effect of anti-rotation devices on biomechanical behaviour of teeth restored with cast...
DESCRIPTION
Aim: To test the hypothesis that the presence of an anti-rotation device (ARD) and its location can influence the biomechanical behavior of root filled teeth restored with cast post-and-cores and metallic crowns.Methodology Fifth two bovine incisor roots were selected and divided into four groups (n = 13): Nd- without ARD; Bd- buccal ARD; Ld- lingual ARD; BLd- buccal and lingual ARD. The specimens were restored with cast post-and-cores and metallic crowns. After a fatigue process (3 · 105 50 N), three strain gauges were attached on the buccal, lingual and proximal surfaces and the samples of each group (n = 3) were submitted to a 0–100 N load. Fracture resistance was assessed in a mechanical testing machine (n = 10). Strain values and fracture resistance data were analyzed by one-way anova and Turkey Honestly Significant Difference (HSD) test (a = 0.05). The failure mode was then evaluated under an optical stereomicroscope. Bi-dimensional models of each group were generated for finite element analysis (FEA) and analyzed using the von Mises criteria.TRANSCRIPT
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Effect of anti-rotation devices on biomechanicalbehaviour of teeth restored with cast post-and-cores
L. H. A. Raposo1, G. R. Silva1, P. C. F. Santos-Filho1, P. V. Soares1, P. B. F. Soares1, P. C.Simamoto-Junior2, A. J. Fernandes-Neto3 & C. J. Soares1
1Department of Operative Dentistry and Dental Materials, Biomechanics Group, Federal University of Uberlandia, Minas Gerais,
Brazil; 2Healthy Technical School, Federal University of Uberlandia, Minas Gerais, Brazil; and 3Department of Fixed Prosthodontics
and Dental Materials, Biomechanics Group, School of Dentistry, Federal University of Uberlandia, Minas Gerais, Brazil
Abstract
Raposo LHA, Silva GR, Santos-Filho PCF, Soares PV,
Soares PBF, Simamoto-Junior PC, Fernandes-Neto AJ,
Soares CJ. Effect of anti-rotation devices on biomechanical
behaviour of teeth restored with cast post-and-cores. Interna-
tional Endodontic Journal, 43, 681691, 2010.
Aim To test the hypothesis that the presence of an
anti-rotation device (ARD) and its location can influ-
ence the biomechanical behaviour of root filled teeth
restored with cast post-and-cores and metallic crowns.
Methodology Fifth two bovine incisor roots were
selected and divided into four groups (n = 13): Nd-
without ARD; Bd- buccal ARD; Ld- lingual ARD; BLd-
buccal and lingual ARD. The specimens were restored
with cast post-and-cores and metallic crowns. After a
fatigue process (3 105 50 N), three strain gaugeswere attached on the buccal, lingual and proximal
surfaces and the samples of each group (n = 3) were
submitted to a 0100 N load. Fracture resistance was
assessed in a mechanical testing machine (n = 10).
Strain values and fracture resistance data were anal-
ysed by one-way anova and Tukey Honestly Signifi-
cant Difference (HSD) test (a = 0.05). The failure modewas then evaluated under an optical stereomicroscope.
Bidimensional models of each group were generated for
finite element analysis (FEA) and analysed using the
von Mises criteria.
Results No significant difference in fracture resis-
tance values and fracture modes occurred between the
four groups. The BLd group had higher stress concen-
trations in the buccal dentine and higher strain values
on the proximal surfaces.
Conclusions The anti-rotation devices did not influ-
ence significantly the fracture resistance and fracture
mode. However, the stressstrain values were increased
when the anti-rotation device was prepared on the
buccal and lingual faces concomitantly.
Keywords: anti-rotation device, cast post-and-core,
finite element analysis, fracture mode, fracture resis-
tance, strain gauge test.
