although the metal - ceramic system is still widely used to fabricate crowns and fixed 
 partial dentures ( fpds ) and is 
 considered as the standard treatment in dentistry , aesthetic concerns have stimulated the development of new dental 
 ceramic systems . the ongoing search 
 for ceramic materials with mechanical properties suitable for use in high - load areas has 
 resulted in the development of new ceramics with high strength ( alumina and 
 zirconia ) . at present , ceramics 
 are the materials of choice for crowns and fpd due to their superior aesthetic and 
 biocompatibility features . in addition to the development of ceramic materials , new processing technologies have 
 been introduced for the production of all - ceramic cores . in contrast to the conventional technique ( lost wax ) 
 employed to produce the metal core , all - ceramic cores are fabricated from heat - pressed , 
 slip - cast , cad / cam ( computer - assisted design / computer - assisted machining ) , and cam 
 technologies , which employ many ceramic - based materials . the slip - cast ( e.g. , alumina ceramic - based ) is a 
 hand - building technique that requires a duplication of the stone die to the stone 
 refractory prior to fabrication of the alumina core . the cad / cam technique employs 
 sophisticated technologies , and the core can be fabricated from densely sintered or 
 partially - sintered ceramic blocks . a scanner digitalizes the prepared tooth , and the 
 framework or a restoration is then fabricated according to the previously established 
 design . additionally , cad / cam systems have been developed to eliminate or minimize 
 potential sources of errors present in conventional manufacturing techniques . the performance of the all - ceramic crown system is complex and depends on many factors , 
 including those controlled by the clinician and those that are 
 patient - dependent . the shape 
 and thickness of the core and veneer porcelain , laboratory processing methods , elastic 
 modulus of the restoration components , and framework design are conditions that are 
 speculated to play a role in prosthesis longevity . moreover , this 
 crown - cement system should be considered a single structure comprising layers of 
 materials with different mechanical properties . recently , a three - dimensional finite 
 element model of a molar crown showed that the combination of different material layers 
 and load conditions could produce negative effects on the maximum principal stress in 
 the core . the internal fit ( if ) is a clinically relevant topic and can affect the strength of a 
 crown - cement system . the if should 
 be uniform to avoid compromising either the retention or the resistance of the crown and 
 should also provide an appropriate luting space . dental cements are used to maintain the position of the 
 restoration for long periods ; however , some factors seem to influence the seat during 
 the cementation procedure , such as the height and convergence of the axial walls , the 
 diameter of the preparations , the presence of retentive channels and the type of cement 
 used . this set of factors can 
 explain the difficulties associated with cement flow and the oblique seating of the 
 crowns . considering that most of the previous investigations did not separate the if into 
 different regions ( occlusal and axial spaces ) and the small number of 
 studies comparing the ifs of all - ceramic systems to that of metal - ceramic , we sought to 
 examine the in vitro if ( occlusal , axial , and total mean ) of a posterior all - ceramic 
 single crown after simulation of the cementation procedure using a silicone - based 
 material . two null hypotheses were tested : 1- there were no differences between 
 occlusal , axial and total mean measurements within each group ; 2- there were no 
 differences in if among the groups . an artificial maxillary first molar was positioned in a full mouth dental model and 
 manually prepared for a complete crown . the preparation comprised a 1.2 mm 
 circumferential chamfer at the gingival margin , 2.0 mm of occlusal reduction , and 1.5 mm 
 of axial wall reduction . the angle of convergence was approximately 8 , and all of the 
 sharp angles were rounded and smoothed ( figure 1 ) . the prepared tooth underwent six impression using vinyl polysiloxane material ( express ; 
 3 m espe , st . paul , mn , usa ) , to which small increments of the composite resin ( filtek 
 z100 ; 3 m espe ) were added until the final composite shape was acquired , resulting in 
 sixty standardized composite resin tooth replicas . the replicas were randomly divided 
 into three groups ( n=20 , each ) according to the core material : group 1 ( control ) - 
 metal - ceramic ( mc , nickel - chromium , albadent , cordelia , ca , usa ) , group 2 - slip - cast 
 alumina ( ica , inceram , vita zahnfabrik , bad sckingen , germany ) , and group 3 - cad / cam 
 ( y - tzp , ips e.max zircad , ivoclar vivadent ag , schaan , liechtenstein ) . schematic representation of the prepared tooth and measurements an impression of each replica was obtained using polyether material ( impregum f ; 3 m 
 espe ) . subsequently , these moulds were poured with stone ( durone , dentsply ind . com . the investment was 
 removed from the framework and cleaned by 110 m aluminum oxide sandblasting . prior to fabrication of the 
