TWO YEAR METAL ION LEVELS
IN PATIENTS WITH
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Introduction |
Metal-on-metal articulation is becoming a more accepted alternative in hip replacement surgery. It is also intended for the younger and more active patient where polyethylene wear and its accompanying osteolysis of the more conventional metal-on-polyethylene bearings is of most concern. Surface arthroplasty of the hip is a conservative procedure designed to retain valuable femoral bone stock in younger patients. In this study we examine serum chromium (SrCr) and cobalt (SrCo) and urine chromium (UrCr) in patients with metal-on-metal surface arthroplasty (SA) of the hip.
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Materials and Methods |
This is a prospective study. Nine patients with metal-on-metal surface arthroplasty of the hip are presented. The patients were implanted with the Conserve+® surface arthroplasty. The acetabular component consists of one piece and has sintered CoCr (F75) beads on the outer surface designed for interference fitting to obtain initial stability. The femoral component is a CoCr (F75) hemispheric shell with a chamfered cylindrical interior design and a short tapered stem to enhance alignment and stability. The femoral head size ranged from 40 to 52 mm. There were 6 males and 3 females with an average age at implantation of 45 years (range 28 - 59 years). The components were manufactured by Wright Medical Technology Inc. (Arlington, TN, USA) Serum and urine samples were collected pre-operatively and at 3, 6 and 12 months post implantation. Serum was assayed for Cr and Co and urine for Cr concentration using graphite furnace atomic absorption spectrophotometry. The detection limits in serum were 0.03 ng/ml (ppb) for Cr and 0.3 ppb for Co and 0.015 ppb for Cr in urine.
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Results |
The concentration for SrCr, UrCr and SrCo for the nine cases, are shown in Figs. 1-3, respectively. The data indicate that for SrCr, levels are still elevated at 12 months postoperatively and then decrease. Five of the nine cases had lower values at 24 months post operatively than their previous sampled time period. A similar trend is seen for SrCo but the change is more dramatic following the 6 month postoperative time period (Fig 3). Of the nine cases 7 had SrCo levels at 24 months post operatively lower than their previous sampled time period, (Fig 2). UrCr levels increase up to 12 months and then begin to decline. Six of the 8 cases for which we had urine levels at 24 months post operatively demonstrated lower values than their previous sampled time period. Discussion This group of patients is a subset of a larger group that we are following prospectively. As can be seen with the exception of SrCo all post-operative time periods had elevated levels compared to their pre-operative levels, and these levels are higher than those observed in patients with metal-on-poly bearings1. However over 50% of these individuals had lower levels at their latest follow-up compared to their previous sampled time period. In the case of SrCo two cases have already returned to their pre-operative levels by 24 months post-operative. The data demonstrate that the values by two years begin to decline. It is still early to tell with any certainty due to the small sample size but the wear in period of these devices is longer than the accepted 1-year or 1 million cycles generally observed in metal-on-poly bearings. |
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While the wear properties of metal-on-metal bearings are better than those observed in metal-on-poly bearings, the wear particles generated by these bearings are greater in number and smaller in size. Therefore for a given wear volume many more particles will be released into the surrounding tissue and be available for corrosion and subsequent release of metal ions into the system. The decline in metal ion levels in most of these patients supports the anticipated low steady state wear in well functioning metal-on-metal bearings.
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References |
1. Jacobs, J.J. et. al. JBJS 80-A:1447-1458.
2. Doom, P.P. et. al. JBMR 442:103-111
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Acknowledgements |
Wright Medical Technology, Inc.
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Articulation: Ions
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12/19/2004
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