A Novel Coping Design to Decrease Maximum Principal Stress in Zirconia Ceramic Restorations MooGyung Sung, DDS PhD,1 Chong-Hyun Han, DDS, PhD,2 & Sunjai Kim, DDS, PhD3 1

Private Practice, Dental Office Drs. Sung & Oh, Seoul, Korea Professor, Department of Prosthodontics, Gangnam Severance Dental Hospital, College of Dentistry, Yonsei University, Seoul, Korea 3 Associate Professor and Chairman, Department of Prosthodontics, Gangnam Severance Dental Hospital, College of Dentistry, Yonsei University, Seoul, Korea 2

Keywords Zirconia; coping; design; stress; finite element analysis. Correspondence Professor Sunjai Kim, Department of Prosthodontics, Gangnam Severance Dental Hospital, Eonju-ro 211, Gangnam-gu, Seoul 135-270, South Korea. E-mail: [email protected], [email protected] The authors declare that they have no conflict of interests. Accepted October 5, 2013 doi: 10.1111/jopr.12147

Abstract Purpose: The objective of this study was to evaluate the influence of coping design modifications on maximum first principal stress (MPS) in a mathematical zirconia ceramic crown model. Materials and Methods: For a nonlinear, 3D finite element analysis, a simplified tooth model was built on the basis of the average dimensions of mandibular second molars. Virtual tooth reduction was performed to model an abutment with a flat occlusal surface and 16° convergence angle between facing walls. The cement layer was set to a thickness of 100 µm. Three different copings—one with 0.5-mm constant thickness; one with constant thickness and extended lingual and proximal collars; and a novel design with zirconia beam reinforcement—were designed to simulate zirconia ceramic restorations. The novel design had strategically positioned zirconia beams on the lingual and marginal ridges to protect veneer ceramics, and was divided into three subdesigns according to the width of the zirconia beam (0.5, 0.8, and 1 mm). Combinations of vertical and horizontal load were applied over the distolingual marginal ridge, and the MPSs were evaluated. Results: The novel design showed the lowest MPS in veneer ceramics under most loading conditions. The only exception to this was the novel design with a 0.5-mm zirconia beam width under mesial horizontal load. Conclusions: Compared to constant thickness coping with or without extended collars, the novel coping design reduced MPS in veneer ceramics; however, narrow zirconia beams should be avoided to prevent elevations in MPS in veneer ceramic layers.

Cohesive (chipping) or adhesive (delamination) failures of veneering ceramics have been reported as frequent events since yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) was introduced as a framework material for fixed dental prostheses. Furthermore, the failure rates of Y-TZP are higher than those of metal ceramics.1-3 Laboratory studies have suggested the following causes for failures: surface flaws in the veneer ceramic,4 insufficient bond strength between the veneer ceramic and the zirconia coping,5 residual stress due to mismatched coefficients of thermal expansion between the veneer ceramic and the zirconia coping,6 inadequate framework support for the veneer ceramic,7 and accumulated residual stress during the cooling procedures for the veneer ceramic.8-10 According to experimental studies, the reliability of zirconia restorations can be improved by including extended proximal and lingual zirconia collars in the coping design.11,12 Lorenzoni 534

et al also reported that zirconia ceramic with this modified coping provided a structural reliability comparable to that of metal ceramic.13 Thus, changing framework designs can be a practical approach to prevent veneer ceramic chipping; however, the modified design was also associated with more extensive fractures in the veneer layers with zirconia coping support, whereas chipping fractures were limited in veneer layers supported by conventional coping designs.11 Moreover, clinicians still encountered chipping fractures with the modified coping design that had extended lingual and proximal zirconia collars. In addition to the extended collar design two other recent modification are a coping design with a zirconia occlusal surface and a complete zirconia (monolithic) restoration.14 No chipping fractures are expected with either design. When zirconia is used on the occlusal surface, clinicians should finish and polish the zirconia surface completely after any occlusal

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Figure 1 (A) CAD image of the novel coping design in occlusal view. Blue; zirconia coping, yellow; veneer ceramic. (B) Novel coping before veneering. (C) Completed zirconia restoration with novel coping design. A slight color difference is noticeable between veneer ceramic and zirconia coping; however, the final esthetic result was not compromised due to the limited use of zirconia.

