Application of Laser Irradiation for Restorative Treatments

The Effects of Laser Irradiation on Healthy Tooth Tissues

The effect of laser irradiation on enamel is observed as follows: at temperature 650°C to 1100°C, decrease in the enamel solubility would be enhanced, which highly depends on the calcium-phosphate ratio. At 1100°C, new crystalline phases would be formed by tetra Calcium Diphosphate Monoxide (alpha tri-calcium phosphate and beta-phases), which is less soluble, with less carbonate contents, and more resistant to demineralization procedures [8]. In the other aspect of view, lasers might induce surface changes in enamel like surface roughness, crazing, cracking and exfoliation [7].

Laser appliances remove dental tissues selectively by causing water evaporation that is unique for mineralized tissue. The Er:YAG laser makes thermal changes in water molecules of the hard tissue, so the absorbed energy results in superheating and evaporation. The increased vapor pressure causes exfoliation and micro explosive expansion of the hard tissue [9]. The laser irradiation influences the dental tissue by making microscopic coarse surfaces without demineralization, and provides open dentinal tubules without formation of smear layer. Also, it can sterilize the surface significantly [10].

The adverse effect of laser devices on the pulpal health might cause severe damages to the pulp due to thermal damage during tooth preparation, such as irreversible pulpitis and pulp necrosis [11]. However, due to demands for ideal laser instruments, researchers identified the appropriate wavelength that made selective impact to tooth tissue by selective absorption of pulsed radiation (Photothermolysis) [12]. Also, water cooling is recommended to reduce heat during tooth preparation [13]. Some animal [14] and human studies [15] showed appropriate response of the pulp to the Er:YAG laser. Niar et al. [12] observed outstanding preservation of pulpal health by means of Er:YAG laser at 2.94 μm wavelength. In another study by Nair [11], the CO₂ laser caused minimal and mild inflammatory responses to pulpal tissue.

The Effect of Lasers on Carious Structures

Re-mineralization and Caries Removal

Dental caries can be installed by laser irradiation due to its effect on chemical, physical and crystalline properties of the enamel. Enamel remineralization can be induced by laser irradiation at a specific wavelength. Laser irradiation might reinforce the enamel structure by causing physical fusion of the surface and reducing enamel solubility by melting, sealing and re-crystallization [7]. Ultra-structural alterations in the enamel crystals might end in reduced enamel solubility. These noticeable enamel alterations are: reduction of water and carbonate content, increased amount of hydroxyl ion contents, formation of pyrophosphates, and decomposition of proteins [20]. Esteves-Oliveira et al. stated that CO₂ laser was capable to eliminate the enamel caries progression [21]. Also, it is reported that Pulsed CO₂ laser irradiation caused enamel surface fusion and inhibition of the subsurface caries-like lesions with 50% success rate [22]. Moreover, increased micro-hardness of enamel surface was detected by several studies after laser application [7, 23].

Laser removes selectively the carious lesions because they contain higher amounts of water than sound dentinal tissue. Laser can produce three sets of absorption bands in dentine, however the 6 μm absorption band attributes to the dominant organic materials of carious structure. More specifically, 5.8 μm of wavelength can positively remove the carious tissue with minimal damage to healthy dentine [24]. Whereas, the Er:YAG and Er,Cr:YSGG lasers operate at 2.94 and 2.79 μm [25], and pulsed CO₂ laser prepares high-speed and precise ablation of dental caries at wavelengths of 9.6 and 9.3 μm [26]. Short pulses and high dentinal absorption reduce the over heat and thermal damage of the pulp tissue. Ishii et al. tried out a nano second pulsed laser at wavelength of 5.85 μm for selective caries removal. Their results manifested minimal damage to sound dentin and successive removal of less Ca content substrates, like carious structure [27].

The ideal cavity preparation relies on removing the affected dentin without sacrificing some healthy or at least potentially remineralizable tissues [28]. However, literature disputes about the mentioned fact for the Er:YAG. Raucci-Neto et al. reported that Er:YAG laser remained demineralized dentine with a network of ultra-structural collagen matrix [29]. Whereas, another study [30] has claimed that Er:YAG laser removes carious tissues non-selectively in which high volume of residual caries is remained in some specimens, and tissues are removed excessively in some others. Different irradiation parameters, specially the frequency, have remarkable effects on the quality of caries removal. For instance, higher frequencies reduce the selectivity removal of the demineralized dentin tissues [29].

