Objective
To explore the healing effect of low intensity pulsed ultrasound with three different timing of intervention after tenotomy of Achilles tendon in rats.

Method
One hundred and thirty-two male rats were divided into experimental and control groups. Both groups were classified according to treatment phase: treatment on inflammatory phase (group I), proliferative phase (group II), and maturation phase (group III). Each groups were divided into 2 subgroups according to tendon excision time: 1 day after 7 consecutive treatment on 3 different phases (I-A, II-A, and III-A) and 30 days after tenotomy (I-B, II-B, and III-B). Three MHz pulsed ultrasound was administered on right tendon for 4 mins at 0.5 W/cm². The excised tendons of all groups were compared histologically and biochemically as control.

Results
The tendons of II-A experimental group revealed increased fibroblasts. The collagen fibers in the neo-tendon of II-B and III-B experimental groups had a tendency to be arrayed more regularly. On I-A group, the neo-tendon showed high immunoreactivity for type I and particularly type III collagen in cytoplasm of fibroblasts and collagen fibers. The imunoreactivity for type III collagen in the neo-tendon of II-A experimental group increased than control. The concentration of collagen of the neo-tendon was significantly increased on I-A and II-A experimental groups compared with control (p＜0.05). Collagen concentration of the neo-tendon of II-B experimental group increased significantly compared with control and I-B and III-B experimental groups (p＜0.05).

Conclusion
These results suggest that low intensity pulsed ultrasound therapy on injured Achilles tendon may be of benefit such as increasing collagen synthesis in the early healing process, especially in proliferative phase.