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The effect of custom adaptation and span-diameter ratio onthe flexural properties of fiber-reinforced composite posts
Nicola M. Grande a, Gianluca Plotino a,*, Pietro Ioppolo b, Rossella Bedini b,Cornelis H. Pameijer c, Francesco Somma a
aDepartment of Endodontics, Catholic University of Sacred Heart, Rome, ItalybTechnology and Health Department, Italian National Institute of Health, Rome, ItalycSchool of Dental Medicine, University of Connecticut, Farmington, CT, USA
Posts are primarily used to connect the root portion of
endodontically treated teeth to the build-up material placed
on the coronal portion of the teeth.1–3 A post may not be
needed if an endodontically treated tooth that requires a
crown, or must be restored with an intracoronal restoration,
has a sufficient amount of remaining dentin to retain the
restoration.4,5
Prefabricated and cast metal posts have been traditionally
used for the restoration of endodontically treated teeth, but
more recently preformed fiber-reinforced root canal posts
have made inroads as an alternative to metal posts.6,7 FRC
posts have demonstrated mechanical properties close to that
of root canal dentin8–11 and aim to restore the dentin–root
complex using materials with matching mechanical proper-
ties, to improve the clinical performance of the restored tooth.
Clinical prospective and retrospective studies on the use of
FRC posts have reported encouraging results during short to
medium clinical use.12–16
One of the disadvantages associated with the use of
preformed posts is the need to adapt the root canal to the
j o u r n a l o f d e n t i s t r y 3 7 ( 2 0 0 9 ) 3 8 3 – 3 8 9
a r t i c l e i n f o
Article history:
Received 10 November 2008
Received in revised form
18 January 2009
Accepted 19 January 2009
Keywords:
Endodontic posts
Root canal anatomy
Anatomical posts
Flexural properties
a b s t r a c t
Objectives: To evaluate whether custom modification resulting in an anatomically shaped
post and whether the span/diameter ratio (L/D) would affect the mechanical properties of
fiber-reinforced composite posts.
Methods: Preformed glass-fiber posts (Group 1) and modified glass-fiber posts (Group 2) and
glass-fiber rods (Groups 3 and 4) (n = 20) were loaded to failure in a three-point bending test
to determine the maximum load (N), flexural strength (MPa) and flexural modulus (GPa). The
span distance tested for Group 3 was 10.0 mm, while for Group 4 was 22.0 mm. Data were
subjected to different statistical analysis with significance levels of P < 0.05.
Results: The maximum load recorded for Groups 1 and 2 was 72.5 � 5.9 N and 73.4 � 6.4 N
respectively, while for Groups 3 and 4 was 215.3 � 7 N and 156.6 � 3.6 N respectively. The
flexural strength for Groups 1 and 2 was 914.6 � 53.1 MPa and 1069.2 � 115.6 MPa, while for
Groups 3 and 4 was 685.4 � 22.2 MPa and 899.6 � 46.1 MPa. The flexural modulus recorded
for Groups 1 and 2 was 32.6 � 3.2 GPa and 33.4 � 2.2 GPa respectively, while for Groups 3 and
4 was 13.7 � 0.3 GPa and 34.4 � 0.3 GPa respectively.
Conclusions: The flexural properties of an anatomically custom modified fiber post were not
affected by the modification procedure and the span-diameter ratio is an important para-
meter for the interpretation of flexural strength and flexural modulus values.
# 2009 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +39 3396910098; fax: +39 068072289.E-mail address: gplotino@fastwebnet.it (G. Plotino).
