Excision is simply incising or dissecting tissue, whereas ablation refers to vaporization of tissue. An incision creates tissue loss the width of the laser beam (usually ≥0.16 mm). Highly concentrated laser energy (i.e. high‐power density) is required to efficiently cut tissue with minimal heating of surrounding tissue. Since laser energy has no mass (i.e. steel blade) to separate tissue, tension on tissue is absolutely required so the incised surfaces separate. Without tension, excess heat will accumulate and the margins will be jagged and eventually necrotic. Collateral heating of tissue can be a substantial contribution to wound dehiscence, because it produces a zone of necrosis along the wound margin. A small zone of necrosis has no effect on an open wound after resecting a mass, but it profoundly affects healing of a primarily sutured incision. Therefore, adequate power density to incise quickly is critical to create a precise incision with healthy adjacent tissue to achieve primary wound healing [9]. Ablation also requires a relatively high power density but laser energy is moved over a surface to “paint” tissue away [2].
With a small spot size, a single efficient pass across tissue with adequate tension on the tissue, 5,000 W/cm2, is a minimally sufficient power density to avoid collateral thermal necrosis (Figure 12.7) [10]. While learning, the tendency is to reduce the power setting and move tentatively or in multiple passes causing the laser to remain on the tissue longer while increasing the width of the wound and collateral heating. Incisions may dehisce due to thermal necrosis of the margins [11]. Experienced surgeons apply a significantly higher power density and work efficiently with a single pass of the laser (Table 12.1) [9]. A carbon dioxide (CO2) laser in continuous mode at 50 W delivered with a 0.16‐mm focused spot size yields a power density of 248,880 W/cm2; a waveguide‐delivered CO2 laser at 8 W through a 0.4‐mm ceramic tip delivers approximately 6,300 W/cm2. The former will produce an incision more efficiently, but should be moved quickly across the tissue to limit penetration beyond the skin. The latter will produce an acceptable incision if tension is adequate to separate tissue and the waveguide is passed once and quickly across the skin. The skin defect will be 0.24 mm wider than the former with a perfect incision, which is clinically insignificant.
Figure 12.6 Absorption length of various wavelengths of surgical lasers in unpigmented skin. Wavelengths commonly used in veterinary medicine are in dark gray; wavelengths (nm) are stated beside the names. The far‐infrared Ho:YAG and CO2 lasers are highly absorbed by water so penetrate minimally into skin, whereas the near‐infrared Nd:YAG or GAA Diode lasers are absorbed more by the darker pigments of the deeper layers [8].
Figure 12.7 Range of tissue changes from laser beam. With sufficient power density, a laser beam has a central area of tissue vaporization/ablation shown by the crater in this drawing. A layer of carbonization occurs when tissue that has been significantly heated cools to produce char. The area of thermal necrosis is where tissue is heated beyond physiological limits and sloughs later. The goal of incisive surgery is to use adequate power density to create as little carbonization and thermal necrosis as possible [10].
Reports of laser research should be examined closely to detect flawed methods [2]. Incisions created with the CO2 laser were reported to have reduced tensile strength upon healing, with more necrosis and inflammation compared to steel (scalpel) incisions, but the laser incisions were created using a power density of 1,990 W/cm2 which resembles comparing a steel scalpel to a hobbyist’s wood burning set [11].
Laser energy can be delivered to the tissue in a noncontact or contact manner. As the term implies, with noncontact delivery, nothing but the laser light touches the tissue, thus imparting a purely optical interaction. Carbon dioxide laser energy is reflected by mirrors or down a highly polished waveguide and delivered in noncontact fashion. Lasers delivered by quartz fibers (Nd:YAG and GAA diode lasers) can deliver energy either way.
Laser energy is often delivered in continuous mode, i.e. uniform throughout application of the energy to tissue; some lasers have no other mode available. However, pulsed modes tremendously increase efficiency and minimize collateral heating of tissue. The principle is that spikes of laser energy at ≥200 Hz increase power density substantially while the interruptions allow tissue to cool slightly, minimizing diffusion of heat into adjacent tissues [12–14]. A CO2 laser in continuous mode at 50 W delivered with a 0.16‐mm focused spot size yields a power density of 248,880 W/cm2. In its pulsed mode, 400‐W power spikes provide intermittent power densities of 1,990,446 W/cm2 while producing identical total fluence (Figure 12.8). The technique depends upon the interval between laser exposure not exceeding the thermal relaxation time of the tissue, which is the time required to cool 50% of the heat applied without conducting heat to the surrounding tissue. By supplying a second pulse before the tissue cools further, potential char is vaporized and tissue debris is evacuated as smoke or steam. This feature produces a cleaner skin incision with less collateral thermal injury than from a continuous wave [15, 16]. The same principle applies to ablating tissue/masses with a computerized scanner on a CO2 laser. Pulsed mode should not be confused with simple gaited mode, which simply turns the laser delivery off and on at specified intervals, which may be useful to prevent overheating of the quartz fiber) [2].
Table 12.1 Common laser techniques and considerations [9].
| Laser | Description | Capacity | Accessories | Preference for skin incision | Comments |
|---|---|---|---|---|---|
| GAA Diode Laser | Quartz fiber delivery | 25–50 W | 600 and 1,000 micron quartz fibers Handpiece to hold fibers | 1,000 micron fiber sculpted down to approximately 600 micron at the tip | 25 W is insufficient for noncontact vaporization 600 micron fiber too fragile for general surgery. Excellent for endoscopic surgery Sterilize fibers for aseptic procedures. |
| Nd:YAG Laser |