Release of Cobalt and Chromium Ions Into the Serum Following Implantation of the Metal-on-Metal Maverick-Type Artificial Lumbar Disc (Medtronic Sofamor Danek)

Alexander Zeh, MD*; Michael Planert, MD*; Gabriele Siegert, MD†; Peter Lattke, PhD; Andreas Held, MD*; Werner Hein, MD*

Disclosures

Spine. 2007;32(3):348-352. 

In This Article

Discussion

The problem of metal ion systemic release following the implantation of total hip endoprosthetics with metal-on-metal combinations is well known and has been proven many times in vivo.[3,10]

Various authors have undertaken absorption spectrometer measurements of the concentration of cobalt and chromium ions following the implantation of total hip endoprosthetics with metal-on-metal combinations made of cobalt/chromium/molybdenum alloys with the interspersion of block carbides (Metasul).

Concomitantly, they reported on the controversial results of the differing concentrations of cobalt and chromium ions.

In patients with McKee-Farrar M/M implants more than 20 years after implantation (n = 8), the average values were 1.28 ng/mL for the chromium measured in the serum and 0.90 ng/mL for cobalt in the serum.[11] Glelzes et al detected 44.6 nmol/L of cobalt in the serum after 12 (n = 41).[12]

Lhotka et al showed that the concentrations of metal ions for chromium and cobalt 4 years after THA were 50 times higher for cobalt and up to 100 times higher for chromium ions compared with a control group.[13]

Maezawa et al also detected a statistically significant rise in the concentration of Cr ions in the serum from 1.05 to 1.61 μg/L within 3 years in 44 patients, following implantation of metal-on-metal total hip endoprostheses (Metasul).[14]

Conversely, the study group of Brodner et al showed a constant concentration of Co ions in the serum over 5 years after the implantation of Metasul Co-28Cr-6Mo-0.2C metal-on-metal hip endoprosthetics.[3]

The search for the cause of a different quantity of ions released in patients with implants has led to controversial results. Schmalzried et al identified the male sex, weight, body size, and physical activity as factors that lead to an increase in wear debris with metal-on-polyethylene combinations in total hip endoprosthetics.[15]

In contrast, Brodner et al were unable to prove any correlation between these factors and the measured concentration of cobalt following the implantation of metal-on-metal total hip endoprosthetics in the course of the 5-year prospective trial.[3]

We are unaware of any in vivo examinations being carried out at the present time to evaluate the exposure of the system to cobalt and chromium ions following the implantation of Maverick artificial lumbar discs.

Our clinical results, after an average period of 67.60 months (SD, 9.86 months) following the implantation of Maverick artificial lumbar discs, are comparable with those of other groups insofar as using absorption spectrometry they proved an increase in the amount of chromium/cobalt in the serum which was significantly higher compared with the control group.

The concentrations of cobalt and chromium ions in the serum as measured in our study amounted on average to 4.75 μg/L for cobalt and 1.93 μg/L for chromium. These values are similar or higher than the values shown in the literature following implantation of total hip endoprosthetics.[3,11,14]

In terms of this investigation, it is critical to note that there are no values before implantation and thus no direct proof of the origin of the ions found in the serum. We are nevertheless convinced that, because we had negative results in patients without implants and excluded patients who may have been pre-exposed to heavy metal ions, we can assume with adequate reliability that friction debris from the implanted Maverick artificial lumbar disc is the cause of the release of chromium and cobalt.

In addition, these results do not allow us to make a conclusion related to the dynamic release of metallic ions. We cannot associate a change in the exposure over time in respect of their constancy. Further investigations are being carried out to answer this question.

Cobalt and chromium are predominantly excreted by the kidneys.[16,17,18] Impaired renal function can result in considerably increased concentration of cobalt and chromium ions in the serum, as is demonstrated by the measurement of these ions in the serum by the Brodner study group in 2 female patients with chronic renal insufficiency following the implantation of hip prostheses with metal-on-metal sliding surfaces.[3]

Since renal insufficiency was excluded in our group of patients by assuring normal preoperative creatinine levels, the differences cannot be attributed to renal insufficiency.

The high concentrations of metal ions that we measured conflict with the results of investigations by Hellier et al, wherein THA one must reckon with the amount of annual and/or cumulative waste debris being greater by a factor of 11 in 31.5 years. In vitro, assuming 325,000 significant movements of the TDA per annum, Hellier et al established wear debris of 0.44 mm2/yr, which must be contrasted with wear debris of 5 mm2 in THA.[19]

The innocuousness of TDA with regard to the systemic release of metallic ions is justified by the company Medtronic with studies by Allen et al,[20] which show good tolerance in respect of concentrations of cobalt/chromium of 0.01 to 1 g/cm2 in vivo and only established inhibition to alkaline phosphatase from 1.0 g/cm2.

Here the authors assumed a release of 0.1 g/cm2 in THA within 61 years of useful life and correspondingly lower levels of exposure in TDA.

We regard this lesser systemic exposure following implantation of the Maverick TDA as disproved by our research within the period of the study.

On the basis of this study, we cannot conclude about the reason of the significant ion release in patients with Maverick TDA.

There are well-reported studies which suggest a genetic mutation caused by cobalt/chromium. On the other hand, there are experimental proofs as well that cobalt/chromium do not induce genetic deviations.[21,22]

There are also controversial analyses regarding the local effects and induced local toxicity.[23,24]

On the basis of the results of our research for the Maverick TDA, we showed a statistical significant systemic release of Cr/Co in the serum compared with the control group. The concentrations of Cr/Co measured in the serum using an absorption spectrometer are similar in terms of their level to the values measured in THA metal-on-metal combinations or exceed those values reported in the literature. Because the evaluation of the systemic and local effects of the release of Cr/Co from orthopedic implants has not yet been concluded, at the present time, we are not making any recommendations to hold back with the implantation of Maverick TDAs. We recommend that an explanation should be given to the patient about these results of the research into metallic orthopedic implants and that the benefits/risks of alternative combinations of gliding contact surfaces should be discussed and weighed in each individual case.

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