 frameworks , the stone dies were duplicated with special silicone and poured with a 
 refractory stone . a slurry of densely packed al2o3 was applied 
 to a surface of the refractory die , followed by sintering for 10 h at 1,120c in a 
 special furnace . the porous core was then infiltrated with lanthanum glass in a 
 second firing cycle at 1,100c for 2.5 h , and the cores were then sandblasted with 
 110 m aluminum oxide to remove excess lanthanum glass from the core surface . y - tzp cores were fabricated using a milling technique with cad / cam system cerec inlab 
 ( sirona , bensheim , germany ) . optical impressions were obtained using a charge - coupled 
 device ( ccd ) camera , which generated a 3d image of each stone die . a negative cement 
 space ( -30 m ) cores were then milled from 
 pre - sintered e.max zircad blocks ( block size of 15c ) and sintered in a sintramat 
 ( ivoclar vivadent ag ) furnace at 1,500c for 8 h. this technique results in an 
 enlarged core that achieves its final dimensions after a linear shrinkage of 20 - 25% 
 during the sintering process . the veneer porcelain was manually layered on the frameworks , where this process was 
 guided by the silicone matrix of the maxillary first molar . each ceramic system 
 required a specific porcelain veneer according to its thermal expansion coefficient . 
 for mc , the feldspatic porcelain veneer vita vmk 95 ( vita zahnfabrik , bad sckingen , 
 germany ) was used , whereas ica cores were veneered with vita vm7 ( vita zahnfabrik , 
 bad sckingen , germany ) , and e.max ceram ( ipse.max zircad - ivoclar vivadent ag , 
 schaan , liechtenstein ) was used for y - tzp cores . the internal surfaces were cleaned with 70% alcohol and 
 then dried with air to remove debris . the crowns were carefully coated with a light 
 body silicone ( express ; 3 m espe ) and placed on the corresponding tooth resin replica 
 with firm hand pressure followed by the application of a force ( 49 n ) to the center 
 and parallel to the long axis of the crown for 10 min . the crown was positioned at one end of the arm with a weight of 5 kg at the 
 top . after setting of the material , the crowns were gently removed , and the thin 
 silicone films typically remained adhered to the tooth replica . heavy body silicone 
 ( express ; 3 m espe ) was used to remove the films , and the light and heavy assemblies 
 were filled with another heavy layer and then sectioned into three regions with a 
 razor blade in the buccal - palatal direction ( figure 
 2 ) , resulting in two faces to be measured . measurements were obtained for 
 two regions of interest ( occlusal and axial spaces ) to better evaluate the results in 
 addition to the total mean ( tm ) . the axial space ( as ) consisted of the entire 
 extension of the axial wall ( from the chamfer line to the transition of the rounded 
 angle to the occlusal surface ) . the occlusal surface ( os ) included the entire 
 extension of this surface between two rounded transition angles and as . nine 
 different points of each slice were measured : 3 occlusal , 3 buccal and 3 palatal , 
 resulting in 54 measurements for each crown ( figure 
 3 ) . a toolmaker 's microscope ( mitutoyo , tokyo , japan ) was used to measure 
 the cement thickness ( light body ) using a 50x magnification , and all measurements 
 were performed by the same examiner . occlusal view of the silicone layer on the replica tooth resin prior to 
 embedding with heavy body silicone . the lines indicate the zones where the 
 silicone will be sectioned positions of the cement line measurements the mean measurements were analyzed separately according to surfaces from which they 
 were obtained ( os , as , and tm ) using two - way anova with tukey 's multiple comparison 
 test at a significance level of p<0.05 . the investment was 
 removed from the framework and cleaned by 110 m aluminum oxide sandblasting . prior to fabrication of the 
 frameworks , the stone dies were duplicated with special silicone and poured with a 
 refractory stone . a slurry of densely packed al2o3 was applied 
 to a surface of the refractory die , followed by sintering for 10 h at 1,120c in a 
 special furnace . the porous core was then infiltrated with lanthanum glass in a 
 second firing cycle at 1,100c for 2.5 h , and the cores were then sandblasted with 
 110 m aluminum oxide to remove excess lanthanum glass from the core surface . y - tzp cores were fabricated using a milling technique with cad / cam system cerec inlab 
 ( sirona , bensheim , germany ) . optical impressions were obtained using a charge - coupled 
 device ( ccd ) camera , which generated a 3d image of each stone die . a negative cement 
 space ( -30 m ) cores were then milled from 
 pre - sintered e.max zircad blocks ( block size of 15c ) and sintered in a sintramat 
 ( ivoclar vivadent ag ) furnace at 1,500c for 8 h. this technique results in an 
 enlarged core that achieves its final dimensions after a linear shrinkage of 20 - 25% 
 during the sintering process . the veneer porcelain was manually layered on the frameworks , where this process was 
 guided by the silicone matrix of the maxillary first molar . each ceramic system 
 required a specific porcelain veneer according to its thermal expansion coefficient . 
