Title: Higher Blood Cobalt and Chromium Levels in Patients with unilateral metal-on-metal total hip arthroplasties compared to hip resurfacing

Abstract

High cobalt and chromium levels in the blood are known to cause negative side-effects in patients with metal on metal hip replacements. This study discusses the risk factors and the overall likelihood that that people with MoM implants are more likely to experience high cobalt and chromium levels.

This study looks at the blood cobalt and chromium levels of 1,748 patients. 692 of these had hip resurfacings and 1,056 of them had total hip replacements, also known as total hip arthroplasties (THA). In this study, “high” levels of metal ions means greater than 7 ppb.

This study found that patients with THA were more likely to have high metal ion levels than patients with hip resurfacings.

This study also found that these risk factors make hip resurfacing patients more likely to experience high metal ion levels:

  • Small femoral head
  • A high acetabular inclination angle
  • Young age.

These risk factors make MoM THA patients more likely to experience high metal ion levels:

  • Being female
  • Longer time between surgery
  • Ion measurement
  • Large femoral head size

The authors conclude that all large diameter MoM THA implants should definitely be supervised closely.

Introduction

MoM hip replacements have very high revision rates, largely because they cause a set of complications known as adverse reactions to metal debris (ARMD). The amount of cobalt and chromium metal ions in patients’ blood samples are often used to determine whether a metal on metal implant is wearing down because they shed tiny metal particles into patients’ bodies over time.

Because of these problems, guidelines have been put in place to monitor the health of patients with MoM implants. However, some experts believe the existing guidelines don’t do enough to predict whether an implant is failing.

Past studies have shown that large-diameter THA devices are more likely to fail, and some MoM brands are considered more dangerous because they fail way too often.

The authors believe there is only one other study that has compared the metal ion levels associated with different MoM brands.

Additionally, although blood metal ion levels are definitely related to implant failure, there is some debate about what amount of metal should be considered dangerous or “too high.”

The authors of this study tried to answer two questions: First, how common is it for MoM patients to experience blood metal ion levels greater than 7 parts per billion (ppb)? Second, what risk factors make patients with MoM hip implants more likely to experience high metal ion levels?

Patients and Methods

This study analyzed the blood metal ion levels of 1,928 patients with unilateral MoM hip implants (meaning only one of their hips had an implant). Of those patients, 751 of them had hip resurfacings, and 1,177 of them had large-diameter head total hip replacements (THR). Additionally, most of the resurfacing patients were relatively young (89% of them were 65 or younger).

Results

In the hip resurfacing group, 48 of the implants were Biomet ReCap devices. In the THA group, 67 of the implants were Biomet M2a devices.

Patients with THA implants were much more likely to have high metal ions than patients with resurfacing implants. Out of the 7 THA brands analyzed, 5 of them caused high metal ion levels in over 20% of their patients, including Biomet. Meanwhile, less than 10% of all resurfacing patients had high metal ion levels.

In the resurfacing group, there was a lower risk of high metal ions for patients that had BHR and Durom devices than patients with ASR implants. Other factors that increased risk in the resurfacing group were female gender, young age, high acetabular inclination angle, and small femoral head size.

In the THA group, patients with ASR devices had the highest risk for high metal ions. Other risk factors in the THA group were female gender, large femoral head size, and length of time since implantation.

In the resurfacing group, 4.7% patients with Biomet ReCap devices had high metal ion levels. In the THA group, 21.6% patients with Biomet M2a devices had high metal ion levels.

Discussion

This study, which is the largest of its kind, confirms that MoM THAs are more likely to cause increased metal ions than MoM resurfacings. This study also contributes important information about how metal ion levels vary across the different implant brands.

The authors recognize that their study had some limitations. For instance, some brands were more represented in this study than others, which could cause some error in the risk reporting. Second, not all blood samples were taken at the same time. This is important because metal ion levels increase over time, especially for THAs. Third, the medical community still isn’t sure what exact amount of metal in the blood indicates implant failure — different medical institutions have different standards. This study used 7 ppb as its cutoff value for “too much” metal, which is actually the highest standard cutoff value in the medical community. Therefore, this study somewhat understates the link between MoM implants and high metal ion levels.

Two other studies have found that MoM THAs produce higher blood metal ion levels than MoM resurfacings. Experts believe the main reason for higher metal ions in THAs is that the taper wears down. Importantly, this study found that Biomet MoM THAs with titanium sleeves produce lower cobalt levels in the blood. However, the authors believe more research is needed to determine the amount of wear caused by the contact between the titanium sleeve and the femoral head in Biomet implants. Additionally, because titanium sleeves often make revision surgery difficult, the authors cannot say whether a titanium sleeve is actually beneficial in the long run. Finally, although this study found relatively low blood levels of cobalt and chromium in patients with Biomet ReCap devices, other studies have actually found that ReCap implants cause high metal ion levels in a large percentage of patients.

Conclusion

This study presents important data based on the results of a large group of patients. Clearly, some brands of MoM implants pose a higher risk than others. Because high metal ion levels are very common among MoM patients, more studies are needed to determine the effects of high cobalt and chromium levels, especially because many patients with high metal ion levels don’t present any symptoms but could still be at risk of health problems.

Reference

Lainaiala, O. Moilanen, T., Hart, A., Huhtala, H., Sabah, S., Eskelinen, A. “Higher Blood Cobalt and Chromium Levels in Patients With Unilateral Metal-on-Metal Total Hip Arthroplasties Compared to Hip Resurfacings.” The Journal of Arthroplasty 31 (2016) 1261e1266.

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Biomet M2a Research, Metal on Metal Hip Research Summaries Study Finds Metal on Metal Hip Implants Cause Higher Metal Ion Levels than Resurfacings