15: Low-Level Lasers in Dentistry

15 Low-Level Lasers in Dentistry

The wavelengths of surgical lasers (Nd:YAG, CO2, erbium, diode) affect tissues not only through ablation, coagulation, and vaporization, but also through stimulation of natural healing processes in the cells. Other lasers, at much lower power than the surgical lasers, act more as “biostimulators.” This chapter discusses the most useful indications for these lasers, often referred to as “therapeutic lasers.” This therapy is generally called low-level laser therapy (LLLT), although the nomenclature is somewhat controversial. The instruments are generally referred to as “therapeutic lasers” or “cold lasers” versus the “surgical lasers.”

Therapeutic Lasers

The therapeutic lasers are typically found in the visible red to near-visible infrared (IR) part of the electromagnetic spectrum, from 630 to 980 nanometers (nm). Output powers typically range from 50 to 500 milliwatts (mW) with either pulsed or continuous-wave (CW) emission. The names of therapeutic lasers, as with surgical lasers, are derived from the active medium, such as the gallium-aluminum-arsenide (GaAlAs) laser.

The simplest way to classify therapeutic lasers is by wavelength. Penetration depths vary; lasers in the red part of the spectrum are more superficially absorbed, whereas IR lasers penetrate as much as 3 to 5 cm, depending on wavelength and target tissue. There is an “optical window” at about 820 nm, which has the greatest depth of optical penetration. Mucosa is quite transparent to the wavelengths (does not absorb light well), skin and bone are fairly transparent, and muscles have the greatest absorption of light. Dosage at the target tissue must be calculated accordingly. Another factor in depth of penetration is the distance from the target tissue, which affects the spot size (see Chapter 2). Irradiation out of contact versus irradiation in contact versus irradiation with pressure on the tissue all deliver different dosages to the tissue. Laser irradiation with tissue pressure causes a slight ischemia in the area, which reduces the hemoglobin concentration in the spot (Figures 15-1 and 15-2).

Mechanisms

The advantage of therapeutic laser light is that it stimulates natural biological processes and mainly affects cells in a decreased oxidation-reduction (redox) reaction. A cell in a low redox stage is acidic, but after laser irradiation the cell becomes more alkaline and able to perform optimally. Healthy cells cannot significantly increase their redox situation and thus will not react strongly to the laser energy, whereas cells in a low redox situation will be stimulated.1,2 The most essential effect may be the increase of adenosine triphosphate (ATP), the “fuel” of the cells, produced in the mitochondria.3 ATP is the end product of the Krebs cycle, where the photon-acceptor enzyme cytochrome-c oxidase is inhibited by nitric oxide (NO). Laser light will dissociate the binding between NO and cytochrome-c oxidase, allowing it to resume ATP production.4 This basic mechanism initiates a cascade of cell signaling, leading to an optimization of body functions.5

Dosage

The most difficult part of LLLT is to find the optimal dosage. The tissue dosage is expressed in fluence, or energy density, measured in joules per square centimeter (J/cm2). Multiplying the output power of the laser in milliwatts by the time of exposure in seconds equals the produced energy; for example, 50 mW × 40 seconds = 2000 millijoules (mJ), or 2.0 J.

Now that we have the energy (2 J), we need to know the size of the area being irradiated. If we are irradiating an area of 2 cm2, the calculation is 2 J over an area of 2 cm2, or 2/2 = a fluence, or energy density, or superficial tissue dose of 1 J/cm2. Suppose the irradiated area was only 0.5 cm2. An inverse relationship exists between spot size (size of irradiated area) and fluence. Decreasing the size of the irradiated area increases the fluence: 2 J divided by 0.5 cm2 = 4. Thus the dose becomes 4 J/cm2, because the energy was emitted over a smaller area, increasing the local intensity. Because the dose depends greatly on the spot size, a thin light probe will create high doses in J/cm2.However, this does not necessarily mean that the energy applied to the tissue is high, only that the intensity of the light energy at the emitting end of the thin probe is high.

Now that we understand the difference between energy and dose, we face a more complex calculation: dose at target. If the target is 1 cm below the surface, there could be reflection, scattering, and absorption of the energy before it arrives at the target. It is therefore necessary to consider the depth of the target area and the type of tissue between the light and the target tissue. The main absorbers of these wavelengths are pigmented chromophores, such as the hemoglobin in blood; therefore highly vascular tissue will absorb these wavelengths well, and less vascular tissue will absorb these wavelengths poorly. Mucosa is transparent to these low-level wavelengths; bone is also rather transparent, whereas muscle tissue, with its rich blood supply, is not. Another complicating factor is the amount of another chromophore in the target tissue, melanin. Because melanin is a strong absorber of these wavelengths, more light energy may be superficially absorbed rather than reaching deeper tissue, which may create local heating and even pain.

