The most useful cutting instruments are scissors (Fig. 23.14, bottom). Because it is difficult to sharpen laparoscopic scissors, most surgeons prefer disposable instruments that can be used until dull and then discarded. Another mechanical cutting tool is the linear stapler–cutter that can simultaneously cut and hemostatically staple the edges of the incision. The cost and large dimensions of the instruments limit their practical use to only a few highly selected situations, such as separation of the uterus from the ovary and fallopian tube during laparoscopic hysterectomy. Devices that coagulate tissue and mechanically transect it are designed to be narrow enough to be practical and effective enough to become the dominant devices for laparoscopic cutting, when concomitant sealing of vessels is a requirement.
Laser and electrical sources of energy manifest their effect by conversion of electromagnetic energy (Fig. 23.14) to mechanical energy, which is then transformed into thermal energy. Highly focused radiofrequency electrical current (high-power or current density), generated by a specially designed electrosurgical generator produces vaporization or cutting by raising the intracellular temperature above 100◦C, which rapidly converts water to steam with a massive increase in intracellular volume. This expansion ruptures the already damaged cell membrane, resulting in cellular and tissue vaporization into a cloud of steam, ions, and protein particles. If the instrument used to focus this energy is moved in a linear fashion, tissue transection or cutting will result. Less focused radiofrequency energy (moderate current density) elevates intracellular temperature, causing desiccation, rupture of hydrogen bonds, and tissue coagulation, but vaporization does not occur.
Figure 23.14 Radiofrequency electrosurgical generator. Displayed is the Force FX generator with unipolar laparoscopic electrodes. The device is capable of outputting high-voltage (“coagulation”) and low-voltage (“cut”) waveforms for unipolar instruments and a low-voltage bipolar circuit for bipolar instruments.
Figure 23.15 Laparoscopic cutting devices. Top and inset left is a bipolar electrosurgical spatula. Middle and center inset is a harmonic scalpel. This device oscillates at 55,000 Hz to cut tissue. Bottom and inset right are laparoscopic scissors. They may be connected to an electrosurgical generator to act as a unipolar electrosurgical instrument.
Monopolar electrosurgical instruments that are narrow or pointed are capable of generating the high-power densities necessary to vaporize or cut tissue. Continuous or modulated and relatively low voltage outputs tend to be the most effective. For optimal results, the instrument should be used in a noncontact fashion, following (not leading) the energy through the tissue. Specially designed bipolar cutting probes that contain both the active and dispersive electrode are available. The active electrode is shaped as a needle, or even a blade, while the other larger-surface-area electrodes are designed to be dispersive (Fig. 23.15, top). Laparoscopic scissors are generally of unipolar design and are intended to cut mechanically; energy may be applied simultaneously for desiccation and hemostasis when cutting tissue that contains small blood vessels (Fig. 23.15, bottom).
Laser energy can be focused to vaporize and cut tissue. The most efficient laser-based cutting instrument is the CO2 laser, which has the drawback of requiring linear transmission because light cannot be conducted effectively along bendable fibers. The potassium-titanyl-phosphate (KTP) and neodymium:yttrium, aluminum, garnet (Nd:YAG) lasers are effective cutting tools. They are capable of propagating energy along bendable quartz fibers but have a slightly greater degree of collateral thermal injury than radiofrequency electrical or CO2 laser energy. These limitations and their additional expense constrict the value of these lasers.