Scholarly article on topic 'Effects of coping designs on stress distributions in zirconia crowns: Finite element analysis'

Effects of coping designs on stress distributions in zirconia crowns: Finite element analysis Academic research paper on "Economics and business"

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Abstract of research paper on Economics and business, author of scientific article — Seung-Ryong Ha, Sung-Hun Kim, Jai-Bong Lee, Jung-Suk Han, In-Sung Yeo, et al.

Abstract The purpose of this study was to evaluate the effects of coping designs on the stress distributions in posterior zirconia crowns by non-linear three-dimensional finite element analysis. Three-dimensional finite element models of a mandibular right first molar with layers of veneering porcelain, zirconia coping, cement, and abutment tooth were designed by computer software (HyperWorks 10.0). Ten zirconia crowns with different designs were produced according to various shoulder positions and heights. The shoulders (1-mm width) exhibited incremental height increases of 1mm, 2mm, and 3mm on the buccal, lingual, and proximal sides, respectively. An axial compressive dynamic load simulating the progressive load was applied until a stainless steel ball model (7mm in diameter) deepened the veneer surface to 0.7mm in depth. Loads were placed on the inner inclines of the mesiobuccal, distobuccal, and mesiolingual cusps. Residual maximum principal stresses (MPSs) at the veneer and coping under progressive loading were determined for each zirconia crown. Reinforcements with the shoulders on the buccal, lingual, and proximal axial walls resulted in lower MPSs in the veneering porcelain but higher MPSs in the zirconia coping. As the shoulder height increased, the tensile stresses decreased, while the compressive stresses increased in the veneering porcelains. It can be concluded that the shoulder height and position in the zirconia coping will affect the MPSs of the crown. Our findings conclusively reveal the critical role of the shoulder design of the coping in preventing veneer fracture on posterior zirconia restorations by reducing tensile stresses in veneering porcelain.

Academic research paper on topic "Effects of coping designs on stress distributions in zirconia crowns: Finite element analysis"

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Ceramics International 42 (2016) 4932-4940

CERAMICS

INTERNATIONAL

www.elsevier.com/locate/ceramint

Effects of coping designs on stress distributions in zirconia crowns: Finite element analysis

Seung-Ryong Haa, Sung-Hun Kimb,n, Jai-Bong Leeb, Jung-Suk Hanb, In-Sung Yeob,

Seung-Hyun Yooc

aDepartment of Dentistry, Ajou University School of Medicine, Suwon, Republic of Korea bDepartment of Prosthodontics and Dental Research Institute, School of Dentistry, Seoul National University, Seoul, Republic of Korea cDepartment of Mechanical Engineering, College of Engineering, Ajou University, Suwon, Republic of Korea

Received 30 September 2015; received in revised form 30 November 2015; accepted 1 December 2015

Available online 10 December 2015

Abstract

The purpose of this study was to evaluate the effects of coping designs on the stress distributions in posterior zirconia crowns by non-linear three-dimensional finite element analysis. Three-dimensional finite element models of a mandibular right first molar with layers of veneering porcelain, zirconia coping, cement, and abutment tooth were designed by computer software (HyperWorks 10.0). Ten zirconia crowns with different designs were produced according to various shoulder positions and heights. The shoulders (1-mm width) exhibited incremental height increases of 1 mm, 2 mm, and 3 mm on the buccal, lingual, and proximal sides, respectively. An axial compressive dynamic load simulating the progressive load was applied until a stainless steel ball model (7 mm in diameter) deepened the veneer surface to 0.7 mm in depth. Loads were placed on the inner inclines of the mesiobuccal, distobuccal, and mesiolingual cusps. Residual maximum principal stresses (MPSs) at the veneer and coping under progressive loading were determined for each zirconia crown. Reinforcements with the shoulders on the buccal, lingual, and proximal axial walls resulted in lower MPSs in the veneering porcelain but higher MPSs in the zirconia coping. As the shoulder height increased, the tensile stresses decreased, while the compressive stresses increased in the veneering porcelains. It can be concluded that the shoulder height and position in the zirconia coping will affect the MPSs of the crown. Our findings conclusively reveal the critical role of the shoulder design of the coping in preventing veneer fracture on posterior zirconia restorations by reducing tensile stresses in veneering porcelain. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Fracture, stress; Finite element analysis; Dental prosthesis design; Zirconium

