Title: Modular to Monoblock: Difficulties of Detaching the M2a-Magnum Head Are Common in Metal on Metal Revisions
Where Are We Now?
Metal-on-Metal total hip arthroplasty (MoM THA) implants consist of many metal parts that rub together over the course of normal use. The medical community used to think that corrosion — which is essentially rusting — didn’t affect metal on metal THAs very much. However, we now know that corrosion is quite common, and it causes complications during revision surgery.
During MoM revision, the implant head needs to be removed from the stem. However, in implants where the head and stem are joined by a titanium taper, there can be so much corrosion that the components of the implant get welded together, making it very difficult to remove the head during revision. The failure rate of MoM implants is already extremely high — sometimes as high as 49% at 6 years – but corrosion could make this rate even higher.
Some MoM implants are more susceptible to this problem than others. A recent study published by Mantyamaki and colleagues demonstrates that Biomet M2a-Magnum devices are frequently hard to revise because it is surprisingly common for the head to get stuck to the stem.
Where Do We Need to Go?
Many studies have shown that removing the implant head can damage the trunnion (which is the tip of the stem that attaches to the head). Mardones, the author of this article, thinks that instead of removing the whole stem during revision, doctors could just place a protective sleeve over the damaged trunnion, and then place a new head.
Alternatively, in cases where the head or taper cannot be removed at all — or if the trunnion is damaged during revision — Mardones believes the stem must be removed at all costs, even if it is very stuck. Mardones also recommends that doctors have a variety of tools available before going into revision surgery so they can deal with any complications that might come up.
How Do We Get There?
Mardones believes doctors need to develop standard skills and tools to remove implant heads without causing damage to the trunnion. Another idea he suggests is creating a replacement trunnion that has a new head attached to it. He notes that this type of implant should be one single structure without interchangeable parts; this is in contrast to most MoM implants, which are made up of many interchangeable parts and are therefore vulnerable to corrosion.
Mardones warns that doctors must be prepared to encounter serious corrosion during revision, so they should always have a variety of skills and tools available during surgery. This way, they can minimize the amount of bone and tissue damage that occurs during revision. Some examples of useful revision tools are a corkscrew head extractor, a diamond saw, cables, and revision stems.
Mardones believes MoM THAs that are composed of many interchangeable parts are mostly looked down upon now. However, he thinks that THA implants could be saved if we can reduce the number of parts they have. He suggests creating one, continuous large-head MoM device could reduce fretting, corrosion, and cold-welding. Mardones concludes that if we cannot develop a MoM THA device with fewer parts, we should discontinue the use of MoM THAs altogether.
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