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Effect of different coping designs on all-ceramic crown stress distribution: A finite element analysis
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Effect of different coping designs on all-ceramic crown stress distribution: A finite element analysis Jian Hu a , Ning Dai b , Yidong Bao b , Weiping Gu a , Junchi Ma a , FeiMin Zhang a,∗ a
Institute of Stomatology, Nanjing Medical University, NanJing 210029, China College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics & Astronautics, NanJing 210029, China
b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the effect of differential coping designs on the stress distributions
Received 1 May 2013
of an all-ceramic crown on, the upper central incisor under varying loads.
Received in revised form 9 July 2013
Methods. 3D finite element models with three differential coping designs of an all-ceramic
Accepted 2 September 2013
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
Keywords:
coverage in lingual margin). Loading that, simulated the maximum bite force (200 N) was
All-ceramic crown
applied to the crown at differential locations (incisal, edge, lingual fossa and lingual margin).
Three-dimensional finite element
The first principal stress values for the full crown were, calculated and expressed as stress
Stress distribution
intensity in MPa.
Coping design
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. Crown Copyright © 2013 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
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 [1–3]. With the use of yttrium-oxide partially stabilized zirconia (Y-TZP),
∗ Corresponding author at: Institute of Stomatology, Nanjing Medical University, 136 Hanzhong Road, Nanjing 210029, People’s Republic of China. Tel.: +86 13801589779; fax: +86 25 86516414. E-mail address: [email protected] (F. Zhang).
0109-5641/$ – see front matter Crown Copyright © 2013 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
http://dx.doi.org/10.1016/j.dental.2013.09.001
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the toughening of materials produced by phase transformation can enhance the clinical performance of all ceramic crowns even further [4]. 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 [5–7]. 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 [4]. The most recent reports have shown that the annual rate of veneer porcelain fracture and chipping has increased from 1 to 8% [4,8]. 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 [9]. 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 [10–13]. One strategy is to improve the mechanical strength of all-ceramic crowns through application of high strength ceramic materials [14–16]. 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 [17]. Optimal zirconia copings have enhanced porcelain thickness, and have been demonstrated to decrease veneer fracture [13,18–22]. In a study by Marchack et al., approximately 150 crowns were placed with no instances of cohesive porcelain or core fractures [21]. 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 [13]. 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 [23]. 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 [24]. 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 []) were obtained from the literature (Table 1) [25,26]. 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
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Fig. 1 – Geometric models CC, MCL, and MCM. (A) CC, conventional coping; (B) MCL, modified coping with lingual thickening; (C) MCM, modified coping with marginal thickening.
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 ).
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
Fig. 2 – Three types of loads simulate occlusal contact with antagonistic tooth: (A) loading on incisal edge; (B) loading on lingual fossa; (C) loading on lingual margin.
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Fig. 3 – First principal stresses (MPa) under loading at the incisal ridge–buccolingual cross-section: CC; MCM; MCL.
Table 1 – Model material properties. Material Dentin [26] Zirconia core [25] Feldspathic ceramics [26] Enamel [26]
Young’s modulus (MPa) 1.86 × 104 21.0 × 104 6.9 × 104 8.41 × 104
Poisson’s ratio 0.30 0.22 0.30 0.30
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 ( 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.
3.
Results
Under the same loading conditions, the stress distribution patterns for the all-ceramic crown with differential coping designs exhibited similarities. Stress was distributed along a symmetric medial line of the crown; max was on the loading area; stress concentration was found on the cervical margin and on the coping/veneer layer interface. However, different max stress values were determined for the three types of coping designs. In particular, a significant decrease in max was observed for the coping layer and veneer layer in MCL compared with MCM and CC. When was the incisal ridge loaded, the stress distribution for veneer layer, in addition to the loading area, exhibited stress concentration on the labial and proximal surfaces. For the coping layer, the max was found on the proximal surface. Stress concentration was located on the incisal 1/3 of the labial surface and lingual protuberance (Figs. 3 and 6). When the lingual fossa was loaded, the veneer layer, in addition to loading area, exhibited stress concentration on the proximal side. The coping layer exhibited stress concentration
on the loading area, proximal margin and labial surface (Figs. 4 and 7) Finally, when loading on the lingual margin, the veneer layer showed stress concentration located on the labial margin; albeit, the stress value was decreased. For the coping layer, the maximum principal stress was found on the lingual and labial margin, not on the proximal margin (Figs. 5 and 8).
4.
Discussion
The performance of the all-ceramic crown was determined by many parameters. These parameters included the bond strength between the veneer and coping layers, fabrication methods, material selection for both the veneer and coping layers, tooth supporting structure, magnitude and direction of occlusal loads, type and thickness of the cement layer, crown thickness and the coping design [16,27–30]. Most studies have confirmed that the stresses increase with increasing loading magnitudes and the orientation of the load application substantially altered stress levels within the crown [30]. Conflicting reports have identified that the type and thickness of the cement layer can cause little or no effect, or substantial effects, but these factors exert the greatest effect on the magnitude of stress. The elastic modulus of the tooth supporting structure also has been shown to exert different effects on stress distribution. Bond strengths between the zirconia core and veneer are affected by the types of zirconia and veneering materials used, and the appropriate surface treatment of the coping layer is essential to maintain a good bond strength [16,31]. In general, crown strength improves with increasingly strong materials and increasing thickness [30]. After the shape of the preparation and materials of the crown had been determined, the only parameter that was manipulated by the operator was the coping design [32]. Fractographic analysis of clinically failed crowns suggests that many failures occur in the veneer layer. An appropriate ratio of veneer and core thickness may decrease internal stress and reduce mechanical failures. As the core layer is stronger and stiffer than the veneer layer, the importance of adequate core thickness may be paramount [33–37]. When crown thickness is insufficient, the complete-contour zirconia crown and zirconia crown with a buccal porcelain facing have been used for posterior restorations [23]. The design of
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Fig. 4 – First principal stresses (MPa) under loading at the lingual fossa–buccolingual cross-section: CC; MCM; MCL.