Received 20 October 2009; accepted 2 April 2010
Introduction
With the development of endodontic therapy, recovery
and maintenance of severely damaged teeth became
possible. However, there can be a major challenge in
restoring these teeth. Structural loss means that post-
and-cores are often required to maintain the restora-
tion in place (Papa et al. 1993, Morgano & Brackett
1999, Joshi et al. 2001). Root canal preparation results
in dentine removal and increases the risk of fracture
when compared to intact teeth, because the resistance
of root filled teeth is directly related to the amount and
quality of the remaining tooth tissue (Reeh et al. 1989,
Morgano & Brackett 1999, Heydecke et al. 2001). More-
over, the posts do not increase the tooth-restoration
Correspondence: Carlos Jose Soares, Faculdade de Odontologia
Universidade Federal de Uberlandia, Area de Dentstica e
Materiais Odontologicos, Av. Para, no 1720, Campus Umu-
arama, Bloco 2B, Sala 2B-24, CEP: 38405-902, Uberlandia,
Minas Gerais, Brazil (Tel.: +55 34 32182255; fax: +55 34
32182279; e-mail: [email protected]).
doi:10.1111/j.1365-2591.2010.01739.x
2010 International Endodontic Journal International Endodontic Journal, 43, 681691, 2010 681
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complex resistance (Sorensen & Martinoff 1984,
Trope et al. 1985, Assif & Gorfil 1994), providing
enough retention and stability only for the prosthesis or
coronal restorative material (Christensen 1996). In
addition, the geometry, extension and mechanical
properties of the post (Caputo & Hokama 1984, Sirimai
et al. 1999, Fokkinga et al. 2006) could have an effect
on dentine stress distribution, reducing the risk of root
fracture.
Posts with mechanical properties similar to dentine,
which may distribute homogenously the stresses
induced by occlusal forces, are favourable in reducing
the likelihood of tooth fracture (Caputo & Hokama
1987, Boschian Pest et al. 2002). However, in
extensive restorative procedures, materials that offer
longevity and avoid subsequent interventions are
commonly chosen by the clinicians. Cast post-and-
cores have been used routinely to restore root filled
teeth with moderate to severe destruction, especially
in single rooted teeth (Ross et al. 1991, Morgano &
Brackett 1999). On the other hand, some forces could
reach the tooth-restoration complex, generating stress
concentrations in this system when the tooth is
under function (Kumagai et al. 1999, Sirimai et al.
1999, Akkayan & Gulmez 2002, Torbjorner &
Fransson 2004, Genovese et al. 2005). These stresses
could be responsible for fractures and subsequent loss
of the tooth (Assif & Gorfil 1994, Pegoretti et al.
2002).
Internal boxes in the mesial and distal faces or in the
greatest bulk of dentine (Tjan & Miller 1984, Hem-
mings et al. 1991) during root canal preparation for
cast post-and-core have been proposed as a way of
dissipating stresses in the longitudinal axis of the tooth
and work as an anti-rotation device (ARD). Shilling-
burg et al. (1997) stated that these devices should be
performed only on the lingual face of incisors with
conical or cylindrical root canals and extensive coronal
destruction.
Destructive mechanical tests, such as fracture tests,
are important for biomechanical analysis of tooth and
restorative materials, as they enhance understanding of
the behaviour of teeth in high loading situations.
However, these tests have limited capacity to clarify the
stressstrain relationship in the tooth-restoration com-
plex (Soares et al. 2006, 2008c). The use of non-
destructive tests, such as strain gauge tests (Ross et al.
1991, Sakaguchi et al. 1991), and finite element
analysis (FEA) (Kishen et al. 2004, Romeed et al.
2004, Jacobsen et al. 2006) is more suitable for
understanding the failure characteristics of the restor-
ative procedures (Ausiello et al. 2001, Lin et al. 2001,
Magne & Belser 2003, Soares et al. 2008b). Several
studies have made comparative investigations using
only FEA (Lin et al. 2001, Pierrisnard et al. 2002,
Magne & Belser 2003, Romeed et al. 2004, Lanza et al.
2005, Jacobsen et al. 2006, Toksavul et al. 2006),
however, this methodology is more representative
when associated with destructive tests (Fennis et al.
2005), or with non-destructive assays such as the
strain gauge test (Palamara et al. 2002, Lertchirakarn
et al. 2003, Soares et al. 2008d).
The aim of this study was to assess ex vivo the
fracture resistance, the strain of the buccal, lingual and
proximal root dentine and the stress distribution of root
filled bovine incisors restored with cast post-and-cores.
The study tested the hypothesis that the presence of
ARDs and their location influences the biomechanical
behaviour of incisors restored with cast post-and-cores
and metallic crowns.
Materials and methods
Tooth selection, preparation and embedding
Fifty-two similar bovine incisor teeth were selected, 40
for fracture resistance and fracture mode evaluation
and 12 for strain measurement tests. External debris
were removed with hand scalers, and teeth were stored
in 0.2% thymol solution (Soares et al. 2007). Teeth of
similar size and shape were selected by crown dimen-
sions after measuring the buccolingual and mesiodistal
widths in millimetres, allowing a maximum deviation
of 10% from the average. The crowns of all the teeth
were sectioned horizontally to the long axis, 15 mm
from the apex, with a water-cooled diamond disk (No.
