wear resistance of denture teeth has been considered as one of the most important requirements for oral rehabilitation of edentulous patients with removable dentures , in order to maintain a stable occlusal support over time13 .
wear of the occlusal surfaces may result in insufficient posterior tooth support25 , loss of chewing efficiency1,21 and nonfunctional activities1,2 .
although wear of acrylic resin teeth has also been related to the loss of vertical dimension of occlusion with complete dentures , the major factor affecting it is the reduction of residual ridges by resorption22 .
initially , denture teeth were made of ceramic material . with the advent of polymethyl methacrylate ( pmma ) in the 1940s ,
denture teeth are currently made of either methacrylate - based resins ( acrylic resin ) or ceramics , although resin teeth have almost eliminated ceramic teeth from the market25 due to a number of advantages , such as the chemical bond to denture base5,6,16 , lower susceptibility to fracture6 and decrease of clicking4,24 .
nonetheless , the wear resistance of acrylic resin teeth has been questioned for being lower than that of ceramic teeth10 - 12,18,20,22,28 .
manufacturers have then tried to develop acrylic resins designed to offer improved wear resistance for resin denture teeth25 .
pmma has been reported to have advanced with the advent of cross - linked agents27 , which are bifunctional monomeric molecules that are added to the polymeric material to allow crossing between linear polymeric chains11 .
the cross - linked polymer was later modified by increasing the quantity of cross - linked agents to ensure a higher degree of cross - linking between the polymers22 , or by blending special polymers and co - polymers.3 other modification of cross - linked polymer was the appearance of resins with interpenetrating polymer network ( ipn ) 22,28 .
ipn resin is formed when a polymer network is crossed inside another network occupied by a second crossed polymer .
the crossed networks coexist in the same volume of the space ( one being physically retained within the other ) and can not be separated without chemical bond rupture22,29 .
these factors may influence the final quality for the acrylic resin and produce denture teeth with better mechanical and physical properties26 .
the search for a more wear - resistant denture tooth material resulted in the development of modified resin teeth that display acceptable wear resistance21 . in present days
, acrylic resin denture teeth can be classified into two main categories : conventional ( with crossed - linked polymeric chains ) and high - strength ( with various polymer technologies that provide different chemical compositions ) .
some studies22,27,28 have found a higher wear resistance for the high - strength acrylic resin denture teeth with modified polymers in comparison to conventional pmma , while others14,15,17,19,29 have found no advantages of such materials with regard to wear properties .
composite resin denture teeth were developed in the 1980s as an effort to achieve greater wear resistance and bond strength to denture bases30 .
tooth materials made with microparticle inorganic fillers immersed in a bis - gma ( bisphenol a glycidyl methacrylate ) matrix1,28 , or nanometric inorganic fillers immersed in a pmma matrix26 have been used for fabrication of composite denture teeth .
it has been reported that composite denture teeth show a higher wear resistance than acrylic resin denture teeth3,26 , although data vary depending on the experimental design3 . moreover , some authors found denture teeth with inorganic fillers to be 40 - 50% more abrasion - resistant than ipn teeth27 . despite the evolution of the mechanical properties of pmma , wear of this material continues to occur with clinical use10,12 .
the wear resistance of acrylic resin denture teeth has been investigated22,27,28 ; however , the constant appearance of resins with modified polymers used for fabrication of denture teeth manufacturing , impose the need for further studies in this context .
furthermore , the results are not clear because the wide variety of wear - testing devices have hindered the comparison of the results19,26 , which reflect the lack of a standardized and reproducible methodology for testing the wear resistance of acrylic resin denture teeth28 . this in vitro study tested the null hypotheses that there is no difference among the wear resistance of one conventional ( trubyte biotone ) and two high - strength pmma denture teeth ( trilux and vivodent ) when opposed to glazed or airborne particle abraded ceramic .
the two different antagonist ceramic surfaces were used to allow the test to be carried out under conditions that would cause minimal and maximal wear of the denture teeth .
both right and left maxillary canines ( n=10 ) of 3 types of pmma denture teeth were purchased : trubyte biotone ( a conventional denture tooth composed by cross - linked pmma ) ; trilux ( a high - strength tooth composed by highly cross - linked ipn - pmma ) ; and vivodent ( a high - strength tooth composed by highly cross - linked pmma ) ( table 1 ) .
all teeth were stored in distilled water at 37c for 24 h before wear testing to allow water absorption . *
the in vitro two - body wear - testing apparatus used in this study was a simulated brushing machine .
