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 mu m(3). Assuming the mineral phase of bone to be hydroxyapatite, Ca-10 (PO4)(6) (OH)(2), the mean cortical mineral concentration (C-M) per tr ansverse section was found to range from 1.33 to 1.47 g cm(-3). Detect able 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 C-M with distance , L, from the distal growth plate was observed and a saturating expone ntial equation, C-M = a - be(-alpha L), was used to describe the chang es. Each section of bone tissue was considered as a population of elem entary volumes of bone (EVE) and L was related to the age of the EVE ( T-EVB). A simple model for the mineralization process of an EVE was th en proposed. Each newly formed EVE accumulated mineral rapidly to give an initial mineral concentration of similar to 1.3 g cm(-3) (paramete r a-b). Their mineral concentrations then increased asymptotically to similar to 1.5 g cm(-3) (parameter a) with a time constant of similar to 330 days. This slow maturation process is attributed to Ostwald rip ening of the bone crystals with further crystal growth using ions from the extracellular fluid.