a successful orthodontic movement is directly related to the ability of orthodontic 
 wires to slide through brackets slots and tubes , and it is well known that the sliding 
 resistance between the bracket slot and archwire can drastically influence the tooth 
 movement . the sliding mechanism is important not only to close 
 the space , but also in the initial phase of the treatment in which leveling and 
 alignment of the teeth occur . in an attempt to fulfill an increasingly aesthetic demand , aesthetic accessories 
 consisting of different materials have been developed , among which are the aesthetic 
 ceramic and polycarbonate brackets . 
 therefore , these accessories alleviate the aesthetic problem despite the limitations of 
 their use , such as fracture of the brackets , abrasion of antagonist teeth and , mainly , 
 the increased friction resulting from the mechanical sliding .
friction is 
 defined as the force opposing the movement of two objects in direct contact to each 
 other , and its direction is tangent to a common interface between both surfaces . the 
 intensity of this force is closely related to the surface characteristics and properties 
 of the materials involved .
therefore , knowing the influence of 
 different materials on the sliding mechanism , the aim of this study was to assess the 
 frictional force existing between conventional and aesthetic brackets in association 
 with stainless steel ( ss ) and titanium - molybdenum alloy ( tma ) wires by correlating the 
 mean roughness values of wires and bracket slots .
conventional right upper canine brackets ( b ) made of pure polycarbonate ( pb ; american 
 orthodontics , sheboygan , wi , usa ) , ceramic ( cb ; american orthodontics , sheboygan , wi , 
 usa ) , and ss ( ssb ; 3 m unitek , monrovia , ca , usa ) with slots of 0.022x0.030-in were 
 used for study .
ss and tma wires ( american orthodontics ) , both measuring 
 0.019x0.025-in , were evaluated ( figure 
 1 ) .
systems assessed and their respective abbreviations a device for mechanical test was developed in order to simulate the sliding movement 
 of the wire through the bracket slots as seen during the orthodontic treatment .
holes were 
 perforated in this glass support to fit high - rotation penholders ( dabi atlante , 
 ribeiro preto , sp , brazil ) into which the brackets were inserted ( figure 2a ) . the penholders were filled with 
 self - curing composite ( 3 m unitek , so paulo , sp , brazil ) , thus creating a base 
 for bonding the brackets .
the brackets were then positioned and bonded to their 
 respective penholders by using cyanoacrylate ester ( super bonder ; loctite , so 
 paulo , sp , brazil ) ( figure 2b ) .
this set was 
 hold within the glass box through lateral rods made from glass lamina in order to 
 keep the system stable enough during the traction test .
the glass box was designed to 
 keep the system immersed in artificial saliva , thus providing a better simulation of 
 the oral cavity .
also , a heater with thermostat was added to this system so that the 
 inner temperature was maintained at 37c during the whole experiment .
a ) device made for stabilizing the combinations of bracket and wire during 
 mechanical test .
b ) detail of the bracket moving through the wire and 
 angulation close to 0 a rectified 8-cm segment from both orthodontic wires was attached to the universal 
 testing machine ( emic dl 10.000 ; so jos dos pinhais , pr , brazil ) , 
 mounted onto the bonded bracket with no active torque and then tied to it with an 
 elastic thread ( 3 m unitek , usa ) by using a ligature elastic applicator ( morelli , 
 sorocaba , sp , brazil ) .
the universal testing machine simulated the sliding movement of the bracket through 
 the orthodontic wire during the retraction movement of the canines , which yielded a 
 total dislocation of 8 mm that corresponds to the mean width of a premolar .
a personal computer connected to the equipment recorded graphic results showing 
 values of maximum load ( n ) generated by a load cell .
such a test was repeated 5 times 
 for each of the 3 combinations of bracket / wire of each group .
the wings of the brackets were removed with a steel diamond discs ( 22-mm diameter , 
 0.15-mm thickness , ref .
