Metasul® Metal-on-Metal Technology

Material

Material Key Factor:  Carbon Content and Material Processing

Chemical composition and material processing are the key material factors controlling the wear behavior of metal-on-metal articulations.

Carbon Content Influences Wear Resistance

Two types of CoCr wrought alloy are available for metal-on-metal articulations: a low-carbon alloy with 0.05%-0.08% carbon concentration, or a high-carbon alloy with 0.20%-0.25% carbon concentration.  The in vitro wear behavior of high- and low-carbon alloys was investigated by numerous groups2,3,4,5 as summarized in this table.  These researchers confirm that the high-carbon alloy is the alloy of choice for metal-on-metal articulations.

Author Wear of High-Carbon Alloy Wear of Low-Carbon Alloy
Wang 1.2 mg after 3 million cycles 8.0 mg after 3 million cycles
Fisher

0.03 mm1 /million cycles

0.33 mm1 /million cycles

Tipper

“The high/high carbon pairing had a significantly lower
(P<0.05) wear rate than the low/low carbon pairing.”

St. John

“After the initial wear-in period, the samples with the higher carbon content exhibited a significantly lower wear rate than those with the lower carbon content.”

Material Processing Matters

Today two process technologies are used for manufacturing of modern metal-on-metal articulations: a cast alloy or a wrought alloy.  Despite the same chemical composition of the material, a wrought high-carbon alloy demonstrates smaller carbide size, homogeneously distributed carbides and a lower surface roughness.

Material processing also plays an important role, because generally, wear resistance improves with hardness of the alloy6.  A high-carbon wrought alloy is harder than a low-carbon wrought or cast alloy and therefore may exhibit improved wear performance.

Cast Alloy Wrought Alloy
Cast Wrought

Comparison of surface and structure for a cast and wrought high-carbon CoCr alloy.

References

  1. Rieker C, et al: In-vitro tribology of large metal-on-metal implants– influence of the clearance. 50th Annual Meeting ORS, 2004, 123
  2. Wang A, et al: Surface characterization of metal-on-metal hip implants tested in a hip simulator. Wear 225–229, 1999, 708–715
  3. Fisher J, et al. Wear and debris generation in artificial hip joints, in: Reliability and Long-term Results of Ceramics in Orthopaedics. Sedel L, Willmann G (eds), Stuttgart-New York, Thieme, 1999, 78–81
  4. 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 10, 1999, 353–362
  5. St. John KR, et al: Comparison of two cobalt-based alloys for use in metal-on-metal hip prostheses: Evaluation of the wear properties in a simulator. Cobalt-base Alloys for Biomedical Applications, ASTM STP 1365, 1999, 145–155
  6. Kato K, et al.: Wear mechanisms, in: Modern Tribology Handbook: Volume 1. Bhusan B (ed). Boca Raton, CRC Press, 2001, 273-300