Physics of lasers and their interactions with tissues

It is important to understand the physics of lasers before using them in surgical procedures. Lasers transfer energy to an electron and then that energy is emitted as laser energy when the atom returns to its lower energy state. This is based on Einstein’s quantum theory of radiation hypothesis. The basis of this theory is that electrons are usually in a low-energy orbit close to the atomic nucleus and that these electrons can move to a higher energy state by absorbing external energy. When the electrons return to their original lower energy state, or orbit level, this absorbed energy is released spontaneously as light or photons. Therefore, a laser is an emission of energy accomplished by transferring energy to the electron of an atom. The low-energy electron is excited by a photon striking the atom; an electron absorbs this energy and sends the electron into an outer electron ring. This moves the electron further away from the nucleus of the atom. The further away the electron is from the atom, the more unstable it becomes. The electron in this higher energy and more unstable state wants to become more stable by returning to its ground state. As the electron returns to this original ground state, it releases electromagnetic energy by spontaneous emission of photons or light. The energy in the laser beam is generated by using an electrical, chemical, or optical source to begin the emission of photons from the atom.

A laser is a device that controls the way that energized atoms release photons. The word “laser” is an acronym that stands for light amplification by stimulated emission of radiation. To produce this beam of light lasers use several components. These components consist of an energy source, an active medium, and a resonant chamber. Most energy sources for lasers are electrical. This energy source flows through the laser medium and excites the electrons to a higher energy state. Lasers in dentistry usually are made up of one of three commonly used mediums. These include the Nd:YAG laser, CO ₂ laser, and dye laser. The Nd:YAG laser is composed of neodymium ions and crystal of yttrium-aluminum-garnet. The CO ₂ laser is a gas laser incorporating carbon dioxide, nitrogen, and helium. Dye lasers are liquid lasers with fluorescent organic dyes injected into a tube. The last component is the resonant chamber, which contains the laser medium and reflective mirrors. There is usually one highly reflective and one partially reflective mirror. These mirrors permit the laser light to exit. Usually, there are intense flashes of electrical discharge that pump this medium and create a large collection of atoms with higher energy electrons. This pumping phenomenon is created by atoms absorbing energy, going to a higher state, and decaying and releasing this energy. This released energy is then absorbed by other atoms, causing these atoms to enter a higher energy state. The atoms are excited to a level that is two to three times greater than their ground state. This leads to population inversion, which is when there are more atoms in the excited state than in the lower energy ground state. As more and more atoms reach the higher energy state they spontaneously decay and release photons to travel within the laser chamber. Pumping continues to maintain the population inversion within the resonant chamber. Eventually, enough photons are released to produce a beam of coherent light that is reflected by the mirrors, giving rise to a laser beam.

Lasers are different from normal light. A laser light has three distinguishing properties. The first is that the light released is monochromatic. The laser contains one specific wavelength of light. The laser is usually characterized by this wavelength and the wavelength is determined by the amount of energy released when the electron drops down to its ground state. The wavelength and absorption of the laser determines the laser’s interaction with the tissue. The second property is that the laser is coherent or organized. The photons move with all the others and launch in unison. The third property is that the light is directional. The laser has a very tight beam and is concentrated. This allows the laser to remain coherent and not scatter over a long distance as opposed to a light bulb that allows light to go in all directions. The photon emitted has a very specific wavelength that depends on the state of the electron’s energy when the photon is released. The wavelength of the laser determines the degree of scattering, tissue penetration, and the amount of energy absorbed by the tissues. If there is more scattering, less energy is transmitted to the tissue. A laser with minimal scatter is more precise and delivered to a single spot on the tissue, allowing it to be used to cut the tissue. A laser with more scatter is better suited for coagulation of the tissue. Another factor that determines the laser’s effect on the tissue is the exposure time. Exposure time is the amount of time the laser is directed at the target tissue. The exposure time can be either continuous, pulsated, or other modes. Continuous mode delivers a constant laser beam to the tissue over a period specified by the operator. Pulsated delivery modifies the delivery of higher laser energy for shorter periods. The pulsation rate is usually measured in pulses per second. This pulsation prevents deeper tissue penetration and minimizes heat buildup. Superpulsation is a type of pulsing used to minimize adjacent tissue damage by using a high peak power pulse at a much shorter width. This is useful for procedures such as skin resurfacing.

The current lasers are the Nd:YAG, CO ₂ , erbium:YAG, and argon ion. The ND:YAG is a solid state laser; the wavelengths of the laser beam are between 1064 and 1320 nm. This laser is minimally absorbed and is able to penetrate tissue to depths of up to 10 mm. This laser causes a great amount of tissue damage when compared with the CO ₂ laser. The Nd:YAG is used for coagulation of angiomas, arthroscopic surgery of the temporomandibular joint (TMJ), and vascular tumor resections. The CO ₂ laser uses a gas medium primarily composed of CO ₂ . This laser produces wavelengths around 10,600 nm. It is an excellent laser due to its properties of minimal scatter, rapid soft tissue vaporization, excellent water absorption, and negligible damage to surrounding tissue. Due to these properties, the CO ₂ laser is able to focus precisely onto its target tissue and can be used as a scalpel type of laser. It also produces a hemostatic cutting because vessels below 0.5 mm are coagulated. The erbium:YAG laser produces a wavelength of around 2940 nm and it has very good energy absorption by water. This laser has a limited depth of penetration and causes less damage to adjacent tissue resulting in less erythema, edema, and pain. These properties make it excellent for intraoral and skin resurfacing procedures. This laser is also capable of cutting both bone and enamel, allowing it to be used in crown elongation, tooth exposure, implant uncovering procedures, and operative dentistry. The argon ion laser has a wavelength of around 488 to nm making it a low wavelength laser. It is primarily used for photocoagulation of small vessels.

