Low-dose X-irradiation promotes mineralization of fracture callus in a rat model

Objectives: This study investigated the hypothesized beneficial effect of low-dose irradiation (LDI) on fracture callus mineralization in a rat model. Methods: Seventy-two male Sprague-Dawley rats were averagely randomized into LDI group (rats treated with LDI) and SHAM group (rats treated with sham irradiation). Right after either LDI or sham irradiation, a standardized closed fracture on the right femur was established. At 2, 3 and 4 weeks postfracture, 12 rats in each group were euthanized. Fracture callus was assessed by using radiography and MicroCT for callus bridging, peripheral quantitative computed tomography (pQCT) for quantifying bone mineral content (BMC) and cross sectional area (CSA), confocal laser scanning microscopy for measuring area fraction of fluorescence labeling (AFFL) and four-point bending test for examining mechanical properties. Results: The CSA and AFFL were found to be 22 and 33% smaller in the LDI group compared to the SHAM group at 2 weeks (P < 0.05 for both), whereas the BMC and AFFL were 15 and 34% higher in the LDI group at 3 weeks (P < 0.05 for both). The changing patterns were consistent with the findings in 3-D MicroCT reconstructions. The mechanical parameters (Max-Load, Stiffness and Energy) were also 18, 30 and 24% higher in the LDI group than in the SHAM group at 3 weeks (P < 0.05 for all). At 4 weeks, there was no difference found for all assessments between the two groups. Conclusion: The results indicated LDI promoted mineralization at the stage of hard callus formation in a rat fracture model.