What's Hip: Materials Matter

Rubbing (sliding with friction) metal on metal is a difficult subject.

In technical applications it is usually mandatory to use different materials as steel on bronze.

This results from the ability of similar or identical materials to fret and locally weld if new (non-oxidised) material is brought into contact with each other.

In seawater or body-fluid (both are water plus sodium-chloride plus other ingredients) the situation is worse as with different metals there may be galvanic corrosion unless inhibition takes place. Inhibition will mean partial or total oxidation as by chromium in stainless steels.

But if the rubbing is too intense there will the oxides be destroyed and dissolved partially. So new oxides are grown and wear will continue until problems arise.

In our body the immune system may make a much more complicated and worse scenario.

So metal on metal will have problems: some patients tolerate these without problems but some have problems and some have severe problems - very likely similar mechanisms as in allergic reactions make the difference.

Ceramic on ceramic is not at all new and will generate fewer problems as the oxides used in these implants are not soluble in our body-fluid. So there may be some wear but this will be tolerated (in most patients).

But: there is the rubbing condition known as squealing, that is in effect a fast stick-slip condition of friction and wear caused by elasticity and/or dependence of friction coefficient on velocity of sliding.

This may generate (elastic) stress in the implant that is initiating cracks, these slowly growing until catastrophic failure.

The engineering approach would be to use better ceramics (tough and shock resistant and not prone to crack growth) but cost and tradition and liability are big obstacles to change.

Metal-PE (UHMW-PE) may be worse than any other variety as the clearance is (has to be) much wider and this will invariably result in wear and our body won't tolerate the PE-squeezed particles. The squeezing and deformation seems to act as a mechano-chemical molecule reshaping method thus producing reactive molecules with any consequence you might not want. Or this is once more an amplified reaction of our immune system. Regardless which version of these is true (may be both) the patient will have problems. Added are the problems of much higher wear in the PE.

So metal-PE will be recommended only for aged patients. Bot those that live long against the odds of any statistics will have problems too.

So todays best is ceramic on ceramic. But to maintain a gap of a very few micrometer in sliding motion with the fluid between the two surfaces acting as a hydrodynamic bearing will require very close tolerances for grinding an polishing of the surfaces.

So anybody who wants to make these is recommended to buy the best possible machines, do not intend to make perfect spheres by x-y machining, use optical tolerances and measurements and try at which speed the load capacity is sufficient to bear twice the body weight. And measure or calculate how long the action of squeeze-film-damping can support the load of 5 times the body weight.

If these data look good the product will be at the front end of quality of surface generation and supporting action.

RHABE