Frequency
The final factor in the equation is the denominator, frequency. Radiosurgical instruments operate at frequencies of 3.8 to
4 MHz. Electrocautery devices operate at much lower frequencies, in the 300- to 500-kHz range. Higher frequencies result in
less lateral distribution of heat from the incision and more accurate application of energy.2 Interestingly, 4 MHz appears to be the ideal frequency, in that frequencies higher than 4 MHz can create channeling, thus
damaging tissue distant to the incision, and higher frequencies increase the risk of sparking, resulting in excessive lateral
heat transfer.7
PROS AND CONS OF VARIOUS CUTTING INSTRUMENTS
Tissue cuts are produced when tissue fibers are divided by a concentrated application of energy. To understand the advantages
and disadvantages of instruments used to divide tissue, you must understand how cuts are made and how different tissues respond
to cuts. Three principal factors are involved in the cutting process: the properties of the tissue to be incised, the shape
of the instrument, and its guidance by the surgeon.8
Tissue properties
A key property of tissue is sectility. Tissue sectility is determined by the degree fibers are cut compared with how much
they shift as energy is applied. Tissues of high sectility are more easily cut, while those of low sectility are apt to remain
intact. Sectility may be enhanced by increasing tension directly in front of the cutting instrument and by increasing the
speed of the cutting device's travel. Placing tension on low-sectility tissue while making an incision requires extensive
mechanical support and may create a final incision that looks quite different from that initially intended by the surgeon.
The eyelids are good examples of low-sectility tissue. They are difficult to incise in their normal state, but cutting is
enhanced through mechanical support (e.g. chalazion forceps).
Instrument shape
The action of a scalpel is determined by the sharpness and shape of the blade. The sharper the cutting edge, the less the
tissue resistance, which results in more efficient cuts. The length of the cutting surface is also important. A longer blade
edge has greater lateral resistance, resulting in the blade's following the path of least resistance when guided parallel
to the blade's cutting surface.8 Shorter blades have less lateral resistance, allowing a surgeon to manipulate the blades around surfaces. So blade length
is directly proportional to the ease with which a surgeon makes a straight incision but inversely proportional to the ease
tissue is incised along contours. Scalpel blades are ideal for cutting tissues of high sectility, especially in cases in which
long straight cuts are desired. Lasers and radiosurgical electrodes offer the advantage of being point cutting devices (devices
with very short blade length, or, in the case of these modalities, no blade length) in which there is no lateral resistance,
allowing the cut to be completely controlled by a surgeon.
Since tissue resistance is not a factor with lasers or radiosurgical devices, tissue sectility is not a factor. This allows
a surgeon to make precise cuts with minimal tissue support. The cutting ability of the laser is determined by the source of
atoms being excited to emit energy as photons. Examples of lasers used in veterinary medicine include carbon dioxide, neodymium:yttrium-aluminum-garnet
(Nd:YAG), and diode lasers. Carbon dioxide lasers are commonly used in veterinary medicine to incise skin and other superficial
tissues. Carbon dioxide lasers produce an infrared light (10,600 nm) that is invisible to people. The slower a surgeon passes
the laser beam across the tissue, the deeper the cut; however, similar to radiosurgical electrodes, the slower the passage,
the greater the lateral heat distribution. The wavelength of light emitted by a laser also determines its effectiveness in
ablating tissue. For example, carbon dioxide laser light is preferentially absorbed by water, and cells with high water content
are vaporized as they absorb carbon dioxide laser energy; thus, high-moisture tissues will be more effectively removed. Diode
lasers emit a light preferentially absorbed by melanin, so pigmented tissue is more effectively removed.
As previously discussed, many factors are involved in determining the incision made with radiosurgery, including the shape
and size of the active electrode. Fine-wire electrodes concentrate the radio wave into tissue with immediate contact. The
radio wave vaporizes cells contacting the electrode. Once the cells are vaporized, the resulting air gap between the electrode
and adjacent tissue serves as insulation to prevent lateral heat transmission. The length of the exposed wire will determine
the depth of the incision. Larger-diameter electrodes transfer the radio wave less efficiently and, while vaporizing cells
in contact with the electrode, allow the transfer of some heat laterally to enhance hemostasis.
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