Ab s t r a c t High-power low-frequency ultrasound in the range 20-60 kHz has wide ranging clinical applications in surgical and medical instruments for biological tissue cutting, ablation or fragmentation, and removal. Despite widespread clinical application and common device operating characteristics, there is an incomplete understanding of the mechanism of tissue failure, removal and damage. The relative contribution of cavitation, direct mechanical impact and thermal effects to each process for specific tissue types remains unclear. Different and distinct mechanisms and rates of tissue removal are observed for interaction with soft and hard tissue types. Device operating parameters known to affect the interaction include frequency, peak-peak tip amplitude, suction and application time. To date, there has been little analysis of the effect of variations in, and interactions of, these parameters on tissue removal and damage for individual biological tissue types. Potential controllable damage mechanisms occurring in tissues include alteration in global biomechanical properties, histomorphological changes, protein denaturation and tissue necrosis. This paper presents a critical review of the literature on the clinical application, mechanism of tissue interaction, removal and residual tissue damage. It describes known mechanisms for distinct tissue types.

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Abstract-The use of therapeutic ultrasound delivered via small diameter wire waveguides may represent an emerging minimally invasive approach in the treatment of chronic total occlusions (CTOs), calcified and fibrous plaques. The distal-tip mechanical vibrations (typically 0-210 µm peak-to-peak) have been reported to debulk rigid calcified and fibrous tissues while healthy elastic arterial tissue remains largely unaffected. The risk of arterial (healthy tissue) perforation with energized waveguides is not fully understood. An ultrasonic apparatus capable of delivering a range of wire waveguide distal-tip displacements, up to 80 µm peak-to-peak (p-p), at an operational frequency of 22.5 KHz (+/-6%) has been developed. For three distal-tip displacement settings (32, 50 and 80 µm p-p) with 1.0 mm diameter waveguides, the force required to perforate healthy porcine aortic tissue was experimentally determined.The results show a distinct two stage perforation, thought to be the result of different mechanical properties of the layers in the arterial wall. The average maximum force (N) required to cause perforation with the 1.0 mm diameter ultrasonic waveguide activated at the three settings was experimentally determined to be 2.7 N (32 µm p-p), 2.6 N (50 µm p-p) and 2 N (80 µm p-p). The force required to cause perforation of the tissue with no ultrasound was found to be approximately 4 N. These results highlight that when ultrasound energy is applied to the waveguide, less force is required to perforate healthy arterial tissue. This reduction in perforation force is more pronounced at higher ultrasonic displacements, similar to those reported in clinical studies for the effective removal of diseased calcified and fibrous plaques.

Abstract. The use of ultrasonic vibrations transmitted via small diameter wire waveguides represents a technology that has potential for minimally invasive procedures in surgery. This form of energy delivery results in distal tip mechanical vibrations with amplitudes of vibration of up to 50 µm and at frequencies between 20-50 kHz commonly reported. This energy can then be used by micro-cutting surgical tools and end effectors for a range of applications such as bone cutting, cement removal in joint revision surgery and soft tissue cutting. One particular application which has gained regulatory approval in recent years is in the area of cardiovascular surgery in the removal of calcified atherosclerotic plaques and chronic total occlusions. This paper builds on previous work that was focused on the ultrasonic perforation of soft vascular tissue using ultrasonically activated mechanical waveguides and the applied force required to initiate failure in soft tissue when compared with non-ultrasonic waveguides. An ultrasonic device and experimental rig was developed that can deliver ultrasonic vibrations to the distal tip of 1.0 mm diameter nickel-titanium waveguides. The operation of the ultrasonic device has been characterized at 22.5 kHz with achievable amplitudes of vibration in the range of 16 -40µm. The experimental rig allows the ultrasonically activated waveguide to be advanced through a tissue sample over a range of feedrates and the waveguide-tissue interaction force can be measured during perforation into the tissue. Preliminary studies into the effects of feedrate on porcine aortic arterial tissue perforation forces are presented as part of this work. A range of amplitudes of vibration at the wire waveguide distal tip were examined. The resulting temperature increase when perforating artery wall when using the energized wire waveguides is also examined. Results show a clear multistage failure of the tissue. The first stage involves a rise in force up to some critical force and tissue displacement whereby the cut is initiated. The results show that with increasing ultrasonic amplitude of vibration the perforation force decreases considerably. The current results show that for the range of feedrates investigated 19-95 mm/min at an amplitude of vibration of 34.3 µm there was no significant effect on the perforation initiation force. The ΔT in the tissue 3.0 mm from the point of entry is also presented for a range of amplitudes of vibration.