an esthetic wire coated with a tooth - colored plastic material , such as a synthetic 
 fluorine - containing resin or an epoxy resin composed mainly of 
 polytetraflueroethlyene , has 
 been used to satisfy esthetic demands .
several problems involving the wearing or peeling 
 of the outer coatings of coated wires have been identified .
kusy ( 1997 ) found that 
 coated , colored wires are routinely damaged by mastication forces and the activity of 
 oral enzymes within 3 weeks of their use in vivo .
elayyan , silikas and 
 bearn ( 2008 ) reported that 
 surface roughness of coated archwires increased after use in vivo . 

kaphoor and sundareswaran ( 2012 ) 
 reported that the forces of some coated wires were significantly lower than those of 
 uncoated wires .
other authors also encountered difficulties with such coated archwires , 
 claiming that the color tended to change with time and that the coating split during use 
 in the mouth , exposing the underlying metal . in addition
, the evaluation of wire properties , such as force and 
 smoothness , would be valuable because the sliding of a wire with problematic surface 
 properties , such as the durability of the coating material , may be inferior to that of 
 an uncoated wire .
some reports have described the influence of wire sliding against a 
 single piece of bracket with soaking solution and the relationship between the cross - sectional dimensions of the 
 wire and load deflection .
we initially use small wires , such as a 0.012-inch nickel - titanium ( ni - ti ) wire , because 
 use of low - friction brackets is widespread . in the initial stages of orthodontic treatment , the wire used must 
 produce a continuous force without interference . although the sliding resistance 
 ( interference ) of the wire depends on its size as compared to the bracket slot 
 room ,
the relationship between 
 force and resistance is important because it affects the movement of the teeth . during 
 leveling of dental arches with irregularities ,
if the force produced by the wire is less 
 than the friction resistance , as interference , the dental arch must be expanded until 
 the placement at which the resistance is released is reached . thus , expansion of the 
 dental arch in an extraction case can lead to poor - quality treatment , such as a 
 prolonged duration .
ni - ti wires are categorized into two types ( austenitic and 
 martensite types ) according to mechanical properties and fabrication .
although both have 
 excellent springback properties , austenitic ni - ti wires also have shape memory and 
 superelasticity .
thus , this 
 issue should be examined using small ni - ti wires with different mechanical properties , 
 and for plain and coated small wires , because coated wires may have sliding issues .
the 
 purpose of this study was to evaluate the relationship between mechanical properties and 
 friction in small esthetic ( including plated ) ni - ti wires using a dental arch model 
 designed with linguoversion of the lateral incisor in the arch and an iso bending test , 
 and to compare esthetic and unesthetic wires .
five esthetic ( three coated and two plated ) and two small plain ni - ti ( base sizes of 
 0.012 and 0.014 inches ) wires were used ( figure 
 1 ) .
a three - point bending test was carried out using the end 30 mm of each 
 archwire . to investigate the relationship between force and deflection in the bending of 
 ni - ti wires ,
briefly , we performed the three - point 
 bending test with an interfulcrum distance of 10 mm , a crosshead speed of 7.5 mm / min , 
 and a temperature of 361c using a testing machine ( 5567 ; instron , norwood , ma , usa ) 
 because the ni - ti wire used displayed no linear elastic behavior during unloading at 
 temperatures up to 50c .
next , we measured the force - deflection curve of each wire and 
 obtained unloading bending forces at deflections of 3.0 , 2.0 , 1.0 , and 0.5 mm during 
 unloading .
the base wires a-1 to a-5 and p-1 were austenitic ni - ti , and p-2 was 
 martensite ni - ti ( abb . :
abbreviation ) to assess friction resistance between brackets and wires in the dental arch , static 
 friction force was measured .
we placed each wire in low - friction passive - ligating 
 brackets ( t-21 ; tomy international , tokyo , japan ; slot size , 0.022 inch ; composition , 
 bracket : polyethylene terephthalate etc , slot cap : polyacetal ) , which were aligned to a 
 dental arch form plate model ( figure 2 ) with 0.5 , 
 1.0 , 2.0 , or 3.0 mm displacement at the lateral incisor and the arch - form plate .
the 
 brackets and the end of the wire were placed in the air - chucks of the testing machine 
 ( 5567 ; instron ; figure 3 ) .
