Varied simulation-based stress analyses on zirconia all-ceramic crowns

LILIANA POROJAN1, FLORIN TOPALĂ2, SORIN POROJAN3

1Department of Dental Prostheses Technology, 2Department of Dental Prosthodontics, 3Department of

Oral Rehabilitation, School of Dentistry, “V. Babeş” University of Medicine and Pharmacy

9 Revolutiei 1989 Blv., 300041 Timişoara

ROMANIA

Abstract: The development of high-strength ceramics and its use in posterior areas has been a field of constant

investigation. The performance of all-ceramic molar crowns fabricated with new CAD/CAM techniques is a

subject of interest. The studies available in literature focused on the analysis of all-ceramic restorations failures,

investigating several parameters involved on the tooth structure. The goal of this study was to investigate the

stress distributions of zirconia - all ceramic crowns, under loads using varied stress distribution analyses. A

static structural analysis was performed to calculate the stress distribution using the computer-aided

engineering software. Equivalent stresses were recorded in the tooth structures and in the restoration for all

these designs. Since ceramic materials exhibit brittle behavior, the first principal stress criterion was adopted to

compare the stress values and distribution with those obtained for the first simulations. Under the same loading

conditions, the stress distribution patterns for the zirconia all-ceramic crown using differential stress analyses

exhibited similarities. Only the values are lower for the maximal principal stresses. The present study suggests

that varied simulation methods are promising to assess the biomechanical behaviour of all-ceramic systems.

Key-Words: zirconia all-ceramic crown, molar, simulation methods, stresses.

1 Introduction

The trend for development of high-strength

ceramics and its use in posterior areas has been a

field of constant investigation [1,2]. Yttria-

Stabilized Tetragonal Zirconia Polycrystals (Y-TZP)

was introduced as a core ceramic in attempt to

reduce restoration bulk fracture. Its high mechanical

properties have resulted in successful use of Y-TZP

as a core ceramic in short- and medium-term

clinical studies, where framework fractures were

seldom reported [3,4]. While Y-TZP provides

strength, the clinical success of these restorations

has been hampered by fractures within the

veneering porcelain. With regard to chipping and/or

delaminating of the veneer, the performance of all-

ceramic molar crowns fabricated with new

CAD/CAM techniques is a subject of interest [5].

This material is indicated for posterior crowns but

due to its high opacity requires veneering with glass

ceramics. High strength zirconia core can be

manufactured through CAD/CAM technology and

subsequently veneered conventionally. According to

in vivo observation, the clinical survival of zirconia-

based restorations are comparable to metal–ceramic

restorations [6]. In recent years it has become

obvious that cohesive and adhesive failures of

zirconia - ceramics veneered restorations often

occur [7,8]. The studies available in literature

focused on the analysis of all-ceramic restorations

failures, investigating several parameters involved

on the tooth structure - restoration complex, in order

to improve clinical performances. Some of the

parameters, like the framework design, are

technique-sensitive and during the manufacturing of

the restorations can easily influence the failure rates

and fracture modes of final restorations, similarly to

metal ceramic crowns. In order to predict the

clinical behavior of porcelain layered zirconia

crowns, some studies evaluating fracture resistance

have been performed [9,10]. The focus was on

framework design and how different coping designs

may influence possible clinical failures. They show

that the coping design affected the fracture load and

the mode of fracture of zirconia all-ceramic crown

[11].

Simulation-based medicine and the development of

complex computer models of biological structures is

becoming ubiquitous for advancing biomedical

engineering and clinical research. Finite element

analysis (FEA) has been widely used in the last few

decades to understand and predict biomechanical

phenomena. Modeling and simulation approaches in

MOLECULAR SCIENCES AND APPLICATIONS

DOI: 10.37394/232023.2022.2.1

Liliana Porojan, Florin Topală, Sorin Porojan

E-ISSN: 2732-9992

Volume 2, 2022

biomechanics are highly interdisciplinary, involving

novice and skilled developers in all areas of

biomedical engineering and biology. While recent

advances in model development and simulation

platforms offer a wide range of tools to

investigators, the decision making process during

modeling and simulation has become more opaque

[12]. Establishing guidelines for model development

and simulation, particularly for complex structures

and different materials poses a challenge in the field

of dental technology.

2 Purpose

The goal of this study was to investigate the stress

distributions of zirconia - all ceramic crowns, under

loads using varied stress distribution analyses.

3 Materials and Method

For the experimental analyses a maxillary right first

molar was chosen in order to simulate the

biomechanical behaviour of the teeth restored with

zirconia - all ceramic crowns. The prepared die was

designed with a chamfer finishing line and an 6°

occlusal convergence angle of the axial walls was

chosen for the preparation.

