digital impression systems using intraoral scanning have been available since the introduction of cerec 1 ( sirona s bluecam ) as part of the single - sitting dental appointment concept 
 . these systems offer remarkable benefits compared with conventional impression methods , including easier data storage , smaller storage requirements , more time - efficient treatment procedures , and an increase in patient comfort 
 . however , restorations fabricated with the first digital impression systems available showed relatively poor marginal adaptation compared to the ones made with conventional impression methods 
 ; therefore , some authors expressed concerns on the longevity of the restorations , since adequate marginal adaptation is a decisive factor for clinical longevity 
 . on the other hand , technical advances have significantly improved the adaptation of restorations fabricated with digital impression systems ; a recent study demonstrated that a complete crown fabricated using intraoral scanning resulted in a better marginal fit than crowns created with conventional techniques 
 . the process of fabricating restorations using digital impressions and  computer - aided - design / computer - aided - manufacturing  ( cad / cam ) technology comprises the following steps : data acquisition , data processing , and manufacturing . these steps are equally important for the fabrication of well - fitting restorations , and errors in any of these steps may result in distortions of the restorations 
 . therefore , the scanning procedure , the specific software , and the milling machine used have all been suggested as factors that could have a detrimental effect on the fit of restorations fabricated with the cad / cam technology 


