Home » Hips » Bearing Surfaces » Metal on Metal Controvery

Statement on Metal on Metal Total Hip Controversy

It has been some time now since the announcement of the DePuy/Johnson and Johnson recall of the ASR hip system. I have never implanted this device, but because it is a metal on metal design, I have had some inquiries about this. With the recently published article in the NY Times, it has generated so much controversy, that I have decided to write this summary of my experience and opinion.

First, it is clear from large European databases that the DePuy ASR hip was failing at a higher than expected rate. So, what does that mean? Failure rates of total hips are very low, so when we saw that the ASR was failing at a rate of 3-4%, this was surprising. Keep in mind that even with this rate, 96-97% of the hips were doing just fine. Be aware also that a "recall" of a hip does not mean a patient is going to have a problem and it does not mean that a patient will require further surgery. It simply means that we should closely observe that patient’s hip. Virtually all major implant manufacturers have a metal on metal hip and the failure rates of these seemed to be within the expected range. Since 1997, metal on metal hips have been the primary type of total hip I use. The vast majority of these are the M2A system manufactured by Biomet which has not been recalled. I have implanted over 1000 of these hips. Second, I have watched carefully the data on other surfaces such as ceramic or polyethylene used in total hips and although there are improvements in these designs I still question if they are better than metal on metal. We’ll talk more about this in a minute, but let’s look at history for a minute.

For many years in my practice, the total hip replacement utilized a small metal ball which articulated with a plastic cup. This of course offered excellent function and good pain relief, but two issues were of concern. One: is the ball was small and therefore at risk of dislocation and two: the metal ball over years of time commonly would wear through the plastic part. As the head wore through the plastic, there were debris of plastic generated which cause an inflammatory reaction known as osteolysis. This reaction may cause severe damage to the surrounding bone. I have had to revise hundreds of hip replacements for this condition and the surgery can be extensive as well as difficult. Due to these issues, there was a major effort to develop alternate bearing surfaces. Three main lines of research followed. One was the metal on metal implant. In fact there have been metal on metal implants for many years, but the technology of metals has improved so dramatically, that it was felt modern techniques would be very successful. A second technology has been ceramic on ceramic products. These implants have also been around for some time but advancing technology has allowed for the ball to be made larger than in the past making it more successful. Concerns continue with regard to breakage of the ceramic which occurs rarely (1/10,000-1/25,000 cases) and also there is a low risk of "squeaking" of the hip. Ceramics have been the subject of several recalls. A third technology was focused on improving the durability of plastics. I have been somewhat leery of this as we have been promised in the past that the "improved" plastics were better and would last longer and we have been disappointed.

We have known from the beginning that metal on metal implants create metal ions. In fact any metal in the body creates metal ions, even a metal hip with a plastic or ceramic liner. However, the level of ions with metal on metal implants is higher. There is no clear evidence that this creates issues in the blood stream. Long term studies have shown that these ions which are so small that they cannot be seen by microscopy are absorbed into the body, and are evacuated by the kidneys. Long term studies also do not show any clear evidence of issues such as increased cancer risk. We have learned however that some patients may have a reaction within the tissue around the hip to these ions. The reaction may be due to a higher than normal amount of ions released or it may be due to a high sensitivity of the patient to metal ions. Unusually high ion levels may be seen in certain designs and this may be due to differences in metallurgy or may be due to positioning of the implant within the hip. The hip which I have used for most of the last 13 years is the M2A. This link notes some data with regard to this hip design. As mentioned, the position of the hip implant may affect the amount of metal ions. There are certain parameters we as surgeons use to place these devices and certain "malpositions" may predispose to excess ion formation. Also, a “hypersensitivity” to metal ions may exist in some individuals. This may be due to an allergic type reaction due to prior exposures. We know from large data bases of hip replacement patients that metal on metal failure is greater in women than in men. See the abstract below.

Gender is a Significant Factor for Failure of Metal-on-Metal Total Hip Arthroplasty.

Latteier MJ | Berend KR | Lombardi AV Jr | Ajluni AF | Seng BE | Adams JB

Source
Joint Implant Surgeons, Inc., New Albany, Ohio.

Abstract Metal-on-metal (MoM) articulations offers low wear, larger head size, and increased stability. Reports of early failure are troubling and include failure of ingrowth and metal articulation problems such as metallosis, hypersensitivity, pseudotumor, and unexplained pain. This study investigates the survivorship of modern MoM articulations by gender. We reviewed 1589 primary MoM THA in 1363 patients, with minimum 2-year follow-up for 1212 hips. Follow-up averaged 60 months. There were 643 female patients and 719 male patients.

