The Pros and Cons Compared to Metal-on-Metal

Updated 8/11/2024

The Polymotion Hip Resurfacing FDA Trial is starting. If you are interested in this implant, please read this entire article that discusses the pros and cons in detail. After you read it and are still interested in Polymotion, I will be available to answer any questions after you submit your forms.

A limited number of patients desiring Polymotion will be able to get this implant as part of the FDA IDE study. In about 4 years, the FDA will review the study data and decide if the implant can be cleared for unlimited sale in the US. The implant that I have been using since 2007, the Zimmer/Biomet uncemented Recap Magnum will continue to be available on an unlimited basis.

The uncemented Polymotion acetabular component is paired with a cemented cobalt-chrome femoral component for hip resurfacing (HR). Design surgeons are Derek McMinn and Ronan Treacy of BHR fame. Their company is Joint Medica, which is now owned by Exactech. The Polymotion cup is mostly made of Vitamin E crosslinked polyethylene, coated with a thin layer of titanium porous coating for bone ingrowth. The femoral component is made of cast cobalt chrome which contains traces of carbon, nickel, and molybdenum. The femoral implant requires methylmethacrylate bone cement for fixation to bone. In the future we hope that an uncemented femoral version will also become available.

My partner Dr. Coleman Fowble and I have joined with Exactech to bring this new resurfacing technology to the US. Between the two of us, our group has done more Metal-on-Metal (MoM) HR than anyone else in North America (>8000 cases). We have published the best outcomes for hip resurfacing in the world. Outcomes that surpass all results for total hip replacement (THR).

Because of persistent concerns of metal ion release with MoM HRA (chiefly Cobalt ions) we are now happy to be able to offer an alternative bearing HR. Even though HR has better outcomes than THR, we are concerned that many people are still opting for the inferior THR, because of a residual concern for cobalt release that is widespread among “thought leaders” in joint replacement as well as on the internet. Functionally, HR has been demonstrated to be superior to THR, allowing patients to return to full unlimited sports activity and heavy labor. HR is also more resistant to dislocation. Both of these advantages compared to stemmed THR result from the fact that a HR more closely biomechanically approximates the normal hip. There is no reason to think that the new materials in the Polymotion will change these facts.

To allow Exactech to sell these implants in the US, the FDA has determined that Exactech must complete an IDE (Investigational Device Exemption) study. We have been consultants for Exactech in this process. We will be one of 10 study sites in the US as part of the planned multicenter study design. We have now received final FDA approval and anticipate that the study can probably begin in July-September 2024. Patients interested in participating in this study will be recruited and placed on a list that we will activate as soon as we get the green light. Surgical fees will be identical if you enroll in this study or decide to stick with our standard uncemented Zimmer Biomet MoM HR that we have been using since 2007.

This new bearing option for hip resurfacing should remove any possibility of failures from inflammatory tissue reaction from metal debris. These types of reactions are variously known as AWRF (adverse wear-related failure), ALTR (altered local tissue reaction), ARMD (abnormal reaction to metal debris), ALVAL (aseptic lymphocytic vasculitis-associated lesion), metallosis, or Pseudotumor.

All artificial joint implants will release small amounts of abnormal metal, plastic, cement, or other debris into the surrounding tissues. This is generally well tolerated. Occasionally the debris volume becomes great enough to cause an inflammatory tissue reaction often given one of the above names. A revision surgery is then required to remove the debris and exchange the implants.

I have been performing metal-on-metal (MoM) hip resurfacing since 1999. Before 2011, I had a 1% rate of failure by 10 years due to AWRF. In MoM, HR We discovered that AWRF occurred with certain acetabular (cup) positions. Since we developed a safe zone for placing these components, AWRF has no longer occurred in my practice since 2009.

