to reproduce highly esthetic tooth - colored fixed dental prostheses , computer aided design / computer aided manufacturing ( cad / cam ) techniques are becoming frequently used . since the commercial introduction of polymeric blanks into the dental market , cad / paul , mn , usa ) are innovative new cad / cam materials that make it possible to achieve superior esthetic results in easy steps . the milled rnc restorations can be individualized intra- or extra - orally , either before or after insertion . in recent years , researchers have tried to achieve a more effective and longer - lasting adhesion between restorative materials and the dental substrate . the adhesive techniques are based on research on the hybrid layer and on chemical and mechanical adhesion . some researchers have attempted to shorten the application time and reduce the number of steps , creating new generations of materials and improving their quality . increasing demand for esthetic restorations has led to greater use of all ceramic materials because of their improved biocompatibility and optical properties , compared with metal - ceramic restorations . advances in cad and cam systems are providing new options for dentistry , creating an alternative to the conventional impression and casting technique for producing dental restorations . a requirement for the successful function of a cad / cam restoration is adequate adhesion between ceramic and tooth substance ; however , the literature is unclear on which cement , ceramic , conditioning treatment , and dentine bonding agent produce the highest bond strength . resin composite cements are used to lute conventional metal crowns , fixed partial dentures , ceramic crowns , and veneers and to repair fractured metal ceramic restorations . resin cements have been selected for their advantageous mechanical and adhesive properties compared with conventional luting agents . the use of resin luting agents also appears to be essential in determining an effective stress distribution , which will prevent crack initiation . bond strength to ceramic material is influenced by the composition of the ceramic substrate as well as by mechanical and chemical interaction between substrate and bonding agent . the aim of this study was to evaluate the influence of three different luting protocols ( total - etch , self - etch , and self - adhesive ) on shear bond strength of cad / cam rnc material , based on cad / cam technology , to dentin . in the present in vitro study , 30 disks ( 5 mm in diameter , 3 mm thick ) were designed with cerec software 4.2 platform ( sirona dental gmbh , salzburg , austria ) and obtained by milling from rnc blocks for cad / cam ( lava ultimate restorative , 3 m espe , st . paul , mn , usa ) with cerec mc xl ( sirona dental gmbh , salzburg , austria ) [ figure 1 ] . the disks were obtained by milling from resin nano ceramic blocks for computer aided design / computer aided manufacturing with cerec mc xl . characteristics of materials tested the disks were subsequently cemented to the exposed dentin of 30 bovine permanent mandibular incisors freshly extracted and stored in a 0.1% ( wt / vol ) thymol solution , which were used as a substitute for human teeth . the criteria for tooth selection included intact buccal enamel with no cracks caused by extraction , the absence of caries , and adequate dimension of the crown . the teeth were cleansed of soft tissue remnants and debris with periodontal curettes , stored in the thymol solution for 1 week , and later in saline solution at room temperature until testing . then the roots of the teeth were embedded in self - curing , fast - setting acrylic resin ( rapid repair , degu dent gmbh , hanau , germany ) . specially fabricated cylindrical teflon mold with an internal diameter of 14 mm were filled with the acrylic resin and allowed to cure , thus encasing each specimen while allowing the buccal surface of dentin to be exposed . each tooth was oriented so that its labial surface was parallel to the shearing force . the buccal enamel was removed using a high - speed carbide rotary instrument ( no . h21 l.314.014 ; komet , lemgo , germany ) under copious water irrigation , to expose midcoronal dentin . the exposed dentin surfaces were finished off with an automated polishing machine ( apl-4 ; arotec s.a . ind com , cotia , sp , brazil ) with a 600-grit silicon carbide abrasive paper disks for 5 s , to obtain a flat and uniform dentin surface and reduce any micromechanical interlocking that could affect the real bonding influence of the tested adhesive cements . before cementation , to remove the smear layer and to treat the dental surface with cleaning solution , the dentin surface was rinsed for 1 min with a cotton pellet impregnated with tubulicid blue ( dental therapeutics ab , saltsj - boo , sweden ) without fluorine . the surface was then rinsed and dried before cementation ; the labial surface of each incisor was cleaned for 10 s with a mixture of water and fluoride - free pumice and rubber polishing cup in a low - speed handpiece . the dentinal surface was rinsed with water to remove pumice or debris and then dried with an oil - free air stream . the bonding surface of each disk was cleaned with alcohol and rinsed with water to remove oil debris contained in the milling liquid . the specimens were randomly assigned to 3 groups of 10 teeth each [ figure 1 ] according to different luting procedures ( total - etch , self - etch , and self - adhesive ) . group 1 : ult - te : disks were cemented on the dentin surface , which had been acid - etched with scotchbond universal etchant phosphoric acid ( 3 m espe , st . paul , mn , usa ) for 15 s , the etched substrates were rinsed with water for 15 s , and gently air dried for 15 s to remove excess water and treated with scotchbond universal adhesive ( 3 m espe , st . paul , mn , usa ) with adhesive resin cement ( relyx ultimate/3 m espe , st . paul , mn , usa)group 2 : ult - se : disks were cemented on the dentin surface , which had been treated with scotchbond universal adhesive , with the adhesive resin cement ( relyx ultimate)group 3 : u2-sa : disks were cemented on the dentin surface with the dual polymerization resin luting agent relyx unicem 2 automix . group 1 : ult - te : disks were cemented on the dentin surface , which had been acid - etched with scotchbond universal etchant phosphoric acid ( 3 m espe , st . paul , mn , usa ) for 15 s , the etched substrates were rinsed with water for 15 s , and gently air dried for 15 s to remove excess water and treated with scotchbond universal adhesive ( 3 m espe , st . paul , mn , usa ) with adhesive resin cement ( relyx ultimate/3 m espe , st . paul , mn , usa ) group 2 : ult - se : disks were cemented on the dentin surface , which had been treated with scotchbond universal adhesive , with the adhesive resin cement ( relyx ultimate ) group 3 : u2-sa : disks were cemented on the dentin surface with the dual polymerization resin luting agent relyx unicem 2 automix . a vinyl ring with an internal diameter of 4.5 mm was applied under the dentin surface to standardize the adhesion area . for all groups , no further procedures were applied in order to avoid bias and confounding factors . during cementation , a thin layer of cement was applied and distributed to the bonding surface of the cylinders by means of a heidemann spatula . on each specimen , five surfaces were identified : mesial , lingual , distal , buccal , and occlusal . as suggested by the manufacturer , every surface was light - polymerized for 20 s at a light intensity of 1000 mw / cm using a light - emitting diode ( led ) curing light in soft start - polymerization mode ( celalux 2 high - power led curing light , voco gmbh , cuxhaven , germany ) . the power output ( light intensity ) of the led was measured with a cure rite radiometer ( caulk - dentsply mod . all samples were stored in distilled water at room temperature for 24 h. after storing , the specimens were all submitted to a shear bond strength test to check the strength of adhesion between the two substrates , dentin , and rnc . this test is defined as a test in which two materials are connected by an adhesive agent and loaded in shear until separation occurs . specimens were placed in a universal testing machine ( model 3343 , instron corporation , norwood , ma , usa ) . teeth were secured in the lower jaw of the machine so that the bonded cylinder base was parallel to the shear force direction . specimens were stressed in an occlusogingival direction at a crosshead speed of 1 mm / min [ figure 2 ] . the maximum load necessary to debond was recorded in newton ( n ) and calculated in mpa as a ratio of newton to the surface area of the cylinder ( the calculated shear bond strength was determined by dividing the strength at which bond failure occurred by the bonding area ) . specimens were placed in a universal testing machine , secured in the lower jaw of the machine so that the bonded cylinder base was parallel to the shear force direction . after the testing procedure , the fractured surfaces were examined with an optical microscope ( stereomicroscope sr , zeiss , oberkochen , germany ) . statistical analysis was performed with stata 12.0 software ( stata , college station , texas , usa ) . descriptive statistics , including the mean , standard deviation , median , and minimum and maximum values were calculated for all groups . analysis of variance ( anova ) was applied to determine whether significant differences in debond strength values existed among the groups . in the present in vitro study , 30 disks ( 5 mm in diameter , 3 mm thick ) were designed with cerec software 4.2 platform ( sirona dental gmbh , salzburg , austria ) and obtained by milling from rnc blocks for cad / cam ( lava ultimate restorative , 3 m espe , st . paul , mn , usa ) with cerec mc xl ( sirona dental gmbh , salzburg , austria ) [ figure 1 ] . the disks were obtained by milling from resin nano ceramic blocks for computer aided design / computer aided manufacturing with cerec mc xl . characteristics of materials tested the disks were subsequently cemented to the exposed dentin of 30 bovine permanent mandibular incisors freshly extracted and stored in a 0.1% ( wt / vol ) thymol solution , which were used as a substitute for human teeth . the criteria for tooth selection included intact buccal enamel with no cracks caused by extraction , the absence of caries , and adequate dimension of the crown . the teeth were cleansed of soft tissue remnants and debris with periodontal curettes , stored in the thymol solution for 1 week , and later in saline solution at room temperature until testing . then the roots of the teeth were embedded in self - curing , fast - setting acrylic resin ( rapid