Collagen and the Mechanical Properties of Bone and Calcified Cartilage

In bone type I collagen is mineralized by very small crystals of carbonated hydroxyapatite. There is usually some water present. These three materials together produce a composite whose mechanical properties are unlike that of any of the constituents. The mechanical behavior of bone is not strange and will eventually be explained in terms of standard composite theory. However, that time is not yet, particularly because there is still considerable dispute about some fundamental features of bone, for instance the size and shape of the mineral crystals and their topographical relationship to the collagen. Calcified cartilage, made by the calcification of type II collagen, is the stiff structural element in the skeleton of many chondrichthyean fish. It shows interesting similarities to and differences from bone.