I recommend ion-level testing at 2 years postoperatively after MoM HR. Seventy (70%) of my patients comply with this recommendation. Of these, 80% have normal cobalt levels for people without implants. 20% have abnormal levels. Although having somewhat elevated cobalt levels has not posed any actual clinical problem, patients are sometimes alarmed by this. They read things on the internet that further alarm them. If they have any residual pain and see other consultants, they are sometimes told that they have metallosis and undergo unnecessary revision surgery that does nothing for them except lower this Cobalt level, but it places them at risk for complications of revision surgery and then they are stuck with an inferior THR.

To put all this in perspective, patients with total knee replacement (TKR) have been found to have slightly higher mean cobalt levels than I report with HR. No one seems concerned about this in the least. A normal Cobalt level is under 1.5 ug/L. There is some evidence that mild toxicity can occur at a level of 20ug/L. Severe cardiac toxicity has been reported rarely in the range of 200-500 ug/L. The lowest level I have seen for AWRF has been 15ug/L, but the mean level in patients with this problem before 2007 was 70ug/L. Before 2009, I have seen a few patients with mild toxicity with AWRF that have resolved after revision. I have never seen cardiac toxicity. Also, the concern that cobalt causes cancer has been debunked in several long-term studies. Now, there are at least 6 controlled studies that have shown patients with MoM HR have at least a 20% lower 10-year all-cause mortality than matched THR patients. In summary, there seems to be no convincing evidence that long-term exposure to low cobalt levels is harmful. But people are still concerned.

The Polymotion HR features a cobalt-chrome head that articulates on a vitamin E crosslinked polyethylene cup. This combination will release primarily plastic particles, and also some cobalt chrome. There will also be some release of titanium from the backside of the cup and methylmethacrylate cement from the backside of the head. Wear is the main source of particle release, and the plastic part is softer, so it will liberate more material. There will be some cobalt release from the harder head, but it should be much less than from a MoM articulation. We emphasize that Polymotion will still shed some cobalt, but less than a MoM device. We hope this will be an advantage. There remains a major advantage of any HR compared to THR because of the absence of any possibility of trunnion corrosion. This is a severe inflammatory reaction which in my experience is far worse than AWRF with a HR. The trunnion is the connector between the head and stem of a THR. This is subject to corrosion and leads to a 1-2% rate of failure of THR by 10 years due to tissue inflammation.

Overall, it seems that Polymotion should have the least chance of causing any type of tissue reaction than any device currently on the market. Polymotion has gone through extensive biomechanical laboratory testing prior to starting the FDA IDE study. But many devices have looked good in the lab, and have even been approved by the FDA, only to be found problematic later. The DePuy MoM ASR THR is the most notorious example of this. This large MoM THR was found to have a 50% failure rate by 5 years compared to a benchmark of 3% for all THRs in the industry. Most other large-bearing MoM THR brands had an 8% failure rate by 5 years: nothing like ASR, but still problematic. This led to widespread disappointment with MoM bearings. But hip resurfacings were different; my results with MoM HR have been far better. With only a 2% failure by 18 years, we have surpassed all benchmarks and most expert surgeon series with any style of THR or any other bearing. In the last 20 years, we have learned a lot about wear failures and much more rigorous testing methodologies have become standard. Therefore, the Polymotion comes to market with a much better chance of success.

We think that the Polymotion will be a resounding success and will remove all fears of cobalt problems from the minds of most joint replacement surgeons, allowing more of them to begin learning how to perform HR safely. I personally believe the long-term goal should be to perform HR on most patients with severe hip arthritis, reserving the THR, which has inferior functional outcomes, to only those cases that cannot be resurfaced. After 7000 MoM resurfacings, I am already at that point. I perform over 400 HR annually and less than 50 THR. With HR, I have a 98% 18-year implant survivorship in all patient categories (age, sex, diagnosis, bone quality). But HR is a more difficult operation and only the best technical surgeons will master it and do better than they can with a THR. Our goal is to assist other surgeons around the world to achieve success with resurfacing. We want every hip arthritis patient in the world to have a chance at having a near-normal hip outcome.

Most likely there will be some new problems with a new implant that we will have to address and solve as we bring this new technology online.

Currently, this is how I would summarize the pros and cons of the two devices.