orthodontic bracket bond failure is a common problem during orthodontic treatment   with a reported incidence of 17.6% [ 24 ] .
the effects of acidic foods , acidic and alcoholic beverages , herbal teas and different chemical solvents on the bond strength of orthodontic brackets have been investigated by researchers [ 69 ] . aside from increasing the risk of bond failure , these substances may increase the incidence of caries and periodontal problems and can lead to patient dissatisfaction . decreased ph and higher lactobacillus and streptococcus mutans count increase the susceptibility to caries . following orthodontic bracket placement , number of microorganisms increases by 610% [ 1214 ] .
plaque formation in orthodontic patients is two to three times more than in nonorthodontic adult patients with a high dental plaque score .
drop in salivary ph due to plaque formation and bacterial activity is considered the main cause of enamel demineralization .
the present study was designed to answer the question whether bond failure of orthodontic brackets occurs more frequently in patients with decreased salivary ph .
in this in vitro study , 80 intact premolar teeth extracted in the past six months for orthodontic reasons were used .
the teeth were examined under dental unit light to exclude the cracked ones , and washed under running water before storage in distilled water . after collecting the teeth ,
they were placed in 0.1% thymol solution for one week for disinfection , and then immersed in distilled water again to prevent dehydration .
the buccal surfaces were cleaned by dental prophylactic brush under running water , dried , etched with 37% phosphoric acid gel for 30 seconds , rinsed for 20 seconds and dried .
the buccal surface of the teeth was cleaned using non - fluoridated pumice powder and prophylactic rubber cups for 15 seconds , rinsed and dried with air spray . after cleaning ,
the teeth were conditioned with 37% phosphoric acid gel ( fine etch co , chungcheongnam - do , south korea ) for 20 seconds and dried with oil- and moisture - free air spray until a frosty white appearance was achieved . stainless steel standard edgewise premolar brackets ( dentaurum gmbh & co. kg , ispringen , germany ) were used in this study .
after etching , a thin adhesive resin layer ( unitek , 3 m espe , st .
paul , mn , usa ) was applied to the buccal surface of the teeth and the bracket bases were coated with composite resin ( unitek , 3 m espe , st .
the brackets were positioned at four - millimeter distance from the buccal cusp tip using a special gauge .
after removal of excess composite with a dental explorer , adhesive was cured using light - emitting diode ( led ) light curing unit ( led curing , morita , kyoto , japan ) for 20 seconds ( five seconds from each of the occlusal , gingival , mesial and distal directions ) . after bonding ,
each group was incubated in artificial saliva with a specific ph in similar conditions for two months .
since the normal salivary ph is 6.8 , and the critical ph for enamel decalcification is 5.5 , the ph level of solutions for groups one ( control ) to four was adjusted at 6.8 , 5.8 , 4.8 , and 3.8 , respectively .
artificial saliva was prepared by dissolving 0.4 g nacl , 1.21 g kcl , 0.78 g nah2po4 , 2h2o ; 0.005 g na2s , 9h2o and 1 g co(nh2)2 in 1000ml of deionized distilled water .
each tooth was embedded in self - cured acrylic block , with the crown exposed .
the sbs was measured using a dartec hc10 universal testing machine ( zwick ltd . ,
herefordshire , uk ) while the buccal surface was parallel to the direction of force at a crosshead speed of 0.5mm / min with a 0.5mm - thick blade .
the sbs was calculated by dividing the force at fracture by the base of the bracket and reported in megapascals ( mpa ) .
after debonding , the enamel surfaces were examined under a stereomicroscope ( carl zeiss ag , oberkochen , germany ) at 10 magnification to evaluate the amount of remaining adhesive .
the adhesive remnant on the surface was scored using adhesive remnant index ( ari ) of oliver .
the scores are : one , 100% ; two , more than 90% ; three , between 1090% ; four , less than 10% ; and five , 0% .
descriptive statistics including the mean and standard deviation ( sd ) of sbs values were calculated by means of statistical package for social sciences ( spss ) ( spss for windows , release 10.0.0 , chicago , il , usa ) .
the anova and tukey s post hoc test were used for multiple comparisons of sbs between the groups . to evaluate differences in ari scores , the kruskal 
descriptive statistics including the mean and standard deviation ( sd ) of sbs values were calculated by means of statistical package for social sciences ( spss ) ( spss for windows , release 10.0.0 , chicago , il , usa ) .
the anova and tukey s post hoc test were used for multiple comparisons of sbs between the groups . to evaluate differences in ari scores , the kruskal 
the obtained sbs values are shown in table 1 . the highest sbs ( 10.7  2.24 mpa ) belonged to group four ( ph of 6.8 ) and the lowest ( 6.12  2.04 mpa ) to group one ( ph of 3.8 ) .
the shear bond strength values in the four groups there were no significant differences in sbs values among groups one , two and three ( 10.7  2.24 , 10.09  2.39 and 8.94  2.17 mpa , respectively ) ( p>0.05 ) , but group four ( 6.12  2.04 mpa ) showed significantly lower value compared to others ( p < 0.05 ) ( table 2 ) .
comparison of shear bond strength among groups descriptive statistics for ari scores are shown in table 3 .
bond failure was mostly at the adhesive - enamel interface ( scores four and five ) in groups one and two , and at the adhesive - bracket interface in groups three and four ( scores one and two ) .
in this study , the highest mean value of sbs belonged to group four ( ph of 6.8 ) , but this value was not significantly different from that of groups three ( ph of 5.8 ) and two ( ph of 4.8 ) . having a higher mean
oncag and coworkers in 2005 reported that acidic beverages , by decalcifying the enamel surrounding the brackets , have negative effects on the bond strength at the bracket - enamel interface .
lee and coworkers in 1996 evaluated the effect of liquids , simulating foods and drinks , on sbs of brackets . in their study ,
extracted intact premolars with bonded brackets were stored in buffered lactic acid with a ph of four , and corn oil for 12 weeks .
their results showed that 50% ethanol and lactic acid with a ph of four caused the lowest sbs .
ulusoy and coworkers in 2009 studied the effects of different kinds of tea on sbs of orthodontic brackets . in their study , 90 intact premolars were assigned to six groups .
four of them were immersed in four different kinds of tea including : black tea , green tea , tea with lime flavor and fruit tea .
two other groups were subjected to a carbonated beverage ( coca cola ) , and distilled water as controls .
the results showed a direct correlation between the ph of beverages and the sbs of orthodontic brackets .
the ari scores showed that in an acidic environment , bond failures were mostly at the bracket - adhesive interface . softened adhesive compromises the bracket  adhesive interlocking .
oral hygiene , nutrition , and bonding technique are the most effective factors determining the degree of erosion .
steffen stated that the cariogenic activity of bacteria in the oral cavity is accelerated in presence of acidic beverages . in the current study , group one ( ph of 3.8 ) showed the least sbs value , which was significantly different from that of other groups .
this study suggests that acidic oral environment , due to poor oral hygiene , malnutrition , and frequent use of acidic beverages , may be the reason of increased incidence of bond failure in these orthodontic patients .
although oncag and coworkers showed that only extremely acidic oral environment ( ph=23 ) may reduce the sbs , in - vitro environment may be different from in - vivo , and also other factors may play roles in reducing the sbs .
we can prevent bond failure and its consequences by encouraging patients to maintain good oral hygiene and limit the use of acidic beverages such as cola .
the results of this study showed that : 
 1- the sbs values of groups two ( ph of 4.8 ) and three ( ph of 5.8 ) were not significantly different from that of group four ( ph of 6.8 ) ( control).2- the sbs value of group one ( ph of 3.8 ) was significantly lower than that of three other groups.3- this study strongly recommends maintaining good oral hygiene and limiting the use of acidic beverages .
1- the sbs values of groups two ( ph of 4.8 ) and three ( ph of 5.8 ) were not significantly different from that of group four ( ph of 6.8 ) ( control ) .
2- the sbs value of group one ( ph of 3.8 ) was significantly lower than that of three other groups .
3- this study strongly recommends maintaining good oral hygiene and limiting the use of acidic beverages .
 objectives : the purpose of this study was to evaluate the effect of salivary ph on the shear bond strength ( sbs ) of orthodontic brackets to tooth surface.materials and methods : eighty intact premolars were randomly divided into four groups of 20 . after bonding a bracket on each tooth , 
 the groups one to four were stored in artificial saliva at a ph of 3.8 , 4.8 , 5.8 , and 6.8 , respectively for two months . 
 the artificial saliva solutions were refreshed weekly . 
 each tooth was then embedded in an acrylic block so that the crown was exposed and its buccal surface was parallel to the direction of the force during sbs testing . 
 all brackets were debonded using dartec universal testing machine , and the mean values of sbs in different groups were compared using one - way analysis of variance ( anova).results : the mean sbs value in group one ( ph 3.8 ) was significantly lower than that in other groups ( p<0.05 ) . 
 the differences between other groups were not significant ( p>0.05).conclusion : decreased salivary ph due to poor oral hygiene and/or frequent consumption of acidic beverages may be responsible for orthodontic bracket bond failure . 
INTRODUCTION
MATERIALS AND METHODS
Statistical analysis
RESULTS
DISCUSSION
CONCLUSION