the abrasion wear of artificial teeth used in prosthetic rehabilitation treatments is of great clinical interest . worn teeth alter the vertical dimension of occlusion , which may lead to craniofacial disorders , reduce chewing efficiency , cause fatigue of masticatory muscles , increase patient discomfort and impair the esthetics4,12,13,19 . in order to maintain adequate function and stable occlusion , more attention
should be given to the choice of the artificial teeth , which should be made considering the material of the antagonist teeth2,6 .
there are three options of materials for posterior artificial teeth : porcelain , acrylic resin and metal . in removable prostheses ,
acrylic resin teeth present some advantages over porcelain teeth , such as : less brittleness , better connection to the denture base material , easier occlusal adjustments and repolishing , more natural appearance and less chewing noises15,18 .
the low resistance to abrasion wear of the acrylic resin teeth consists in a limitation of these teeth .
they do not resist parafunctional habits , and many times it is hard to maintain the vertical dimension . in order to improve abrasion wear resistance , acrylic artificial teeth with better mechanical properties
have been developed , such as the interpenetrated polymer network ( ipn ) resin teeth , with polymer doublecrossed links ( dcl ) , and resin teeth with addition of inorganic agents in their composition15 - 17 .
however , a recent study reported that these teeth , when compared to conventional ones , have worse adhesion to the denture base resin11 .
the objective of this study was to evaluate the in vitro abrasion wear resistance of 7 commercial brands of artificial denture teeth ( biolux , trilux , blue dent , biocler , postaris , orthosit , gnathostar ) , opposed to metallic antagonists ( m  nickel - chromium alloy ) and composite antagonists ( c  solidex indirect composite ) .
although some of these artificial teeth are commercially available only in the brazilian market , all of them have formulations found worldwide and can therefore indicate the expected results when used in dental prostheses .
seven groups of 12 specimens each were prepared and divided in two subgroups : one group was tested opposed to metallic antagonists ( m  ni - cr alloy [ vera bond ii , aalba dent .
inc . , cordelia , ca , usa ] ) , and another group was tested opposed to composite antagonists ( c  indirect composite , ( solidex [ shofu inc . , tokyo , japan ] ) .
the ni - cr alloy was selected because it is extensively used in oclusal surfaces of fixed partial dentures , as clinically observed in partially edentulous patients .
the 84 specimens were included in 1-inch pvc rings with self - curing acrylic resin , using a custom - made parallelometer ( ribeiro preto dental school , university of so paulo ) , with a griping tip that stabilized the specimen during the inclusion . for preparation of the antagonist tablets ,
a 20-mm - diameter polytetrafluoroethylene matrix was manufactured in the department of dental materials and prosthodontics of ribeiro preto dental school , university of so paulo ( ddmp ) , with 2- and 3-mm - thick spacers . using 2-mm - thick spacers and picodip wax ( renfert gmbh , hilzingen , germany ) liquefied in a wax dipping pot ( hotty led , renfert , hilzingen , germany ) ,
patterns were obtained , and were included with phosphate - based investment ( termocast , polidental ind . bras .
ltda , so paulo , brazil ) , and cast with ni - cr alloy in the discovery plasma casting machine ( edg equipments and controls ltda , so carlos , sp , brazil ) , under vacuum and inert argon atmosphere .
the castings obtained were sandblasted with 100-m aluminum oxide stream , under 80 psi ( 5.62 kgf / cm ) pressure . using the same matrix , but with the 3-mm - thick spacer , 12 other tablets were obtained with the solidex composite .
each tablet was obtained using an incremental technique , according to which 2 composite increments were built , each one subjected to a 90-s curing cycle in a light - curing unit ( unixs , heraeus kulzer gmbh , hanau , germany ) .
the polymerized specimen was retrieved from the matrix and subjected to more 90-s of light curing . according to the din method stated by the iso
/ ts 14569 - 2 standard , the surface of the antagonist tablet must have a surface roughness of 0.75 m .
thus , all antagonist tablets were prepared with 300- , 600- and 1200-grit silicon carbide paper in a decreasing sequence of abrasiveness , and their surface roughness was read in a surface roughness meter ( spj-2 , mitutoyo , tokyo , japan ) . before abrasion wear tests , the specimens were taken to a profile projector ( nikon 6c , tokyo , japan ) with a 20 magnification and were individually placed in a positioning platform that allows further placement of the specimen in the same position for the post - test measurements .
the abrasion wear tests were performed in the mechanical loading device developed at the ddmp . in this device ,
an acrylic recipient is attached to the device 's lever arm , and performs a 10-mm course , resulting in a linear speed of 88 mm / s .
specimens were placed in a pole with vertical adjustment , allowing it to be positioned over the antagonist .
once the pole was completely released , a load of 5 n , corresponding to the whole weight of the assembly , was applied to the specimens .