we applied tensile 
 loading under a crosshead speed of 0.5 mm / min and measured the maximum loading as the 
 static friction force . a dental arch - form plate model designed for the linguoversion of the lateral 
 incisor in the arch a test of static frictional force . a dental archform plate and the end of an 
 archwire , placed in the arch , are held by the air - chucks of the testing 
 machine descriptive statistics , including means and standard deviations , were calculated for the 
 unloading bending force and static friction force using statistical analysis software 
 ( ver . 16.0 ; spss , chicago , il , usa )
additionally , the scheff test and games - howell 
 test were used for multiple comparisons among the products . a p value 
 < 0.05 was considered to indicate statistical significance .
bending forces produced from the wires and friction forces at displacements of 3 , 2 , 
 1.0 , and 0.5 mm during unloading are shown in tables 
 1 - 4 . at a displacement of 0.5 mm , 
 unloading bending forces were 27.8 - 85.6 cn and friction forces were 7.0 - 50.5 cn . at a 
 displacement of 1.0 mm , unloading bending forces were 42.7 - 123.2 cn and friction forces 
 were 11.2 - 93.6 cn . unloading bending forces at a displacement of 2.0 mm were 50.3 - 165.4 
 cn and friction forces were 96.1 - 444.9 cn . at a displacement of 3.0 mm ,
unloading 
 bending forces were 53.5 - 164.5 cn and friction forces were 164.4 - 950.7 cn .
unloading 
 bending and friction forces in each wire , except the 0.014-inch p-2 wire , did not differ 
 significantly .
different letters ( roman type : unloading bending 
 force group ; italics : friction force group ) indicate a significant difference 
 ( p<0.05 ) within the single - wire - size and identical - force groups unloading bending and friction forces at a displacement of 1.0 mm s.d . :
different letters ( roman type : unloading bending 
 force group ; italics : friction force group ) indicate a significant difference 
 ( p<0.05 ) within single - wire - size and identical - force groups unloading bending and friction forces at a displacement of 2.0 mm s.d . :
different letters ( roman type : unloading bending 
 force group ; italics : friction force group ) indicate a significant difference 
 ( p<0.05 ) within the single - wire - size and identical - force groups unloading bending and friction forces at a displacement of 3.0 mm s.d . :
different letters ( roman type : unloading bending 
 force group ; italics : friction force group ) indicate a significant difference 
 ( p<0.05 ) within the single - wire - size and identical - force groups the relationships between unloading bending and friction forces for each wire size are 
 shown in figures 4 and 5 . for 0.012- and 0.014-inch wires ,
all unloading bending forces at 
 0.5 and 1.0 mm were larger than friction forces .
however , all friction forces at 
 displacements exceeding 2.0 mm were larger than the unloading bending forces .
unloading bending forces at 0.5 and 1.0 mm were larger than friction forces , but 
 all friction forces at displacements exceeding 2.0 mm were larger than unloading 
 bending forces relationships between unloading bending and friction forces for 0.014-inch 
 wires
at the leveling stage , tooth movement behavior depends on the force produced by the 
 sliding wire and the friction resisting it . in this experiment ,
the friction force at 
 displacements exceeding 2.0 mm was larger than the force caused by the wire .
that is , 
 teeth are likely to move with the expansion of the dental arch up to a displacement of 
 1.0 mm .
resistance to sliding has been reported to be slightly greater in the wet state 
 than in the dry state .
even if our experiments were performed 
 in the wet state , the relationship should not differ because most friction forces at 
 displacements exceeding 2.0 mm were markedly larger than unloading bending forces . regarding the friction force ,
many studies have reported the use of experimental models 
 to measure the force of a wire drawn from a bracket .
on the other 
 hand , henao and kusy ( 2004 ) used a 
 lower typodont malocclusion model with damon brackets , and reported friction forces of 
 250 - 675 cn with a 0.014-inch ni - ti wire .
kim , kim and baek ( 2008 ) reported that the static friction force of a 
 0.014-inch austenitic ni - ti wire was 89.5 - 2249.0 cn in a mandibular typodont with 0 - 3.0 
 mm tooth displacement , identical to that used in this study .
however , kim , kim and 
 baek(2008 ) used a mandibular 
 typodont with lateral incisors that could be displaced lingually . in our experiment , 
 friction forces with the 0.014-inch wire were 11.4 - 950.7 cn . thus ,
if our dental arch 
 model had bilateral tooth displacement , the friction force would be approximately 
 doubled , although the ability to determine the actual friction force is limited in many 
 cases by the simplicity of the model . regarding the experimental model , we used a mandibular dental arch that was placed in 
 passive self - ligating brackets to measure friction force .
