syed rashid habib* digital microscopic evaluation of...

Biomed. Eng.-Biomed. Tech. 2018; aop

Syed Rashid Habib*

Digital microscopic evaluation of vertical marginal discrepancies of CAD/CAM fabricated zirconia coreshttps://doi.org/10.1515/bmt-2017-0234Received December 27, 2017; accepted April 16, 2018

Abstract

Objective: The aim of this in vitro research study was to evaluate the vertical marginal discrepancies of zirconia (Zr) cores fabricated by five different computer-aided design and manufacturing (CAD/CAM) systems using a digital microscope.Materials and methods: A total of 60 specimens were prepared and randomly divided into five groups (n = 12 each) using the following systems: Ceramill Motion 2 (CM, Amanngirrbach, Germany); Weiland (WI, Ivoclar Vivadent, USA); Cerec (CS, Sirona Dental, USA); Zirkonzahn (ZZ, Gmbh Bruneck, Italy) and Cad4dent (CD, Canada). The specimens were numbered and the vertical marginal discrepancies were evaluated with a digital microscope at 50× magnification.Results: A one-way analysis of variance showed a sta-tistically significant difference (p = 0.002) between the groups. The CM group exhibited the lowest values for the marginal gaps (31.30 ± 15.12 μm), while the ZZ group exhibited the highest values for the marginal gaps (44.83 ± 28.76 μm) compared to other groups. A post hoc Tukey’s test for multiple comparisons between the experi-mental groups showed a statistically significant differ-ence (p < 0.05) between the group CM and group CD with group ZZ. The rest of the groups showed no significant differences between them. Variations in the values were observed for the four sites measured with the highest and the least mean marginal gap value of 43.19 ± 23.84 μm and 32.49 ± 12.21 μm for buccal and lingual sites, respectively.Conclusion: Variations existed in the marginal discrep-ancy values for the CAD/CAM systems investigated in the study. Vertical marginal discrepancy values observed for

various systems investigated in the study were well within the clinically acceptable range.

Keywords: digital microscope; marginal discrepancy; marginal fit; zirconia; zirconia cores; zirconia crowns.

IntroductionMetal free and excellent esthetics combined with superior biocompatibility make the all-ceramic crowns the choice of material to be used in the anterior esthetic zone [1]. Historically, resin-based crowns were the first metal-free crowns to be used, but they were abandoned because of their low fracture resistance. Newer metal-free crowns are increasingly being used in dental practice; these crowns are made from different ceramic materials such as lithium disilicate, zirconia (Zr), leucite-reinforced glass and glass-infiltrated alumina [2].

The need for a material which is tooth-like in color and at the same time having strength equal to metal resulted in the development of all-ceramic crowns with Zr cores. Transformation-toughened Zr is a successful alternative for use in clinical situations where esthetics and strength are required equally [3]. Zr demonstrated a flexural strength of 900–1200 MPa and a fracture tough-ness of 9–10 MPa. The restorations are milled either by machining of semi-sintered blocks followed by complete sintering at high temperature, or by machining of com-pletely sintered hard blocks. Both layered Zr as well as the monolithic Zr are popular due to improvements in their mechanical properties [4].

Precise marginal fit of all-ceramic crowns is an essen-tial requirement in reducing the prevalence of diseases such as caries, gingivitis/periodontitis of the abutment teeth and to increase the long-term prognosis of these res-torations [5]. A marginal discrepancy of 120 microns (µm) is considered clinically acceptable [6]. At the same time, a minimum gap of approximately 25–50 µm is required between the restoration and the tooth to ensure complete seating of the restoration and to provide an even layer of luting cement [6].

*Corresponding author: Dr. Syed Rashid Habib, BDS, FCPS, Associate Professor, Department of Prosthetic Dental Sciences, College of Dentistry, King Saud University, P.O. Box 60169, King Abdullah Road, Riyadh, 11545, Saudi Arabia, Phone: +966-1-467 7441, Mobile: +966-534750834, Fax: +966-1-467 8548, E-mail: [email protected]. http://orcid.org/0000-0002-4398-3479

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2 S.R. Habib: Marginal discrepancies of zirconia

The marginal discrepancy or marginal misfit can be detected by various techniques described in the literature, all with individual pros and cons [7]. Sorensen [8] classi-fied the different techniques into four categories as follows: direct view; cross-sectional view; impression technique and use of explorer with visual examination (X-rays) [8]. At the same time, “misfit” can be measured and detected at many different locations between the tooth and the restoration. These misfits at different locations are geometrically related to each other and defined as the following: internal gap; mar-ginal gap; overextended margin; under extended margin; absolute marginal discrepancy; vertical marginal discrep-ancy; horizontal marginal discrepancy and seating discrep-ancy [9]. The method employed in this current study is based on the direct measurement of vertical marginal discrepancy as described by Holmes et al. [10], and which probably is the largest measurement of error at the margin and reflects the misfit at different points. The gap measured is the vertical distance measured parallel to the path of withdrawal of the cores and their corresponding three-dimensional (3D) resin dies at mid of buccal, mesial, lingual and distal walls [10].

