Soft Tissue Cutting Abilities of CO2 and Diode Veterinary Lasers
The cutting and ablation of soft tissue by CO2 and diode veterinary lasers have been extensively studied and reported.[1-3] This web-page illustrates the obvious practical differences between CO2 and diode veterinary lasers with respect to their soft tissue cutting and ablation abilities.
Water Absorption Spectrum
It is important to understand how the wavelength-dependent light of different lasers is absorbed by water (the main component of soft tissue), in order to understand how it cuts the soft tissue (See Figure 1).
Figure 1. Water Absorption Spectrum; Comparing laser wavelengths.
- The CO2 laser wavelength (10,600 nm) has an absorption/penetration depth of 0.01 mm in water, which enables high precision when removing tissue and provides for sufficient hemostasis. The short penetration depth also allows for an extremely thin sub – 0.1 mm thermal damage zone on the margins of the incision.[1]
- The diode veterinary laser wavelength (800-1,100 nm) has an absorption/penetration depth one thousand times greater than the CO2 laser wavelength in water. Although hemoglobin and melanin strongly absorb light in this wavelength, their low concentrations in soft tissue result in a wide thermal damage zone of up to 8 mm.[2,3]
CO2 Laser-Tissue Interaction
Figure 2 Laser Incision with Aesculight 0.25 mm spot size CO2 laser.
Figure 2 illustrates the interaction of CO2 laser light at 10,600 nm with fresh poultry muscle tissue. The CO2 laser beam from the Aesculight laser-focused to a 0.25 mm spot size at 5-watts continuous wave in SuperPulse mode, produces a clean incision with char-free (carbon-free with no evidence of burning) margins with minimal thermal damage.
The CO2 laser-tissue interaction is always predictable and is based on:
- Laser beam spot size
- Laser beam power
Why Not a Diode Laser?
Because their 800-1,000 nm wavelengths are poorly absorbed by soft tissue, diodes are used exclusively as THERMAL devices. There is no diode laser beam-tissue interaction beyond glass tip “activation,” and such an activated diode laser glass tip is essentially a “hot tip.” Diodes are never used surgically in order to avoid a massive spread of thermal necrosis. The red-hot glass tip is used to very slowly “poke, burn and pick” tiny amounts of oral-mucosa, similar to electrosurgery, branding irons, or hot picks. I.e. the diode is a thermal instrument and not a laser.
Diode Laser Beam-Tissue Interaction Is Non-Surgical
Figures 3-6 illustrate the use of the diode veterinary laser at 810 nm with the same tissue sample, the same average laser power (7-watt continuous wave) and similar spot size (0.3 mm) as used for CO2 laser cutting settings presented in Figure 2. The use of the diode veterinary laser in “non-contact” mode is represented in Figure 3A (laser beam ON) and Figure 3B (laser beam OFF); no tissue is removed regardless of exposure time, while sub-surface thermal necrosis may extend up to 8 mm in depth.[2] The use of the diode veterinary laser in “contact” mode with a fresh and clean distal glass fiber tip firmly pressed against the soft tissue surface is illustrated in Figure 4A (laser beam ON) and Figure 4B (post-lasing). Just like with “non-contact” mode, no tissue is removed regardless of exposure time, while sub-surface thermal damage may be very widespread and extensive.[2]
| 
| 
| 
|
Why Should the Diode Laser Glass Tip Be Activated?
The key to soft tissue removal with the diode laser is the carbon-rich black ink or char deposited on the diode laser fiberglass tip in order to “initiate” or “activate” it – see Figure 5A. The char absorbs the diode laser light and blocks it inside the glass tip. The glass tip then heats up to a temperature at which it can burn the soft tissue upon contact – see Figure 6B. Such thermal tissue removal is a slow heat-conduction process that depends on how charred and hot the glass tip is. That is to say, THERE IS NO DIODE LASER BEAM-TISSUE INTERACTION BEYOND TIP ACTIVATION. Slow tissue removal induced thermal necrosis up to 6 mm deep[3] is manifested by 1) extensive char left at the margins of incision and by 2) white “seared” discoloration outside of the charred margins of incision seen in Figure 5B. An often overlooked aspect of using a hot charred glass tip for soft tissue removal is the thermal stress induced fracture of the fiber and loss of the broken tip inside the tissue – see Figure 6.
| 
|
Figure 6. Diode laser tip breaking due to thermal stress.