the core is emerging as a significant factor in the survival and the extent of damage after a restoration. However, related literature regarding improvements to the mechanical properties by coping design of anterior all-ceramic crown is limited. In this study, we offer a solution to avoid veneer failure of an anterior all-ceramic crown by limiting the porcelain coverage of zirconia copings; a lesson taken from the differential coping design for metal ceramic restorations from many decades ago [21,23,37]. For the upper incisor, the bite force was loaded on the lingual side of the crown. Therefore, based on the concepts presented in the metal ceramic literature and the known physical properties of zirconia, we investigated the reduction of veneer stress by minimizing or eliminating the coverage of veneering porcelain on the lingual side. This customized coping design, in which a partial/full lingual surface of zirconia was used to support the veneering porcelain, utilized CAD/CAM technology without the requirement of full contour waxing and cut back. The objective was to determine which coping design could reduce mechanical failures of an anterior all-ceramic crown. There are many contact areas between the upper and inferior incisor when the incisor exerts functional occlusion. Therefore, differential loading can alter the stress distribution of an all-ceramic crown. We investigated three characteristic
loading locations: the incisal edge, lingual fossa and lingual margin. This is due to the fact that while chewing, load was applied on the incisal edge between the upper and inferior incisor; the lingual fossa was the most common contact area of the incisor in the central occlusion; while deep overlap in the anterior tooth caused abnormal occlusal forces and thus leaded the occlusal location to lie mainly on the lingual margin. These loads exhibit the extremes of stress distribution and therefore can be used to determine any weak regions. Our results showed that when loading was transferred from the incisal edge to the lingual margin, the highest stress value simultaneously moved from the incisal edge to the cervical margin. In line with previous investigations, our results imply that load is the major factor that influences stress and its distribution on the all-ceramic crown [31,38]. Measures should be applied to avoid abnormal load, such as adjustment of occlusal relation and changes to chewing habits. However, when abnormal load is inevitable, rational coping designs should be explored. As shown in Figs. 3 and 6, under loading on the incisal edge, similar stress distributions were detected. These results are in agreement with those from a previous 3D-FEA study in which stress concentration occurred at the loading area of the veneer layer and all coping layers; the highest stress
Fig. 5 – First principal stresses (MPa) under loading at the lingual margin–buccolingual cross-section: CC; MCM; MCL.
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maximum principal stress(MPa)
60 50
labial margin(veneer) 40
proximal margin(veneer) loading area(veneer)
30
labial margin(coping) proximal margin(coping)
20
loading area(coping) 10 0 CC
MCM
MCL
Fig. 6 – First principal stresses (MPa) in veneer and coping layer for CC, MCM and MCL models under loading on incisal ridge.
levels were directly observed beneath the loading area [38]. In a separate in vitro study, the failures of zirconia-based ceramic crowns were confined within the porcelain veneer in which the crack propagated from the sliding contact area [2]. As the loading area was on veneer porcelain and exhibited the highest stress values, it represents a high fracture risk. Hence, where possible, porcelain veneer contact with load should be avoided. In this vein, when the incisal edge is covered with veneer porcelain for esthetic reasons; chewing food with allceramic crown restored incisors is not advisable. From Figs. 4 and 7, the stress distribution demonstrated that when the load transferred to the lingual fossa, the deformation areas on the lingual fossa and labial margin were intensified. The results also showed that coping design slightly influenced the stress value. Although the stress value of the loading area was similar for the three loading locations, the lowest stress value for the veneer and coping layer was at the buccal surface of the finish line that occurred in the MCL model. In analyzing Fig. 4, we observed a negative correlation between the volume of coping layer and the stress value. When the volume of coping layer in the lingual side increased, the stress value decreased in the whole coping layer. The fracture strength of the all-ceramic crown mainly depended on two key variables: material and geometry. Accordingly, it has been shown that tougher, harder and stronger materials exhibit superior damage resistance, while critical loads for radial fracture strongly depend on crown net thickness [39]. In our study, we demonstrated that coping design modifications that provided stronger ceramics with increased thickness decreased the stress concentration.