1. Introduction

Dental materials have undergone considerable improvement, and a variety of new systems have become widely used. Currently, there are several dental ceramic materials available on the market, including glass ceramics and polycrystalline ceramics. Their use is advantageous not only due to their favorable optical properties but also due to their adequate clinical function, favorable mechanical properties, and longevity [1-4]. Among these ceramic systems, the mechanical

*Corresponding author. Tel.: + 82 220722664; fax: + 82 220723860. E-mail address: ksh1250@snu.ac.kr (S.-H. Kim).

properties of zirconia are the highest ever reported, resembling those of metals. It has been termed 'ceramic steel' [5].

Zirconia can exist in three crystallographic polymorphs depending on the temperature: monoclinic, tetragonal, and cubic. Its structure is monoclinic below 1170 °C, tetragonal between 1170 and 2370 °C, and cubic above 2370 °C and up to the melting point [3]. When the tetragonal phase transforms to the monoclinic phase on cooling, a volumetric change in the crystal (circa 4.5% volume increase) may lead to fracture [3]. To retain the tetragonal structure at room temperature, stabilizing oxides, such as CaO, MgO, CeO2, or Y2O3, are alloyed with pure zirconia [3]. The transformation from the tetragonal (t) phase to the monoclinic (m) phase at room temperature is due to the application of an external stress on the zirconia

http://dx.doi.org/10.1016/j.ceramint.2015.12.007

0272-8842/© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

surface accompanied by a volumetric change. This stress-induced unique phenomenon, known as a t-m transformation, arrests crack propagation by crystalline expansion, which seals the crack and leads to high fracture resistance [5,6].

Zirconia has been introduced to prosthodontic dentistry for the manufacture of fixed dental prostheses (FDPs) and implant abutments in combination with computer-aided design and computer-aided manufacturing (CAD/CAM) techniques. Increased demand for metal-free restorations in posterior areas has increased our focus on zirconia restorations because of its advantages in patient comfort and acceptance and excellent mechanical properties. In this system, the crown coping is fabricated from high-strength zirconia ceramic materials and is veneered with feldspathic porcelain. It has been speculated that cracks originate on the internal surfaces of all-ceramic crowns, leading us to believe that it reinforces the crowns with high-strength ceramic copings [7]. However, in several reports, the failure rate of the crowns replaced in posterior areas has been described to be 3% to 4% per year [8-12]. The chipping or delamination of the veneering porcelain in zirconia prostheses is a problematic issue in prosthodontic dentistry, occurring in as many as 25% of all cases [13-22]. Therefore, the restoration of a molar with this material remains controversial.

Clinical studies have demonstrated that cohesive failure of veneering porcelain on zirconia prostheses is related to several contributing factors: fatigue, overloading, residual stress in veneering porcelain, mismatch of coefficients of thermal expansion (CTE), poor wettability of veneering porcelain, improper porcelain-coping thickness ratio, flaws on the veneering porcelain, porosities, and poor framework design [3,2327]. Clinical studies with modified coping designs in zirconia crowns reported promising results [28-34]. Brittle veneering porcelains were fractured under tensile loads because of its low tensile strength, even with high compressive strength. Therefore, the design concepts of those studies focused on decreasing tensile stresses in veneering porcelains by the supportive

structures of zirconia copings: a high palatal shoulder [29], a palatal and midproximal shoulder [28], a 2.5-mm-high lingual and proximal shoulder [32], and a proximal and lingual shoulder [30,31,33,34]. However, these studies did not offer information on the influence of variations of shoulder positions and heights on fracture resistance of veneering porcelain. Moreover, little on the variation of shoulder heights and positions in stress distribution of zirconia crown models has been researched [24].