 . as a consequence , a number of studies have been performed to compare marginal and internal adaptation of restorations scanned with different intraoral scanners or fabricated made with different milling machines . however , to perform accurate comparisons it is also essential to take into account the software version used with each cad / cam system and the specific parameter settings ( i.e. spacer settings ) selected . despite the importance of these factors , therefore , the purpose of this study was to evaluate marginal and internal adaptation of restorations fabricated with different versions of the cad / cam software and the effect of different parameter settings . an acrylic model of the maxillary left second molar was prepared to receive a single ceramic crown restoration ( nissin dental model , nissin dental prod . the preparation featured a 1-mm - deep chamfer finish line with a total convergence angle of 12 degrees . to verify a standardized tooth reduction , an initial impression was taken with a vinyl polysiloxane material ( aquasil soft putty , dentsply detrey gmbh , konstanz , baden - wrttemberg , germany ) before preparation . this impression was sectioned vertically ( at a width of 2 mm ) , and set in the place of the prepared model . the depth needed for tooth preparation was estimated by comparing the impression body and the prepared model . for repeated measurements , a titanium replica of the acrylic model was made using a customized cad / cam system ( myplant , addtech co. , seoul , seoul , korea ) . a thin uniform layer of an antireflection powder ( cerec optispray , sirona dental systems gmbh , bensheim , hesse , germany ) was sprayed on the reference model . digital impressions were taken using a cerec ac with bluecam ( sirona s bluecam , sirona dental systems gmbh , bensheim , hesse , germany ) system according to the scanning protocol of ender and mehl 
  ( 2013 ) , including occlusal views with scans at an angle of 30 degree from the buccal and lingual surfaces 
 . one single dentist trained in taking digital impressions acquired 40 digital impressions of the reference model . the 40 digital impressions acquired were first divided into two groups ( n=20 each ) , depending on the software version that was used in the processing ( i.e. , cerec version 3.8 or version 4.2 ) . each group was further divided into two subgroups ( n=10 each ) , depending on the spacer parameter settings ( i.e. , 40 or 80 m ) ( table 1 ) . the acquired datasets were transmitted to a milling unit ( cerec mcxl , sirona dental systems gmbh , bensheim , hesse , germany ) , and crowns were fabricated using lavaultimate restorative milling blocks ( 3 m espe , st . table 1two - way anova analysis showing that software version and spacer parameter significantly affected the fit of the crowns ( p<.05)independent variablesignificancesoftware versionp<0.05space parameterp<0.05 
  the replica technique previously described by molin and karlsson 
  ( 1993 ) , and validated by rahme , et al . the crowns were filled with a low viscosity silicone material ( fit checker , gc dental , tokyo , tokyo , japan ) , and placed in the titanium reference model using a shimadzu universal tester ( ag-10knx , shimadzu co , tokyo , tokyo , japan ) with a 20 n load . after 5 minutes , the silicone material attached to the internal surface of the crowns was removed and then stabilized with the impression material of higher viscosity ( examixfine regular type , gc dental , tokyo , tokyo , japan ) . all replicas were cut along the abutment axis both in the buccolingual and in the mesiodistal direction , using a razor blade , yielding four fragments per crown . for each cross section , four points were measured , thus 16 thickness points of the titanium replica were measured ( figure 1 ) . the points from  1  to  8  were included in the buccolingual direction section , and the points from  a  to  h  were included in the mesiodistal direction section . the measuring points were divided into 4 categories considering the location of tooth : margin ( 1 , 8 , a , h ) , lower axial wall ( 2 , 7 , b , g ) , upper axial wall ( 3 , 6 , c , f ) , and occlusal surface ( 4 , 5 , d , e ) . figure 1groups defined for the study 
  all analyses were performed using a double - blind protocol . replica film thickness was measured with a video measuring system ( optical video measuring system , seven ocean optical technology , donnguan , guangdong , china ) at a 10x magnification with external light source . the mean and the standard deviation of the fit accuracy were calculated for each group . the influence of independent variables , including the ones from the software and from the spacer parameters , were analyzed using a friedman two - way analysis of variance ( anova ) ( p=.05 ) . the groups within each category ( margin , lower axial wall , upper axial wall , occlusal surface ) were merged , and the mean and the standard deviation of each category were calculated . the comparisons between groups within each category were conducted using paired t - tests ( p<05 ) . all statistical analyses were carried out with medcalc version 12.5.0 ( medcalc software , ostend , vlaanderen , belgium ) . an