Title: Modular to Monoblock: Difficulties of Detaching the M2a-Magnum Head Are Common in Metal-on-metal Revisions

Abstract

Metal-on-Metal total hip arthroplasty (MoM THA) implants are made up of a variety of components. One common component is the modular head; it is supposed to be easy to remove during revision surgery. However, it can actually be incredibly difficult – sometimes impossible – to remove because it can get welded to other components over time. This happens frequently in implant devices that have a titanium sleeve between the head and stem. The authors of this study have had a lot of difficulties removing the titanium sleeve adapter in the Biomet M2a Magnum hips. These difficulties have been reported to the FDA, but the frequency and complications of these difficulties are unknown.

This study analyzes the results of 124 Metal-on-Metal THA revision surgeries, all of them involving Biomet devices.

The authors of this study wanted to answer two questions. First, are Biomet M2a-Magnum implants more difficult to revise than the Biomet M2a-38? Second, in cases where the M2a-Magnum was harder to revise because of problems with the modular head, were there more surgical complications (such as operative time, bleeding, a risk of fracture, or joint infection)?

In response to their first question, the authors found that, yes, Biomet M2a-Magnum implants are more difficult to revise than Biomet M2a-38 devices. In response to their second question, the authors found that difficult revisions did result in longer operative times, more bleeding, and a higher risk of bone fracture and joint infection after surgery.

Introduction

MoM THAs often fail and require revision surgery. The survival rate of large-diameter MoM THA implants can be as low as 51% after 6 years. Revisions occur because of pseudotumors (which are enlarged masses of tissue that resemble tumors), dislocations, bone fractures around the implant, and infections. Although the use of MoM THAs has gone down, many people still have them and will require revisions in the future, so it is important to understand what might happen during revision.

One common MoM device is the Biomet M2a-Magnum, which is made up of several components: a cup, a femoral head, a titanium taper adapter, and a stem. Another Biomet device, the M2a-38, is similar to the M2a-Magnum, but one key difference is that it does not have a separate titanium taper like the M2a-Magnum.

Usually, during revision surgery, it is not necessary to remove the stem, and the modular head can be removed easily by using a punch and a mallet. However, with the Biomet M2a Magnum device, there is so much grinding and corrosion that occurs between the head and the titanium adapter that the two can get stuck together. This makes it very difficult to remove the head during revision.

Materials and Methods

This study looks at 124 MoM THA implants that underwent revision surgery. 70 of them were M2a-Magnum heads, and 54 of them were M2a-38 heads.  Additionally, all patients had Biomet-brand stems.

The authors of this study measured the difficulty of removing the femoral head based on how many special tools they had to use. The authors also measured operation time, blood loss, infection,  bone fractures, and any health complications after revision surgery.

The authors then compared the results of the M2a-Magnum group to the M2a-38 group.

Results

The authors did not experience difficulties removing any of the M2a-38 heads. However, the authors experienced difficulties in 29% of the M2a-Magnum revisions and had to use special tools to extract the heads. Additionally, 17% of the Magnum hips needed unplanned stem revisions because of the difficulties experienced during head removal. This means that the patients received a more invasive surgery than they had initially planned.

For the M2a Magnum revisions that had removal difficulties, operation time and amount of bleeding increased. Additionally, 9 of the M2a-Magnum patients experienced health complications one year after revision, including 6 infections and 3 bone fractures around the implant.

Discussion

MoM THA implants have been known to cause a number of problems. Patients with M2a-Magnum MoM implants may experience additional problems because this brand of implant is difficult to revise, as this study shows.

The authors note that their study faced some limitations. For instance, they only measured metal ion levels for 68% of the patients. Therefore, they can’t reach a definite conclusion about the relationship between head removal difficulties and metal ion levels. However, their data showed higher average metal ion levels for patients who didn’t experience difficulties, which suggests there may not be any relationship between head-removal problems and metal ion levels.

As mentioned earlier, all of the difficulties were experienced in the M2a-Magnum heads. These heads are larger than the M2a-38 heads, but it is not clear whether the size of the head is the cause of the difficulties. Cold welding – or the joining of two metal pieces without the use of heat – seems to happen a lot in MoM THA implants. However, it is not yet clear how this cold welding occurs.

This study found that the titanium-titanium taper can be difficult to separate during revision surgery. If doctors don’t anticipate these difficulties, this can result in complicated revision surgeries in patients with M2a-Magnum devices. These difficulties can occur even if implantation occurred recently, and even if the patient has low metal ion levels.

Importantly, the authors note that these difficulties might occur in more sensitive cases, such as emergency revisions due to infections. For these cases, it is especially important that patients and physicians be prepared for challenging revisions.

Doctors should inform their patients that their revision surgeries may be longer and more complicated — and may even require bone-cutting — if they have M2a-Magnum devices because of the high rate of difficulties reported by this study.

Additionally, doctors revising M2a-Magnum devices should go into surgery with special tools like femoral head extraction tools and diamond saws.

A previous study found that the use of titanium sleeves is increasing, but this study shows that these sleeves can cause problems. The authors believe more research is needed about titanium sleeves.

Reference

Mantymaki, H., Makela, K., Vahlberg, T., Hirviniemi, J., Niinimaki T. “Modular to Monoblock: Difficulties of Detaching the
M2a-MagnumTM Head Are Common in Metal-on-metal Revisions.” Clin Orthop Relat Res (2016) 474:1999–2005.

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Biomet M2a Research, Metal on Metal Hip Research Summaries Study: Biomet M2a Magnum Heads Increase Risks of Bleeding, Infection, Fracture During Revision Surgery