repair , degu dent gmbh , hanau , germany ) . specially fabricated cylindrical teflon mold with an internal diameter of 14 mm were filled with the acrylic resin and allowed to cure , thus encasing each specimen while allowing the buccal surface of dentin to be exposed . each tooth was oriented so that its labial surface was parallel to the shearing force . the buccal enamel was removed using a high - speed carbide rotary instrument ( no . h21 l.314.014 ; komet , lemgo , germany ) under copious water irrigation , to expose midcoronal dentin . the exposed dentin surfaces were finished off with an automated polishing machine ( apl-4 ; arotec s.a . ind com , cotia , sp , brazil ) with a 600-grit silicon carbide abrasive paper disks for 5 s , to obtain a flat and uniform dentin surface and reduce any micromechanical interlocking that could affect the real bonding influence of the tested adhesive cements . before cementation , to remove the smear layer and to treat the dental surface with cleaning solution , the dentin surface was rinsed for 1 min with a cotton pellet impregnated with tubulicid blue ( dental therapeutics ab , saltsj - boo , sweden ) without fluorine . the surface was then rinsed and dried before cementation ; the labial surface of each incisor was cleaned for 10 s with a mixture of water and fluoride - free pumice and rubber polishing cup in a low - speed handpiece . the dentinal surface was rinsed with water to remove pumice or debris and then dried with an oil - free air stream . the bonding surface of each disk was cleaned with alcohol and rinsed with water to remove oil debris contained in the milling liquid . the specimens were randomly assigned to 3 groups of 10 teeth each [ figure 1 ] according to different luting procedures ( total - etch , self - etch , and self - adhesive ) . group 1 : ult - te : disks were cemented on the dentin surface , which had been acid - etched with scotchbond universal etchant phosphoric acid ( 3 m espe , st . paul , mn , usa ) for 15 s , the etched substrates were rinsed with water for 15 s , and gently air dried for 15 s to remove excess water and treated with scotchbond universal adhesive ( 3 m espe , st . paul , mn , usa ) with adhesive resin cement ( relyx ultimate/3 m espe , st . paul , mn , usa)group 2 : ult - se : disks were cemented on the dentin surface , which had been treated with scotchbond universal adhesive , with the adhesive resin cement ( relyx ultimate)group 3 : u2-sa : disks were cemented on the dentin surface with the dual polymerization resin luting agent relyx unicem 2 automix . group 1 : ult - te : disks were cemented on the dentin surface , which had been acid - etched with scotchbond universal etchant phosphoric acid ( 3 m espe , st . paul , mn , usa ) for 15 s , the etched substrates were rinsed with water for 15 s , and gently air dried for 15 s to remove excess water and treated with scotchbond universal adhesive ( 3 m espe , st . paul , mn , usa ) with adhesive resin cement ( relyx ultimate/3 m espe , st . paul , mn , usa ) group 2 : ult - se : disks were cemented on the dentin surface , which had been treated with scotchbond universal adhesive , with the adhesive resin cement ( relyx ultimate ) group 3 : u2-sa : disks were cemented on the dentin surface with the dual polymerization resin luting agent relyx unicem 2 automix . a vinyl ring with an internal diameter of 4.5 mm was applied under the dentin surface to standardize the adhesion area . for all groups , no further procedures were applied in order to avoid bias and confounding factors . during cementation , a thin layer of cement was applied and distributed to the bonding surface of the cylinders by means of a heidemann spatula . on each specimen , five surfaces were identified : mesial , lingual , distal , buccal , and occlusal . as suggested by the manufacturer , every surface was light - polymerized for 20 s at a light intensity of 1000 mw / cm using a light - emitting diode ( led ) curing light in soft start - polymerization mode ( celalux 2 high - power led curing light , voco gmbh , cuxhaven , germany ) . the power output ( light intensity ) of the led was measured with a cure rite radiometer ( caulk - dentsply mod . after storing , the specimens were all submitted to a shear bond strength test to check the strength of adhesion between the two substrates , dentin , and rnc . this test is defined as a test in which two materials are connected by an adhesive agent and loaded in shear until separation occurs . specimens were placed in a universal testing machine ( model 3343 , instron corporation , norwood , ma , usa ) . teeth were secured in the lower jaw of the machine so that the bonded cylinder base was parallel to the shear force direction . specimens were stressed in an occlusogingival direction at a crosshead speed of 1 mm / min [ figure 2 ] . the maximum load necessary to debond was recorded in newton ( n ) and calculated in mpa as a ratio of newton to the surface area of the cylinder ( the calculated shear bond strength was determined by dividing the strength at which bond failure occurred by the bonding area ) . specimens were placed in a universal testing machine , secured in the lower jaw of the machine so that the bonded cylinder base was parallel to the shear force direction . after the testing procedure , the fractured surfaces were examined with an optical microscope ( stereomicroscope sr , zeiss , oberkochen , germany ) . statistical analysis was performed with stata 12.0 software ( stata , college station , texas , usa ) . descriptive statistics , including the mean , standard deviation , median , and minimum and maximum values were calculated for all groups . analysis of variance ( anova ) was applied to determine whether significant differences in debond strength values existed among the groups . anova showed the presence of significant differences among the various groups ( p < 0.0001 ) . as showed in figure 3 , post - hoc tukey testing showed that the highest shear strength values ( p < 0.001 ) were reported in group 2 ( ult - se ) . the lowest data ( p < 0.001 ) were recorded in group 3 ( u2-sa ) . descriptive statistics ( mpa ) of the different groups shear bond strength values ( mpa ) obtained in the three groups . patients needs and desires and developments in adhesive dentistry have made the use of all - ceramic restorations increasingly frequent , particularly silica - based ones such as crowns , inlay - onlays , and laminate veneers . in order to improve the impression and casting procedure steps and to produce indirect restorations faster and easier , without the need for provisional restorations and dental laboratories , cad / paul , mn , usa ) is a new composite / nanoceramic material for cad / cam manufacturing . this material allows the possibility to use composite materials to characterize and adjust the restoration after milling . unlike conventional ceramic restorations , customization and glaze firing industrially , prefabricated cad / cam restorations are polymerized by standardized methods and improving material properties , in particular , predictability and consistency . comparing these machinable prostheses to laboratory - handmade restorations , it has been advocated that , due to a highly homogeneous quality crystalline content , the bond strength to hard tooth tissues and the clinical longevity of these cad / cam restorations have been increased . in contrast , conventional manual polymerization and processing is greatly influenced by the operator and can cause a high level of variations . however , to achieve a long duration of restoration and , therefore , its long - term success , durable bond strength between the tooth and the restorative material is fundamental . to date , no study has evaluated the shear bond strength between the new nanoceramic lava ultimate material and tooth structures . the aim of this study was to investigate the influence of different luting protocols ( total - etch , self - etch , and self - adhesive ) on the shear bond strength values between dentin and this innovative rnc material , based on cad / cam technology . although it is preferable to use extracted human teeth for bonding research , it has become increasingly difficult to obtain such samples for laboratory studies in italy . to compare data from the current study with that reported in previous bovine enamel bond strength tests , bovine teeth have some advantages , as they are easy to obtain in large quantities , are in good condition and have less composition variables than human enamel . bovine teeth also have large , flat surfaces and are unlikely to have undergone prior caries challenges that could affect the test result . the mineral distribution within the carious lesions in bovine teeth is reportedly similar to that found in human teeth , and the structural changes that occur in human and bovine teeth are also similar . for the cementation procedure , three different luting protocols have been evaluated in this study : total etch / etch and rinse protocol ( group 1 - ult - te : sco35% phosphoric acid + scotchbond universal adhesive + relyx ultimate conventional resin cement ) ; self - etch protocol ( group 2 - ult - se : scotchbond universal adhesive + relyx ultimate conventional resin cement ) , and self - adhesive protocol ( group 3 - u2-sa : relyx unicem 2 automix self - adhesive resin cement ) . these three different cementation techniques have been selected according to contemporary adhesive systems classification into etch - and - rinse and self - etch adhesives . while the etch - and - rinse approach requires a separate acid - etch step to promote dentin and enamel demineralization before monomer infiltration , demineralization and infiltration occur simultaneously in the self - etch approach , although with no perfect synchronism . in addition , the separate etch - and - rinse step completely removes the smear layer , while the combined etch and bonding step in self - etch adhesive systems only partially dissolve the smear layer . complete removal of the smear layer may allow for more intimate contact of the hydrophilic primer and hydrophobic bonding agent to the tooth . many articles related to adhesive procedures used for the cementation of ceramic to tooth structure have shown that the presence of a hybrid layer between adhesive resin and dentin seems to adequately seal the dentinal tubules and allows a cellular reorganization of the pulpal tissues . in the present study , all the tested cements are based on adhesive procedures , which determine the formation of the hybrid layer and lead to the creation of a stronger link between dental structure and composite cement . in general two different types of resin composite cements exist : the conventional and the self - adhesive resin composite cements . to bond cad / cam restoration to dentin , relyx ultimate conventional resin cement was used together with a etch and rinse adhesive or with a self - etch adhesive ; while relyx unicem 2 automix self - adhesive resin cement was used alone . a pretreatment of tooth abutment is necessary in case of use of conventional resin composite cements . for this reason , since they do not necessitate any pretreatment of tooth substrate , self - adhesive resin composite cements have been developed . self - adhesive resin composite cements contain acid monomers , resulting in an initial lower ph value for the infiltration into the demineralized collagen network , in our study significant differences were found between conventional and self - adhesive resin cements . the lowest shear bond strength values were recorded in group 3 ; i.e. , lava ultimate rnc disks bonded to dentin with self - adhesive cements . this is in accordance with a study by stawarczyk et al . , which reported lower tensile bond strength with self - adhesive resin composite cements to polymeric crowns , compared to the bonding with conventional resin cements . while some products have equal bond strength of self - adhesive resin cement to dentin , other products show an inferior bond to enamel . the success of the restoration depends not only on the bond between tooth and resin cement but also on the bond between restoration and resin composite cement . according to some authors to achieve a resistant bond , further conditioning of the restoration material is needed . when comparing the shear bond strength values of conventional resin cements used together with an etch and rinse adhesive or with a self - etch adhesive , significantly higher bond strength values were recorded in group 2 ( self - etch protocol ) . accurate and meticulous procedures during the cementation phase may play an essential clinical role in achieving a valuable connection between the dentin and the ceramic restoration . retention form of the preparation , marginal integrity , and clinical micro - leakage are the key parameters used to judge the effectiveness of a resin composite cement system . further studies should be conducted to test the correlation of bond strength for both tested materials to 48 h , 1 week , 1 month , and 1 year to evaluate the time factor related to the effective quality of bonding between ceramic material and dentin . additional research should also be conducted to test the correlation between porosity , poor wetting , high viscosity , and failures . within the limitations of this in vitro study , conventional resin cements ( coupled with etch and rinse or self - etch adhesives ) showed better shear strength values compared to self - adhesive resin cements . then , for the tested polymeric cad / cam materials , the use of additional adhesives for conditioning is necessary . furthermore , conventional resin cements used together with a self - etch adhesive reported the highest values of adhesion . however , future researches are needed to evaluate the bond strength to new ceramic / polymer prosthetic materials . the authors of this manuscript declare that they have no conflicts of interest , real or perceived , financial or non - financial in this article the authors of this manuscript declare that they have no conflicts of interest , real or perceived , financial or non - financial in this article
background : the purpose of this study was to evaluate the influence of three different luting protocols on shear bond strength of computer aided design / computer aided manufacturing ( cad / cam ) resin nanoceramic ( rnc ) material to dentin.materials and methods : in this in vitro study , 30 disks were milled from rnc blocks ( lava ultimate/3 m espe ) with cad / cam technology . 
 the disks were subsequently cemented to the exposed dentin of 30 recently extracted bovine permanent mandibular incisors . 
 the specimens were randomly assigned into 3 groups of 10 teeth each . in group 1 , disks were cemented using a total - etch protocol ( scotchbond universal etchant phosphoric acid + scotchbond universal adhesive + relyx ultimate conventional resin cement ) ; in group 2 , disks were cemented using a self - etch protocol ( scotchbond universal adhesive + relyx ultimate conventional resin cement ) ; in group 3 , disks were cemented using a self - adhesive protocol ( relyx unicem 2 automix self - adhesive resin cement ) . 
 all cemented specimens were placed in a universal testing machine ( instron universal testing machine 3343 ) and submitted to a shear bond strength test to check the strength of adhesion between the two substrates , dentin , and rnc disks . 
 specimens were stressed at a crosshead speed of 1 mm / min . 
 data were analyzed with analysis of variance and post - hoc tukey 's test at a level of significance of 0.05.results:post-hoc tukey testing showed that the highest shear strength values ( p < 0.001 ) were reported in group 2 . 
 the lowest data ( p < 0.001 ) were recorded in group 3.conclusion:within the limitations of this in vitro study , conventional resin cements ( coupled with etch and rinse or self - etch adhesives ) showed better shear strength values compared to self - adhesive resin cements . 
 furthermore , conventional resin cements used together with a self - etch adhesive reported the highest values of adhesion .