 for mc , the feldspatic porcelain veneer vita vmk 95 ( vita zahnfabrik , bad sckingen , 
 germany ) was used , whereas ica cores were veneered with vita vm7 ( vita zahnfabrik , 
 bad sckingen , germany ) , and e.max ceram ( ipse.max zircad - ivoclar vivadent ag , 
 schaan , liechtenstein ) was used for y - tzp cores . the internal surfaces were cleaned with 70% alcohol and 
 then dried with air to remove debris . the crowns were carefully coated with a light 
 body silicone ( express ; 3 m espe ) and placed on the corresponding tooth resin replica 
 with firm hand pressure followed by the application of a force ( 49 n ) to the center 
 and parallel to the long axis of the crown for 10 min . a device with a vertical arm 
 was used . the crown was positioned at one end of the arm with a weight of 5 kg at the 
 top . after setting of the material , the crowns were gently removed , and the thin 
 silicone films typically remained adhered to the tooth replica . heavy body silicone 
 ( express ; 3 m espe ) was used to remove the films , and the light and heavy assemblies 
 were filled with another heavy layer and then sectioned into three regions with a 
 razor blade in the buccal - palatal direction ( figure 
 2 ) , resulting in two faces to be measured . measurements were obtained for 
 two regions of interest ( occlusal and axial spaces ) to better evaluate the results in 
 addition to the total mean ( tm ) . the axial space ( as ) consisted of the entire 
 extension of the axial wall ( from the chamfer line to the transition of the rounded 
 angle to the occlusal surface ) . the occlusal surface ( os ) included the entire 
 extension of this surface between two rounded transition angles and as . nine 
 different points of each slice were measured : 3 occlusal , 3 buccal and 3 palatal , 
 resulting in 54 measurements for each crown ( figure 
 3 ) . a toolmaker 's microscope ( mitutoyo , tokyo , japan ) was used to measure 
 the cement thickness ( light body ) using a 50x magnification , and all measurements 
 were performed by the same examiner . occlusal view of the silicone layer on the replica tooth resin prior to 
 embedding with heavy body silicone . the lines indicate the zones where the 
 silicone will be sectioned positions of the cement line measurements the mean measurements were analyzed separately according to surfaces from which they 
 were obtained ( os , as , and tm ) using two - way anova with tukey 's multiple comparison 
 test at a significance level of p<0.05 . figure 4 shows the means and standard deviations 
 of the film thicknesses in each group and the statistical differences among the groups . the maximum deviations for the groups were 222.79 m ( os ) and 139.72 m ( as ) for mc 
 group , 246.67 m ( os ) and 112.40 m ( as ) for y - tzp group and 637.01 m ( os ) and 152.27 
 m ( as ) for ica group . no differences were detected among os ( 95.42 m ) , as ( 83.12 m ) 
 and tm ( 89.76 m ) in the mc group ( p<0.001 ) . for y - tzp , as ( 83.93 m ) was 
 significantly lower than both oc ( 180.90 m ) and tm ( 132.54 m ) ( p<0.05 ) . for ica , as 
 ( 83.14 m ) was significantly different compared to os ( 229.42 m ) and tm ( 158.27 m ) 
 ( p<0.001 ) . thus , differences were detected among all surfaces analyzed in both 
 all - ceramic groups . values with the 
 same letter were not significantly different ( p<0.05 ) among the groups , no differences were observed with respect to as ( p<0.001 ) . the mc 
 group showed significantly lower values for os compared to ica ( p<0.001 ) and y - tzp 
 ( p<0.001 ) groups , and ica was significantly higher than y - tzp ( p<0.001 ) . for tm , 
 the all - ceramic groups were statistically equivalent , but the value obtained for the mc 
 group was significantly lower than ica ( p<0.001 ) and y - tzp ( p<0.05 ) . crowns and fpd are routinely fabricated using indirect techniques , and are , thus , 
 subject to deformations of the materials . these alterations are somewhat expected and 
 are important for proper seating of the prosthesis onto its abutment tooth . thus , the 
 space between the internal crown and the prepared tooth surfaces is required to 
 accommodate the cement . if is a 
 controversial topic in the literature , mainly with respect to its thickness and whether it can modify the fracture 
