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Systemic Metal Exposure in Large- and Small-diameter Metal-on-Metal Total Hip Replacements

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Abstract

Large-diameter metal-metal total hip replacement (THR) offers the advantages of low wear and low dislocation risk. The aim of this investigation was to compare metal levels in large- and small-diameter metal-metal hip replacements. Whole blood concentrations and daily output of cobalt and chromium in 28 patients with unilateral large diameter (42- to 54-mm) metal-on-metal hip replacements at 1-year follow-up were compared with levels in patients with 28-mm metal-on-metal THRs. Both bearings were made of high-carbon cobalt-chrome alloy. The larger bearing is as-cast and the smaller wrought alloy. High-resolution inductively coupled plasma mass spectrometry was used for analysis. The patients had either a cemented polished tapered stainless steel stem or a cementless porous ingrowth titanium alloy stem. Mean whole blood levels in the small- and large-diameter THRs are not significantly different at 1 year (cobalt, 1.7 vs 2.3 µg/L and chromium 1.7 vs 1.4 µg/L). Daily urinary output of cobalt and chromium was also in the same range and without a significant mean difference (cobalt 11.6 µg/24 h in large-diameter and 12.3µg/24 h in small-diameter THRs and chromium 3.7 and 4.1µg/24 h, respectively).

Second-generation metal-on-metal bearings were introduced as small-diameter (28 and 32 mm) total hip replacements (THRs) nearly 20 years ago.¹ The wear rates in these modern metal-on-metal devices are lower compared with conventional metal-on-polyethylene devices.^2,3

Later, large-diameter metal-on-metal bearings were introduced into clinical use in the form of modern hip resurfacing bearings.^4,5 Large-diameter metal-on-metal bearing THRs were initially envisaged as revision systems for femoral failures after a resurfacing. The femoral component at revision was replaced with a modular revision component that matched the existing cup and was fixed on a conventional stem through a modular junction cone. Good outcomes resulting from such revisions encouraged the use of these large-diameter metal-on-metal THRs as primary procedures in patients whose poor femoral head bone quality precluded the possibility of a resurfacing, but in whom a low-wear bearing is highly desirable.

In addition to a low wear rate, large-diameter THRs also offer a more favorable head-neck ratio compared with both the small-diameter metal-on-metal THRs and the hip resurfacings. This allows an increased range of motion (ROM) before impingement and a reduced rate of dislocation. These advantages have led some surgeons to argue against the use of resurfacings.^6-8 Instead, they advocate large-diameter metal-on-metal THRs as the first option in young patients with severe hip arthritis.⁸ It is argued that this combination avoids the potential risks of stress shielding, loss of femoral head vascularity, and neck fractures. However, this comes at a cost. Compared with hip resurfacing, hip replacement is not only a more invasive procedure but also has more potential sites for metal release, ie, from the modular junction between the femoral component and the stem and from the stem-cement interface or implant-bone interface. Metal ion studies^9,10 from conventional bearing hip replacements confirm the possibility of metal release from nonbearing sites.

In an earlier report,¹¹ we presented the daily output of metal ions and whole blood metal levels in a cohort of patients with Birmingham Hip Resurfacings and compared them with the levels seen in patients with 28-mm Metasul THRs at the same periods of follow-up. We did not find a significant difference between the metal ion levels generated by these two implants. In this study, we compare the metal ion output and blood levels in patients with Metasul THRs, as published earlier, with a cohort of patients with large-diameter metal-on-metal THRs.

Patients and Methods

The subjects in this study are those with unilateral, well-functioning, large-diameter (42- to 54-mm) metal-on-metal THRs (Smith & Nephew Orthopaedics, Warwick, United Kingdom) and those with well-functioning 28-mm Metasul THRs (Sulzer Orthopaedics, now Zimmer GmbH, Winterthur, Switzerland). Whole blood concentrations and daily output of cobalt and chromium in 28 patients (mean age at operation, 60.6 years; range, 37-76.7 years) with unilateral large-diameter metal-on-metal THRs at 1-year follow-up were obtained. The blood levels were compared with whole blood concentrations in 20 patients with unilateral 28-mm Metasul THRs at 1-year follow-up (mean age at operation, 63.3 years; range, 55-74.7). The metal ion daily output was compared with that in 28 patients with unilateral 28-mm Metasul THRs at 1- to 3-year follow-up (mean age at operation, 56.6 years; range, 39 to 63). Because we had fewer patients with Metasul THRs than with large-diameter metal-on-metal THRs available for study at our center, the follow-up period extended over a wider range for those with the Metasul bearings.

Both bearings are made of high-carbon cobalt chrome alloy and are not subjected to heat treatment processes in the later stages of manufacture. The larger bearing is as-cast, and the smaller is wrought alloy. The acetabular components were cementless in both systems. In patients with the Metasul THRs, cementless porous ingrowth titanium sockets with a polyethylene sandwich construct were used.¹² The large-diameter THR had a monoblock hydroxyapatite-coated porous uncemented acetabular component with a nominal thickness of 3 or 4 mm. These components are the same as those routinely used with the Birmingham Hip Resurfacing system (Smith & Nephew Orthopaedics).