Therefore, the use of J/cm2 to describe the dose in owner’s manuals can be confusing. The J/cm2 (dose, fluence) indicates the intensity at the surface of the tissue but not the dose at the underlying target. A simpler approach is to use the term energy per point, calculating only the number of joules in each point. For clinical use, this is acceptable, but for scientific investigations, unacceptable. A “point” is often referred to as the size of the tip of the laser probe (spot size). The relationship between spot size and power density holds true for low-level lasers as well. A small spot size creates a larger concentration of the power per square millimeter or centimeter of tissue irradiated, whereas a wider spot size dilutes the same energy over a larger area.

Acute vs. Chronic conditions

The general rule is to apply high doses of laser energy to acute conditions that present with inflammation and edema, treating chronic conditions (e.g., wounds, paresthesias, pain) more conservatively. Initially, acute conditions can often be treated until they resolve, whereas chronic conditions should be treated only once or twice weekly. Low-level laser doses are cumulative, which means that the dose given on day 1 lingers in the tissues on day 2 and adds to the long-term buildup of inhibitory levels.

Patients with long-standing chronic pain conditions may experience a flare of pain after LLLT. This is transient and actually shows that the patient is responding well to the treatment. The chronic situation is believed to be transformed into an acute phase, allowing healing to begin. Pain levels are reduced below baseline within 24 hours. The patient should be informed about this possibility before treatment. Other chronic pain patients may respond with deep fatigue, interpreted as an accumulated lack of rest, surfacing when the pain subsides. As with pharmaceuticals, the reaction and dosage must be individualized for each patient (Figure 15-6).

Pulsing

The importance of pulsing the light is obvious when applied to cell monolayers in the laboratory.911 The pulse repetition rate (PRR) also helps in the clinical setting, although little is known about pulsing in vivo and animal/clinical studies are inconclusive. It is not yet known how to control these mechanisms through different PRRs. Also, the biological effects of a “chopped” CW laser beam and a superpulsed beam are different. At this time, use of a continuous beam is therefore recommended in units that have CW emission. The 904-nm GaAs laser does not have a CW mode, so a PRR must be selected, relying on anecdotal evidence to select pulse parameters.

Moriyama et al.12 found more expression of the inducible nitric oxide synthase (iNOS) gene after 905-nm superpulsed laser irradiation. This suggests a different mechanism in activating the inflammatory pathway response in superpulsed mode compared with CW mode.

Side Effects and Contraindications

Doses of laser energy near the therapeutic window will not cause negative effects. The worst result with LLLT is that nothing happens. There are few absolute contraindications for LLLT, but a few caveats.

Because LLLT affects blood flow in undefined ways, irradiation of patients with coagulation disorders should be avoided.13,14 Presence of known malignancies is another contraindication, because LLLT stimulates cell growth. The literature also discusses pregnancy as a contraindication,15 although dentists work exclusively in the oral and head/neck regions. Also, although sometimes listed as a contraindication, pacemakers are electrical and not influenced by light. Some traditional safety regulations and contraindications seem to have been transferred from electrosurgical and other therapies to surgical lasers.

A contraindication especially relevant to dentistry is irradiation over the thyroid gland, located within the dental treatment area. Dentists generally are not informed about possible hyperthyroid or hypothyroid conditions, so direct irradiation over this area should be avoided. However, LLLT for thyroid disorders has been studied.16

Choosing the “Right” Laser

The lasers available not only offer different wavelengths but also different combinations of wavelengths, powers, and probe sizes. Some are battery operated and therefore small and handy, but with all the usual battery problems. Lasers plugged into sockets are more stable and durable, but the cords might further obstruct a cluttered treatment room.

The choice of a laser is therefore not “one size fits all.” Any therapeutic laser can be used for many indications, but some combinations of wavelength and power are optimal for selected indications; clinicians should focus on their special interests. For intraoral tissue regeneration, red and IR lasers with outputs below 100 mW are useful. For muscular treatment and temporomandibular joint pathology, 300 to 500–mW, 810-nm diode or even GaAs diode lasers provide better results. The output power can be adjusted in some lasers. When choosing a laser, other practical considerations include probe sterilization, education, technical service, and guarantee period.