7016 , kg sorensen , so paulo , sp , brazil ) at low speed 
 on a handpiece ( dabi atlante ) for reading through a profilometer ( surf test sj , 201 , 
 mitutoyo co. , kawasaki , honshu , japan ) .
three milliliters of slots of the 3 types of 
 brackets were analyzed , yielding three readings for each bracket .
similarly , 5-mm 
 segments of each wire submitted to mechanical traction test were sectioned , and 3 mm 
 of them were analyzed , also yielding three readings for each wire .
the orthodontic brackets and wires were randomly selected , and sectioned for analysis 
 of their surface with a jeol scanning electron microscope ( 2000 fx , tokyo , japan ) . 

next , the 
 orthodontic bracket and wires were positioned on a double - faced adhesive tape whose 
 sequence was carefully recorded .
the samples were then placed in the sample chamber 
 of the microscope for visualization of the surfaces of the bracket slots and 
 wires .
the data obtained were submitted to simple anova with tukey s test for 
 post - hoc multiple comparison tests between the systems . 

conventional right upper canine brackets ( b ) made of pure polycarbonate ( pb ; american 
 orthodontics , sheboygan , wi , usa ) , ceramic ( cb ; american orthodontics , sheboygan , wi , 
 usa ) , and ss ( ssb ; 3 m unitek , monrovia , ca , usa ) with slots of 0.022x0.030-in were 
 used for study .
ss and tma wires ( american orthodontics ) , both measuring 
 0.019x0.025-in , were evaluated ( figure 
 1 ) .
a device for mechanical test was developed in order to simulate the sliding movement 
 of the wire through the bracket slots as seen during the orthodontic treatment .
holes were 
 perforated in this glass support to fit high - rotation penholders ( dabi atlante , 
 ribeiro preto , sp , brazil ) into which the brackets were inserted ( figure 2a ) . the penholders were filled with 
 self - curing composite ( 3 m unitek , so paulo , sp , brazil ) , thus creating a base 
 for bonding the brackets . the brackets were then positioned and bonded to their 
 respective penholders by using cyanoacrylate ester ( super bonder ; loctite , so 
 paulo , sp , brazil ) ( figure 2b ) .
this set was 
 hold within the glass box through lateral rods made from glass lamina in order to 
 keep the system stable enough during the traction test .
the glass box was designed to 
 keep the system immersed in artificial saliva , thus providing a better simulation of 
 the oral cavity .
also , a heater with thermostat was added to this system so that the 
 inner temperature was maintained at 37c during the whole experiment .
a ) device made for stabilizing the combinations of bracket and wire during 
 mechanical test .
b ) detail of the bracket moving through the wire and 
 angulation close to 0 a rectified 8-cm segment from both orthodontic wires was attached to the universal 
 testing machine ( emic dl 10.000 ; so jos dos pinhais , pr , brazil ) , 
 mounted onto the bonded bracket with no active torque and then tied to it with an 
 elastic thread ( 3 m unitek , usa ) by using a ligature elastic applicator ( morelli , 
 sorocaba , sp , brazil ) .
the universal testing machine simulated the sliding movement of the bracket through 
 the orthodontic wire during the retraction movement of the canines , which yielded a 
 total dislocation of 8 mm that corresponds to the mean width of a premolar .
a personal computer connected to the equipment recorded graphic results showing 
 values of maximum load ( n ) generated by a load cell .
such a test was repeated 5 times 
 for each of the 3 combinations of bracket / wire of each group .
the wings of the brackets were removed with a steel diamond discs ( 22-mm diameter , 
 0.15-mm thickness , ref .
7016 , kg sorensen , so paulo , sp , brazil ) at low speed 
 on a handpiece ( dabi atlante ) for reading through a profilometer ( surf test sj , 201 , 
 mitutoyo co. , kawasaki , honshu , japan ) .
three milliliters of slots of the 3 types of 
 brackets were analyzed , yielding three readings for each bracket .
similarly , 5-mm 
 segments of each wire submitted to mechanical traction test were sectioned , and 3 mm 
 of them were analyzed , also yielding three readings for each wire .
the orthodontic brackets and wires were randomly selected , and sectioned for analysis 
 of their surface with a jeol scanning electron microscope ( 2000 fx , tokyo , japan ) . 

