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Serum Cobalt Levels After Metal-on-Metal Total Hip Arthroplasty
Wolfram Brodner, MD1; Peter Bitzan, MD1; Vanee Meisinger, P1; Alexandra Kaider, MS1; Florian Gottsauner-Wolf, MD, P2; Rainer Kotz, MD, P1
1 Department of Orthopaedics (W.B., P.B., and R.K.), Clinical Department of Occupational Medicine, Department of Internal Medicine IV (V.M.), and Department of Medical Computer Sciences (A.K.), Vienna General Hospital, University of Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria. E-mail address for W. Brodner: wolfram.brodner@akh-wien.ac.at
2 Department of Orthopaedics, General Hospital Krems, Mitterweg 10, 3500 Krems an der Donau, Austria
View Disclosures and Other Information
In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Centerpulse Orthopedics. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at the Vienna General Hospital, University of Vienna, Vienna, Austria

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2003 Nov 01;85(11):2168-2173
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Abstract

Background: Systemic cobalt dissemination from the Metasul Co-28Cr-6Mo-0.2C metal-on-metal total hip prosthesis has been demonstrated in the first year after implantation. The aim of this prospective study was to monitor the serum cobalt concentrations in patients during the first five years after total hip arthroplasty with a metal-on-metal articulation.

Methods: A total hip arthroplasty was performed without cement in 100 consecutive patients who had either unilateral osteoarthritis or unilateral osteonecrosis. Fifty patients were randomized to be treated with a metal-on-metal articulation, and fifty patients, with a ceramic-on-polyethylene bearing. The femoral stem was made of a Ti-6Al-7Nb alloy, and the threaded acetabular cup was made of commercially pure titanium. Blood samples were taken before the operation and at multiple time-points for five years after the operation. Serum cobalt concentrations were measured with use of atomic absorption spectrometry.

Results: In the metal-on-metal group, the median serum cobalt concentration was 1 µg/L at one year after surgery and 0.7 µg/L at five years. The median of the serum cobalt concentrations measured from three to twelve months did not differ from the median of subsequent measurements, with the numbers available. The median serum cobalt level in the control group of patients treated with the ceramic-on-polyethylene articulation was below the detection limit at all time-points.

Conclusions: Systemic cobalt release from Metasul metal-on-metal articulations was demonstrated throughout the five-year study period. The median serum cobalt concentrations were found to be slightly above the detection limit and remained in a constant range. The serum cobalt concentrations did not reflect a so-called run-in wear period of the metal-on-metal articulations.

Level of Evidence: Therapeutic study, Level I-1a (randomized controlled trial [significant difference]). See Instructions to Authors for a complete description of levels of evidence.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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    Wolfram Brodner, M.D.
    Posted on March 04, 2004
    Dr. Brodner responds:
    Vienna General Hospital, University of Vienna, Wahringer Gurtel 18-20, 1090 Vienna, Austria

    To the Editor:

    We are certainly aware of that there is a run-in wear phenomenon. It was demonstrated in-vitro (1, 2), and from retrieved implants (3). The run-in wear phenomenon is based on higher initial wear rates of the bearing components followed by a steady state of lower wear. Chan et al. suggest that one of the reasons might be "achievement of some constant value of surface roughness" (2). The question posed by Dr. Silva and colleagues is whether the serum cobalt concentrations that we measured reflected this run-in wear period.

    Our patients usually are quite inactive during the first 6 weeks after implantation of a total hip arthroplasty. Immediately after surgery, most of them spend their days either lying in bed or later sitting in a chair and over the next weeks the patients slowly increase their walking distance using two crutches. If we had to correlate the immediate post- surgical activity levels with the UCLA Activity-Level Rating (4) our patients start out with level 1 and reach about an estimated level of 2 to 3 after 6 weeks (More active younger patients might reach an activity level of 4 or eventfully even 5 at 6 weeks.). Based on the research of Dr. Schmalzried and co-workers we know that "wear is a function of use, not of time" (5). This is why we thought it would be prudent to exclude the initial post-surgical rather inactive period, namely the 3 and 6 weeks values, from our analysis, and to compare "the individual patients' mean values from 3 to 12 months and from 1.5 to 5 years as summary measures of the cobalt concentration".

    Our hypothesis was that the run-in wear phenomenon would probably demonstrate higher serum cobalt concentrations in the first year after implantation, compared to later serum cobalt concentrations after a steady wear rate of the bearing components was reached. This hypothesis simply did not turn out to be true. It is not clear to us how Drs. Silva and colleagues concluded that we omitted 300.000 cycles from our analysis. Obviously, they wanted to correlate hip simulator wear data from Chan at el. (2) with the serum cobalt concentrations of our trial. But it is evident that machine testing is performed with a constant frequency throughout the testing period starting on day one. For example 1.13 Hz was used by Chan et al. (2). This does not at all reflect the clinical situation with patient inactivity after surgery.