7020; KG Sorensen, Barueri, SP, Brazil). The roots were
divided into four groups (n = 13): Nd control without
ARD; Bd buccal ARD; Ld lingual ARD; BLd buccal
and lingual ARD.
Root canals were instrumented with size 50 master
apical files (K-files; Dentsply Maillefer, Ballaigues,
Switzerland) in association with 1.0% sodium hypo-
chlorite (Cloro Rio 1.0%; Sao Jose do Rio Preto, SP,
Brazil), filled with gutta-percha and calcium hydrox-
ide-based cement (Sealer 26; Dentsply, Petropolis, RJ,
Brazil). The post space was created initially with a
heated instrument and the residual gutta-percha
was then removed with Gattes-Gliden burs (2, 3, 4,
Dentsply Maillefer), standardizing the post space to
10 mm and preserving 5 mm of root filling at
the apex. Root canal walls were then enlarged with
Anti-rotation devices biomechanics Raposo et al.
International Endodontic Journal, 43, 681691, 2010 2010 International Endodontic Journal682
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a 1.5-mm-diameter bur (Largo Peeso Reamer, No. 5,
Dentsply Maillefer).
The selected roots were embedded in self-polymerizing
polystyrene resin (AM 190 Resin; AeroJet, Santo Amaro,
SP, Brazil) to a level 2 mm below the coronal margin.
The periodontal ligament was simulated, using a polye-
ther-based impression material (Impregum Soft; 3M-
ESPE, Saint Paul, MN, USA). To carry out this procedure
root surfaces were dipped into molten wax 2 mm
apically to the coronal surface, resulting in a 0.2 to
0.3mm-thick wax layer. A radiographic film with a
centralized circular hole was used to stabilize teeth for
the embedding procedure. This assembly was placed
with the crown faced down into a hole in a wooden
board leaving the root in a vertical position perpendic-
ular to the supporting radiographic film. Then, a plastic
cylinder (25 mm diameter) was placed around the root
and fixed in position with cyanoacrylate-based adhesive
(Super Bonder; Loctite, Itapev, SP, Brazil) and wax. The
resin was manipulated according to manufacturers
instructions and inserted into the cylinder. After resin
polymerization, the teeth were removed from the cylin-
der and the wax was removed from both root surface and
the cylinder. The impression material was placed into
the resin cylinders, thus the tooth was re-inserted and
the excess of polyether material was removed with a
scalpel blade (Soares et al. 2005).
Anti-rotation device, post-and-core and crown
restoration
The ARDs box preparations were performed in a cavity
preparation machine (Soares et al. 2008a) consisting of
a high-speed handpiece coupled to a mobile base. The
mobile base moves vertically and horizontally with
three precision micrometric heads (152-389; Mitutoyo,
Suzano, SP, Brazil), with a 0.002 mm level of accuracy.
For the preparations, a smooth tapered carbide bur (No.
170; KG Soresen), 1.6 mm diameter and 4.0 mm
depth, was used with its whole active point (Shilling-
burg et al. 1997), following each group designation:
Nd- without ARD; Bd- buccal ARD; Ld- lingual ARD;
BLd- buccal and lingual ARD.
For the cast post-and-core fabrication, pre-fabricated
polycarbonate patterns were used (Nucleojet; Angelus,
Londrina, PR, Brazil). Reline of patterns was carried out
using autopolymerizing acrylic resin in the individual
root canal (Duralay; Reliance Dental Mfg. Co., Worth,
IL, USA) until passive retention was achieved. Subse-
quently, the individual patterns were adjusted in each
respective specimen, standardizing their height at
6.0 mm. The patterns were invested, cast in copper-
aluminium alloy (Cu-Al alloy; Goldent, Sao Paulo, SP,
Brazil) and sandblasted with aluminium oxide particles
(50 lm) under two bars pressure for 10 s (Fig. 1). Priorto cementation, the root canals were cleaned with
distilled water and dried with absorbent paper points
(Dentsply Maillefer). The cast post-and-cores were
cemented with zinc-phosphate cement (Zinc Cement;
SS White, Rio de Janeiro, RJ, Brazil) under a constant
pressure of 50 N for 10 min.
