STUDIES OF BONE BIOPHYSICS USING ULTRASOUND VELOCITY

INTRODUCTION The biological processes in bone are expressed in its final structure, achieved to fulfill specific physical functions. Thus a biophysical study of bone is a study of the relationship between its organization (at various scales) and its functions. Bone tissue primarily provides structural support and mineral homeostasis. Its microstructure has a hierarchical organization and a symmetry (transverse isotropy) optimized to meet a variety of structural needs over the entire skeleton; the material is thus heterogeneous and anisotropic with its mechanical properties varying with site and orientation. The two compartments (types) of bone, cortical and cancellous, are made of the same material but vary in the composition of the components. Bone material is constituted from organic matter which consists mostly of proteins (primarily the fibrous protein Type I collagen), glycoproteins and proteoglycans and from inorganic matter, the bone mineral crystals. The microstructural organization of these two phases is not very well described but it has been demonstrated that the mineral crystals are normally preferentially oriented with their major axis along the long fibrous collagen proteins1. The properties of these components also change with location and age. This complexity in the microarchitecture of bone matrix makes it extremely difficult to diagnose causes of mechanical or functional failure of the tissue. The strength of bone is a clinically relevant biophysical property, the assessment of which is currently carried out by calculating the amount of mineral present in bone from a projected X-ray image of the entire bone. Biochemical markers such as collagen degradation products have also been used to assess bone physiology but give no insight into location or compartment-specific responses. Biopsies of bone can provide valuable information about bone status but this is an invasive technique and repeated measurements are generally not possible. Magnetic resonance microscopy has been used at peripheral sites to quantitate bone structure, through which the functional status of bone can be inferred2. However, all of the above techniques provide indirect measures of the mechanical properties of bone.