When loading on the lingual margin (Figs. 5 and 8), our results indicated that stress concentration could be found mainly in the lingual margin. Apart from the loading area, both the veneer and coping layers showed no exacerbation in Von Mises stresses. However, the tensile stress values for the veneer and coping layers in the labial margin were higher for CC and MCM than for MCL. Much of the literature has demonstrated the cervical margin of an all-ceramic crown to be a weak region and that loading on the lingual margin could increase stress on the cervical margin [38]. Our results indicate that the coping design in MCL reduces the stress concentration on the veneer layer. Of the three types of coping designs, the veneer layer in CC exhibited the highest stress value during differential loading, especially within the loading area, which implies that CC is at high risk of brittle fracture. For MCM, loading on the lingual margin avoided contact with the veneer porcelain. High stress was shown when loading on the lingual fossa and lingual margin. Moreover, of the three coping designs, MCL always maintained the lowest stress in similar regions when loading on the lingual fossa and lingual margin. Therefore, our results indicate that MCL increases the survival rate of the all-ceramic crown. Some abnormal results also were observed. Under loading on the incisal edge, MCL exhibited higher stress than CC and under loading on the lingual margin, MCM exhibited the highest stress value overall. One reasonable explanation is that the loading area is located near the veneer/coping layer interface. Some previous in vitro studies have shown that higher stress levels were directly observed at the zirconia core/cement layer
maximum pricipal stress(MPa)
80 70 60
labial margin(veneer)
50
proximal margin(veneer) loading area(veneer)
40
labial margin(coping)
30
proximal margin(coping)
20
loading area(coping)
10 0 CC
MCM
MCL
Fig. 7 – First principal stress (MPa) in veneer and coping layer for CC, MCM and MCL models under loading on lingual fossa.
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maximum principal stress(MPa)
80 70 60
labial margin(veneer)
50
proximal margin(veneer) loading area(veneer)
40
labial margin(coping)
30
proximal margin(coping)
20
loading area(coping)
10 0 CC
MCM
MCL
Fig. 8 – First principal stress (MPa) in veneer and coping layer for CC, MCM and MCL models under loading on lingual margin.
interface; however, Von Mises stress values at the interface were lower than the rupture strength for the all-ceramic crown [26,28]. In our study, stress concentration could also be found at the interface between the core layer and veneer layer, especially at the proximal side and incisal ridge. Chemical bonding and mechanical interlock were insufficient at the interface between the core layer and veneer layer. This interface was also a weak area and thus could lead to cracks and interface failures. Therefore, the veneer/coping layer interface should be designed far away from occlusal contact. When monolith configuration was designed in the loading area, cone cracks originate from the ceramic surface as a result of pressure. When the bilayered configuration was created in the loading area, a major crack developed in the veneer/coping layer interface as a result of tensile stress [39]. As it is a more brittle material, ceramic is more susceptible to the initiation of cracking under tensile stress than under pressure stress. Our results indicate that in the loading area, monolith configuration zirconia coping layer is the preferred coping design.
5.
Conclusion
Our hypothesis of whether a customized coping design could reduce the stress on veneers was partially accepted, as the present study showed that the stress value in the MCL decreased with the use of the lingual modified coping design that expanded the cover area and increased the thickness of the coping layer. However, the MCM design merely resulted in a slight decrease in stress on the lingual margin under loading. The present study suggests that the modified coping design is a promising method to avoid veneer porcelain fracturing. It may be advisable in the case of anterior allceramic crowns that the core material is not veneered on lingual surface where esthetic considerations are not crucial. Our study indicates that FEA results can assistant in rebuilding the design guidelines in CAD/CAM systems for all-ceramic restorations. The major limitation of our study was that the absence of a cementation layer in the simulation may have had potential effects on the stress distribution in the core. In fact, all-ceramic crowns have to endure changing and cycling occlusal forces under mastication. The load type was static in our finite element analysis; therefore, further studies based on
our static results should be conducted regarding fatigue analysis. Our results are also ideal for further in vitro laboratory testing and clinical research.
Acknowledgments This work was supported by grants from the Outstanding Medical Academic Leader Program and Creative team of Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), National High Technology Research and Development Program of China (863 Program, Grant No. 2012AA030309).
references
[1] Rekow ED, Silva NR, Coelho PG, Zhang Y, Guess P, Thompson VP. Performance of dental ceramics: challenges for improvements. J Dent Res 2011;90(8):937–52. [2] Silva NR, Bonfante EA, Zavanelli RA, Thompson VP, Ferencz JL, Coelho PG. Reliability of metalloceramic and zirconia-based ceramic crowns. J Dent Res 2010;89(10):1051–6. [3] Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater 2009;25(9):1122–7. [4] Silva NRFA, Sailer I, Zhang Y, Coelho PG, Guess PC, Zembic A, et al. Performance of zirconia for dental healthcare. Materials 2010;3(2):863–96. [5] Fradeani M, Aquilano A, Corrado M. Clinical experience with In-Ceram Spinell crowns: 5-year follow-up. Int J Periodont Restorative Dent 2002;22(6):525–33. [6] Ereifej N, Rodrigues FP, Silikas N, Watts DC. Experimental and FE shear-bonding strength at core/veneer interfaces in bilayered ceramics. Dent Mater 2011;27(6):590–7. [7] Coelho PG, Bonfante EA, Silva NR, Rekow ED, Thompson VP. Laboratory simulation of Y-TZP all-ceramic crown clinical failures. J Dent Res 2009;88(4):382–6. [8] Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24(3):299–307. [9] Sailer I, Gottnerb J, Kanelb S, Hammerle CH. Randomized controlled clinical trial of zirconia–ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up. Int J Prosthodont 2009;22(6):553–60.
