methacrylate - based resin cements are popular for attaching indirect restoratives , appliances , and fiber posts to the prepared teeth due to their less technique sensitivity , good esthetics , high bond strength , and a potential for fluoride release.1 - 4 as luting agents for indirect restorations , resin cements are needed to keep the restoration in place and prevent dislodgement by forming a hard mass with sufficient strength to resist functional forces . in the oral cavity , however , shear forces that interact with the restoration inevitably induce dislodgement and retention loss . in addition , a low flexural strength , high shrinkage , water uptake , and expansion of the restorative materials also provoke debonding.5 - 7 among the resin cements , dual - cure resin cements have been adopted widely because of their dual cure nature by light and chemical agents . since restorative materials with opacity or dark shades are common , insufficient light transmission to the underlying cement and insufficient curing ( degree of conversion ) can occur .
so any additional curing will be beneficial to compensate insufficient curing of resin cement because the curing conditions can be affected by the external light ( light intensity , distance between the light - guide and resin cement , light - curing unit , etc . ) and restoration thickness.8 - 11 in a dual - cure system , light - curing process is initiated by the light - activated photoinitiators and terminates the process mostly after the end of light irradiation . on the other hand , the self - curing process initiated by a chemical activator ( benzoyl peroxide ) lasts longer , even after the termination of light irradiation .
therefore , in a situation of insufficient light transmission , the free radicals formed by the benzoyl peroxide / amine system compensate for the lack of monomer to polymer conversion.12 the dual - cure resin cements give higher hardness or better bond strength than those of the chemically - activated resin cements.13,14 the overall properties of resin cements using the dual - cure system seem to be superior when compared to the single - cure system .
although the temperature in the oral cavity remains constant in most times , it can vary from 0 to 60 - 70 due to the foods served and smoking.15,16 such a variation in temperature may chill or heat the restorative materials and teeth and can change their mechanical properties . the short - term thermal effect on the resin cements
can be understood indirectly by examining the effect of pre - heating on the resin cement . according to the limited studies ,
the results of preheating were inconsistent and product - dependent.17,18 thus long - term thermal effect on the dual - cure resin cements in conjunction with their mechanical properties was not widely studied .
the present study investigated the effect of temperature on the light - cured resin cements that were immersed in water .
the hypothesis was that the mechanical properties of dual - cure resin cements consistently depend on the temperature of immersion solution .
for the study , five dual - cure resin cements ( maxcem elite ( mc ) , biscem ( bc ) , calibra ( ca ) , relyx arc ( rx ) , and multilink automix ( ma ) ) were selected , and their compositions are summarized in table 1 . for light curing ,
a quartztungsten - halogen ( qth ) light - curing unit ( hilux 601 , first medica , greensboro , nc , usa ) of 900 mw / cm was used . to evaluate the temperature effect on the specimens ,
the light - cured specimens were immersed in deionized water of three different temperatures ( 4 , 37 and 60 ) .
the control specimens were aged only for 24 hours in a 37 dry and dark chamber . to measure the surface microhardness of the specimens ,
a metal mold ( 4  2  3 mm ) was filled with the resin cement and light cured for 40 seconds .
the cured specimens was removed from the mold and aged for 24 hours in a 37 dry and dark chamber .
the specimens were then immersed in deionized water ( 4 , 37 and 60 ) for 7 days . at this time , each specimen ( n=10 for each temperature condition ) was placed in a 1.5 ml tube , and the solution in the tube was replaced daily .
after 7 days , specimens were removed from the tube and the solution on the surfaces was blotted away .
the microhardness of the light cured top side ( z=0 ) and bottom side ( z=3 mm ) surfaces was measured using a vickers hardness tester ( mvk - h1 , akashi , tokyo , japan ) by making a microindentation ( n=12 for each test condition ) under a 200 gf load and 10 seconds dwell time .
a three - point bending test was performed to determine the flexural properties [ flexural strength ( fs ) and modulus ( fm ) ] . to make the specimens , a metal mold ( 25  2  2 mm ) was filled with the resin cement according to the iso 4049.19 after filling the mold , both top and bottom surfaces were covered with slide glasses to make a flat surface .