this mechanical loading device was designed according to the iso / ts 14569 - 2 standard ( dental materials  guidance on testing of wear  part 2 : wear by two  and/or three - body contact , 2001 ) under the freiburg method .
the tests completed 40,000 cycles , simulating approximately 120 days of normal oral function4 ( 150 min , 4 hz ) , with specimens and antagonists completely immerse in deionized water ( figure 2 ) .
specimens and composite antagonists were stored in distilled water at 37  1c during 7 days before the tests . in the test machine ,
immediately after removal of the water storage , the specimens were dried with absorbent paper and a gentle air stream , and had their final contour traced in the profile projector ( nikon 6c , tokyo , japan ) , over the initial profile traced .
the measure of the abrasion wear was determined by the linear difference between the two profiles , utilizing a digital paquimeter ( mitutoyo cd 
15c , japan , tokyo ) accurate to the nearest 0.01 mm ( figure 3 ) .
the wear values m ) of all tested specimens , grouped by antagonist , were subjected to one - way anova and tukey 's test .
the paired t - test was used for comparisons within each specimen group , among different antagonists , all statistics were performed using the spss statistical package ( spss inc . , chicago , il , usa ) .
the results obtained in the abrasion wear resistance tests by the artificial teeth are shown in figure 4 . there were statistically significant differences ( p=0.001 ) for comparisons among the different artificial teeth against composite antagonist .
po ( 2.33  0.91 m ) and or ( 2.77  0.69 m ) , which had the lowest wear mean values , were significantly different from groups bc ( 4.48  0.80 m ) and gn ( 4.50  0.73 m ) , which had the highest wear mean values . statistically significant differences ( p=0.000 ) were found when the groups were compared against metallic antagonist . or ( 10.45  1.42 m ) presented the highest wear mean value , and differed significantly from all other groups ( p<0.001 ) .
there were significant differences ( p<0.05 ) between bc ( 6.70  1.37 m ) and tri ( 4.30  0.43 m ) , bl ( 3.70  0.61 m ) and po ( 1.78  0.42 m ) .
significant differences ( p<0.05 ) were also found when bd ( 5.37  1.65 m ) and tri ( 4.30  0.43 m ) were compared to po ( 1.78  0.42 m ) , which had the lowest wear mean values ( figure 3 ) . in order to compare the wear for each group caused by the two types of antagonists ,
significant differences were observed for bc ( p=0.007 ) , which presented mean wear of 4.48  0.80 m against composite and 6.70  1.37 m against metal , and for or ( p=0.000 ) , which presented mean wear of 2.77  0.69 m against composite and 10.45  1.42m against metal .
this study excluded clinical variables , such as neuromuscular forces and movements , saliva ph and lubrication , oral hygiene habits and diet , because according to previous studies , those factors are very difficult to control , and are costly and time consuming10,15 .
prosthetic rehabilitation success depends on adopting correct procedures in order to obtain comfort , function and esthetics .
the choice of the materials that will replace missing dental structures is a critical clinical issue for both the professional and the patient .
the dentist has to have knowledge and discernment to adjust the patient 's expectations to the characteristics of the materials indicated for the case .
also , the dentist must be familiar with the properties of the various artificial teeth available in the market and must be aware of patient 's characteristics , such as diet and presence of parafunctional habits , in order to obtain the best possible treatment prognosis1,7 .
stober , et al.15 ( 2006 ) has stated that natural enamel is an unsuitable antagonist material for standardized wear tests because the composition , shape and wear properties of biological substances are highly variable .
however , it should be considered that the interaction with the enamel provide important data for the evaluation of artificial teeth . in the two - body wear test used in the present study ,
there is direct contact between the tested material and its respective antagonist , with no intermediate material , but running water .
kadokawa , et al.9 ( 2006 ) evaluated the abrasion wear and other abrasive properties of porcelain , gold alloy , composite resin and human enamel using 2-body and 3-body conditions ( pmma slurry ) .
the authors concluded that the wear values in 3-body wear were significantly smaller . according to whitman ,
et al.17 ( 1987 ) and hirano , et al.7 ( 1998 ) , artificial teeth made of improved acrylic resin ( ipn , dcl ) , or polymers with addition of inorganic agents are more resistant than conventional polymethylmetacrylate ( pmma ) teeth . in a recent abrasion wear resistance test , cross - linked teeth or teeth with addition of an inorganic layer presented greater abrasion wear resistance than conventional pmma teeth16 .
other clinical studies , however , did not find significant differences among the wear presented by conventional pmma teeth and improved acrylic teeth ( dcl , ipn)5,8,10,12 .
based on the findings of ghazal , et al.5 ( 2008 ) , the test performed in this study correspond to an approach of two - body wear with direct contact between the artificial teeth tested and the antagonist , which produces a mixed wear of adhesion , attrition and fatigue .
some authors have stated that this condition is especially important when considering complete dentures with a bilaterally balanced occlusion6,15 .
