Title: The Effect of Different Bearing Surfaces on Metal Ion Levels in Urine Following 28mm Metal-on-Metal and 28mm Metal-on-Polyethylene Total Hip Arthroplasty
Total hip arthroplasty (THA), or hip replacement surgery, is known to increase levels of metal ions in the body. This is because the implant devices can shed metal particles into the body. This can have many negative effects on THA patients.
One way to measure the levels of metal ions in the body is to test patients’ urine. This study compares two different types of THA devices to see which one produces higher metal ion levels, as measured by patients’ urine. The two types of implants they compare are metal-on-metal (MoM) bearings and metal-on-polyethylene (MoP) bearings.
This study looks at 92 hip replacement patients; some of them had MoM implants and others had MoP implants. It measures the amount of cobalt, chromium, and molybdenum ions in their urine one year after getting their implants and two years post-surgery.
The patients with MoP implants had only a slight increase in cobalt and chromium levels two years post-surgery. For the patients with MoM devices, chromium levels were 15 times greater two years after surgery, and cobalt levels had increased by 26 times.
The authors conclude MoM devices cause very high cobalt and chromium levels; they are higher than those allowed for on-the-job exposure in Finland, where this study was conducted.
Metal-on-metal hip implant devices that are made out of cobalt and chromium became really popular because they were thought to be more durable. Many improvements have been made since the first generation of MoM devices came out in the 1970s. However, there are still many health problems associated with these devices, such as adverse reactions to metal debris (ARMD). ARMD is a type of hip implant failure that results in pseudotumors and fluid collections around the hip implant. ARMD is caused by metal particles rubbing off of the implant device and into the body.
Patients and Methods
92 patients were divided into two groups, those who had hip replacements with MoM implants and those who had MoP implants. All of the patients had Biomet hip implants with 28-mm head diameters.
The authors collected urine samples from the patients before their hip replacement surgery, one-year post-surgery, and two years post-surgery.
X-rays taken two years after surgery showed that none of the patients’ implants had moved or been misplaced since surgery. Only 5 implants had fractured, and they were fixed.
Before hip replacement surgery, average chromium levels for both MoM and MoP patients were less than .01 µmol/L. One year after surgery, average chromium levels had risen to .12 µmol/L for MoM patients but were still .01 µmol/L for MoP patients. Two years after surgery, average chromium levels rose to .15 µmol/L for MoM patients and .02 µmol/L for MoP patients. Essentially, chromium levels increased a lot more for MoM patients than they did for patients with MoP devices.
Average cobalt levels before hip replacement surgery were 21 nmol/L for MoM patients and 19 nmol/L for MoP patients. One year after surgery, average cobalt levels had risen to 417 nmol/L for MoM patients but only 33 nmol/L for MoP patients. Two years after surgery, cobalt levels had risen to 538 nmol/L for MoM patients and 57 for MoP patients. In sum, cobalt levels were dramatically higher for MoM patients than they were for MoP patients.
Molybdenum levels were also measured for all patients, but there were no significant changes in molybdenum levels.
Overall there was a much greater increase in cobalt and chromium levels for MoM patients than there was for MoP patients. Cobalt and chromium levels for MoM patients ended up being much higher than what is considered safe in Finland.
The authors found that 28-mm MoM bearings in THA patients yield chromium and cobalt concentrations in urine that are higher than those recommended for workplace exposure.
The authors used urine samples because metal ion levels in urine are a good indicator of the levels of metal ions in the entire body.
Although cobalt is necessary for the body, excessive amounts of it can cause goiter and heart failure.
Ever since MoM implants were introduced, there have been concerns about their possible link to cancer; in animal studies, high levels of cobalt and chromium particles produced cancer cells in the animals. Recent studies have also suggested that hip implants (MoM and non-MoM) can cause changes to patients’ DNA and chromosomes.
There is also concern about the effects that MoM implants could have on the fetuses of patients who become pregnant because cobalt and chromium can cross the placenta; this could cause developmental problems in fetuses.
In theory, people who have THA surgery should face higher metal ion levels than people who receive hip resurfacing (HRA) surgery. However, the studies on this subject show conflicting results. One study showed that cobalt and chromium levels for patients who received HRA were higher than those who got THA surgery.
Another study found that cobalt and chromium levels in HRA patients and small-diameter head THA patients were significantly higher than in large-diameter head THA patients 6 months after surgery. However, this same study found that one-year post-surgery, all of the patients’ cobalt and chromium levels were more or less the same.
Some studies have shown that MoM implants often wear down very quickly in the first one to two years after surgery. After this initial period, they enter what the authors call a “steady-state” of wear for many years, which means the implants shed metal particles at a much lower rate.
MoP devices seem to be safer than MoM devices when it comes to metal ion levels.
**It is important to note that this study was funded and sponsored by Biomet UK Limited, a company that manufactures MoM devices.
Tiusanen, H., Makela, K., Kiilunen, M., Sarantsin, P., Sipola, E., Pesola, M. Scandinavian Journal of Surgery 102: 197-203, 2013.
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