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Year : 2015  |  Volume : 26  |  Issue : 1  |  Page : 15-20
Marginal and internal discrepancies of zirconia copings: Effects of milling system and finish line design

1 Department of Restorative Dentistry, Federal University of Paraíba (UFPB), João Pessoa/PB, Brazil
2 Department of Dental Materials and Prosthodontics, São José dos Campos Dental School (UNESP/SJC), São José dos Campos-SP, Brazil
3 Private Clinical, Natal/RN, Federal University of Rio Grande do Norte (UFRN), Natal/RN, Brazil
4 Department of Dentistry, Federal University of Rio Grande do Norte (UFRN), Natal/RN, Brazil

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Date of Submission27-Oct-2013
Date of Decision12-Jul-2014
Date of Acceptance20-Jan-2015
Date of Web Publication11-May-2015


Context: Discrepancies at the abutment/crown interface can affect the longevity of zirconia restorations.
Aim: The aim was to evaluate the marginal and internal discrepancies (MD and ID) of zirconia copings manufactured by two milling systems with different finish lines.
Materials and Methods : Three aluminum-master-dies (h = 5.5 mm; Ψ =7.5 mm; 6΀), with different finish lines (large chamfer [LC]; tilted chamfer [TC]; rounded shoulder [RS]) were fabricated. Twenty impressions were made from each master die and poured. Sixty zirconia copings were manufactured and divided according to the factors "finish line" and "milling system" (n = 10): CAD LC = Computer-aided design/computer-aided manufacturing (CAD/CAM) + LC; CAD TC = CAD/CAM + TC; CAD RS = CAD/CAM + RS; MAD LC = manually aided design/manually aided manufacturing (MAD/MAM) + LC; MAD TC = MAD/MAM + TC; and MAD RS = MAD/MAM + RS. For MD analysis, each coping was fixed, and the distance between the external edges of the coping and the edge of the cervical preparation was measured (50 points). Using the same copings, the ID of each coping was evaluated, by the replica technique, at 12 points equally distributed among the regions (n = 10): Ray (R), axial (A), and occlusal (Occl). The measurements were performed by optical microscopy (Χ250). The data (μm) were subjected to parametric and non-parametric statistical analyses.
Results: For the MAD/MAM system, the "finish line" (P = 0.0001) affected significantly the MD median values (μm): LC = 251.80 a , RS = 68.40 a and TC = 8.10 b (Dunn's test). For the CAD/CAM system, the median MD values (μm) were not affected by the factor "finish line" (P = 0.4037): LC = 0.82 a , RS = 0.52 a , and TC = 0.89 a . For the ID, it was observed interaction between the finish line types and the region (P = 0.0001) and between region and milling system (P = 0.0031) (RM-ANOVA).
Conclusions: The CAD/CAM system presented lower MD values, regardless the finish line. However, the MAD/MAM system showed ID values smaller than those of CAD/CAM.

Keywords: Computer-aided design/computer-aided manufacturing system, internal discrepancy, manually aided design/manually aided manufacturing system, marginal discrepancy, replica technique

How to cite this article:
Arrais Ribeiro IL, Campos F, Sousa RS, Lima Alves ML, Rodrigues DM, Assunção Souza RO, Bottino MA. Marginal and internal discrepancies of zirconia copings: Effects of milling system and finish line design. Indian J Dent Res 2015;26:15-20

How to cite this URL:
Arrais Ribeiro IL, Campos F, Sousa RS, Lima Alves ML, Rodrigues DM, Assunção Souza RO, Bottino MA. Marginal and internal discrepancies of zirconia copings: Effects of milling system and finish line design. Indian J Dent Res [serial online] 2015 [cited 2023 Oct 4];26:15-20. Available from:
Zirconia ceramics has been extensively studied because of their excellent mechanical properties, which are much greater compared with those of other dental ceramics. [1],[2],[3]

Many manufacturing processes - such as the computer-aided design/computer-aided manufacturing (CAD/CAM) and the manually aided design/manually aided manufacturing (MAD/MAM) systems - can be used to produce zirconia restorations. [4] In the CAD/CAM system, zirconia ceramic blocks are machine-standardized by a milling unit, assisted by a computer program. [1],[5],[6] In contrast, the MAD/MAM system, also utilizes ceramic blocks, but the milling system is similar to a pantograph.

However, discrepancies at the abutment/crown interface can affect the longevity of zirconia restorations. Excessive marginal gaps increase dental plaque retention, [7],[8] favor the development of recurrent caries or pulp lesions, [9],[10],[11],[12] and yield to bone resorption. [7] In addition, excessive internal discrepancies (IDs) can reduce the fracture strength of full-ceramic restorations since these areas experience different load concentrations. [13],[14]

Thus, the aim of the present study was to evaluate the marginal discrepancy and ID of zirconia copings manufactured by two milling systems - CAD/CAM and MAD/MAM - with 3 different finish line designs. The hypotheses tested were that: (1) The milling system and (2) the type of finish line affect the MD and ID of zirconia copings, and that (3) the ID varies among the regions measured.

   Materials and methods Top

The brands, manufacturers, and batch numbers of the materials used for this experiment are presented in [Table 1].
Table 1: The commercial names, manufacturers, and batch numbers of the materials used in this study

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From an aluminum aeronautic bar (diameter = 15 mm; length = 69 mm) (AMS 4050F, SAE Aerospace International Group, Brazil), 3 master dies were milled with different finish line designs [Figure 1]. To standardize the MD measurements, 25 laser marks were made with 0.5 mm thickness, approximately 1.5 mm below the finish line, each being 0.5 mm distant from the others.
Figure 1: Schematic drawing of the master dies, showing the three different finish lines: (a) Occlusal view, (b) side view (values in mm)

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Twenty impressions were made from each master die by the simultaneous impression technique, in which the putty body and the light body of the silicone-based impression material were placed in a single-step (Panasil, Kettenbach GmbH and Co. KG, Eschenburg, Germany). A surface-tension-reducing agent (Surfacer, Polidental Ltd., Sγo Paulo, Brazil) was applied to each impression 24 h after they were poured with vacuum-mixed Type IV dental stone (Tuff rock formula 44, Talladium, Inc., Valencia, CA, USA), under vibration.

Sixty copings were made of a zirconia ceramic material and were divided among six groups according to the factors "finish line" (large chamfer [LC], tilted chamfer [TC], and rounded shoulder [RS]) and "milling system" (CAD/CAM - Neo Shape and MAD/MAM - Zirkonzahn) (n = 10): CAD LC , CAD/CAM system ± LC finish line; CAD TC , CAD/CAM system ± TC finish line; CAD RS , CAD/CAM system and RS finish line; MAD LC , MAD/MAM system ± LC finish line; MAD TC , MAD/MAM system ± TC finish line; and MAD RS , MAD/MAM system and RS finish line.

For the CAD/CAM system, the stone dies were positioned in a metallic device placed on a three-dimensional (3D) scanner (Model D700, 3DShape® , Erlangen, Germany). By strip light projection, the scanning and design procedures for all 30 stone dies were carried out with the 3D-Shape Dental program (version, Sirona Dental Systems, Long Island City, NY, USA) and milled in an automatic milling machine. The ceramic copings were designed as frameworks at a thickness of approximately 1.0 mm. After, the copings were sintered according to the manufacturer's recommendations.

For the MAD/MAM system (Zirkonzahn GmbH, Gais, Italy), the copings were manufactured on each stone die with a composite resin supplied by the manufacturer. After polymerization, the zirconia blocks were milled by a manual Zr-Zahn milling unit (Zirkograph 025 ECO, Zirkonzahn GmbH, Gais, Italy). The copings were then sintered in a furnace (Zirkonofen 600/V2, Zirkonzahn GmbH, Gais, Italy) at 500°C/8 h. Based on a pilot study, the luting space was established at 20 μm for both systems.

For marginal fit between the coping and preparation margins, each metallic die was fixed on a cylindrical metallic base which allowed for 360° of rotation of the die around its axis. A constant load was applied on the coping during the entire analytical process. The analysis was done by 3D optical microscopy at a precision of 1 μm (Roi, RAM Optical Instrumentation, Irvine, CA, USA) at ×200 magnification. Fifty measurements were made along the coping margins, based on 25 laser-marked reference points. All measurements were carried out by only one operator, and MD values below 1 μm were considered as 0 μm.

Internal discrepancy of the copings was measured by a replica technique. [15] Each coping was filled with light-body silicone (Panasil, Kettenbach GmbH and Co. KG, Eschenburg, Germany) and inserted into the respective master die under a constant load (750 g) for 10 min, by means of a modified parallelometer. After the light-body silicone had set, the coping was removed. Since it was not possible to remove the light-body silicone from the interior portions of the crown without distorting it, a heavy-body silicone was used to stabilize the light-body silicone. Using a razor blade (n°. 15c), the replicas were carefully sectioned into four equal segments.

From the four sections obtained from each replica, two opposite sections were used to measure internal fit, with three regions measured on each section (R = ray, A = axial, and Occl = occlusal), yielding 12 internal measurements for each coping. Using 3D optical microscopy at ×250 magnification and precision of 1 μm (Roi, RAM Optical Instrumentation, Irvine, CA, USA), the light-body silicone thickness for all replicas was measured, representing the distance between the internal surface of the coping and the external surface of the preparation.

All MD data (average of 50 measurements per coping-μm) were analyzed by Kruskal-Wallis, Mann-Whitney and Dunn (5%) tests. All ID data (average of 12 measurements per coping) were analyzed by RM ANOVA and Tukey's test (5%) (Statistix 8.0, Analytical Software, Tallahassee, FL, USA).

