Title: A 5-year survival analysis of 160 Biomet Magnum M2a metal-on-metal total hip prostheses
Large-head metal-on-metal (MoM) total hip arthroplasties (THA), or hip implants, have high failure rates. This study looks at the conditions of 160 hip devices 5 years after implantation in order to determine their failure rate; all 160 devices were Biomet Magnum M2 large-head MoM total hip articulations.
Overall, a total of 13 implants – or 8.1% – failed and had to be revised. Additionally, this study found that 14 – or 8.75% – of the implants caused pseudotumors in their patients. Pseudotumors are enlarged masses of tissue that resemble tumors, and they’re a known side-effect of MoM implants. Pseudotumors were the leading reason for revision in this study.
The authors of this study conclude that doctors need a rigorous follow-up strategy for MoM THA patients in order to identify and treat implant failure early on.
When MoM THA implants were first introduced, they were thought to have a lot of benefits and quickly became popular. However, multiple studies have shown that MoM devices shed metal debris into patients’ bodies, causing many painful and dangerous complications such as pseudotumors and elevated levels of metal ions (cobalt and chromium) in the blood and serum.
Pseudotumor formations can lead to the early failure of hip implants, and there are many risk factors that make people more likely to develop pseudotumors, ranging from the positioning of the implant to the gender of the patient. Because of the well-documented negative consequences of MoM hip implants, the Netherlands recalled these devices and ordered active follow-ups of all patients who received them. This study analyzes the patients who came for follow-ups after the recall.
Materials and Methods
All patients received the Biomet Magnum (M2a-Magnum) prostheses with Recap cup and Taperloc (Taperloc®Hip Stem) or Mallory stem (Mallory®).
All patients had their cobalt and chromium blood levels tested; normal cobalt levels were considered to be below 40 nmol/l. Patients were also asked to complete the Harris Hip Score (HHS) survey. The HHS asks a series of questions that are intended to measure how well an implant is working; after answering these questions, the patient is given a score ranging from 0-100 to rank how well the implant is working. In addition to these examinations, the patients got X-rays, MARS-MRI scans, and CT scans.
In total, this study looked at 160 MoM implants from 150 patients; 10 patients had bilateral MoM implants (meaning they had implants on both hips).
From this group of 160 hips, a total of 15 implants (9.4%) had to be revised. Revisions occurred, on average, 3.7 years after implantation, but some revisions occurred in as little as 6 months after implantation.
Seven of the revision patients experienced groin pain, but 21 of the non-revised patients also experienced groin pain. Additionally, two revisions were conducted due to the loosening of the implant device. Finally, a total of 14 implants developed pseudotumors.
All patient statistics considered, the authors conclude that the survival rate of the large-head Biomet MoM THA device at 5 years is 93.1%.
The patients with revised implants had an average HHS score between 70-80, which is considered “fair.”
The X-rays did not show any bone loss, bone resorption, or fracture. An important finding in this study is that patients with pseudotumors had much higher levels of cobalt in their blood. Finally, 26 patients had abnormal fluid around their joints.
Many studies have shown that large-head MoM THA devices fail early on, develop pseudotumors, and have high revision rates. For instance, one study showed that Biomet implants tend to fail specifically because of their cup, which is a component in the device that’s implanted in the pelvis. Another study found that the Biomet M2a-Magnum had a survival rate of 88% after 3.6 years.
When it comes to pseudotumors, there is some confusion in the literature. Some authors believe that any patient with a pseudotumor should undergo revision surgery, while other authors believe that a pseudotumor alone is not a sufficient reason to revise an implant. It is also unclear whether pain in the hip always indicates the presence of a pseudotumor.
In this study, a total of 14 hips developed pseudotumors after 6.1 years, meaning that patients had an 8.75% chance of developing pseudotumors. Some warning signs of pseudotumors include pain and discomfort in the groin, deafness, fear, and dizziness.
This study also found that patients with pseudotumors experienced more pain in the groin and had lower HHS scores. However, the authors note that they may have missed pseudotumors in patients who did not show any pseudotumor symptoms because some patients did not receive CT scans.
The authors of this study also found that pseudotumors are linked to higher levels of cobalt and chromium in the blood.
The authors believe that large-head MoM devices and high inclination angles are more likely to result in elevated cobalt and chromium blood levels because they cause the implants to shed metal debris more quickly.
The effects of metal debris on the body are not completely understood yet. Some studies have shown that women are more likely to develop pseudotumors than men, and they tend to have higher serum levels of cobalt and chromium.
The authors end their study with multiple recommendations. First, they conclude that doctors should follow up with their patients frequently and measure their metal ion levels each time because these are useful in determining the presence of pseudotumors. They also recommend that patients with pseudotumors undergo revision surgery in order to prevent future complications. Additionally, the authors state that doctors should follow-up very closely with patients who are experiencing pain. Next, they recommend close follow-ups for patients with non-MoM hip devices because those can also have negative side-effects. Finally, they discourage the use of MoM implants outside of a research setting.
Koper M., Mathijssen N., Vehmeijer, S. “A 5-year survival analysis of 160 Biomet Magnum M2 metal-on-metal total hip prostheses.” Hip Int 2016; 26(1): 50-56.
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