On the femoral side, either a cemented polished tapered stainless steel stem or a cementless porous ingrowth titanium alloy stem was used with the respective modular femoral head component (Figure 1). Patients with any other metal devices were excluded, and there were no patients with a history of compromised renal function.

Specimen collection was similar in the two groups. All specimens were analyzed for cobalt and chromium ion levels using high-resolution inductively coupled plasma mass spectrometry. With regard to the Metasul THR patients, the details are published in an earlier report.¹¹ After informed consent was obtained, patients with large-diameter metal-on-metal THRs were sent follow-up hip outcomes questionnaires, urine containers, and clear instructions to collect a 12-hour specimen of urine. They brought the completed questionnaires and the urine specimen to their review appointment. Daily output of metal ions was calculated from these specimens.

In the follow-up clinic, a plain anteroposterior radiograph of the pelvis was taken, and a clinicoradiologic assessment was carried out. Whole blood samples were also obtained for metal ion assessments with precautions to prevent contamination. These were drawn into two 6-mL Lithium Heparin Vacutainer tubes (Sarstedt Ltd, Leicester, United Kingdom). One tube was batched for analysis, and the other was stored in a freezer at -18ºC as a reserve.

The total volume of urine in 12 hours was recorded, and two small aliquots were separated. One aliquot was batched and sent for analysis and the other held as a reserve. The limits of detection for cobalt and chromium were 0.02 µg/L in the urine study, and the reporting limit was three times the limit of detection (ie, 0.06 µg/L). The reporting limits in the whole blood study were 0.1 µg/L (limit of detection, 0.03 µg/L) for cobalt and 0.2 µg/L for chromium (limit of detection, 0.067 µg/L), respectively.

Results

The differences between mean whole blood levels in the 28-mm Metasul THRs and the large-diameter metal-on-metal THRs were not statistically significant at 1 year (chromium, 1.7 vs 1.4 µg/L and cobalt, 1.7 vs 2.3 µg/L, respectively; P > .1) (Figure 2). Daily urinary output of cobalt and chromium in both bearings were also without a significant mean difference (P > .05). Mean cobalt output was 11.6 µg/24 h in the 28-mm Metasul THRs and 12.3 µg/24 h in the large-diameter metal-on-metal THRs (P > .5), and mean chromium output was 3.7 and 4.1µg/24 h (P > .5), respectively (Figure 3).

Discussion

Daily output of metal ions in urine serves as a measure of in vivo metal ion release from bearing wear and corrosion. Blood metal ion level monitoring is a means of assessing systemic exposure to metal after metal-on-metal bearing arthroplasty¹³

Tribologic theory^14,15 and hip simulator studies^16,17 suggest that large-diameter metal-on-metal bearings wear less than small-diameter bearings. This phenomenon is believed to be due to the beneficial effect of fluid film generation in large-diameter bearings. However, controversy exists regarding in vivo wear as evidenced by metal ion levels. Some studies suggest no difference in metal ion levels^11,18 between large-diameter resurfacings and small-diameter THRs, whereas others suggest that large-diameter resurfacings generate more wear.^19,20 Regrettably, none of those studies compare small- and large-diameter metal-on-metal THRs.

In our previous report,¹¹ we did not find a significant difference between metal ion levels in patients with Birmingham Hip Resurfacings and 28 mm Metasul metal-on-metal THRs. Both these bearings have been widely used globally and have a proven clinical record in the medium term.^21-23 It is possible that the lack of difference between the resurfacings and the hip replacements is attributable to metal release from nonbearing wear in the THRs, offsetting a higher bearing wear from the Birmingham Hip Resurfacings. To allow for metal release from nonbearing surfaces, we performed this study comparing patients who had undergone 28-mm Metasul-bearing THRs with patients who have primary large-diameter metal-on-metal THRs using the same bearing as the Birmingham Hip Resurfacings. This metal-on-metal THR combination has not been approved by the FDA for use in primary hip arthroplasty in the United States. It is, however, used in Australia and in countries in Europe, Africa, and Asia, especially in high-demand patients who are unsuitable for a resurfacing because of poor femoral head bone quality.

Although there are minor differences between mean metal ion levels in the two cohorts, with blood chromium being slightly higher in the Metasul THRs and blood cobalt being slightly higher in the large-diameter metal-on-metal THR group, the differences are only marginal and not statistically significant.

The disadvantages of this study are 1) it is a retrospective study, and 2) the sizes of the cohorts are small. At our center, we believe that the advantages offered by large-diameter THRs in the form of reduced dislocation rate and increased ROM are so compelling that they leave us limited scope to continue the use of smaller-diameter metal-on-metal THRs. This preliminary study, however, could form the basis on which to design a larger prospective series in centers where both small- and large-diameter metal-on-metal bearing hip replacements with a good clinical record are routinely used.

Conclusion

This study supports our previous findings that there is no significant difference between metal release in small- and large-diameter bearings in hip arthroplasty in the early years after implantation.