the specimen was irradiated for 40 seconds using a light - curing unit . since the specimen was much wider ( 25 mm ) than the tip size ( 8 mm ) , five light exposures were performed on each side by overlapping the curing light . after light curing , one group of specimens
was removed from the mold and aged for 24 hours in a 37 dry and dark chamber ( control ) , the other groups of specimens were immersed in the three test solutions ( 4 , 37 and 60 ) for 7 days . after aging or immersing in the test solutions ,
the specimens ( n=10 for each test condition ) were loaded to a universal testing machine ( instron 3345 , grove city , pa , usa ) at a crosshead speed of 1 mm / min .
the fs ( f in mpa ) was obtained using the following formula where d is the distance between the supports ( 20 mm ) , p is the maximum failure load ( n ) , w is the width ( 2 mm ) , and h is the height ( 2 mm ) of the tested specimen .
the fm ( e in gpa ) was obtained using the following formula e = ( p / d )  ( d / ( 4wh ) ) where p / d is the slope in the linear portion of the load - displacement curve . to measure the compressive properties [ compressive strength ( cs ) and modulus ( cm ) ] , a metal mold ( 3 mm in diameter , 6 mm in height ) was filled with the resin cement .
after filling the mold , both the top and bottom surfaces were covered with slide glasses to make the surface flat and then irradiated for 5 seconds ( since light does not reach to the bottom surface , light curing would be better through the lateral surface after exposure ) .
the opposite side was light cured for 40 seconds again after removing the other part . after light curing , one group of specimens
was removed from the mold and aged for 24 hours in a 37 dry and dark chamber ( control ) .
the other group of specimens was immersed in the testing solutions ( 4 , 37 and 60 ) for 7 days . after aging or immersing in the test solutions ,
compression tests were performed using a universal test machine at a crosshead speed of 1 mm / min .
the cs ( c in mpa ) was obtained using the following formula where p is the maximum failure load ( n ) and a is the cross - sectional area of the specimen .
the cm of the specimens is the slope of the linear portion of the load - displacement curve .
the data acquired from the microhardness , three - point bending test , and compression test was analyzed by two way anova and post - hoc tukey tests at the 0.05 level of significance .
table 2 shows the microhardness of the top and bottom surfaces before and after immersion in the solutions of different temperatures . before immersion
, ma and mc showed the highest ( 50.9/47.6 hv ) and lowest ( 33.1/30.1 hv ) microhardness , respectively , among the specimens .
after immersion for 7 days , the specimens showed a significant decrease in microhardness compared to the control case regardless of the solution temperatures ( p<.001 ) . on the top surface , ca and bc showed the lowest ( 4.8 - 15.9% ) and highest ( 42.4 - 54.6% ) microhardness difference between the control and immersed specimens , respectively .
rx and ma showed a consistent microhardness decrease as the solution temperature increased . on the bottom surface , for the control case , the microhardness decreased approximately 3.3 - 12.2% compared to that of the top surface .
the change of microhardness for solutions of different temperatures was not consistent both on the top and bottom surfaces .
flexural properties ( fs and fm ) of the resin cements immersed in the solutions of different temperatures are shown in table 3 .
the highest and lowest fs , before ( control ) and after immersion , were achieved by ca ( 116.8 - 147.1 mpa ) and bc ( 51.9 - 85.5 mpa ) , respectively .
after immersion for 7 days , in most cases , fs decreased with increasing solution temperature . among the specimens , bc and ca showed the highest ( 39.3% ) and lowest ( 20.6% ) fs decrease , respectively . before immersion
, ma and rx showed the highest ( 12.95 gpa ) and lowest ( 10.72 gpa ) fm , respectively .
after immersion , most specimens ( except ca ) showed a significant decrease in modulus with increasing solution temperature ( p<.001 ) . among the specimens , ca and bc showed the highest ( 10.82 - 11.49 gpa ) and lowest ( 6.62 - 6.81 gpa ) fm , respectively .
table 4 shows the compressive properties ( cs and cm ) of the resin cements immersed in the solutions of different temperatures . before immersion ,
rx and bc showed the highest ( 299.3 mpa ) and lowest ( 211.7 mpa ) cs , respectively .
after immersion for 7 days , cs of the specimens decreased significantly ( p<.001 ) . among the specimens , ca and ma showed the highest ( 22.6% ) and lowest ( 2.2% ) cs decrease , respectively .
for cm , before immersion , bc and rx showed the highest ( 4.19 gpa ) and lowest ( 3.62 gpa ) modulus , respectively .
however , after immersion , specimens showed a significant decrease in modulus ( p<.001 ) . among the specimens
, bc showed the lowest modulus ( 2.16 gpa ) when immersed in the solution of 60.
the present study evaluated the mechanical properties of dual - cure resin cements under different solution temperatures , which can minimally reflect the oral situation .
the choice of solution temperature was arbitrary because the range of temperatures in the oral cavity is complicated and dynamic .
in most situations , a constant temperature is maintained if there is no eating or drinking .
however , the temperature can vary from 0 to 60 - 70 once cold or hot food is served.15,16 the range in this case can change depending on the dietary habits of each individual . in that sense , the choice of three different temperatures ( 4 , 37 and 60 ) appears to be minimally appropriate for mimicking the oral situation .
the microhardness test is a useful indirect assessment for evaluating the degree of cure of methacrylate - based specimens .
the microhardness of the specimens is affected by many other factors which constitute the specimens . among the factors , the filler content ,
monomers types and ratios were found to be the determinant factors.20,21 regarding the filler content , in the present study , manufacturers released only the weight% data . according to the data , the resin cements have much lower filler content than many other composite resins . generally , in light - curing composite resins , the microhardness is positively correlated with the filler content .
the degree of cure is basically the degree of monomer conversion to the polymer networks .
most methacrylate - based dental restorative materials contain monomers , such as bis - gma , bis - ema , udma , and tegdma , with many different combinations and ratios .
this means that the degree of cure ( conversion ) changes inconsistently depending on the commixtures and their combination ratios.22,23 in the present study , the filler content of the specimens showed a low correlation with the measured microhardness ( r<0.4 ) regardless of the solution temperatures .
such a low correlation may be due to the low filler content of the tested specimens . within the resin matrix ,
the distributed fillers are the source of light dissipation because fillers induce light scattering and absorption , so low filler content may result in less light loss within the specimen .
actually , resin cements are not used for the restorations in which a direct mastication load is impressed , so the situation of low filler content would be tolerable both by itself and due to the dual - cure nature .
the process of dual - cure is the curing of resin cements both by the external light and chemical agent .
the dual - cure polymerization is beneficial in which light transmission is not sufficient . in this process
, light - induced polymerization occurs first and then chemical agent - induced polymerization occurs later .
the latter process compensates incomplete polymerization due to insufficient light transmission at the lower part of the specimen . on the control case ( before immersion ) , minor microhardness difference ( 3.3 - 12.2% depend on resin brand ) between the top and bottom surfaces would be due to the additional curing by chemical agent .
such low microhardness difference is not common in the light - curing composite resins of 3-mm thick .
after immersion in the test solutions for 7 days , the specimens showed a significant decrease in microhardness both on the top and bottom surfaces .
such decrease in microhardness has also been observed in composite resins due to softening of the surface.24 once the specimens are immersed in the test solutions , water uptake and subsequent attacks at the interface between the inorganic fillers and resin matrix will occur .
weakening of the bond between the filler and matrix and leaching of the surface by water make the specimen soft , which leads to a decrease in hardness .
in the case of rx and ma , the microhardness decreased gradually with increasing solution temperature . on the other hand ,
in the case of bc , the microhardness increased gradually with increasing solution temperature even though the microhardness had significantly decreased after immersion at 4 solution .
moreover , any additional curing by the external heat after the termination of light curing will depend on the residual monomers . in dual - cure resin cements , the residual monomers will be polymerized further by the subsequent chemical curing , so the remaining monomers will be reduced further with time ( even though the situations can be different in each specimen ) . in the present study , hot water may enhance further polymerization , but , it may also enhance the softening and leaching of specimen through the specimen expansion and dissolution , so subsequent weakening is possible . on the other hand , if chemical curing after light curing is not complete and any monomers remain , external heat from the immersion solution may lead to some further polymerization , resulting in a subsequent increase in microhardness .
fs and fm , the flexural properties , are determined using a three - point bending test .
these flexural properties are related to the resistance to the mastication stress without fracture or permanent deformation .
generally , fs depends on the internal defects or voids that were generated during manufacturing process.25 - 27 in the present study , fs and fm showed a significant decrease after immersion in the test solution , but the decrease was not consistent in the specimens .
the fs of the resin cements tested ranged approximately 52 - 147 mpa depending on the conditions ( control or immersion ) .