the interfulcrum distance , 
 which is standardized to 10.0 mm according to iso 15841 , matches the interbracket distance in mandibular 
 anterior teeth .
kim , kim and baek 
 ( 2008 ) and henao and kusy(2004 , 2005 ) reported friction forces 
 obtained with bimaxillary models .
although friction force depends on the degree of tooth 
 displacement and the brackets used , the force in the maxillary arch must be smaller than 
 that in the mandibular arch because the interbracket distance in the mandible is 
 shorter . however , additional experiments are needed to understand the relationship 
 between the maxillary dental arch and friction resistance because the force produced by 
 the wire must also be smaller . with regard to
the relationship between periodontal tissue and orthodontic force , 
 schwartz ( 1932 ) reported that 
 a safe force for tooth movement was 20 - 26 g / cm . using a rat experiment 
 model , noda , et al .
( 2000 ) found 
 that the optimal force corresponding to a human premolar was 41.4 g. in this experiment , 
 the smallest unloading bending force produced by the wire was 27.8 cn .
the force 
 produced from a wire in the three - point bending test is translated as reciprocal action . 
 because the force that reaches the teeth becomes half of the unloading bending force , 
 the forces of all 0.012-inch wires at a displacement of 0.5 mm and those of some 
 0.012-inch wires at a displacement of 1.0 mm were likely of doubtful use for optimal 
 tooth movement . in product comparisons , no significant difference in unloading bending or friction force 
 was detected between esthetic and p-1 wires or among esthetic wires .
however , 
 significant differences in force - deflection curves were detected between plain p-1 and 
 p-2 ni - ti wires .
the p-2 wire ( nitinol classic ) is a stabilized ( work - hardened ) 
 martensitic ni - ti wire , whereas the p-1 and other wires have curves that convey 
 superelasticity , resulting in an almost even force during unloading .
thorstenson and kusy ( 2002 ) reported that the regression 
 lines in the sliding of a stabilized martensitic ni - ti wire differed from those of 
 active austenitic ni - ti wires . in our study , at a displacement of 3.0 mm , the ratio 
 between the friction and unloading bending forces of a 0.014-inch p-2 wire was the 
 largest among wires , and the friction force was approximately 5.8-fold larger than the 
 unloading bending force .
thus , the surface treatment of the wire used is likely 
 unimportant for austenitic ni - ti wires .
furthermore , the stabilized martensitic ni - ti 
 wire , which exhibited a larger ratio , is not likely to be superior to other wires with 
 respect to the relationship between force and friction , at least within the limitations 
 of this experiment .
within the limitations of this study , the following conclusions were reached : because the friction force at displacements exceeding 2.0 mm is larger than the force 
 produced by the wire , teeth are likely to move with the expansion of the dental arch up 
 to a displacement of 1.0 mm .
no significant difference in unloading bending or friction force was detected between 
 esthetic and austenitic plain ni - ti wires .
the ratio between friction and unloading bending forces was smaller for austenitic ni - ti 
 wires than for martensitic ni - ti wires ; thus , austenitic ni - ti wires are likely more 
 appropriate for clinical use .
 the relationship between orthodontic force and friction produced from an archwire and 
 brackets affects the sliding of the wire in the leveling stage.objectivethe purpose of this study was to evaluate the relationship between force and 
 friction in a small esthetic nickel - titanium ( ni - ti ) wire . material and methodsfive esthetic wires ( three coated and two plated ) and two small , plain ni - ti wires 
 ( 0.012 and 0.014 inches ) were used . 
 we performed a three - point bending test 
 according to iso 15841 and the drawing test with a dental arch model designed with 
 upper linguoversion of the lateral incisor in the arch ( displacements of 0.5 , 1.0 , 
 2.0 and 3.0 mm ) , and evaluated the relationship between them . 
 resultsunloading bending forces of all wires at displacements of less than 1.0 mm were 
 larger than friction forces , but all friction forces at displacements exceeding 
 2.0 mm were larger than unloading bending forces . 
 the arch likely expands when 
 displacement from the proximal brackets exceeds 1.0 mm . 
 the friction force of a 
 martensite 0.014-inch ni - ti wire was significantly greater than those of the other 
 esthetic and austenitic wires . 
 conclusionsa wire with the smallest possible friction force should be used in cases with more 
 than 1.0 mm displacement . 
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS