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Figure 29: Principle of Laser Action

Mechanism of Laser Action:

Lasers can concentrate light energy and exert a strong effect, targeting tissue at an energy level that is much lower than that of natural light. The photon emitted has a specific wavelength that depends on the state of the electron’s energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical wavelengths. Lasers can interact with their target material by either being absorbed, reflected, transmitted, or scattered. Absorbed light energy gets converted to heat and can lead to warming, coagulation, or excision and incision of the target tissue. Although the wavelength of the laser is the primary determinant of how much energy is absorbed by the target tissue, optical properties of the tissue, such as pigmentation, water content, and mineral content, can also influence the extent of energy absorbed.

Laser Effects On Tissue:

Tissue Interactions and Biological Effects:

Once a laser beam is produced it is aimed at tissue to perform a specific task. As the energy reaches the biological interface one of four interactions will occur; reflection, transmission, scattering, or absorption.

• Absorption – Specific molecules in the tissue known as chromophores absorb the photons. The light energy is then converted into other forms of energy to perform work.
• Reflection – The laser beam bounces off the surface with no penetration or interaction at all. Reflection is usually an undesired effect, but a useful example of reflection is found when Erbium lasers reflect off titanium allowing for safe trimming of gingiva around implant abutments.
• Transmission – The laser energy can pass through superficial tissues to interact with deeper areas. Retinal surgery is an example; the laser passes through the lens to treat the retina. The deeper penetration seen with Nd:YAG and diode lasers is an example of tissue transmission as well.
• Scattering – Once the laser energy enters the target tissue it will scatter in various directions. This phenomenon is usually not helpful, but can help with certain wavelengths biostimulative properties ^(12).

Figure 30:  Optical behavior of a tissue layer during irradiation with laser light

Absorption is the most important interaction. Each wavelength has specific chromophores that absorb their energy. This absorbed energy is converted into thermal and and/or mechanical energy that is used to perform the work desired. Near infrared lasers like diodes and Nd:YAGs are mostly absorbed by pigments such as hemoglobin and melanin. Erbium and CO₂ lasers are predominantly absorbed by water and hydroxyapatite. The shorter, near infrared wavelengths of diodes and Nd:YAG lasers also penetrate tissue more deeply than the longer, mid infrared wavelengths of the erbium and CO₂ lasers.

There are five important types of biological effects that can occur once the laser photons enter the tissue:

• Fluorescence,
• Photothermal,
• Photodisruptive,
• Photochemical, and
• Photobiomodulation

Fluorescence happens when actively carious tooth structure is exposed to the 655nm visible wavelength of the Diagnodent diagnostic device. The amount of fluorescence is related to the size of the lesion, and this information is useful in diagnosing and managing early carious lesions.

Photothermal effects occur when the chromophores absorb the laser energy and heat is generated. This heat is used to perform work such as incising tissue or coagulating blood. Photothermal interactions predominate when most soft tissue procedures are performed with dental lasers. Photothermal ablation is also at work when CO₂ lasers are used on teeth as hard tissue is vaporized during removal. Heat is generated during these procedures and great care must be taken to avoid thermal damage to the tissue.

Photodisruptive effects (or photoacoustic) can be a bit more difficult to understand. Hard tissues are removed through a process known as photodisruptive ablation. Short-pulsed bursts of laser light with extremely high power interact with water in the tissue and from the handpiece causing rapid thermal expansion of the water molecules. This causes a thermo-mechanical acoustic shock wave that is capable of disrupting enamel and bony matrices quite efficiently. Erbium lasers’ high ablation efficiency results from these micro-explosions of superheated tissue water in which their laser energy is predominantly absorbed. Thus tooth and bone are not vaporized but pulverized instead through the photomechanical ablation process. This shock wave creates the distinct popping sound heard during erbium laser use. Thermal damage is very unlikely as almost no residual heat is created when used properly, particularly when the concept of thermal relaxation is considered.

Photochemical reactions occur when photon energy causes a chemical reaction. These reactions are implicated in some of the beneficial effects found in biostimulation discussed below.

