Post on 13-Jul-2018
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
lilianasandu@gmail.com
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.
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Mathematical Methods and Techniques in Engineering and Environmental Science
ISBN: 978-1-61804-046-6 410