The assessment of marginal discrepancies of com-monly used computer-aided design and manufacturing (CAD/CAM) fabricated Zr cores can be of help to the dental clinicians in predicting the long-term success of their all-ceramic Zr restorations. Thus, the aim of this study was to evaluate the vertical marginal discrepancies of Zr cores fabricated by five different CAD/CAM systems using a digital microscope. The null hypothesis was that there is no difference in the marginal integrity of the Zr cores fab-ricated by these CAD/CAM systems.

Materials and methodsThe study was conducted at the Department of Prosthetic Dental Sciences/College of Dentistry Research Center, College of Dentistry, King Saud University and approved by the ethical committee of Col-lege of Dentistry Research Center (FR 0404).

Preparation of the tooth

A lower right first molar ivorine tooth in a lower jaw dentoform (D85DP-CHO.1, Nissin Dental Products, Inc., Kyoto, Japan) was pre-pared for an all-ceramic Zr crown (Figure 1). The preparation was based on the standard guidelines for all-ceramic crowns as described in the textbooks of Shillingburg et al. [11] and Rosenstiel et al. [12]. A silicone putty index (Virtual®, Ivoclar, Vivadent Inc., NY, USA) was used for the assessment of preparation, which included the follow-ing: axial reduction = 1.5 mm; occlusal reduction = 2 mm; functional cusp bevel; marginal design = deep chamfer; taper between axial and proximal walls = 6°; overall smooth finish and rounded line angles.

Fabrication of the experimental dies

The prepared tooth was used as the master die. It was scanned and 60 similar solid dies were printed in resin using 3D laser printing (DWX-50 5-Axis Dental Mill, Roland, USA) by one dental laboratory (Figure 2). The prepared specimens were then randomly divided into five groups of (n = 12 each, according to the results of power analysis) and categorized according to the CAD/CAM systems to be investigated in the study as following: Ceramill Motion 2 (CM, Amanngirrbach, Germany); Weiland (WI, Ivoclar Vivadent, USA); Cerec (CS, Sinrona Dental, USA); Zirkonzahn (ZZ, Gmbh Bruneck, Italy) and Cad4dent (CD, Canada). All these prepared dies were numbered from 1 to 12 in each group and used as working dies.

Fabrication of the experimental cores

All the Zr cores for each system were manufactured according to their manufacturer’s instructions by an experienced CAD/CAM technician accustomed to the specific system (Figure 3). For all the test material groups, partially sintered blocks of Zr were used for the milling of the cores. The complete process included the scanning, digitalized designing, milling and complete sintering of the cores for dies of each group with their corresponding scanners, soft wares, milling units and sintering furnaces, respectively. The cores for all the groups were designed with similar wall thickness of 0.5 mm and a cement space of 20 μm. All the cores were tried on their respective dies and the fit was verified by visual examination and tactically with a sharp explorer (Figure 4). After careful evaluation, all the cores were found to have a satisfactory fit and all of the prepared cores were included in this experimental study.

Figure 1: Lower right first molar ivorine tooth mounted in resin block with standardized preparation for an all-ceramic zirconia crown.


S.R. Habib: Marginal discrepancies of zirconia 3

Locking of cores on respective dies

A custom designed device was fabricated in stainless steel and used for locking/securing the position of the cores on their respective dies during the microscopic measurements (Figure 5). The device was designed to use a manual torque wrench (Manual Torque Wrench Prosthetic, Nobel Bio Care, Switzerland) for applying a vertical load of 15 Ncm over the core placed on the die. For standardizing the seat-ing of the cores over the resin dies, the conical tip of the device was adapted over the central groove of the occlusal surface. However,

the device allowed rotation of the whole specimen for multiple measurements around the margins. Each sample was then locked in this position and made ready for the microscopic measurements (Figure 5). The outer restoration margin and the cavosurface angle of the finish line was perpendicular to the optical axis of the digital microscope.

Digital microscopic measurements

Marginal discrepancy was assessed by measuring the vertical dis-tance between the core margin and the prepared cavosurface angle of the die parallel to the path of withdrawal of the core on their

Figure 2: A sample of 3D printed solid replica of the prepared tooth in resin.

Figure 3: A sample of CAD/CAM fabricated zirconia core.

Figure 4: Zirconia core placed over the corresponding resin die.

Figure 5: The complete specimen of zirconia core along with the resin die locked with 15 Ncm force in the customized locking device.



respective dies at the mid of buccal, mesial, lingual and distal sur-faces. All the samples locked in the locking device were examined at 50× magnification under the lens of a digital microscope (Digi-tal Microscope, KH-7700, Hirox-USA, Inc., USA). The digital camera, light source, liquid-crystal display (LCD) monitor, computer and software are all integrated in this microscope. After focusing each site to be measured, an optical image was captured and automati-cally transferred to the digital imaging software of the microscope (2D Image File Viewing Software, KH-7700, Ver.2.10c©Hirox Co. Ltd. 2010, USA). The measurements (Figure 6) were recorded in microm-eters with the help of a micrometer ruler placed in the field of view to calibrate the computer software program. All the imaging were captured and measurements done at four sites by an expert digital microscope technician who was also familiarized initially with pilot samples.

Data analysis

The mean of the four sides together (buccal, mesial, lingual and dis-tal) were recorded and taken as the final mean for each sample. The mean of the samples (n = 12) for each group was considered as the final mean for each group. Mean values, standard deviations (SDs) and 95% confidence intervals were calculated as well as analysis of variance (ANOVA) and the comparisons between groups with a post hoc Tukey’s test were completed using SPSS version 21 software (SPSS, Inc., Chicago, IL, USA) with a predetermined significance level at p < 0.05.

Results

The current study showed the marginal discrepancy values to be less than 50 μm for all the experimental groups investigated in the study. The overall mean values and SD of the marginal discrepancies for the five experi-mental groups are presented in Table 1. A one-way ANOVA showed a statistically significant difference (p = 0.002) between the groups. The CM group exhibited the lowest values for the marginal gaps (31.30 ± 15.12 μm) thus dem-onstrating its superior marginal adaptation, while the ZZ group exhibited the highest values for the marginal gaps (44.83 ± 28.76 μm) compared to other groups.

The largest variation between the four measurements sites was observed for the CD group and was confirmed with a one-way ANOVA indicating non-significant differ-ences between the four readings for all the groups except for the CD group (p = 0.000) (Table 2).

Table 3 shows the assessment by a post hoc Tukey’s test for multiple comparisons between the experimental groups. It showed a statistically significant difference (p < 0.05) between the CM group and the CD group with the ZZ group. The rest of the groups showed no significant differences between them.

Figure 6: Examples of digital microscopic measurements at 50–100× magnification.

Table 1: Descriptive statistics plus ANOVA result for the five experimental groups.

Group Mean (μm)

Standard deviation

95% Confidence interval for mean Minimum Maximum ANOVA p-value

Lower bound Upper bound

CM (n = 48) 31.30 15.12 26.91 35.69 5.39 80.06 0.002WI (n = 48) 42.33 16.98 37.40 47.26 17.05 99.12 CS (n = 48) 36.55 17.81 31.38 41.73 7.62 95.38 ZZ (n = 48) 44.83 28.76 36.48 53.19 7.62 110.62 CD (n = 48) 31.36 19.69 25.64 37.08 3.81 92.84 Total (n = 240) 37.28 20.83 34.63 39.93 3.81 110.62



The mean values for the four sites of measurements and results of the ANOVA are presented in Table 4. Varia-tions in the values were observed for the four sites meas-ured with the highest and least mean marginal gap value of 43.19 ± 23.84 and 32.49 ± 12.21 μm for buccal and lingual sites, respectively. The ANOVA showed a statistically sig-nificant difference (p = 0.005) between the means of the four sites. The possible explanation for this variation could be related to the morphology of the tooth.

Table 2: Mean values plus ANOVA results between four sites of measurements for each experimental group.