The present study aims to examine the stress distribution and localize the critical sites within posterior mandibular zirconia crowns in different coping designs under progressive loading using three-dimensional (3D) finite element analysis (FEA). We hypothesized that there are differences in the stress distributions of the veneering porcelain and zirconia coping with various shoulder positions and heights.

2. Materials and methods

2.1. Tooth, zirconia coping, veneering porcelain solid models generation

The 1.2-mm-deep and 8°-convergence-angle chamfer was prepared on a mandibular right first molar resin model (D85DP-500B.1, Nissin Dental, Kyoto, Japan) using a carbide bur (Komet H 356 RGE 103.031, Gebr. Brasseler GmbH, Lemgo, Germany). The carbide bur was affixed to a surveyor (F1, DeguDent GmbH, Kanau, Germany) to ensure a standardized preparation. The crowns were completed by digitizing the unprepared and prepared resin models using an optical scanner (Optical 3D Scanner Activity 101, smart optics Sensortechnik GmbH, Bochum, Germany).

The variations in the zirconia coping designs were as follows: 1-mm-wide shoulder had incremental height increases of 1 mm, 2 mm, and 3 mm on the buccal, and lingual and proximal sides (Fig. 1 and Table 1).

Fig. 1. Schematic image of the variations of shoulders in the zirconia coping designed in computer-aided design software. The 1-mm-wide shoulder variations in the copings were incremental increases of 1 mm, 2 mm, and 3 mm in the buccal (B) height and proximal and lingual (PL) height. (a) Model 1: no shoulder, (b) Model 2: PL 1 mm, (c) Model 3: PL 1 mm and B 1 mm, (d) Model 4: PL 2 mm, (e) Model 5: PL 2 mm and B 1 mm, (f) Model 6: PL 2 mm and B 2 mm, (g) Model 7: PL 3 mm, (h) Model 8: PL 3 mm and B 1 mm, (i) Model 9: PL 3 mm and B 2 mm, and (j) Model 10: PL 3 mm and B 3 mm.

Table 1

Ten experimental models used in this study.

Model Shoulder height from margin (mm)

Proximal and lingual Buccal

10 3 3

Model 1: No shoulder (Fig. 1a).

Model 2: 1-mm-high shoulder on the proximal/lingual cervical area of the crown (Fig. 1b).

Model 3: 1-mm-high shoulder on the proximal/lingual and the buccal cervical area of the crown (Fig. 1c).

Model 4: 2-mm-high shoulder on the proximal/lingual cervical area of the crown (Fig. 1d).

Model 5: 2-mm-high shoulder on the proximal/lingual and 1-mm-height shoulder on the buccal cervical area of the crown (Fig. 1e).

Model 6: 2-mm-high shoulder on the proximal/lingual and the buccal cervical area of the crown (Fig. 1f).

Model 7: 3-mm-high shoulder on the proximal/lingual cervical area of the crown (Fig. 1g).

Model 8: 3-mm-high shoulder on the proximal/lingual and

1-mm-height shoulder on the buccal cervical area of the crown (Fig. 1h).

Model 9: 3-mm-high shoulder on the proximal/lingual and

2-mm-height shoulder on the buccal cervical area of the crown (Fig. 1i).

Model 10: 3-mm-high shoulder on the proximal/lingual and the buccal cervical area of the crown (Fig. 1j).