additionally , in a 2-body wear test only direct interactions between surfaces of sample and antagonist cause a substance loss15 . in the present study ,
the group po ( double cross - linking [ dcl ] polymethylmetacrylate ) presented the smallest wear mean values , for both metallic and composite antagonists .
the group or ( polymer with inorganic filling ) presented the highest wear mean values against metallic antagonists among all other groups tested .
however , it presented small wear mean values against composite antagonists , similar to the best results obtained by the other groups , which suggests a different type of interaction , possibly justified by the fact that both types of artificial teeth are based on the same polymer type .
brigago , et al.2 ( 2005 ) obtained smaller values of weight and height loss for po teeth .
however , these authors did not obtain enough wear on the tested specimens , and no significant differences were observed among the groups , using 200,000 cycles , which corresponded to 1 year of clinical function14 , with 400 g load . possibly due to the load used in this study , which was low
( 5 n ) , no wear was measured in the antagonists . because the rough surface characteristics ( 75 m , accordingly to the din method of the iso / ts 14569 - 2 standard ) just a mark was visible . the vertical substance loss measured in this study , as a consequence of the sliding contact of a buccal cusp of artificial teeth and the antagonist tablets , represents the abraded area of the teeth . each artificial teeth used in this study
has a particular cusp shape , and the contact area increased as the abrasion test was performed . in order to avoid higher influence of this
increased contact area , the tests performed were limited to simulate a 120-day use of the teeth .
in addition , the cusp shape in this study was in accordance with the sample dimensions described in the iso / ts 14569 - 2 standard ( dental materials  guidance on testing of wear  part 2 : wear by two  and/or three - body contact , 2001 ) under the freiburg method . however , this is a limitation of the method used3 .
other evaluations , with higher loads , must be performed to determine the effects on both the artificial teeth and antagonist materials .
reis , et al.13 ( 2008 ) used a canine tooth to test the in vitro wear resistance for three types of pmma artificial teeth in a similar way , and observed that tested pmma denture teeth have significantly different wear resistance against abraded ceramic .
the highest values were found for high - strength pmma teeth than conventional pmma teeth .
the findings of present in vitro study are important for the comparison of the abrasion wear resistance of different commercially available artificial teeth against different types of antagonists
in addition , new studies should be accomplished to establish the wear resistance of artificial teeth when they are used opposing materials with the same wear characteristics .
based on the methodology applied to this study , considering the inherent limitations and the obtained results , it may be concluded that the antagonist material present on ( or planed for ) a clinical rehabilitation is a factor of major importance to be considered for the choice of the artificial teeth to be used in the prosthesis . or was the group that presented the greatest result variance regarding the abrasion wear resistance ; it also presented the worst result opposed to metallic antagonist ( 10.45  1.42 m ) , and a result similar to the best ones opposed to composite antagonists ( 2.77  0.69 m ) .
the group po presented the smallest wear mean values , both for metallic ( 1.70  0.42 m ) and composite ( 2.30  0.91 m ) antagonists . in the light of the obtained results , dentist 's decision to indicate any of the artificial teeth tested in this study can be made based on the analysis of important characteristics in determining its functionality and esthetics , allied to the cost of the material .
 one of the most important properties of artificial teeth is the abrasion wear resistance , which is determinant in the maintenance of the rehabilitation 's occlusal pattern.objectives:this in vitro study aims to evaluate the abrasion wear resistance of 7 brands of artificial teeth opposed to two types of antagonists.material and methods : seven groups were prepared with 12 specimens each ( biolux  bl , trilux  
 tr , blue dent  
 bd , biocler  
 bc , postaris  
 po , orthosit  or , gnathostar  
 gn ) , opposed to metallic ( m  nickel - chromium alloy ) , and to composite antagonists ( c  solidex indirect composite ) . 
 a mechanical loading device was used ( 240 cycles / min , 4 hz speed , 10 mm antagonist course ) . 
 initial and final contours of each specimen were registered with aid of a profile projector ( 20x magnification ) . 
 the linear difference between the two profiles was measured and the registered values were subjected to anova and tukey 's test.results:regarding the antagonists , only or ( m = 10.45  1.42 m and c = 2.77  0.69 m ) and bc ( m = 6.70  1.37 m and c = 4.48  0.80 m ) presented statistically significant differences ( p < 0.05 ) . 
 best results were obtained with po ( c = 2.33  0.91 m and m = 1.78  0.42 m ) , followed by bl ( c = 3.70  1.32 m and m = 3.70  0.61 m ) , statistically similar for both antagonists ( p>0.05 ) . 
 greater result variance was obtained with or , which presented the worse results opposed to ni - cr ( 10.45  1.42 m ) , and results similar to the best ones against composite ( 2.77  0.69 m).conclusions : within the limitations of this study , it may be concluded that the antagonist material is a factor of major importance to be considered in the choice of the artificial teeth to be used in the prosthesis . 
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS