since the report of buonocore in 1955 , phosphoric acid etching has been a standard protocol to treat tooth enamel for bonding resins and orthodontic attachments . however , decalcification is the one potential disadvantage , which leaves the enamel susceptible to caries attack , especially under orthodontic attachments . consequently , alternative methods that decrease the decalcification risk and provide clinically acceptable bonding strength have been sought . , after maiman introduced the ruby laser , several types of laser have been applied in dentistry , such as carbon dioxide , neodymium - doped : yttrium aluminum garnet ( nd : yag ) and diode lasers . the first - generation lasers were suitable for soft - tissue treatment , especially in periodontology . when these lasers were used in dental hard - tissue , they resulted an inflammatory pulpal response . recently the development of mid - infrared lasers ( erbium - doped yttrium aluminum garnet [ er : yag ] and erbium , chromium : yttrium , scandium , gallium , garnet [ er , cr : ysgg ] ) allow ablation of both soft and hard - tissues with minimal thermal side - effects . the er : yag laser with a wavelength of 2940 nm can ablate enamel and dentin effectively , because its light is highly and efficiently absorbed by water and hydroxyapatite . it has been suggested that er : yag laser was able to decrease acid dissolution and increase fluoride uptake ; thus , produce a surface , which is less susceptible to caries . the result of the studies in the past two decades on the application of laser etching for increasing bond strength of restorative material have been controversial . some studies indicate that acid - etched teeth had significantly more bond strength than laser - etched teeth . on the other hand , others suggested that laser etching could result in bond strength comparable with or even stronger than acid etching . these differences could be attributed to the various types of lasers or various irradiation parameters used , because the laser - hard tissue interaction is dependent on wavelength and irradiation energy . according to the controversial findings regarding the use of erbium lasers for enamel etching , the aim of this study is to evaluate the shear bond strength ( sbs ) between the acid etched enamel and laser irradiated enamel and to investigate the fracture mode of the bond . in this experimental in vitro study , 15 caries - free , intact human upper premolars extracted for orthodontic purposes in the oral surgery department of isfahan university of medical sciences were used . then , they were stored in hanks balanced salt solution ( hank 's balanced salt solution [ hbss ] , sigma - aldrich , st . antibiotics ( penicillin 100 , metronidazole 100 , gentamycin 100 , amphotericin 100 ) with 1% volume ratio was added to hbss to prevent bacterial and fungal growth . the teeth were mounted in self - cure acrylic resin ( rapid repair , detrey dentsply ltd , surrey , u.k ) up to the cementoenamel junction . the buccal enamel surface was cleaned and polished with non - fluoridated pumice and rubber cup , then washed with an oil free air spray . the buccal enamel surfaces of the premolars were divided into mesial and distal regions with masking tape ( approximately 1 mm width ) . one region was randomly etched with er : yag laser with a wavelength of 2940 nm ( fotona , fidelis plus , ljubljana , slovenia ) . laser energy is delivered through a r14 handpiece with fiber optic system with 900 m diameter [ figure 1 ] . it operates at 2 w power with 100 mj energy output , 20 hz of frequency and short pulse mode . the beam was directed perpendicular to enamel at 1 mm distance ( contact mode ) and was moved in a sweeping fashion by hand over an approximately 3  3 mm during an exposure time of 20 s , which was enough to scan this area . the irradiated teeth were dried with an oil free air spray for 15 s. the r14 handpiece used in this study the other region was etched with conventional 37% phosphoric acid gel ( american orthodontics , sheboygan , usa ) for 15 s , then washed for 20 s and dried with an oil - free source for 20 s. the frosty white appearance of enamel was visible in both regions for all specimens . transbond xt primer ( 3 m unitek , monrovia , ca , usa ) was then applied to both etched surfaces ; afterward an air jet was lightly applied to the enamel . then , transbond xt composite ( 3 m unitek , monrovia , ca , usa ) was applied onto the base of the two metal mandibular incisors brackets ( american orthodontic , standard edgewise.018 , 380 - 0008 , sheboygan , usa ) . according to the manufacturer 's instruction these brackets had a surface bonding area of 8.78 mm . the brackets were placed onto mesial and distal regions of the buccal tooth surface immediately and adjusted to final position and pressed firmly . excessive adhesives were removed from the periphery of the bracket base to keep the bond area of each tooth uniform and light - cured with a light - emitting diode ( starlight pro , mectron , carasco , italy ) for 20 s ( 10 s from each proximal side ) . then the masking tape was removed [ figure 2 ] . one of the samples which , the right side was etched by 37% phosphoric acid and the left side etched by erbiumdoped