e298
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[10] Fischer J, Stawarzcyk B, Trottmann A, Hämmerle CH. Impact of thermal misfit on shear strength of veneering ceramic/zirconia composites. Dent Mater 2009;25(4):419–23. [11] Anunmana C, Anusavice KJ, Mecholsky Jr JJ. Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-noCCh test. Dent Mater 2010;26(2):111–7. [12] Tan JP, Sederstrom D, Polansky JR, McLaren EA, White SN. The use of slow heating and slow cooling regimens to strengthen porcelain fused to zirconia. J Prosth Dent 2012;107(3):163–9. [13] Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29(3):307–16. [14] Schmitter M, Mueller D, Rues S. Chipping behaviour of all-ceramic crowns with zirconia framework and CAD/CAM manufactured veneer. J Dent 2012;40(2):154–62. ¨ [15] Luthardt RG, Holzhuter M, Sandkuhl O, Herold V, Schnapp JD, Kuhlisch E, et al. Reliability and properties of ground Y-TZP-zirconia ceramics. J Dent Res 2002;81(7):487–91. [16] Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Effect of zirconia type on its bond strength with different veneer ceramics. J Prosthod 2008;17(5):401–8. [17] Taskonak B, Mecholsky Jr JJ, Anusavice KJ. Residual stresses in bilayer dental ceramics. Biomaterials 2005;26(16):3235–41. [18] Baldassarri M, Zhang Y, Thompson VP, Rekow ED, Stappert CF. Reliability and failure modes of implant-supported zirconium-oxide fixed dental prostheses related to veneering techniques. J Dent 2011;39(7):489–98. [19] Giannetopoulos S, van Noort R, Tsitrou E. Evaluation of the marginal integrity of ceramic copings with different marginal angles using two different CAD/CAM systems. J Dent 2010;38(12):980–6. [20] Isgro G, Addison O, Fleming GJ. The deformation and strength of a dental ceramic following resin-cement coating. J Dent 2011;39(2):122–7. [21] Marchack BW, Futatsuki Y, Marchack CB, White SN. Customization of milled zirconia copings for all-ceramic crowns: a clinical report. J Prosthet Dent 2008;99(3): 169–73. [22] Silva NR, Bonfante EA, Rafferty BT, Zavanelli RA, Rekow ED, Thompson VP, et al. Modified Y-TZP core design improves all-ceramic crown reliability. J Dent Res 2011;90(1):104–8. [23] Marchack BW, Sato S, Marchack CB, White SN. Complete and partial contour zirconia designs for crowns and fixed dental prostheses: a clinical report. J Prosthet Dent 2011;106(3):145–52. [24] Rafferty BT, Janal MN, Zavanelli RA, Silva NR, Rekow ED, Thompson VP, et al. Design features of a three-dimensional molar crown and related maximum principal stress. A finite element model study. Dent Mater 2010;26(2):156–63.
[25] De Jager N, de Kler M, van der Zel JM. The influence of different core material on the FEA-determined stress distribution in dental crowns. Dent Mater 2006;22(3): 234–42. [26] Ausiello P, Franciosa P, Martorelli M, Watts DC. Mechanical behavior of post-restored upper canine teeth: a 3D FE analysis. Dent Mater 2011;27(12):1285–94. [27] Swain MV. Unstable cracking (chipping) of veneering porcelain on all-ceramic dental crowns and fixed partial dentures. Acta Biomater 2009;5(5):1668–77. [28] Anunmana C, Anusavice KJ, Mecholsky Jr JJ. Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-notch test. Dent Mater 2010;26(2):111–7. [29] Fischer J, Stawarczyk B, Sailer I, Hämmerle CH. Shear bond strength between veneering ceramics and ceria-stabilized zirconia/alumina. J Prosthet Dent 2010;103(2):267–74. [30] Rekow ED, Harsono M, Janal M, Thompson VP, Zhang G. Factorial analysis of variables influencing stress in all-ceramic crowns. Dent Mater 2006;22(2):125–32. [31] Ozkurt Z, Kazazoglu E, Unal A. In vitro evaluation of shear bond strength of veneering ceramics to zirconia. Dent Mater J 2010;29(2):138–46. [32] De Jager N, Pallav P, Feilzer AJ. The influence of design parameters on the FEA-determined stress distribution in CAD–CAM produced all-ceramic dental crowns. Dent Mater 2005;21(3):242–51. [33] Wakabayashi N, Anusavice KJ. Crack initiation modes in bilayered alumina/porcelain disks as a function of core/veneer thickness ratio and supporting substrate stiffness. J Dent Res 2000;79(6):1398–404. [34] Esquivel-Upshaw JF, Anusavice KJ. Ceramic design concepts based on stress distribution analysis. Compend Contin Educ Dent 2000;21(8), 649–52, 654. [35] Proos KA, Swain MV, Ironside J, Steven GP. Influence of core thickness on a restored crown of a first premolar using finite element analysis. Int J Prosthodont 2003;16(5):474–80. [36] Lawn BR, Deng Y, Lloyd IK, Janal MN, Rekow ED, Thompson VP. Materials design of ceramic-based layer structures for crowns. J Dent Res 2002;81(6):433–8. [37] Pogoncheff CM, Duff RE. Use of zirconia collar to prevent interproximal porcelain fracture: a clinical report. J Prosthet Dent 2010;104(2):77–9. [38] Rocha EP, Anchieta RB, Freitas Jr AC, de Almeida EO, Cattaneo PM, Chang Ko C. Mechanical behavior of ceramic veneer in zirconia-based restorations: a 3-dimensional finite element analysis using microcomputed tomography data. J Prosthet Dent 2010;105(1):14–20. [39] Lawn BR, Pajares A, Zhang Y, Deng Y, Polack MA, Lloyd IK, Dianne RE, Thompson VP. Materials design in the performance of all-ceramic crowns. Biomaterials 2004;25(14):2885–92.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of different coping designs on all-ceramic crown stress distribution: A finite element analysis Jian Hu a , Ning Dai b , Yidong Bao b , Weiping Gu a , Junchi Ma a , FeiMin Zhang a,∗ a
Institute of Stomatology, Nanjing Medical University, NanJing 210029, China College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics & Astronautics, NanJing 210029, China
b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the effect of differential coping designs on the stress distributions
Received 1 May 2013
of an all-ceramic crown on, the upper central incisor under varying loads.
