today , acid etching is the most common method of preparing enamel for bonding orthodontic brackets .
although this method results in high bond strength , its great disadvantage is the potential for caries formation .
acid etching removes and demineralizes the most superficial layer of enamel and makes the teeth more susceptible to long - term acid attack , especially when resin monomers can not sufficiently fill the demineralized area due to saliva contamination or air bubbles .
considering the high prevalence of white spot lesions in orthodontic patients , and with respect to the fact that white spot lesions can form in as early as 4 weeks in the presence of inadequate oral hygiene , prevention of enamel demineralization is of great importance during treatment .
there have been ample attempts to find a method to decrease the incidence of white spot lesions in orthodontic patients .
fluoride mouth rinses have been shown to significantly reduce caries formation , but excellent cooperation in using a mouth rinse can be achieved in only 13% of the patients .
another method to prevent demineralization is to use fluoride - releasing composite resins and conventional or resin - modified glass ionomer cements , but the lower bond strengths of these materials compared with conventional acid etching[68 ] have raised questions on their clinical efficiency .
the physical and chemical changes in laser - irradiated surfaces hold promise for prevention of enamel demineralization .
although different laser systems have been used in previous studies for conditioning dental hard tissues,[914 ] it is well clear that with available technology , only erbium family lasers ( er : yag and er , cr : ysgg ) are suitable for this purpose .
the wavelength of er : yag laser is highly absorbed by water and hydroxyapatite , making it suitable for both hard and soft tissue ablation .
laser irradiation removes smear layer and creates surface roughness ; both of them are favorable for monomer infiltration and adhesion process .
reported an increase in the calcium to phosphorus ratio during laser irradiation , which resulted in caries inhibition . in the study of kim et al .
, laser - treated enamels showed improvement in crystalline structure and had the lowest mineral dissolution compared to control and phosphoric acid - etched specimens .
another study showed that er : yag laser treatment reduced the carbonate content and modified the organic matrix , thus providing caries - preventive effect on enamel .
however , some studies did not find any significant difference between er : yag - lased and non - lased groups with respect to the enamel demineralization .
observed that er : yag laser was unable to achieve any notable reduction in acid solubility of dental enamel .
some authors concluded that the application of sub - ablative erbium lasers solely for preventive caries treatment does not seem to be sensible under the conditions they studied . if it was possible to reduce the prevalence of demineralization , while using er : yag laser for etching the enamel surface , this would be of great advantage for orthodontic patients .
therefore , the objective of this study was to evaluate the caries resistance potential of er : yag laser - etched surfaces after exposure to an in vitro caries challenge , compared to acid - etched specimens .
in this in vitro study , 50 intact and caries - free human premolars were used .
any remaining soft tissue was removed with a scaler and the teeth were kept in 0.1% thymol solution to prevent bacterial growth .
the teeth were randomly assigned into two equal groups based on the etching procedure . prior to the experiment
, the roots were sectioned 2 mm below the cementoenamel junction using a high - speed drill and a diamond bur , to facilitate further microscopic sample preparation .
the enamel surfaces of the teeth were cleaned with pumice slurry and rubber prophylactic cups , then rinsed with running water and dried with a moisture - free air syringe . to limit and standardize the etching area ,
the facial surfaces of the teeth were painted with a thin coat of nail varnish except a 4  4 mm window which was left exposed on the center of the clinical crown .
the enamel surfaces of the teeth in group 1 were etched with a 37% phosphoric acid gel for 15 seconds , rinsed for 15 seconds with copious amount of water , and dried with oil - free air for another 15 seconds . the frosty white appearance was observed on all the teeth .
the teeth in group 2 were etched with an er : yag laser device ( fidelis plus ii , fotona , slovenia ) emitting a wavelength of 2.94 mm .
the laser was irradiated using short pulse mode ( 180 s ) , 300 mj of energy per pulse , and 10 pulses per second under air and water spray ( 5 ml / min ) .
the laser beam was directed manually at 1 mm distance and was delivered with a sweeping motion perpendicular to the enamel surface for 10 seconds with the use of ro7 handpiece . after laser etching ,
the frosty white appearance was noticed on all the specimens similar to the acid - etched group . to compare the acid resistance of the teeth exposed to er : yag laser with acid - etched group , the teeth in both groups
were subjected to 4-day ph cycling process to form caries - like lesions . in this process
, the teeth were immersed in demineralization and remineralization solutions for 18 hours and 6 hours per day , respectively . the demineralization solution ( ph = 4.6 ) consisted of 0.05 m acetic acid , 2.2 mm calcium , and 2.2 mm phosphate ions , and the remineralization solution ( ph = 7.0 ) contained 0.15 m potassium chloride , 1.5 mm calcium , and 0.9 mm phosphate ions
between the two phases and at the end of the process , the teeth were washed with running water for 5 minutes .
