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Finite Element Analysis of Molars Restored with Complete Cast Crowns SORIN POROJAN, LILIANA SANDU, FLORIN TOPALĂ School of Dentistry “V. Babeş” University of Medicine and Pharmacy 9 Revolutiei 1989 Blv., 300041 Timişoara ROMANIA [email protected] Abstract: Complete cast crowns are good alternatives for the restoration of damaged posterior teeth. They have the best longevity of all fixed restorations. Their incomplete fit remains a critical problem for dentists, leading many researchers to study this problem. Gold alloys complete crowns can prove to be extremely long-lived restorations because of the excellent properties of these alloys. The geometry of tooth preparation has been the subject of many debates without clear evidence that one type of tooth preparation or method of fabrication provides consistently superior marginal fit. Three margin designs may be used for complete cast crowns: shoulderless, chamfer, and shoulder. The objective of this study was to evaluate, by finite element analysis, the influence of different marginal geometries (shoulderless, chamfer, shoulder) on the stress distribution in teeth prepared for cast metal crowns and in the gold alloy restorations. A 3D model of a molar was created: intact teeth, unrestored teeth different marginal geometries: shoulderless, with chamfer, with shoulder preparations; the same tooth restored full cast metal crowns. These were exported in Ansys finite element analysis software for structural simulations. Maximal equivalent stresses were recorded in the tooth structures and in the restoration for all preparation types. In all cases the values were higher in the crowns. Chamfer preparation is the recommended preparation tooth design for cast gold alloy crowns, from biomechanical point of view. It is followed by the shoulder preparation. Consideration should be given to the designs also from prophylactic and biological points of view. Chamfer margins are also favorable from this point of view. Key-Words: molar, complete cast crown, gold alloy, finite element analysis, accuracy, preparation design. 1 Introduction Even if the esthetic factors may limit their applications, complete cast crowns are good alternatives for the restoration of damaged posterior teeth. They have the best longevity of all fixed restorations [1, 2]. The incomplete fit of full cast crown restorations remains a critical problem for dentists, leading many researchers to study this problem. Marginal and internal accuracy of fit is valued as one of the most important criteria for the clinical quality and success of complete crowns. The geometry of tooth preparation has been the subject of many debates without clear evidence that one type of tooth preparation or method of fabrication provides consistently superior marginal fit [3]. Gold alloys complete crowns can prove to be extremely long-lived restorations because of the excellent properties of these alloys. A well-designed preparation has a smooth and even margin. Rough, irregular margins substantially reduce the adaptation of the restoration. The cross- section configuration of the margin has been the subject of much analysis and debate. The minimization of crown marginal gaps is an important goal in prosthodontics [1, 4]. The geometry of tooth preparation has been the subject of many debates without clear evidence that one type of tooth preparation or method of fabrication provides consistently superior marginal fit [2, 5]. Three margin designs may be used for complete cast crowns: shoulderless, chamfer, and shoulder. Although they are conservative for tooth structure, shoulderless crown preparations should be avoided because they fail to provide adequate bulk at the margins. Overcontoured restorations often result from shoulderless preparations. Under most circumstances, these kinds of margins are unacceptable. A chamfer margin is particularly suitable for cast metal crowns. It is distinct and easily identified, provides space for adequate bulk of material, although care is needed to avoid leaving a ledge of unsupported enamel. Shoulder margins always offer space for the crown material. It should form a 90 degree angle with the unprepared tooth surface [3]. Mathematical Methods and Techniques in Engineering and Environmental Science ISBN: 978-1-61804-046-6 407

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Finite Element Analysis of Molars Restored with Complete Cast

Crowns

SORIN POROJAN, LILIANA SANDU, FLORIN TOPALĂ

School of Dentistry

“V. Babeş” University of Medicine and Pharmacy

9 Revolutiei 1989 Blv., 300041 Timişoara ROMANIA

[email protected]