Impressions of the coronal portion of the specimens
were taken with a 2-step technique, using a polyether
impression material (Impregum Soft; 3M-ESPE). After
1 h, the impressions were poured with a type IV dental
stone (Durone IV, Dentsply). A standard crown with a
lingual plateau 1.0 mm in thickness for load applica-
tion was constructed in composite resin (Filtek Z250;
3M-ESPE) and from which a laboratory silicone matrix
was produced (IQ 428 Rubber, Aerojet). Heated liquid
wax (Green wax; Kota Imports, Sao Paulo, SP, Brazil)
was inserted in this matrix, followed by one of the
individual stone casts, resulting in the formation of the
crown wax pattern. The patterns were invested and
cast in nickel-chromium alloy (Kromalit; Knebel, Porto
Alegre, RS, Brazil). The crowns were adjusted and then
cemented with zinc-phosphate cement (Zinc cement, SS
White), under a constant pressure of 50 N for 10 min.
To simulate the fatigue and mechanic deterioration of
the restorative materials (Huysmans et al. 1993, Isidor
et al. 1996, Mannocci et al. 1999, Reagan et al. 1999),
the specimens were submitted to a cyclical loading of
50 N directed at 135 to the lingual elevation of themetallic crowns. At a frequency of 1.25 Hz, 3 105
cycles were undertaken (Isidor et al. 1996, Naumann
et al. 2007) using a fatigue-testing machine (ER-LA-11000;
Figure 1 Experimental groups: Nd- control without ARD; Bd-
buccal ARD; Ld- lingual ARD; BLd- buccal and lingual ARD.
Raposo et al. Anti-rotation devices biomechanics
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ERIOS, Sao Paulo, SP, Brazil) with a constant tem-
perature of 37 C in 100% of humidity (Fig. 2).
Strain measurement tests
For strain measurements tests, three strain gauges (PA-
06-060OBG-350LEN; Excel Sensores, Sao Paulo, Brazil)
were fixed on three samples per group, 1.0 mm below
the crown cervical limit of each specimen, two parallel
to the root long axis (one on the buccal root surface
and the other on the lingual root surface) and one
transversely to the root, on the proximal root surface
(Fig. 3). According to the manufacturer, the base
material of these gauges consists of a polyimide and
metal constantan film, with temperature self-compen-
sation for steel. The strain gauge grid had an area of
4.1 mm2 and an electrical resistance of 350 X. Thegauge factor is a proportional constant between elec-
trical resistance variation and strain, and the strain
gauges used for this study had a gauge factor of 2.12
(Santos-Filho et al. 2008, Soares et al. 2008b). For the
strain gauge attachment, the root surface was etched
with 37% phosphoric acid for 15 s (Cond AC 37; FGM,
Joinville, SC, Brazil), rinsed with water and air-dried.
Sequentially, the strain gauges were bonded with a
cyanoacrylate-based adhesive (Super Bonder, Loctite)
and connected to a data acquisition system (AD-
S0500IP; Lynx, Sao Paulo, Brazil). A control specimen,
with three strain gauges attached but not subjected to
load application, was mounted adjacent to the test
tooth as a compensator for temperature fluctuations
because of the gauge electrical resistance or local
environment.
The specimens fitted with strain gauges were sub-
jected to a non-destructive ramp-load from 0 to 100 N
at a cross-head speed of 0.5 mm per min using a
mechanical testing machine (EMIC DL-2000; EMIC,
Sao Jose dos Pinhais, PR, Brazil). The load was applied
at the lingual elevation with a wedge-shaped tip. The
data obtained were transferred to a computer using
specific acquisition, signal transformation and data
analysis software (AqDados 7.02 and AqAnalisys;
Lynx). During load application, one strain value was
recorded for each strain gauge every 0.3 s until a
maximum load was attained. Data for each region
showed normal and homogenous distribution and were
statistically analysed by one-way analysis of variance
(anova) and Tukey Honestly Significant Difference test
(HSD).
Fracture tests
Fracture resistance tests were performed in all speci-
mens using the same compressive design used for the
strain gauge test (Fig. 4). The results were obtained in
Figure 2 Mechanical fatigue: cyclical loading of 50 N directed
at 135 to the lingual elevation of the metallic crowns.
Figure 3 Strain gauges attached to buccal (A) lingual (B) and
proximal (C) faces.
Figure 4 Fracture resistance set up.