Photobiomodulation or Biostimulation refers to lasers ability to speed healing, increase circulation, reduce edema, and minimize pain. Many studies have exhibited effects such as increased collagen synthesis, fibroblast proliferation, increased osteogenesis, enhanced leukocyte phagocytosis, and the like with various wavelengths. The exact mechanism of these effects is not clear but it is theorized they occur mostly through photochemical and photobiological interactions within the cellular matrix and mitochondria. Biostimulation is used dentally to reduce postoperative discomfort and to treat such as recurrent herpes and aphthous stomatitis. Low Level Laser Therapy (LLLT) is another term used to describe this phenomenon ^(12).

When a dental laser is employed it can be used in contact mode or non-contact mode. The laser tip directly touches the target tissue in contact mode. In non-contact mode the laser is pointed at a distance from the target tissue anywhere from a few millimeters, such as in operative dentistry, or up to several centimeters when performing biostimulation.

When a laser heats oral tissues certain reversible or irreversible changes can occur:

• Hyperthermia – below 50º C
• Coagulation and Protein Denaturation – 60+ ºC
• Vaporization – 100+ º C
• Carbonization – 200+ ºC

Irreversible effects such as denaturation and carbonization result in thermal damage that cause inflammation, pain, and edema.

Effect of Temperature on Tissue:

Tissue Composition:

The tissue consists of collagen, water, haemoglobin and perhaps a few other chromophores, such as melanin. This is a gross over simplification of tissue biology, but is a useful reduction in complexity which can help us Understand more easily some of the thermal effects taking place in the tissue.

• Cells: This is the part of this tissue that biologists have conventionally concentrated on for our purposes they are often treated as water-filled, although the proteins in certain cells can be crucial to some applications (such as the haemoglobin in red blood cells is to port wine stain treatment).
• Extracellular matrix (ECM): this is a fibrous scaffold among which the cells nestle, and which gives tissue most of its stiffness and structure. It is made from collagen and elastin and other glycoproteins and proteoglycans. The ratio of the amount of ECM to number of cells varies widely depending on the type of tissue. Liver and muscle, for instance, are low in ecm, whereas bone, tendon and the retina are largely ECM. The collagen in ECM is of interest when considering thermal effects because it breaks down at temperatures well below 100^{◦ (13)}

Figure 31: Effect of Temperature On Tissues

Temperature Effects:

• 37^◦c is normal body temperature, and for the first 5^◦c or so of heating few irreversible changes occur.
• At 42.5^◦c a number of effects, covered with the blanket term hyperthermia, begin. Cell protein such as membrane and cytoplasmic proteins – undergo conformational (shape) changes. They change conformation because the hydrogen bonds keeping them in their native state are broken by the increasingly violent vibrations of the molecule as the temperature increases. When a protein molecule changes shape it can often no longer fulfill its function within the cell. For instance, when enzymes, whose catalytic functions depend crucially on their shape, begin to deform, reaction rates within cells slow down. Even at small increases in temperature some cells will die because of these effects. The rate at which cells necrose (or apoptosis) increases with the temperature.
• From ≈ 45◦c the collagen fibers forming the ECM begin to shrink as the collagen’s tri helical Structure breaks apart. The optical scattering in the tissue increases then the collagen then softens and gelatinizes. (gelatin is just tangled, random coils of collagen) Tissue starts to coagulate, and blood clots form.
• At 100◦c the water in the cells and extracellular fluid boils. The huge volume expansion as the water changes phase (vaporizes) can lead to tissue being expelled from the skin surface.
• Once all the water has boiled off, the remaining organic material may char (carbonize, Blacken). At very high temperatures it will eventually evaporate. ⁽¹³⁾

Laser Safety:

Safe use of lasers is also one of the important concerns in the use of laser therapy. With the availability, utilization and future development of different laser wavelengths and methods of pulsing, much interest is developing in this growing field. Diodes, Neodymium-Doped Yttrium Aluminum Garnet (Nd: YAG), Erbium and Carbon Dioxide Laser (CO₂) lasers are Class 4 lasers, which are considered high-powered dental lasers. They are a hazard to eyes and skin and require special precautions.

