Finite element stress analysis of short-post core and over restorations prepared with different restorative materials.

Abstract

The present study was conducted to determine the effect on the distribution of stress with the use of short-post cores and over restorations composed of different materials. The restorative materials used were namely two different composite resin materials (Valux Plus and Tetric Flow), a polyacid-modified resin material (Dyract AP), and a woven polyethylene fiber combination (Ribbond Fiber + Bonding agent + Tetric Flow). Finite element analysis (FEA) was used to develop a model for the maxillary primary anterior teeth. A masticatory force of 100 N was applied at 148 degrees to the incisal edge of the palatal surface of the crown model. Stress distributions and stress values were compared using von Mises criteria. The tooth model was assumed to be isotropic, homogeneous, elastic, and asymmetrical. It was observed that the highest stress usually occurred in the cervical area of the tooth when Tetric Flow was used as the short-post core and over restoration material. The same maximum stress value was also obtained when Ribbond fiber + Tetric Flow material was used for the short-post core. The results of FEA showed that the mechanical properties and elastic modulus of the restorative material influenced the stresses generated in enamel, dentin, and restoration when short-post core restorations were loaded incisally. Resin-based restorative materials with higher elastic moduli were found to be unsuitable as short-post core materials in endodontically treated maxillary primary anterior teeth.

Full text

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Dental Materials Journal 2008; 27(4): 499－507
Original Paper

Finite Element Stress Analysis of Short-post Core and Over Restorations
Prepared with Different Restorative Materials

Taskin GURBUZ1, Fatih SENGUL2 and Ceyhan ALTUN3
1Department of Pedodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey
2Department of Pedodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey
3Department of Pedodontics, Center of Dental Sciences, Gulhane Medical Academy, Ankara, Turkey
Corresponding author, Taskin GURBUZ; E-mail: gurbuzt25@yahoo.com

The present study was conducted to determine the effect on the distribution of stress with the use of short-post cores and
over restorations composed of different materials. The restorative materials used were namely two different composite
resin materials (Valux Plus and Tetric Flow), a polyacid-modified resin material (Dyract AP), and a woven polyethylene
fiber combination (Ribbond Fiber + Bonding agent + Tetric Flow). Finite element analysis (FEA) was used to develop a
model for the maxillary primary anterior teeth. A masticatory force of 100 N was applied at 148° to the incisal edge of the
palatal surface of the crown model. Stress distributions and stress values were compared using von Mises criteria. The
tooth model was assumed to be isotropic, homogeneous, elastic, and asymmetrical. It was observed that the highest stress
usually occurred in the cervical area of the tooth when Tetric Flow was used as the short-post core and over restoration
material. The same maximum stress value was also obtained when Ribbond fiber + Tetric Flow material was used for the
short-post core. The results of FEA showed that the mechanical properties and elastic modulus of the restorative material
influenced the stresses generated in enamel, dentin, and restoration when short-post core restorations were loaded incisally.
Resin-based restorative materials with higher elastic moduli were found to be unsuitable as short-post core materials in
endodontically treated maxillary primary anterior teeth.
Key words: Finite element analysis, Short-post core, Caries

Received Mar 18, 2007: Accepted Jan 21, 2008
INTRODUCTION
In many populations, early childhood caries is a
common dental problem found in preschool-age
children. The maxillary primary anterior teeth are
generally the most severely involved, and lesions can
lead to total destruction of the crowns of the teeth1).
It is important to restore crowns that have been
destroyed by caries in order to preserve the integrity
of the primary dentition until the eruption of
permanent teeth2). The restoration of primary
incisors with extensive tissue loss due to early
childhood caries is often a difficult procedure and
presents a particular challenge to dentists. A
restorative technique that is able to provide efficient,
durable, and functional restoration and which is
simple to perform would most certainly improve the
management of patients with carious maxillary
primary incisors3). Such restorations would provide
adequate resistance to bite forces and retention to
tooth tissue4).
　　Several decades ago, severely carious maxillary
primary incisors were often extracted. However,
owing to increasing demand by parents, restorative
procedures such as stainless steel crowns, open-face
stainless steel crowns, pre-veneered stainless steel
crowns, polycarbonate crowns, and strip crowns
combined with resin composites have been used for
the restoration of primary teeth that have been
severely damaged by early childhood caries. On the
other hand, it has also been reported that such
crowns are difficult to use in the case of extensive
caries due to retention problem5-7). The results of
various studies have demonstrated that short-post
cores and over restorations can be used to treat such
patients3,7-9).
　　Initially, pins or orthodontic wires combined with
composite resin were used in the short-post core
technique prior to over restorations with polycarbon-
ate crowns6,10). With the advent of new adhesive
materials and techniques, different composite resin
short posts and fiber-reinforced composite resin short
posts have been available for the restoration of
severely decayed primary anterior teeth7,11,12).
　　Resin-based dental restorative materials are the
materials of first choice for many clinicians, mainly
because of their superior esthetic properties and the
ease of repair if the crown subsequently chips or
fractures. However, the success of restorations using
these materials is highly sensitive to the technique
used. Contamination of the site with moisture from
blood or saliva may interfere with the bonding, and
hemorrhage can alter the shade or color of the
material. Additionally, adequate tooth structure
must remain after removal of caries to ensure that
there is sufficient surface area for bonding4).

