Metal on Metal Bearing
https://www.nashvillejointreplacement.com/hip-implants
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.
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