Biostimulation

Although dental lasers such as the neodymium-doped yttrium-aluminum-garnet (Nd:YAG), carbon dioxide (CO2), and erbium family are considered “hard” or “surgical” lasers, they may be producing a degree of biostimulation in the areas peripheral to the focal spot, where the energy is reduced to stimulatory levels (Figure 15-8).

Some of the positive effects observed with the hard lasers may be explained by biostimulation. Pourzarandian et al.20,21 reported stimulation of human gingival fibroblasts by low-level Er:YAG irradiation, as well as increased prostaglandin E2 (PGE2) production through induction of cyclooxygenase-2 (COX-2) messenger ribonucleic acid (mRNA) in human gingival fibroblasts. Additionally, Er:YAG lasers may be used at lowest output and scanned at a slight distance (out of focus) to produce biostimulation. The problem with using these hard lasers in this manner is that no microprocessor informs the practitioner how to control the dosage, and the fibers are not adapted for biostimulation.

Biostimulation is not limited to the traditional near-IR wavelength window; such effects are even reported when using defocused CO2 lasers.2224 The CO2 laser has an extremely poor penetration through tissue because it is so well absorbed at the surface, and the biological effects reported on deeper tissues may first appear improbable. However, the coherent light is absorbed in peripheral microvessels, and the clinical effect observed shows that LLLT has primary effects at the target as well as systemic effects through blood and lymph circulation. Thus, with minimal calculation, the owner of a “hard” laser can have a “soft” laser for free. Least complicated is the surgical diode laser, with simpler calculations and wavelengths within the traditional range of biostimulation.

Curing Light

All dentists have a composite curing light, typically with a wavelength peak of about 470 nm. Energies delivered are within the low-level laser therapeutic window, but studies of the photobiological effect on the surrounding tissues are largely lacking.

In a 2009 in vitro study, Enwemeka et al.29 demonstrated that light-emitting diode (LED) energy with a 470-nm peak successfully kills methicillin-resistant Staphylococcus aureus (MRSA). Irradiation produced a statistically significant, dose-dependent reduction in both the number and the aggregate area of colonies formed by each strain of cells. The higher the dose, the more bacteria were killed, but the effect was not linear, being more impressive at lower than at higher doses. Almost 30% of both strains were killed with as little as 3 J/cm2. As much as 90.4% of the two different colonies were killed with an energy density of 55 J/cm2. This study requires further in vivo investigation but suggests an innovative use of an available dental light source to treat MRSA infection, which has high morbidity and high mortality.

Noncoherent Light

Blue light, as in curing lights, has a bactericidal effect in general. Importantly, curing lights are noncoherent, whereas lasers are coherent. Whether or not coherence is important has long been discussed, and some claim coherence is not needed. This misunderstanding is based on in vitro studies using cell monolayer that emphasize wavelength and intensity, as discussed earlier.5 The in vivo situation is different, however, and thus far a relative equivalence between coherent and noncoherent light has only been documented for superficial conditions such as open wounds.

All light has a biological effect, even broadband noncoherent light, given the correct parameters and conditions. Studies comparing coherent and noncoherent light for deeper tissues to date favor coherent light. The excellent biological effects of noncoherent light have not been compared to coherent light.

The length of coherence is also important. The length of coherence from a gas laser such as a helium-neon (HeNe) laser is very long, whereas that from a diode laser of the same wavelength is much shorter. A comparison of the effect on gingivitis using HeNe on one side and red diode laser on the other side (both lasers emitting at the same wavelength) showed better results using the longer coherence but same wavelength HeNe. The difference probably can be minimized using higher energies with the diode. Coherence is not lost when light penetrates tissue. However, the length of coherence is split into small, coherent spots called “speckles,” found in all areas reached by the laser light.

Acupuncture

Few dentists use acupuncture and, unless trained, should avoid it. However, the therapeutic laser seems to produce effects similar to those of acupuncture needles, with some safe points to use. For example, P6 (Neiguan) on the wrist is an excellent point to reduce nausea and vomiting.30 The location of this point is 2.5 cm up from the wrist and 0.5 to 1.0 cm deep. The point is between two tendons. Delivery of 3 to 4 J to this point often allows a more relaxed environment for taking impressions or working in the molar area, especially in patients with a heightened gag reflex. Another easily accessible point is the Li4 (Hegu), a pain-reducing point located in the middle of the second metacarpal bone on the radial side (Figure 15-9).

Functional magnetic resonance imaging (fMRI) has confirmed the similarity between needles and lasers on the same acupuncture point.31 However, understanding this phenomenon becomes more difficult because there is no “qi” effect with the laser. The “gate control” theory would not explain the effects because there is no pain stimulus in the acupuncture point when using a laser.

