Introduction

The purpose of this chapter is to show that the responses to osteotropic influences of different bones in the skeleton vary according to a ‘survival value’ associated with that particular bone. An understanding of the mechanisms by which these differences occur, particularly the ways in which some bones are resistant to resorption, could have pervasive consequences in the treatment of diseases which are characterised by functionally inappropriate bone loss.

Function of the skeleton

The skeleton has numerous functions, but the one which accounts predominantly for its shape, mass and material properties is the requirement that it should bear loads. Bone is unique among the tissues of the body in the level of its resistance to compressive forces, which results from its composition. While collagen and other organic components give tensile strength, hydroxyapatite is responsible for the resistance to compression. The relative amounts of these two components vary at different stages in life. Young animals have bones which are compliant as a result of a high proportion of organic components and relatively little mineral. This has been suggested to confer resistance to falls which would result in fractures in individuals with more rigid skeletons. Of course, the consequence of such compliance is greater dissipation of energy during loading. This translates into a structure which wastes more energy during locomotion than a more rigid bone (Currey, 1969). During maturation, mineralisation of the skeleton increases, and the individual usually becomes more coordinated, so that falls are less likely, and efficient locomotion is a more important requirement than high fracture threshold.