Effects of differences in m ineralization on the m echanical properties of bone

There is a considerable variation in the m ineralization of bone; norm al, nonpathological com pact bone has ash masses ranging from 45 to 85% by mass. This range of m ineralization results in an even greater range of mechanical properties. The Young modulus of elasticity can range from 4 to 32 GPa, bending strength from 50 to 300 M Pa, and the work of fracture from 200 to 7000 Jm ~ 2. It is not possible for any one type of bone to have high values for all three properties. Very high values of m ineralization produce high values of Young modulus but low values of work of fracture (which is a measure of fracture toughness). R ather low values of m ineral­ ization are associated with high values of work of fracture but low values of Young modulus and interm ediate values of bending strength. The reason for the high value for the Young modulus associated with high m ineralization is intuitively obvious, but has not yet been rigorously modelled. The low fracture toughness associated with high m ineralization may be caused by the failure of various crack-stopping mechanisms that can act when the mineral crystals in bone have not coalesced, but which become ineffective when the volume fraction of mineral becomes too high. The adoption of different degrees of m ineralization by different bones, leading to different sets of mechanical properties, is shown to be adaptive in most cases studied, but some puzzles still remain.