Effects of Low Magnitude High Frequency Vibration on Blood Flow and Angiogenesis during Fracture Healing in Normal and Osteoporotic Bones

INTRODUCTION: Bone fracture, especially osteoporotic fracture, has become a major health issue due to its increase in morbidity, mortality and the financial burden of medical care. Our previous studies confirmed low magnitude high frequency vibration (LMHFV) (magnitude=0.3g, frequency=35Hz), providing non-invasive, systemic mechanical stimulation, can promote both normal and osteoporotic fracture healing in rats, however, the mechanism of its positive osteogenic effect is still unclear. We hypothesized LMHFV could enhance the blood flow of hind limb and promote the angiogenesis at the fracture site in both normal and osteoporotic rats, hence to accelerate the healing process.;MATERIALS AND METHODOLOGY: Nine-month-old ovariectomy-induced (OVX) and sham-ovariectomized (Sham) rats were created closed fractures on right femoral mid-shaft. Five days after fracture surgery, totally 72 rats were randomized into LMHFV (Sham-V, OVX-V) (35Hz, 0.3g, 20min/day, 5days/week) and control (Sham-C, OVX-C) groups. The external callus width (CW) and callus area (CA) were measured by radiography weekly to monitor the status of fracture healing. Immunohistochemistry was performed to evaluate the expression of vascular endothelial growth factor (VEGF) in external callus.;RESULTS: (1) Vibration groups had larger CW and CA than the corresponding controls, and higher CW and CA was also observed in Sham-C than OVX-C. OVX-V had the largest CW and CA than other groups in the early phase of fracture healing.;(2) Pulsed-wave Doppler showed an increasing blood flow velocity of injured femoral artery from weeks 2 to 8. At each time point, it indicated a higher blood flow velocity in vibration groups than control ones.;(3) 3D-HF-PDU demonstrated an enhanced blood volume at the fracture site by LMHFV treatment compared to the controls during the early phase of fracture healing (week 2: Sham-V > Sham-C, p=0.021). The microcirculation of OVX groups was inferior to the corresponding Sham ones.;(4) MicroCT-based microangiography also confirmed increased vascular volume (VV) of fracture site in vibration groups (week 2: OVX-V > OVX-C, p=0.009; week 4: OVX-V > OVX-C, p=0.034), and an inferior level of angiogenesis was found in OVX groups as compared with Sham groups (week 2: Sham-V > OVX-V, p=0.014; Sham-C > OVX-C, p=0.014). The ratio of vascular volume to total tissue volume (VV/TV) showed a similar trend as above.;(5) Immunohistochemistry assessment also indicated higher level of VEGF expressions in vibration groups than controls within external callus in the early phase of fracture healing, and the OVX groups had lower level of expressions as compared with Sham ones.;DISCUSSION: Ovariectomy-induced osteoporotic rats had suboptimal femoral blood supply than normal rats because estrogen deficiency would increase blood viscosity, thus decreased the blood flow velocity. LMHFV could reduce the peripheral resistance by widening small vessels in muscles, which resulted in an increase of blood flow velocity. Vibration also promoted angiogenesis in both normal and osteoporotic fractures. This might be vibration increased the blood flow shear forces at vascular endothelium, which augmented the functions of VEGF by up-regulating VEGFR-2. The percentage of increase in angiogenesis by LMHFV in OVX groups was higher than Sham ones, which suggested osteoporotic bone might have higher sensitivity of angiogenic response to mechanical stimulation. With the consistent findings between angiogenesis and osteogenesis and the significant positive linear correlation between VV and BV1, it indicated angiogenesis was associated with osteogenesis of fracture healing process, especially in the early stage, which suggested LMHFV therapy should be applied from the early healing phase.;CONCLUSION: LMHFV can augment blood flow of fractured hind limb and enhance angiogenesis at the fracture site with different extent in normal and osteoporotic rats, which indicates the promotion of both systemic and local blood circulation is one of the mechanisms for LMHFV to accelerate the fracture healing. (Abstract shortened by UMI.).