The degree of mineralization of bone tissue measured by computerized quantitative contact microradiography

Strength of bones depends on bone matrix volume (BMV), bone microarchitecture, but also bone mineralization, and we have recently shown in osteoporotic patients treated with alendronate that fracture risk and bone mineral density (BMD) were changed without modifications of BMV or bone microarchitecture. Mineralization of bone matrix implies two successive steps: a rapid primary mineralization on the calcification front followed by a slow process of secondary mineralization progressively adding about one-half of the mineral content on bone matrix. These two steps are clearly illustrated by microradiographs of compact and cancellous bone tissue from transiliac human biopsies. Our working hypothesis is based on the impact of changes in bone remodeling rate on the degree of mineralization of bone, i.e., on the BMD measured at the tissue level. Contact quantitative microradiography using a computerized microdensitometric method, is described and allows the measurement of the mean degree of mineralization of bone (MDMB). This parameter may be quantitatively evaluated by exposing an aluminum calibration step-wedge and a plane-parallel calcified tissue section simultaneously to the same beam of X-rays, then determining, from the resulting microradiograph, the thickness of aluminum that produces the same X-ray absorption as a given region of the bone tissue section. To be used as a control group, iliac bone samples were taken at necropsy from 43 subjects (30 women aged 48.4 +/- 3.7 years and 13 men aged 66.0 +/- 4.4 years) who died suddenly showing no apparent bone disease. A control MDMB, which does not change with age, and a control distribution of these values are thus established. These control values are necessary for interpreting the changes in MDMB observed in bone conditions untreated or treated.