discussion applies to both. Many Nd:YAG lasers have been replaced by the less expensive and more compact diode units. Nd:YAG lasers are generally sold with outputs up to 100 W, while diode lasers are most often in the 15–50 W range. Higher power output is a reason why some continue to use Nd:YAG lasers [2].
In their purely optical forms, these lasers are absorbed by dark pigment such as melanin and hemoglobin and poorly absorbed by water (Figure 12.2). These wavelengths and any other delivered by quartz fiber can be used under water [9]. When the tissue is not obviously dark, the laser energy will convert to heat more slowly as it encounters sufficient deeper pigment or protein, which may take several seconds. That distance could be a few millimeters in pale skin or mucous membrane or longer in an eye if only cornea and clear aqueous or vitreous humor is encountered. As tissue blackens, more laser energy is absorbed until black char accumulates and limits penetration. To continue, the char must be physically removed or time for tissue slough must be allowed [2]. All this occurs with considerable potential tissue heat accumulation.
Figure 12.10 (a) Preoperative image of large mixed sarcoid covering the scapular region of a horse. (b) Computerized scanner attached to a CO2 laser performing a partial (skin) thickness ablation of the sarcoid shown in Figure 12.9a. The surface is even and there is no char formation. The entire lesion will be treated. Leaving the dermis intact facilitates healing and minimizes chance of recurrence. Topical fluorouracil was also used. (c) End result of lesion shown in Figures 14.9 a and b.
Source: Kenneth E. Sullins.
Coagulation results in physical contraction of tissue, which will slough during the ensuing several days if the protein has been denatured and the blood supply has been coagulated. Vascular stasis occurs when melanin‐rich tissues absorb the laser energy and conduct heat to the vascular endothelium where the coagulation cascade is activated. In tissues with low melanin concentrations, hemostasis occurs when hemoglobin absorbs the laser energy and conducts thermal energy to plasma protein [25, 26].
Deeply scattered laser energy can damage subsurface tissues such as nerves or vessels or coagulate darkly pigmented skin on the ear after passing through white cartilage of the pinna. Misdirected Nd:YAG laser energy in the pharynx can leave a horse dysphagic from damage to the pharyngeal branch of the vagus nerve, which lies deep to the dorsolateral pharyngeal wall. When deeper tissues are at risk, a contact technique should be used with care and the beam should be directed tangentially across the surface, and the integrity of the sculpted fiber or sapphire tip should be ensured (see below) [2].
Diode and Nd:YAG lasers are the instruments of choice for equine endoscopic surgery because the energy is delivered through flexible quartz fibers, which can be inserted through the biopsy channels of video endoscopes. Two types of quartz fibers are in general use [2].
The “bare” fiber is covered with a plastic coating similar to insulation on an electrical wire. That plastic must be stripped from the tip before use because it will burn. After stripping, the end is cleaved by scoring the quartz and fracturing the fiber or cutting with scissors to yield a symmetric circle from the aiming beam. A uniformly circular shape of the aimed beam indicates the coherence of the light emitting from the fiber, which is important for uniform delivery of laser energy in a non‐contact fashion. With normal use, bare fibers gradually crystallize and burn out requiring cleaving back to a new area of the fiber, a continuous process until they are too short to use. Bare quartz fibers are commonly available in diameters of 600–1,000 microns [2].
Bare quartz fibers (Figure 12.11a) used in contact fashion and may be “sculpted” to a point to maximize the power density for incisive surgery. The sculpted tip burns away rapidly leaving a fiber that is the same diameter as the entire fiber. The free beam (noncontact) effect of the fiber returns when the tip wears out. Adequate power density for cutting is generally provided with a 600‐micron fiber at an output of 20 W. Larger diameter fibers require more laser output or sculpting to maintain effective power density for incision and may emit excess laser energy into the deeper tissues at higher power settings. Sculpted 1,000‐micron fibers cut very well, and the sculpting will last for approximately one procedure. They are stiff enough to have a real tissue feel but may have difficulty bending to reach tissue during endoscopic surgery. Blackening the tip of a bare fiber by firing it on a tongue depressor or, more conveniently, with a black permanent marker, causes the energy to be absorbed at the fiber tip so it cuts efficiently (Figure 12.11b). Activating the laser only when the fiber is in contact with tissue significantly prolongs the fiber life because tissue dissipates the heat.
Figure 12.11 (a) Bare quartz fibers (1,000‐μ) for use with Nd:YAG or diode lasers. The fiber on the left is a plain cylindric tip for free beam (non‐contact) transmission of laser energy. The fiber on the right has been sculpted into a chisel point to increase power density for contact laser surgery. Both ends eventually burn out requiring stripping back of the plastic coating and cleaving of the quartz in a fresh site. Although possible to manually resculpt the tip, it is tedious and not as accurate as replacing the fiber. (b) A bare quartz fiber is being blackened with a permanent marker. The black pigment absorbs the laser energy for an immediate effect on tissue and limiting deeper penetration of laser energy. As the marker pigment burns off, the heat itself and tissue char blackens the fiber continuing until the tip must be cleaved again. (c) Gas cooled fiber for use with Nd:YAG laser. The quartz fiber inside the plastic tubing can transmit 50–100 W of energy without burning out, because the gas circulating in the tubing cools it. The ports in the tips (inset) must remain clean for cooling to continue. The fiber can be used in non‐contact fashion with the bare tip only or sapphire tips of various types can be screwed onto the tip. Illustrated in the inset left to right are right‐angle, conical and end‐on sapphire tips. The conical tips are used for incisions whereas the others are used for contact ablation of tissue.
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