 strengths of crowns . no differences were observed among the 
 measured spaces ( os , as , and tm ) in the mc group , but in the all - ceramic groups , 
 significant differences were detected among these spaces . no significant differences in 
 as were observed among the groups , but os and tm showed statistically significant 
 differences . remarkably , the ica and y - tzp groups displayed high values for os , as 
 compared to the values obtained for the mc group . moreover , the ica group differed from 
 y - tzp group with respect to os . theoretically , the internal space necessary for the cement is 20 to 40 m . an if ranging between 50 to 100 m has 
 been considered acceptable , but some 
 researchers ( without scientific evidence ) propose that a cement space ranging between 
 200 and 300 m is also suitable . ( 2008 ) found a lack of significant 
 differences of margin fit values among zinc phosphate , resin composite and resin 
 modified glass ionomer cements . this data indicates that the minimum space is 
 independent of the cement type . in the present study , the smallest if was observed in 
 the mc group ( 89.76 m ) and the largest in the ica group ( 158.27 m ) . y - tzp crowns 
 displayed an if average of 132.54 m , which is consistent with the outcomes ( 49 to 136 
 m ) reported by previous studies evaluating y - tzp specimens . therefore , all 
 groups presented tms within the previous recommendations . only the mc group showed 
 positive results for if and uniformity . the absence of uniformity of the if in the y - tzp 
 group was not expected , because the cad / cam technology is used to eliminate potential 
 source of errors , and even choosing 
 a negative value for cement space ( -30 m ) in order to improve the internal fit , the 
 y - tzp if did not provide measures similar to mc if . thus , the present results suggest 
 the need to improve the quality of the if for all - ceramic crowns . the largest if is often located at the os , which can range in size between 100 and 160 m . overall , this value is in agreement 
 with the present findings , although the results obtained for ica ( 229.41 m ) and y - tzp 
 ( 180.90 m ) exceeded the range of the reported values . a recent study evaluating y - tzp 
 pfd ( without veneer porcelain ) demonstrated an average value of 93.2 m for the occlusal 
 space , which differs from the present finding ( 180.90 m ) . the relationship between the fracture strength and the 
 if in the os remains unclear . whereas some researchers have agreed that os does not 
 influence the crown strength , others have stated that a smaller film 
 thickness is preferable , because a greater internal film thickness results in a lower 
 resistance to crown fracture and the strength of a crown - cement - tooth system can be dramatically 
 changed when the if in os is increased . taking into account the positive in vitro and in vivo 
 records for y - tzp core survival ( considering a core thickness of 0.5 mm ) 
 together with the observation that the cohesive fracture of porcelain is the most 
 important type of failure , one might 
 conclude that if is not a key factor for the success of the y - tzp framework . similarly , 
 due to the ductility of the metal core , the if is also not an important topic for 
 determination of the success of the mc core . on the other hand , in vivo 
 studies have shown that the most common glass - infiltrate alumina failure for 
 tooth - supported posterior crowns is bulk fracture , because alumina is more susceptible to radial cracks due to 
 surface cementation than is y - tzp . this contrast between y - tzp and glass - infiltrate 
 alumina crowns is due to differences in core mechanical properties . alumina has a 
 strength of 550 mpa and a fracture toughness of 3 mpa / m , which are 
 substantially lower than those of y - tzp ( 1450 mpa and 5.4 mpa / m , 
 respectively ) . therefore , the larger if in os could be a negative factor for the success 
 of the alumina core , and approaches such as framework modification , cement with a lower elasticity 
 modulus , and tooth substrates 
 with a higher elasticity modulus 