One theory is that women have more exposure to metal due to jewelry and thus may have developed a sensitivity to certain metals. There is a test called the “Melisa” test which can test for metal sensitivity. A reaction to metal ions is in fact rare. In over 1000 cases I have done with metal on metal, I have seen only one case of metal sensitivity in a female patient. This occurred about 8 years ago with some pain and swelling of the hip. With medication, this resolved and the patient is doing great after all of these years. I have not had to revise a single M2A hip for metal sensitivity. I follow my patients regularly and take x-rays on each visit. Many of these patients are now 10-13 years post surgery with absolutely no sign of symptoms, failure or bone damage due to metal ions or metallosis.

One article which is just being published (see below) compares the outcome of metal on metal hips to hips done with so-called improved polyethylene liners. As noted, the failure rate of both is low, but the failure rate of the metal on metal is slightly greater than with improved polyethylene. Note however that the risk of dislocation is higher with polyethylene. The reason for this is that the metal on metal head can be made very large and thus the risk of dislocation is very low. Over the years that I have done metal on metal hips, the risk of dislocation was almost forgotten. However, since there has been more concern about metal on metal, I have started using the polyethylene more commonly especially in women. Since then we have seen an increase in our dislocation rate. Also, this study is not a long term study and I fear that as time goes on and we see more polyethylene failure and that at 10-15 years I would expect the failure rate of poly to far exceed metal. Time will tell!

Metal-on-Metal vs Metal-on-Improved Polyethylene Bearings in Total Hip Arthroplasty.

Molli RG | Lombardi AV Jr | Berend KR | Adams JB | Sneller MA

Source
Joint Implant Surgeons, Inc., New Albany, Ohio.

Abstract Major weaknesses of total hip arthroplasty (THA) in the 20th century were polyethylene wear and dislocation. Efforts to improve THA took several directions including improvement of polyethylene and use of metal-on-metal (MoM) articulations. Metal-on-metal articulations showed excellent early results, but concerns mount over hypersensitivity and increasing failure. This study compares our experience with MoM and metal-on-improved polyethylene (MoIP) bearings in primary THA. We identified 1362 patients undergoing 1589 MoM THA and 693 patients undergoing 779 MoIP THA (all compression molded and sterilized in argon). Overall, more MoM than MoIP cups have been revised (4.0% vs 2.2%; P = .0241). Revision incidence for infection was similar; higher with MoM for loosening (P = .0020), metal complication (P = .0017), and combined aseptic causes (P = .0067); and higher for dislocation in MoIP (P = .0246). Metal-on-improved polyethylene devices had superior results compared with MoM in this study. Furthermore, 5 MoIP revisions were liner exchanges, a conservative option not available with MoM, reported 1 piece metal-on-metal system with a significant failure rate at early follow-up. This particular class of implants has inherent design flaws that lead to early failure.

Note also from the NY Times article that 75% of reported complaints about metal on metal are from ASR by DePuy hips. The other 25% are from a variety of hips of other manufacturers and I would venture to guess that many of the complaints are in fact made by patients who have simply been alarmed by the press. To my patients, I will say I am still using the M2A hip on many patients. We are using it more in men than in women. If a woman has a prior M2A I have found they usually want another just like it if we must replace the opposite hip. Many female patients are now choosing ceramic or polyethylene hips. Some men are as well. I will say that if I needed a new hip today, for me I believe metal on metal would be the best choice. If I have learned anything in the last 31 years in this field is that nothing is perfect, there is a downside and a risk to everything and time always reveals the truth. Until that time comes, common sense and close observation are key. For all of my years in practice I ask my patients to see me at least every other year for an exam and x-ray and I asked them to let me know if there are any problems. I believe this is good medicine. Well, remember what I said “recall” means when it comes to a total hip. It means the patient should report any problems and that we as surgeons should “observe” for problems. As mentioned in the NY Times article, the FDA has instructed other manufacturers including Biomet (the maker of the M2A) to do post-market surveillance. This means that the manufacturers must keep track of patients who have had their products implanted. To me this is common sense. Even patients with the recalled ASR/DePuy hip have over a 95% chance of long term success. Metal on metal hips by other manufacturers seem to have an even better chance than that!