Abstract： Objective:To investigate the distribution of 99Tc m-methylene diphosphonate (MDP) at different stages of bone injury repair. Methods:A total of 30 rabbit models of femur injury were established by the method of electric drill and perforation of femur. According to the different stages of bone injury repair (at 1, 2 and 3 week), rabbits were divided into group A, B and C ( n=10 each group). Femoral SPECT/CT imaging was performed on the last day of different stages of bone injury repair to obtain radioactivity counts in the region of interest (ROI) on the test side and control side and to calculate target/background ratio (T/B). The light intensity of 3 groups was analyzed by phosphor screen imaging and the distribution of 99Tc m-MDP in bone cells was observed by autoradiography. One-way analysis of variance and paired t test were used to analyze the data. Results:The T/B values of group A, B and C were 1.16±0.14, 1.39±0.23 and 1.18±0.10, respectively ( F=5.83, P<0.01). There were significant differences of the maximum radiation count between the test side (50.00±12.45, 59.50±12.83 and 55.10±9.26) and the control side (43.20±9.57, 50.00±12.30 and 44.30± 6.50) in group A, B and C ( t values: 3.24, 2.28 and 5.77, all P<0.05). There were significant differences in the light intensity of bone specimens in group A, B and C by phosphor screen imaging (37 324.67±6 481.50, 60 950.33±9 781.72 and 43 905.00±4 957.92; F=8.25, P=0.02). 99Tc m-MDP were deposited in both intracellular and extracellular during different stages of bone repair in osteocytes and osteoblasts under autoradiography. Conclusion:At different stages of bone injury repair, the concentration of 99Tc m-MDP is significantly distributed, suggesting that there are other ways of concentration mechanism of 99Tc m-MDP in bone tissue besides the chemical adsorption with hydroxyapatite.