Figure 2 (A) CAD image of novel coping design in proximal view. Proximal contact areas are covered with veneer ceramic for ease of adjustment and polishing. (B) Proximal view of a completed zirconia restoration with the novel coping design. Veneer ceramic was used on both mesial and distal proximal contact areas. A definite color difference can be seen between veneer ceramic and zirconia coping.

adjustment. Well-polished zirconia resulted in less enamel wear than feldspathic porcelain;15,16 however, rough zirconia surfaces can cause a significant amount of enamel wear.17 Although more translucent blocks were introduced to enhance the esthetic results, monolithic zirconia crowns still have limited esthetic outcomes compared to veneered zirconia restorations. Based on previous clinical and laboratory studies, the authors concluded that the following are requirements for a desirable zirconia coping design: 1. It should protect veneer ceramic (proximal, lingual, and occlusal). 2. It should provide good esthetic results. Optimal esthetics can be easily achieved by using veneer ceramic over the zirconia coping. Veneer ceramic should be applied over the zirconia coping on any areas that can be seen by others (i.e., occlusal, buccal, and mesial surfaces of the restoration). 3. It should be easy to adjust and polish. Most clinical adjustments are made on the proximal and occlusal surfaces. Therefore, these surfaces should be veneered. 4. It should be simple and economical. The laboratory and clinical procedures should be easy for both technicians and clinicians and should not require additional expensive equipment. To overcome the limitations of conventional zirconia coping designs, the present study suggested a novel coping design that could prevent or minimize ceramic chipping fracture. The purpose of the current study was to evaluate and compare the

amount of maximum first principal stress (MPS) in ceramic crown restorations supported by different Y-TZP coping designs under a simulated load. A nonlinear, 3D finite element analysis (FEA) was used to measure the amount of stress in the veneering ceramic. The null hypothesis tested was that there was no difference between the maximum first principal stresses in the veneer ceramic supported by three coping designs.

Materials and methods A novel coping design

A novel coping design was proposed based on the requirements of the abovementioned desirable coping designs. To minimize lingual or proximal chipping of the veneer ceramics, three widths of “zirconia beam” were provided on all surfaces except for the buccal surface (Fig 1). The novel coping was veneered with feldspathic ceramic to provide easy adjustment and polishing while ensuring excellent esthetics. Proximal cutbacks were also designed to build up the veneer ceramic on the proximal contact area for ease of adjustment and polishing (Fig 2). For better esthetics, no zirconia was exposed on the buccal surface, and for ease of fabrication, no veneer ceramic was used on the lingual surface. Building 3D FEA models

Based on the average dimensions of natural teeth, a 3D FEA model of a mandibular second molar was constructed using CAD software (Solidworks 2012; Dassault Syst`emes SolidWorks Corp., Lowell, MA). For the convenience of modeling,

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Figure 3 Design EC coping has extended lingual and proximal zirconia collar to provide better supportive configuration for veneer ceramic.

Figure 5 Cross-sectional view of design EC. Teal represents veneer ceramic, purple for coping, yellow for cement layer. The thickest part of the veneer ceramic was 1.5 mm, and the thinnest part was 0.8 mm (all in mm). Table 1 Mechanical properties of FEA materials used Material IPS e.max Ceram Zirconia Resin cement Dentin

Young’s modulus (GPa)

Poisson’s ratio

64 210 6.48 18.6

0.23 0.3 0.27 0.3

All the values were based on work by Lu et al.22

Figure 4 Design ZB has a complete lingual zirconia contour with proximal and lingual zirconia beams of three widths. Proximal contact areas were covered with veneer ceramic. Design ZB also divided into three subgroups. ZB05, ZB08, and ZB10, had 0.5-, 0.8-, and 1.0-mm-wide lingual and proximal zirconia-encompassing beams, respectively.

the anatomic shape of the mandibular second molar was simplified. The cervical line of the crown was on a single plane, perpendicular to the long axis of the clinical crown. An abutment was modeled by applying a 16° convergence angle between the facing axial walls (buccal to lingual, mesial to distal). The height of the abutment was set as 4.5 mm with a flat occlusal surface. Three kinds of coping designs were modeled in the current study. The first coping design had a 0.5-mm constant thickness of coping over the abutment model (constant thickness design, design CT). For the second coping design, proximal and lingual collars, 2 mm high, were added to design CT to provide an additional supporting configuration for the veneer ceramic (extended collar design, design EC, Fig 3). The novel coping design had a complete zirconia contour at the lingual and marginal ridge areas to encompass and protect veneer ceramic (zirconia beam design, design ZB, Fig 4). The novel coping design had three subdesigns based on the width of the zirconia beam. To model design ZB, virtual occlusal 536

