Tribology of Metal-on-Metal Articulations

Lubrication

Importance of Lubrication

Lubrication controls the amount of wear in metal-on-metal articulations. Studies of lubrication behavior suggest that strict control over manufacturing processes and component design can produce conditions favorable for fluid film lubrication, especially for large-diameter metal-on-metal bearings.

Types of Lubrication

Three types of lubrication are possible for a total hip prosthesis: boundary, mixed or fluid film. A natural healthy hip joint is an optimally functioning tribological system in which synovial fluid minimizes contact between the articulating surfaces. In artificial joints some contact between the surfaces is possible, but the tribological characteristics of the surfaces can be controlled by:                                                                             

  • Surface roughness                                                               
  • Articulation diameter, and
  • Diametrical clearance

The Lambda Coefficient

The Lambda coefficient captures all critical factors that influence metal-on-metal wear performance, including design, load, articulation diameter, material, roughness and diametrical clearance of the components. This coefficient is the ratio between the minimum film thickness (hc ) and the root mean square roughness (Ra) of the two components. Complete fluid film lubrication is possible if Lambda is >3. Calculation of the Lambda coefficient demonstrates that fluid film lubrication is theoretically possible for large Metasul® articulations, as long as some relative motion occurs in the articulation.

In order to achieve the lowest possible amount of wear, the Lambda coefficient must be as high as possible. Thus, central film thickness (h c) has to be as high as possible and roughness (Ra) as low as possible. To minimize roughness, Metasul components are manufactured from wrought CoCr alloys. The central film thickness for metal-on-metal bearings is controlled by articulation diameter and the diametrical clearance of the bearing.

The fluid film thickness increases significantly with an increase of the articulation diameter. For example, for a specific load on 3000N, a specific viscosity of 0.005 Pas and assuming a diametrical clearance on 100 μm, the fluid film thickness for a 22mm bearing would be 11nm, and for a 60mm bearing it would be 97nm (a factor of 9). This explains the favorable performance in regard to lubrication for large diameter metal-on-metal articulations.

Diametrical clearance also significantly influences the film thickness. An increase of fluid film thickness occurs when the diametrical clearance decreases. For example, with the same load, viscosity and articulation diameter of 40mm, decreasing the diametrical clearance from 500 μm to

50 μm would result in fluid film thicknesses of 12nm and 68nm, respectively (a factor of 5.5). To achieve sufficient film thickness for fluid film lubrication, the diametrical clearance for large diameter components must be as small as possible.

Lubrication is controlled by the surface roughness, the diameter of the articulation and the clearance. Metasul implants are designed to maximize the Lambda coefficient by minimizing surface roughness, reducing diametrical clearance, and producing the largest possible articulation diameter.