Effects of Low-intensity Pulsed Ultrasound on Bone Remodeling during Repair Process of Bone Defects in Rat Tibiae

Abstract

It is well known that mechanical forces have beneficial effects on bone regeneration after implant and periodontal treatment. Recently, LIPUS (Low-intensity Pulsed Ultrasound) treatment, which is one method of inducing mechanical force, has been shown to facilitate repair of bone defects, because ultrasound can not only provide good stimulus for bone formation, but also induce the differentiation of osteoblasts. However, cellular events induced in bone cells by LIPUS have not yet been clarified. This study examined the biological effects of LIPUS on bone remodeling in bone defects using morphological and immunohistochemical analyses. Female SD rats were used in the experiment. Bone defects were created in the tibiae by mechanical drilling using dental burs. Then LIPUS irradiation was repeatedly applied 24hrs post-operatively. The left tibiae (LIPUS groups) were treated daily with 15min of LIPUS (1.0MHz, 0.24W) for 1, 2, 3, and 4 weeks, and the right tibiae (control group) were treated without LIPUS. The tibiae were used for the following analyses; three-point bending test, dual energy X-ray absorptiometry (DEXA), soft X-ray, bone morphometry, and histochemistry. Soft X-ray pictures demonstrated a greater amount of new bone formation in the bone defect regions in the LIPUS groups than in the control groups 2 and 3 weeks post-operatively. The tibiae in the LIPUS group were stiffer than those in the control group on the three-point bending test 2 weeks post-operatively, but there were no significant differences BMD between the control and LIPUS groups in DEXA analysis during all experimental periods. On H-E stained sections, the thickness of the cortical bone in the defect region of the LIPUS groups was greater than that in the control groups 2, 3 and 4 weeks post-operatively. Bone morphometric analysis using calcein labeling in the bone defect regions demonstrated that the mineral apposition rate in the LIPUS group was twice that in the control group. The anti-osteopontin antibody was localized in osteoblasts and osteocytes in the newly formed bone tissues in the bone defect regions, and there were no differences in the distribution of bone tissues in the defect regions between the LIPUS and control groups. These findings indicate that LIPUS stimulates osteoblast in periosteum to form bone matrix, and suggests that the repair process in bone defects was accelerated by LIPUS as a result of stimulating bone formation in the cortical bone.