Mechanism of action of the ultrasonic tissue resectors disclosed using high-speed and thermal imaging techniques

During surgery, selective resection of soft and hard tissue is obtained using devices based on ultrasound induced cavitation bubbles. Building upon the experience of an earlier study, real-time high speed and thermal imaging techniques were applied to expand the understanding of the mechanism of action in relation to irrigation and aspiration and driving frequency. The Cavitational Ultrasonic Surgical Aspirator (CUSA, Valleylab, Boulder, CO) and the Selector (NMT Neurosciences, UK) equipped with a 2.3 mm hollow titanium needle (frequencies 24 and 35 kHz) were investigated. Close-up photography (1 microsecond(s) ) showed a ring of imploding cavitation bubbles around the rim of the tip which fragmented tissue within a well defined radius. Using Schlieren techniques (10 ns resolution), multiple shock waves generated by imploding cavitation bubbles were observed up to 5 mm inside the transparent tissue without leaving damage. The combined irrigation and aspiration is essential for effective tissue removal. The irrigation provides cooling of the tip and enables cavitation formation. The aspiration draws soft tissue into the area of fragmentation and removes debris. Without irrigation, friction and thermal conduction will result in undesired thermal damage and inefficient tissue removal. The impact of the shock waves and difference in driving frequency are expected to be minimal.