StomatologyEduJ 5(1) SEJ_4-2017r | Page 21

IN VITRO WEAR OF THREE BULK FILL COMPOSITES AND ENAMEL
Figure 1 . Total wear of 3 Bulk-fill composites and enamel ( control ) as well as the steatite antagonists after 12 x 10 5 cycles . Letters below the columns indicate Tuckey ’ s group .
while in the former experiment the load was 58.86 N , which seems to be too much since fractures of the samples had occurred . It is difficult to determine the actual chewing force under function . Literature data show high variability ( 20 – 120 N ). The decision to use 49.05 N was based on a paper by Gibbs et al . 26 who reported that value to be the average chewing force under normal function . To measure the wear facets a laser scanner was used . Heintze et al . 27 have indeed shown that there was no significant difference between a mechanical or optical profilometer and a laser scanner . As in a former experiment , 25 the wear behavior in the first 5x10 3 cycles was inconsistent and had a higher variability . This is a known effect called “ running in ”. Therefore , the analysis of the data began at 5x10 3 cycles . From that point on , the wear development was linear with an excellent fit ( R2 > 0.98 ; Fig 2 ), which confirms the findings from Heintze et al ., 21 , 27 Wang et al . 28 and Matias et al . 25 It allows to calculate a wear rate (= volume loss / cycle ) which is best expressed in µ m3 / cycle ( Table 3 ). It is thus possible to make direct comparisons independently from the number of cycles run . Nevertheless , it is recommended to run at least 1.2x10 5 cycles , to exclude an unnoticed change in the slope of the wear rate . The latter may be in fact due to fatigue-induced catastrophic failure , as we have seen with a glass ionomer cement ( unpublished data ). 4.2 . Results With the exception of QuiXX , the results of this study can confirm Heintze ’ s , 21 , 27 and Matias ’ s et al data 25 that the antagonist ’ s wear is about half the wear of the composite materials ( Fig . 1 ). This may be explained by a different composition of QuiXX compared to the other composites , especially in its glass filler . That filler seems to be less hard than the fillers used in the other composites , thus being easier abraded , but at the same time being gentler with the antagonists . QuiXX was worn three times more than the other two materials ( p < 0.001 ). Looking at the composition of QuiXX as indicated in the directions for use , one can see that besides UDMA and TEGDMA , Di-
Figure 2 . Linear regression of wear vs cycles for the tested composites and enamel ( p < 0.0001 ). Superscript letters next to the name of the materials in the legend show Tuckey ’ s group .
and Trimethacrylate resins , also a Carboxilic acidmodified dimethacrylate resin has been added . In addition , silanated strontium aluminum sodium fluoride phosphate silicate glass was used as filler . Furthermore , the material is delivered in a blister , obviously to prevent a ionomeric reaction between the carboxylic acid hydrolyzed by water that may diffuse into the material and the glass , which would make the material harden in its package The manufacturer claimed fluoride release as well . Both facts lead to the suspicion that compomer technology was used for that product , and that could at least partly explain the increased wear of that material . 29-32 X-tra fill is characterized by the manufacturer as a hybrid composite with 70.1 % vol filler content and BiS-GMA , UDMA , BHT and TDMA as resins . Multimodal filler distributions with prepolymerized composite particles have been used for that material . Similar composition can be found in Tetric N Ceram Bulkfil , which could explain the same wear behavior . It is not known by the authors , if Voco uses similar filler technology . Looking at the antagonist wear one may speculate that the filler used by Voco might be of a conventional type , and in average coarser than the one used in the bulk-filled material by Ivoclar Vivadent . An interesting fact , the composite with the highest wear ( QuiXX ) has worn the antagonists the least and the composite with the least wear ( Xtra ) has worn the antagonist the most . This could be partly explained by the particle size , particle size distribution , the properties of the fillers ( composition , hardness ) and the filler load . If the particle distributions and the composition of the fillers used were known , this statement could be verified . Besides showing the least wear , enamel also showed the least antagonist wear . This can be explained with the structure of enamel , which is very dense . The size of the hydroxyapatite crystals is much smaller than the ones of the fillers used in the tested bulkfill composites . Once polished , the enamel surface is very smooth and generates low friction . Since there are considerable differences in the

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