zirconia ceramic , a structured oxide ceramic , has been widely used for construction of fixed dental prostheses such as veneers , crowns , and fixed partial dentures .
its mechanical properties are comparable to stainless steel as its bending strength is 900 - 1,200 mpa and its compressive strength can range to 2,000 mpa . therefore , zirconia ceramic can be utilized in cases of long span fixed partial dentures on both anterior and posterior teeth .
it also has a high corrosion resistance and chemical stability , as well as biological compatibility to oral tissue .
however , as zirconia ceramic has high corrosion resistance and surface stability , this makes it difficult to achieve a good bonding .
al - amleh et al . , reported a 54% incidence of veneering ceramic fracture on a zirconia substructure , especially in posterior teeth .
there are several reasons which can explain this failure including mismatch of the coefficient of thermal expansion between the veneering ceramic and the zirconia substructure , porosity of the zirconia surface , improper support of the zirconia substructure , and residual stress from cooling . after a veneering ceramic fracture
, restoration replacement may be required , which results in issues related to clinical and laboratory time , as well as additional cost .
the principles of repairing zirconia restorations are similar to repairing fractured ceramics of metal - ceramic restorations .
the second approach is to repair indirectly through overcasting , inlays , or onlay restorations .
between these two repairing techniques , repairing the fractured ceramic with resin composite is the most commonly used method for this task .
the properties of currently available resin composite materials have also been and are being continuously improved for this task . bonding resin composite to ceramic surfaces requires ceramic surface treatments .
etching with hydrofluoric acid in combination with silane , a well - accepted pretreatment for preparing ceramic surfaces for resin composite bonding , are often not successful in anchoring the zirconia surface , due to high corrosion resistance and surface stability .
one of the zirconia ceramic surface treatments for enhanced bonding ( intraoral use ) is coating with a primer .
a primer is easy to apply and cost - effective , and does not require the use of a proprietary tool .
numerous studies support the use of a 10-methacryloxydecyl dihydrogen phosphate ( mdp)-containing primer on the zirconia surface .
the results have shown that phosphate monomers are assuring chemical agents for improving zirconia bonding .
the plausible mechanism is the ability of phosphate monomers to form chemical bonds with the zirconia surface and have polymerizable resin terminal end groups ( e.g. , methacrylate ) , which enable cohesive bonding to appropriate resin cements .
in addition to a mdp - containing primer for zirconia bonding , a silane primer or silane plus phosphoric acid methacrylate and a sulfide methacrylate component have also been proposed to expand their use to zirconia surfaces .
therefore , the objective of the present study was to evaluate the effect of various primers on the shear bond strength of zirconia ceramic and resin composite .
forty zirconia ceramic specimens , 10 mm in diameter and 2 mm thick , were prepared from a presintered zirconia ceramic block ( zeno , wieland dental , germany ) .
the block was cut into pieces by a cutting machine ( isomet 4000 , buehler , usa ) .
then the specimens were heated at 1,450c for 12 h. after the specimen had cooled to room temperature ( 23c ) , the specimens were mechanically polished ( phoenix 4000 , buehler , usa ) under running water using 600- and 1200-grit silicon carbide paper ( 3 m espe , usa ) .
each specimen was embedded in a clear acrylic resin cylinder , 3 cm in diameter and 3 cm in height ( takilon , rodent srl , italy ) .
subsequently , all specimens were sandblasted with 50 m alumina with a 25 mm distance between the specimen and the abrasion tip for 15 s. finally , the specimens were ultrasonically cleaned ( pc3 , l&r mfg co , usa ) in distilled water for 10 min .
