Mineral concentration gradients in rat femoral diaphyses measured by X-ray microtomography.

The bone mineral concentrations of five rat femora were measured as a function of distance from the distal metaphysis by quantitative X-ray microtomography (XMT) at a resolution of approximately 23 x 23 x 15 microns3. Assuming the mineral phase of bone to be hydroxyapatite, Ca10 (PO4)6(OH)2, the mean cortical mineral concentration (CM) per transverse section was found to range from 1.33 to 1.47 g cm-3. Detectable variations in the bone mineral concentration between sections of femora from different animals could not be attributed to the age when the particular animal was sacrificed. An increase in CM with distance, L, from the distal growth plate was observed and a saturating exponential equation, CM = a - be-alpha L, was used to describe the changes. Each section of bone tissue was considered as a population of elementary volumes of bone (EVB) and L was related to the age of the EVB (TEVB). A simple model for the mineralization process of an EVB was then proposed. Each newly formed EVB accumulated mineral rapidly to give an initial mineral concentration of approximately 1.3 g cm-3 (parameter a-b). Their mineral concentrations then increased asymptotically to approximately 1.5 g cm-3 (parameter a) with a time constant of approximately 330 days. This slow maturation process is attributed to Ostwald ripening of the bone crystals with further crystal growth using ions from the extracellular fluid.