the samples were embedded in epoxy resin ( struers , copenhagen , denmark ) using plastic tubes ( 2 cm diameter and 1.5 cm height ) as molds .
the crowns were mounted in such a way that the facial surface of each tooth was parallel to the bottom of the mold .
the microscopic sections were obtained via trimming the teeth by a ground section apparatus from both mesial and distal aspects in a buccolingual direction . to do this , the tooth was first trimmed from one side until it reached the middle of the treatment window .
then , the tooth was fixed on a glass slab with a transparent stone adhesive and the other half was trimmed until the thickness of the specimen reached approximately 100 . for thickness evaluation , another slab was placed on top of the specimen , and the specimen was compared with a matrix strap ( thickness , 100  ) , which was placed next to it on the slab .
the sections were evaluated under a polarized light microscope ( olympus , model bh2 , dualmont corporation , minnepolis , minn ) and the photograph of each section was taken with maximum illumination at 40 magnificence .
the resulting pictures were analyzed with the aid of adobe photoshop cs software ( adobe system incorporated , usa ) . using this software , the demineralized zone on the enamel surface of each sample
was captured with magic wand tool ( photoshop software ) and the surface area of the lesion was calculated and expressed in pixel .
the lesion areas were compared between the laser - etched and acid - etched groups by student t - test .
statistical calculation was performed with spss software ( version 11.5 , spss inc , chicago , ill ) , and p value less than 0.05 was considered significant .
the samples were embedded in epoxy resin ( struers , copenhagen , denmark ) using plastic tubes ( 2 cm diameter and 1.5 cm height ) as molds .
the crowns were mounted in such a way that the facial surface of each tooth was parallel to the bottom of the mold .
the microscopic sections were obtained via trimming the teeth by a ground section apparatus from both mesial and distal aspects in a buccolingual direction . to do this ,
the tooth was first trimmed from one side until it reached the middle of the treatment window .
then , the tooth was fixed on a glass slab with a transparent stone adhesive and the other half was trimmed until the thickness of the specimen reached approximately 100 . for thickness evaluation , another slab was placed on top of the specimen , and the specimen was compared with a matrix strap ( thickness , 100  ) , which was placed next to it on the slab .
the sections were evaluated under a polarized light microscope ( olympus , model bh2 , dualmont corporation , minnepolis , minn ) and the photograph of each section was taken with maximum illumination at 40 magnificence .
the resulting pictures were analyzed with the aid of adobe photoshop cs software ( adobe system incorporated , usa ) . using this software , the demineralized zone on the enamel surface of each sample
was captured with magic wand tool ( photoshop software ) and the surface area of the lesion was calculated and expressed in pixel .
the lesion areas were compared between the laser - etched and acid - etched groups by student t - test .
statistical calculation was performed with spss software ( version 11.5 , spss inc , chicago , ill ) , and p value less than 0.05 was considered significant .
photomicrographs of representative lesions in the laser - etched and acid - etched groups are demonstrated in figures 1 and 2 , respectively . in general
, the demineralized tissue seems dark under polarized light microscope , while the intact enamel shows a yellowish or bluish appearance .
student t test revealed no significant difference between the laser - etched and acid - etched groups regarding the area of demineralization ( p = 0.914 ) .
photomicrograph of enamel lesion representing the er : yag laser - etched group at 40 magnification under polarized light microscope photomicrograph of enamel lesion representing the acid - etched group at 40 magnification under polarized light microscope lesion area measurements in the study groups
this in vitro study evaluated the acid resistance of er : yag laser - etched specimens , compared with those prepared by conventional acid - etching technique .
the main disadvantage of phosphoric acid etching , i.e. , the potential for producing decalcification , has prompted many clinicians to find an alternative conditioning method .
enamel pretreatment with polyacrylic acid has been suggested by some authors to reduce the rate of enamel loss during etching , but this method failed to achieve adequate bond strength to resist intraoral forces .
the application of laser for tooth conditioning may be a suitable alternative for acid etching because it can save chair time and provide caries prevention effect .
the bond strengths of adhesive resins bonded to laser - etched enamel surfaces have been a matter of concern for many clinicians .
some studies have found no significant differences between bond strengths of the teeth etched with erbium lasers or phosphoric acid,[2932 ] while others reported the reverse finding .
however , it seems reasonable that with suitable laser parameters , bond strengths of laser - etched surfaces should be comparable to those prepared by conventional acid etching . the caries protective effect of laser light has been attributed to the heat produced during laser irradiation , which can cause changes in the chemical and crystalline structure of the enamel .
previous studies have reported a minimum solubility of enamel and smallest lesion depths after heating the dental enamel to between 300 and 400c .
most authors believe that carbonate decomposition and loss of water are important contributing factors to caries prevention .
