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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: [email protected] (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


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.



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.

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 9 385


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-



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



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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