yttrium aluminum garnet laser after storing the specimens in distilled water at 37c for 24 h , they were thermocycled for a total of 500 cycles at 5 - 55c with a dwell time of 30 s and a 10 s transfer time between baths , to simulate the heat and humidity conditions of the oral cavity . the shear bond test was accomplished with a chisel edge , mounted on the crosshead of a universal testing machine ( walter + bai ag , lhningen , switzerland ) . the edge was aimed at the bracket - enamel interface with a crosshead speed of 1 mm / min and the debonding forces were recorded for each specimen in newtons and then converted to megapascals . the shear bond test was performed by a technician who was blinded about the preparation procedures undertaken for the groups . the debonded buccal surface of each tooth was evaluated under 10 magnification and the adhesive remnant index ( ari ) was quantified according to the criteria established by artun and bergland , i.e. , 0 = no adhesive left on tooth , 1 = less than half of the adhesive left on tooth , 2 = more than half of the adhesive left on tooth , 3 = all the adhesive left on tooth . statistical comparisons were performed with software statistical package for the social sciences for windows ( version 19.0 , chicago , il , usa ) . the frosty white appearance of enamel was visible in both regions for all specimens . transbond xt primer ( 3 m unitek , monrovia , ca , usa ) was then applied to both etched surfaces ; afterward an air jet was lightly applied to the enamel . then , transbond xt composite ( 3 m unitek , monrovia , ca , usa ) was applied onto the base of the two metal mandibular incisors brackets ( american orthodontic , standard edgewise.018 , 380 - 0008 , sheboygan , usa ) . according to the manufacturer 's instruction these brackets had a surface bonding area of 8.78 mm . the brackets were placed onto mesial and distal regions of the buccal tooth surface immediately and adjusted to final position and pressed firmly . excessive adhesives were removed from the periphery of the bracket base to keep the bond area of each tooth uniform and light - cured with a light - emitting diode ( starlight pro , mectron , carasco , italy ) for 20 s ( 10 s from each proximal side ) . then the masking tape was removed [ figure 2 ] . one of the samples which , the right side was etched by 37% phosphoric acid and the left side etched by erbiumdoped yttrium aluminum garnet laser after storing the specimens in distilled water at 37c for 24 h , they were thermocycled for a total of 500 cycles at 5 - 55c with a dwell time of 30 s and a 10 s transfer time between baths , to simulate the heat and humidity conditions of the oral cavity . the shear bond test was accomplished with a chisel edge , mounted on the crosshead of a universal testing machine ( walter + bai ag , lhningen , switzerland ) . the edge was aimed at the bracket - enamel interface with a crosshead speed of 1 mm / min and the debonding forces were recorded for each specimen in newtons and then converted to megapascals . the shear bond test was performed by a technician who was blinded about the preparation procedures undertaken for the groups . the debonded buccal surface of each tooth was evaluated under 10 magnification and the adhesive remnant index ( ari ) was quantified according to the criteria established by artun and bergland , i.e. , 0 = no adhesive left on tooth , 1 = less than half of the adhesive left on tooth , 2 = more than half of the adhesive left on tooth , 3 = all the adhesive left on tooth . statistical comparisons were performed with software statistical package for the social sciences for windows ( version 19.0 , chicago , il , usa ) . descriptive statistics for the comparison of sbs for the two groups are given in table 1 . descriptive statistics of shear bond strength ( in mpa ) for the phosphoric acid and laser etching groups the acid - etched group had higher sbs means ( 18.86  6.09 mpa ) than laser - etched group ( 16.61  7.70 mpa ) , but no significant difference was found between both groups ( p = 0.41 ) . the location of the fracture for each group was evaluated with the ari index [ table 2 ] . three possible types of fractures may be observed : cohesive fracture ( within the body of the composite ) , adhesive fracture ( at the composite - bracket base or enamel - composite interface ) and mixed fracture . residual adhesive rating according to ari for laser - etched and acid - etched groups in the acid - etched group , 40% of fractures were located at the bracket - adhesive interface ( ari 3 ) , 20% at enamel - adhesive interface ( ari 0 ) and 40% were mixed fractures ( ari 1 and 2 ) . in the laser etched group , 47% of bond failures were located at the enamel - adhesive interface ( ari 0 ) , 13% at bracket - adhesive interface ( ari 3 ) and 40% were mixed fractures ( ari 1 and 2 ) . no cohesive failures within the body of the resin and no enamel tooth fractures were found . the mann - whitney u test showed no significant differences ( p = 0.08 ) between two groups according to the ari evaluation . in the present study , the laser - etched group had similar loading strengths with the acid - etched group ( p = 0.41 ) . maijer and smith reported that 8 mpa of bonding strength