References

1. Schmidt M, Weber H, Schön R. Cobalt chromium molybdenum metal combination for modular hip prostheses. Clin Orthop Relat Res. 1996 Aug;(329 suppl):S35-S47. Review.
2. Clarke IC, Good V, Williams P, et al.Ultra-low wear rates for rigid-on-rigid bearings in total hip replacements. Proc Inst Mech Eng [H]. 2000;214:331-347.
3. Sieber HP, Rieker CB, Köttig P. Analysis of 118 second-generation metal-on-metal retrieved hip implants. J Bone Joint Surg Br. 1999 Jan;81(1):46-50.
4. McMinn D, Treacy R, Lin K, Pynsent P. Metal on metal surface replacement of the hip. Experience of the McMinn prosthesis. Clin Orthop Relat Res. 1996 Aug;(329 suppl):S89-S98.
5. D.J.W. McMinn Development of Metal/Metal Hip Resurfacing Hip International 2003; 13: S41-S53.
6. Cuckler JM. The optimal metal-metal arthroplasty is still a total hip arthroplasty: in the affirmative. J Arthroplasty. 2006 Jun;21(4 suppl 1):74-76.
7. Berend KR. Hip resurfacing: time for a second look. Orthopedics Today. 2007;27:82.
8. Berend M, Bertrand T. Metal-metal hip resurfacing: solution to a nonexistent problem. Orthopedics. 2007;30:724.
9. Jacobs JJ, Skipor AK, Patterson LM, et al. Metal release in patients who have had a primary total hip arthroplasty. A prospective, controlled, longitudinal study. J Bone Joint Surg Am. 1998 Oct;80(10):1447-1458.
10. Hart AJ, Hester T, Sinclair K,et al. The association between metal ions from hip resurfacing and reduced T-cell counts. J Bone Joint Surg Br. 2006;88: 449-454.
11. Daniel J, Ziaee H, Salama A, Pradhan C, McMinn DJ. The effect of the diameter of metal-on-metal bearings on systemic exposure to cobalt and chromium. J Bone Joint Surg Br. 2006 Apr;88(4):443-448.
12. Liu F, Jin ZM, Grigoris P, Hirt F, Rieker C. Contact mechanics of metal-on-metal hip implants employing a metallic cup with a UHMWPE backing. Proc Inst Mech Eng [H]. 2003;217(3):207-213.
13. Daniel J, Ziaee H, Pynsent PB, McMinn DJ. The validity of serum levels as a surrogate measure of systemic exposure to metal ions in hip replacement. J Bone Joint Surg Br. 2007 Jun;89(6):736-741.
14. Jin ZM, Dowson D, Fisher J. Analysis of fluid film lubrication in artificial hip joint replacements with surfaces of high elastic modulus. Proc Inst Mech Eng [H]. 1997;211(3): 247-256.
15. Udofia IJ, Jin ZM. Elastohydrodynamic lubrication analysis of metal-on-metal hip-resurfacing prostheses. J Biomech. 2003 Apr;36(4):537-544.
16. Smith SL, Dowson D, Goldsmith AA. The effect of femoral head diameter upon lubrication and wear of metal-on-metal total hip replacements. Proc Inst Mech Eng [H]. 2001;215(2):161-170.
17. Dowson D, Hardaker C, Flett M, Isaac GH. A hip joint simulator study of the performance of metal-on-metal joints. Part II: Design. J Arthroplasty. 2004;19:124-130.
18. Moroni A, Savarino L, Cadossi M, Baldini N, Giannini S. Does Ion Release Differ Between Hip Resurfacing and Metal-on-metal THA? Clin Orthop Relat Res. 2008 Mar;466(3):700-707.
19. Clarke MT, Lee PT, Arora A, Villar RN. Levels of metal ions after small- and large-diameter metal-on-metal hip arthroplasty. J Bone Joint Surg Br. 2003 Aug;85(6):913-917.
20. Witzleb WC, Ziegler J, Krummenauer F, Neumeister V, Guenther KP. Exposure to chromium, cobalt and molybdenum from metal-on-metal total hip replacement and hip resurfacing arthroplasty. Acta Orthop. 2006 Oct;77(5):697-705.
21. Long WT, Dorr LD, Gendelman V. An American experience with metal-on-metal total hip arthroplasties: a 7-year follow-up study. J Arthroplasty. 2004;19(8 suppl 3):29-34.
22. Daniel J, Pynsent PB, McMinn DJ. Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg Br. 2004 Mar;86(2):177-184.
23. Treacy RB, McBryde CW, Pynsent PB. Birmingham hip resurfacing arthroplasty. A minimum follow-up of five years. J Bone Joint Surg Br. 2005 Feb;87(2):167-170.

Authors

Drs Daniel, Pradhan, and McMinn, and Hena Ziaee, BSc (Hons), are from The McMinn Centre, Birmingham, United Kingdom.

Drs Daniel and Pradhan and Hena Ziaee, BSc (Hons), received research grant support from Smith & Nephew Orthopaedics. Dr McMinn is a consultant and nonexecutive director for Smith & Nephew.

Correspondence should be addressed to Joseph Daniel, FRCS, The McMinn Centre, 25 Highfield Road, Edgbaston, Birmingham, B15 3DP United Kingdom.