the denture tooth was fixed on the top of a tooth brush using a screw and composite resin to allow the tooth be fixed on the wear - testing apparatus . the tooth was securely mounted on the upper stainless steel holder of the apparatus ( movable part ) , whereas the antagonist plate was mounted on the fixed bottom part so that they contact tightly on each surface ( figure 1 ) . glazed or airborne particle abraded metalloceramic plates ( 2.5 x 1.2 x 0.15 cm ) were used as antagonist surfaces and replaced after each change of denture tooth .
the opposing plates were cast in ni - cr alloy ( durabond universal , odonto comercial importadora ltda ) and trimmed with aluminum oxide abrasive burs at low rotation .
thereafter the surface was sandblasted with 100 m aluminum oxide particles ( trijato odonto larcon , maring , pr , brazil ) and the conventional application of porcelain was made , including the glaze final process ( duceram plus , degudent ind .
the antagonist ceramic was planed and polished using silicon carbide paper up to 600 ( extec corp .
, enfield , usa ) and then sandblasted with 100 m aluminum oxide particles . at each change of paper ,
the metalloceramic plates were cleaned in an ultrasonic cleaner at 40 hz for 2 min .
wear testing was performed by repeated sliding contact at 4.5 cycles / s with 20 mm sliding distance per cycle in the buccolingual direction with a load of 300 g. during the wear test , continuous rinsing with demineralized water ( 37c ) was used to remove abraded particles from the sample surface and to simulate the wet environment of the oral cavity .
the height ( mm ) of each tooth was measured using a digital measuring microscope accurate to 0.001 mm ( tm-505 , mitutoyo corporation , japan ) before and after the wear test .
the wear resistance assessed as height loss ( mm ) was calculated as the decrease in height after 100,000 cycles .
the mean wear values were analyzed individually by two - way analysis of variance ( anova ) and the differences between the denture teeth were determined by the tukey 's test ( =0.05 ) .
the mean wear values obtained after 100,000 cycles of all denture teeth tested against the glazed or airborne particle abraded ceramic plates are summarized in table 2 .
there were no statistically significant differences ( p>0.05 ) in wear among the 3 denture teeth evaluated against glazed ceramic .
there were significant differences among the teeth tested against abraded ceramic , with trubyte biotone presenting significantly more wear compared to the high - strength denture teeth ( p<0.0001 ) , which , in turn , did not differ significantly from each other ( p>0.05 ) when tested against either glazed or airborne particle abraded ceramic .
all teeth showed significantly more wear against airborne particle abraded than against glazed ceramic ( p<0.05 ) . * data with the same letters are not statistically different within the same antagonist ceramic at 5% significance level .
a large number of chewing simulators serving to determine the in vitro wear of acrylic resin denture teeth have been described8,24,30 . in vitro methods
three - body wear tests are frequently used to measure wear characteristics . in these tests ,
in addition to the antagonists , various factors are taken into consideration , namely the abrasive properties of the intermediate medium or ph - related dissolution processes .
on the other hand , in two - body wear tests , all substance losses result from a direct interaction between the specimen surface and the antagonist surface31 .
clinically , this situation arises , for example , during swallowing , empty mastication , parafunctional habits and dynamic occlusion movements . especially in the case of full dentures with bilaterally balanced occlusion , these direct occlusal contacts are a factor that contributes to the wear of denture teeth . therefore , a two - body wear simulation was performed in the present this study25 . the wear - testing apparatus used in this study was developed to evaluate two - body wear of the pmma teeth abraded directly against glazed or airbone particle abraded opposing ceramic without an intermediate abrasive medium .
the obtained data showed that the wear of resin denture teeth was influenced considerably by the texture of the opposing material .
the least wear indicated as the total height loss was observed with trilux tooth opposing the glazed ceramic .
however , a very small amount of wear occurred in all denture teeth tested against glazed ceramic , which suggests that the difference on wear among the teeth could not have been detected in this way .
indeed , no significant differences were observed on the wear of the teeth tested against the glazed ceramic ( p>0.05 ) . nonetheless , with regard to the data obtained from the airborne particle abraded ceramic ,
trubyte biotone ( a conventional denture tooth made of cross - linked pmma ) showed significantly more wear than trilux ( a high - strength denture tooth made of highly cross - linked ipn - pmma ) and vivodent ( a high - strength denture tooth made of highly cross - linked pmma ) .
since higher mean wear values were found against the abraded ceramic compared to the glazed ceramic , these results appear to be more consistent in this current wear test series .
