Structural and functional assessment of intense therapeutic ultrasound effects on partial Achilles tendon transection

Tendinopathies and tendon tears heal slowly because tendons have a limited blood supply. Intense therapeutic ultrasound (ITU) is a treatment modality that creates very small, focal coagula in tissue, which can stimulate a healing response. This pilot study investigated the effects of ITU on rabbit and rat models of partial Achilles tendon rupture. The right Achilles tendons of 20 New Zealand White rabbits and 118 rats were partially transected. Twenty-four hours after surgery, ITU coagula were placed in the tendon and surrounding tissue, alternating right and left legs. At various time points, the following data were collected: ultrasound imaging, optical coherence tomography (OCT) imaging, mechanical testing, gene expression analysis, histology, and multiphoton microscopy (MPM) of sectioned tissue. Ultrasound visualized cuts and treatment lesions. OCT showed the effect of the interventions on birefringence banding caused by collagen organization. MPM showed inflammatory infiltrate, collagen synthesis and organization. By day 1428, all tendons had a smooth appearance and histology, MPM and OCT still could still visualize residual healing processes. Few significant results in gene expression were seen, but trends were that ITU treatment caused an initial decrease in growth and collagen gene expression followed by an increase. No difference in failure loads was found between control, cut, and ITU treatment groups, suggesting that sufficient healing had occurred by 14 days to restore all test tissue to control mechanical properties. These results suggest that ITU does not cause harm to tendon tissue. Upregulation of some genes suggests that ITU may increase healing response.