Evaluation of marginal and internal adaptations of posterior all-ceramic crowns fabricated with chair-side CAD / CAM system : an in vitro study

OBJECTIVE: Advances in chair-side Computer-Aided Design / Computer-Aided Manufacturing (CAD/CAM) technology and materials science currently enable the fabrication of highly esthetic restorations for the anterior and posterior teeth. However, there is a lack of evidence regarding the marginal and internal adaptations of new CAD/CAM materials. The objective of this study was to evaluate the marginal and internal adaptations of posterior all-ceramic restorations fabricated from contemporary restorative materials with chair-side CAD/CAM system. MATERIALS AND METHOD: An artificial mandibular right first molar tooth was prepared according to standard tooth preparation procedures, and standard models of the prepared teeth were obtained. All-ceramic restorations (n=10) were fabricated from seven different CAD/CAM blocks (IPS e.max CAD, Lava Ultimate, Incoris TZI, Incoris ZI, Vita Suprinity, Vita Enamic, and GC Cerasmart). The marginal and internal adaptations were measured with silicone replicas, which were sectioned with a thin lancet. The discrepancy between the die and the inner surface of the restoration was examined at 50× magnification by using a light microscope with digital camera. Four reference points were examined at each buccal-lingual section and eight reference points were examined at each mesial-distal section. The results were evaluated by two-way analysis of variance (ANOVA) followed by Tukey HSD test (α=0.05). RESULTS: The values obtained from marginal-internal areas were generally greater than those in the marginal areas. Significant differences were found between the materials. The statistical analysis revealed that there was an interaction between the material type and the location of the reference points (p<0.05); the lowest values were observed in axial areas, and the highest values were observed in occlusal areas. CONCLUSION: All materials showed low marginal and internal discrepancies which were considered clinically acceptable.


INTRODUCTION
Computer-aided design (CAD) and computer-aided manufacturing (CAM) have been popular in recent years.These digital techniques afford several advantages in the dental practice. 1Capturing images of prepared, adjacent, and opposing teeth, which is the first step in the CAD/CAM process, eliminates the need for making an impression using elastomeric impression material.3][4] The design and milling stages of the CAD/CAM process would decrease manufacturing costs by reducing the time spent in the laboratory by technicians and allowing clinicians to fabricate chair-side restorations. 5Milling the restoration from an industrially sintered ceramic block with a very homogenous structure improves the quality of the material compared with conventional ceramic processing techniques. 4,6[12][13][14][15] The adaptation of a restoration is determined by measurements of its marginal and internal gaps, which are important factors for the long-term clinical success of restorations.7][18][19] An ideal internal adaptation improves mechanical properties, such as retention, strength, and resistance. 20,21The adaptation of conventionally-fabricated all-ceramic restorations in the laboratory is a sensitive technique that may be affected by several factors such as the impression material and technique, disinfection, the storage time and conditions of the impression before pouring the stone cast, application of the die spacer, and the investment and casting or pressing process. 1,223][24] However, recent CAD/ CAM systems use highly accurate scanners, advanced software, and precise milling devices with advanced technology. 25The accuracies of scanners 26,27 and precision of milling devices 3,28 have been confirmed by recent studies.Therefore, restorative material properties have gained interest for the accuracy of the CAD/CAM fabrication process.
12][13][14][15]29 The mechanical properties of some of these materials have been investigated. 23However, there is little information about the marginal and internal adaptations of CAD/ CAM ceramic materials in the literature; moreover, such studies have been limited to lithium disilicate and zirconia.The marginal and internal adaptations of CAD/CAM fabricated lithium disilicate crowns have been comparable or better than that of conventionally fabricated crowns. 1,20,22Zirconia copings have also exhibited marginal and internal adaptation values falling within the clinically acceptable range. 6,7,19To the best of our knowledge, there are no studies pertaining to the adaptation of recently introduced CAD/CAM materials (e.g.monolithic zirconia and hybrid ceramics).
The objective of this in vitro study was to evaluate the marginal and internal adaptations of chair-side CAD/CAM ceramic crowns fabricated from different prefabricated blocks.The null hypothesis for this study was that marginal and internal adaptations of chairside posterior crowns would be similar for different materials.