Anti-rotation devices biomechanics Raposo et al.
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Newtons (N) showed normal and homogenous distri-
bution and were submitted to one-way anova and
Tukey HSD tests. For all tests, an alpha level of 0.05
was used. The failed samples were analysed under an
optical stereomicroscope to determine the fracture
mode according to the location of the failure in the
tooth structure (Akkayan & Gulmez 2002, Toksavul
et al. 2006, Naumann et al. 2007). Fractures were
classified as type I, proximal to buccal coronal third
fracture; type II, proximal coronal third fracture; type
III, proximal to buccal medium or apical third fracture
and type IV, proximal medium or apical third fracture
(Fig. 5).
Finite elements analysis
Bidimensional models were created from a longitudinal
cut of one tooth of each experimental groups simulat-
ing the dimensions of the dental structure, using
computer-aided design software (Mechanical Desktop,
AutoCAD 6; Autodesk Inc, San Rafael, CA, USA)
(Soares et al. 2008b). The external outline of the tooth
and its support structures were included in the model.
The data obtained were exported to a software appli-
cation (ANSYS 9.0; ANSYS Inc, Canonsburg, PA,
USA). In this software, the areas corresponding to each
structure were plotted and then meshed with isopara-
metric elements (Plane183). This element is defined by
eight nodes having two degrees of freedom at each
node: translations in the nodal X and Y directions. The
mechanical properties of each structure and materials
used in the analysis are described in Table 1 (Farah
et al. 1975, Ko et al. 1992, Holmes et al. 1996,
Suansuwan & Swain 2001). All tooth structures and
materials used in the models were considered iso-
tropic, elastic, linear and homogeneous. The boundary
conditions were defined with a 10 N load applied at a
135 angle in the centre of the concavity formed onthe lingual face, 1.0 mm bellow the incisal edge,
simulating the load application used on the fracture
resistance test (Lanza et al. 2005). Model displacements
were restricted at the external lateral and top outlines.
Stress distribution analysis was performed by means of
the quantitative association of the main maximum
stress by von Mises criteria. In 10 monitoring points
positioned among the restorative system, the quantita-
tive stress were obtained (Fig. 6).
Results
The one-way anova indicated no significant difference
between the groups in the fracture resistance test
Figure 5 Schematic representation of the failure mode: Type
I- proximal to buccal coronal third fracture; Type II- proximal
coronal third fracture; Type III- proximal to buccal medium or
apical third fracture; and Type IV- proximal medium or apical
third fracture.
Table 1 Mechanical properties of dental structures and
restorative materials
Structure/Material
Youngs
modulus (MPa)
Poissons
ratio
Dentine (Ko et al. 1992) 18600 0.31
Ligament (Holmes et al. 1996) 68.9 0.45
Cortical bone (Ko et al. 1992) 13700 0.30
Cancellous bone (Ko et al. 1992) 1.370 0.30
Gutta-percha (Ko et al. 1992) 6.9 0.45
Zinc-phosphate cement
(Farah et al. 1975)
22000 0.35
NiCr alloy (Suansuwan
& Swain 2001)
203600 0.30
CuAl alloya 109080 0.33
aManufacturers information.
Figure 6 Monitoring points to quantify the von Mises stress
values (MPa).
Raposo et al. Anti-rotation devices biomechanics
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(P = 0.605). The mean fracture resistance values are
shown in Table 2. Fracture mode distribution is
described in Table 3. Prevalence of types III and IV
(catastrophic fractures) was observed in all groups.
One-way anova revealed no significant difference in
the strain values recorded on the buccal (P = 0.43) and
lingual (P = 0.54) surfaces. However, on the proximal
surface, significant difference between groups was
observed with high strain values for the BLd group,
followed by Bd and Ld groups, and lowest values for Nd
group (Table 4).
Finite element analysis showed greater similarity in
the stress distribution levels within tooth structure
between Nd, Ld and Bd groups, which presented lower
stress concentration into buccal dentine than BLd
group. Bd group showed a stress concentration area on
the buccal face, near to the ARD. The BLd group
presented the higher stresses between all groups
(Fig. 7). In the quantitative analysis, lower stresses
were observed in the Ld and Nd groups, while Bd group
presented a similar pattern to Nd group with higher
stress. The BLd group showed the highest stress
concentration (Fig. 8).
