Tribology of Metal-on-Metal Articulations

Friction

Frictional torque was a problem in first-generation metal-on-metal implants due to limitations in manufacturing and a poor understanding of bearing design and function. However, with modern manufacturing techniques, proper diametrical clearances and low surface roughness, today’s metal-on-metal components minimize frictional torque and demonstrate improved lubrication.

On a friction simulator, the friction coefficient of different metal-on-metal articulations was examined. A friction coefficient of 0.11 and 0.06 respectively was measured using a Metasul® 28mm and 32mm articulation during simulated immediate postoperative situation. As reported by R.M. Streicher, “The reduction of the ball diameter to 32 and 28mm and the use of the CoCrMo forged alloy, Protasul ®-II, gave a reduction of the friction loses to the same values as for the polymer-metal combinations in the lubricated state.”1  

Despite optimized clearance, every Metasul Bearing undergoes a period of run-in wear while the articulating surfaces adapt to one another. This period differs in every patient, depending on weight and activity as well as on the inclination angle of the cup. Run-in wear and friction can be tested and quantified with a hip joint simulator1.

References

  1. Streicher RM, et al: Untersuchung des tribologischen Verhaltens von Metall/Metall-Kombinationen für künstliche Hüftgelenke. Biomedizinische Technik 35, 1990, 107–111