Smith chapter 1 problem 2, Total hip arthroplasty

a) Hip replacement components: see figure HipReplacement.jpg

· The stem is inserted into the patient’s upper leg bone

· The ball is attached to the stem

· The cup is attached to the patient’s pelvic bone and is composed of liner and a shell.

b) and c) Materials use in the manufacture of these components and factors the materials engineer should consider in their selection:

Stem and cup shell:

Made of metal, such as a Ti-6Al-4V alloy or a CoCr alloy. These alloys are selected for high strength, toughness (not brittle), not corrodible in oxygenated saline body fluids and do not kill many cells. The outer surfaces of the metal are treated to promote adhesion of either bone cement or bone tissue itself: grit blasted to roughen the surface or plasma sprayed with a porous metal layer and/or coated with a layer of calcium hydroxyapatite which is the inorganic component of human bone, to promote bone growth.

Ball:

Made of metal or ceramic, chosen for strength, toughness, corrosion resistance and biocompatibility, for example a Ti alloy, stainless steel (FeCrC alloy) or CoCr alloy or alumina (Al2O3) ceramic. The surface of a metal ball may be treated to harden it, for example by implanting nitrogen ions. The surface must also be polished to an extremely smooth finish. The ball material, in combination with the cup liner material must provide both low friction and low wear over the life of the joint, at least 10 years and preferably 20 or more.

Cup liner:

Typically made of either ultra high molecular weight polyethylene (UHMWPE) plastic or alumina. This material, in combination with the ball, must give low friction and low wear over the life of the joint. This material must also resist corrosion in body fluids. If the ball is metal, the cup liner is typically UHMWPE. If the ball is alumina, the cup liner is typically alumina too. In the case of UHMWPE, creep is an issue. Over a period of years, the joint may loosen by this mechanism. Experimentation has shown that increased cross-linking of the UHMWPE reduces wear. This can be accomplished by irradiation of the material. Also, oxidation of the UHMWPE is known to increase wear and so should be minimized. When the cup liner wears, debris particles are generated. These should not be toxic or damaging to human cells.

Note: Bone cement may be used to hold the stem and/or cup shell in place. This material is primarily polymethyl methacrylate, also known as PMMA or Plexiglas. About 10% barium sulfate is added to make the cement show up well on x-rays. Some antibiotics may also be added.