Specimen preparation
An artificial mandibular right first molar tooth (ANA-4 ZE Standard Replacement Teeth, Frasaco GmbH, Tettnang, Germany) was prepared for an all-ceramiccrown according to standard tooth preparation procedures.The tooth preparation yielded a 1.2 mm deep chamfer, a 2 mm occlusal reduction, and 6-degree angled axial walls.The prepared tooth was mounted on a Typodont (ANA-4V Advanced Standard Typodont, Frasaco GmbH).Impressions were taken with a polyvinyl siloxane (PVS; Hydrorise Monophase, Zhermack Spa, Badia Polesine, Italy) impression material and a custom impression tray.Custom trays were fabricated using visible light polymerizing acrylic resin (Triad VLC, Dentsply Caulk, York, PA, USA), which ensured a minimum impression material thickness of 3 mm.One-step dual viscosity impression was made.A lightbody PVS impression material (Hydrorise light body, Zhermack Spa) was mixed using a manual dispenser system and injected on and around the prepared tooth.Next, a heavy body impression material (Hydrorise Maxi Heavy, Zhermack Spa) was mixed using a dynamic mixer.A custom impression tray was filled with this heavy body impression material.Then, the impression tray was positioned on the typodont.All impressions were taken under standard room conditions by the same investigator.To ensure proper polymerization of the impression material, the impressions were allowed to set three times longer than the duration recommended by the manufacturer.The impressions were visually inspected using a magnifying glass (Loupe opt-on, Orange Dental, Biberach, Germany).Then, the casts were poured using a type IV dental stone.The dental stone was mixed in accordance with the water/ powder ratio recommended by the manufacturer and poured into the impressions under constant vibration.The casts were allowed to set for 1 h.Next, the casts were separated from the impressions and the prepared tooth was visually inspected (Loupe opt-on; Orange Dental) for irregularities by a single operator.A total of 70 standard casts were fabricated for 7 groups.Each group was assigned to a particular CAD/CAM material to be milled in the CEREC system.The casts were mounted in semi-adjustable articulator (Stratos 100; Ivoclar Vivadent, Schaan, Liechtenstein) in a maximum intercuspal position.Full-arch digital impressions were obtained using an intraoral scanner (CEREC Omnicam, Sirona Dental Systems, Bensheim, Germany) from the mandibular and maxillary casts and the buccal side of the casts.The maximum intercuspal positions of virtual models were calculated using buccal images after models were generated by the CAD software (inLab SW 4.2; Sirona Dental Systems).Then, the model axis was set, and the restoration margins were determined.The restorations were designed by selecting a cement thickness of 80 µm.Manual adjustment was not made to the marginal aspects of the designs except suggestions by the software.Minor adjustments were made to the occlusal surface of the restoration to ensure a similar material thickness for all specimens, if necessary.All of the ceramic crowns were milled in a milling unit (CEREC MC XL; Sirona Dental Systems).The crowns were cleaned with water in an ultrasonic cleaner to remove milling residue from the CAD/CAM milling machine.Then, each group was processed according to the procedures recommended by the manufacturer as follows: The milled crowns in Groups 1 and 4, which were in a crystalline intermediate phase, were crystallized in a porcelain furnace (Programat P300; Ivoclar Vivadent).The milled crowns in Group 3, which were in a partially sintered phase, were dried and sintered in a sintering furnace (InFire HTC; Sirona Dental Systems).No processing was required for the milled crowns in Groups 2, 5, and 7.For the crowns in Group 6, the zirconia cores were veneered with CAD-CAMfabricated feldspathic ceramic (CEREC Block; Sirona Dental Systems), which was bonded to the cores using a resin cement (Panavia 2.0; Kuraray, Osaka, Japan).Removable dies were separated from the casts and duplicated from a Co-Cr alloy powder (Eos Cobalt Chrome SP2; Eos GmbH, Krailling, Germany) using direct-laser sintering technology (Eosint M 270; Eos GmbH).The metal dies were used for measuring the internal and marginal adaptations of the crowns.