these values meet the minimum fs requirement for dual - cure luting materials ( 50 mpa ) set by iso 4049.19 as to the many methacrylate - based composite resins from more than 70 proprietary products ( approximately 63 - 161 mpa ) , the tested resin cements showed a similar range of fs values.28 after immersion for 7 days , the initial ( control ) fm ( 10.72 - 12.95 gpa ) decreased to 6.62 - 11.49 gpa depending on the solution temperature and brand . in the oral cavity , the preferred fm of the resin cements should be near the value of dentin and the restorative materials in order to have a similar deformation nature against the external load . according to the studies , the modulus of methacrylate - based restoratives and dentin was approximately 3 - 12 gpa and 17 - 25 gpa , respectively.28 - 30
a similar fm of the tested resin cements as to the many other restoratives is desirable for more durable cementation .
otherwise , any resin cement may cause an adverse effect on the dentin - restorative structure due to the improper resistance at the interface .
a compression test is an important in vitro test that evaluates the material 's sustained resistance against a heavy load during mastication .
after immersion , the compressive properties ( cs and cm ) significantly decreased , but the correlation with solution temperature was not consistent .
mc and ma showed minor cs change ( 2.2 - 4.4% decrease after immersion compared to that of the control case ) . on the other hand
, the rest resin cements showed much greater cs change ( 15.8 - 22.6% ) than mc and ma .
generally , as the solution temperature increases , the immersed specimens expand and it results in the increase of water uptake through the filler - matrix interface .
also , dissolution of fluoride from the contained fluoride - containing fillers will occur . as a result ,
weakening of the bond between the filler and matrix and the structure itself can lead to a decrease in cs .
however , if there is any additional curing by the hot solution , such curing may compensate the weakening of the specimens by the absorbed water . the similar cs pattern in mc and ma may be due to a combination of these two effects . in the present study , cs after immersion for 7 days ranged approximately 178 - 299 mpa .
this range matches with most methacrylate - based composite resins ( approximately 100 - 290 mpa based on the study with more than 70 proprietary products).28 specimens showed much lower cm ( 2.2 - 4.2 gpa ) than that of dentin ( 11.0 - 18.5 gpa ) whether they were immersed or not.31,32 as a luting material , since resin cement forms a thin layer at the interface , such low cm may not make any serious mechanical problem after bonding . according to the evaluation , immersion in solution made the specimens to have significantly lower microhardness , flexural and compressive properties ; yet , the values inconsistently changed for varying solution temperature .
the mechanical properties of the dual - cure resin cements were tested for different solution temperatures .
the microhardness , flexural properties ( fs and fm ) , and compressive properties ( cs and cm ) decreased significantly after immersion in the deionized water regardless of change in temperature .
however , the effect of the solution temperature on the mechanical properties was not consistent and was rather brand - dependent .
the flexural and compressive properties of the dual - cure resin cements were similar to those of the most methacrylate - based composite resins and dentin .
 purposethis study was to evaluate the effect of the solution temperature on the mechanical properties of dualcure resin cements.materials and methodsfor the study , five dual - cure resin cements were chosen and light cured . to evaluate the effect of temperature on the specimens , 
 the light - cured specimens were immersed in deionized water at three different temperatures ( 4 , 37 and 60 ) for 7 days . 
 the control specimens were aged in a 37 dry and dark chamber for 24 hours . 
 the mechanical properties of the light - cured specimens were evaluated using the vickers hardness test , three - point bending test , and compression test , respectively . both flexural and compressive 
 properties were evaluated using a universal testing machine . 
 the data were analyzed using a two way anova with tukey test to perform multiple comparisons ( =0.05).resultsafter immersion , the specimens showed significantly different microhardness , flexural , and compressive properties compared to the control case regardless of solution temperatures . 
 depending on the resin brand , the microhardness difference between the top and bottom surfaces ranged approximately 3.3 - 12.2% . among the specimens , biscem and calibra showed the highest and lowest decrease of flexural strength , respectively . 
 also , calibra and multilink automix showed the highest and lowest decrease of compressive strength , respectively compared to the control case.conclusionthe examined dual - cure resin cements had compatible flexural and compressive properties with most methacrylate - based composite resins and the underlying dentin regardless of solution temperature . 
 however , the effect of the solution temperature on the mechanical properties was not consistent and depended more on the resin brand . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION