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Biomaterials

When a metal with a Young's modulus far higher than that of bone is used for the stem of an artificial hip joint, which statement correctly describes the resulting problem of stress shielding?

  1. ABone carries most of the load, so excessive stress acts on the bone and it fractures easily
  2. BThe implant carries most of the load and the stress on the surrounding bone falls, so the bone is resorbed and thins
  3. CThe passive film on the implant surface breaks under stress, and the release of metal ions increases
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Answer: B. The implant carries most of the load and the stress on the surrounding bone falls, so the bone is resorbed and thins

When bone and an implant carry a load together, both strain by nearly the same amount, so the one with the larger Young's modulus carries most of the stress. The Young's modulus of cortical bone is about 10–30 GPa, but Co-Cr alloys at about 210 GPa and Ti-6Al-4V at about 110 GPa are far higher, so the stress on the bone becomes smaller than it should be. Bone changes the balance of formation and resorption according to the mechanical stimulus it receives (Wolff's law), so where the stimulus falls, bone is resorbed and thins, which causes implant loosening and fractures around the implant. Excessive stress on the bone is the opposite situation. Metal ion release due to damage to the passive film is a separate problem, such as fretting corrosion.

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