Measurement of marginal and internal gaps
Silicone replica technique was used to measure the internal and marginal adaptation.To obtain replicas, a light-body impression material (Variotime; Heraeus Kulzer GmbH, Hanau, Germany) was filled in all of the ceramic crowns thoroughly and then the crowns were placed on the metal dies.After the light-body impression material was set, the crowns were removed from the metal dies.Heavy-body impression material (Variotime; Heraeus Kulzer GmbH) was placed in the crowns on the light-body impression material.Replicas of the cast teeth specimens were sectioned with a thin lancet to 1 mm thickness and 5 sections were obtained for each replica (Figure 1).For sections 1-4, four reference points (Figure 2a) were determined, and for section 5, eight reference points (Figure 2b) were determined.The reference points were set at the marginal (M), marginal-internal (MI), axial (A), and occlusal (O) areas, and the mean values were calculated for each reference point.Silicone replicas were examined using a light microscope at 50× magnification (Figure 3) and Leica Qwin Plus program (Leica Microsystems Imaging Solutions Ltd, Cambridge, UK).

Statistical analysis
The statistical analysis was performed using software (SPSS version 20.0, SPSS Inc, Chicago IL, USA).The data were evaluated using two-way analysis of variance (ANOVA; α=0.05).The Tukey HSD test was used to measure differences between groups.

RESULTS
The ANOVA test revealed that there was an interaction between the material type and the location of the reference points (p<0.05;Table 1).The mean values and standard deviations of the materials as a function of the reference points are given in Table 2.It was observed that the lowest and highest values were in the axial, and the occlusal areas, respectively.Larger gaps were observed in Groups 3 (TZI) and 7 (GC), followed by Group 1 (IC).The values of Group 2 (LU), Group 4 (VS), Group 5 (VE), and Group 6 (ZI) were similar.The values in the marginal-internal areas were generally higher than those in the marginal areas.Pairwise comparisons between the tested materials are shown in Table 3.

DISCUSSION
In this in vitro study, the marginal and internal adaptations of CEREC-fabricated ceramic crowns were investigated.A wide range of prefabricated blocks including glass ceramics, zirconia cores, translucent zirconia, and hybrid materials which are suitable for posterior crowns were used as crown material.To the best of our knowledge, no previous study has evaluated the marginal and internal adaptations of crowns fabricated from these materials.The data from this in vitro study led to the rejection of the research hypothesis that marginal and internal adaptations of posterior CAD/CAM-fabricated crowns would be similar for different materials.
Adaptation between the tooth and the restoration is important for the long-term success of restorations. 5,7o measure the marginal and internal adaptations, several techniques have been used 1,5,19,20,22 ; however, a single standard protocol is not available in the literature.In the present study, the silicone replica technique, which is used to determine the in vivo gap between tooth and crown surfaces, has been used to evaluate the marginal and internal adaptation.This method is reliable and non-invasive for indirect restorations. 3,6,9,28evertheless, some shortcomings have been reported for this technique.First, difficulties related to stabilizing the thin and fragile replicated space have been noted. 8n the present study, all of the silicone replicas remained in their crowns, a finding that may be attributed to the fact that silicone adheres to the rough inner surfaces of the crowns, not to the smooth surfaces of the metal dies of the prepared tooth.Another, difficulty pertains to standardizing the seating force. 17However, different seating forces do not appear to significantly affect the  thickness of the silicone layer. 18In this study, light-body silicone impression material was applied to the inner surface of the crowns and placed on the prepared teeth by a single operator using the same finger pressure. 28e marginal and internal adaptations of CAD/ CAM-fabricated restorations may have been affected by several factors including fabrication technique, preparation design, spacer thickness, the scanning method and its accuracy, the software, the restorative material, and the properties of milling machine. 1 The aim of this study was to evaluate marginal and internal adaptations of posterior CAD/CAM crowns fabricated from contemporary restorative materials.Therefore, an effort to standardize remaining factors from abutment teeth preparation to milling of the crowns has been made.A standard tooth preparation was replicated to obtain gypsum dies that were used to fabricate the CAD/ CAM crowns.Therefore, the effects of the preparation geometry and the margin design on adaptation of crowns could be eliminated.3 Next, the gypsum dies were replicated with laser-sintering technology to obtain metal dies for measuring the adaptation of the restorations.With these metal dies, more uniform measurements could be achieved compared with natural teeth and resin dies.3,30 We opted for a chamfer preparation in light of the findings of previous studies evaluating the effect of different marginal preparation designs on the adaptation of all-ceramic restorations.These studies revealed no difference between the marginal adaptations of chamfer and shoulder margins, and the chamfer design has clinical advantages over the shoulder design.[31][32][33] In the zirconia core group (ZI), a circumferential collar was designed in the cores to avoid the influence of a veneer layer on the marginal adaptation of crowns.CAD/CAM-fabricated feldspathic veneer luted to the zirconia core by a resin cement was preferred in the zirconia core group to eliminate the effect of firing procedures on the marginal adaptation of zirconia cores.34 The influence of different veneering methods such as conventional porcelain firing, the heatpress technique, and CAD/CAM-fabricated veneers on the internal and marginal adaptations of zirconia cores may be of interest for future research.Since we were attempting to standardize milling conditions and the quality of specimen fabrication, we used a new set of diamond burs. Te cooling and lubricating fluid was also refreshed for each group of specimens.Scanning of the gypsum dies of the prepared teeth in the diagnostic casts was also performed by the same clinician using an intraoral camera (CEREC Omnicam) based on triangulation technology as its capture method.fundamental factors in the clinical success of partial fixed dental prostheses, the ideal gap for ceramic crowns is a controversial topic.3,35 Several studies have reported that marginal gap values ranging from 100-200 µm 2,5,36 and internal gaps ranging from 200-300 µm are clinically acceptable for cemented restorations.3,37,38 Considering CAD/CAM-fabricated restorations, previous studies focused on marginal and internal adaptations of zirconia copings and lithium disilicate crowns, and both materials yielded clinically acceptable marginal and internal crown adaptation results.1,[6][7][8]11,16,[19][20][21][22] The marginal and internal gap values of zirconia copings and lithium disilicate crowns found in this study are slightly higher than those of previous studies.This finding may be due to the spacer thickness setting of 80 µm used in this study.In conventional fabrication techniques, it is known that providing a space between the dies and the restorations for the cement improves adaptation.CAD/ CAM technology makes it possible to set this cement space using software.It has been reported that different settings have an impact on the resulting marginal and internal adaptation.[13][14][15] A small gap may lead to widening the marginal gap in consequence of premature contacts between restoration and abutment tooth.13,39 Mously et al. 1 evaluated the effect of spacer thickness settings on adaptation of restorations fabricated using E4D CAD/ CAM system, and the authors recommended a space thickness of 30 or 60 µm.However, no information is available about the effect of spacer thickness settings on adaptation of CEREC crowns.Additional studies are necessary to determine the optimum spacer thicknesses for CEREC crowns milled from different materials.