Oral and maxillofacial surgery

Oral and maxillofacial surgery

Skin rejuvenation

Cosmetics have become increasingly popular in recent decades. People want to look younger and more attractive. With this rising trend has come an increase in the demand for skin rejuvenation procedures. Lasers have been used effectively for this purpose. Lasers are used for the revision of facial scars, treatment of aging, sun-related skin damage, and pathologic conditions of the face. The use of lasers for skin rejuvenations is often done without anesthesia, although pretreatment with a topical anesthetic is used by some clinicians. There is no need for wound closure, suction, or evacuation of a laser plume. Patients often get this procedure done during lunch breaks because there is minimal to no posttreatment down time.

Photoaging is a common cosmetic defect that results from environmental UV exposure. This UV exposure can lead to dry, rough, and course skin along with squamous cell carcinoma, basal cell carcinoma, and melanoma. Many treatments are available for photoaging, including the use of lasers. Lasers are used for skin resurfacing procedures by producing a controlled wound stimulates new skin growth. The depth of treatment is usually through the epidermis and into a specific layer of the dermis. The treatments are usually divided into superficial, mid, and deep, depending on what layer of the dermis they penetrate. The superficial depth makes it to the stratum corneum and papillary dermis, middepth to the upper reticular dermis, and deep to the midreticular dermis. After the laser treatment, the new skin replaces the damaged epidermal layer and a tighter dermal layer. The dermal layer becomes tighter due to the shortening of the existing collagen and the production of new collagen. Laser skin resurfacing is best used for the treatment of acne scars, actinic keratosis, shallow rhytids, irregular pigmentation, and fine wrinkles. Lasers that target water, such as CO ₂ lasers, produce nonspecific heating of the upper dermis, with the goal of promoting new collagen production. Laser resurfacing most commonly uses CO ₂ lasers that are pulsed, although the CO ₂ laser should not be used for the treatment of deep rhytids due to the possibility of suboptimal scarring and hypopigmentation. The CO ₂ laser vaporizes the epidermis and causes collagen shrinkage of the dermis, which results in contraction of the dermis. The ability to change the settings of the laser, such as pulsation and delivery time, gives the clinician better control than the use of chemical peels. Other than the CO ₂ laser the erbium:yttrium-aluminum-garnet (Er:YAG) laser can be used for skin rejuvenation procedures of patients with dark skin color or pigmentation. The Er:YAG laser is better absorbed by water than the CO ₂ laser and results in less thermal injury.

Newer techniques for facial rejuvenation with lasers have emerged including nonablative laser resurfacing. Nonablative facial rejuvenation is designed to reduce facial rhytids, improve skin texture, decrease pore size, and aims to achieve a younger, healthier appearance. This type of resurfacing limits the epidermal damage and simultaneously applies selective energy to the dermis. The epidermis is spared due to selective cooling and leads to improved healing and less need for anesthesia. This technique can be used for all skin types and is best for the treatment of fine skin lines and shallow rhytids.

Patients may also have scars that they want eliminated or minimized. For scars that are hypertrophic in nature, lasers capable of a wavelength in the range of 585 nm are beneficial. This is usually accomplished with pulse dye lasers. The laser produces new collagen and remodels the existing collagen. Simultaneously the fibroblast scar response is minimized. Usually this is not done in one sitting and requires multiple laser scar revision therapies. It has been shown that laser scar revision therapy is more successful when used with other treatments such as steroid injections.

There are several contraindications and possible complications associated with skin resurfacing and scar revision procedures. The contraindications include the use of certain medications, skin type, recent facial procedures, and active medical or skin disorders. If the patient is currently taking retinoids, steroids, Dilantin, or d-penicillamine, then laser resurfacing is contraindicated. Facial cosmetic procedures that have recently been performed such as face lifts, rhinoplasty, and brow lifts are contraindications. This is because these procedures disrupt the lymphatic drainage system and can lead to severe edema. Medical conditions that are contraindications include prior radiation treatment of the head and neck, psychiatric disorders such as body dysmorphic syndrome, and a history of keloid scar formation. If the patient has active skin disorders, including acne, psoriasis, rosacea, or verrucae infections, laser skin resurfacing procedures should be postponed. Smoking is a relative contraindication because it can delay healing.

Most complications from laser resurfacing procedures arise from overaggressive treatment. Complications include persistent erythema, hypopigmentation, and hyperpigmentation. Hyperpigmentation is usually seen in darker skinned individuals such as Hispanics, Asians, and African Americans. Usually this resolves on its own over time but it can be treated with Retin-A, hydroquinone, and topical steroids. Hypopigmentation is due to overtreatment and destruction of melanocytes. This complication usually manifests as a late complication and is much more serious because it can be permanent. No specific treatment is available for hypopigmentation. Hypertrophic scarring is another possible complication that is usually caused by overaggressive treatment into the underlying deep dermal layers. It is usually seen in dark skin types and is best treated by prevention. If it does occur, treatment is the same as for other hypertrophic scars, including steroid injections, topical steroids, and compression dressings.

Vascular lesions of the face are best treated with lasers because tissue does not have to be excised, which can result in scarring and cosmetic defects. Lasers that target the hemoglobin chromophore, such as pulsed dye lasers at 580 nm with short pulses, are the preferred treatment of vascular lesions, including hemangiomas, vascular malformations, port-wine stains, ectasias, and telangiectasias. It is thought that these lasers cause the release of inflammatory mediators by heating of the microvasculature while sparing the adjacent normal tissue. These mediators may stimulate fibroblasts, which results in new collagen production and remodeling. Many of these smaller lesions, such as the ectasias, can be treated in one session, but the larger port-wine stains require multiple sessions.