    We would also ask why the authors of the letter used our data to construct mean values and standard deviations? Based on the great patient-to-patient variability, which they also noticed, it seemed more appropriate to us to use the non parametric medians and quartiles values.

    Drs. Silva, Heisel, and Schmalzried stated that the average serum cobalt level increased over the first year and was relatively constant for the subsequent four years. But why did they conclude that their constructed plot was consistent with a run-in period for a metal-on-metal articulation? We feel that Chan's plot (2) probably was incorrectly interpreted by Drs. Silva, Heisel, and Schmalzried. Chan's plot describes total volumetric wear and wear does not increase a lot after reaching a steady state after 1 million cycles, meaning that there simply is not a lot of wear between 1 and 3 million cycles (2). Schmidt et al. described wear rates rather than total volumetric wear and their plotted graph started at an initial high value and then tangentially got closer to zero, but did not reach zero (1). Therefore if our data would show consistence with a run-in period for a metal-on-metal articulation it should rather resemble the plot by Schmidt et al. (1) than the plot by Chan et al. (2).

    Jacobs et al. demonstrated metal dissemination due to corrosion at the modular head neck interface of total hip arthroplasties (6). We believe that the dissemination of cobalt observed in our trial is much more likely due to wear of the metal-on-metal bearing surfaces. However, it is difficult to correlate serum cobalt concentrations directly to wear as biologic factors also have to be considered. After metal dissemination - either by particle transport or by distribution of soluble cobalt ions - there is also storage of particles in distant organs (7) and presumably also redistribution of particles or ions from these storage sites. Furthermore renal function might influence serum metal concentrations as most metals are excreted with the urine (8, 9).

    We totally agree that articles published in the Journal have tremendous impact on the orthopaedic community. But we disagree with the proposal of Drs. Silva, Heisel, and Schmalzried to consider "setting the records straight on this issue". We would be happy if Drs. Silva, Heisel, and Schmalzried could share their own experience with the run-in wear phenomenon and serum metal concentrations after metal-on-metal hips with us.

    Sincerely, Dr. Wolfram Brodner Dr. Peter Bitzan Dr. Vanee Meisinger Alexandra Kaider, MSc Florian Gottsauner-Wolf, M.D. Rainer Kotz, M.D.

    References: 1) Schmidt M, Weber H, Sch6n R. Cobalt chromium molybdenum metal combination for modular hip prostheses. Clin Orthop. 1996;329S:35-47. 2) Chan FW, Bobyn JD, Medley JB, Krygier JJ, Tanzer M. The Otto Aufranc Award. Wear and Lubrification of Metal-on-Metal Hip Implants. Clin Orthop. 1999;369:10-24. 3) Sieber HB, Rieker CB, Kottig P. Analysis of 118 second-generation metal on metal retrieved hip implants. J Bone Joint Surg Br. 1999;81:46-50. 4) Amstutz HC, Thomas BJ, Jinnah R, Kim W, Grogan T, Yale C. Treatment of primary osteoarthritis of the hip. A comparison of total joint and surface replacement arthroplasty. J Bone Joint Surg Am. 1984;66:228-41. 5) Schmalzried TP, Shepherd EF, Dorey FJ, Jackson WO, dela-Rosa M, Fa'vae F, McKellop HA, McClung CD, Martell J, Moreland JR, Amstutz HC. The John Charnley Award. Wear is a function of use, not time. Clin Orthop. 2000;381:36-46. 6) Jacobs JJ, Urban RM, Gilbert JL, Skipor AK, Black J, Jasty M, Galante JO. Local and distant products from modularity. Clin Orthop. 1995;319:94- 105. 7) Michel R, Nolte M, Reich M, Loer F. Systemic effects of implanted prostheses made of cobalt-chromium alloys. Arch Orthop Trauma Surg. 1991;110:61-74. 8) Barceloux DG. Cobalt. J Toxicol Clin Toxicol. 1999;37:201-16. 9) Brodner W, Grohs JG, Bitzan P, Meisinger V, Kovarik J, Kotz R. . _Serumcobalt- _und Serumchromspiegel beizwei chtonisch niereninsuffiziente.n Patientinnen mit Hiafttotalendoprothese and Metal l- Metall-Gleitpaarung. [Serum cobalt and serum chromium levels in two patients with chronic renal failure and total hip arthroplasty with metal- on-metal articulations]. Z Orthop Ihre Grenzgeb. 2000;138:425-30. German.