the specimens were then divided into four groups of 10 : group c was the control group ( no primer treatment ) , group cp was coated with a relyx ceramic primer ( 3 m espe ) , group ap was painted with an alloy primer ( kuraray medical inc . , japan ) , and group mp was covered with monobond plus ( ivoclar vivadent ag , liechtenstein ) . prior to bonding with resin composite ,
the zirconia ceramic specimens were treated with primers ( according to the above defined three groups ) for 1 min with a clean brush ; two layers were applied and then air - dried . a bonding agent ( adper scotchbond multi - purpose plus adhesive , 3 m espe ) was applied and light - polymerized with visible light ( 600 mw / cm at a wavelength of 400 - 500 nm ; optilux 501 , kerr gmbh , germany ) for 20 s. a thin plastic tube ( ap extrusion , usa ) with 3 mm inner diameter and 2 mm thickness was positioned at the center of each porcelain specimen , and the tube was held in place with plastic pliers fixed onto a metal lab stand .
resin composite ( filtek supreme xt , 3 m espe ) was filled into the tube and light - polymerized for 40 s , and again for 40 s after the plastic tube had been cut with a blade and removed .
subsequently , all ceramic specimens were stored in 100% humidity at 37c for 24 h before testing .
the shear bond strength of zirconia ceramic specimens to resin composite was tested by using a single - bladed instron machine ( model 5583 , instron corp .
the load at failure was recorded and converted to shear bond strength expressed in mega pascals ( mpa ) as shown in the following formula : shear bond strength = f/r , where f is a load force at fracture in newtons , and r is the radius of the resin composite cylinder in meters .
the surfaces of the specimens were subsequently examined under a stereoscope ( model smz 1500 m , nikon instech , kanagawa , japan ) at 40 magnification in order to determine the mode of failure .
mode of failure was recorded by one observer as either adhesive ( between zirconia ceramic or resin composite and the bonding agent ) , cohesive ( in the ceramic , resin composite or bonding agent ) or a combination of adhesive and cohesive fractures .
a one - way analysis of variance ( anova ) was used to find differences between groups .
tukey 's honestly significant difference ( hsd ) test was used for post hoc comparisons (  = 0.05 ) .
results of the one - way anova as well as the results of multiple comparisons using tukey 's hsd tests revealed that the shear bond strength did not differ significantly among groups ( p = 0.079 ) .
the mean values of the shear bond strength of the ceramic to resin composite at fracture are presented in table 1 .
group ap yielded the highest mean and standard deviation ( sd ) value of shear bond strength ( 16.8  2.5 mpa ) , whereas group c presented the lowest mean and sd ( 15.4  1.6 mpa ) .
mean shear bond strength ( mpa ) and standard deviation ( sd ) of zirconia ceramic to resin composite the mode of failure for all specimens , evaluated under a stereoscope , was found to be adhesive failure at the zirconia ceramic and bonding agent interface .
sandblasting the ceramic surface before applying a primer is an appropriate bonding preparation method for oxide ceramic and precious alloys .
this produced an activated microroughened zirconia surface , increased the bonding area , and modified the surface energy and wettability ; hence improving the formation of resin - ceramic bond strength .
three primers used in this study were selected according to the different type of functional monomers .
relyx ceramic primer has a silane functional monomer , whereas an alloy primer is composed of 6-(4-vinylbenzyl - n - propyl ) amino-1,3,5-triazine 2,4-dithione ( vbatdt ) and mdp .
in addition , the monobond plus primer which is claimed to be a universal primer for all dental alloys and dental ceramics , has three components : an alcohol solution of silane methacrylate , phosphoric acid methacrylate , and sulfide methacrylate .
the adhesion mechanisms of alloy primer and monobond plus are described below . for alloy primer , which is recommended for noble alloys surface treatment , sulfur atoms of vbatdt ( a thione - thiol tautomer ) bonds chemically to the noble metal atoms , while the double bonds on the other end of the molecule copolymerize with the resin monomers .
the presence of mdp helps the reaction between vbatdt and noble metals , resulting in increased bond strength .
the results in this study presented the highest shear bond strength in group ap , which might indicate that alloy primer can be repaired on zirconia surface treatments .
