The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 85, Issue 11

Scientific Articles | November 01, 2003

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

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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.

Figures in this Article

Metal-on-metal articulations in total hip arthroplasty were reintroduced in 1988¹, as an alternative to polyethylene, to reduce wear and avoid polyethylene particle-induced osteolysis. More than 140,000 Metasul metal-on-metal articulations (Centerpulse Orthopedics, Baar, Switzerland) have been implanted². The in vitro observation of markedly reduced wear^3,4 was verified by an analysis of retrieved implants showing an annual linear wear rate of about 5 µm per year for the whole articulation after the first year of implantation⁵. Because of the minimal amount of wear and the radiographic appearance of the metal articulations, an in vivo determination of wear rates is not possible. Furthermore, little is known about the possible systemic effects of metal-on-metal articulations. Detectable metal concentrations in the serum and urine of patients with total hip replacements were described as early as 1973⁶ and were later confirmed by other investigators^7-9.

A preliminary report from our institution described a median serum cobalt concentration of 1.1 µg/L at one year after implantation of a metal-on-metal prosthesis in twenty-seven patients¹⁰ and indicated the presence of systemic cobalt disseminated from Metasul metal-on-metal articulations. The aim of this prospective study was to determine the extent of the metal dissemination over a five-year period. We analyzed the samples for the presence of cobalt because cobalt makes up the major part of the Co-28Cr-6Mo-0.2C-alloy articulation.

Materials and Methods

Materials and Methods | Results | Discussion | Appendix | References

This prospective, randomized study was performed in patients with the same type of total hip arthroplasty but two different types of articulations. Patient-selection criteria included the absence of additional cobalt-containing orthopaedic or dental implants and no involvement of other major joints by rheumatoid arthritis or osteoarthritis, as it was possible that those other joints would have to be replaced subsequent to the enrollment of the patient. We implanted an uncemented Alloclassic total hip prosthesis (Centerpulse Orthopedics) in 100 consecutive patients with unilateral osteoarthritis or unilateral osteonecrosis of the femoral head who met the selection criteria. We did not apply for institutional review board approval when we started the study in 1993. All patients were informed of the aim of the study, and all verbally consented to take part in the investigation.

The Alloclassic system consists of the SL stem, which is a tapered rectangular Ti-6Al-7Nb alloy stem, and the CSF cup, which is a conical threaded cup made of commercially pure titanium (Fig. 1). According to a previously established randomization list, fifty patients were treated with a metal-on-metal articulation and fifty were treated with a ceramic-on-polyethylene bearing and functioned as the control group. The articulation was the only implant parameter that differed between the groups. We used the Metasul metal-on-metal articulation in the study group and an articulation consisting of an Al₂O₃ aluminum oxide ceramic head (Biolox; CeramTec AG, Plochingen, Germany) and an ultra-high molecular weight polyethylene liner in the control group (Fig. 1). Metasul is made of a wrought cobalt-based Co-28Cr-6Mo alloy with a carbon content of 0.2%; the carbon is present as carbides of 2 to 3 µm in size embedded in the metal surface⁷. All polyethylene parts were sterilized by gamma irradiation in an inert gas atmosphere. The head diameter was 28 mm in both groups (except in three patients in the ceramic-on-polyethylene group, who had a 32-mm head). We reasoned that since the Alloclassic SL stem is made of a Ti-6Al-7Nb alloy, the CSF threaded cup is made of commercially pure titanium, and other possible cobalt sources in the body were excluded, any measured serum cobalt concentration would have been derived from the metal-on-metal components.

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Alloclassic total hip system with the tapered rectangular SL stem on the left side and the conical threaded CSF metal cup on the right side. The Metasul articulation consists of a metal head and a metal acetabular component, which is preattached to a polyethylene liner. A Biolox alumina ceramic head and an ultra-high molecular weight polyethylene liner were used in the control group. Both types of liners are inserted into the threaded CSF cup during surgery.

Thirty-one female and nineteen male patients were randomized to be treated with the metal-on-metal articulation. At the time of implantation, their mean age was 58.3 years (range, sixteen to eighty-one years). Thirty-nine female and eleven male patients were treated with the ceramic-on-polyethylene articulation, and their mean age was 62.0 years (range, twenty-six to eighty-seven years). The two groups did not differ with regard to gender distribution (p = 0.08, chi-square test) or age distribution (p = 0.22, t test), with the numbers available.

Blood samples were taken before implantation of the prosthesis and three and six weeks, three and six months, and then every six months after the surgery, until the five-year end point was reached. We used cobalt-free Vacutainer needles and Vacutainer glass tubes (Becton, Dickinson and Company, Franklin Lakes, New Jersey) without additives for blood collection. The serum cobalt level was determined with use of a 5100-ZL atomic absorption spectrometer (PerkinElmer, Shelton, Connecticut) at a wavelength of 242.5 nm. All analyses were performed by a laboratory technician blinded to the randomization. The detection limit of cobalt in serum is 0.3 µg/L in our laboratory. All concentrations below that limit were defined as 0.15 µg/L to allow statistical calculation.

In order to avoid any bias of the serum cobalt concentrations during the course of the study, we subsequently excluded patients who had a contralateral total hip replacement, implantation of any additional cobalt-containing orthopaedic or dental device, a dislocation, or replacement of the articulation. On the basis of these criteria as well as loss to follow-up for a variety of reasons, fourteen patients in the metal-on-metal group and thirteen in the control group were excluded during the course of observation (Table I).

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TABLE I Reasons for Exclusion of Patients During the Course of the Study

Reason for Exclusion	No. of patients
Contralateral total hip arthroplasty	8
Unwilling to return for follow-up	5
New cobalt-containing dental prosthesis	4
Dislocation	3
Death	3
Revision surgery due to infection	1
Revision surgery due to cup loosening	1
Unable to return for follow-up because of age	1
Lost to follow-up	1

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TABLE I Reasons for Exclusion of Patients During the Course of the Study

Reason for Exclusion	No. of patients
Contralateral total hip arthroplasty	8
Unwilling to return for follow-up	5
New cobalt-containing dental prosthesis	4
Dislocation	3
Death	3
Revision surgery due to infection	1
Revision surgery due to cup loosening	1
Unable to return for follow-up because of age	1
Lost to follow-up	1

For each time-point, the median as well as the twenty-fifth and seventy-fifth percentiles of the cobalt concentrations were calculated for the two groups. For comparison, we used the mean postoperative cobalt concentration of each patient as a summary measure and applied the Wilcoxon rank-sum test to those individual mean concentrations.

We also addressed the issue of whether a so-called run-in wear phenomenon in the first year after implantation of a metal-on-metal prosthesis was linked to higher serum cobalt 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.

Age, body-mass index, serum creatinine level, activity level, cup inclination, and Harris hip score¹¹ were correlated with the individual mean serum cobalt concentrations from the three-month to five-year follow-up evaluations with use of a Spearman correlation coefficient. Only patients reaching the five-year end point were included in this analysis. The activity level was rated on a scale of 1 to 10 according to the University of California at Los Angeles (UCLA) system¹². Individual mean cobalt concentrations were compared between female and male patients with use of the Wilcoxon rank-sum test. A p value of <0.05 was regarded as significant for all tests.

Radiographs made at five years or the latest available follow-up radiographs were analyzed for evidence of loosening and osteolysis.

Results

Materials and Methods | Results | Discussion | Appendix | References

The median postoperative serum cobalt concentration in the metal-on-metal group ranged from 0.55 to 1.0 µg/L, with the highest median values at six months and one year and the lowest at 2.5 years (Tables II and III, Appendix, and Fig. 2). The median as well as the seventy-fifth percentile cobalt concentrations in the ceramic-on-polyethylene control group were below the detection limit at all time-points. The difference in the individual mean postoperative serum cobalt concentrations between the two groups was significant (p < 0.0001).

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TABLE II Twenty-fifth Percentile, Median, and Seventy-fifth Percentile of Serum Cobalt Concentrations at Each Time-Point as Determined with Groupwise Nonparametric Analysis*

Articulation/Percentile	Serum Cobalt Concentration (µg/L)
Preop.	3 Wk	6 Wk	3 Mo	6 Mo	1 Yr	1.5 Yr	2 Yr	2.5 Yr	3 Yr	3.5 Yr	4 Yr	4.5 Yr	5 Yr
Metal-on-metal
25th	0.15	0.225	0.3	0.45	0.5	0.6	0.3	0.4	0.3	0.15	0.15	0.4	0.4	0.225
Median	0.15	0.6	0.7	0.7	1	1	0.7	0.75	0.55	0.6	0.7	0.9	0.7	0.7
75th	0.15	0.8	0.9	1.05	1.4	2.2	1.7	2.85	1.55	1.7	1.4	1.7	1.5	1.75
Ceramic-on-polyethylene
25th	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15
Median	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15
75th	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15

Discussion

Materials and Methods | Results | Discussion | Appendix | References

Knowledge about the in vivo behavior of so-called second-generation metal-on-metal articulations is limited. Dissemination of cobalt and chromium has been demonstrated^10,13-18, and wear has been measured on retrieved implants⁵. In this study, we quantified dissemination of cobalt from metal-on-metal articulations prospectively and continuously over a five-year period; thus, our results should provide baseline information for future research.

In the ceramic-on-polyethylene control group, the median and the seventy-fifth percentile of serum cobalt levels were below the detection limit at all time-points. Therefore, we concluded that the reported cobalt levels in the study group were not biased by external factors. The median cobalt concentration in the metal-on-metal group ranged from 0.55 to 1.0 µg/L throughout the five-year observation period. A previously described, so-called run-in wear phase for this particular articulation^3-5 was not reflected by a corresponding higher serum cobalt concentration in the initial phase. Our observations also contrast with reports of increasing serum metal concentrations as a function of implant life in the patient^14,16 or during the first two years after surgery¹⁵.

In a study of eight patients with a McKee-Farrar metal-on-metal total hip arthroplasty¹³, the mean serum cobalt concentration more than twenty-five years after implantation was found to be 0.9 µg/L. Recently, various so-called second-generation metal-on-metal total hip prostheses have been studied. Skipor et al. reported a mean serum cobalt concentration of 2.29 µg/L in fourteen individuals at twelve months after metal-on-metal arthroplasty¹⁷, and Gleizes et al. reported a median serum cobalt concentration of 1.5 µg/L (mean, 2.6 µg/L) in forty-one patients at one year after total hip arthroplasty¹⁴. In the study by Schaffer et al. twenty-two patients had a median cobalt concentration of 1.5 µg/L in whole blood at one year after total hip arthroplasty¹⁵, and in the study by Lhotka et al. twenty-three patients had a mean erythrocyte cobalt content of 3.46 µg/L at 2.9 years after total hip arthroplasty¹⁶.

Schmalzried et al. identified male gender, height, weight, and activity as risk factors for increased wear in patients with a total hip replacement with a metal-on-polyethylene articulation¹⁹. The serum cobalt concentrations in our patient group did not correlate with male gender, body-mass index, or the UCLA rating of the activity level. Also, cup inclination, the Harris hip score, and the serum creatinine level (after the patient with end-stage chronic renal failure was excluded) did not influence serum cobalt levels. However, we found a weak correlation between the serum cobalt level and increasing patient age (r = 0.36).

We did not determine what the serum cobalt levels reflect or whether they corresponded to wear of the articulating metal surfaces. We believe that the concentrations that we observed mainly reflect wear, as there is in vitro^3,4 and in vivo⁵ evidence of wear of the articulating surfaces. Corrosion at the head-neck interface might also contribute to the serum cobalt concentrations⁹. There seems to be little doubt that renal function also influences serum metal concentrations, as 90% of parenterally administered cobalt is cleared from the body in the urine²⁰ and markedly elevated serum cobalt and serum chromium levels were measured in two patients with end-stage chronic renal disease²¹.

We do not believe that routine determination of serum metal levels or routine monitoring of renal function is necessary for patients with a metal-on-metal articulation. However, we regard end-stage chronic renal diseases as a contraindication for the use of a metal-on-metal articulation. Furthermore, we no longer use metal-on-metal articulations in patients with other types of chronic renal disease. We believe that allergy to cobalt or chromium and advanced age are also reasons to avoid metal-on-metal hip replacements. We furthermore recommend careful follow-up with monitoring of serum cobalt concentrations as well as renal function when renal disease develops in a patient with a metal-on-metal articulation.

Until more information is available, we are concerned about patients with high levels of serum cobalt, and we believe that it is prudent to observe such patients. Revision surgery to replace the articulation does not seem to be warranted at this time. We recommend informing patients that, so far, elevated serum cobalt concentrations after total hip replacement with a metal-on-metal articulation have not been linked to adverse health reactions but the effects of long-term elevation of serum cobalt levels are not yet known.

Appendix

Materials and Methods | Results | Discussion | Appendix | References

Tables showing the serum cobalt concentrations for each patient and the mean serum cobalt levels are available with the electronic versions of this article, on our web site at (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM).

References

Materials and Methods | Results | Discussion | Appendix | References

Weber BG. [Metal-metal total prosthesis of the hip joint: back to the future]. Z Orthop Ihre Grenzgeb.1992;130: 306-9. German.130306 1992 [CrossRef]

Personal communication. Centerpulse Orthopedics Ltd., Baar, Switzerland, 2000. 2000

Streicher RM, Schön R, Semlitsch MF. [Tribologic behavior of metal/metal combinations for artificial hip joints]. Biomed Tech (Berl).1990;35: 107-11. German.35107 1990 [PubMed][CrossRef]

Schmidt M, Weber H, Schön R. Cobalt chromium molybdenum metal combination for modular hip prostheses. Clin Orthop.1996;329 Suppl: S35-47.329S35 1996 [PubMed][CrossRef]

Sieber HB, Rieker CB, Kottig P. Analysis of 118 second-generation metal-on-metal retrieved hip implants. J Bone Joint Surg Br.1999;81: 46-50.8146 1999 [PubMed][CrossRef]

Coleman RF, Herrington J, Scales JT. Concentration of wear products in hair, blood, and urine after total hip replacement. Br Med J.1973;1: 527-9.1527 1973 [PubMed][CrossRef]

Bartolozzi A, Black J. Chromium concentrations in serum, blood clot and urine from patients following total hip arthroplasty. Biomaterials.1985;6: 2-8.62 1985 [PubMed][CrossRef]

Michel R, Nolte M, Reich M, Löer F. Systemic effects of implanted prostheses made of cobalt-chromium alloys. Arch Orthop Trauma Surg.1991;110: 61-74.11061 1991 [PubMed][CrossRef]

Jacobs JJ, Urban RM, Gilbert JL, Skipor AK, Black J, Jasty M, Galante JO. Local and distant products from modularity. Clin Orthop.1995;319: 94-105.31994 1995 [PubMed]

Brodner W, Bitzan P, Meisinger V, Kaider A, Gottsauner-Wolf F, Kotz R. Elevated serum cobalt with metal-on-metal articulating surfaces. J Bone Joint Surg Br.1997;79: 316-21.79316 1997 [CrossRef]

Harris WH. Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An end-result study using a new method of result evaluation. J Bone Joint Surg Am.1969;51: 737-55.51737 1969 [PubMed]

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: 228-41.66228 1984

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 Suppl: S256-63.329S256 1996 [PubMed][CrossRef]

Gleizes V, Poupon J, Lazennec JY, Chamberlin B, Saillant G. [Value and limits of determining serum cobalt levels in patients with metal on metal articulating prostheses]. Rev Chir Orthop Reparatrice Appar Mot.1999;85: 217-25. French.85217 1999 [PubMed]

Schaffer AW, Pilger A, Engelhardt C, Zweymueller K, Ruediger HW. Increased blood cobalt and chromium after total hip replacement. J Toxicol Clin Toxicol.1999;37: 839-44.37839 1999 [PubMed][CrossRef]

Lhotka C, Steffan J, Zhuber K, Szekeres T, Zweymüller K. Whole-blood cobalt and chromium levels in patients managed with total hip replacements involving different metal-on-metal combinations. In: Willmann G, Zweymüller K, editors. Bioceramics in hip joint replacement: proceedings of the 5th International Ceram Tec symposium 2000. New York: Thieme Medical; 2000. p 107-11.107 2000

Skipor AK, Patterson LM, Gitelis S, Berger RA, Jacobs JJ.Metal levels in patients with metal-on-metal total hip replacements. Read at the Annual Meeting of the American Academy of Orthopaedic Surgeons; 2002 Feb 13-17; Dallas, TX. 2002

MacDonald SJ, McCalden RW, Chess DG, Bourne RB, Rorabeck CH, Cleland D, Leung F. Metal on metal versus metal on polyethylene liners in total hip arthroplasty: clinical and metal ion results of a prospective randomized clinical trial. Read at the Thirtieth Open Scientific Meeting and the Eighth Combined Open Meeting of the Hip Society and AAHKS; 2002 Feb 16; Dallas, TX. 2002

Schmalzried TP, Shepherd EF, Dorey FJ, Jackson WO, dela Rosa M, Fa'vae F, McKellop HA, McClung CD, Martell J, Moreland JR, Amstutz HC. Wear is a function of use, not time. Clin Orthop.2000;381: 36-46.38136 2000 [PubMed][CrossRef]

Elinder CG, Friberg L. Cobalt. In: Friberg L, Nordberg GF, Vouk VB, editors. Handbook on the toxicology of metals. Amsterdam: Elsevier/North-Holland Biomedical Press; 1979. p 399-410.399 1979

Brodner W, Grohs JG, Bitzan P, Meisinger V, Kovarik J, Kotz R. [Metal-on-metal bearing with THA and chronic renal failure]. Z Orthop Ihre Grenzgeb.2000;138: 425-9. German.138425 2000 [PubMed][CrossRef]

Topics

cobalt ; metals ; hip replacement arthroplasty ; polyethylene

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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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Wolfram Brodner, Peter Bitzan, Vanee Meisinger, Alexandra Kaider, Florian Gottsauner-Wolf, Rainer Kotz; Serum Cobalt Levels After Metal-on-Metal Total Hip Arthroplasty. The Journal of Bone & Joint Surgery. 2003 Nov;85(11):2168-2173.

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