Principles of Ultrasound in Surgical Applications

Surgery is a medical specialty focused on the repair or removal of specific anatomical components. Crucial elements to the basic operative procedures of a surgeon are the abilities to cut, coagulate, fragment, excise and destroy tissues in a precise and controlled manner. Over the last 30 years, devices based on ultrasonic principles have been developed to dramatically advance modalities of tissue dissection and ablation, ultimately leading to improved surgical outcomes. Misonix has been at the forefront of innovation in the development and manufacturing of these ultrasonic devices for a variety of surgical applications ranging from

• Liposuction
• Surgical aspiration,
• Laparoscopic cutting and coagulation (ultrasonic shears),
• Lithotripsy,
• To minimally and even non-invasive treatment of cancers.

Basic Terminology

For a more accurate understanding of the beneficial effects of ultrasound in surgery, a few fundamental concepts must first be understood.

Sound is vibration or pressure waves transmitted through media, and perceived by the sense of hearing.

Ultrasound is sound waves vibrating at frequencies greater than what can be detected by human hearing, which is approximately 18 kilohertz (18,000 Hertz) and higher.

Frequency is the number of cycles per unit time of a wave, most commonly measured in Hertz (cycles per second)

Period is the time required to complete one cycle, or pulse length.

Amplitude is the maximum value of a periodic wave.

Intensity is the amount of energy, or power, transferred to a specific unit of area.

Cavitation is the formation and collapse of microscopic vacuum bubbles in a liquid due to sudden pressure differentials. The bubble implodes on itself, instantaneously releasing tremendous amounts of temperature and pressure changes at the point of collapse, resulting in shock waves and strong shearing forces.

Ultrasonic waves can be generated by applying electrical energy to a piezoelectric or magnetostrictive transducer which transforms the alternating electrical current into mechanical vibrations. These mechanical vibrations are then directed through a probe element, which vary in geometry and shape depending on the surgical application. By controlling the variables of an ultrasonic wave such as frequency, amplitude and intensity, a broad spectrum of ultrasonic effects cab be obtained.

Ultrasound is most often associated with diagnostic imaging, which employs high frequency sound waves delivered at low intensities and has no damaging effects on tissue. In contrast, therapeutic ultrasound delivered at high intensities will cause tissue damage in a controllable manner. There are three predominant mechanisms by which ultrasound impacts tissue in surgical applications: they are the mechanical, cavitational and thermal effects.

Mechanical Effect - Ultrasonic Cutting and Coagulation

When a blade is vibrated at ultrasonic frequencies and in the direction of its relatively blunt working surface, the result is a clean cutting mechanism at higher speed compared to an un-powered scalpel operated by hand. In addition, because the blade element is moving back and forth at such high speeds, nothing can stick to the blade and less force is required to cut tissues. This effect is best utilized by surgeons when cutting through tougher tissues, such as muscle, or when dissection is required in small, confined areas next to vital organs as is the case with most minimally invasive procedures (laparoscopy).

The AutoSonix™ is an example of an ultrasonic device that cuts soft tissues through mechanical means and coagulates through thermal effects. This combination of cutting and coagulation is beneficial in mobilizing organs and separating blood vessels (see thermal effects)

Cavitational Effect - Ultrasonic fragmentation and emulsification

When a blunt tip vibrating at ultrasonic frequencies is introduced into a liquid medium, the back and forth motion of the tip will cause sudden compression and then decompression of the surrounding liquid. As a result, increasing and decreasing pressure waves are generated and cavitation bubbles will form, filling those voids created by the pressure differentials. The bubbles grow with each excursion and if the amplitude of vibration of the probe tip is high enough, the bubbles will collapse and release forces strong enough to fragment and disintegrate tissue.

Utilizing cavitation as a mechanism for tissue fragmentation and emulsification offers significant benefits for surgical applications. First of all, cavitation is tissue selective in the sense that varying tissues respond differently to imploding cavitation bubbles. For example, in the case of liposuction, since fat cells have significantly higher water content than muscle, skin and interconnective tissue cells, they break more easily when subjected to cavitational shearing forces. Secondly, cavitation can destroy tissues in a very precise and controlled manner, thus enabling the surgeon to work in very delicate areas such as the brain.

The Lysonix 3000 (liposuction unit), the Sonastar and FS-1000 (surgical aspirator) as well as the SonicOne (wound care system) are examples of devices which harness the cavitational effect of ultrasound to fragment and emulsify tissues. In the case of the Lysonix 3000 and FS-1000, built in aspiration allows the resulting emulsion to be removed from the surgical site.

Thermal Effect - Noninvasive ablation

As an ultrasonic wave propagates into tissue, it is absorbed and the energy is converted into heat. If the ultrasonic beam has enough energy and is brought into a tight focus, a local temperature rise will occur within the focus area alone. Coagulative necrosis occurs when the energy creates a heat build up above the threshold level of protein denaturation, which is 47 degrees Celcius. The AutoSonix laparoscopic shears utilize this thermal effect to prevent and control bleeding when cutting tissue or blood vessels. It is also the principle used in HIFU (High Frequency Focused Ultrasound) to cause very localized cell death by focusing the ultrasound beam in a targeted volume of tissue. Applying ultrasound in this manner is extremely appealing as it offers an opportunity for less or even non-invasive surgery without mechanically penetrating the target organ and preserving healthy tissue and nerve structures. The Sonablate® 500, which is manufactured by Misonix, Inc., is an example of a non-invasive HIFU device which is capable of destroying cancerous cells in the prostate without a single incision being made.