is adequate for orthodontic brackets . therefore , bond forces range in both group were within acceptable limit and laser etching at these setting seems acceptable for clinical use . on the other hand , our results are not in agreement with those of umez et al . . these controversies could be due to different tooth structures , type of laser and different laser setting such as power output , wavelength , emission mode , contact or non - contact mode , irradiation time , water cooling and irradiation distance . the ari values indicated no significant difference in the bond failure site among the two groups . although in the laser group more adhesive remained on the brackets , which is not suitable for orthodontic purposes . less chair time would be needed with less adhesive left on the enamel after debonding , but some authors state that bond failure at the bracket - adhesive interface or within the adhesive is more acceptable ( safe ) than failure at the adhesive - enamel interface , because enamel fracture and cracking have been reported at bracket debonding , especially with ceramic brackets . the ability of er : yag laser to ablate dental hard tissue is ascribed to its 2940 nm wavelength , which is coincident with the absorption band of water and hydroxyapatite of enamel . the irradiation is highly absorbed by the water molecules in the enamel , causing sudden heating and water evaporation . consequence is high stream pressure that leads to the micro - explosions with ejection of tissue particles , which are characteristic of the ablation process and determine the micro - crater like appearance of lased surface . the majority of irradiation is consumed in the ablation process and leaving very little residual energy for adverse thermal interactions with the pulp tissue and peripheral soft- and hard - tissues . moreover , er : yag laser can be applied in wet conditions and the clinician has more control of the area to be etched . although , gel acids are more stable than liquid acids , there is always a shift of gel acid on the enamel surface . often , laser etching leaves the tooth with a rough surface , so time saved with laser etching might be used performing additional polishing after debonding . furthermore , laser radiation can lead to the micro - crack formation in the enamel surface , which acts as starting points for fracture , acid attack and demineralization . consequently , the possible positive effect of erbium laser irradiation in preventing enamel demineralization around the brackets is reduced or eliminated . in this study , the irradiation was performed manually in order to simulate the clinical condition . to ensure identical conditions for both groups , the buccal surface of each sample was divided into mesial and distal regions using a masking tape then ; one region was etched with acid and the other with laser . the results show that laser etching procedure produce clinically acceptable bond strengths , but this was an in vitro study and the result may be different when the procedures are performed on the patients . further investigation to evaluate structure and mechanical properties of the enamel after bonding with laser is suggested . according to the results of this study , the mean bond strength and bond failure mode of 37% phosphoric acid - etched group and er : yag laser - etched group were not significantly different . therefore , er : yag laser may be an alternative method for conventional acid - etching .
background : laser ablation has been suggested as an alternative method to acid etching ; however , previous studies have obtained contrasting results . 
 the purpose of this study was to compare the shear bond strength ( sbs ) and fracture mode of orthodontic brackets that are bonded to enamel etched with acid and erbium - doped yttrium aluminum garnet ( er : yag ) laser.materials and methods : in this experimental in vitro study , buccal surfaces of 15 non - carious human premolars were divided into mesial and distal regions . 
 randomly , one of the regions was etched with 37% phosphoric acid for 15 s and another region irradiated with er : yag laser at 100 mj energy and 20 hz frequency for 20 s. stainless steel brackets were then bonded using transbond xt , following which all the samples were stored in distilled water for 24 h and then subjected to 500 thermal cycles . 
 sbs was tested by a chisel edge , mounted on the crosshead of universal testing machine . 
 after debonding , the teeth were examined under 10 magnification and adhesive remnant index ( ari ) score determined . 
 sbs and ari scores of the two groups were then compared using t - test and mann - whitney u test . significant level was set at p < 0.05.results:the mean sbs of the laser group ( 16.61  7.7 mpa ) was not significantly different from that of the acid - etched group ( 18.86  6.09 mpa ) ( p = 0.41 ) . there was no significant difference in the ari scores between two groups ( p = 0.08 ) . however , in the laser group , more adhesive remained on the brackets , which is not suitable for orthodontic purposes.conclusion:laser etching at 100 mj energy produced bond strength similar to acid etching . 
 therefore , er : yag laser may be an alternative method for conventional acid - etching .