avai lab le at www.sc iencedi rec t .com
journal homepage: www.intl.elsevierhealth.com/journals/jden
0300-5712/$ – see front matter # 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.jdent.2009.01.007
Author's personal copy
shape and size of the post to be used. The adaptation of the
canal to the post requires the sacrifice of sound dentin tissue
which conflicts with one of the universally accepted concepts
in the restoration of endodontically treated teeth: prognosis
improves proportionally to the amount of sound tooth
structure present, regardless of the type of restoration
chosen.2 The loss of dentin is in fact the primary cause for
fracture susceptibility in endodontically treated teeth.1,2,17–19
Preformed root canal posts have often circular cylindrical or
progressively tapered shapes, yet are placed in canals that are
oval or ribbon shaped. Furthermore, anatomical variability of
the teeth is often a complicating factor in root canal treatment
and restorative procedures,20 while cone-shaped anatomy
with a circular base is not the most common shape in radicular
canals, which are more laminar rather than circular, espe-
cially in the coronal and middle third.21–26
A semi-direct chairside clinical procedure has been
recently reported, that permits the use of an anatomically
shaped post in order to unify the advantages of a fiber post
with those of an anatomical post27 This is possible due to the
characteristics of the preformed fiber post, which can more
easily be modified than a metallic preformed post.28
This technique aims to give the post a shape as close as
possible to the anatomy of the root canal. The technique
requires a straight low-speed hand piece and medium-grit
diamond bur to modify a fiber post of the largest available
diameter to a shape approximating the anatomy of the root
canal itself. The post has to be modified in such a way as to
passively occupy the entire length of the post-space, using a
cast of the post-space as a guide.27
A SEM study demonstrated that commercially available
fiber posts that are machined to a particular shape resulted in
posts that had a surface that was not different from unaltered
parallel-sided commercially available fiber posts. It has also
been demonstrated that no differences in surface morphology
were observed between fiber posts that were modified by
grinding with a slow-speed medium-grit diamond bur28 and
machined commercial posts. However, a question that
remains unanswered is, whether the custom modification
of these posts affects the mechanical properties.
Previous studies have demonstrated that, when measuring
the flexural properties of fiber posts using a three-point
bending test, the results may be biased by the geometrical
design of the test set-up: the distance between supports (span
length—L), the diameter (D) of the specimens tested, and the
ratio between these values (L/D) which were identified as span/
diameter ratio.10,11,29–34 This parameter could be particularly
unfavorable for specimen with a large diameter (>2 mm).
Moreover, when testing root canal posts, the span length is
limited by the length and the shape of the specimens that are
commercially available.11
The aim of this study was to evaluate whether custom
modification resulting in an anatomically shaped post would
affect the mechanical properties of the post and whether the
span/diameter ratio (L/D) would affect the flexural properties
of fiber-reinforced composite posts. The null hypotheses
tested were that the flexural properties of fiber posts are not
affected by the custom modification procedure and that the L/
D ratio has no influence when using a three-point bending
tests for analysis.
1. Materials and methods
Four different groups of specimens were tested (Fig. 1):
� Group 1: preformed glass-fiber posts (Periodent, Milano,
Italy) with a diameter of 1.2 mm, a length of 19 mm and
having a cylindrical shape with a tapered end.
� Group 2: modified glass-fiber posts (Periodent, Milano, Italy)
with a diameter of 1.2 mm, a length of 19 mm and a
cylindrical shape. The specimens were obtained from a
cylindrical glass-fiber rod 2.2 mm in diameter. The mod-
ification of the rods was obtained using a straight low-speed
hand piece without water spray and a medium-grit conical
diamond bur (Gebr. Brasseler, Lemgo, Germany), in order to
give to the specimens a shape and size similar to those of
specimens from Group 1.
� Groups 3 and 4: glass-fiber rods (Periodent, Milano, Italy)
with a diameter of 2.2 mm, a length of 30 mm and a
cylindrical shape.
Eighty specimens, 20 for each group (n = 20) were tested. A
description of the test specimens, cross-section diameters,
Fig. 1 – The specimens that were tested in the study. (a)
Preformed glass-fiber posts, (b) modified glass-fiber posts,
(c) glass-fiber rod tested with 10 mm span length, and (d)
glass-fiber rod tested with 22 mm span length.
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span lengths of the three-point loading test and L/D ratios for
each group are presented in Table 1.
The diameter of the specimens was measured with an
electronic digimatic 500-311 caliper (Mitutoyo, Tokio, Japan).
Measurements were taken at three locations (coronal, middle
and apical) in order to verify the information provided by the
manufacturers and to ensure accuracy of the modified
specimens (Group 2). Dimensions of the middle portion of
the modified posts were further determined at five different
locations in order to verify whether the diameter corre-
sponded to that chosen for the test. Samples that did not meet
the criteria (mean diameter measured �0.1 mm) were
Table 1 – Description of test specimens, diameters of the specimens, span lengths, L/D ratios and mean (S.D.) of theflexural properties obtained with three-point bending test (asterisk represent groups not significantly different, P > 0.05using Bonferroni multiple comparison).
Group Description Diameter,D (mm)
Span,L (mm)
L/D ratio Maximumload, (N)
Flexuralstrength, (MPa)
Flexuralmodulus, (GPa)
1 Non-adapted posts 1.2 10 8.3 72.5* (6) 914.6* (53.1) 32.6* (3.2)
2 Adapted posts 1.2 10 8.3 73.4* (6.4) 1069.2 (115.6) 33.4* (2.2)
3 Non-adapted bars 2.2 10 4.5 215.3 (7) 685.4 (22.2) 13.7 (0.3)
4 Non-adapted bars 2.2 22 10 156.6 (3.6) 899.6* (46.1) 34.4* (0.3)
Fig. 2 – Representative images from the three-point bending test used to measure the maximum load, flexural strength and
flexural modulus values of the specimens tested.
