Uptake and biodistribution of technetium-99m-MD³²P during rat tibial bone repair

The present study was carried out in order to test the hypothesis that intravenously injected Tc-MDP separates into its technetium and methylene diphosphonate components in the bone, and that the technetium is preferentially taken-up by the newly-formed osteoid, while the methylene diphosphonate is taken up by the forming mineral. Uptake of Tc-MDP was studied in a rat model of primary bone formation following tibial bone marrow ablation. Each of five radiopharmaceuticals (^99mTcO₄, ^99mTc-MDP, Tc- MD³²P, ^99mTc-MD³²P or MD³²P) was injected and their uptake was followed in the whole bone as well as in the organic and inorganic phases of the bone. Irrespective of the radionuclides injected, ^99mTc was always taken-up preferentially by the organic phase, while the ³²P was preferentially taken-up by the inorganic phase. When ^99mTcO₄ was injected, it was not taken up by the bone at all. These results indicate that the increased incorporation of ^99mTc, when administered as ^99mTc-MDP during bone healing, reflects an enhancement in the formation of the organic matrix and not of the calcification process. The study also suggests that the ^99mTc-MDP dissociates into its technetium and methylene diphosphonate moieties, which are then adsorbed onto the organic and inorganic phases respectively.