Tribology of Metal-on-Metal Articulations

Wear Performance

Hardness Influences Wear Performance

Optimized clearance and low surface roughness can result in good fluid film lubrication of metal-on-metal bearings as long as some relative motion occurs in the articulation, e.g. during normal gait. However, during many daily activities metal-on-metal articulations operate with boundary or mixed lubrication, allowing contacts between the two components. In these two lubrication modes, the main mechanism of wear is abrasion, where hard particles generate scratches on the loaded zone.

According to Kato3 the wear volume is inversely proportional to the hardness of the alloy, meaning that harder alloys may have a higher wear resistance.

Protasul®-21WF, has a high hardness (470 HV6 ), and is the alloy of choice for minimizing the abrasive wear of metal-on-metal articulations. Zimmer uses this alloy for all Metasul® acetabular components.                                                                                         

The hardness value for a cast alloy is 310 HV5, and for wrought low-carbon alloy is 410HV4.

In Vitro Wear Performance

The wear behavior of metal-on metal articulations was investigated with an AMTI hip simulator. These studies showed that wear occurs in two distinct phases, a run-in phase followed by a steady-state phase. The run-in volumetric wear of a 50mm Metasul large-diameter articulation was slightly higher than that of the 28mm articulation. Lower wear rates after the run-in phase confirmed that the Metasul large-diameter articulations achieved better lubrication than the 28mm articulation

Metasul Articulations Run-in Wear Steady State Wear Rate
28mm

1.7mm1

0.40mm1 / 102cycles
50mm

5.1mm1

0.05mm1 / 102 cycles

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. Semlitsch M, et al: Wear behaviour of cast CoCrMo cups and balls in long-term implanted total hip prostheses. Orthopaedics 18, 1989, 377–381
  3. Kato K, et al.: Wear mechanisms, in: Modern Tribology Handbook: Volume 1. Bhusan B (ed). Boca Raton, CRC Press, 2001, 273-300
  4. Data on file at Zimmer
  5. 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
  6. Jazrawi LM, et al: Wear rates of ceramic-on-ceramic bearing surfaces in total hip arthroplasty. A 12-year follow-up study. J Arthroplasty 14 (7), 1999, 781–787