recall of a small number of designs recently.
Cuckler: Surgeons need to understand the evolving preponderance of evidence that implant position — particularly proper version of the acetabular component and avoidance of a vertical cup position in excess of approximately 45· — that is critical to avoidance of accelerated wear which has been associated with both pseudotumors and increased risk of hypersensitivity reactions. Impingement of the femoral neck, whether prosthetic or natural in the case of resurfacing, is also a probable source of failure due to pseudo-subluxation of the implants in extreme positions of flexion or internal rotation. Small-diameter acetabular components, particularly when used in the setting of resurfacing, also appear to be at greater risk for impingement and pseudosubluxation.
The other critical (and poorly defined) issue is that of implant and instrument design. Anecdotal evidence suggests that reamer design and precision, acetabular component design and porous coating technology may also affect the success of individual implants. The surgeon should select a MoM component system based on intermediate-term clinical follow-up data available in implant registries from countries such as the United Kingdom and Australia.
Lombardi: Not all MoM bearings are created equal. While manufacturers have paid particular attention to sphericity, surface finish and clearance, controversy exists over the degree of carbon content with recent literature showing that high carbon content has lower wear. Additionally, there has been discussion on the negative affect of heat treatment, which destroys the blocky carbides and, therefore, negatively effects wear. Perhaps the most important feature of large femoral head MoM arthroplasty is the acetabular component design. While components are generally a 180· hemisphere on the outer surface, on the inner surface they are less than 180· and vary anywhere from 150· to 170·. The components were manufactured in this fashion in order to increase their strength and prevent deflection. Unfortunately, this has changed the amount of coverage and therefore has made the position of the acetabular component extremely critical. In a 2009 study, Jeffers and colleagues found that devices may function at up to 16· greater abduction than the angle at which they are placed. For example, some components placed at 45· may actually act as if they were placed at 61·. This vertical nature has led to edge loading and increased metallic debris.
Murray: Metal-on-metal articulations appear to be very unforgiving with minor changes in design, manufacture and metallurgy causing 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 5 years. I would, therefore, recommend that surgeons only use designs that have been shown to work well for at least five or ideally 10 years.
Greenwald: In your practice, what is the current percentage of MoM use in THA procedures and if applicable, surface replacement?
Barrack: I utilize MoM surface replacement for young active patients who meet the criteria for appropriate indications. This would constitute only about 5% to 10% of a typical total joint practice, but constitutes about one-fourth of my practice. I reserve MoM total hip replacement for patients who are specifically seeking MoM surface replacement but have a contraindication such as a large cyst and want to avoid a traditional total hip replacement.
Cuckler: I rarely perform resurfacing, and have used it with great success only in situations of extreme deformity of the metaphysis of the femur which precludes the use of conventional (or custom) femoral stems.
Approximately 25% of my primary THRs are MoM, large head implants. These are performed primarily in younger, active and heavy men.
Lombardi: I have performed a limited number of surface replacements in high demand men who actually requested this type of procedure. With respect to MoM THA, I was performing approximately 95% MoM bearings 1 year ago. Currently, I have decreased to 2%, again, only in high-demand, active men.
Murray: I only use metal-metal bearings for hip resurfacing and then only for young fit active men with relatively normal hip anatomy.
Greenwald: The overall percentage use of MoM bearings has diminished in the United States over the past 3 years. Has this occurred in your practice and for what reason(s)?
Barrack: Based on new data, I have decreased the application of MoM hip arthroplasty to a modest degree. New data indicate that small sized resurfacing components have a higher failure rate, so I advise patients of this if it is anticipated that their femoral head size will be less than 50 mm. I seriously discourage using heads less than 46 mm, and have not implanted a device this small in the past year or more.
Cuckler: The evidence for the success of highly crosslinked polyethylene (XLPE) is now quite compelling at 7 or more years follow-up. When MoM implants were introduced approximately 10 years ago in the U.S. market, no such data existed. Given the increased cost of MoM relative to most XLPE components, surgeons in the United States are making bearing surface decisions in a fiscally responsible and scientifically sound manner.
Lombardi: Yes, the overall use of MoM bearings has decreased in my practice over the past several years. While my patient population has had a relatively low incidence of adverse tissue reaction to metal debris, there have been a number of articles published documenting this phenomena. Additionally, I have had patients referred to our practice with significant soft tissue damage secondary to MoM bearings. Concomitant with this has been the introduction of second-generation XLPE. This polyethylene maintains the wear characteristics of first-generation XLPE, however without the compromise in mechanical properties. Therefore, I have switched my utilization to this type of polyethylene, and in very active individuals I have combined it with a ceramic femoral head.
Murray: My usage of MoM bearings has decreased because of concern about reactions to metal debris.
References:
- Glyn-Jones S, Pandit H, Kwon YM, et al. Risk factors for inflammatory pseudotumour formation following hip resurfacing. J Bone Joint Surg (Br). 2009;91(12):1566-1574.
- Jeffers JR, Roques A, Taylor A, Tuke MA. The problem with large diameter metal-on-metal acetabular cup inclination. Bull NYU Hosp Jt Dis. 2009;67(2):189-192.
- Ollivere B, Darrah C, Barker T, et al. Early clinical failure of the Birmingham metal-on-metal hip resurfacing is associated with metallosis and soft-tissue necrosis. J Bone Joint Surg (Br). 2009;91(8):1025-1030.
- Robert L. Barrack, MD, can be reached at Washington University School of Medicine, 660 S. Euclid Ave., Campus Box 8233, Department of Orthopedic Surgery, St. Louis, MO 63110; 314-727-2592; e-mail: barrackr@wustl.edu.He , has received royalties and research support from Smith & Nephew in the past 12 months.
- John M. Cuckler, MD, can be reached at Suite 164, 100 Club Dr., Burnsville, AL 28714; 205-936-9199; e-mail: jcuckler@charter.net. He is a consultant for Biomet and Iconacy and receives royalties from Biomet for IP and hip and knee designs.
- A. Seth Greenwald, DPhil(Oxon), can be reached at Orthopaedic Research Laboratories, 2310 Superior Ave. East, Cleveland, OH 44114; 216-523-7004; e-mail: seth@orl-inc.com. He has no direct financial interest in any products or companies mentioned in this article.
- Adolph V. Lombardi Jr., MD, FACS, can be reached at Joint Implant Surgeons Inc., 7277 Smith’s Mill Road-Suite 200, New Albany, OH 43054: 614-221-6331; e-mail: LombardiAV@joint-surgeons.com. He is a consultant for and has intellectual property rights with Biomet.
- David W. Murray, MD, FRCSC, can be reached at Nuffield Orthopaedic Centre, University of Oxford, Oxford, OX3 7LD, United Kingdom; e-mail: david.murray@ndorms.ox.ac.uk. He receives institutional support from Biomet, DePuy, Smith & Nephew, Styker, Wright Medical and Zimmer.
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