Abstract
Objective
To investigate the effect of differential coping designs on the stress distributions of an all-ceramic crown on, the upper central incisor under varying loads.
Methods
3D finite element models with three differential coping designs of an all-ceramic crown on, the upper central incisor were constructed using CAD (computer aided design) software. The coping, designs included: CC (conventional coping), MCL (modified coping without veneer coverage in lingual, surface) and MCM (modified coping without veneer coverage in lingual margin). Loading that, simulated the maximum bite force (200 N) was applied to the crown at differential locations (incisal, edge, lingual fossa and lingual margin). The first principal stress values for the full crown were, calculated and expressed as stress intensity in MPa.
Results
The simulations showed the stress distribution tendencies of the all-ceramic crown with, differential coping materials were similar. The stress concentration was found in the cervical region, coping/veneer layer interface and the loading area for both the coping layer and the veneer layer. Maximal stress value was observed in the loading area. Stress values varied for the three types of, coping designs; however, compared with CC and MCM, MCL exhibited the lowest stress values.
Significance
Modified coping without veneer coverage in the lingual side (MCL) proved promising in, preventing all-ceramic crown failures that originate from veneering porcelain, especially under, abnormal occlusal force.
1
Introduction
All-ceramic systems exhibit greater esthetics and biocompatibility compared with conventional metal ceramic restorations, and therefore is regularly used as long-lasting fixed prosthodontics. Various forms of dental ceramics can be used, including glass ceramics, alumina-based ceramics and zirconia-based ceramics. A crystalline phase has improved their mechanical properties by increasing ceramic strength levels, of which the most popular core material is zirconia. This produces the highest mechanical strength with a single-cycle load only failing at 1227 ± 221 N . With the use of yttrium-oxide partially stabilized zirconia (Y-TZP), the toughening of materials produced by phase transformation can enhance the clinical performance of all ceramic crowns even further .
However, as a consequence of the tensile stress caused by external loading, all-ceramic crown failures can frequently occur to the veneer or the core/veneer interface, which is the weakest component in these structures . In zirconia, bulk fractures rarely occur. The most common mode of failure after zirconia-based restorations is an inner cone crack of the veneer porcelain beneath the cusp of the tooth. As water can become trapped in these cracks, they progress to create a chip or dental fracture .
The most recent reports have shown that the annual rate of veneer porcelain fracture and chipping has increased from 1 to 8% . Zirconia-based restorations are more vulnerable to veneer fractures than metal-ceramic crowns (MCRs). The chip size of zirconia cores is much more common (minor chips in 19.4% of MCRs vs. 25% of zirconia-based restorations) and tend to be greater in size; in one study, extended veneer fractures were only observed in the zirconia-based restorations .
Strategies developed to resolve such failures have included improvements in the strength of all-ceramic crowns via the optimization of core surface treatments, fabrication methods and thermal compatibility between core and veneer dental ceramics . One strategy is to improve the mechanical strength of all-ceramic crowns through application of high strength ceramic materials . In this instance, esthetics was compromised with the crown becoming more opaque. Another strategy has involved redesigning the geometric configurations of the coping design. Since cracks initiate from the veneer surface, flexural strength and fracture toughness in veneer restoration depend on the veneer layer . Optimal zirconia copings have enhanced porcelain thickness, and have been demonstrated to decrease veneer fracture . In a study by Marchack et al., approximately 150 crowns were placed with no instances of cohesive porcelain or core fractures . Moreover, mechanical testing of anatomic core design modification revealed a significant increase in the reliability of the coping design, and resulted in reduced chip sizes in the veneer porcelain . Methods that limit the porcelain coverage of zirconia copings have also been implemented in veneer strengthening designs. Here, the buccal surface of an anatomic contour waxing is cut back to obtain a uniform 1 mm space for veneering porcelain, and thus the porcelain to zirconia junction is beyond occlusal contact .
Taken together, these studies indicate that coping and veneer designs that minimize tensile loading of porcelain may indeed reduce porcelain fracture. However, these studies have only focused on posterior all-ceramic restorations; structure optimization designs of anterior all-ceramic crowns remain lacking. Moreover, the results seem to be based more upon empirical guidelines than upon scientific data.
Designs for restorations created using CAD/CAM (computer aided designed/computer aided manufactured) are digitally available and more amenable to analysis of stresses . The prediction of which designs will fail is more achievable. However, little information is available regarding the behavior of customized coping designs compared to conventional designs.
In this study, we assessed the effect of customized coping designs of an anterior all-ceramic crown on protection of the porcelain from fracture and chipping, using 3-D FEA. We build on lessons already learned from metal ceramic restorations, whereby metal lingual plates were designed to avoid cohesive failures of veneer layers in inadequate occlusal spacing. Our customizing design incorporated a ceramic core contoured with differential veneer coverage on the lingual surface of an all-ceramic crown.
The hypothesis of present study is that stress value of veneer porcelain of all-ceramic crown can be minimized by customizing coping design.
2
Materials and methods
2.1
3D CAD modeling
An artificial maxillary central incisor tooth was supplied (KaVo Dental Products Inc., Biberach, Germany) and used as the primary model. The tooth was prepared according to the clinical rules listed as follows: incisal 2 mm reduction, 0.8 mm deep reduction chamfer margin, 1–1.5 mm proximal wall width, 12° convergence angle. The tooth surface was perfectly smooth and without flaw. The tooth preparation and unprepared tooth were scanned with a 3D scanning system (MCS-30, 3D Camega Co. Ltd, Beijing, China) to create two groups of point-cloud digital data (.ftl) of the exterior surface. These data were imported into the CAD software (Geomagic Studio 8, Raindrop Corp., Morrisville, NC, USA). A digital model of the bilayer crown was designed to occupy the space between the original tooth form and the prepared tooth form using the registration technique.
Following this, three geometric models with differential coping designs were developed. The first model was a CC (conventional coping) model, which had a homogenous coping thickness of 0.5 mm and full porcelain coverage. The MCL (modified coping without veneer coverage on the lingual surface) and MCM models (modified coping without veneer coverage on the lingual margin) were developed using a similar digital model of the bilayer crown, in which a thickening coping without veneer coverage on the lingual surface and lingual margin were created, respectively. The coping layer on the lingual surface fitted in with the geometry of the primary tooth.
2.2
Static analysis of three dimensional finite element models
A static structural analysis was performed to calculate the stress distribution from different coping designs. All numerical simulations were performed using ANSYS Workbench 10.0 (ANSYS, Inc. Pittsburgh, PA, USA). The mesh was composed of 0.4 mm tetrahedral elements. The models had 100914 (MCL), 80396 (MCM) and 105497 (CC) elements, with 24369 (MCL), 19196 (MCM) and 25444 (CC) nodes, respectively ( Fig. 1 ).
Mechanical properties (modulus of elasticity [ E ] and Poisson’s ratio [ ν ]) were obtained from the literature ( Table 1 ) . Three types of loads were applied that simulated occlusal contact with the antagonistic tooth during mastication. In the first case, the load was applied on the incisor ridge at 15° to the tooth axis. In the second case, the load was applied on the lingual fossa and at 45° to the tooth axis. In the third case, the load was applied on the lingual margin at 45° to the tooth axis. Both loading conditions were applied over a 3 mm diameter circle area and set at a bite force of 200 N ( Fig. 2 ).
| Material | Young’s modulus (MPa) | Poisson’s ratio |
|---|---|---|
| Dentin | 1.86 × 10 4 | 0.30 |
| Zirconia core | 21.0 × 10 4 | 0.22 |
| Feldspathic ceramics | 6.9 × 10 4 | 0.30 |
| Enamel | 8.41 × 10 4 | 0.30 |
The following assumptions were included in the finite element model: (1) all solids were homogeneous, isotropic and linearly elastic, (2) no slip was permitted between components (perfect bonding), (3) there were no flaws in any of the components and (4) the tooth root was fully constrained. The boundary conditions constrained all six degrees of freedom within the tooth preparation root surface, located at 1.5 mm below the most cervical cement-tooth preparation boundary.
Since ceramic materials exhibit brittle behavior, we adopted the first principal stress criterion. First principal stress ( σ max ) regions and values for three types of loads in two layers (veneer layer and coping layer) were determined through 3D graphs and software output values. For the veneer layer and coping layer, three points (labial margin, proximal margin and loading area) were selected separately to show the first principal stress value.
2
Materials and methods
2.1
3D CAD modeling
An artificial maxillary central incisor tooth was supplied (KaVo Dental Products Inc., Biberach, Germany) and used as the primary model. The tooth was prepared according to the clinical rules listed as follows: incisal 2 mm reduction, 0.8 mm deep reduction chamfer margin, 1–1.5 mm proximal wall width, 12° convergence angle. The tooth surface was perfectly smooth and without flaw. The tooth preparation and unprepared tooth were scanned with a 3D scanning system (MCS-30, 3D Camega Co. Ltd, Beijing, China) to create two groups of point-cloud digital data (.ftl) of the exterior surface. These data were imported into the CAD software (Geomagic Studio 8, Raindrop Corp., Morrisville, NC, USA). A digital model of the bilayer crown was designed to occupy the space between the original tooth form and the prepared tooth form using the registration technique.
Following this, three geometric models with differential coping designs were developed. The first model was a CC (conventional coping) model, which had a homogenous coping thickness of 0.5 mm and full porcelain coverage. The MCL (modified coping without veneer coverage on the lingual surface) and MCM models (modified coping without veneer coverage on the lingual margin) were developed using a similar digital model of the bilayer crown, in which a thickening coping without veneer coverage on the lingual surface and lingual margin were created, respectively. The coping layer on the lingual surface fitted in with the geometry of the primary tooth.
2.2
Static analysis of three dimensional finite element models
A static structural analysis was performed to calculate the stress distribution from different coping designs. All numerical simulations were performed using ANSYS Workbench 10.0 (ANSYS, Inc. Pittsburgh, PA, USA). The mesh was composed of 0.4 mm tetrahedral elements. The models had 100914 (MCL), 80396 (MCM) and 105497 (CC) elements, with 24369 (MCL), 19196 (MCM) and 25444 (CC) nodes, respectively ( Fig. 1 ).
Mechanical properties (modulus of elasticity [ E ] and Poisson’s ratio [ ν ]) were obtained from the literature ( Table 1 ) . Three types of loads were applied that simulated occlusal contact with the antagonistic tooth during mastication. In the first case, the load was applied on the incisor ridge at 15° to the tooth axis. In the second case, the load was applied on the lingual fossa and at 45° to the tooth axis. In the third case, the load was applied on the lingual margin at 45° to the tooth axis. Both loading conditions were applied over a 3 mm diameter circle area and set at a bite force of 200 N ( Fig. 2 ).
| Material | Young’s modulus (MPa) | Poisson’s ratio |
|---|---|---|
| Dentin | 1.86 × 10 4 | 0.30 |
| Zirconia core | 21.0 × 10 4 | 0.22 |
| Feldspathic ceramics | 6.9 × 10 4 | 0.30 |
| Enamel | 8.41 × 10 4 | 0.30 |
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