it is thus assumed that a connection between the chemical composition and the wear resistance of these denture teeth may rather exist .
these data are consistent with those other studies that have also found a lower wear resistance for conventional pmma denture teeth evaluated in comparison to high - strength denture teeth8,21,24,28,27,26 . however ,
regarding the high - strength teeth evaluated in the present study ( trilux and vivodent ) , no significant differences were found in wear for either of the opposing ceramics .
this finding was in accordance with some authors15,19,21 and had previously been reported by other study3 . in spite of that , differences in the wear of high - strength denture teeth formulations have been reported27 .
therefore , the results of previous in vitro studies on wear resistance of acrylic resin denture teeth seem to be rather inconsistent25 .
the findings of whitman , et al.28 , hirano , et al.13 , and suzuki26 have suggested that denture teeth made of ipn resin , highly cross - linked polymers or composite resin are more wear - resistant than the conventional acrylic resin denture teeth made of cross - linked pmma .
25 , on the other hand , did not find any connection between the chemical composition and the wear resistance of the tested denture teeth .
other investigations also failed to show differences among denture teeth made of polymers with a high degree of cross - linking or polymers with inorganic fillers and teeth made of conventional pmma3,14,15,19,22,28,29 .
these evidences put in question the superiority of high - strength denture teeth and raises doubts about the advantageous clinical use of these products over time .
indeed , recent clinical investigations have not revealed any statistically proven difference in wear behavior between polymers with a high degree of cross - linking or with inorganic fillers and conventional polymethyl methacrylate15,19,22 .
these conflicting data may be due to the large variety of experimental designs , measuring instruments and wear - testing methods used in these investigations .
the large number of denture tooth brands with different chemical compositions has created additional difficulties to the analysis of these data . in spite of the controversy
the results of this study may assist dentists in selecting pmma denture teeth from the standpoint of wear resistance .
nonetheless , it is important to consider that there is not a determinant factor for predicting the wear of pmma denture teeth . on the contrary , besides the chemical composition of the acrylic resin , several other factors should be taken into account on the abrasive process to allow the scientific understanding of the complex phenomenon of wear , namely the chewing pattern10,18 , chewing frequency14 , occlusion force , food abrasion10,18 , non - functional tooth - grinding habits11 , abrasive cleansers3 , materials ' mechanical properties1,5,20 and dusty atmosphere12 .
under the experimental conditions and within limitations of this study , the null hypothesis was rejected , since the wear resistance of the three types of polymethyl methacrylate denture teeth was different against glazed or airborne particle abraded ceramic .
 the wear resistance of denture teeth is important to the longevity of removable prostheses of edentulous patients . 
 the ability of denture teeth to maintain a stable occlusal relationship over time may be influenced by this property . 
 the purpose of this in vitro study was to evaluate the wear resistance of polymethyl methacrylate ( pmma ) denture teeth based on their chemical composition when opposed by a ceramic antagonist . 
 the maxillary canines ( n=10 ) of 3 pmma denture teeth ( trubyte biotone , cross - linked pmma ; trilux , highly cross - linked ipn ( interpenetrating polymer network)-pmma ; and vivodent , highly cross - linked pmma ) were secured in an in vitro 2-body wear - testing apparatus that produced sliding contact of the specimens ( 4.5 cycles / s , sliding distance of 20 mm , under 37c running water ) against glazed or airborne particle abraded ceramic . 
 wear resistance was measured as height loss ( mm ) under 300 g ( sliding force ) after 100,000 cycles , using a digital measuring microscope . 
 mean values were analyzed by 2-way anova and tukey 's test ( =0.05 ) . 
 the wear of trubyte biotone ( 0.93  0.14 mm ) was significantly higher than that of both other types of teeth tested against abraded ceramic ( p<0.05 ) . 
 the vivodent tooth ( 0.64  0.17 mm ) exhibited the best wear resistance among the denture teeth tested against airborne particle abraded ceramic . 
 there were no statistically significant differences ( p>0.05 ) in wear among the 3 denture teeth evaluated against glazed ceramic . 
 trilux and vivodent teeth tested against either glazed or airborne particle abraded ceramic did not differ significantly from each other ( p<0.05 ) . 
 all teeth showed significantly more wear against airborne particle abraded ceramic than against glazed ceramic ( p<0.05 ) . in conclusion , the three types of pmma denture teeth presented significantly different wear resistance against the abraded ceramic . 
 the high - strength pmma denture teeth were more wear - resistant than the conventional pmma denture tooth . 
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS