lingual appliances were developed in the 1980s as an esthetic treatment option for adult orthodontic patients.123 however , there are several disadvantages to using this technique such as longer chair time , tongue discomfort , speech problems , a relatively insufficient bonding area caused by the short clinical crown height of the lingual surface , variations in anatomy on the tooth lingual surface , shorter arch perimeter , reduced interbracket distance on the lingual side versus the labial side , bowing effects , and finishing difficulty.2456789 therefore , several modifications to lingual appliances have been introduced to overcome these disadvantages , including a low - profile bracket design for patient comfort , a narrow bracket width design to increase interbracket distance , a customized bracket base for variations in tooth surface anatomy , and self - ligating lingual brackets for reduced chair time . sliding resistance during tooth movement occurs between both lingual brackets and the archwire and between conventional labial brackets and the archwire . this has been attributed to bracket type , size and geometry of the bracket slot , size and alloy of the archwire , and method of ligation.1011121314 in cases with crowding , rotation , angulation , or vertical discrepancy , the initial leveling and alignment archwire is usually deflected and contacts the edges of the bracket slots . therefore , the effect of sliding resistance is important in the leveling and alignment stage as well as in the space closure stage . although numerous studies have investigated the implications of friction between conventional labial brackets and archwires,111215161718 information about the frictional properties of lingual brackets is still limited , and some drawbacks exist in experimental design and methodology . for example , one or several lingual brackets aligned in a straight line have been used to measure frictional force.1920 therefore , it is necessary to perform an experiment using whole dentitions with initial malocclusion statuses to mimic clinical situations . currently , the lingual straight wire technique is used for lingual orthodontic treatment , which uses conventional ligating lingual brackets ( cl - lbs ) with narrow bracket widths.21 small - sized lingual brackets have narrow and shallow slots that make it extremely difficult to perform a double over - tie ligation , which is usually performed with cl - lbs.22 however , few studies have compared frictional properties between cl - lbs with a narrow bracket width and self - ligating lingual brackets . therefore , the purpose of this in vitro mechanical study was to evaluate the effects of tooth displacement on frictional properties when cl - lbs , cl - lbs with a narrow bracket width , and self - ligating lingual brackets were used with an initial leveling and alignment wire . the null hypothesis was that there is no significant difference in the effects of tooth displacement or lingual bracket type on static or kinetic frictional forces ( sff and kff , respectively ) . one type of cl - lb ( 7th generation ; ormco , orange , ca , usa ) , one type of cl - lb with a narrow bracket width ( stb ; ormco ) , and one type of self - ligating lingual bracket ( in - ovation l ; gac , dentsply corp . , york , pa , usa ) were tested under three tooth displacement conditions : no displacement ( control ) ; a 2-mm palatal displacement of the maxillary right lateral incisor ( mxli ) ; and a 2-mm gingival displacement of the maxillary right canine ( mxc ) ( figure 1 ) . therefore , a total of nine groups were created by the combination of these factors ( n = 7 per group ) . in this study , this typodont system had a full maxillary dentition fixed to an archshaped metal frame , which allowed each tooth to move in the occluso - gingival ( up and down ) and labio - palatal ( forward and backward ) directions . at the zero position , all teeth were aligned in the ideal position according to an ovoid arch form ( orthoform iii - ovoid , reference no . each tooth had its periodontal ligament space filled with imprint ii garant light body vinyl polysiloxane impression material ( 3 m espe , seefeld , germany ) , which emulates the mobility of human teeth and absorbs mechanical stress.121718 the characteristics of the lingual brackets tested in this study are listed in table 1 . after the 7th generation , stb , and in - ovation l brackets were positioned with full - size preformed straight lingual archwire at the center of the lingual surface , customized resin bases for the brackets were fabricated by curing transbond xt ( 3 m unitek ) . then , the archwire was removed and individual transfer trays were made . to minimize wire - related bias , 0.013-inch copper - nickel - titanium ( cu - niti ) preformed lingual archwires were used ( stb straight wire small , 204 - 2101 ; ormco ) . for ligation of the maxillary anterior teeth , a double over - tie of powerchain was used for the 7th generation group ( clear generation ii power chain , 639 - 0002 ; ormco ) and a single tie of elastic modules was applied to the stb group ( alastik easy - to - tie ligature ; 3 m unitek ) according to the manufacturer 's guide.2122 for ligation of the maxillary posterior teeth , elastic modules ( alastik easy - to - tie ligature ; 3 m unitek ) were used in both cl - lb groups ( the 7th generation and stb groups ) according to the manufacturer 's guide.2122 after the ligation of all brackets , a 3-minute waiting period was allowed to obtain reproducible amounts of stress relaxation and ligation force.1112151618 since the in - ovation l bracket was self - ligating , it was closed with an active clip . the typodont was then attached to a metal plate fixed to a mechanical testing machine ( model 4466 ; instron , canton , ma , usa ) . seoul , korea ) was sprayed onto the bracket , the end of the archwire extruding from the maxillary right second molar tube was gripped with a custom - designed adaptor . sff and kff were measured while drawing the archwire through the brackets at a speed of 0.5 mm / min for 5 minutes . tests were conducted in a chamber maintained at 36.5  0.3 ( figure 2 ) . after each test , the typodont system was immediately washed with distilled water and alcohol to remove the artificial saliva and then dried with an air syringe . each group was tested seven times , and a new wire was used each time . kff was calculated by averaging frictional forces from after the maximal point of the initial rise to the end of the test.121718 a power analysis was performed to determine the sample size using a sample size determination program ( version 2.0.1 ; seoul national university dental hospital , registration no . the shapiro - wilk test was performed to assess the normality of the distributions in the experimental groups . if equal variances were assumed by the levene 's test , a one - way analysis of variance ( anova ) with tukey 's honest significant difference ( hsd ) post hoc test was performed for the statistical analysis . when equal variances were not assumed by levene 's test , welch 's variance - weighted anova with dunnett 's t3 post hoc test was used . the 7th generation group showed the highest sff and kff , followed by the stb group and the in - ovation l group ( p < 0.001 ; table 2 and figure 4 ) . the 7th generation group showed the highest sff and kff , followed by the stb and in - ovation l groups ( p < 0.001 ) . there was no significant difference between the stb and in - ovation l groups in sff . however , the stb group demonstrated higher kff than the in - ovation l group ( p < 0.001 ; table 2 and figure 4 ) . the 7th generation group demonstrated higher sff and kff than the stb and in - ovation l groups ( p < 0.001 ) . there was no significant difference between the stb and in - ovation l groups in sff . however , the stb group showed higher kff than the in - ovation l group ( p < 0.001 ; table 2 and figure 4 ) . in the 7th generation group , a 2-mm palatal displacement of the mxli was associated with significantly lower sff and kff than in the control group and with a 2-mm gingival displacement of the mxc ( all p < 0.001 ; table 3 and figure 5 ) . interestingly , the control group exhibited higher sff and kff than a 2-mm palatal displacement of the mxli ( all p < 0.001 ) . in addition , there was no significant difference in sff between the control group and a 2-mm gingival displacement of the mxc . in the stb group , a 2-mm gingival displacement of the mxc demonstrated higher sff than the control group and a 2-mm palatal displacement of the mxli ( p < 0.001 ) . similarly , a 2-mm gingival displacement of the mxc produced the highest kff , followed by a 2-mm palatal displacement of the mxli and the control group ( p < 0.001 ) . in the in - ovation l group , the highest sff and kff were observed with a 2-mm gingival displacement of the mxc , followed by a 2-mm palatal displacement of the mxli and the control group ( all p < 0.001 ) . the 7th generation group showed the highest sff and kff , followed by the stb group and the in - ovation l group ( p < 0.001 ; table 2 and figure 4 ) . the 7th generation group showed the highest sff and kff , followed by the stb and in - ovation l groups ( p < 0.001 ) . there was no significant difference between the stb and in - ovation l groups in sff . however , the stb group demonstrated higher kff than the in - ovation l group ( p < 0.001 ; table 2 and figure 4 ) . the 7th generation group demonstrated higher sff and kff than the stb and in - ovation l groups ( p < 0.001 ) . there was no significant difference between the stb and in - ovation l groups in sff . however , the stb group showed higher kff than the in - ovation l group ( p < 0.001 ; table 2 and figure 4 ) . in the 7th generation group , a 2-mm palatal displacement of the mxli was associated with significantly lower sff and kff than in the control group and with a 2-mm gingival displacement of the mxc ( all p < 0.001 ; table 3 and figure 5 ) . interestingly , the control group exhibited higher sff and kff than a 2-mm palatal displacement of the mxli ( all p < 0.001 ) . in addition , there was no significant difference in sff between the control group and a 2-mm gingival displacement of the mxc . in the stb group , a 2-mm gingival displacement of the mxc demonstrated higher sff than the control group and a 2-mm palatal displacement of the mxli ( p < 0.001 ) . similarly , a 2-mm gingival displacement of the mxc produced the highest kff , followed by a 2-mm palatal displacement of the mxli and the control group ( p < 0.001 ) . in the in - ovation l group , the highest sff and kff were observed with a 2-mm gingival displacement of the mxc , followed by a 2-mm palatal displacement of the mxli and the control group ( all p < 0.001 ) . the present study showed that the self - ligating lingual bracket ( in - ovation l ) group produced lower sff under conditions of no displacement ( p < 0.001 ; table 2 and figure 4 ) and lower kff under all displacement conditions ( the control group , 2 mm of palatal displacement of the mxli , and 2 mm of gingival displacement of the mxc ; all p < 0.001 ; table 2 and figure 4 ) than the cl - lb groups ( 7th generation and stb ) . this might be attributed to differences in ligation methods and in the original and effective slot dimensions as follows . first , the in - ovation l brackets use a self - ligating clip for ligation of the maxillary anterior and posterior teeth . however , the 7th generation brackets use the double over - tie method , while stb brackets use a single - tie method for ligation of the maxillary anterior teeth . both 7th generation and stb brackets use the single - tie method for ligation of the maxillary posterior teeth . second , when an archwire was placed in the 7th generation and stb brackets , the effective slot width was increased over the original slot dimension because of elastomeric ligature material surrounding the bracket wings ( figure 6 ) . however , the in - ovation l brackets ( self - ligating type ) demonstrated no difference between the original and effective slot dimensions ( figure 6 ) . ozturk ortan et al.20 reported that the in - ovation l bracket generated lower frictional force than the stb bracket , in agreement with the results of the present study ( table 2 and figure 4 ) . however , lombardo et al.13 demonstrated that the stb bracket produced significantly lower friction than the in - ovation l bracket . the reason for the disagreement between the results of their study and those of the present study seems to originate from differences in experimental design , and specifically the use of three anterior teeth and a straight archwire in the study by lombardo et al.13 rather than the whole maxillary dentition and preformed lingual archwire used in the present study . the finding that the stb ( cl - lbs with a narrow bracket width ) group produced lower sff and kff than the 7th generation ( cl - lbs ) group under all displacement conditions ( the control group , 2 mm of palatal displacement of the mxli , and 2 mm of gingival displacement of the mxc ; all p < 0.001 ; table 2 and figure 4 ) can be explained by the narrow bracket width and difference in ligation method . first , stb brackets have a narrower mesiodistal bracket width and thinner bracket pad than 7th generation brackets , which increases the interbracket distance and thus reduces both the force transmitted by the archwire and resistance to sliding mechanics.1321 second , stb brackets use a single - tie method for ligation of the maxillary anterior teeth . the double over - tie method for ligation of the maxillary anterior teeth used with 7th generation brackets can generate more friction than the single - tie method used with stb brackets . there was no significant difference in sff between the in - ovation l and stb groups for a 2-mm gingival displacement of the mxc and 2-mm palatal displacement of the mxli ( table 2 and figure 7 ) . because of the incorporation of a 0.33-mm passive ligation step on each side of the stb bracket slot ( figure 6),21 effective interbracket distances could be increased and critical contact angles might also be affected . this phenomenon might induce less deflection of the archwire and reduce the degree of binding and frictional forces between the archwire and bracket slot . therefore , cl - lbs with a narrow bracket width and passive ligation step ( stb ) exhibited similar amounts of sff as self - ligating lingual brackets ( in - ovation l ) under conditions of tooth displacement . in the present study , a 2-mm gingival displacement of the mxc produced higher sff and kff than a 2-mm palatal displacement of the mxli and the control group among the cl - lbs ( 7th generation and stb ) , and even in a self - ligating lingual bracket ( in - ovation l ) ( all p < 0.001 ; tables 2 , 3 , and figure 5 ) . this seems to result from differences in the patterns of contact and degrees of binding between the archwires and bracket slots . in a 2-mm gingival displacement of the mxc , the 7th generation , stb , and in - ovation l groups showed full contact between the archwire and gingival wall of the bracket slot of the mxli and the maxillary first bicuspid , and between the archwire and the incisal wall of the bracket slot of the mxc , resulting in strong binding of the archwire within the bracket slot ( figure 7 ) . however , in a 2-mm palatal displacement of the mxli , these experimental groups demonstrated only partial contact between the archwire and the vertical wall of the bracket slot of the mxc and the maxillary central incisor , although full contact between the archwire and the vertical wall of the bracket slot was observed in the mxli ( figure 7 ) . interestingly , the 7th generation group produced lower sff and kff in a 2-mm palatal displacement of the mxli than the control group ( all p < 0.001 ; table 3 ) . the engagement of an archwire into the bracket slot of a palatally displaced mxli produced an internal shear force in the wire from deflection , which might exceed the seating force of a double over - tie ligation , resulting in partial disengagement of the archwire from the bracket slot and eventual decreases in sff and kff than in the control group ( figure 7 ) . all experimental groups showed higher kff than sff , which was consistently found , even when the experiments were repeated several times . this phenomenon might be related to mechanical differences such as shorter arch perimeter , shorter interbracket distance , and smaller curvature of the anterior segment than the labial appliance.49 these differences affect the load deflection characteristics of the wire , increase wire stiffness , and produce a higher binding force between the bracket and wire , resulting in a greater increase in kff than sff.47 this in vitro study exhibited new findings that cl - lbs with narrow bracket widths can reduce sff as effectively as self - ligating lingual brackets in a 2-mm gingival displacement of the mxc and a 2-mm palatal displacement of the mxli . however , special precautions should be taken when interpreting the findings of this study because of several limitations . first , the material used to emulate the periodontal ligament 's stress - absorbing mechanism could not truly replicate the biological tooth - periodontal ligament - bone complex.23 second , archwire movement can occur in various ways , and not strictly in a single direction in an intraoral situation . third , different ligation methods were used for the maxillary anterior teeth with the cl - lbs . therefore , it would be prudent to develop an experimental design improving on these drawbacks in further studies . the null hypothesis was rejected . since conventional - ligating lingual brackets with a narrow bracket width exhibited higher kff than self - ligating lingual brackets under conditions of tooth displacement , it is necessary to develop conventional - ligating lingual brackets and ligation methods that reduce sff and kff as effectively as self - ligating lingual brackets during the initial and leveling stage .
objectivewe evaluated the effects of tooth displacement on frictional force when conventional ligating lingual brackets ( cl - lbs ) , cl - lbs with a narrow bracket width , and self - ligating lingual brackets ( sl - lbs ) were used with initial leveling and alignment wires.methodscl-lbs ( 7th generation ) , cl - lbs with a narrow bracket width ( stb ) , and sl - lbs ( in - ovation l ) were tested under three tooth displacement conditions : no displacement ( control ) ; a 2-mm palatal displacement ( pd ) of the maxillary right lateral incisor ( mxli ) ; and a 2-mm gingival displacement ( gd ) of the maxillary right canine ( mxc ) ( nine groups , n = 7 per group ) . a stereolithographic typodont system and artificial saliva were used . 
 static and kinetic frictional forces ( sff and kff , respectively ) were measured while drawing a 0.013-inch copper - nickel - titanium archwire through brackets at 0.5 mm / min for 5 minutes at 36.5.resultsthe in - ovation l exhibited lower sff under control conditions and lower kff under all displacement conditions than the 7th generation and stb ( all p < 0.001 ) . 
 no significant difference in sff existed between the in - ovation l and stb for a 2-mm gd of the mxc and 2-mm pd of the mxli . 
 a 2-mm gd of the mxc produced higher sff and kff than a 2-mm pd of the mxli in all brackets ( all p < 0.001).conclusionscl - lbs with narrow bracket widths exhibited higher kff than sl - lbs under tooth displacement conditions . 
 cl - lbs and ligation methods should be developed to produce sff and kff as low as those in sl - lbs during the initial and leveling stage .