removal of carbonate improves the crystalline structure , and thus decreases the enamel susceptibility to demineralization , because carbonate fit less well in the lattice and produce more acid - soluble apatite - phase .
indicated that decomposition of carbonate begins from 400c onward , but fowler ad kuroda reported substantial loss of carbonate and water at temperatures between 100 and 400c .
another theory for the protective effect of laser light is based on the blocking of enamel diffusion pathway . in lased enamel
, the decomposed organic materials can block the interprismatic and intraprismatic spaces that act as ion diffusion channels during the demineralization process , making the enamel less vulnerable to mineral loss .
one other explanation for the laser - induced caries prevention is the formation of microspaces and microfissures in lased enamel .
these spaces are believed to trap the calcium , phosphorus , and fluoride ions released from the tooth during the demineralization process .
the size of these spaces can vary with the laser energy ; higher energies leading to the formation of deeper or more extensive spaces in the enamel .
however , some authors suggest that these spaces can also act as open channels , facilitating the acid attack to subsurface , thus enhancing the mineral loss .
the present study indicates that er : yag laser parameters for enamel conditioning can not simultaneously prevent demineralization when compared to phosphoric acid etching .
this finding is in agreement with the studies of apel et al . and chimello et al . who found no significant differences between er : yag - lased and unlased groups with respect to the enamel demineralization
apel et al . found fine cracks in the enamel surface after sub - ablative erbium laser irradiation which can act as starting points for acid attack and demineralization , therefore counteracting the positive effect of laser light in caries prevention .
the results of this study are in contrast with the study of hossain et al . who reported significant caries resistance in er : yag laser - irradiated specimens , compared to control group .
liu et al . exposed molars to er : yag laser irradiation using 100 , 200 , and 300 mj pulse energies and found that laser treatments of 100 and 200 mj provided significant protection of enamel demineralization , but not the treatment with 300 mj .
evaluated the different settings of er : yag laser on enamel acid resistance and reported that lower energies ( 60 - 80 mj ) caused a significant reduction in enamel solubility .
can be attributed to the higher energy per pulse ( 300 mj ) we employed for enamel etching .
some previous studies did not use water cooling during er : yag laser treatment , while in the present study , water - cooling was employed to closely simulate the clinical condition used for etching the enamel surface .
a previous study indicated that er : yag laser is effective for increasing the caries resistance either used with or without water cooling , but the degree of caries prevention was considerably higher without water mist than with water mist .
it is apparent that the application of er : yag laser without water cooling may increase the enamel surface temperature higher than that achieved with water application .
therefore , the difference between the results of this study compared to those of liu et al .
may be related to the different surface temperature of the enamel , achieved with different laser parameters .
however , it should be noted that the time of laser exposure in the studies that did not use water cooling was shorter than that needed for effective enamel conditioning .
the time of phosphoric acid etching in the clinical condition may vary between 15 to 60 seconds . in the present study ,
15 seconds of acid etching was selected in order to minimize the adverse effect of acid etching on the enamel surface .
it is expected that longer etching times , as are used commonly by many clinicians , may make the teeth more susceptible to demineralization , and in this case , the difference between the caries resistance of acid - etched and laser - etched surfaces would be more remarkable .
further research should focus on finding suitable range of er : yag laser parameters to achieve both optimal caries resistance and high bond strength in the lased enamel .
the present study indicates that er : yag laser can not increase the enamel resistance to demineralization when using for enamel conditioning .
 background : several studies have shown that laser - etching of enamel for bonding orthodontic brackets could be an appropriate alternative for acid conditioning , since a potential advantage of laser could or might be caries prevention . this study compared enamel resistance to demineralization following etching with acid phosphoric or er : yag laser for bonding orthodontic brackets.materials and methods : 
 fifty sound human premolars were divided into two equal groups . in the first group , 
 enamel was etched with 37% phosphoric acid for 15 seconds . in the second group , er : yag laser ( wavelength , 2 940 nm ; 300 mj / pulse , 10 pulses per second , 10 seconds ) was used for tooth conditioning . 
 the teeth were subjected to 4-day ph - cycling process to induce caries - like lesions . 
 the teeth were then sectioned and the surface area of the lesion was calculated in each microphotographs and expressed in pixel . 
 the total surface of each specimen was 196 608 pixels.results:mean lesion areas were 7 171 and 7532 pixels for laser - etched and acid - etched groups , respectively . 
 the two sample t - test showed that there was no significant difference in lesion area between the two groups ( p = 0.914).conclusion : although er : yag laser seems promising for etching enamel before bonding orthodontic brackets , it does not reduce enamel demineralization when exposed to acid challenge . 
INTRODUCTION
MATERIALS AND METHODS
Slide preparation and microscopic observation
RESULTS
DISCUSSION
CONCLUSION