Although marginal and internal adaptations are
Various restorative materials for CAD/CAM systems differing in chemical structure and indications are currently available on the dental market. 12Furthermore, advances in materials have led scientists to develop new classifications for all-ceramic and ceramiclike materials. 12,23Some of these materials require additional processing after milling, necessitating specialized equipment for firing and glazing, 25 other products do not necessarily require those steps and can generally be finished using the readily available armamentarium in a dental office. 40These differences in production process may affect marginal and internal adaptations of restorations.The literature does not present any information about marginal and internal adaptations of crowns milled from monolithic zirconia and resin-ceramic hybrid blocks.In this study, resinceramic hybrid materials demonstrated clinically acceptable marginal and internal adaptations; however, discrepancies were high for monolithic zirconia crowns.This excessive marginal gap may be attributed to sinterization shrinkage of thick zirconia material of the full contour of the monolithic zirconia crowns.Monolithic zirconia crowns are thicker than zirconia cores, a fact that may lead to significantly more sintering contraction.Additional studies are necessary to evaluate the effect of zirconia thickness on the adaptation of restorations.
There are some limitations to this study.Although the silicone replica technique has been accepted as a precise method to measure adaptation of restorations, the cementation process may affect the definitive marginal and internal adaptations due to differences in luting agent viscosity.Therefore, examining the marginal and internal adaptation of the crowns does not simulate the clinical conditions in this study.In the present study, we assessed the marginal and internal adaptations of single mandibular first molar crowns.Because of differences in morphology and shape, the marginal and internal adaptations may be different for anterior or premolar teeth.Furthermore, different results might have been observed if the fixed partial prostheses had been investigated in terms of precision of the adaptation instead of single crowns.

CONCLUSION
Within the limitations of the study, all-ceramic crown materials that are used for chair-side CAD/CAM systems demonstrated clinically acceptable marginal and internal adaptations.Gaps were higher in monolithic zirconia crowns.The lowest gap values were observed in axial areas, and the highest values were observed in occlusal areas.

Table 1 .
Tests of between-subjects effects

Table 3 .
Pairwise comparisons of tested materials