acrylic model of the maxillary left second molar was prepared to receive a single ceramic crown restoration ( nissin dental model , nissin dental prod . the preparation featured a 1-mm - deep chamfer finish line with a total convergence angle of 12 degrees . to verify a standardized tooth reduction , an initial impression was taken with a vinyl polysiloxane material ( aquasil soft putty , dentsply detrey gmbh , konstanz , baden - wrttemberg , germany ) before preparation . this impression was sectioned vertically ( at a width of 2 mm ) , and set in the place of the prepared model . the depth needed for tooth preparation was estimated by comparing the impression body and the prepared model . for repeated measurements , a titanium replica of the acrylic model was made using a customized cad / cam system ( myplant , addtech co. , seoul , seoul , korea ) . a thin uniform layer of an antireflection powder ( cerec optispray , sirona dental systems gmbh , bensheim , hesse , germany ) was sprayed on the reference model . digital impressions were taken using a cerec ac with bluecam ( sirona s bluecam , sirona dental systems gmbh , bensheim , hesse , germany ) system according to the scanning protocol of ender and mehl 
  ( 2013 ) , including occlusal views with scans at an angle of 30 degree from the buccal and lingual surfaces 
 . one single dentist trained in taking digital impressions acquired 40 digital impressions of the reference model . the 40 digital impressions acquired were first divided into two groups ( n=20 each ) , depending on the software version that was used in the processing ( i.e. , cerec version 3.8 or version 4.2 ) . each group was further divided into two subgroups ( n=10 each ) , depending on the spacer parameter settings ( i.e. , 40 or 80 m ) ( table 1 ) . the acquired datasets were transmitted to a milling unit ( cerec mcxl , sirona dental systems gmbh , bensheim , hesse , germany ) , and crowns were fabricated using lavaultimate restorative milling blocks ( 3 m espe , st . table 1two - way anova analysis showing that software version and spacer parameter significantly affected the fit of the crowns ( p<.05)independent variablesignificancesoftware versionp<0.05space parameterp<0.05 the replica technique previously described by molin and karlsson 
  ( 1993 ) , and validated by rahme , et al . the crowns were filled with a low viscosity silicone material ( fit checker , gc dental , tokyo , tokyo , japan ) , and placed in the titanium reference model using a shimadzu universal tester ( ag-10knx , shimadzu co , tokyo , tokyo , japan ) with a 20 n load . after 5 minutes , the silicone material attached to the internal surface of the crowns was removed and then stabilized with the impression material of higher viscosity ( examixfine regular type , gc dental , tokyo , tokyo , japan ) . all replicas were cut along the abutment axis both in the buccolingual and in the mesiodistal direction , using a razor blade , yielding four fragments per crown . for each cross section , four points were measured , thus 16 thickness points of the titanium replica were measured ( figure 1 ) . the points from  1  to  8  were included in the buccolingual direction section , and the points from  a  to  h  were included in the mesiodistal direction section . the measuring points were divided into 4 categories considering the location of tooth : margin ( 1 , 8 , a , h ) , lower axial wall ( 2 , 7 , b , g ) , upper axial wall ( 3 , 6 , c , f ) , and occlusal surface ( 4 , 5 , d , e ) . figure 1groups defined for the study 
  all analyses were performed using a double - blind protocol . replica film thickness was measured with a video measuring system ( optical video measuring system , seven ocean optical technology , donnguan , guangdong , china ) at a 10x magnification with external light source . the mean and the standard deviation of the fit accuracy were calculated for each group . the influence of independent variables , including the ones from the software and from the spacer parameters , were analyzed using a friedman two - way analysis of variance ( anova ) ( p=.05 ) . the groups within each category ( margin , lower axial wall , upper axial wall , occlusal surface ) were merged , and the mean and the standard deviation of each category were calculated . the comparisons between groups within each category were conducted using paired t - tests ( p<05 ) . all statistical analyses were carried out with medcalc version 12.5.0 ( medcalc software , ostend , vlaanderen , belgium ) . the accuracy of the fit at each measuring point for each experimental group is summarized in figures 2 to 6 . the results drawn from the average values of the same categorized measuring points are shown in figure 9 . the two - way anova analysis showed the software version and the spacer parameter significantly affected the fit of crowns ( p<.05 ) ( table 1 ) . the points 1 to 8 are positioned on the buccolingually sectioned surface , and a to h are on the mesiodistally sectioned surface . points of 1 , 8 , a , h are positioned on the margin , 2 , 7 , b , g are on the lower axial wall , 3 , 6 , c , f are on the upper axial wall , and 4 , 5 , d , e are on the occlusal surface 
 
