Laser Tissue Interaction

The key to the success of soft tissue lasers is their ability to cut and coagulate the soft tissue at the same time. Laser light’s wavelength determines how it is absorbed by objects. Lasers of different wavelengths produce different effects on tissue.

For practical surgical lasers on the market today (diode, erbium and CO₂ lasers), the laser light energy is transformed, through absorption, into the heat inside the tissue leading to elevated tissue temperatures that, in turn, can result in tissue ablation and coagulation. Such laser-tissue interaction is referred to as photo-thermal.

Comparing laser wavelengths

As illustrated with the photographs on this page, not all lasers are efficient at both cutting and coagulating the soft tissues. Some laser wavelengths (such as erbium lasers) are great at cutting but are not efficient at coagulating. Other laser wavelengths (such as of diode lasers) are highly efficient coagulators but are poor scalpels. There are also lasers (such as a CO₂ laser) that are efficient at both cutting and coagulating soft tissue.

The key to understanding how the laser light ablates (cuts, incises, excises) and coagulates is through the absorption of laser light by the soft tissue. The wavelength-dependent Absorption Depth [mm] for the sub-epithelium connective oral soft tissue is illustrated in Figure 3; it is derived from absorption and reduced scattering coefficients of oral soft tissue dominant chromophores: water, hemoglobin, and oxyhemoglobin. ^[1-4]

The cooling efficiency of the tissue irradiated by laser light is largely determined by the tissue’s own thermal conductivity (or thermal diffusivity) to dissipate (or diffuse) the heat away from the irradiated tissue.

The rate at which the irradiated tissue diffuses heat away is defined through the thermal diffusion time, or Thermal Relaxation Time (also presented in Figure 3) as T_R = A²/K,^[5] where A is optical absorption depth (also from Figure 3), and K is tissue’s thermal diffusivity.

Practical implications of the Thermal Relaxation Time concept are important for the appropriate application of laser energy. The most efficient heating of the irradiated tissue takes place when laser pulse duration t is much shorter than thermal relaxation T_R. The most efficient tissue cooling takes place if the time duration between laser pulses T is much greater than T_R.

Soft tissue photo-thermal ablation (or photovaporolysis) is a process of vaporization of intra- and extra-cellular water.

Figure 4 illustrates a laser beam irradiating the tissue surface from the left. Inside the tissue, the laser light intensity is exponentially attenuated: I = I₀ Exp[-x/A], where A is absorption depth from Figure 3. When laser intensity I₀ is greater than ablation threshold intensity, soft tissue ablation takes place in Ablation Zone 0<x<x_a. Immediately below the Ablation Zone, there is a heat affected Coagulation Zone, where coagulation depth H = x_c – x_a, is defined by 60-100° C temperature range inside.

For a fixed laser beam diameter (or spot size), the volume of tissue exposed to the laser beam is proportional to the optical penetration (i.e. absorption or Near-IR attenuation as defined above) depth. The shorter the penetration depth – the less energy required to ablate the tissue. The longer the optical penetration depth – the greater the volume of irradiated tissue and, therefore, more energy is required to ablate the tissue within the irradiated volume of tissue.

The ablation threshold energy density E_TH spectrum for sub-epithelium connective oral soft tissue is illustrated in Figure 5.

Diode Laser Wavelengths

The diode laser wavelengths 800-1,100 nm are characterized by 1,000s times greater photo-thermal ablation threshold energy densities than Mid-IR and IR wavelengths because of much weaker Near-IR absorption by the soft tissue.

Erbium and Carbon Dioxide Laser Wavelengths

In sharp contrast to diode laser wavelengths, the erbium and the CO₂ wavelengths are highly energy-efficient at ablating the soft tissue photo-thermally with very low ablation thresholds due to extremely small volumes of irradiated tissue because of extremely short absorption depths.

Coagulation occurs as a denaturation of soft tissue proteins that takes place in the 60-100°C temperature range leading to a significant reduction in bleeding (and oozing of lymphatic liquids) on the margins of ablated tissue during laser ablation (and excision, incision) procedures.

The coagulation depth value H relative to the blood vessel diameter B is an important measure of coagulation efficiency; and is presented in Figure 6 for B = 21-40 micrometers (sub-epithelium oral gingival soft tissue), where absorption/attenuation depth A from Figure 3 is utilized to calculate H.

Erbium laser wavelengths

For H<<B (erbium laser wavelengths), optical absorption and coagulation depths are significantly smaller than blood vessel diameters; coagulation takes place on a relatively small spatial scale and cannot prevent bleeding from the blood vessels severed during tissue ablation. For this reason, bleeding control is relatively low, when erbium lasers are used.

Diode laser wavelengths

For H>>B (diode laser wavelengths), optical absorption (Near-IR attenuation) and coagulation depths are significantly greater than blood vessel diameters; coagulation takes place over extended volumes – far away from ablation sites where no coagulation is required.

Carbon dioxide laser wavelengths

For H ≥ B (CO₂ laser wavelengths), coagulation extends just deep enough into a severed blood vessel to stop the bleeding; the coagulation is more efficient than for the above two cases H<<B, and H>>B.

Coagulation depth can be extended with longer laser pulses that allow heat diffusion away from the irradiated tissue.

Figure 7 is an illustration of vastly different absorption coefficients around 1,000 nm (sub 1 cm^-1), around 3,000 nm (3,000-10,000 cm^-1), and around 10,000 nm (400-800 cm^-1) by the main chromophores in the sub-epithelium connective oral soft tissue:

CO₂ laser wavelengths are a highly efficient and spatially accurate photo-thermal ablation tool with excellent coagulation efficiency (close match between coagulation depth and oral soft tissue blood capillary diameters). The CO₂ laser is THE ONLY practical soft-tissue surgical laser, which uses the laser beam directly to both cut and coagulate the soft tissues.

The LightScalpel CO₂ laser is used in a broad range of clinical applications.  The laser can augment and even, in certain instances, replace, traditional instruments and methods. The LightScalpel CO₂ laser is useful in procedures where:

• Surface penetration is desired
• Soft tissue is the target

The 10,600 nm CO₂ laser is cleared by the FDA for soft tissue procedures only. The laser is an effective hemostatic tool for vascular tissue. When the CO₂ laser is used for muscle dissection there is minimal heating or contraction of the muscle. This helps facilitate certain procedures and reduce post-surgical pain and edema.

The 10,600 nm CO₂ laser is not cleared by the FDA for use on bone or in hard tissue procedures in the United States. Proper use of CO₂ lasers such as the LightScalpel within the FDA-cleared indications for use may offer some of the following advantages over conventional treatment:

• Improved access to some areas, compared to the scalpel
• Reduced operative time in some procedures
• Tissue sculpting ability
• Easier removal of lesions without distortion of surrounding tissue
• Less bleeding, often with less trauma
• Less need for suturing
• Reduced postoperative discomfort
• Minimized edema

CO₂ laser surgery wounds may initially take slightly longer to heal than scalpel incisions.  However, several studies indicate that there is no significant difference in the healing of wounds made by the two modalities after two weeks.  Patients often report less postoperative pain with laser wounds.

Finally, the LightScalpel laser is more versatile than conventional surgical instruments because it can:

• Incise, excise, or cut
• Vaporize or ablate
• Provide coagulation and hemostasis