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replaced. The posts dimensions were assessed with Student’s
t-test to demonstrate any significant difference between
Groups 1 and 2 and between Groups 3 and 4 (P > 0.05).
A three-point bending test with 10.0 mm span distance, a
loading tip with 2 mm cross-sectional diameter, and 1.0 mm/
min crosshead speed was used to measure the maximum load,
flexural strength and flexural modulus values of specimens
from Groups 1–3 (Fig. 2); while the span distance tested for
Group 4 was 22.0 mm. In order to reduce the influence of the
tapered end of the posts in Group 1, a 10-mm span length was
used to ensure testing of the parallel coronal portion of the
post only. The L/D ratio was calculated for all the specimens
tested.
For mechanical testing an electronic dynamometer (Lloyd
Instruments Ltd. LR30K, Fareham, England) equipped with a
500 N � 0.5% load cell was used. The load–deflection curves
were obtained by means of Nexygen Mt v4.5 PC-software
(Lloyd Instruments Ltd., UK) and the maximum load (N), the
flexural strength (MPa) and the flexural modulus (GPa) mean
values with the relative standard deviations were calculated.
At the start of the test of each post, the specimen dimensions
were entered following a command from Nexygen.
Data analysis was performed using SPSS software (Statis-
tical Package for the Social Sciences, SPSS Inc., IL, USA) and
subjected to one-way ANOVA to determine significant
differences between groups. When the overall F-test showed
a significant difference, the multiple comparison Bonferroni t-
test procedure was applied to determine which mean values
differed from one another with a significance level of P < 0.05.
Three multivariate linear regression models using the back-
ward elimination procedure described by Hosmer and
Lemeshow35 were performed to investigate the effects of
the geometrical parameters of the test set-up considered as
independent variables (distance between supports (L), dia-
meter of the specimens (D) and the resulting L/D ratio) on the
flexural properties of the specimens tested (maximum load,
flexural strength and flexural modulus) considered as depen-
dent variables. The goodness of fit was determined using the
R2 values.
We considered all the differences significant when the
experimental F had a significance of P < 0.05.
2. Results
Measurements of the diameter of the specimens from Groups
1, 3 and 4 corresponded rather precisely to the dimensions
specified by the manufacturer with slight variations in the
range of 0.01–0.02 mm. Mean diameters were respectively
1.20 � 0.014 mm (Group 1), 2.20 � 0.018 mm (Group 3), and
2.20 � 0.022 mm (Group 4). Mean diameter for the custom
modified posts (Group 2) was 1.20 � 0.029 mm. The t-test did
not show a significant difference between Groups 1 and 2
(P > 0.05). Furthermore the mean specimen dimension of
Groups 3 and 4 did not demonstrate a statistically significant
difference (P > 0.05).
The mean and standard deviation of maximum load (N),
flexural strength (MPa) and flexural modulus (GPa), are
illustrated in Table 1. ANOVA overall tests showed significant
differences among groups in maximum load (P < 0.000,
F = 1240.56), flexural strength (P < 0.000, F = 41.48) and flexural
modulus (P < 0.000, F = 198.11) mean values. With respect to
maximum load values, a subsequent Bonferroni t-test multi-
ple comparison did not show a statistically significant
difference (P = 1.000) between Groups 1 and 2, while a
statistically significant difference was demonstrated among
the other groups (P < 0.000). With respect to the flexural
strength values, the Bonferroni t-test multiple comparison did
not show a statistically significant difference (P = 1.000)
between Groups 1 and 4 with the other groups being
statistically significantly different (P < 0.000).
Analysis of the flexural modulus values by means of the
Bonferroni t-test multiple comparison did not show a
statistically significant difference between Groups 1 and 2
(P = 1.000), between Groups 1 and 4 (P = 0.449) and between
Groups 2 and 4 (P = 0.449). Only Group 3 was statistically
significantly different from all other groups (P > 0.000).
The overall regression model for the maximum load as
dependent variable was statistically significant (F = 2046.47;
P = 0.000; R2 = 0.992). Furthermore, among the independent
variables considered, diameter of the specimens D and L/D
ratio were statistically significant (P < 0.000), while the span
length L was excluded by the model because it showed
collinearity with the other variables. D positively affects the
dependent variable (standard b = 0.848) while L/D ratio
negatively affected it (standard b = �0.338).
