99mTc-MDP accumulation mechanisms in bone.

The accumulation mechanisms of technetium-99m methylene diphosphonate were investigated using fetal mice calvaria, osteoblast-like cells, collagen sponges, and hydroxyapatite powder. Technetium-99m methylene diphosphonate accumulation was found consistently in the area of cells growing out from calvarial fragments cultured for 10 days. Accumulation was also seen on autoradiographs of calvarial fragments immediately fixed with 90% ethanol. The accumulated technetium-99m methylene diphosphonate in cultured calvarial fragments was removed by 100 mmol/L pyrophosphate and 378 mmol/L ethylenediaminetetraacetic acid, however, residual accumulations were still clearly visible. There was no relation between the osteoblast-like cell number and technetium-99m methylene diphosphonate accumulation. Technetium-99m methylene diphosphonate did not accumulate in collagen sponges. These findings suggest that the accumulation of technetium-99m methylene diphosphonate is by both chemical adsorption onto the surface of the hydroxyapatite in bone and incorporation into the crystalline structure of hydroxyapatite.