 figure 3accuracy of fit of each measuring point in group 1 ( cerec sw 3.8 , 40 mm ) . the top of box and bottom of box indicate the 75% and 25% values of results . the midline in the box means the average of the results 
 
 figure 4accuracy of fit of each measuring point in group 2 ( cerec sw 3.8 , 80 mm ) . the top of box and bottom of box indicate the 75% and 25% values of results . the midline in the box means the average of the results 
 
 figure 5accuracy of fit of each measuring point in group 3 ( cerec sw 4.2 , 40 mm ) . the top of box and bottom of box indicate the 75% and 25% values of results . the midline in the box means the average of the results 
 
 figure 6accuracy of fit of each measuring point in group 4 ( cerec sw 4.2 , 80 mm ) . the top of box and bottom of box indicate the 75% and 25% values of results . the midline in the box means the average of the results 
 
 figure 7mean internal fit ( mm ) of each measuring points on buccolingually sectioned surface . * indicates statistically significant differences 
 
 figure 8mean internal fit ( mm ) of each measuring points on mesiodistally sectioned surface . * indicates statistically significant differences 
 
 figure 9the results of each average values of measuring points categorized as margin ( 1 , 8 , a , h ) , lower axial wall ( 2 , 7 , b , g ) , upper axial wall ( 3 , 6 , c , f ) and occlusal suface ( 4 , 5 , d , e ) . * indicates statistically significant differences 
  the accuracy of the fit of all measuring points on the margin ( 1 , 8 , a , h ) showed no statistical significant difference between groups , and the average values of measuring points categorized as the margin also showed no significant difference ( p>.05 ) . on the lower and on upper axial wall , some specific measuring points ( i.e. , points 2 and g of the lower axial wall ; points 3 and f of the upper axial wall ) showed a significantly larger gap when data were processed using the version 3.8 of the software compared with the version 4.2 of the software , regardless of the spacer settings used . measuring points 6 ( upper axial wall ) and 7 ( lower axial wall ) showed a significantly larger gap using the version 3.8 of the software compared with the 4.2 of the software when the spacer parameter was set at 40 m ( p<.05 ) . the comparison of the average values of the measuring points categorized as lower axial wall and upper axial wall showed a significantly larger gap using the version 3.8 of the software , regardless of the spacer parameter used ( p<.05 ) . in the occlusal surface , measuring points d and e showed a significantly larger gap when the data were processed using the version 3.8 of the software compared with the version 4.2 of the software , regardless of the spacer parameter used ; while measuring point 4 showed the same result only when the spacer parameter was set at 80 m ( p<.05 ) . the comparison of the average values of the occlusal surface showed the group processed with the version 3.8 of the software had a larger gap compared with the group processed with the version 4.2 of the software only when the spacer parameter was set at 40 m ( p<.05 ) . although we expected the larger spacer parameter setting would have caused a larger gap between crown and tooth , the use of 40 m as the spacer parameter setting in the version 3.8 of the software produced a larger gap when compared to the setting of the spacer at 80 m for the six measuring points , including 5 ( occlusal surface ) , 6 ( upper axial wall ) , a ( margin ) , b ( lower axial wall ) , c ( upper axial wall ) , and h ( margin ) . on the contrary , for the version 4.2 of the software , one measuring point ( i.e. , point 6 of upper axial wall ) showed a smaller gap with a 40 m spacer setting when compared with the group selecting 80 m , while the group selecting 80 m in the version 4.2 of the software showed the least variations among the results . the process of dental restorations using cad / cam technology involves data acquisition , data processing , and manufacturing . the aim of this study was to evaluate the effect of data processing on the adaptation of crown restorations . variables such as different versions of software and space parameters were analyzed in the four groups of crown impressions , and the fit of the crown was measured using the replica technique . as the results showed , the crowns designed using the version 4.2 of the software produced a better fit than those designed using the version 3.8 of the software , particularly in the axial wall . the spacer parameter was more accurately represented in the version 4.2 of the software than in the version 3.8 . in addition , the use of the version 4.2 of the software combined with the spacer parameter set at 80 m showed the least variation . on the other hand , such parameters indicate the setting factors to determine particular features of a restoration in cad / cam system . the cement space , the contact strength , the occlusion strength , and the minimum thickness of a restoration can be easily changed as providing different parameter in the cad / cam system . space parameter can be set from -100 to 100 m in the version 3.8 , and from 0 to 100 m in the version 4.2 . among the overlapped space parameter ( from 0 to 100 m ) , forty m is commonly used as space parameter because resin cements range from 20 to 35 m concerning the thickness of their film ; the 80 m is the double value of 40 m to evaluate the effect of space parameter . to fabricate well - fitting dental crowns using cad / cam systems in the clinic , the optimal combination involving the intraoral scanner , the software version , and although many studies have compared the fit of crown restorations fabricated with different intraoral scanners 


  and milling machines 
 , there have been few studies evaluating the effect of different versions of the software . the results of this study showed the fit of a restoration could be affected by the software version and the spacer parameter setting , even when the same scanner and milling machine were used . compared to the version 3.8 of the cad / cam software , the version 4.2 of this software can be recommended for the fabrication of well - fitting crown restorations , and for the appropriate regulation of the spacer parameter . even though the 80-m spacer setting produced a larger gap , this setting could be recommended for the version 4.2 of the cad / cam software because it showed a good repeatability . adequate marginal gaps and internal fits are decisive factors for the clinical longevity of crown restorations 
 . a large marginal gap can cause discoloration , disintegration of cement , plaque retention , and gingival inflammation 


 . the recommended marginal gap for the prevention of the above mentioned complications 


  is below 100 m . the results obtained in this study showed a clinically acceptable marginal fit could be achieved regardless of the software version and of the spacer parameter settings , with an average gap of 35 m . a larger internal gap could produce restorative fractures and postoperative sensitivity . the results of this study demonstrated the version 3.8 of the software and the spacer parameter set at 40 m produced a relatively larger internal gap when compared with the version 4.2 of the software and with the spacer parameter set at 80 m . the use of a sufficient amount of cement with harder mechanical properties is recommended in these cases to avoid the complications caused by large internal gaps . although zirconia is one of the most popular materials used in the fabrication of crowns by utilizing the cad / cam system , the additional process of sintering may introduce variability among crowns . for the accurate evaluation of the effect of the version of the software and of the spacer parameter settings , lava ultimate restorative , a resin of nano ceramic material that does not require the sintering process was selected as the restoration material in this study 
 . compared with previous studies 


 , the smaller marginal and internal gaps reported in this study could be due to the use of a different restoration material . although measurements using a digital 3d scanner are precise , convenient , and easily visualized , the replica technique is the appropriate measurement tool for the accurate assessment of multiple points in the internal surface of restorations fabricated using the cad / cam system 
 . the generally recognized improvement introduced by the version 4.2 of the software compared with the version 3.8 is an increased convenience for the use of the cerec cad / cam system . the version 4.2.5 is already available these days , being a supplementary version of the 4.2 and it does not seem to have big functional differences with the version 4.2 . the results of this study showed that a smaller internal gap could be obtained using the version 4.2 of the software , particularly at the axial wall , despite the fact the marginal gap was almost the same . the improvement of the adaptation obtained with the version 4.2 of the software could be in part due to differences in the pre - set space , basically providing space regardless of the parameter setting at the internal part between software versions . it has been shown that a greater internal space resulted in less marginal gap leading to less internal premature contact 
 ; a greater internal space might be necessary for achieving an adequate marginal gap in crown restorations , taking into account the accuracy and reproducibility of intraoral scanners and milling machines that are currently used with the version 3.8 of the software . another possible explanation could be the improvement in resolution of the version 4.2 of the software during the processing of the acquired data through the intraoral scanner . the application of the cad / cam technology in dentistry have been producing improvement not only in dental clinics but also in dental laboratories 
 . many studies are using the ad / cam technology to compare it with conventional methods or to manage it as a tool to measure specimens . our results suggest that , to achieve accurate comparisons across studies , all reports should specify the software version and the space parameter settings used ( additional studies of reference on the effect of various software and parameters will be necessary ) . the fit of a crown restoration can be affected by the specific cad / cam software version and by the parameter settings selected , even if the same scanner and milling machine were used . in comparison with the version 3.8 of the cad / cam software , the version 4.2 of this software can be recommended for the fabrication of well - fitting crown restorations , and for the appropriate regulation of the spacer parameter . even though the 80-m spacer setting produced a larger gap , this setting can be recommended for the version 4.2 of the cad / cam software because it has shown a good repeatability .
objective this study investigated the marginal and internal adaptation of individual dental crowns fabricated using a cad / cam system ( sirona s bluecam ) , also evaluating the effect of the software version used , and the specific parameter settings in the adaptation of crowns.material and methods forty digital impressions of a master model previously prepared were acquired using an intraoral scanner and divided into four groups based on the software version and on the spacer settings used . 
 the versions 3.8 and 4.2 of the software were used , and the spacer parameter was set at either 40 m or 80 m . 
 the marginal and internal fit of the crowns were measured using the replica technique , which uses a low viscosity silicone material that simulates the thickness of the cement layer . 
 the data were analyzed using a friedman two - way analysis of variance ( anova ) and paired t - tests with significance level set at p<0.05.results the two - way anova analysis showed the software version ( p<0.05 ) and the spacer parameter ( p<0.05 ) significantly affected the crown adaptation . 
 the crowns designed with the version 4.2 of the software showed a better fit than those designed with the version 3.8 , particularly in the axial wall and in the inner margin . 
 the spacer parameter was more accurately represented in the version 4.2 of the software than in the version 3.8 . 
 in addition , the use of the version 4.2 of the software combined with the spacer parameter set at 80 m showed the least variation . on the other hand , 
 the outer margin was not affected by the variables.conclusion compared to the version 3.8 of the software , the version 4.2 can be recommended for the fabrication of well - fitting crown restorations , and for the appropriate regulation of the spacer parameter .