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 .