Both the overall regression models for flexural strength and
flexural modulus as dependent variables were statistically
significant (respectively F = 33.40; P = 0.000; R2 = 0.680 and
F = 330.61; P = 0.000; R2 = 0.951). In these models the L/D ratio
showed collinearity with the other independent variables and
it was also excluded by the model, while diameter of the
specimens D and span length L were statistically significant
(P < 0.000). D negatively affected both the dependent variables
(standard b = �1.018 for flexural strength and standard
b = �1.149 for flexural modulus), while span length L positively
affected them (standard b = 0.570 for flexural strength and
standard b = 1.020 for flexural modulus).
3. Discussion
The aim of this study was to determine whether custom
grinding fiber posts with a medium diamond bur would cause
irreversible damage, thus negatively affecting its mechanical
properties. The procedure to grind and modify a fiber post
could potentially alter the micromorphology of the continuous
reinforcing fibers embedded in the polymer matrix, thus
altering its mechanical properties.
Based on the diameter measurements of all posts that were
tested it can be concluded that the test data has validity in
spite of the larger S.D. in Group 2 with modified posts, which
upon statistical analysis was determined not to be statistically
significant (P > 0.05).
The present study reported no statistically significant
differences in the maximum load values of modified posts
versus untouched commercially available preformed fiber
posts of the same diameter (P = 1.000). Moreover, the flexural
modulus and flexural strength of the modified posts, the
untouched preformed posts and the fiber-reinforced compo-
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site rods were similar. A Bonferroni t-test multiple comparison
did not show a statistically significant difference (P < 0.05)
among these groups except for the modified posts (Group 2)
which showed a slightly higher flexural strength value than
the other groups (1069.2 � 115.6 MPa). The non-modified
preformed posts (Group 1) were found to be no more flexible
or more resistant to fracture than the modified circular posts
of the same diameter (Group 2).
The values of flexural strength and flexural modulus
recorded in the present study are mostly in agreement with
previously published studies8–11,34,36 The results demonstrate
that the flexural properties of fiber-reinforced composite root
canal posts were not affected by grinding with a medium-grit
diamond bur. Thus, the first null hypothesis tested in this
study cannot be rejected.
A previous study reported37 that fiber posts can be cut using
a medium-grit diamond bur in a low-speed hand piece under
copious water coolant. That study focused on the surface
morphology, however, it did not address whether these
procedures had an effect on the mechanical properties of
the posts.
In the present study the posts were cut dry as several
articles have reported that humidity can alter the mechanical
properties of fiber posts.8,36,38–41 It still remains to be
established whether the use of water cooling affects the
mechanical properties of fiber posts.
The advantages of the technique that was investigated in
the present study are the following. First, the most important
advantage is the preservation of tooth structure.42–46 A
modified fiber post adapts to the root canal rather than
adapting the root canal to the post, usually by enlarging it. This
is a significant advantage as after endodontic treatment root
canals are usually oval-shaped rather than perfectly
round.47,48 An anatomically adapted fiber post significantly
reduces the thickness of the cement layer,48 thus reducing
polymerization stress caused by a large amount of cement
around a post.49–52 The formation of bubbles or voids,
representing areas of weakness within the material, is less
likely to occur in a thin and uniform layer of cement.48 If a post
does not fit well, especially at the coronal level, the cement
layer will be too thick and bubbles or voids are likely to be
present.48 A thin and even thickness of a cement layer and the
absence of voids increase the retention of the post, thus
reducing the risk of debonding.53,54 It has been reported that
debonding is more likely to occur in excessively thick layers of
cement or in weak areas within the material, such as are
caused by air bubbles.14,15 Using a prefabricated fiber post in a
flared or severely excavated extended canal due to caries may
result in an excessively thick layer of resin cement in the
coronal region of the post preparation, which would not be
strong enough to resist occlusal loading. The thicker portion of
the extraradicular anatomically adapted fiber post may
preserve the margin on the tension side from opening during
cyclic loading and the margin on the compressive side from
crushing cement and dentin, thus maintaining marginal
integrity under function.55 Moreover, insufficient dowel
stiffness will lead to excessive deformation of the post and
localization of stresses during function, allowing marginal
failure.55 Finally, some authors have reported that the greater
thickness of the coronal portion of the post results in higher
core debonding load values and that the adhesion between
post and composite core material may be enhanced by using
an anatomically modified fiber post.56 In addition, the final
shape of a modified post is more resistant to rotational forces
than a round post.57,58
The flexural modulus parameter defines the flexibility of a
sample; higher values indicate more stiffness, while lower
values indicate more flexibility. The flexural modulus is
calculated by taking into account the elastic behavior of a
sample within a load range that will not cause plastic
deformation. The flexural strength parameter determines
the resistance to fracture of a sample constructed with a
known material. Higher values indicate that a material is more
resistant to fracture, lower values that it is less. The flexural
strength is determined by the highest load a sample can
withstand. Both these values should be independent from the
size and the shape of the specimens tested, because they are
used to describe the properties of a material per se. In spite of
this observation, previous studies have reported that, in a
three-point bending test, the distance between supports (L),
the diameter of the specimen (D) and the resulting L/D ratio
can affect flexural properties of the specimens.10,11,29–34 This
then could represent a systematic error in determining the
flexural strength and flexural modulus values.