next , the 
 orthodontic bracket and wires were positioned on a double - faced adhesive tape whose 
 sequence was carefully recorded .
the samples were then placed in the sample chamber 
 of the microscope for visualization of the surfaces of the bracket slots and 
 wires .
the data obtained were submitted to simple anova with tukey s test for 
 post - hoc multiple comparison tests between the systems . 

ss wire had a lower mean value of roughness value compared to tma wire . on the other 
 hand , the polycarbonate bracket had a greater roughness compared to the ceramic and 
 metallic ones ( table 1 ) .
the results regarding 
 roughness of brackets and wires after be submitted to mechanical traction test 
 demonstrated a reduction in the roughness in all materials studied , with the metallic 
 brackets having the most significant reduction ( 61.7% ) and the polycarbonate brackets 
 having the lowest ( 47.6% ) .
reading regarding 3-mm segment of each 
 sample comparing the maximum mean friction values for the bracket systems using ss wire ( table 2 ) , no statistically significant differences 
 were found between metallic ( ssb - ss ) and polycarbonate ( pbss ) brackets .
there were 
 significant differences between the systems of ceramic ( cb - ss ) and metallic brackets 
 using ss wire ( ssb - ss ) ( p<0.001 * ) as well comparing the systems 
 cb - ss and pb - ss ( p=0.001 ) .
statistical data on friction coefficient of the systems of bracket and wire 
 ( n=15 ) significance level of 95% ( * p<0.05 ; anova and tukey 's post hoc test ) . 

sd = standard deviation with regard to systems of brackets using tma wire , no statistically significant 
 differences were found between metallic ( ssb - tma ) and polycarbonate ( pb - tma ) brackets or 
 between metallic ( ssb - tma ) and ceramic ( cb - tma ) brackets .
however , statistically 
 significant differences were observed between the pb - tma and cb - tma systems 
 ( p=0.038 ) . the ssb - ss system had the lowest friction coefficient , differing significantly from the 
 ssb - tma ( p=0.012 ) .
however , when the systems pb - ss and cb - ss were 
 compared to the systems pb - tma and cb - tma , respectively , no statistical differences were 
 found .
the ssb - ss and cb - ss systems had the lowest and highest friction coefficients , 
 respectively .
figure 3a shows that the friction 
 coefficient tends to be even higher in the cb - ss group . on the other hand , the pb - tma 
 system had the lowest friction coefficient , followed by the ssb - ss and cb - tma systems , 
 with the highest friction coefficient among all systems ( figure 3b ) .
box plot showing maximum frictional coefficients between metal , polycarbonate , and 
 ceramic brackets in association with different wires : a ) stainless steel wire ; b ) 
 titanium - molybdenum alloy ( tma ) wire .
simple analysis of variance ( anova ) and 
 tukey s test as post hoc multiple comparison test at confidence interval of 95% ( * 
 p<0.05 ) the tma wire ( figure 5a , b ) has less smooth surfaces compared to ss wire ( figure 4a , b ) . 

the conventional metallic bracket ( figure 6a , 
 b ) also exhibits a smoother surface than the 
 polycarbonate ( figure 7a , b 
polymers homogeneously distributed ) and ceramic ( figure 8a , b  
 the most irregular surface among all ) brackets . scanning electron microscopy micrograph showing surface topography of tma wire 
 ( associated with mb ) before ( a ) and after ( b ) mechanical traction scanning electron microscopy micrograph showing surface topography of ss wire 
 ( associated with mb ) before ( a ) and after ( b ) mechanical traction scanning electron microscopy micrograph showing surface topography of mb slot 
 ( associated with ss wire ) before ( a ) and after ( b ) mechanical traction scanning electron microscopy micrograph showing surface topography of pb slot 
 ( associated with ss wire ) before ( a ) and after ( b ) mechanical traction scanning electron microscopy micrograph showing surface topography of cb slot 
 ( associated with ss wire ) before ( a ) and after ( b ) mechanical traction after the mechanical traction test ,
the surfaces of all materials studied were more 
 polished , which was more evident in the polycarbonate brackets and less evident in the 
 ceramic brackets .
