Changes in the mineral density distribution in human bone with age: image analysis using backscattered electrons in the SEM.

We report a study to test the feasibility of studying mineral density distributions in bone using the backscattered electron signal in scanning electron microscopy. Samples were human sixth ribs ranging in age from 8 weeks to 59 years, embedded in polymethylmethacrylate (PMMA), cut, polished, and carbon coated. The proportions of pixels falling in a uniform set of gray level slices of the BSE signal were determined using a microcomputer-based image analysis system interfaced directly to the SEM. The amount of high-density bone gradually increased with age at the expense of low-density bone, and there was an associated compression of the range of the mineral density distribution. Age-related differences were noted between the density distributions in the outer and inner rib cortices. The distribution in the inner cortex in neonates was influenced by the inclusion of densely mineralized endochondral bone and cartilage trabeculae formed at the growth cartilage zone. In adults it appeared that greater bone turnover occurred in the outer cortex, perhaps reflecting a differential mechanical loading across the rib. The technique enabled rapid, unbiased discrimination between the bone of neonates, children, and adults.