4 Selection and Use of Lasers and Their Safety

10.1055/b-0034-82080

4 Selection and Use of Lasers and Their Safety

In 1905, Dr. Albert Einstein introduced the concept of a photon as a constituent of light. This opened the door for one of the most fascinating technologies of modern days: LASER. The word LASER is an acronym for light amplification by stimulated e mission of radiation. Unlike normal light, laser light has certain qualities:

It is monochromatic.
It is a coherence of photons with the same wavelength and energy level.
It can be made very intense or not intense at all.
It can be focused at a minute point to deliver its energy.

Basic Physics

Amplification of light means a very intense beam of light can be created. The laser may be activated by a few photons, and many more photons are generated. Electrons surrounding the atom’s nucleus occupy many different energy levels in different orbits (the larger the orbit, the higher the energy level). When an electron moves from a higher energy level to a lower energy level, the lost energy is released in the form of a photon. In an ordinary light source, these photons are generated randomly; hence the energy is scattered.

Stimulation to produce a photon with a specified energy level is the core concept of laser light. In stimulated emission, an electron in higher energy is brought to a lower energy level by the presence of a photon of exactly the same energy as the energy difference between the two levels. When this happens, a second photon identical to the first is produced. In this manner many more photons can be generated and then focused into a beam of monochromatic light with a specified energy level. This beam can channel through a fiberoptic system to be delivered to a target point.

Laser–Tissue Interaction

The biological effect of a laser is the function of three elements:

Laser wavelength
Energy density
Tissue absorption

For the surgeon it is helpful to look at wavelengths as the nature or character of the surgical instrument and energy density as dosage. Laser light may be scattered by particles in liquid surrounding the tissue, transmitted through and scattered within, or absorbed by the tissue. Two important components of tissue absorption are water and pigments such as hemoglobin and melanin.

Tissue Ablation Mechanisms

Five types of tissue ablation mechanisms have been postulated: photochemical interaction, thermal interaction, photoablation, plasma-induced ablation, and photo disruption.

Photochemical Interaction

Light can induce chemical effects and reactions within macromolecules or tissues.
Photochemical interaction takes place at very low densities (typically 1 W/cm2) and long exposure times ranging from seconds to continuous wave.

Thermal Interaction

Depending on the duration and the peak value of the tissue temperature achieved, different effects like coagulation, vaporization, cutting, carbonization, and melting may be distinguished.
Coagulation for homeostasis is best achieved by lower energy density obtained by enlarging the spot size or reducing the absolute power or exposure duration.
Vaporization is possible by a large spot size and high power density to achieve a higher rate of tissue removal.
Cutting is basically a thin layer of vaporization produced by a combination of high power density with the smallest possible spot size.
Carbonization of the tissues starts at a temperature above ~150°C, leading to blackening of tissue.

Photoablation

Absorption of photons directly dissociates the molecular bonds.
The photoablative process is only possible for ultra-violet (UV) laser wavelengths (e.g., excimer laser, argon laser, fluoride laser, 193 nm).

Plasma-Induced Ablation

Ablation by ionizing plasma formation

Photo Disruption

Fragmentation and cutting of the tissues by mechanical forces when the deposited energy leads to increased stresses within the tissue.

Lasers for Endoscopic Spine Surgery

The ability of the laser to be transmitted through flexible optic fibers can be regarded as a mandatory requirement for an efficient endoscopic surgical procedure. Therefore CO₂ lasers, even though they have excellent ablative properties with minimal scattering, cannot be used through an endoscope. The lasers available for use in endoscopic surgery are the erbium:yttrium-aluminum-garnet (Er:YAG), neodymium:yttrium-aluminum-garnet (Nd:YAG), holmium:yttrium-aluminum-garnet (Ho:YAG), and potassium titanyl-phosphate (KTP) lasers ( Fig. 4.1 ). Ho:YAG laser is the most commonly used laser.

Holmium:YAG Laser

The Ho:YAG laser has its wavelength in the midinfrared range, a range that is absorbed well by water.
It is fiberoptic.
An effective dose of energy can be introduced into the disk via fibers introduced percutaneously through a needle or catheter.
The Ho:YAG laser is a pulsed laser, in contrast to the continuous-wave near-infrared lasers, and therefore has the theoretic advantage of producing minimal amounts of heat in adjacent tissues.
With a pulse width of ~250 microseconds at 10 Hz and 1.6 J per pulse, virtually no temperature rise is noted in adjacent tissues.
When 1200 J of Ho:YAG laser energy was introduced into the disk through a 400-μm fiber with the same parameters, it consistently produced a 2.0 × 1.5 × 1.0–cm defect in the nucleus pulposus.
Ho:YAG laser ablation is faster than Nd:YAG laser ablation because the latter operates on the principle of coagulation.
It has an ability to resect, cut, coagulate, vaporize, and ablate cartilaginous tissues.
Its major advantage is excellent hemostatic ability, which greatly improves endoscopic visibility.