the friction on a given surface is closely related to both material involved and surface 
 characteristics . with regard to roughness ,
the ss wire 
 exhibited less irregular surface compared to that of tma wire , thus corroborating other 
 findings in the literatureand confirming that the greater the 
 roughness , the higher the friction coefficient . in the present study ,
the systems using tma wire had a higher 
 friction coefficient regardless of the type of bracket used .
in addition , tma has been 
 shown to present lower modulus of elasticity , springback greater than that of 
 steel . with regard to the 
 brackets , the slots of metallic brackets exhibited less roughness
however , the polycarbonate bracket showed a friction 
 coefficient lower than that of ceramic bracket either with ss wire 
 ( p=0.01 ) or tma wire ( p=0.038 ) , both statistically 
 significant .
this may be explained by the fact that the profilometer could not detect 
 small irregularities which were easily observed in the sem images , and the ceramic 
 bracket showed more irregularities than the polycarbonate accessory , thus justifying the 
 similarity of roughness between ss and tma wires .
the authors developed an apparatus that was immersed in artificial saliva during 
 mechanical traction test , which not only reduces the friction but also simulates the 
 sliding mechanism as seen in vivo and control an important variable , 
 the lubrication .
furthermore , studies demonstrated that lubrication 
 reduces significantly the friction between bracket and wire .
also , a device was developed to correct the small vertical 
 angulations so that the angle formed between wire and bracket was close to 0 , thus 
 helping to reduce the friction coefficient . according to the literature , friction 
 coefficient increases as the angulation between bracket and wire increases .
generally , when extraction is indicated during orthodontic treatment , closing of 
 extraction spaces can be performed mainly by canine retraction through distal 
 movement .
wire was attached to the bracket 
 with elastic ligatures because it promotes an additional force in comparison to wire 
 ligature .
in addition , attachment to the bracket with elastic 
 guarantees a standard force in comparison to the wire ligature .
the different combinations comparing the ssbss and pb - ss systems showed that both 
 yielded the lowest friction coefficients without statistically significant differences . 

this fact is explained by the use of brackets and wires made from the same material , 
 which reduces friction as surfaces from the same material tend to have less friction 
 compared to surfaces from different materials , in addition to their low roughness 
 compared to other groups . with regard to the pb - ss system , despite using different 
 materials , the low friction coefficient observed may be explained by the fact that the 
 polymeric chains ( macromolecules consisting of simple molecules ) are small spheres 
 homogeneously distributed on the surface of the polycarbonate bracket , being partially 
 responsible for its roughness .
these factors may explain the lack of statistically 
 significant difference in the low friction coefficient between these both systems .
on the other hand , the cb - ss system had the highest friction coefficient , being 
 statistically significant different from the ssb - ss system , which is also in agreement 
 with the literature .
this can be 
 explained by the higher surface roughness of the ceramic brackets and also by the 
 different wire material ( ss ) , thus 
 making the sliding movement of the bracket through the wire difficult .
in addition , the 
 ceramic bracket showed higher friction coefficient compared to the polycarbonate 
 bracket , since the former consists of crystal grains that are clearly larger than the 
 monomers of the polycarbonate .
as can be seen in the sem images , the ceramic surface 
 also exhibits more irregularities , thus increasing the friction .
similarly , no statistically significant difference between polycarbonate and metallic 
 brackets was found when they were used in combination with tma wire , with both systems 
 having a lower friction coefficient .
significant difference was observed in relation to 
 the ceramic brackets ( p=0.038 ) , but not between metallic and ceramic 
 brackets .
this is due to the fact that both ceramic bracket and tma wire have higher 
 friction coefficients because they have a rougher topography compared to other systems . 

