By David Duclos, DVM, DACVD, and Peter Vitruk, Ph.D., DABLS

Ideally, an effective soft tissue surgical laser should be able to vaporize and coagulate soft tissue simultaneously, thus simplifying procedures performed with this technology and making them far more enjoyable for practitioners.^1-11

Figure 1 presents the known optical absorption coefficient spectra of the main three sub-epidermal/sub-epithelial soft tissue chromophores^3-6—water, hemoglobin (Hb), and oxyhemoglobin (HbO₂). Chromophore is a material in tissue that absorbs particular laser wavelength based on its absorption coefficient.) The optical absorption coefficient spectra help explain photothermal ablation (or photovaporolysis^3,4) and photothermal coagulation (or photopyrolysis^3,4) efficiencies for soft tissue lasers⁷ on the market today. The following classifies laser wavelengths and their cutting and coagulating efficiencies:

• Infrared (IR) carbon dioxide (CO₂) lasers circa 10,000 nm are efficient at both incising and coagulating soft tissue;^1-11
• Mid-IR Erbium lasers circa 3000 nm efficiently cut soft tissue, though coagulate it poorly;^3-5
• Near-IR diodes circa 1000 nm are highly inefficient cutters, but are excellent at coagulating soft tissue; ^2-6,8 and
• Diodes are hot glass-tip cautery devices.⁹ Near-IR wavelengths increase the "initiated" (blackened, charred) distal end of the fibreglass tip to 500 to 900 C⁹ and conduct heat to soft tissue.

Thermal relaxation time

Soft tissue ablation and coagulation efficiencies are influenced not only by absorption/attenuation spectra, but also by laser pulse duration and tissue thermal conductivity. The rate of how fast the irradiated tissue diffuses the heat away is defined through the thermal relaxation time (TRT), which equals approximately 1.5 msec for CO₂ lasers (see Figure 1). The most efficient heating of the irradiated tissue takes place when laser pulse energy is high and its duration is much shorter than TRT. The most efficient cooling of the tissue adjacent to the ablated zone takes place if time duration between laser pulses is much greater than TRT.

For instance, short pulse super pulse CO₂ lasers allow for the most efficient ablation of the irradiated tissue with minimum coagulation and hemostasis underneath the ablated tissue. The long pulse and continuous wave (CW) CO₂ lasers are less efficient cutters but provide for greater depth of coagulation for excising/incising in highly vascular and inflamed tissues such as hemangioma.

Depth of laser vaporization/ablation/incision

For CO₂ laser cutting applications, greater laser fluence (i.e., energy density, or power density times the duration it applied to the target) results in greater depth and rate of soft tissue removal. The vaporization/ablation depth equals A (E – E_th) / E_th, where A is the absorption depth and E_th is the ablation threshold (15 micrometers and 3 J/cm², respectively, derived from data in Figure 1), and E is the fluence delivered to the tissue. For laser beam that is moving over the target tissue, the vaporization/ablation depth^2,5,6 is:

• Proportional to laser average power, and
• Inversely proportional to laser beam width and surgeon's hand speed.

Cutting at the speed of scalpel

Figure 2 presents the calculated depth of incision with super pulse CO₂ laser with 0.25 mm focal diameter of the beam (the smallest diameter with practically acceptable depth of focus in veterinary laser surgery). A requirement for multi-millimeter deep incisions with hand speed exceeding
10 mm/sec is laser power above 35 to 45 Watts (VetScalpel CO₂ laser model VS-4530 is used in surgeries presented in this paper).

Adjustable laser beam geometry: 0.25, 0.4, and 0.8 mm diameters, and 0.4x2.5 mm² "paintbrush" wide ablation. Figure 3 shows the latest generation of CO₂ laser handpiece technology that allows for easy switch between four different geometries of laser beam on the target tissue. Handpiece is easy to clean and autoclave. Small incisional diameters 0.25 and 0.4 mm, combined with high power of VetScalpel CO₂ laser model VS-4530, allow for high speed, deep cutting, e.g., full-skin-depth incisions, high-speed amputations, etc. Larger sizes 0.8-mm diameter and 0.4-by-2.5-mm² rectangle, combined with high-laser power, allow for high speed, large area, superficial ablation of lesions.

Large beam size = sub-mm shallow vaporization/ablation. VetScalpel wide ablation nozzle, displayed in Figures 3 through 5, features approximately a 2.5-mm-by-0.4-mm large spot area of approximately 1 mm². Such wide ablation nozzle, shown in use in Figures 4 and 5, allows for approximately 2.5 mm wide pass of the laser beam over the target tissue with sub-mm depth of ablation. Wide ablation beam area is 20 times larger than focal area of a 0.25-mm diameter beam in Figure 2, which allows for up to 20 times shallower ablations at high power with fast handspeed.

Small beam size = multi-mm deep incision. VetScalpel adjustable handpiece, displayed in Figures 3 and 6, features three spot-size diameters 0.25, 0.4, and 0.8 mm. The smallest focal area of 0.05 mm² (20 times smaller than wide ablation nozzle laser spot area) is used for fast, multi-mm deep, 0.25-mm narrow incisions (illustrated in Figure 2) with char-free margins when super pulse mode is utilized for the most efficient cutting (Figure 6A). Adjustable handpiece allows for instant switch to a larger spot size of 0.8 mm diameter with 0.5-mm² area (10 times larger area than for a 0.25-mm spot) for a shallow coagulation when CW mode of operation is utilized (Figure 6B), or for shallow ablation with super pulse mode of operation.

Efficiency and depth of laser coagulation/hemostasis

Coagulation occurs as a denaturation of soft tissue proteins that takes place in the 60 to 100°C temperature range^3-5 leading to a significant reduction in bleeding (and oozing of lymphatic fluids) on the margins of ablated tissue. Photothermal coagulation also is accompanied by hemostasis due to shrinkage of the walls of blood and lymphatic vessels resulting from collagen shrinkage at increased temperatures. For super pulse CO₂ lasers, the sub 100-µm^5,7 coagulation depths (see Figure 1) extend just deep enough into severed blood vessel capillaries to stop the bleeding; coagulation depth can be extended for longer laser pulses and for CW laser operation.

Summary

A combination of the CO₂ laser wavelength, super pulse settings, and a variety of beam geometries, allows for a char-free and bloodless surgery with:

• Highly controllable speed and depth of vaporization/ablation/incision with dynamic range from micrometers to millimeters. The depth is proportional to laser power and inversely proportional to laser beam width and handspeed. ^1,2,6
• Sub-100 µm coagulation/hemostasis depth, which closely matches the blood capillary diameters.⁵ It allows for an instant hemostasis during high-speed ablation/cutting. It affords the clinician with the improved visibility of the surgical field and therefore allows for more precise and accurate tissue removal;⁷ coagulation depth can be increased by utilizing a long pulse or CW laser operation.
• Minimal post-operative pain, discomfort, swelling, and edema; significantly reduced post-surgery production of myofibroblasts; diminished wound contraction and scarring. ^1,2,11 As observed in our surgeries, healing with the CO₂ laser is markedly different from the other surgical modalities, it is uncomplicated and predictable. 

References

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