Abstract: Complete cast crowns are good alternatives for the restoration of damaged posterior teeth. They have

the best longevity of all fixed restorations. Their incomplete fit remains a critical problem for dentists, leading

many researchers to study this problem. Gold alloys complete crowns can prove to be extremely long-lived

restorations because of the excellent properties of these alloys. The geometry of tooth preparation has been the

subject of many debates without clear evidence that one type of tooth preparation or method of fabrication

provides consistently superior marginal fit. Three margin designs may be used for complete cast crowns:

shoulderless, chamfer, and shoulder. The objective of this study was to evaluate, by finite element analysis, the

influence of different marginal geometries (shoulderless, chamfer, shoulder) on the stress distribution in teeth

prepared for cast metal crowns and in the gold alloy restorations. A 3D model of a molar was created: intact

teeth, unrestored teeth different marginal geometries: shoulderless, with chamfer, with shoulder preparations;

the same tooth restored full cast metal crowns. These were exported in Ansys finite element analysis software for structural simulations. Maximal equivalent stresses were recorded in the tooth structures and in the

restoration for all preparation types. In all cases the values were higher in the crowns. Chamfer preparation is

the recommended preparation tooth design for cast gold alloy crowns, from biomechanical point of view. It is

followed by the shoulder preparation. Consideration should be given to the designs also from prophylactic and

biological points of view. Chamfer margins are also favorable from this point of view.

Key-Words: molar, complete cast crown, gold alloy, finite element analysis, accuracy, preparation design.

1 Introduction Even if the esthetic factors may limit their

applications, complete cast crowns are good

alternatives for the restoration of damaged posterior

teeth. They have the best longevity of all fixed

restorations [1, 2]. The incomplete fit of full cast

crown restorations remains a critical problem for

dentists, leading many researchers to study this

problem. Marginal and internal accuracy of fit is

valued as one of the most important criteria for the

clinical quality and success of complete crowns. The

geometry of tooth preparation has been the subject

of many debates without clear evidence that one

type of tooth preparation or method of fabrication

provides consistently superior marginal fit [3].

Gold alloys complete crowns can prove to be

extremely long-lived restorations because of the excellent properties of these alloys.

A well-designed preparation has a smooth and even

margin. Rough, irregular margins substantially

reduce the adaptation of the restoration. The cross-

section configuration of the margin has been the

subject of much analysis and debate. The

minimization of crown marginal gaps is an

important goal in prosthodontics [1, 4]. The

geometry of tooth preparation has been the subject

of many debates without clear evidence that one

type of tooth preparation or method of fabrication

provides consistently superior marginal fit [2, 5].

Three margin designs may be used for complete cast

crowns: shoulderless, chamfer, and shoulder.

Although they are conservative for tooth structure,

shoulderless crown preparations should be avoided

because they fail to provide adequate bulk at the

margins. Overcontoured restorations often result

from shoulderless preparations. Under most

circumstances, these kinds of margins are

unacceptable. A chamfer margin is particularly

suitable for cast metal crowns. It is distinct and

easily identified, provides space for adequate bulk

of material, although care is needed to avoid leaving

a ledge of unsupported enamel. Shoulder margins

always offer space for the crown material. It should

form a 90 degree angle with the unprepared tooth

surface [3].

Mathematical Methods and Techniques in Engineering and Environmental Science

ISBN: 978-1-61804-046-6 407

Traditional tooth preparation margin designs are still

advised by most manufacturers for indirect

restorations [6-10].

The shoulder preparation emerged as the

recommended preparation design from both

mechanical and periodontal points of view. As for a

less invasive preparation design, the slight chamfer

preparation would be the recommended option [11].

2 Purpose The objective of this study was to evaluate, by finite

element analysis, the influence of different marginal

geometries (shoulderless, chamfer, shoulder) on the

stress distribution in teeth prepared for cast metal

crowns and in the gold alloy restorations.

3 Materials and Method For the experimental analysis, a 3D model of a

molar was created: intact teeth, unrestored teeth

different marginal geometries: shoulderless, with

chamfer, with shoulder preparations; the same tooth restored with full cast metal crowns. The geometry

of the intact tooth were obtained by 3D scanning

using a manufactured device (Fig. 1). The

nonparametric modeling software (Blender 2.57b)

was used to obtain the shape of the teeth structures

(Fig. 2).