The veneering porcelains and zirconia copings were designed with an external shape of the resin model by CAD (FreeForm modeling systems, Sensable-Geomagic, MA, USA). After exporting the STL files, 10 experimental models were completed in accordance with the coping designs. The solid models were generated by copying the models using CAD software (HyperWorks 10.0, Altair Engineering, Ontario, Canada): a 1-mm-thick veneer layer (veneering porcelain), a 0.5-mm-thick coping layer (zirconia), a 100-^m-thick cement layer (resin cement), and an abutment tooth. All layers were joined in the final model.

2.2. Three-dimensional finite element model generation

The solid models were exported and uploaded to FEA software (ABAQUS/CAE 6.9, Dassault Systèmes, Vélizy-Vil-lacoublay, France). Then, they were meshed with four node linear tetrahedral elements (Fig. 2). The material properties of the veneering porcelain, zirconia coping, resin cement, and abutment tooth were allocated to each element. Table 2 shows

Fig. 2. Computer-aided designed stainless steel ball and veneered zirconia crown system, simulating a progressive load test. (a) Stainless steel ball 7 mm in diameter, (b) veneering porcelain, (c) zirconia coping, (d) cement layer, and (e) prepared tooth.

the material properties inputs used for this study. Thus, 10 different three-dimensional finite element models were constructed. Tables 3 and 4 show elements and nodes in 3D FEA. The models were tested for convergence before simulation.

2.3. Boundary condition

The assumptions for the FEA model were that (1) all solids were presumed to be isotropic, linear elastic and homogeneous

Table 2

Material properties input for finite element analysis.

Component Material Young's Poisson's Density

modulus (GPa) ratio (g/mL)

Veneer Porcelain 70 0.19 2.40

Ceramic coping Zirconia 200 0.19 2.40

Cement layer Resin 8 0.33 2.19

cement

Tooth Dentin 16 0.31 2.14

(prepared)

Table 3

Elements in the three-dimensional finite element model.

Model Number of elements

Veneer Zirconia Cement Dentin

1 57,748 34,842 30,150 72,977

2 65,841 41,966 30,150 72,977

3 62,217 48,528 30,150 72,977

4 62,703 53,049 30,150 72,977

5 66,232 60,442 30,150 72,977

6 58,144 50,988 30,150 72,977

7 67,293 63,596 30,150 72,977

8 57,211 60,218 30,150 72,977

9 56,825 66,809 30,150 72,977

10 58,028 73,954 30,150 72,977

Table 4

Nodes in the three-dimensional finite element model.

Model Number of nodes

Veneer Zirconia Cement Dentin

1 15,909 10,575 10,200 18,383

2 14,020 12,433 10,200 18,383

3 16,632 14,020 10,200 18,383

4 17,004 15,017 10,200 18,383

5 17,598 16,881 10,200 18,383

6 15,250 14,502 10,200 18,383

7 18,458 17,632 10,200 18,383

8 15,293 16,719 10,200 18,383

9 15,124 18,340 10,200 18,383

10 15,318 19,972 10,200 18,383

over the entire deformation, (2) perfect bonding among components, (3) uniform 100-^m-thick cement layer, (4) uniform 0.5-mm-thick coping, except for the shoulder, (5) no flaws in any components, and (6) all degrees of freedom confined at the root component surface [35].

2.4. Numerical simulations

Progressive loading situations were simulated. In this simulation, the abutment tooth model was confined apically to the chamfer finish line. A load was applied by a 3D finite element ball model (7 mm in diameter) at the three loading points: two points on the inner inclines of the mesiobuccal and

Fig. 3. Schematic representation of loading points and directions on the crown. (a) Two points on the inner inclines of the mesiobuccal and distobuccal cusps, one point on the inner incline of the mesiolingual cusp. (b) Load was applied until the stainless steel ball model (7 mm in diameter) was lower onto the veneer surface to 0.7 mm in depth.

1? 14000 a.

