Title: Explant Analysis of the Biomet Magnum/ReCap metal-on-metal hip joint
Metal-on-Metal (MoM) hip resurfacing implants and MoM total hip arthroplasty (THA) implants have recently been questioned by the medical community because of their high revision rates. Understanding the reasons for failure can provide insight into how implants can be improved. Furthermore, looking at the wear (amount of material lost) on implant devices can help us determined why they failed.
The authors of this study used a special 3D scanner called a Coordinate Measuring Machine to measure the wear on five failed Biomet M2a-Magnum/Recap/Taperloc large-head MoM THAs and one Biomet ReCap resurfacing implant.
Of the 6 devices tested, the average amount of material lost was 6.1 mm3 per year, which is higher than the wear rates known to cause implant failure in similar metal-on-metal hips. In addition, the average wear rate of the tapers in the Magnum tapers was .054 mm3/year. The taper is a cylindrical piece of titanium at the tip of the implant stem, and it connects to the head.
The authors of this study conclude that patients with Biomet hip implants should be monitored closely by their doctors, regardless of whether they have a MoM THA device or a resurfacing device.
MoM implants were popular from the 1990s through the mid-2000s. They were supposed to offer many benefits, such as increased range of motion and lower rates of failure. Young, active patients often received these types of implants. MoM implants come in two forms: hip resurfacing and total hip replacement.
However, some hip implant brands, such as the DePuy Articular Surface Replacement (ASR) and the ASR XL THA device have very high rates of wear and revision, which caused a lot of concern about whether MoM implants are actually beneficial.
A lot of these implants failed due to a set of conditions called adverse reaction to metal debris, or ARMD. ARMD is caused when the implant’s components rub against each other and shed metal debris, primarily cobalt and chromium, into the surrounding tissue. Symptoms of ARMD include pain, dead tissue, metal poisoning, and fluid accumulations around the hip joint.
However, the DePuy ASR and ASR XL THA implants are not the only MoM devices with high revision rates; studies have reported high failure rates among brands like Zimmer Durom and Birmingham Hip Resurfacing (BHR). Because MoM THAs fail at such high rates, they have become a lot less common, but they are still in use.
The authors of this study believe that all patients with MoM implants should be monitored closely in order to understand which implants are working well and which ones are not.
One way to asses the performance of hip implants that have already been removed is to measure the amount of wear they experienced (or how much material rubbed off of the implant). This is measured in cubic millimeters (mm3).
A previous study found that MoM wear rates higher than 2.3 mm3 per year can cause implant failure and require revision. Another study found that wear rates above just 1 mm3 per year can lead to dangerously high metal ion levels in the blood. High metal ion levels can lead to a number of health consequences, ranging from fatal heart disease to dementia.
Importantly, a previous study stated that “the Biomet ReCap and Magnum components are not susceptible to the same design problems” that caused high failure rates among the Durom and ASR hip implants. The authors of this study wanted to test this statement and see if Biomet implants actually functioned better than others.
Patients and Methods
This study examines 5 Biomet M2a-Magnum/Recap/Taperloc MoM implant systems, and one Biomet ReCap resurfacing device. These implants were all extracted during revision surgery after the implant failed.
Reasons for revision were pain, ARMD, and/or dangerously high blood metal ion levels. Blood tests showed that two patients had metal ion levels above what is considered safe (7 µg/l). Additionally, four of the patients had symptoms of ARMD.
The total wear for each implant device was between 30 mm3 and 57.3 mm3. This means that each device shed between 30 mm3 and 57.3 mm3 of metal. The average wear rate for the implant devices was 6.1 mm3 per year.
In all of the implants, the femoral head experienced much more wear than the other components.
Magnum devices have an extra component called a taper, which is made out of titanium. On average, each taper lost about .38 mm3 of material in total. The average wear rate for the tapers was .054 mm3 per year
The authors found that the inclination angles — or the way in which the implant components were positioned — did not cause the high wear rates. In other words, the high wear rates were not caused by doctors positioning the implants incorrectly during implantation surgery; rather, the wear rates were the result of the implant design. However, they did find that the implant with the lowest inclination angle had the highest wear rate and produced the highest metal ion levels.
A previous study found that a wear rate of just 2.3 mm3 per year could be enough to cause implant failure. A 2003 study found that a wear rate as low as .55 mm3 per year could cause wear-related failure. The Biomet implants in this study all had wear rates significantly higher than these, as the lowest wear rate in this study was 4.1 mm3 per year and the average was 6.1 mm3.
In 2016, the National Joint Registry (NJR) reported that the probability of failure for the ReCap implant was 7.79% in the first seven years, which is much higher than BHR’s 5.47% but lower than ASR’s 20.88%. In contrast, the Australian Orthopaedic Association (AOA) found that the seven-year failure rate of the ReCap device was 12.2%.
For the Magnum THA system, the AOA found that its revision rate was 8.6% in the first seven years, which is slightly higher than the BHR THA device’s revision rate at seven years.
The NJR found that. in general, revision rates for MoM THAs were higher than revision rates for MoM resurfacings. Some studies concluded that the reason for the higher revision rates in THAs was primarily taper wear, which is why the authors of this study also measured the wear rates of the Magnum tapers.
This study found that, on average, the wear rate for the Magnum taper was .054 mm3 per year, which is relatively low compared to the wear rates of other tapers. The authors believe the reason for the relatively low taper wear is that the Magnum taper is made out of titanium, which has proven to shed less. However, the authors warn that titanium tapers should not be seen as the solution to implant wear because titanium tapers have their own problems. For instance, they are known to weld themselves to other implant components, which can cause complications during revision surgery.
To summarize, the authors conclude that the wear rates for the 6 Biomet devices they studied were higher than the rate known to cause wear-related failures (which is generally considered to be 2.3 mm3 per year). Finally, the authors state that all patients with Biomet implants should be monitored closely by their doctors.
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