reduction was performed except for the lingual and proximal line-angle area, leaving the zirconia beam with three different widths (0.5, 0.8, and 1.0 mm) on the area. Proximal cutbacks were also made to provide room for veneer ceramic, but for ease of fabrication, the entire lingual contour was zirconia. The cement gaps differed according to the measurement location. In general, the marginal gaps were the smallest, the mid-axial gaps were greater, and the occlusal gaps were the largest.18,19 However, for the convenience of model fabrications, the cement space was set to an even thickness of 100 µm based on the average mid-axial gap values from previous studies.20,21 Figure 5 represents a cross-sectional view of design EC, and Table 1 shows the mechanical properties of materials used in each FEA model. The following assumptions were made for the 3D FEA model: (1) all solids are homogeneous, isotropic, and linear elastic; (2) no slip was permitted between the components (perfect bonding); (3) there were no flaws in any component; and (4) the abutment is fully constrained. Simulation of occlusal loading

Four loading conditions were simulated over a 1-mm-diameter circle area on the marginal ridge of the distolingual cusp for each FEA model. The conditions were classified as purely vertical or a combination of vertical and horizontal. The vertical direction of loading (V) was not perpendicular to the load-receiving surface but parallel to the long axis of the

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Table 2 MPS in veneer ceramics and zirconia copings under different coping designs and loading conditions (MPa) Coping design Design CT

Design EC

Design ZB05

Figure 6 Purely vertical load of 30 N was applied on the design ZB10 model. The applied load was parallel to the long axis of the clinical crown, not perpendicular to the load-receiving surface. Therefore, a horizontal vector of load was produced at the buccal slope of the distolingual cusp even as the pure vertical load was applied. The majority of the stresses were noted at the lingual side of the load receiving area in the zirconia beam.

anatomic crown to produce horizontal vector components. The additional horizontal loads were in the mesial (Hm; horizontalmesial) and lingual (Hl; horizontal-lingual) directions. Therefore, the loading conditions were (1) purely vertical (V); (2) purely vertical combined with mesial horizontal (VHm); (3) purely vertical combined with lingual horizontal (VHl); or (4) purely vertical combined with mesial and lingual horizontal (VHml). A previous study evaluated the estimated stress required to cause chipping failures by examining clinically failed restorations, and the estimated stress was 26.8 ± 9 MPa.4 Therefore, the amount of loading for each vertical and horizontal was set for 30 N for this study. Under each loading condition, the first principal stresses were computed for both the veneer ceramic and the zirconia coping.

Results Table 2 shows the MPSs introduced in the veneer ceramic and zirconia coping under all loading conditions, and Figure 6 represents the first principal stress in design ZB10 when a purely vertical load (V) was applied. Under all the loading conditions, the novel design resulted in a smaller MPS in the veneer ceramic than both the constant thickness coping (design CT) and the coping with extended collars (design EC), unless the mesial horizontal load was combined with any other loading conditions. Under such conditions (VHm and VHml), design ZB05 resulted in the greatest MPS in both the veneer ceramic and the zirconia coping. Considering the MPS in the zirconia coping, all three novel designs resulted in a six- to eight-fold greater MPS than design CT or EC. By increasing the zirconia beam width from 0.5 mm to 1.0 mm, the MPS was decreased in the veneer ceramic as well as in the zirconia coping when the mesial horizontal load was combined with other loading conditions (VHm and VHml);

ZB08

ZB10

Load direction

Veneer

Coping

V VHm VHl VHml V VHm VHl VHml

34.9 27.6 61.4 55.6 35.2 29.7 61.5 56.2

5.1 8.2 9.4 13.6 5.1 8.3 9.2 12.6

V VHm VHl VHml V VHm VHl VHml V VHm VHl VHml

30.0 35.9 44.7 72.3 31.1 26.1 40.1 54.6 24.5 8.2 43.4 47.1

31.1 66.8 33.5 99.6 41.6 49.3 53.9 94.1 33.5 39.7 60.3 71.7

V: 30 N vertical load only; VHm: 30 N vertical + 30 N mesial direction horizontal load; VHl: 30 N vertical + 30 N lingual horizontal load; VHml: 30 N vertical + 30 N mesial direction horizontal load + 30 N lingual direction horizontal load. ZB05: novel design with 0.5 mm zirconia beam width; ZB08: novel design with 0.8 mm zirconia beam width; ZB10: novel design with 1.0 mm zirconia beam width.

however, the MPS in the zirconia coping increased when the vertical load was combined only with the lingual horizontal load (VHl). Under this condition (VHl), the MPS in the veneer ceramic was not significantly influenced by the width of the zirconia beam.