Why Not a YAG laser?
The Er:YAG laser wavelength’s extinction depth in soft tissue is ten times shorter than the CO2 laser wavelength. The Er:YAG is sufficient at cutting soft tissue but is detrimental for hemostasis, so it is most useful in water-rich hard tissue applications where hemostasis is not needed. Furthermore, Er:YAG lasers require complex power supplies, cooling systems, and beam delivery devices that are expensive for the end-user and have high maintenance costs.
Why a CO2 Veterinary Laser?
The CO2 laser has been used for veterinary procedures since the 1960s and millions of CO2 laser procedures have been performed with Luxar-Aesculight-VetScalpel flexible fibers to date since 1991.
The CO2 laser wavelength of 10,600nm is universally regarded by surgical specialists as best suited for soft tissue laser surgery, due to its very thin extinction depth and excellent hemostasis. Its wavelength is absorbed by soft tissue 10-100 times better than the diode wavelength.
Why Are CO2 Veterinary Lasers Better than “All-Tissue Lasers”?
Currently, there is no laser wavelength that works equally well on both hard and soft tissues.
Char-Free CO2 Laser Surgery: What is SuperPulse mode and what are its clinical advantages?
The SuperPulse mode was invented in the 1990s. Instead of an uninterrupted flow of laser power, SuperPulse mode produces a continual series of short, high peak power bursts or pulses. SuperPulse provides a greater rate of tissue ablation, with less heat propagation into surrounding tissues. SuperPulse CO2 lasers minimize collateral thermal damage to adjacent tissue, which leads to considerably less char, wound contraction, and faster healing. Because SuperPulse mode has a higher ablation rate than regular CW mode, it is ideal for procedures in which thermal damage needs to be minimized while the tissue removal rate needs to be maximized (e.g. faster and deeper cutting).
Figure 7. VetScalpel’s SuperPulse mode: high power, short laser pulses are spaced further apart than soft tissue’s thermal relaxation time. SuperPulse mode maximizes the rate of soft tissue removal while keeping adjacent tissue cool.
Summary
CO2 laser light’s ability to ablate and cut the water-rich soft tissue with maximum precision and minimal collateral thermal effects[1] makes it a true “What You See Is What You Get” surgical laser with a short learning curve and a great variety of uses in dentistry, OMS, ENT, dermatology, general surgery, podiatry, etc.
Diode veterinary laser light does not ablate the water-rich soft tissue such as muscle tissue or low pigmentation skin; it is used indirectly to heat up the optically “black” char on the tip of glass fiber. Then the hot charred glass tip cuts the tissue by burning it away upon contact. Excessive char and thermal necrosis along with the possibility of a thermal stress-induced fracture of the glass tip inside the surgical site make the diode veterinary laser a “What You Don’t See Can Hurt You” tool for soft tissue surgery. A diode’s fiber hot glass tip is best used surgically where the use of CO2 lasers is limited (fluid-filled surgical site or with flexible endoscopes), while diode laser light has a number of non-surgical applications such as Biostimulation, etc.
References:
- P. Wilder-Smith et al. “Incision properties and thermal effects…“, Oral Surgery Oral Medicine Oral Pathology, 1995, p.685.
- P.W.A. Willems et al. “Contact laser-assisted neuroendoscopy…“, Lasers in Surgery and Medicine, 2001, p. 324.
- L. B. Rizzo et al. “Histologic comparison of skin biopsy…“, JAVMA, 2004, p. 1562.