— 12000

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 Model

Fig. 4. Highest maximum principal stresses of the veneer and zirconia in each model. M1, Model 1; M2, Model 2; M3, Model 3; M4, Model 4; M5, Model 5; M6, Model 6; M7, Model 7; M8, Model 8; M9, Model 9; and M10, Model 10.

distobuccal cusps and one point on the inner incline of the mesiolingual cusp (Fig. 3). Axial compression was applied with a crosshead speed of 0.7 mm/s until the 3D finite element ball model moved into the veneer surface 0.7 mm in depth.

3. Results

The highest maximum principal stress values in the simulated models of the veneering porcelains and zirconia copings are shown in Fig. 4. As the shoulder height increased, the maximum principal stress values in the veneering porcelain decreased and those in the zirconia coping increased. Irrespective of shoulder positions and heights, the peak maximum principal stress levels were observed at the loading points for both veneering porcelains and zirconia copings. In addition, high maximum principal stress levels were observed at the shoulder region.

The maximum principal stress distributions of each model that simulated the progressive load test are shown in Figs. 5 and 6. The presence of shoulder had a dramatic effect. Whether there was a shoulder on the buccal side (Fig. 5a and b, d and g and Fig. 5c, e, f, and h-j, respectively) suggested that the tensile stress was concentrated on the shoulder of the zirconia coping when there was one. When there was no

Fig. 5. Buccal view of maximum principal stress distributions within zirconia crowns for all models. (a) Model 1, (b) Model 2, (c) Model 3, (d) Model 4, (e) Model 5, (f) Model 6, (g) Model 7, (h) Model 8, (i) Model 9, and (j) Model 10.

Fig. 6. Lingual view of maximum principal stress distributions within zirconia crowns for all models. (a) Model 1, (b) Model 2, (c) Model 3, (d) Model 4, (e) Model 5, (f) Model 6, (g) Model 7, (h) Model 8, (i) Model 9, and (j) Model 10.

Fig. 7. Resultant view of maximal principal stress distributions within Model 10 by non-linear 3D FEA. As more time passed (from left to right in the picture), more tensile stress was concentrated on the shoulder of the model.

shoulder on the buccal or lingual side (Fig. 5 a), the tensile stress was concentrated on not only the buccal side but also the lingual proximal side of the middle third area. Moreover, when there was no shoulder on the buccal or lingual side (Fig. 6a), tensile stress was observed more on the occlusal side than the cervical side. As the shoulder height of the zirconia coping increased, more tensile stress was absorbed by it. Within each model, as the more time passed, that is, the further the stainless steel ball model was lowered vertically into the surface of the porcelain, more tensile stress was concentrated on the shoulder of each model (Fig. 7). The mesial shoulder exhibited a greater

concentration of tensile stress than the distal, while the stress was more concentrated on the middle of the proximal shoulder at the beginning of loading but it spread bucco-lingually as time went on (Fig. 7). In models with shoulders, more tensile stress was observed on the proximal margin, especially on the mesial shoulder margin, and it began to spread upward and bucco-lingually as time went on.

The minimum principal stress distributions of each model in the progressive load test are shown in Figs. 8 and 9. The existence of a shoulder influenced the compressive stress distribution. When there was no shoulder (Fig. 8a, b, d, and g),

Fig. 9. Lingual view of minimum principal stress distributions within zirconia crowns for all models. (a) Model 1, (b) Model 2, (c) Model 3, (d) Model 4, (e) Model 5, (f) Model 6, (g) Model 7, (h) Model 8, (i) Model 9, and (j) Model 10.

the compressive stress was mainly present around the margin. However, when there was a shoulder (Fig. 8c, e, f, and h-j), compressive stress was concentrated on the veneering porcelain near the shoulder by which it was supported. In addition, when there was the same height of the shoulder on the lingual, (Figs. 8D-f and g-j), a higher buccal shoulder produced stronger compressive forces of the veneering porcelain.

4. Discussion

The objective of the current study was to examine the effects of the coping designs on the stress distributions within posterior mandibular zirconia crowns under progressive loading using 3D FEA. We hypothesized that there are differences in the stress distributions of the veneering porcelain and zirconia coping with various shoulder positions and heights. The results of the current study show that there were variations in the stress distributions within the posterior mandibular zirconia crowns.