Discussion
The hypothesis of this study was partially supported by
the results. The presence and location of ARDs did not
influence the fracture resistance and fracture mode of
incisors restored with cast post-and-cores. However,
the ARDs influenced the strain values at the proximal
face and the stress distribution within tooth structure,
mainly with its application on the buccal face (Bd) or
simultaneously on the buccal and lingual faces (BLd
group).
Anti-rotation devices have been described as impor-
tant means to avoid dislodgment of cast posts-and-cores
in teeth with round root canals (Tjan & Miller 1984,
Hemmings et al. 1991, Shillingburg et al. 1997). Pre-
vious studies, concluded that this class of devices could
indeed increase the torsion resistance of teeth submit-
ted to these forces (Tjan & Miller 1984, Hemmings et al.
1991, Joshi et al. 2001) and improve stress distribution
over the restorative system (Shillingburg et al. 1997).
Despite the fact of the fracture resistance of root filled
teeth being proportional to the remaining structure
(Assif & Gorfil 1994), the tooth loss caused by the ARD
preparation did not affect the fracture resistance of
incisor roots. This probably occurred because of the low
sensitivity of the fracture resistance test, which is not
capable of measuring adequately the influence of such
small alterations.
Despite significant differences on the proximal
strains, the results of this region were lower than the
buccal and lingual ones. This occurred probably
because of the core and crown major supporting area,
which is located mainly at the buccal and lingual faces
of the root, instead of the proximal sites, where the
dentinal support for the metallic components is con-
siderably reduced. This can result in high strain values
in the referred areas during the specimen loading. The
reduced amount of dentine of the proximal regions and
the ARD preparation could explain the differences in
the strain values between groups. The fracture mode
analysis showed failures starting mainly on the prox-
imal faces, continuing normally towards the buccal
face. The removal of sound tooth structure in the
preparation of the anti-rotation increased dentine
strain, favouring higher strains on the proximal site.
However, strain gauge method records the strains on a
surface just before these forces dissipate throughout the
whole body. Consequently, strains that occur inside a
body with load application reach higher values than
those measured on its surface.
The post inserted into the root canal modifies the
tooth stressstrain distribution (Assif & Gorfil 1994).
Under loading, stress will concentrate in the more rigid
material in a system with materials that present
different stiffness. A cast post-and-core presents differ-
ent mechanical properties when compared to a
non-metallic post system and to the inner dentine
(Pierrisnard et al. 2002, Toksavul et al. 2006). The
Table 2 Mean fracture resistance values (SDs) and results of
Tukey Honestly Significant Difference (HSD) test (n = 10)
Groups Fracture resistance (N) Tukey Categorya
Nd 708.6 (110.1) A
Bd 716.8 (217.3) A
Ld 766.9 (270.0) A
BLd 823.80 (221.9) A
aTukey categories with same uppercase letters are not statisti-
cally significant from each other (P < 0.05)
Table 3 Fracture mode distribution among the groups
Fracture type
Group I II III IV
Nd 1 0 5 4
Bd 0 0 6 4
Ld 0 0 4 6
BLd 1 1 4 4
Total (%) 5% 2.5% 47.5% 45%
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International Endodontic Journal, 43, 681691, 2010 2010 International Endodontic Journal686
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stiffness imposed on the root by the metallic post does
not allow the root structure to flex and consequently
increases the stress concentration along the whole
system. On this way, the stress primary distributed to
the root periphery concentrate inside the regions where
the micro-dislodgment is restricted as in the dentine-
cement-post interfaces (Genovese et al. 2005, Lanza
et al. 2005). The collapse of the interface induces
dissipation of the energy accumulated in the metallic
post to the dentine. This could explain the strains
observed on the proximal surface, predominantly when
anti-rotations devices are used on the buccal and
lingual faces. Strain values on proximal surfaces seem
to be particularly more important when comparing the
fracture resistance and fracture mode. This tensile
strain starts inside the root canal and can spread
through the cracks leading to catastrophic fractures of
the structure (Santos-Filho et al. 2008).
In a normal occlusion, the dentine exhibits consid-
erable flexibility to elastic strains, withstanding several
forces applied from different angles (Kishen et al. 2004,
Naumann et al. 2007). However, these forces could
exceed the dentine strength as well as the proportional
limit, and fracture may occur (Kishen et al. 2004).