Although many regulations and standards relating to laser safety are in effect, there continue to be an average of 35 laser injuries per year in U.S. This can be attributed to unmonitored use of lasers in many solo practices. Furthermore, the level of training and experience of dental staff is generally far less than that of the laser surgical nurse or hospital laser safety officer. Therefore, the present review article focuses on the various hazards that can be encountered by the patients and the dentist while using laser and control measures that can be adopted to minimize the injuries caused by the same.

Laser Hazard Classification:

A hazard is something with the potential to cause injury. There are a number of hazards associated with laser use in a clinical environment, the most obvious being the laser light itself. Accidental exposure could be caused by a misaligned or misdirected laser beam, laser light escaping from the protective housing of the unit, or a broken or detached optical fiber. There are various international laser standards and classifications.  The Centre for Devices and Radiological Health (CDRH) of Food and Drug Administration (FDA) of USA sets forth the standards governing the manufacture of lasers in the Code of Federal Regulations (CFR).

This standard categorizes all laser devices into one of the four classes based on their total energy output and wavelength. The laser used in dentistry generally fall into the class IV category, which is considered the most hazardous group of lasers. Another classification is set by the ‘The International Electrotechnical Commission’ (IEC), a global organization that prepares and publishes international standards for all electrical, electronic and related technologies. The Laser Institute of America, which serves as a secretariat for the American National Standard Institute (ANSI), has developed both general and clinical standards that currently serve as voluntary guidelines for the safe use of lasers in dentistry and medicine.

Class	Dental Purpose	Eye Hazards	Skin Hazards	LSO Requirement
1 1M	Caries Detector	Safe		No May Be
2 2M	Caries Detector Aiming Beam	Blink or Aversion Response Potential Hazards		No May Be
3r	Low Level Laser			No
3b	Low Level Laser	Eye Damage		Yes
4	ARGON 488-514nm DIODE 810-980nm Nd:YAG 1064nm Er,Cr:YSGG 2,780nm And Er:YAG 2940 Nm Co2 10,600nm	Retinal Lesion And Corneal Damage	Excessive Dryness And Skin Burns	Yes

Table 2: Laser Safety Classification

Hazards of Laser:

1. Ocular Hazards

Potential injury to the eye can occur either by direct emission from laser or from the reflection from mirror like surfaces. Dental instruments have been capable of producing reflections that may result in tissue damage to both the operator and the patient. The use of carbonized or non-reflective instruments has been recommended during laser treatment. Several structures of the eye may be injured as a result of laser emissions. The site of injury is directly dependent on the preferential absorption of various wavelengths by specific structures of the eye. The primary ocular injury that may result from a laser accident is a retinal or corneal burn. Retinal injury is possible with emissions in the visible and near infrared spectral regions. Even low intensity beams can cause damage because of the focusing effect of the lens and cornea. Approximately 95% of the incident radiation entering the eye is absorbed by pigmented epithelium of the retina and choroids layer. Irreversible retinal burns resulting in permanent blindness can occur by conversion of incident radiation to heat energy within a fraction of a second. Other potential ocular injuries from various wavelengths may occur e.g., injury to the sclera, aqueous humor, cataract etc.

2. Tissue Damage

Laser induced damage to the skin and other non-target tissue (oral tissue) can result from thermal interaction of radiant energy with tissue proteins. Temperature elevations of 21ºC above normal body temperature (37ºC) can produce cell destruction by denaturation of cellular enzymes and structural proteins, which interrupts basic metabolic processes. Histologically, the thermal effect of absorbed radiant energy is manifested as thermal coagulation necrosis for wavelengths above 400 nm.

Other non-thermal tissue interactions are thought to induce injury through photochemical and photo acoustic mechanisms. They occur with single or repetitive pulses of very soft duration. Although they have been no reports of laser-induced carcinogenesis to date, the potential for mutagenic changes have been questioned. Of clinical significance is the potential damage to deeper tissue from penetration of specific wavelengths such as the continuous wave Nd: YAG laser. With prolonged exposures of low power density from this type of laser, excess thermal necrosis that may not be apparent at the tissue surface can occur.

