There are important differences among the metal alloys used for metal-on-metal bearings. Zimmer is the only manufacturer using a high-carbon, wrought-forged cobalt-chromium (CoCr) alloy for all metal-on-metal bearings.
- The high-carbon, wrought-forged CoCr alloy used for Metasul Material has a higher hardness
- Several in vitro wear studies show a significant difference in wear between high-carbon alloys (0.20%-0.25% carbon concentration) and low-carbon alloys (0.05%-0.08% carbon concentration)2-8
- A recent independent study demonstrates a clear trend that components manufactured from wrought CoCr have a lower surface roughness than those manufactured from cast CoCr.9 This is because Metasul material contains finer carbides with a more homogenous carbide distribution.
- Lower roughness has been shown to facilitate the creation of a thicker lubrication film,10,11 providing more efficient lubrication and contributing to wear reduction
Recent clinical reports confirm a number of earlier Metasul material clinical studies that showed excellent results, with survival rates from 96.1% to 100%.12-19
Additional Considerations-Metal Ions
There has been much discussion in the literature about metal-on-metal articulations and the potential for metal ion release. Metal-on-metal articulations have been associated with an increased level of cobalt (Co) and chromium (Cr) in the blood or serum of these joint replacement patients. While metal concentrations are higher in patients who have metal-on-metal articulations than those who do not, it has been concluded that the risk of cancer is not any higher than those who do not have a metal-on-metal articulation.20
For full disclosure on patient risks, please refer to the Instructions For Use of Metasul Implants.
Differences in metal-on-metal bearing systems may include chemical composition (carbon content), material processing methods, and geometry (surface roughness, articulation diameter, sphericity, and diametrical clearance). Any of these differences may affect the incidence of osteolysis and aseptic loosening, as the causes of these complications are multifactorial.21
Aggregated worldwide experience with high-carbon CoCr metal-on-metal bearings indicates that the incidence of hypersensitivity is approximately two per 10,000.22
Although some researchers suggest that metal debris may elicit a type IV hypersensitivity response, it is unclear whether hypersensitivity reactions affect implant performance in the majority of patients.8
According to a 2006 JBJS editorial, "The evidence linking osteolysis and aseptic loosening with metal hypersensitivity is circumstantial; cause and effect have not been established."21
Although studies have shown slightly elevated cobalt and chromium concentrations in the blood or serum of patients with metal-on-metal implants, data from 110,792 THAs and 29,800 TKAs do not support a causal link between metal-on-metal articulations and the development of cancer.23
Long-term exposure to metals from metal-on-metal bearings is well tolerated with respect to cancer risks by the elderly THA population.20
- Cawley J, et al., A tribological study of cobalt chromium molybdenum alloys used in Metal-on-Metal resurfacing hip arthroplasty. Wear 255. 2003:999-1006.
- Wang A, Yue S, Bobyn JD, et al., Surface characterization of Metal-on-Meta implants tested in a hip simulator. Wear 225. 1999: 708-715.
- Fisher J, Ingham E, Stone MH, et al, Wear and debris generation in artificial hip joints. In: Reliability and Long-term Results of Ceramics in Orthopaedics. Sedel L, William G (eds), Stuttgart-New York, Thieme. 1999: 78-81.
- Streicher RM, Semlitsch M, Schön R, et al: Metal-on-metal articulation for artificial hip joints: laboratory study and clinical results. Proceedings of the Institution of Mechanical Engineers, Part H 210, 1996: 223-232.
- Tipper Jl, et al., Quantitative analysis of the wear and wear debris from low and high-carbon content cobalt chrome alloy used in metal on metal hip replacements. J Mat Sci: Mat Med. 1999; 10(6): 353-362.
- Scholes SC, Unsworth A: Pin-on-plate studies on the effect of rotation on the wear of Metal-on-Metal samples. J Mater Sci Mater Med, 2001: 12, 299-303.
- St. John KR, Zardiackas LD, Poggie RA: Wear evaluation of cobalt-chromium alloy for use in a metal-on- metal hip prosthesis. J Biomed Mater Res 68B, 2004: 1-14.
- Firkins PJ, Tipper JL, Saadatzadeh MR, et al: Quantitative analysis of wear and wear debris from metalon- metal hip prostheses tested in a physiological hip joint simulator. Biomed Mater Eng. 2001; 11: 143-57.
- Data on file at Zimmer.
- Chan FW, Bobyn JD, Medley JB, et al. Wear and lubrication of Metal-on-Metal implants. Clin Orthop. 1999; 369: 10-24.
- Jin ZM, Analysis of mixed lubrication mechanism in Metal-on-Metal hip joint replacements. Proc Instn Mech Engrs. 2002; 216 (part H): 85-89.
- Sharma S, et al., Metal-on-Metal total hip joint replacement: a minimum follow-up of five years. Hip Int, 2007; 17: 70–77.
- Migaud H, et al., Cementless Metal-on-Metal hip arthroplasty in patients less than 50 years of age. Comparison with a matched control group using ceramic-on-polyethylene after a minimum 5-year follow- up. J Arthroplasty. 2004; 19 (8, suppl 3): 23–28.
- Long WT, et al., 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.
- Jessen N, et al., Metal/Metal – A new (old) hip bearing system in clinical evaluation. Prospective 7-year follow-up study. Orthopäde. 2004; 33:594–602.
- Delaunay CP, Metal-on-metal bearings in cementless primary total hip arthroplasty. J Arthroplasty. 2004; 19 (8, suppl 3): 35–40.
- Grübl A, et al., Long-term follow-up of Metal-on-Metal total hip replacement. J Orthop Res. 2007; 25: 841–848.
- Eswaramoorthy V, et al., The Metasul Metal-on-Metal articulation in primary total hip replacement: clinical and radiological results at ten years. J Bone Joint Surg Br. 2008; 90B: 1278–1283.
- Delaunay CP, et al., THA using Metal-on-Metal articulation in active patients younger than 50 years. Clin Orthop Relat Res. 2008; 466: 340-346.
- Visuri T, et al., Does Metal-on-Metal total hip prosthesis have influence on cancer? A long-term follow-up study, World Tribology Forum in Arthroplasty, Rieker C, et al. (eds). Bern, Hans Huber, 2001: 181-187.
- Jacobs JJ, HallabNJ. Loosening and osteolysis associated with metal-on-metal bearings: a local effect of metal hypersensitivity? (Editorial). J Bone Joint Surg. June 2006;88-A(6):1171-1172.
- Silva M, Heisel C, Schmalzried P. Metal-on-metal total hip replacement. Clin Orthop. 2005;430:53-61.
- Tharani R, Dorey FJ, Schmalzried TP. The risk of cancer following total hip or knee arthroplasty. J Bone Joint Surg. 2001;83-A(5):428-436.