however , no statistically significant difference was observed between polycarbonate and 
 metallic brackets regarding the friction coefficient because of the increased friction 
 observed in relation to the ss wire .
however , even in the final phases of the treatment in 
 which the long - range movements are not the main goal , tma wire should be carefully used 
 as the results of the present study have shown that its friction is significantly higher 
 than that of ss wire for metallic brackets , a finding also corroborated 
 elsewhere . however , when polycarbonate and ceramic brackets are 
 used in combination with tma wire , the friction coefficient decreases compared to 
 systems using ss wire .
it is suggested that despite the brackets being of different 
 materials and tma wire having a rougher surface compared to ss wires , it is possible 
 that the flexibility of the tma wire might have produced less friction , reducing the 
 frictional resistance for small inclination angles , although the brackets were in a 
 rotating platform . therefore , no statistically significant difference between metallic 
 and polycarbonate brackets was observed , whereas significant difference was found 
 between the polycarbonate and ceramic brackets .
this latter system also showed higher 
 friction coefficient compared to the other systems , but with no statistically 
 significant difference .
one can observe that the ss wire has a lower friction coefficient when used with 
 metallic bracket compared to the tma wire , which is well described in the 
 literature . as the materials composing brackets and wires of each 
 system are different , no significant differences between the systems using polycarbonate 
 and ceramic brackets in association with ss and tma wires were observed .
the orthodontic brackets and wire submitted to mechanical traction tests were then 
 evaluated by using a profilometer in order to verify changes on the surface .
the results 
 showed a decrease in the roughness of all materials tested : ss wire ( 52.2% ) , tma wire 
 ( 84.1% ) , polycarbonate bracket ( 47.6% ) , ceramic bracket ( 60.1% ) , and metallic bracket 
 ( 61.7% ) .
based on the data obtained the present study , it is important to emphasize that in 
 sliding mechanics the effective force to be clinically applied to metallic brackets 
 should be increased when using tma wire instead of ss wire in order to maintain a 
 satisfactory speed of the tooth movement . on the other hand ,
some undesirable consequences such as loss of 
 anchorage , tooth inclination , root absorption , and intense pain may occur , among other 
 complications that may cause short- and long - term irreversible damages .
future studies 
 are suggested to evaluate how human salivary components act on the friction between 
 bracket and wire during orthodontic treatment .
the following conclusions can be drawn : the system formed by metallic bracket and ss 
 wire exhibited less roughness and lower friction .
the use of ceramic brackets with tma wire should be judiciously used as this system was 
 found to have a higher friction coefficient .
 objectivethe objective this work was to assess the friction coefficient between brackets 
 and wires of different materials under conditions simulating the oral 
 environment.material and methodsstainless steel ( ss ) and titanium - molybdenum alloy ( tma ) wires of 0.019x0.025-in 
 diameter ( american orthodontics ) and polycarbonate bracket ( american 
 orthodontics ) , ceramic bracket ( american orthodontics ) , and metal bracket ( 3 m 
 unitek ) with slots of 0.022x0.030-in were used . 
 the friction coefficient was 
 assessed by means of mechanical traction with the system immersed in artificial 
 saliva . 
 the mean roughness of both wire surface and bracket slots was evaluated by 
 using a surface profilometer.resultsthe system using tma wire and polycarbonate bracket had the highest roughness 
 ( p<0.05 ) . 
 ss wire with ceramic bracket had the highest friction 
 coefficient , whereas the use of metallic bracket yielded the lowest 
 ( p<0.05 ) . 
 however , it was observed a statistically significant difference 
 in the system using tma wire and ceramic bracket compared to that using tma wire 
 and polycarbonate bracket ( p=0.038).conclusionceramic brackets in association with ss wire should be judiciously used , since 
 this system showed a high friction coefficient . 
INTRODUCTION
MATERIAL AND METHODS
Materials
Mechanical Traction Test
Surface Roughness
Scanning Electronic Microscopy (SEM)
Statistical Analysis
RESULTS
DISCUSSION
CONCLUSIONS