3. Respiratory/Environmental Hazards

Another class of hazards involves the potential inhalation of air borne biohazard: materials may be released as a result of surgical application of lasers. These secondary hazards belong to a group of ‘potential laser hazards’ (also called as ‘non beam hazards’). They do not pertain to injuries resulting from direct exposure to laser beam. Inhaled air borne contaminants can be emitted in the form of smoke or plume generated through the thermal interaction of surgical lasers through tissue or through the accidental escape of toxic chemical and gases from the laser itself.

Toxic gases and chemicals are a more common hazard in dental research facilities and laboratories. Some of the toxic gases and chemicals are: fluorine, hydrochloride gases, toxic dyes and solvent. A study by the National Institute for Occupational Safety and Health (NIOSH) evaluated the air that operating room personnel were exposed to during laser procedures and found that detectable levels of ethanol, isopropanol, anthracene, formaldehyde, cyanide and airborne mutagenic particles were found. Inhalations of these toxic aerosols have been found to be potentially damaging to the respiratory system.

4. Combustion Hazards:

In the presence of flammable materials, laser may pose other significant hazards. Flammable solids, liquids and gases used within the dental surgical setting can be easily ignited if exposed to the laser beam. Toxic fumes released as a result of combustion of flammable materials present an additional hazard. The use of flame resistant materials and other precaution is therefore recommended. Some of the common flammable materials found in the dental treatment areas are:

• Solids: clothing, paper products, plastic, waxes and resins.
• Liquids: ethanol, acetone, methyl methacrylate, solvents.
• Gases: oxygen, nitrous oxide, general anesthetics, aromatic vapors.

5. Electrical Hazards:

Most laser systems involve high potential, high current electrical supplies. The most serious accidents with lasers have been electrocutions. There are several associated hazards that may be potentially lethal.

Electrical hazards are grouped as:

• Shock hazards
• Fire hazards or explosion hazards

Safe manufacturing practices offer adequate protection from these hazards. Insulation, shielding, grounding and housing of high voltage electrical components provide adequate protection under most circumstances from electrical injury. Installation and servicing of laser equipment should always be performed by qualified personnel and not by the dentist. It is a good practice to have at least two persons in an area while working on high energy power systems. In labs where laser power supplies are opened or serviced by lab personnel, dental and other auxiliary staff should be trained in cardiopulmonary resuscitation^{. (14)}

Laser Hazard Control Measures:

According to OSHA guidelines and ANSI standards, for the safe use of lasers in dentistry, control measures are required to reduce the possibility of unwanted exposure of patient and personnel to laser radiation.

Four categories of control measures are:

1. Engineering controls
2. Personal protective equipment
3. Administrative and procedural controls
4. Environmental controls

1. Engineering Controls:

Engineering controls are normally designed and built into the laser equipment to provide safety.

Engineering controls such as enclosures, interlocks and beam stops are very effective at eliminating hazards (if not defeated). Some of the important engineering controls recommended by the ANSI are as follows.

• Protective Housing: A laser shall have an enclosure around it that limits access to the laser beam or radiation at or below the applicable Maximum Permissible Exposure (MPE) level. A protective housing is required for all classes of lasers except, of course, at the beam aperture.
• Master Switch Control: All Class IV lasers (including dental lasers) and laser systems require a master switch control. The switch can be operated by a key or computer code. When disabled (key or code removed), the laser cannot be operated.
• Optical Viewing System Safety: Interlocks, filters, or attenuators are to be incorporated in Conjunction with beam shutters when optical viewing systems such as telescopes, microscopes, viewing ports or screens are used to view the beam or beam-reflection area.
• Beam Stop or Attenuator: Class IV lasers require a permanently attached beam stop or attenuator which can reduce the output emission to a level at or below the appropriate MPE level when the laser system is on ‘standby’.
• Laser Activation Warning System: An audible tone or bell and/or visual warning (such as a flashing light) are recommended as an area control for Class III b laser operation. Such a warning system is mandatory for Class IV lasers.

