An electron probe X-ray microanalytical study of bone mineral in osteogenesis imperfecta

A semiquantitative electron probe X-ray microanalytical (XRMA) technique, in conjunction with transmission electron microscopy, was used to compare the calcium to phosphorus (Ca/P) molar ratios in calcium phosphate standards of known composition, in normal bone and in bone from patients with osteogenesis imperfecta (OI). Using a modified routine processing and resin embedding schedule, the measured Ca/P molar ratio of calcium phosphates standards of known composition were found to correlate well with the Ca/P molar ratio based on their respective chemical formulae. This technique was then used to compare the Ca/P molar ratio in normal human bone and in OI bone. The Ca/P ratio values for normal bone (Ca/P = 1.631) correlated well with those for chemically prepared hydroxyapatite (Ca/P = 1.602), but in bone from OI patients, the Ca/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 the carbanoapatite usually found in bone. Isomorphous substitutions in the carbanoapatite lattice could account for this although this study has neither proved nor disproved this. The altered bone mineral is another factor that could contribute to the increased fracture rate observed in OI.