Received in revised form 9 July 2013
Methods. 3D finite element models with three differential coping designs of an all-ceramic
Accepted 2 September 2013
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
Keywords:
coverage in lingual margin). Loading that, simulated the maximum bite force (200 N) was
All-ceramic crown
applied to the crown at differential locations (incisal, edge, lingual fossa and lingual margin).
Three-dimensional finite element
The first principal stress values for the full crown were, calculated and expressed as stress
Stress distribution
intensity in MPa.
Coping design
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. Crown Copyright © 2013 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
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 [1–3]. With the use of yttrium-oxide partially stabilized zirconia (Y-TZP),
∗ Corresponding author at: Institute of Stomatology, Nanjing Medical University, 136 Hanzhong Road, Nanjing 210029, People’s Republic of China. Tel.: +86 13801589779; fax: +86 25 86516414. E-mail address: [email protected] (F. Zhang).
0109-5641/$ – see front matter Crown Copyright © 2013 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
http://dx.doi.org/10.1016/j.dental.2013.09.001
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the toughening of materials produced by phase transformation can enhance the clinical performance of all ceramic crowns even further [4]. 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 [5–7]. 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 [4]. The most recent reports have shown that the annual rate of veneer porcelain fracture and chipping has increased from 1 to 8% [4,8]. 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 [9]. 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 [10–13]. One strategy is to improve the mechanical strength of all-ceramic crowns through application of high strength ceramic materials [14–16]. 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 [17]. Optimal zirconia copings have enhanced porcelain thickness, and have been demonstrated to decrease veneer fracture [13,18–22]. In a study by Marchack et al., approximately 150 crowns were placed with no instances of cohesive porcelain or core fractures [21]. 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 [13]. 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 [23]. 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 [24]. 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 []) were obtained from the literature (Table 1) [25,26]. 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
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Fig. 1 – Geometric models CC, MCL, and MCM. (A) CC, conventional coping; (B) MCL, modified coping with lingual thickening; (C) MCM, modified coping with marginal thickening.
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 ).
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
Fig. 2 – Three types of loads simulate occlusal contact with antagonistic tooth: (A) loading on incisal edge; (B) loading on lingual fossa; (C) loading on lingual margin.
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Fig. 3 – First principal stresses (MPa) under loading at the incisal ridge–buccolingual cross-section: CC; MCM; MCL.
Table 1 – Model material properties. Material Dentin [26] Zirconia core [25] Feldspathic ceramics [26] Enamel [26]
Young’s modulus (MPa) 1.86 × 104 21.0 × 104 6.9 × 104 8.41 × 104
Poisson’s ratio 0.30 0.22 0.30 0.30
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 ( 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.
3.
Results
Under the same loading conditions, the stress distribution patterns for the all-ceramic crown with differential coping designs exhibited similarities. Stress was distributed along a symmetric medial line of the crown; max was on the loading area; stress concentration was found on the cervical margin and on the coping/veneer layer interface. However, different max stress values were determined for the three types of coping designs. In particular, a significant decrease in max was observed for the coping layer and veneer layer in MCL compared with MCM and CC. When was the incisal ridge loaded, the stress distribution for veneer layer, in addition to the loading area, exhibited stress concentration on the labial and proximal surfaces. For the coping layer, the max was found on the proximal surface. Stress concentration was located on the incisal 1/3 of the labial surface and lingual protuberance (Figs. 3 and 6). When the lingual fossa was loaded, the veneer layer, in addition to loading area, exhibited stress concentration on the proximal side. The coping layer exhibited stress concentration
on the loading area, proximal margin and labial surface (Figs. 4 and 7) Finally, when loading on the lingual margin, the veneer layer showed stress concentration located on the labial margin; albeit, the stress value was decreased. For the coping layer, the maximum principal stress was found on the lingual and labial margin, not on the proximal margin (Figs. 5 and 8).
4.
Discussion
The performance of the all-ceramic crown was determined by many parameters. These parameters included the bond strength between the veneer and coping layers, fabrication methods, material selection for both the veneer and coping layers, tooth supporting structure, magnitude and direction of occlusal loads, type and thickness of the cement layer, crown thickness and the coping design [16,27–30]. Most studies have confirmed that the stresses increase with increasing loading magnitudes and the orientation of the load application substantially altered stress levels within the crown [30]. Conflicting reports have identified that the type and thickness of the cement layer can cause little or no effect, or substantial effects, but these factors exert the greatest effect on the magnitude of stress. The elastic modulus of the tooth supporting structure also has been shown to exert different effects on stress distribution. Bond strengths between the zirconia core and veneer are affected by the types of zirconia and veneering materials used, and the appropriate surface treatment of the coping layer is essential to maintain a good bond strength [16,31]. In general, crown strength improves with increasingly strong materials and increasing thickness [30]. After the shape of the preparation and materials of the crown had been determined, the only parameter that was manipulated by the operator was the coping design [32]. Fractographic analysis of clinically failed crowns suggests that many failures occur in the veneer layer. An appropriate ratio of veneer and core thickness may decrease internal stress and reduce mechanical failures. As the core layer is stronger and stiffer than the veneer layer, the importance of adequate core thickness may be paramount [33–37]. When crown thickness is insufficient, the complete-contour zirconia crown and zirconia crown with a buccal porcelain facing have been used for posterior restorations [23]. The design of
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Fig. 4 – First principal stresses (MPa) under loading at the lingual fossa–buccolingual cross-section: CC; MCM; MCL.
the core is emerging as a significant factor in the survival and the extent of damage after a restoration. However, related literature regarding improvements to the mechanical properties by coping design of anterior all-ceramic crown is limited. In this study, we offer a solution to avoid veneer failure of an anterior all-ceramic crown by limiting the porcelain coverage of zirconia copings; a lesson taken from the differential coping design for metal ceramic restorations from many decades ago [21,23,37]. For the upper incisor, the bite force was loaded on the lingual side of the crown. Therefore, based on the concepts presented in the metal ceramic literature and the known physical properties of zirconia, we investigated the reduction of veneer stress by minimizing or eliminating the coverage of veneering porcelain on the lingual side. This customized coping design, in which a partial/full lingual surface of zirconia was used to support the veneering porcelain, utilized CAD/CAM technology without the requirement of full contour waxing and cut back. The objective was to determine which coping design could reduce mechanical failures of an anterior all-ceramic crown. There are many contact areas between the upper and inferior incisor when the incisor exerts functional occlusion. Therefore, differential loading can alter the stress distribution of an all-ceramic crown. We investigated three characteristic
loading locations: the incisal edge, lingual fossa and lingual margin. This is due to the fact that while chewing, load was applied on the incisal edge between the upper and inferior incisor; the lingual fossa was the most common contact area of the incisor in the central occlusion; while deep overlap in the anterior tooth caused abnormal occlusal forces and thus leaded the occlusal location to lie mainly on the lingual margin. These loads exhibit the extremes of stress distribution and therefore can be used to determine any weak regions. Our results showed that when loading was transferred from the incisal edge to the lingual margin, the highest stress value simultaneously moved from the incisal edge to the cervical margin. In line with previous investigations, our results imply that load is the major factor that influences stress and its distribution on the all-ceramic crown [31,38]. Measures should be applied to avoid abnormal load, such as adjustment of occlusal relation and changes to chewing habits. However, when abnormal load is inevitable, rational coping designs should be explored. As shown in Figs. 3 and 6, under loading on the incisal edge, similar stress distributions were detected. These results are in agreement with those from a previous 3D-FEA study in which stress concentration occurred at the loading area of the veneer layer and all coping layers; the highest stress
Fig. 5 – First principal stresses (MPa) under loading at the lingual margin–buccolingual cross-section: CC; MCM; MCL.
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maximum principal stress(MPa)
60 50
labial margin(veneer) 40
proximal margin(veneer) loading area(veneer)
30
labial margin(coping) proximal margin(coping)
20
loading area(coping) 10 0 CC
MCM
MCL
Fig. 6 – First principal stresses (MPa) in veneer and coping layer for CC, MCM and MCL models under loading on incisal ridge.
levels were directly observed beneath the loading area [38]. In a separate in vitro study, the failures of zirconia-based ceramic crowns were confined within the porcelain veneer in which the crack propagated from the sliding contact area [2]. As the loading area was on veneer porcelain and exhibited the highest stress values, it represents a high fracture risk. Hence, where possible, porcelain veneer contact with load should be avoided. In this vein, when the incisal edge is covered with veneer porcelain for esthetic reasons; chewing food with allceramic crown restored incisors is not advisable. From Figs. 4 and 7, the stress distribution demonstrated that when the load transferred to the lingual fossa, the deformation areas on the lingual fossa and labial margin were intensified. The results also showed that coping design slightly influenced the stress value. Although the stress value of the loading area was similar for the three loading locations, the lowest stress value for the veneer and coping layer was at the buccal surface of the finish line that occurred in the MCL model. In analyzing Fig. 4, we observed a negative correlation between the volume of coping layer and the stress value. When the volume of coping layer in the lingual side increased, the stress value decreased in the whole coping layer. The fracture strength of the all-ceramic crown mainly depended on two key variables: material and geometry. Accordingly, it has been shown that tougher, harder and stronger materials exhibit superior damage resistance, while critical loads for radial fracture strongly depend on crown net thickness [39]. In our study, we demonstrated that coping design modifications that provided stronger ceramics with increased thickness decreased the stress concentration.