2. Personal Protective Equipment:

All people within the dental treatment room must wear adequate eye protection, including the patient. When selecting the protective eyewear, several factors should be considered. They are as follows.

1. Wavelength of laser emission
2. Maximum permissible exposure limits
3. Degradation of absorbing media or filter
4. Optical density of the eyewear
5. Radiant exposure limits
6. Need for corrective lenses
7. Multiple wavelength requirements
8. Restriction of peripheral vision
9. Comfort and fit

Optical density is one of the most important factors to consider when choosing laser eye protection. The attenuation should be to reduce the beam exposure to the eye to relatively safe levels. Laser protective eyewear is intended to provide a level of protection that may be used to stare directly into the beam.

3. Administrative and Procedural Controls:

 If general anesthesia is administered during any dental procedure, in place of the standard P.V.C Intubation tube, a red rubber or silastic tube should be used. For further protection, the tube can be wrapped with 1/3-1/2-inch aluminum tape. Highly reflective instruments and those with mirrored surfaces should be avoided since they cause damage to the non-target tissues. A wax spatula or periosteal elevator can be inserted into the gingival sulcus to serve as an effective shield when lasing soft tissue near teeth. For most applications it may be advisable to use low power time settings initially before progressing to higher and faster times. When lasers are not actually been used for treatment or if long pauses occur between use, the unit should be placed in a standby mode to prevent inadvertent firing of the laser beam. Most manufacturers provide a cover or metal hood to prevent accidental activation of the laser beam. The foot switch should be cleaned and inspected prior to use to avoid getting stuck in a position while operating.

Most laser accidents and injuries can be prevented if appropriate control measures are recognized and implemented.

4. Environmental Controls:

Evaluation of environmental hazards involves an assessment of three primary aspects of the laser. Treatment area that should be considered to establish adequate control measures for the particular application.

These include:

1. Physical environment in which the laser is used should be confined to controlled areas with restricted access. Use of protective laser curtains should be considered to prevent accidental exposures to passers-by. Fail-safe mechanisms that prohibit the laser from firing when doors are opened are also useful to prevent accidental exposure to persons entering the operating room during laser procedures. All entrances to the operating theatre should be clearly marked with a removable warning signs that contains the words ‘Danger’ and ‘Laser Radiation.’ Highly reflective instruments and surfaces should be avoided to prevent reflection of the laser beam onto the non-target tissues.
2. Potential for injury attributed to direct exposure from the laser beam output and delivery mechanism. To avoid an electrical hazard during the operation of the laser unit, the floor of the operating room must be kept dry. Because laser energy generates heat, care must be taken to avoid the use of flammable and explosive liquid or gases in the operating room. Flammable materials such as surgical drapes and gauze sponges may be soaked in sterile saline to reduce the potential of burning by accidental exposure to the laser beam.
3. Level of training and knowledge of laser safety of the persons all staff members should receive objective and recognized training in the safety aspects of laser use within dentistry, as with other specialties. The ANSI Z136.1 standard states that the management (employer) has the fundamental responsibility for the assurance of the safe use of lasers owned and/or operated by the employer. There are three main types of training programs-
   • • Laser Safety Awareness
     • Laser Safety Refresher
     • Laser Safety Update ⁽¹⁴⁾

Role of Laser Safety Officer (LSO):

The Laser Safety Officer (LSO) is an individual designated to be responsible for a laser or system of lasers and for the preparation and enforcement of a safety plan, including standard operating procedures for the safe operation of lasers. The LSO has the authority and responsibility to monitor and enforce the control of laser hazards and to effect the knowledgeable evaluation and control of laser hazards. Dental practices offering Class IIIb and IV laser treatment, must appoint a laser protection advisor (LPA) and a LSO.

The LPA is usually a medical physicist who will advise on the protective devices required, MPE and nominal ocular hazard distance (NOHD) for any given wavelength being used. The LSO is appointed to ensure that all safety aspects of laser use are identified and enforced. Ideally, this could be a suitably trained and qualified dental surgery assistant. According to ANSI, the LSO and other employees routinely working with or around lasers are strongly recommended to participate in laser safety training programmes. ⁽¹⁴⁾

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