Apart from passing current through the patient to produce localized heating and tissue destruction (either cutting or coagulation), there are other means of transferring electrical potential energy into energy for surgery. Two of the most prominent ways of ultrasound surgery are dependent on the production of ultrasonic vibrations, although each produces its effect in a unique manner.
Ultrasonic Scalpels & Clamps
Principles of focused ultrasound surgery
Several types of ultrasonic “scalpels” and clamps allow cutting and coagulation of tissue in a technique completely different from that employed in electrosurgery. In these instruments, electrical energy from a power source is transformed into ultrasonic vibrations by a transducer, a unit that expands and contracts in response to electrical current at a frequency of up to 55,500 cycles per second. This is amplified in the shaft of the instrument to magnify the vibrating distance of the blade, which moves longitudinally. The blade tip vibrates through an amplitude of around 200 m. As the blade tip vibrates, it produces cellular friction and denatures proteins. These denatured proteins form a coagulum, which allows sealing of coapted blood vessels. When the instrument is left in place longer, secondary heat is produced, and larger blood vessels may be sealed. By producing cellular disruption in this fashion, the temperatures achieved are between 50 °C and 100 °C. In contrast, in conventional electrosurgery, tissues are subject to temperatures between 150 °C and 400 °C. Thus, using an ultrasonic device, tissues can be dissected without burning or oxidizing tissues, without producing an eschar, and there is less potential to disrupt the coagulum when removing the instrument. When one is cutting using the clamp portion of the instrument, energy is transferred to the tissue through the active blade under applied force, minimizing lateral spread. Additionally, the motion of the blade induces cavitation along the cell surfaces, whereby low pressure causes cell fluid to vaporize and rupture.
Advantages for focused ultrasound surgery
The advantages of an ultrasonic scalpel system are clearest when one is operating in tight spaces, with the attendant risks of damage to adjacent structures. This makes such instruments especially suited for laparoscopic and other types of minimally invasive procedures. Additionally, while the potential still exists for damaging adjacent tissue with an inadvertent contact of an active tip with tissue, there is no risk of current inadvertently arcing to adjacent structures, since the current is converted into mechanical energy in the handpiece. Further, there is no neuromuscular stimulation produced, since no current passes through the patient. Since the tissue effects are exerted through mechanical disruption of the cells, and coagulation occurs at much lower temperatures than used in conventional electrosurgery, lateral thermal tissue damage is minimized. Further, since the tissues are not heated to the point of combustion or carbonization of proteins, there is no eschar formation on the blade, and less smoke is produced.
Disadvantages for focused ultrasound surgery
A primary disadvantage of this system is that the components are more expensive than those used for conventional electrosurgery, and with more mechanical parts, there are more potential areas of breakage. Further, whereas electrosurgery can be applied throughout an operation, ultrasonic scalpels are typically used for more controlled dissection around the site of interest.
Applications for focused ultrasound surgery
The primary applications of ultrasonic instruments are found when traditional electrosurgery is unsuitable or not desirable. As mentioned above, they are particularly useful during minimally invasive procedures, due to the risks of running an active electrode through a cannula and into a body cavity. The lack of current flow mitigates this risk. Additionally, the reduced smoke production by instruments of this type is advantageous in this setting. In patients where there is a concern about current flow due to questions about electrophysiology (such as with implantable cardiac defibrillators or pacemakers), these instruments eliminate a source of concern by avoiding the hazard of passing current through the patient’s body.
Cavitational Ultrasonic Surgical Aspiration
Cavitational ultrasonic surgical aspirators work on many of the same principles as the ultrasonic scalpels mentioned above. In the handpiece, current passes through a coil and induces a magnetic field. The magnetic field excites a transducer of a nickel alloy (either a piezoelectric or magnetostrictive device), expanding and contracting, resulting in an oscillating motion (vibration) in the longitudinal axis with a frequency of 23 or 36 kHz. These ultrasonic mechanical vibrations are magnified over the length of the handpiece. The amount of oscillation varies: With low frequency, there is greater amplitude, with high frequency, there is lower amplitude. The oscillating tip, when brought into contact with tissue, causes fragmentation of tissue via producing cavitation at the cell surface, with low pressure outside the cell leading to cellular disruption. This high-frequency vibration produces heat, which is reduced via a closed, recirculating cooling water system. This system maintains the temperature of the tip at approximately 40 °C.
As tissue is fragmented, the debris must be carried away, and this is another function of these instruments, as implied by their name. For irrigation, IV fluid (water or saline) is fed through tubing to the handpiece, where it irrigates the surgical site and suspends the fragmented tissue debris. Removal of this debris is possible because the instrument contains a vacuum pump that provides suction. Suction pulls irrigation fluid, fragmented tissue, and other material through the distal tip of the handpiece. The material is contained in a separate canister.
Some ultrasonic surgical aspirator instruments have technology that allows the surgeon to influence the selectivity of the disruption induced by the instrument itself. Some surgeons attempt to gain extra control by lowering the amplitude of the tip oscillations. Lowering amplitude in an attempt to gain greater selectivity when fragmenting tissue near critical structures, however, only results in a reduced speed of tissue removal. By using a mode in which on/off power intervals are supplied, the reserve power (which governs the tip response when encountering tissue) is reduced. The total amount of power in the oscillating hollow tip is determined by the amount of reserve power available. Reserve power maintains tip oscillation when a resistive load is placed on the tip, as occurs when it contacts tissue. As the resistance increases, more power is supplied to the tip.
Ultrasonic surgical aspirator systems have their primary application for focused ultrasound surgery
in situations where fragmentation, emulsification, and aspiration of a significant amount of tissue is desirable. Since minimal additional hemostasis is provided, this instrument is not as versatile in its application to general surgery as electrosurgery or the more high-power ultrasonic scalpel. In general surgery, its primary application is in liver resection, where it can disrupt parenchyma while leaving major vasculature and the biliary ducts intact.