 should be considered for the clinical success of the veneered alumina crown . laboratory processing steps were key factors in the differences observed between the mc 
 and all - ceramic groups , in particular with respect to os . for the slip - cast technique , 
 high results for all spaces were expected due to fabrication method . in contrast to mc 
 and y - tzp cores , the slip - cast technique requires an additional duplication of the 
 master die prior to construction of the core , and this process may have affected the 
 results . y - tzp demonstrated os and tm values that were significantly higher than those 
 in the metal - ceramic group . this result was not expected , considering that a cad / cam 
 system was used to eliminate potential sources of error that can occur using 
 conventional techniques . therefore , 
 the occlusal surface seems to be affected by factors that can be speculated as problems 
 related to the accuracy of the optical images acquired , which are not consistently 
 reliable . other explanations could be related to the machining of 
 the y - tzp blocks such as milling machine , tool - path generation , tool shapes , and 
 sintering processes used to generate the cores and also the tooth preparation 
 design . finally , though marginal fit has been associated with marginal gingivitis , occlusal fit 
 may play a significant role on structural durability of all - ceramic crowns and the axial 
 wall cement space can influence the retention of fixed prostheses . in addition , there is a lack of studies 
 that measure the internal fit and further differentiate the measures of occlusal and 
 axial fit , which hinder any comparison with other studies outcomes . moreover , the replica technique used in 
 the present study to evaluate the if of crowns has been extensively used as a 
 non - destructive , reliable and valid method . further investigations to improve and enhance all - ceramic 
 manufacturing techniques are necessary , and the relationship between the reliability of 
 crowns and the if could be the topic of future studies by the application of sliding 
 contact fatigue to specimens that simulate the complex geometry of molar crowns . this investigation showed that metal - ceramic crowns present similar if , while 
 all - ceramic groups presented differences in if within the as and os regions . among 
 groups , thus , 
 hypothesis 1 , which postulated that within each group if would be similar in both 
 regions and hypothesis 2 , which postulated that there would be no differences in 
 internal fit among the groups , were both partially accepted . the total mean achieved by 
 all groups was within the range of clinical acceptability .
objectives the aim of this study was to investigate the internal fit ( if ) of 
 glass - infiltrated alumina ( ica - in - ceram alumina ) , yttria - stabilized tetragonal 
 zirconia polycrystals ( y - tzp - ips e.max zircad ) , and metal - ceramic ( mc - ni - cr 
 alloy ) crowns . 
 material and methods sixty standardized resin - tooth replicas of a maxillary first molar were produced 
 for crown placement and divided into 3 groups ( n=20 each ) according to the core 
 material used ( metal , ica or y - tzp ) . 
 the if of the crowns was measured using the 
 replica technique , which employs a light body polyvinyl siloxane impression 
 material to simulate the cement layer thickness . 
 the data were analyzed according 
 to the surfaces obtained for the occlusal space ( os ) , axial space ( as ) and total 
 mean ( tm ) using two - way anova with tukey 's multiple comparison test ( p<0.05 ) . 
 results no differences among the different areas were detected in the mc group . 
 for the 
 y - tzp and ica groups , as was statistically lower than both os and tm . 
 no 
 differences in as were observed among the groups . however , os and tm showed 
 significantly higher values for ica and y - tzp groups than mc group . 
 comparisons of 
 ica and y - tzp revealed that os was significantly lower for y - tzp group , whereas no 
 differences were observed for tm . 
 conclusions the total mean achieved by all groups was within the range of clinical 
 acceptability . 
 however , the metal - ceramic group demonstrated significantly lower 
 values than the all - ceramic groups , especially in os .