When loading on the lingual margin (Figs. 5 and 8), our results indicated that stress concentration could be found mainly in the lingual margin. Apart from the loading area, both the veneer and coping layers showed no exacerbation in Von Mises stresses. However, the tensile stress values for the veneer and coping layers in the labial margin were higher for CC and MCM than for MCL. Much of the literature has demonstrated the cervical margin of an all-ceramic crown to be a weak region and that loading on the lingual margin could increase stress on the cervical margin [38]. Our results indicate that the coping design in MCL reduces the stress concentration on the veneer layer. Of the three types of coping designs, the veneer layer in CC exhibited the highest stress value during differential loading, especially within the loading area, which implies that CC is at high risk of brittle fracture. For MCM, loading on the lingual margin avoided contact with the veneer porcelain. High stress was shown when loading on the lingual fossa and lingual margin. Moreover, of the three coping designs, MCL always maintained the lowest stress in similar regions when loading on the lingual fossa and lingual margin. Therefore, our results indicate that MCL increases the survival rate of the all-ceramic crown. Some abnormal results also were observed. Under loading on the incisal edge, MCL exhibited higher stress than CC and under loading on the lingual margin, MCM exhibited the highest stress value overall. One reasonable explanation is that the loading area is located near the veneer/coping layer interface. Some previous in vitro studies have shown that higher stress levels were directly observed at the zirconia core/cement layer
maximum pricipal stress(MPa)
80 70 60
labial margin(veneer)
50
proximal margin(veneer) loading area(veneer)
40
labial margin(coping)
30
proximal margin(coping)
20
loading area(coping)
10 0 CC
MCM
MCL
Fig. 7 – First principal stress (MPa) in veneer and coping layer for CC, MCM and MCL models under loading on lingual fossa.
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maximum principal stress(MPa)
80 70 60
labial margin(veneer)
50
proximal margin(veneer) loading area(veneer)
40
labial margin(coping)
30
proximal margin(coping)
20
loading area(coping)
10 0 CC
MCM
MCL
Fig. 8 – First principal stress (MPa) in veneer and coping layer for CC, MCM and MCL models under loading on lingual margin.
interface; however, Von Mises stress values at the interface were lower than the rupture strength for the all-ceramic crown [26,28]. In our study, stress concentration could also be found at the interface between the core layer and veneer layer, especially at the proximal side and incisal ridge. Chemical bonding and mechanical interlock were insufficient at the interface between the core layer and veneer layer. This interface was also a weak area and thus could lead to cracks and interface failures. Therefore, the veneer/coping layer interface should be designed far away from occlusal contact. When monolith configuration was designed in the loading area, cone cracks originate from the ceramic surface as a result of pressure. When the bilayered configuration was created in the loading area, a major crack developed in the veneer/coping layer interface as a result of tensile stress [39]. As it is a more brittle material, ceramic is more susceptible to the initiation of cracking under tensile stress than under pressure stress. Our results indicate that in the loading area, monolith configuration zirconia coping layer is the preferred coping design.
5.
Conclusion
Our hypothesis of whether a customized coping design could reduce the stress on veneers was partially accepted, as the present study showed that the stress value in the MCL decreased with the use of the lingual modified coping design that expanded the cover area and increased the thickness of the coping layer. However, the MCM design merely resulted in a slight decrease in stress on the lingual margin under loading. The present study suggests that the modified coping design is a promising method to avoid veneer porcelain fracturing. It may be advisable in the case of anterior allceramic crowns that the core material is not veneered on lingual surface where esthetic considerations are not crucial. Our study indicates that FEA results can assistant in rebuilding the design guidelines in CAD/CAM systems for all-ceramic restorations. The major limitation of our study was that the absence of a cementation layer in the simulation may have had potential effects on the stress distribution in the core. In fact, all-ceramic crowns have to endure changing and cycling occlusal forces under mastication. The load type was static in our finite element analysis; therefore, further studies based on
our static results should be conducted regarding fatigue analysis. Our results are also ideal for further in vitro laboratory testing and clinical research.
Acknowledgments This work was supported by grants from the Outstanding Medical Academic Leader Program and Creative team of Jiangsu Province, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), National High Technology Research and Development Program of China (863 Program, Grant No. 2012AA030309).
references
[1] Rekow ED, Silva NR, Coelho PG, Zhang Y, Guess P, Thompson VP. Performance of dental ceramics: challenges for improvements. J Dent Res 2011;90(8):937–52. [2] Silva NR, Bonfante EA, Zavanelli RA, Thompson VP, Ferencz JL, Coelho PG. Reliability of metalloceramic and zirconia-based ceramic crowns. J Dent Res 2010;89(10):1051–6. [3] Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater 2009;25(9):1122–7. [4] Silva NRFA, Sailer I, Zhang Y, Coelho PG, Guess PC, Zembic A, et al. Performance of zirconia for dental healthcare. Materials 2010;3(2):863–96. [5] Fradeani M, Aquilano A, Corrado M. Clinical experience with In-Ceram Spinell crowns: 5-year follow-up. Int J Periodont Restorative Dent 2002;22(6):525–33. [6] Ereifej N, Rodrigues FP, Silikas N, Watts DC. Experimental and FE shear-bonding strength at core/veneer interfaces in bilayered ceramics. Dent Mater 2011;27(6):590–7. [7] Coelho PG, Bonfante EA, Silva NR, Rekow ED, Thompson VP. Laboratory simulation of Y-TZP all-ceramic crown clinical failures. J Dent Res 2009;88(4):382–6. [8] Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24(3):299–307. [9] Sailer I, Gottnerb J, Kanelb S, Hammerle CH. Randomized controlled clinical trial of zirconia–ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up. Int J Prosthodont 2009;22(6):553–60.