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Dent Mater J 2008; 27(4): 499－507500
　　The shrinkage associated with the polymeriza-
tion of resin composites generates stresses at the
tooth-restoration interface, and may lead to marginal
gap formation, microleakage, marginal discoloration,
and secondary caries13). In addition to considerations
such as the type of restorative material, these factors
may also influence stress formation: restorative
technique and cavity preparation; polymerization
shrinkage, elastic modulus, and flow of the resin
composite; adhesion of resin composite to the cavity
walls; and configuration factor of the restoration14,15).
Indeed, polymerization shrinkage and flow of the
resin composite have been found to be significant
determinants of gap formation
composite restorations15).
　　As a result of biting forces, restored teeth are
subjected to different levels of mechanical stress.
The durability of restored teeth largely depends on
these stresses; hence, extensive research has been
conducted to understand the underlying mechanisms
that cause and contribute to these stresses. Various
types of research have been performed on this issue,
including experimental studies that use mechanical
tests and experimental stress analyses16-18), in
addition to theoretical techniques such as three-
dimensional finite element analysis (FEA)19-21). FEA
is a popular numerical method used in stress
analysis. It demonstrates the internal stresses of a
system, and on that basis predictions about failure
can be made. The use of FEA in dental research has
around resin
been significantly refined during the last decade21-24).
　　Against this backdrop of mounting concerns
about severely damaged primary teeth in tandem
with advances in stress analysis tools, the aim of the
present study was to determine, using FEA, the
stress distributions in short-post cores and over
restorations made of two different composite resin
materials, a polyacid-modified resin material, and a
woven polyethylene-fiber combination material.
MATERIALS AND METHODS
Three-dimensional tooth model
An extracted, sound maxillary primary central
incisor with a fully formed root and an absence of
cracks, fractures, and caries was used for the
preparation of the three-dimensional tooth model.
Images of the tooth with 0.5 mm cross-sections were
acquired using a spiral CT scanner (Aquillion 16,
Toshiba, Tokyo, Japan). Contours of the enamel,
dentin, and pulp boundaries were determined based
on the CT images, and a three-dimensional model
was thus prepared using Pro/Engineer software (Fig.
1). The crown was excluded from the model from 1
mm above the dentino-enamel junction of the root.
Root canal paste, glass ionomer base material (0.5
mm average thickness), bonding agent (15 μm
average thickness), and a mushroom-shaped undercut
region (2.4 mm diameter, 1 mm length) at the level of
the root canal system were prepared. A short-post
Fig. 1 (a) Load applied to the incisal edge of three-dimensional tooth model meshes. (b) Three-
dimensional solid tooth model parts. (c) Three-dimensional bonding layer model.
(d) Three-dimensional post-core and over restoration model. (e) Three-dimensional post-
core model.

