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.