Abstract: BACKGROUND: In recent years, increasing studies have shown that low-intensity pulsed ultrasound can promote the healing of acute tendon injuries, but the specific mechanism is still unclear. 
OBJECTIVE: To observe the effect of low-intensity pulsed ultrasound on early angiogenesis after acute tendon injury, and to detect the regulatory relationship of low-intensity pulsed ultrasound with vascular endothelial growth factor-related signaling pathways, so as to reveal its potential mechanism of action. 
METHODS: Animal models of acute Achilles tendon injury were established using local injection of type I collagenase for 3 days in SPF male Sprague-Dawley rats aged 8-12 weeks, and were then randomly divided into ultrasound group and control group. In the ultrasound group, low-intensity pulsed ultrasound was treated daily with a small ultrasonic probe with an effective radiation area of 1 cm2 perpendicular to the Achilles tendon. No intervention was performed in the control group. Ultrasound imaging examination was performed 2 weeks later to observe the early healing of the tendon. Hematoxylin-eosin staining and CD31 immunohistochemical staining were performed to observe the changes in the number of blood vessels in the tissues after 1 and 2 weeks of treatment. The expression of vascular endothelial growth factor-related signaling pathway molecules in Achilles tendon tissues was detected by western blot or qRT-PCR. 
RESULTS AND CONCLUSION: Compared with the control group, the Achilles tendon in the ultrasound group was more continuous, the echo intensity was lower and more uniform, and the tendon thickness was significantly reduced (P < 0.05). Hematoxylin-eosin staining and CD31 immunohistochemical staining results showed that after 2 weeks of treatment, the number of new vessels in the ultrasound group was significantly increased compared with the control group (P < 0.05). Western blot and qRT-PCR results showed that after 2 weeks of continuous ultrasound intervention, the protein or mRNA expressions of vascular endothelial growth factor, Yes-associated protein, angiopoietin-2 and cysteine-rich angiogenic inducer 61 in the Achilles tendon of the ultrasound group were significantly higher than those of the control group (P < 0.05). These finding indicate that low-intensity pulsed ultrasound significantly increases the number of blood vessels in the early stage of acute tendon injury and accelerate tendon healing by up-regulating vascular endothelial growth factor expression.

Key words: low-intensity pulsed ultrasound, acute tendon injury, tendon healing, angiogenesis, vascular endothelial growth factor