Conventional posterior esthetic restorations have two components: the veneering porcelain and the coping material. However, porcelain chipping occurs frequently, while coping materials are often not affected [4]. As a result of natural brittleness and inherent residual stress, the veneering porcelain on zirconia crowns is subject to fracture. It has been demonstrated that porcelain chipping rates were 3% to 36% [17,20] for FDPs and 8% to 53% [36-38] for implant-supported zirconia FDPs. The lack of a periodontal ligament, the rigidity

of implants, and impaired proprioception were responsible for higher chipping ratios in implant-supported FDPs.

The causes of chipping may be material-related and non-material-related, such as the porcelain-coping thickness ratio and framework design [3]. In clinical studies on the fracto-graphic examination of failed zirconia crowns, it has been suggested that roughness on the veneering porcelain surface may be associated with chipping, due to the cracks originating from the occlusal surface of veneering porcelain [15,38,39]. Therefore, thorough polishing must take place after occlusal adjustments of zirconia restorations to avoid chipping of veneering porcelains. Moreover, the incorporation of air bubbles during veneering porcelain build-ups by hand may result in surface roughness. Accordingly, the use of high-strength heat-pressed ceramics has increased to improve the bond strength to the zirconia coping and to reduce the occurrence of chipping of zirconia restorations. Nevertheless, it has also been demonstrated that chipping occurs in heat-pressed zirconia FDPs [17,40].

Other possible causes for chipping are the mismatch of CTE between zirconia coping and veneering porcelain, residual stresses in veneering porcelain during the cooling process, poor wettability of veneering porcelain, flaws on the veneering porcelain, porosities, fatigue, overloading, and improper support of veneering porcelains [23-27]. Zirconia copings of uniform thickness have been milled, which result in excessive veneering porcelain thickness ( > 2 mm) and ultimately lead to the cohesive fracture of veneering porcelain. It has been

demonstrated that the poor design of the framework, which does not offer uniform, appropriate support to the veneering porcelain, would contribute to chipping [28,29]. Therefore, it has been suggested that the zirconia coping should be designed to provide appropriate support to the veneering porcelain. The design concept was derived from porcelain-fused-to-metal restorations [41,42]. Brittle materials, such as veneering porcelains, should be subject to mostly compressive stresses rather than deleterious tensile stresses as dictated by the coping design. Coelho et al. [43] examined the clinical failure of Y-TZP crowns using 3D FEA. They designed standard posterior zirconia crowns and applied a 1200 N vertical load on one point of the distobuccal cusp of the veneering porcelain. They described that high maximum principal stress values were observed at both the veneering porcelain and zirconia coping directly below the loading point and at the cervical marginal region, including the proximal region. Silva et al. [31] investigated fatigue failure of modified and conventional zirconia compared to metal ceramic crowns under sliding loads using 3D FEA. They designed a uniform 0.5-mm-thick coping for standard crowns, which was overall 0.5 mm thick, 2.5 mm high, and 1 mm thick at the lingual flange for modified copings. The authors applied a 110-N vertical plus a 200-N horizontal load at the mesiolingual cusp ridge and distobuccal cusp ridge toward the central fossa. They demonstrated that high maximum principal stress levels were observed at the veneering porcelain and zirconia coping immediately below the loading point, and substantially higher maximum principal stress levels were shown for the modified coping design. However, these experiments were performed under constant loading conditions by linear FEA. Therefore, it would be premature to compare these results directly with the influence of dynamic forces that occur in the actual oral environment.