Besides the metallic post stiffness, root canal dentine is
arranged radially and the stress tends to be parallel to
the tubules direction, generating a wedge effect and
consequent root failures. Catastrophic fractures (types
III and IV) occurred in all groups, mainly in the Bd
group samples. This probably happened because of the
load applied on the lingual face, which concentrates
compressive stresses on the buccal face (Assif & Gorfil
1994), increasing the fractures on this region. FEA
showed a high stress concentration on the buccal
dentine in the BLd model. The increase in the volume of
the metallic post may explain this behaviour. This
finding did not have a direct effect on the fracture
resistance. However, as shown previously, this could
explain the highest strain values on the proximal
surface during strain gauge tests verified on this group.
Table 4 Mean values and standard deviations of strain values (lS) for the groups, maximum loading of 100 N
Buccal Lingual Proximal
Groups lS
Tukey
Categorya lS
Tukey
Categorya lS
Tukey
Categorya
Nd 549.9 (74.2) A 346.9 (263.6) A 125.5 (16.5) B
Bd 386.5 (216.1) A 333.7 (159.9) A 144.9 (16.5) AB
Ld 562.5 (266.2) A 472.9 (380.4) A 133.9 (65.9) AB
BLd 345.4 (145.9) A 363.7 (110.3) A 311.5 (244.7) A
aTukey categories with same uppercase letters are not statistically significant to each other into the same region (P = 0.0001).
Figure 7 von Mises stress values (MPa):
Nd control without anti-rotation device
(ARD); Bd buccal ARD; Ld lingual
ARD; BLd buccal and lingual ARD.
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The fracture resistance values of all groups are higher
than the incident anterior forces of the oral cavity,
quoted in the literature between 27.8 and 65.3 N
(Kumagai et al. 1999). In addition, the failures that
occur in restorative procedures involving cast post-and-
cores are frequently related to fractures because of the
mechanical fatigue of the system, caused by the forces
concentrated out of the long axis of the tooth (Torbjorner
& Fransson 2004). Consequently, a well planned and
developed prosthesis with favourable biomechanics is
crucial to the longevity of root filled teeth. To conserve
intact dental structure, ARDs should only be realized
when really necessary. Moreover, the lingual face should
be the unique region of choice because of the lower
strains and stress values were observed on this area.
This study was conducted in laboratorial conditions
and it presents some intrinsic limitations, such as the
static loading and absence of thermal cycling. Despite
using bovine teeth, several studies show large accep-
tance of these teeth for laboratory investigations
because of its similarity to human dentine and
geometric root configuration (Schilke et al. 2000,
Fonseca et al. 2008). Additionally, as bovine teeth
have greater availability, it is possible to standardize
specimen size and shape (Santos-Filho et al. 2008,
Soares et al. 2008c), which is essential when obtaining
comparable results as deformation and fracture resis-
tance are dependent on tooth geometry.
The FEA analysis also had limitations, such as the
bidimensional models and the materials assumed as
being elastic and isotropic. The linear elastic analysis is
also another limitation, because the periodontal liga-
ment and interfaces behaviour differently when anal-
ysed using non-linear analysis. However, the
combination of non-destructive (strain gauge) and
destructive experimental mechanical tests (fracture
resistance) with numerical analyses (FEA) can be
effective in predicting the biomechanical behaviour of
dental restorative procedures. FEA using 3D models is
recommended, as it allows anatomic alterations and
devicesample contact to be shown with greater
accuracy (Soares et al. 2008b). Further clinical evalu-
ations, taking into account the remaining dentine
thickness and the presence and location of ARDs,
Figure 8 von Mises stress distribution quantitative analysis.
Anti-rotation devices biomechanics Raposo et al.
International Endodontic Journal, 43, 681691, 2010 2010 International Endodontic Journal688
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would be of benefit. In addition, the use of restorative
materials that could resemble the mechanical behav-
iour of sound teeth should be incorporated into the
clinical choices when indicated.
Conclusions
Within the limitations of this laboratory study, the
following conclusions were drawn:
1. The presence and location of an ARD did not affect
significantly the fracture resistance and fracture mode
of tooth restored with cast post-and-core;
2. Higher strains were observed on the proximal
surface of teeth in the presence of ARDs. The associ-
ation of buccal and lingual ARDs produced the highest
strains;
3. Lower stresses were observed when the ARD was
positioned on the lingual face; despite higher stress
concentrations being observed when this feature was
positioned concomitantly on the buccal and lingual
faces.
Acknowledgements
This study was supported by the Research Support
Foundation of the State of Minas Gerais (FAPEMIG\
Brazil).
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