Group Site n Meana Std. deviation

95% Confidence interval for mean ANOVA p-value


CM Buccal 12 34.34 19.50 21.94 46.73 0.670 Distal 12 32.52 12.51 24.56 40.47 Lingual 12 31.52 12.66 23.47 39.56 Mesial 12 26.82 15.70 16.85 36.80 Total 48 31.30 15.12 26.91 35.69

WI Buccal 12 47.70 21.73 33.89 61.50 0.540 Distal 12 43.71 19.23 31.49 55.93 Lingual 12 39.10 9.97 32.76 45.43 Mesial 12 38.82 15.27 29.11 48.52 Total 48 42.33 16.98 37.40 47.26

CS Buccal 12 43.08 21.35 29.52 56.65 0.536 Distal 12 35.48 17.65 24.27 46.70 Lingual 12 33.19 16.45 22.74 43.65 Mesial 12 34.45 15.91 24.34 44.57 Total 48 36.55 17.81 31.38 41.73

ZZ Buccal 12 42.88 32.21 22.41 63.35 0.219 Distal 12 57.06 36.86 33.63 80.48 Lingual 12 32.60 10.01 26.24 38.97 Mesial 12 46.80 26.53 29.94 63.66 Total 48 44.83 28.76 36.48 53.19

CD Buccal 12 47.97 23.87 32.80 63.14 0.000 Distal 12 35.26 17.73 24.00 46.53 Lingual 12 26.04 8.65 20.54 31.54 Mesial 12 16.18 9.92 9.87 22.48 Total 48 31.36 19.69 25.64 37.08

aMean gap was measured in micrometers (μm).

Table 3: Multiple comparisons of the experimental groups with a post hoc Tukey’s test.

Groups CM WI CS ZZ CD

CM – 0.062 0.709 0.011 1.00WI 0.062 – 0.630 0.974 0.064CS 0.709 0.630 – 0.268 0.719ZZ 0.011a 0.974 0.268 – 0.011a

CD 1.00 0.064 0.719 0.011 –

ap-Value was significant at p < 0.05.

Table 4: Descriptive statistics plus ANOVA results for four areas measured.

Site of marginal gap

Meana Standard deviation

95% Confidence interval for mean ANOVA p-value


Buccal (n = 60) 43.19 23.84 37.03 49.35 0.005Mesial (n = 60) 40.81 23.44 34.75 46.86Lingual (n = 60) 32.49 12.21 29.34 35.65Distal (n = 60) 32.61 19.93 27.46 37.77Total (n = 240) 37.28 20.83 34.63 39.93

aMean gap was measured in micrometers (μm).



Multiple comparisons with a post hoc Tukey’s test revealed a statistically significant difference (p < 0.05) between the mean values of buccal with lingual and distal sites of measurements (Table 5).

DiscussionIn this research study, the vertical marginal discrepancies of Zr cores fabricated by five different CAD/CAM systems were evaluated using a digital microscope under a con-stant vertical load. The results revealed variations and dif-ferences between the mean marginal gap values of these systems, thus rejecting the null hypothesis of no differ-ence in the marginal integrity of the Zr cores fabricated by these CAD/CAM systems.

Currently, there is a lack of consensus relating to fabrication of dies over which the crowns/cores to be tested for the marginal integrity are constructed. Various methods/techniques are employed for fabricating the specimen such as autopolymerizing resin dies [13], metallic dies [14, 15], use of natural teeth [16] and CAD/CAM fabricated dies [7]. In the present study, solid dies printed in resin using 3D laser printing (DWX-50, Roland, USA) were used as dies over which the Zr cores were fab-ricated for the experimental groups. This was to ensure the standardization of each specimen in shape, geometry and dimension and to avoid any human error during the preparation of the samples. In a research study by Lee et al. [17], the accuracy of 3D printing for manufacturing replica teeth were found to be accurate.

In the literature various methods have been employed by researchers for the assessment of marginal discrepan-cies of indirect restorations [4]. The destructive methods in which the samples are completely destroyed [18] and the nondestructive methods in which the specimens remain intact [19] have their advantages and disadvantages. The replica technique [19] and direct microscopic examination [20] are two common examples of nondestructive methods. In the current study, the direct microscopic examination

with a digital microscope attached to a computer system for the measurement of the marginal discrepancies was employed. This method is widely used, is nondestruc-tive, there is no need for intermediate materials such as impression materials or luting cement in between and the measurements can be carried out at various sites [5, 14]. The disadvantage of this technique is that the horizontal marginal discrepancy cannot be investigated. However, there is a large risk of luting agent’s exposure because of the vertical marginal discrepancy, while horizontal mar-ginal discrepancy affects the plaque control and compro-mises the periodontal health of the abutment tooth.