A geometric model of a bilayer crown with a

uniform thickness of 0.5 mm for the framework and

ceramic veneer designed to occupy the space

between the original tooth form and the prepared

tooth form was developed. At first a nonparametric

modeling software (Blender 2.57b) was used to

obtain the 3D tooth shape. The collected data were

used to construct three dimensional models using

Rhinoceros (McNeel North America) NURBS

(Nonuniform Rational B-Splines) modeling

program. In order to obtain a 3D solid model of the

tooth, a surface following the cervical line was

achieved, to close the surfaces.

The geometric models were imported in the finite

element analysis software ANSYS, meshed and

finite element calculations were carried out.

In order to simulate the stress distribution, the

Young’s module and Poisson’s ratios were

introduced: Young’s modulus (GPa) 18 for dentin,

64 for veneering ceramics, and 205 for zirconia and

Poisson’s ratio 0.27 for dentin, 0.21 for veneering

ceramics, and 0.31 for zirconia.

To simulate physiological mastication behavior five

loading areas were defined on the occlusal surface.

Each defined loading area had a diameter of 0.5

mm. A total force of 250 N was allocated to these

areas as pressure load normal to the surfaces in each

point. The bottom of the abutment teeth model was

fully constrained for all simulations.

A static structural analysis was performed to

calculate the stress distribution using the computer-

aided engineering software. Equivalent stresses

were recorded in the tooth structures and in the

restoration for all these designs. Since ceramic

materials exhibit brittle behavior, the first principal

stress criterion was adopted to compare the stress

values and distribution with those obtained for the

first simulations.

3 Results and Discussions

Stresses were calculated for all compunds of

zirconia all-ceramic crown and teeth structures (Fig.

1-3, Table 1).

Fig. 1. Equivalent stress and principal stress

distribution in the ceramic veneer.

Fig. 2. Equivalent stress and principal stress

distribution in the zirconia framework.

MOLECULAR SCIENCES AND APPLICATIONS

DOI: 10.37394/232023.2022.2.1

Liliana Porojan, Florin Topală, Sorin Porojan

E-ISSN: 2732-9992

Volume 2, 2022

Fig. 3. Equivalent stress and principal stress

distribution in the dentin.

Table 1. Maximal equivalent stress and principal

stress in zirconia all-ceramic crown compounds and

dentin.

Stress values in

the structures

Maximal

equivalent

stress [MPa]

Maximal

principal stress

[MPa]

Ceramic veneer

206.74

45.89

Zirconia

framework

47.79

31.59

Dentin

16.96

2.70

In both cases the values were higher in the veneers.

In the veneers stresses were distributed around the

contact areas with the antagonists. The values of the

maximal stresses in the frameworks are low and

distributed occlusal and in the cervical areas buccal

and oral. In dentin stresses are concentrated around

the marginal preparation line. Within the studied

range stresses are not influenced by the veneer

thickness in case of an anatomical design of the

framework.

Under the same loading conditions, the stress

distribution patterns for the zirconia all-ceramic

crown using differential stress analyses exhibited

similarities. Only the values are lower for the

maximal principal stresses.

According to different authors, the material will fail

when the values of the equivalent stresses exceed

the tensile strength of the material [13].

Factorial analysis performed studies showed that

material and thickness of prosthetic crowns are of

primary importance in stress magnitude. The higher

the tensile strength of crown material, the thinner

can be the crown’s walls [14].

With the increasing number of FEA studies, FEA

practice in biomechanics continues to pose a

challenge for model development, sharing and

reporting. In FEA, model definitions and

development procedures are tightly coupled to the

simulation method and the solver capabilities. FEA

software commonly relies on embedded

mathematical models of physical phenomena, e.g.,

solid mechanics. In many cases, decisions made

during model development depend on the specific

solver capabilities [12].

Because most FEA share common features during

model development and simulation process, it is

possible to compile parameters for reporting items

that may be important for model reproducibility and

may help the scientific community to assess the

overall quality, scientific rigor, and utility of the

model [12].

4 Conclusion

Within the limitations of the present study, the

following conclusions can be drawn:

1. The present study suggests that varied

simulation methods are promising to assess the

biomechanical behaviour of all-ceramic

systems.

2. FEA results can be used in rebuilding the

design guidelines in CAD/CAM systems for

zirconia all-ceramic restorations.

5 Acknowledgements

This work was supported by a grant of the

Romanian National Authority for Scientific

Research and Innovation CNCS-UEFISCDI, project

number PN-II-RU-TE-2014-4-0476.

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Liliana Porojan, Florin Topală, Sorin Porojan

E-ISSN: 2732-9992

Volume 2, 2022

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MOLECULAR SCIENCES AND APPLICATIONS

DOI: 10.37394/232023.2022.2.1

Liliana Porojan, Florin Topală, Sorin Porojan

E-ISSN: 2732-9992

Volume 2, 2022