Metal on Metal Bearing

 

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Metal on Metal e.g. Cobalt Chromium 

Advantages over a conventional bearing surface 

    1. large wear ; linear wear rate of < 0.01 mm/year compared with metal on poly UHMWPE bearings with a wear rate of 0.1-0.2 mm/year.

    2. Hydrodynamic lubrication -capable of fluid film lubrication 

                                                    - likely that boundary lubrication occurs when hip at rest and fluid film                                                           not at motion.

  

What factors influence type of lubrication achieved ?

  1.    Radial Clearance - gap between acetabular and femoral bearing surfaces.

                                   - increased radial clearance leads to polar bearing and boundary lubrication.

                                    - small radial clearance leads to equitorial bearing -requires optimal femoral                                                  component       

 2.    Femoral Head Diameter - large femoral head produces fluid film lubrication.

 3.    Component Position     - high cup abduction angle leads to increased risk of edge loading and                                                             boundary lubrication.      

What are disadvantages of Metal on Metal THR ?

        Cancer risk

            Large number of very small wear particles.

            Higher level of cobalt chrome on blood. 

             No definite evidence , but linked to increased risk of cancer. 

       Inflammatory masses due to metal wear debris.