It has been demonstrated that chipping mostly occurred in premolars and molars, in connectors of mandibular posterior FDPs, and in second molars of FDPs [16,44]. In addition, it has been reported that chipping occurred in non-load bearing sites, such as the lingual side of FDPs [13] and the mesiolingual cusps of mandibular second molars [20]. Therefore, the loading points used in the present study were the mesiobuccal, distobuccal, and mesiolingual cusps of mandibular molars, depending on the published locations of chipping of zirconia crowns. We adopted modified coping designs from published studies [29,31,32,35]. However, these studies did not offer information on stress distribution with variations in shoulder height and position. Additionally, the literature regarding the influence of shoulder position and height on system biome-chanics is limited. Accordingly, in the present study, variations in the positions (proximal/lingual, buccal) and heights (0 mm, 1 mm, 2 mm, and 3 mm) of the shoulders were fabricated in the zirconia copings. Moreover, the changes in stress distribution and stress propagation pathways were analyzed by employing progressive loading as an experimental condition in the current study. Many factors, such as the boundary conditions of the FE models, meshing quality, and element type, influence the validity and reliability of the FEA. Thus, a

preliminary convergence test was performed to minimize these effects in the present study.

It was observed that the tensile stresses in the veneering porcelain were decreased, while compressive stresses were increased, as the height of shoulder increased. The presence of a shoulder was significant in that it reduced the detrimental tensile stress in veneering porcelains. When the shoulder did not exist, the tensile stress was concentrated in the middle and occlusal third of the entire crown. It was noted that the shoulder of the zirconia coping bore more tensile stress as the height of the shoulder increased. These areas corresponded with the chipping sites of zirconia crowns found in clinical practice. Even in the same model, we could see that more tensile stress was generated in the zirconia shoulder area as the loading ball came further down in the vertical direction. In addition, the mesial shoulder had more tensile stress than the distal shoulder. This was probably due to the loading position. In models with a shoulder, the proximal areas, especially the mesial shoulder margin areas, showed more tensile stress. Then, it spread upward and bucco-lingually. Our FEA results show that higher maximum principal stress levels were present in two areas of the zirconia coping. As we expected, one high-stress area in the coping was right beneath the occlusal loading area. Other high-stress areas were very close to the shoulder. In addition, the tensile stress produced in the bucco-cervical area of the coping increased considerably. The higher tensile stress areas in crowns caused by the variations in shoulder design along with the stress areas in the occlusal (seen in all simulations) could increase more if flaws are present. In clinical practice, shoulder-supported zirconia crowns may diminish the likelihood of chipping because of the better support of the veneering porcelain. This is shown in Figs. 8 and 9, with compressive stress being the dominant force in the shoulder area.

The results of in vitro simulation testing cannot be translated to the clinical situation because this experimental study design did not contemplate typical factors of the oral cavity, such as fatigue loading or the dynamic forces of mastication. Obviously, the directions of occlusal loading are associated with the anatomy of the teeth during mastication. However, this was not considered in the present study. This was a comparative study in which all variables but the shoulder designs were the same. Hence, it should be noted that the progressive load test was only one of many simulations and that maximum strength is just a single property of the many that are found in zirconia crowns. In addition, the design of the current study provided no data on cyclic loading or thermal cycling. Further investigations are needed to assess the effect of thermal cycling on stress distributions.

5. Conclusion

Within the many limitations that this study possessed, we concluded that the shoulder height and position in the zirconia coping affect the maximum principal stress of the crown. Our findings revealed the critical role of the shoulder design of the

coping in the success of posterior zirconia restorations in terms of reducing tensile stresses in veneering porcelain. The shoulder support of a veneering porcelain may be crucial to the durability of mandibular posterior zirconia crowns. Raising the height of shoulder worked as a stress absorber adjacent the restoration margins. This suggests that this area may be an important point to support the prosthesis during function. We recommend that the shoulder height be greater than 3 mm long, especially on the lingual side, to reduce tensile stresses in mandibular first molars.

Acknowledgments

The authors would like to thank Mr. Se-Chul Jeong for his assistance in finite element analysis.

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