Confirming previous observations on post-polymerized
specimens fabricated from E-glass-fiber prepregs impregnated
with light-polymerizable resin (everStick and Stick Resin, Stick
Tech Ltd., Turku, Finland),34 the present study showed that,
even when testing fiber-reinforced posts manufactured
commercially, an increase in the L/D ratio decreased the
maximum load values (as was expected), but flexural strength
and flexural modulus values increased.
The maximum load, expressed in N, is the highest load a
sample can withstand and depends on the specimen config-
uration and size. In fact, the multivariate regression model
with maximum load as dependant variable showed that an
increase in diameter of the specimens (D) positively affected
the outcome of the test (standard b = 0.848, P < 0.000), while an
increase in the L/D ratio negatively affected it (standard
b = �0.338, P < 0.000). Moreover the other multivariate linear
regression models have shown that an increase in diameter of
the specimens (D) negatively affects both the flexural strength
(standard b = �1.018, P < 0.000) and flexural modulus (stan-
dard b = �1.149, P < 0.000), while an increase in the span
length (L) of the test set-up positively affected both the flexural
strength (standard b = 0.570, P < 0.000) and flexural modulus
(standard b = �1.020, P < 0.000). Even if L/D ratio was the
excluded variable in these models for its collinearity, the
multivariate linear regression showed that the span/diameter
ratio L/D negatively affected both flexural strength and
flexural modulus. As the L/D ratio increased, the flexural
strength and flexural modulus values increased and maximal
load values decreased. Based on the foregoing, the second null
hypothesis tested in this study can be rejected.
It could be hypothesized that when using L/D ratio <5 the
flexural strength and flexural modulus values may become
unreliable. However, the values registered for Group 3
conspicuously differed from all the other group means, both
for flexural strength and flexural modulus (P < 0.05), despite
the fact that the test materials were the same. This was
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attributed to an increased shear stress within the specimen
rather than a tensile stress with low L/D ratios.
Maintaining L/D ratios in a range of 8.3–10 seems to ensure
correct measurements using a three-point bending load test.
The values of flexural strength and flexural modulus registered
for Groups 1, 2 and 4 (respectively 8.3, 8.3, and 10 L/D ratio) did
not show statistically significant differences (P < 0.05). The
values registered for these groups are mostly in agreement with
previouslypublished studies inmeasuring flexural propertiesof
fiber-reinforced composite materials.8–11,34,36
It has been reported that for high strength engineered
composites a high L/D ratio (40 or 60) should be used to
eliminate shear effect during bending test.29–31 It remains to be
determined if the use of a L/D ratio >10 can further influence
the outcome of the three-point flexural tests. In a study by
Alander et al.34 this was also observed with specimens with a
rectangular cross-section. It was determined that even the
cross-section configuration (rectangular or circular) of speci-
mens can influence the outcome of a three-point bending test.
4. Conclusion
Within the limitations of this study, it can be concluded that
the flexural properties of an anatomically custom modified
fiber post were not affected by the modification procedure.
Whether the use of a custom modified post influences the
mechanical properties and clinical behavior of a restored
endodontically treated tooth remains to be seen and warrants
future investigations. Furthermore, it can be concluded that L/
D ratio is an important parameter for the interpretation of
flexural strength and flexural modulus values of fiber-
reinforced composite specimens using a three-point bending
test. Interpretation and comparison of the results obtained by
different three-point bending test set-ups need to be carefully
considered.
Acknowledgements
The authors wish to express their gratitude to Dr. Riccardo Ilic
for providing the test materials as these prototypes of the
glass-fiber root canal posts and glass-fiber rods were speci-
fically produced by Periodent for Dr. Riccardo Ilic S.p.A.
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