Restorative Qualities after Laser Irradiation

The Er:YAG laser tends to leave dentinal tubules open with minor smear layer production during cavity preparation. Hence, higher dentin permeability could be achieved by laser rather than the rotary device or the manual excavation [31, 32]. The Er:YAG laser makes dentinal water evaporates. Also, it causes some structural changes in composition of collagen (incorporation of an OH radical) [33, 34], which may in turn promote the increased dentin permeability.

Some experiments are focused on the marginal leakage and bonding strength of composite resins to prepare dentin by laser irradiation. Korkmaz Y et al. studied the marginal leakage of different adhesive systems in cavities prepared by Er:YAG laser. Their results reflected higher microleakage in cervical margins specially in cementum and dentin [35] which might be related to the technique sensitivity of bonding systems and the quality of formed hybrid layer in that regions [10]. Kimyai et al., surveyed the microleakage of three different adhesive systems in class V cavities prepared by Er,Cr:YSGG laser. They found higher microleakage of gingival margins rather than occlusal margins, specifically in self-etched adhesive system rather than etch and rinse two step (total etch) [36]. In contrast, Baghalian et al. conducted a study to compare the microleakage of restored class V cavities prepared by Er:YAG laser and conventional burs. They claimed that preparation by Er:YAG laser demonstrated a better marginal seal on occlusal and gingival margins [37]. Another study confirmed mentioned results, too [38]. It has been noticed that the degree of microleakage might be affected by several factors such as the type of adhesive systems, the type of self-etching adhesive, the cavity margin location, and the technique of preparation [37]. Also, higher microleakage of cervical margins might be due to the lack of dentinal tubules in the first 100 nm of cervical margins and higher organic content of the dentine [39].

Bond strength of composite resin is another concerning issue in released literature [40-44]. Some studies experienced “laser etching effect” by irradiating laser to prepare sufficient surface roughness and permitting mechanical interlocking with adhesive resins [22, 45]. However, for two or three step etch-and-rinse adhesives, phosphoric acid etching has been suggested prior to the application of mentioned adhesive systems to enhance higher bond strength [46].

Parhami et al. compared the effect of Er:YAG laser and high speed diamond burs on the shear bond strength of flowable composite to enamel. The final conclusion was that the Er:YAG laser preparation reduced shear bond strength of tested composite resin compared to the conventional cavity preparation [40]. Shirani et al reported negative effects of Er:YAG laser irradiation on shear bond strength of composite resins, either [42]. In contrast, Altunsoy et al. evaluated the effect of different modes Er:YAG laser on the micro tensile bond strength of two different self-etch adhesive systems and flowable composite resins to dentin. Their final results indicated that pulse mode laser irradiation increased the micro tensile bond strength of both adhesive systems and flowable composite resins [44]. Moreover, other researches notified the improved bond strength of composite resins after laser irradiation [47-49]. Both peri- and intertubular dentine would be demineralized during acid etching and collagen exposure. This procedure needs enough surface moisture after etchant removal, otherwise hybrid layer formation would decrease because of the collapse of collagen fibers [50]. On one hand, the Er:YAG laser does not demineralize dentin surface and no collagen will be exposed [10], in the other hand, remaining peri tubular dentin after irradiation might be helpful to obtain stronger adhesion to the dentin due to its more mineral content in comparison with inter tubular dentine [51].

Vital Pulp Therapy

Application of laser instruments was studied on vital pulp therapy and pulpotomy of primary teeth. Low intensity lasers can be indicated for vital pulp therapy with the success rate up to 90% in comparison with traditional methods [52]. The advantage of laser therapy is based on its bactericidal activity, hemostatic activity, reduction in elevated pulp temperature, reducing inflammation, stimulating dentinogenesis [53, 54], healing improvement by enhancing collagenic protein, formation of a fibrous matrix, and inducing hard tissue barriers [54-56]. In an animal study by Handa et al. [57], three different strategies for Direct Pulp Capping were evaluated. They concluded that there was no significant difference among utilization of Er:YAG laser + MTA, Er:YAG laser + Ca(OH)₂, and MTA. However, Er:YAG + MTA had better results. Based on this study, Er:YAG laser could be useful in combination with two common Direct Pulp Capping materials.

In some animal study the effect of lasers on the pulpotomy process was evaluated. An investigation by Shoji et al. [58], showed that the CO₂ laser led to no detectable damages to the dogs’ dental pulp. In some human studies, laser application in comparison or with incorporation of some common methods was evaluated. In a study by Elliott et al. [59], the CO₂ laser had comparable response than formocresol in human primary teeth. In another study, the clinical success rate of palpotomy was compared between the Nd:YAG laser and formocresol. The results revealed that the Nd:YAG laser had significantly higher success rate than formocresol [60].