Practitioners have several options for incising tissue when performing surgery. The modality they choose depends on their experience, the type of surgery they are performing, the type of tissue being cut, and even the patient's species. The cost of the instrumentation may also play an important role in the decision-making process. This article discusses the physics of radiosurgery and briefly compares radiosurgery with other surgical devices.
Figure 1. A small-diameter wire electrode is used to concentrate the radio wave, producing a precise incision with minimal distribution of heat laterally into the tissue.
Historically, hemorrhaging tissue was cauterized with metal that had been heated in an open flame. A modern innovation of this firing iron concept was electrocautery, which used an electric current passed through a wire loop to generate heat. In the 1920s, Bovie developed the first practical electrosurgical device. Electrosurgery uses an electrode to channel electric current through a patient to an indifferent electrode attached to the patient. The indifferent electrode then channels the electric current back to the electrosurgery device. Radio frequency machines convert alternating current to direct current, which then passes through a coil or rectifier to generate a radio wave. The radio waves then pass through a high-frequency waveform adapter to modify the radio wave's shape and amplitude. The radio waves are transferred from the electrode tip to the patient.
HOW RADIOSURGERY WORKS
Radiosurgery uses a 4-MHz radio wave that passes from an active electrode (hand-held instrument) to a passive electrode (ground plate beneath the patient). The 4-MHz frequency is similar to that of marine band radios because it is above the AM spectrum but below the FM spectrum.1,2 Tissue resistance to the radio wave transmission volatilizes cells at the tip of the active electrode. Damage to tissue adjacent to the incision is limited provided a good technique is used. Similar to a scalpel blade incision, a radiosurgical incision should be made by using a smooth continuous motion. Unlike a scalpel blade, a radiosurgical electrode cuts without pressure. The character of the cut made with radiosurgery is determined by the type of electrode, contact time with the tissue, intensity of power, nature of the radio wave (waveform mode), and radio wave frequency. The above formula demonstrates how these parameters interact:
Electrode size and configuration
The active electrode transmits radio waves when energized. Numerous electrode configurations are available. The proper electrode to use depends on the type of tissue to be incised and the surgeon's preference.
Figure 2. A large-diameter wire electrode is used once the skin incision is made to incise subcutaneous layers. The larger-diameter needles dissipate more heat laterally into the tissue, sealing small blood vessels to minimize hemorrhage.
Small-diameter electrodes require lower power settings and produce less lateral heating of tissue adjacent to the incision. Thinner-wire electrodes (wire diameter 0.009 to 0.004 in) are generally used to incise skin (Figure 1). The depth of the incision is determined by the length of wire exposed from the insulated housing of the electrode.
Large-diameter electrodes require higher power settings and produce more lateral heat. Large-diameter electrodes are ideal for subcutaneous dissection because lateral heat is useful for hemostasis (Figure 2).