AN ELECTRON-PROBE X-RAY MICROANALYTICAL STUDY OF BONE-MINERAL IN OSTEOGENESIS IMPERFECTA

A semiquantitative electron probe X-ray microanalytical (XRMA) techniq ue, in conjunction with transmission electron microscopy, was used to compare the calcium to phosphorus (Ca/P) molar ratios in calcium phosp hate standards of known composition, in normal bone and in bone from p atients with osteogenesis imperfecta (OI). Using a modified routine pr ocessing and resin embedding schedule, the measured Ca/P molar ratio o f calcium phosphates standards of known composition were found to corr elate well with the Ca/P molar ratio based on their respective chemica l formulae. This technique was then used to compare the Ca/P molar rat io in normal human bone and in OI bone. The Ca/P ratio values for norm al bone (Ca/P = 1.631) correlated well with those for chemically prepa red hydroxyapatite (Ca/P = 1.602), but in bone from OI patients, the C a/P molar ratio was significantly lower (Ca/P = 1.488). This study has shown that there is a lower Ca/P molar ratio in OI bone compared with normal, matched bone. This suggests that the mineral deviates from th e carbanoapatite usually found in bone. Isomorphous substitutions in t he carbanoapatite lattice could account for this although this study h as neither proved nor disproved this. The altered bone mineral is anot her factor that could contribute to the increased fracture rate observ ed in OI.