Dental Indications

Because LLLT can influence so many pathological conditions, the use of the therapeutic laser is not limited to the following indications. More than 30 conditions have been described in the dental literature; the most important are briefly described here.

Anesthesia

Applying LLLT to the mucosa before an injection results in a slight anesthetic effect, although this cannot be obtained in the hard palate.32 LLLT applied before the injection will also improve healing, should the needle cause trauma to a vessel or nerve. LLLT improves local microcirculation,33,34 so the effect of the numbness can be shortened if LLLT is applied to the site after completing the dental procedure. From 4 to 6 J is needed in both cases.

Aphthous Ulcers

The healing time of aphthous ulcers can be shortened and the immediate pain reduced by administering 4 to 6 J over the lesion.35,36 LLLT does not seem to be as effective for aphthous ulcers as the more aggressive approach using surgical lasers (see Chapter 6). Patients with aphthous ulcers should avoid toothpastes containing sodium lauryl sulfate, which may trigger these lesions in predisposed persons.

Endodontics

Use of LLLT does not have a bactericidal effect, and surgical lasers are the preferred instruments to reduce bacteria in infected root canals. Few manufacturers of therapeutic lasers offer probes that could reach the root canals. However, IR light can reach all apices, and visible red light can reach the more superficial apices through the mucosa and produce an antiinflammatory and pain-reducing effect.

Sousa et al.42 analyzed the effect of LLLT on the secretory activity of macrophages activated by interferon gamma (IFN-γ) and lipopolysaccharide (LPS) and stimulated by substances leached from an epoxy resin–based sealer (AH-Plus) and a calcium hydroxide sealer (Sealapex). The production of tumor necrosis factor alpha (TNF-α) was significantly decreased by LLLT, regardless of experimental group. The level of secretion of matrix metalloproteinase-1 (MMP-1) was similar in all groups.

Laser application after overinstrumentation and overfilling is a good example of an indication for LLLT in endodontics. Because LLLT can stimulate bone formation, apical bone resorption probably can heal faster after completed endodontic treatment if LLLT is applied. Energy needed is related to the depth of the apex, ranging from 4 to 8 J per apex. For the same reason, intraoperative and postoperative LLLT in apical surgery has potential,43,44 but solid scientific documentation is still lacking. However, irradiation over the suture line will stimulate fibroblast proliferation and increase tensile strength.45

In patients with acute pulpitis, when the affected tooth or root is difficult to pinpoint, the laser can be applied over the apices in the involved area. The affected tooth may react by a pain increase, probably because of increased microcirculatory pressure in the pulpal chamber. LLLT can also be used as an adjunct therapy in pulp capping46,47 and pulpotomy48,49 (see Chapter 12). In both cases the exposed pulp is irradiated at low intensities, applying 2 to 3 J before traditional methods are used. The irradiation will reduce inflammation, preserve odontoblastic integrity, and stimulate cellular proliferation.

Extractions

Nontraumatic approaches and good postoperative procedures are the key to satisfactory healing after extractions. However, the occasional complication is unavoidable. Adding LLLT after the extraction will reduce the inflammatory phase, induce pain reduction, stimulate the fibroblasts in the wound periphery, and stimulate the osteoblasts in the socket.5055 High doses of laser energy can focus on the postoperative pain, but to obtain a good healing process, lower power and longer time are more advantageous.

The major goal of LLLT after extractions is to stimulate the fibroblasts to seal the socket. In cases of failure (dry socket), the traditional methods are used in combination with high doses of LLLT to reduce patient discomfort. When the dressing is changed during subsequent appointments, lower doses are given to stimulate fibroblast growth. Not only does LLLT stimulate fibroblast proliferation, but the cells are arranged in parallel bundles as well, creating a smoother area.2 This is of particular interest in extraoral surgery when the cosmetic aspect is important (Figures 15-11 to 15-13).

Herpes Simplex Virus

Patients with a herpes simplex virus type 1 (HSV-1) eruption may be reluctant to visit the dentist. However, LLLT is the most efficient method of treating this infection.36,5658 In particular, if treated during the initial prodromal stage (when the patient feels the initial tingling), the healing will only take a few days or may even disappear within hours. Importantly, patients with recurrent HSV-1 attacks will experience longer intervals between the outbursts. Large blisters can be opened with surgical lasers (e.g., erbium, CO2) to empty the fluid/>

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Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 15: Low-Level Lasers in Dentistry
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