Three analogies to explain the mechanical properties of bone

Abstract

• 1.

The mechanical properties of bone are described. It is shown that these properties are not explicable on the assumption that the apatite and the collagen of bone are acting as a compound bar, such as reinforced concrete.

2.

Knese’s hypothesis that bone is a prestressed material, explaining some of the anomalous properties of bone, is discussed. It is shown that:

• (a)

  There is no good evidence that bone is prestressed.

• (b)

  It is difficult to see how the very large initial prestresses could be applied.

• (c)

  For the prestressing to be effective in increasing the apparent tensile strength of the apatite sufficiently would mean that during compression the apatite would be subjected to a load higher than it can probably bear.

3.

It is suggested that bone is a two-phase material, like fibreglass. Similarities between bone and engineering two-phase materials are discussed. The apatite crystals are very small, so dangerous flaws could form in them only with difficulty, and any running crack would immediately come to the edge of the crystal and run into collagen, which would deform but not split.

4.

The possible occurrence of other two-phase materials in nature is discussed.