   Results Top

It was observed that MAD/MAM groups were statistically different of the CAD/CAM groups (P < 0.0001). The CAD/CAM groups promoted significantly lower MD than the MAD/MAM groups for all finish lines [Table 2], [Figure 2] [Figure 3] [Figure 4]. It was also observed that in all MAD/MAM groups, the MD mean values varied greatly in the same sample. The MD varied from 4.08 to 18.30 μm in the TC group, from 6.20 to 574.90 μm in the LC group, and from 47.40 to 265.40 μm in the RS group.
Figure 2: Box plot for marginal discrepancy data for MAD/MAM by finish line

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Figure 3: Box-plot for marginal discrepancy data for CAD/CAM by finishing line

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Figure 4: Representative optical microscopic images (×250) at the marginal interface between the metallic die and the coping, showing higher marginal discrepancy for an MADLC sample (a) compared to a CADRS sample (b)

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Table 2: Marginal discrepancy values (median in μm) according the factors

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In [Table 3], it can be seen the variation of ID values for the two milling systems. It was observed that the interaction between the finish line types and the region (P = 0.0001) and region and milling system (P = 0.0031) were statistically significant (RM-ANOVA).
Table 3: The mean internal discrepancy values (ìm) according the factors

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   Discussion Top

The present study revealed that the MDs of zirconia copings changed according to the milling system and for the MAD/MAN system, according to the finish line. The low MD values for CAD/CAM system reflect the precision of the milling process. However, according to Hamza et al. [16] the marginal fit of CAD/CAM crowns could depend on the system utilized since the software and the milling machine accuracy have a primordial role in this adaptation.

Moreover, there were significant discrepancies along the margin of a single coping and between one coping and another of the same MAD/MAM group. Based on these data, it can be expected that MAD/MAM systems offer greater possibility for human error during the manufacturing process than do CAD/CAM systems. The higher MD in MAD/MAM groups is related to the use of resin-based materials for making the copings. Since the resin contracts after polymerization, the greater discrepancies in the LC and RS groups reflect the increased amount of resin used in the finish line region, compared with the TC group. Corroborating these data, Azar et al. [15] evaluated the differences in preparation depth influenced by the size of the marginal gap between the vestibular-oral and mesiodistal regions, resulting in decreased marginal precision. According to those authors, the CAD/CAM group showed the best results (shoulder, 90° [23.08 μm]; and chamfer [25.77 μm]), which were statistically different from those of the MAD/MAM group (shoulder, 36.11 μm). Similarly, Grenade et al. [17] evaluated the influence of the milling system on the MD of ceramic copings made by a CAD/CAM process (Procera) and by a mechanized manufacturing process (Ceramill; Amann Girrbach, Baden-Württemberg, Germany), and observed that the mean marginal gap for Procera copings (51 μm) was significantly smaller than for Ceramill (81 μm).

Several studies that evaluated the influence of finish line design on the marginal fit of ceramic crowns showed very different and sometimes contradictory results. Some studies demonstrated that marginal fit was not influenced by finish line design. [18],[19],[20],[21],[22],[23] However, several researchers showed differences in the performance of marginal fit according to finish line designs. Pera et al. [21] observed better marginal fit for crowns fabricated on a 50° chamfer or shoulder tooth preparation compared with a 90° shoulder finish line. Lin et al. [22] further reported that the feather-edge finish line resulted in the largest MDs as compared with the chamfer, 0.8-μm RS, and 0.5-μm RS finish lines. Karatasli et al. [24] reported a preference for the chamfer type for ceramic crowns. In contrast, Bindl and Mörmann [25] recommended shoulder, shoulder-bevel, and chamfer as the optimal finish line types for marginal adaptation. Souza et al. [26] also observed that ProCad (Ivoclar, Vivadent, Schaan, Liechtenstein) crowns with a RS (28.24 ± 11.42 μm) finish line presented significantly lower MD values, followed by LC (64.71 ± 25.64 μm) and TC (99.92 ± 18.32 μm), with all groups being statistically different from one another. Moreover, in all groups, MD values showed significant variation within the same sample, ranging from 0 to 283 μm, 0-208 μm, and 0-122 μm for the TC, LC, and RS groups, respectively. A recent study showed even better values of marginal fit for zirconia crowns - In-Ceram YZ (12.24 ± 3.08 μm), Cercon (13.15 ± 3.01 μm), and Procera (8.67 ± 3.96) - and, despite the statistically significant differences among the groups, the authors considered the values obtained to be satisfactory. In this study, all the finish line designs showed satisfactory results, except the LC of the MAD/MAM system. In contrast, Sorensen et al., [27] evaluating the marginal fit of zirconia crowns made by a MAD/MAM system, observed better results for shoulder and chamfer, with mean gap values of 24 and 32 μm, respectively.

According to the literature, marginal fit plays an important role in the clinical success of ceramic restorations since great MDs lead to more dental plaque retention, recurrent caries, and bone resorption. [12],[14],[28] Clinical observations of zirconia-based restorations have shown that the occurrence of marginal gaps results in problems with the fit of computer-manufactured frameworks. [29],[30],[31],[32] Therefore, several investigators have recommended an ideal marginal fit for restorations, to achieve the best clinical performance. Christensen [33] and Andersson et al. [34] recommended marginal gaps of 25 and 40 μm. To Sulaiman et al., [35] marginal gaps between 1 and 165 μm are considered acceptable. Boening et al. [36] suggested that 110-150 μm is acceptable for indirect restorations. However, McLean and von Fraunhofer [37] recommended 120 μm as the maximum acceptable marginal gap value, when they analyzed 1000 restorations after 5 years in service. This last study has been used by several investigators as the "gold standard" for their research. [38] Based on these reports, the marginal gap values observed in this study are clinically acceptable, except for the LC finish line of the MAD/MAM system.

Karatasli et al. [24] reported that even in optimized conventional systems, such as the MAD/CAM, marginal gap values for zirconia crowns were significantly higher than those reported with CAD/CAM systems. These authors also related that the CAD/CAM system rapidly provides standard results and minimizes technical failures compared with the MAD/CAM systems. However, MAD/CAM is easy to use, low-cost, and has not been associated with software difficulties. These better results for CAD/CAM systems can also be attributed to laser scanning, which accurately measures the margin line of the stone model, resulting in a precision procedure during the shrinkage adjustment of milled zirconia blanks. [39]

The ID was affected by the milling system, the finish line, and the region. The ID values observed in this study were higher for the CAD/CAM than for the MAD/MAM system, for the occlusal region. Moreover, the highest values for finish line type were obtained for the RS, followed by the LC and the TC. Similarly, Souza et al. [26] observed that the mean ID values of ceramic crowns were shorter for LC (183.01 μm) and were statistically significantly different (P < 0.05) compared with those of the TC (216.26 μm) and RS groups (219.12 μm); however, TC and RS were not statistically significantly different from each other. Several investigators have also reported higher ID values for CAD/CAM zirconia copings than for conventional metal-ceramic crowns. [40],[41] Grenade et al. [17] evaluating the influence of the milling system on the ID of ceramic copings made by a CAD/CAM process (Procera) and a mechanized manufacturing process (Ceramill; Amann Girrbach), observed that the values for Procera and Ceramil were similar. In contrast, Coli and Karlsson [42] supported the CAD/CAM technique as providing high precision for internal space in the manufacture of zirconium dioxide copings.

The ID changed among the regions measured. For the regions analyzed, this study showed that the greatest ID values were found in the occlusal region in the CAD/CAM system (860.79 μm) and in the MAD/MAM system (690.44 μm), and that lower values were found in the axial and ray region for both milling systems [Table 2]. Similar data were obtained by Colpani et al., [41] who found greater ID values in the occlusal area than in the other regions for both zirconia- and glass-infiltrated-based alumina ceramic crowns (In-Ceram YZ/Vita and Vita In-Ceram Zirconia), and lower mean ID values for the axial wall for the same two ceramic systems. In addition, metal crowns showed better mean ID gap values compared with the ceramic systems. Similar results were reported by Martins et al., [40] who found that ID values at the occlusal (246.67 μm) region were greater than those at the axial region (112.40 μm) and no differences among the areas were detected in the metal-ceramic group.

Although great IDs can decrease the fracture strength of full-ceramic restorations, [13],[14] no conclusive evidence is available concerning the ideal ID. Several articles [43],[44],[45] reported that 50-100 μm is acceptable, with due respect for the physical and clinical properties of resin-based luting agents. In vitro studies of all-ceramic machine-milled crowns reported that the mean internal gaps ranged from 30 to 204 μm. [25],[42],[46],[47] Other in vitro studies demonstrated clinically acceptable restorations with ID between 200 and 300 μm. [44],[48] Thus, it can be stated that the ID data obtained in this study are appropriate for the generation of satisfactory clinical performance in the axial regions of all the finish lines in the two evaluated systems.

To date, limited studies are available identifying a correlation between the longevity of ceramic restorations and MD and/or ID in zirconia crowns. Moreover, fatigue behavior of crowns with high discrepancies warrants further research.

Based on our results, it can be concluded that despite the higher occlusal ID values of CAD/CAM system, its whole adaptation seems to be better and more consistently repeatable.

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Correspondence Address:
Rodrigo Othavio Assunção Souza
Department of Dentistry, Federal University of Rio Grande do Norte (UFRN), Natal/RN
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0970-9290.156790

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