Fig. 1. Geometry of the intact molar.

Fig. 2. Modeling of the teeth structures shape.

The collected data were used to construct three

dimensional models using Rhinoceros (McNeel

North America) NURBS (Nonuniform Rational B-

Splines) modeling program (Fig. 3).

Fig. 3. 3D model of the molar after NURBS

modeling.

The tooth preparations: shoulderless, chamfer,

shoulder were designed (Fig. 4).

a

b

c

Fig. 4. Tooth preparations: a. shoulderless,

b. shoulder, c. chamfer.

Mathematical Methods and Techniques in Engineering and Environmental Science

ISBN: 978-1-61804-046-6 408

Complete cast crowns were designed for all

preparation types.

Models were exported in Ansys finite element

analysis software for structural simulations. An

occlusal load of 200 N was applied in 10 points,

according to the contact points with the antagonists

(Fig. 5).

The forces were applied perpendicular to the tooth

surface in each point. The mesh structure of the

solid 3D model was created using the computational

simulation of Ansys finite element analysis

software.

Fig. 5. Points selected for loading on the restored

molar.

Von Mises equivalent stresses were calculated and

their distribution was plotted graphically.

3 Results and Discussions Maximal equivalent stresses were recorded in the

tooth structures and in the restoration for all preparation types. In all cases the values were

higher in the crowns.

The stresses are distributed around the contact areas

with the antagonists (Fig. 6). The stress distribution

areas are larger for shoulderless preparation,

followed by shoulder and chamfer.

The values of the maximal equivalent stress in the

tooth structures were higher for the shoulder

preparations, but distributed on the occlusal surface.

Regarding the stress distribution for the other two

preparation designs, the areas are larger (Table 1,

Fig. 7).

For the last two a disadvantage is that more stresses

are present around the marginal area.

Margin design is a determining factor in

establishing the extent of the minimal preparation

for a cast metal crowns.

Table 1. Maximal Von Mises equivalent stress

values in the cast crowns and in the restored molars.

Preparation

type

Maximal Von Mises equivalent

stress [Pa]

total crown prepared

tooth

chamfer 1.21E+08 1.21E+08 1.31E+07

shoulder 1.12E+08 1.12E+08 1.59E+07

shoulderless 1.35E+08 1.35E+08 1.45E+07

a

b

c

Fig. 7. Von Mises equivalent stress in the complete

cast crown for different marginal designs:

a. chamfer, b. shoulder, c. shoulderless.

Mathematical Methods and Techniques in Engineering and Environmental Science

ISBN: 978-1-61804-046-6 409

a

b

c

Fig. 7. Von Mises equivalent stress in the restored

teeth with different marginal designs:

a. chamfer, b. shoulder, c. shoulderless.

4 Conclusion Within the limitations of the present study, the

following conclusions can be drawn:

1. In all cases the maximal equivalent stress

values were higher in the crowns. The stresses

are distributed around the contact areas with the

antagonists and are smaller for the chamfer

preparation.

2. Chamfer preparation is the recommended

preparation tooth design for cast gold alloy

crowns, from biomechanical point of view. It is

followed by the shoulder preparation.

3. In light of these results, consideration should be

given to the designs also from prophylactic and

biological points of view, with emphasis on

conserving tooth structure and preventing

preparation trauma. Chamfer margins are also

favorable from this point of view.

4 Acknowledgements This work was supported by CNCSIS-UEFISCSU,

project number PN II-RU TE_217/2010.

References:

[1] S. Rosenstiel, M. Land, J. Fujimoto,

Contemporary fixed prosthodontics, 3rd ed.

Mosby, St. Louis, 2001.

[2] N. De Jager, P. Pallav, A.J. Feilzer, The

influence of design parameters on the FEA-

determined stress distribution in CAD-CAM

produced all-ceramic dental crowns, Dent

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[3] A.B. Olivera, T. Saito, The Effect of Die

Spacer on Retention and Fitting of Complete

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[4] P.L. Tan, D.G. Gratton, A.M. Diaz-Arnold,

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Mathematical Methods and Techniques in Engineering and Environmental Science

ISBN: 978-1-61804-046-6 410