    Mauricio Silva, M.D.
    Posted on December 08, 2003
    Serum Cobalt Levels after Metal on Metal Total Hip Arthroplasty
    Joint Replacement Institute at Orthopaedic Hospital, Los Angeles, CA 90007

    To the Editor:

    We read with great interest the article by Brodner et al., "Serum Cobalt Levels After Metal-on-Metal Total Hip Arthroplasty" published in the November 2003 issue of the Journal. This work provides valuable longitudinal data on serum ion levels. One of the conclusions is that the serum cobalt concentrations did not reflect a so-called run-in wear period of the metal-on-metal articulations. We respectfully disagree.

    The method of combining data from defined early and late time periods and comparing median values suggests that the authors had a bias or hypothesis regarding how a run-in phenomenon would be manifest in serum ion levels, "We used the mean values of the individual patients from three to twelve months and from 1.5 to five years as summary measures of cobalt concentration. Then we calculated the medians at these two time-periods and applied the Wilcoxon signed-rank test for comparison." It appears that the authors assumed that the ion levels would be higher in the short term and then lower over time. When no difference in the defined data sets was seen, the authors concluded that there was no run-in phenomenon. It is unclear why the authors chose to exclude the data prior to the third post-operative month. A better analysis would assess the change in serum cobalt levels over the first year to the change in serum cobalt levels over the subsequent years.

    Using the Appendix material, we constructed a plot (Figure 1) of all the authors' data (patients with metal-on-metal articulations) with the means and standard deviations at each time point. Looking at all the data, it is clear that the average serum cobalt level increases over the first year and is then relatively constant for the subsequent four years, but with great patient-to-patient variability (large standard deviation). This is consistent with a run-in period for a metal-on-metal articulation.

    Figure 2 is published data from hip wear simulator studies of metal-on-metal articulations (2). Manufacturing variables such as radial clearance and surface roughness affect the wear rate during the run-in period, but all of the bearings have a higher initial wear rate followed by a low steady-state wear rate. These plots have the same shape as the plots of the ion data presented by Brodner et al. Excluding the first 300,000 cycles from the analysis would effectively exclude the run-in. Retrieval studies indicate that similar tribologic behavior occurs clinically. The wear rate of metal-on-metal bearings retrieved after a year or less in vivo is more than 5x higher than that seen on bearings retrieved after 3 or more years (5) and bearings retrieved after more than 20 years have only a couple of microns of wear per year 1,4.

    The clinically critical implication of these tribologic characteristics is that the great majority of wear (and wear particles) are generated during the run-in phase. Whether these particles are retained locally or variably disseminated to regional or distant tissues, they are all in a saline-based environment and undergo dissolution with the release of Co and Cr ions. Because the great majority of wear occurs during the run-in phase, the "shower" of wear particles generated during the run-in phase is an important determinant of serum Co and Cr ion levels. Ion data from well-functioning hips with metal-on-metal bearings that have been in situ for more than 20 years suggest that the serum Co and Cr ion levels slowly decrease, consistent with the total volume of particles decreasing over time 1,3,3. The fact that there was no difference between the 3 to 12 month and 1.5 to 5 year median ion levels, is consistent with a run-in phenomenon that occurred within the first year, followed by a relatively low steady-state wear rate - hence no change in the ion levels.

    Articles published in the Journal have tremendous impact on the orthopaedic community. We would appreciate your consideration in setting the record straight on this issue.

    Mauricio Silva, M.D. Christian Helsel, M.D. Thomas P. Schmalzried, M.D.

    References 1.Campbell PA, Urban RM, Catelas I., Skipor A., Schmalzried T.P. Autopsy analysis thirty years following metal-on-metal total hip replacement: A case report. JBone Joint Surg Am. 2003, In press. 2.Chan FW, Bobyn JD, Medley JB, Krygier JJ, Tanzer M. Wear and lubrication of metal-on-metal hip implants. Clin Orthop. 1999; 369:10-24. 3.Jacobs JJ, Skipor AK, Doorn PF, Campbell P, Schmalzried TP, Black J, Amstutz HC. Cobalt and chromium concentrations in patients with metal on metal total hip replacements. Clin Orthop. 1996; 329 Supp1:S256-S263. 4.Schmalzried TP, Peters PC, Maurer BT, Bragdon CR, Harris WH. Longduration metal-on-metal total hip arthroplasties with low wear of the articulating surfaces. JArthroplasty. 1996; 11:322-331. 5.Sieber HP, Rieker CB, Kottig P. Analysis of 118 second-generation metal-onmetal retrieved hip implants. JBone Joint Surg Br. 1999, 81-13:46-50.

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