Discussion Chipping failure is one of the most frequent technical complications with zirconia restorations.23,24 Experimental studies have suggested that inadequate support for veneer ceramic could potentially lead to high chipping rates. When the coping designs were modified to allow uniform thickness of veneer ceramic or when they had extended zirconia collars, improved reliability was reported.11,25 In addition to the experimental studies, a 3-year clinical study reported a comparable chipping rate of zirconia restorations to metal ceramics. In that study, zirconia restorations received modified framework designs and firing/cooling cycles for veneer ceramic.26 Therefore, modification of coping design can be used as an effective approach to minimize the chipping failure of zirconia-ceramic restorations. The novel coping design (design ZB) used in the current study placed zirconia at strategically chosen areas, encompassing the lingual cusps as well as the marginal ridges, to prevent chipping failures. The lingual cusps of mandibular molars were described as a common location for chipping failures,1,27,28 and previous studies reported that nonfunctional molar cusps

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were more vulnerable to fracture because they were narrower and smaller than functional cusps.11,29,30 Providing a zirconia framework on the vulnerable areas can be an effective design without compromising esthetics. The results of the current study are promising with regard to the MPS in veneer ceramic. When the width of the zirconia beam was set to 1 mm, the MPS decreased to approximately 30 to 85% of those noted in the constant thickness coping design (design CT) or the constant thickness coping design with extended lingual and proximal collars (design EC) in all loading conditions. The stress decrease was most significant when the simulated load was purely vertical combined with mesial horizontal direction (8.2 MPa for design ZB10 vs. 29.7 MPa for design EC). A previous study compared the MPS between a modified and a conventional design when occlusal loads were simulated on the tip of the mesiolingual cusp. In that study, the modified design had constant thickness occlusal veneer ceramic with lingual and proximal zirconia collars, whereas the conventional design had a 0.5-mm constant coping thickness.12 The modified design resulted in a 37% decreased peak stress in the veneer ceramic than in the conventional design when the loading direction was purely vertical to the load-receiving surface; however, when a horizontal load was combined with the purely vertical load, the modified design resulted in a greater MPS than the conventional design in both the veneer ceramic and the zirconia coping. In the current study, when the simulated load was either purely vertical or vertical combined with horizontal directions, designs ZB08 and ZB10 resulted in a smaller MPS in the veneer ceramic than designs CT and EC. Therefore, the novel design can be a safer choice to decrease the MPS in the veneer ceramic, regardless of loading conditions. The current study also presented the effect of the zirconia beam width on the MPS. By increasing the width of zirconia beam from 0.5 mm to 1.0 mm, the MPS was decreased in every loading condition; however, the 0.5-mm zirconia beam model resulted in greater MPS than designs CT or EC when mesial-horizontal loading was combined with any other loading direction. The maximum stress was noted at the veneer/zirconia interface next to the loading area. Therefore, a thin zirconia wall design should be avoided to prevent stress concentration in the veneer ceramics. The new coping design also has a disadvantage. It has a complicated configuration compared with conventional coping designs, and therefore additional time on the CAD process is required. Most current CAD software can design anatomically shaped copings by controlling the thickness of the veneer ceramic, but cannot mathematically control the width of the zirconia beam. Therefore, CAD operators have to determine the width arbitrarily. Because a thin zirconia beam design could increase the MPS in the veneer ceramic, the operator must be careful not to produce a narrow zirconia beam. The present study compared the MPS in zirconia restorations with different coping designs, and the novel coping design showed promising results. Long-term clinical studies should be performed to confirm the results of the current study. 538

Conclusion Compared to conventional zirconia coping designs, the novel coping design reduced the maximum first principal stress in the veneer ceramic by encompassing the vulnerable veneer area with a zirconia framework. A narrow zirconia beam should be avoided to prevent stress concentration.

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A novel coping design to decrease maximum principal stress in zirconia ceramic restorations.

The objective of this study was to evaluate the influence of coping design modifications on maximum first principal stress (MPS) in a mathematical zir...
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