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Prosthetic Articular Bearing

S. Zafar Naqui (25.12.05) Miller, Orthoteers

Hard on soft
Traditional hip bearing has been ‘hard on soft’ couple. Metallic (Co-Cr alloy/Ti alloy) heads or ceramic (alumina or zirconia) head mated to Polyethelene cup (PE).

Of the metallic heads – the Co-Cr considered the best. Ti alloy susceptible to scratching from third-body debris causing rapid wear of PE. (However use Ti in nickel-allergy patients)

For both metallic and ceramic-PE couples, optimum wear is related to roughness of head surface and sphericity of head. In Co-Cr heads roughness due to residual carbide asperites that stick up; in ceramics the residual pits cause surface roughness.

Lubrication for hard on soft is BOUNDARY lubrication. The lubricant (synovial fluid) cannot prevent the aspirites (high points) between the 2 surfaces from touching, but can separate the 2 surfaces enough to prevent severe wear.

Ceramic on PE bearing surfaces – Urban et al Nov. JBJS 2001 17 to 21 yr results –linear wear rate at 0.034 mm/yr and volumetric at 28mm3/yr with survivorship of 95%  at 5 and 10yrs – wear rates lower than metal on poly.

hard on hard Bearings
Impetus for this has been failure from PE osteolysis.  Particel size 0.2µm -0.7µm shown to trigger immune response leading to osteolysis.  Hard on hard debris is 0.015 to 0.012µm – therefore less intense reaction.

NB – although have very low liner wear rate and lower wear volume – no of particles generated still more than hard on soft bearings. (But particles so much smaller that overall aggregate is greater.)

The small debris are absorbed regionally and disseminated systemically via lymphatics – which is a concern for metal-on metal – incr. Co-Cr ions in blood and urine. However, no long-term studies suggest incr neoplasia risk. Ceramics are inert so cancer risk not an issue.

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