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[10] Fischer J, Stawarzcyk B, Trottmann A, Hämmerle CH. Impact of thermal misfit on shear strength of veneering ceramic/zirconia composites. Dent Mater 2009;25(4):419–23. [11] Anunmana C, Anusavice KJ, Mecholsky Jr JJ. Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-noCCh test. Dent Mater 2010;26(2):111–7. [12] Tan JP, Sederstrom D, Polansky JR, McLaren EA, White SN. The use of slow heating and slow cooling regimens to strengthen porcelain fused to zirconia. J Prosth Dent 2012;107(3):163–9. [13] Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29(3):307–16. [14] Schmitter M, Mueller D, Rues S. Chipping behaviour of all-ceramic crowns with zirconia framework and CAD/CAM manufactured veneer. J Dent 2012;40(2):154–62. ¨ [15] Luthardt RG, Holzhuter M, Sandkuhl O, Herold V, Schnapp JD, Kuhlisch E, et al. Reliability and properties of ground Y-TZP-zirconia ceramics. J Dent Res 2002;81(7):487–91. [16] Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Effect of zirconia type on its bond strength with different veneer ceramics. J Prosthod 2008;17(5):401–8. [17] Taskonak B, Mecholsky Jr JJ, Anusavice KJ. Residual stresses in bilayer dental ceramics. Biomaterials 2005;26(16):3235–41. [18] Baldassarri M, Zhang Y, Thompson VP, Rekow ED, Stappert CF. Reliability and failure modes of implant-supported zirconium-oxide fixed dental prostheses related to veneering techniques. J Dent 2011;39(7):489–98. [19] Giannetopoulos S, van Noort R, Tsitrou E. Evaluation of the marginal integrity of ceramic copings with different marginal angles using two different CAD/CAM systems. J Dent 2010;38(12):980–6. [20] Isgro G, Addison O, Fleming GJ. The deformation and strength of a dental ceramic following resin-cement coating. J Dent 2011;39(2):122–7. [21] Marchack BW, Futatsuki Y, Marchack CB, White SN. Customization of milled zirconia copings for all-ceramic crowns: a clinical report. J Prosthet Dent 2008;99(3): 169–73. [22] Silva NR, Bonfante EA, Rafferty BT, Zavanelli RA, Rekow ED, Thompson VP, et al. Modified Y-TZP core design improves all-ceramic crown reliability. J Dent Res 2011;90(1):104–8. [23] Marchack BW, Sato S, Marchack CB, White SN. Complete and partial contour zirconia designs for crowns and fixed dental prostheses: a clinical report. J Prosthet Dent 2011;106(3):145–52. [24] Rafferty BT, Janal MN, Zavanelli RA, Silva NR, Rekow ED, Thompson VP, et al. Design features of a three-dimensional molar crown and related maximum principal stress. A finite element model study. Dent Mater 2010;26(2):156–63.
[25] De Jager N, de Kler M, van der Zel JM. The influence of different core material on the FEA-determined stress distribution in dental crowns. Dent Mater 2006;22(3): 234–42. [26] Ausiello P, Franciosa P, Martorelli M, Watts DC. Mechanical behavior of post-restored upper canine teeth: a 3D FE analysis. Dent Mater 2011;27(12):1285–94. [27] Swain MV. Unstable cracking (chipping) of veneering porcelain on all-ceramic dental crowns and fixed partial dentures. Acta Biomater 2009;5(5):1668–77. [28] Anunmana C, Anusavice KJ, Mecholsky Jr JJ. Interfacial toughness of bilayer dental ceramics based on a short-bar, chevron-notch test. Dent Mater 2010;26(2):111–7. [29] Fischer J, Stawarczyk B, Sailer I, Hämmerle CH. Shear bond strength between veneering ceramics and ceria-stabilized zirconia/alumina. J Prosthet Dent 2010;103(2):267–74. [30] Rekow ED, Harsono M, Janal M, Thompson VP, Zhang G. Factorial analysis of variables influencing stress in all-ceramic crowns. Dent Mater 2006;22(2):125–32. [31] Ozkurt Z, Kazazoglu E, Unal A. In vitro evaluation of shear bond strength of veneering ceramics to zirconia. Dent Mater J 2010;29(2):138–46. [32] De Jager N, Pallav P, Feilzer AJ. The influence of design parameters on the FEA-determined stress distribution in CAD–CAM produced all-ceramic dental crowns. Dent Mater 2005;21(3):242–51. [33] Wakabayashi N, Anusavice KJ. Crack initiation modes in bilayered alumina/porcelain disks as a function of core/veneer thickness ratio and supporting substrate stiffness. J Dent Res 2000;79(6):1398–404. [34] Esquivel-Upshaw JF, Anusavice KJ. Ceramic design concepts based on stress distribution analysis. Compend Contin Educ Dent 2000;21(8), 649–52, 654. [35] Proos KA, Swain MV, Ironside J, Steven GP. Influence of core thickness on a restored crown of a first premolar using finite element analysis. Int J Prosthodont 2003;16(5):474–80. [36] Lawn BR, Deng Y, Lloyd IK, Janal MN, Rekow ED, Thompson VP. Materials design of ceramic-based layer structures for crowns. J Dent Res 2002;81(6):433–8. [37] Pogoncheff CM, Duff RE. Use of zirconia collar to prevent interproximal porcelain fracture: a clinical report. J Prosthet Dent 2010;104(2):77–9. [38] Rocha EP, Anchieta RB, Freitas Jr AC, de Almeida EO, Cattaneo PM, Chang Ko C. Mechanical behavior of ceramic veneer in zirconia-based restorations: a 3-dimensional finite element analysis using microcomputed tomography data. J Prosthet Dent 2010;105(1):14–20. [39] Lawn BR, Pajares A, Zhang Y, Deng Y, Polack MA, Lloyd IK, Dianne RE, Thompson VP. Materials design in the performance of all-ceramic crowns. Biomaterials 2004;25(14):2885–92.