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Dent Mater J 2008; 27(4): 499－507501
core portion of the model (2 mm base diameter, 3.6
mm length) was also prepared, and the crown of the
original primary tooth was used to shape the over
restoration. Similarly, three-dimensional models of
the supporting tissues of the tooth were prepared
using Pro/Engineer software (Fig. 1).
Restorative materials used in this study
On the distributions of stress in different restorative
materials, they were investigated using FEA with a
software program, ANSYS 7.0 (Swanson Ansys Inc.,
Houston, PA, USA). Table 1 presents the materials
used in the study. They were namely a universal
hybrid resin-based composite (Valux Plus, 3M ESPE,
St. Paul, MN, USA), a flowable resin composite
(Tetric Flow, Ivoclar
Liechtenstein), a polyacid-modified resin composite
(Dyract AP, Dentsply/De Trey, Konstanz, Germany),
and a woven polyethylene fiber combination [Ribbond
Vivadent, Schaan,
Fiber (Ribbond Inc., Seattle, WA, USA) + Bonding
agent + Tetric Flow]. These were selected as test
materials because of their different elastic moduli.
Finite element stress analysis
Using finite element method, a load of 100 N was
applied to the short-post core and over restorations.
The model of the sound primary tooth (without post-
core restorations) was used as a control. A point load
at 148° to the inter-incisal angle of the primary
incisors was applied to the incisal edge of the three-
dimensional model to simulate ideal primary
occlusion (Fig. 1). The Young’s modulus and
Poisson’s ratio values of the tooth and supporting
tissues are given in Table 225-32).
　　In FEA, each element was assigned a predeter-
mined number of nodes. The elements were
connected to each other by means of their nodes.
Each model comprised 39,274 elements and 72,890
Material name ManufacturerBatch numberIngredients
Valux Plus 3M Dental Products
St Paul, MN, USA
5540SB Bis-GMA
TEGDMA
Tetric FlowIvoclar-Vivadent
AG, Schaan, Liechtenstein
D08465Bis-GMA, UDMA, TEGDMA, barium
glass, silica, titanium dioxide
Dyract APDentsply, De Trey
GmbH, Konstanz, Germany
0203001191UDMA, TCB resin, strontium-fluoro-
silicate glass, strontium fluoride, photo
initiators, stabilisers
Ribbond Fiber Ribbond Inc,
Seattle, WA, USA
9518UHMWPE
Bis-GMA: bisphenol-A-diglycidylmethacrylate; TEGDMA: triethylene glycol dimethacrylate; UDMA: urethane
dimethacrylate; TCB: a reaction product of butane tetracarboxylic acid and hydroxyethyl methacrylate; UHMWPE:
ultra high molecular weight polyethylene
Table 1 Materials used in this study
Materials
Mechanical properties
Poisson’s ratio
0.33
0.31
0.45
0.30
0.30
0.45
0.30
0.24
0.28
0.28
0.32
Young’s modulus(MPa)
80350
19890
Ref. No
25
25
26
27
27
28
29
30
31
31
32
Enamel
Dentine
Pulp
Spongiose bone
Cortical bone
Periodontal ligament
Glass-ionomer cement
Valux Plus
Tetric Flow
Dyract AP
Ribbond fiber + bonding agent+
Tetric Flow
2
490
14700
69
10800
19700
5300
10700
23600
Table 2 Mechanical properties of the materials and the tooth used in this study

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Dent Mater J 2008; 27(4): 499－507502
nodes.
　　The following assumptions were made with
regard to the properties of the materials studied:
(a) The model comprised
periodontal ligament, spongiose bone, cortical
bone, root canal filling material, base cement,
bonding agent, and the post-core and over
restoration materials.
(b) The materials were
homogeneous, isotropic, and linearly elastic.
(c) The effect of cementum was ignored because of
the thinness of the layer.
(d) The alveolar bone that supported the tooth
was assumed to be rigid at the sliced surface.
　　The stress values and patterns that occurred in
the tooth were evaluated according to the three-
dimensional criterion of Von Mises (σe), which
enamel, dentin,
assumed to be
always yields positive results, using the equation
below33):
　　　 1
　σe = 　 [(σ1－σ2)2 + (σ2－σ3)2 + (σ3－σ1)2]1/2,
　　　 2
where σ1, σ2, and σ3 represent the principal
stresses33).
RESULTS
Assessments were made based on the color patterns
in Figs 2－6, where warm colors denote higher
stresses. Table 3 presents the maximum von Mises
stress values obtained from the three-dimensional
tooth models prepared with different restorative
materials.
Restoration
materials
In toothIn short-post coreIn over restorationIn bonding agent
Valux Plus83.010.6100.046.4
Tetric Flow85.8 8.599.043.6
Dyract AP84.09.299.645.8
Ribbond fiber +83.120.199.766.8
bonding agent
+ Tetric Flow
Vital Tooth82.5
－－－
Table 3 Maximum von Mises stress values in the tooth and with different restorative materials (MPa)
Fig. 2 Distribution of von Mises stress within the bonding layer.

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Dent Mater J 2008; 27(4): 499－507503
　　As shown in Fig. 2 for the bonding layer, major
stress developed near the crown surface of the root
canal for all the materials. The highest stress value
at the surface of the bonding layer and the undercut
area was determined to be 66.8 MPa, which was
obtained when Ribbond Fiber was combined with the
Tetric Flow material. The lowest stress value, 43.6
MPa, was obtained when Tetric Flow restorative
material was used (Fig. 2). In the sound primary
tooth, stress was found to be distributed throughout
the enamel. Maximum value of von Mises stress was
found to be 82.5 MPa, and this occurred in the
cervical area. Among the experimental groups, the
lowest maximum von Mises stress value, 83.0 MPa,
was obtained near the dentino-enamel junction when
Valux Plus restorative material was used. However,
Fig. 3 Labial view of tooth showing distribution pattern of von Mises stress.
Fig. 4 Distribution of von Mises stress within dentin and enamel.