Furthermore, in the current study, the digitalized measurements were performed directly on the specimen comprising the core placed over its corresponding die and secured in its place with a customized locking device to ensure its adaptation. This method of measurement has been employed by Martínez-Rus et al. [5] and Torabi et al. [14], and was found to be useful. Following the protocols described by Martínez-Rus et al. [5] and Quintas et al. [21] the core specimens were not cemented over the dies to rule out the effect of the luting agent’s physical behavior and the cementation procedure on the marginal discrepancies.

The current study compared the vertical marginal discrepancies of five different CAD/CAM fabricated Zr cores. It also compared the findings with other reported studies [5, 7, 9, 13–21]. The acceptability of marginal dis-crepancies for crowns varies between 20 μm and 200 μm with different researchers. However, the majority of them agreed that a marginal discrepancy below 120 μm should be considered clinically acceptable for cast, porcelain fused to metal and all-ceramic crowns [4, 6, 8, 10]. The results of the current study revealed an overall mean mar-ginal discrepancy of 37.28 ± 20.83 μm for all the groups with highest values of 44.83 ± 28.76 μm for one of the groups. These values are well within the clinical accept-ability level and for the long-term successful prognosis of the all-ceramic Zr crowns.

Many investigators reported a significantly higher marginal gap after cementation than that before cemen-tation [22, 23]. Furthermore, the cementation techniques such as uncontrolled finger pressure or overfilling of the crown with cement can cause uneven flow of cement with one axial wall having a thick film and the opposite wall having a thin film [24]. In order to evaluate the accuracy of each CAD/CAM system investigated in this study, the microscopic measurements were performed without any luting agent. The results of the current study recommend using resin luting cement with the least viscosity for cementation of Zr crowns with a proper cementation tech-nique for achieving the best vertical marginal adaptation

Table 5: Multiple comparisons of the four areas measured for the experimental groups with a post hoc Tukey’s test.

Groups Buccal Mesial Lingual Distal

Buccal – 0.919 0.023a 0.025a

Mesial 0.919 – 0.118 0.127Lingual 0.023a 0.118 – 1.00Distal 0.025a 0.127 1.00 –

ap-Value was significant at p < 0.05.



of the Zr crowns. Use of the luting cement with high vis-cosity and a faulty cementation technique will result in increased marginal discrepancies and future complica-tions [21, 22].

Zr cores fabricated with ZZ exhibi ted the highest verti-cal marginal discrepancy among the groups investigated in the study and this was statistically significant with the cores fabricated with CM and CD. Kim et al. [23] reported that the chances of increase in the marginal discrepan-cies for the CAD/CAM fabricated restorations are present at each and every step. These differences may be a result of differences in the hardware equipment such as digital scanners, milling units and inter-technician variations. The variations in the marginal discrepancies for the dif-ferent brands of Zr cores/copings have been reported by Ha and Cho [7] and Ortega et al. [15]. However, the varia-tions in the marginal discrepancies are within fractions of micrometers and carry no clinical significance. Based on the results of our study, the high precision of the CAD/CAM systems can be verified irrespective of the brand of the systems investigated.

Possible limitations of this research included the 2D measurements recorded with the digital microscope where the accuracy of measurements was dependent on the viewing angle. Four measurements were recorded for each sample and could have been increased for more accuracy. It has been reported that individual measure-ments at different locations of the margin may reveal deviations from the mean [24]. In addition, the larger the number of measurements per specimen, the greater the precision of the analysis will be [25]. Veneering process and its associated heat treatment [14] are associated with the marginal discrepancies of all-ceramic crowns and the current study focused only the discrepancies result-ing from the cores.

ConclusionWithin the limitations of the current study it can be con-cluded that;

– Variations existed for the marginal discrepancy values for the CAD/CAM systems investigated in the study.

– The CM system presented the best mean marginal adap-tation (31.30 ± 15.12 μm) while the ZZ system produced the greatest mean marginal gap (44.83 ± 28.76 μm).

– Vertical marginal discrepancy values observed for various systems investigated in the study were well within the clinically acceptable range.

Acknowledgments: The author is thankful to Mr. Bong for his help with the digital microscope measurements.

Author StatementResearch funding: The research project was approved and supported by the College of Dentistry Research Center (CDRC) and deanship of scientific research at King Saud University (Registration # FR 0404).Conflict of interest: Author state no conflict of interest.Informed consent: Informed consent is not applicable.Ethical approval: The study was conducted at the Depart-ment of Prosthetic Dental Sciences/College of Dentistry Research Center, College of Dentistry, King Saud Univer-sity and approved by the ethical committee of College of Dentistry Research Center (FR 0404).

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