for monobond plus , methacrylate monomers with a functional phosphoric acid group are used to establish a bond to zirconia ( a formed methacrylate zirconium oxide compound ) which has high affinity to phosphoric acid ; a typical trait of zirconia ceramic . while for the relyx ceramic primer
, the role of silane in adhesion is that it increases the bond strength by performing two functions .
firstly , silane provides a chemical link between dental ceramic and resin composite as already described above .
secondly , the organic portion of the silane molecule increases the wettability of the ceramic surface and thus , enhances the flow of the resin cement into the complicated archetype of the microundercuts of the ceramic surface , resulting in a more intimate micromechanical bond .
this also effectively reduces the size and numbers of the surface flaws , and thus strengthens the ceramic .
however , previous studies showed that these roles are not amenable to adhesion in zirconia ceramic due to its inert properties . even though these primers have mechanisms to bond to zirconia , results from this present study did not present that .
therefore , as the results in the present study revealed no significant difference in bond strength values , these might imply that the role of primers used have no effect on shear bond strength . in the control group ( group c , no primer treatment ) ,
it was observed that the shear bond strength values received might from the sandblasted surfaces , to some degree .
even though there was no significant difference in bond strength values , the highest shear bond strength was found in group ap , which had vbtdt and mdp functional monomers . in agreement with recent studies
, this study showed that phosphate monomers are chemical agents that provide a good bond strength to zirconia .
it has been reported that if the bond strength between the ceramic and the adhesive resin is greater than 13 mpa , the ceramic will fracture . in this study , all groups obtained values greater than 13 mpa , which resulted in adhesive failures .
this observation is important because if refracture occurred after repairing , it might not cause redamage to the ceramic surfaces , which would affect the esthetics and strength of the restoration . in regard to clinical recommendation for repairing fractured zirconia ceramic restoration , data from the present study advocates treating the ceramic with sandblasting using 50 m alumina and applying either type of primer with adhesive resin .
even though there was no significant difference in bond strength values , bond strength in primer groups were greater than the untreated group , which may be clinically helpful .
a higher shear bond strength between the zirconia ceramic and the resin composite was found in all groups .
for example , the presence of water , temperature change , and ph level in the oral cavity may considerably affect bond strengths of ceramics to resin composite .
in addition , the present study evaluated only one type of resin composite and adhesive bonding ( filtek supreme xt with scotchbond multi - purpose plus adhesive ) , and only three primers were used .
within the limitations of this study , the shear bond strength values between zirconia ceramic and resin composite using various primers were not significantly different .
the mode of failure for all specimens was found to be adhesive failure at the ceramic and bonding agent interface .
 aims : to determine the in vitro shear bond strengths ( sbs ) of zirconia ceramic to resin composite after various primer treatments.materials and methods : forty zirconia ceramic ( zeno , wieland dental ) specimens ( 10 mm in diameter and 2 mm thick ) were prepared , sandblasted with 50 m alumina , and divided into four groups ( n = 10 ) . 
 three experimental groups were surface treated with three primers ; cp ( relyx ceramic primer , 3 m espe ) , ap ( alloy primer , kuraray medical ) , and mp ( monobond plus , ivoclar vivadent ag ) . 
 one group was not treated and served as the control . 
 all specimens were bonded to a resin composite ( filtek supreme xt , 3 m espe ) cylinder with an adhesive system ( adper scotchbond multi - purpose plus adhesive , 3 m espe ) and then stored in 100% humidity at 37c for 24 h before sbs testing in a universal testing machine . 
 mean sbs ( mpa ) were analyzed with one - way analysis of variance ( anova ) and the tukey 's honestly significant difference ( hsd ) test (  = 0.05).results : group ap yielded the highest mean and standard deviation ( sd ) value of sbs ( 16.8  2.5 mpa ) and group c presented the lowest mean and sd value ( 15.4  1.6 mpa ) . the sbs did not differ significantly among the groups ( p = 0.079).conclusions : within the limitations of this study , the sbs values between zirconia ceramic to resin composite using various primers and untreated surface were not significantly different . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION