Assessment of Push Out Bond Strength and Cement Thickness for Oval Root Canals Restored with Different Post Techniques

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Fixed Prosthodontics, Dental materials, Conservative Dentistry and Endodontics
EGYPTIAN
DENTAL JOURNAL
Vol. 65, 3855:3870, October, 2019
* Assistant Professor, Fixed Prosthodontics Division, Oral and Maxillofacial Department, Faculty of Dentistry,
Umm Al-Qura University.
**Associate Professor, Fixed Prosthodontics Department, Faculty of Dentistry, Cairo University, Egypt, Associate
Professor, Fixed Prosthodontics Division, Oral and Maxillofacial Department, Faculty of Dentistry, Umm Al-Qura
University.
ASSESSMENT OF PUSH OUT BOND STRENGTH AND CEMENT
THICKNESS FOR OVAL ROOT CANALS RESTORED WITH
DIFFERENT POST TECHNIQUES
Khaled A. Elbanna *, Zeinab N. Emam ** and Shereen M. El Sayed **
ABSTRACT
Objectives: The aim of the present study was to evaluate the push out bond strength of ber
posts to oval shaped root canals using different post techniques and correlation of bond strength
values to different cement thicknesses at different post level in different post techniques.
Methods: A total of thirty-two recently extracted human mandibular rst premolars with oval
root canals were selected and stored in normal saline solution. The teeth were horizontally sectioned
2 mm coronal to the cementoenamel junction with a double-faced diamond disk in a slow-speed
handpiece with copious coolant. Root canal treatment was done for all teeth using a standardized
technique. The teeth were then randomly divided into four equal groups (n=8) according to the post
and core system used: Group I: Single circular post technique, Group II: Relined post technique,
Group III: Double circular post technique, Group IV: Oval post technique. For each sample, the
roots were cut horizontally, perpendicular to the long axis of the root using low speed diamond
saw under water coolant to obtain three 2 mm ± 0.1 thick root specimens representing cervical,
middle and apical third of the root. The resin cement thickness around each specimen obtained
after slicing were measured using scanning electron microscope (SEM) at eight standardized points
around the circumference of the slice; between the canal wall and the ber post perimeter. Each
root slice was subjected to compressive loading at a crosshead speed of 1 mm/min via a computer
controlled Universal testing machine to record the maximum failure load in Newton. The push out
bond strength was calculated by dividing the recorded peak load by the surface area of each slice.
Results: As regards to the mean push out bond strength values for the post techniques, there
was a statistically signicant difference between mean push out bond strength of different post
techniques where oval post technique showed the statistically signicant highest mean push out
bond strength. Relined technique showed statistically signicant lower mean value followed by
double circular technique. Single circular technique showed the statistically signicantly lowest

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INTRODUCTION
Restoration of endodontically treated teeth have
always been a challenge in prosthodontics. These
teeth are considered at a high risk of unfavorable
fractures due to extensive loss of coronal tooth
structure due to caries and the endodontic procedure
itself. Most likely, these teeth will require the
placement of posts for retention of the core material
before the preparation of teeth to receive an extra-
coronal full coverage restoration.
Numerous post systems have been offered
over the years, from cast posts to different shapes
of ready-made metallic posts and lately the most
widely used nowadays the glass ber posts (1). Cast
metal posts have been used long time ago by the
clinicians and was considered the gold standard
due to the perfect adaptation of these posts to any
shape of the root canal. However, the use of these
posts had declined owing to the extra complicated
clinical and laboratory procedures needed for their
fabrication (2). Additionally, the big drawback of cast
metal posts had been reported in the literature to be
their high modulus of elasticity (200 Gpa) resulting
in uneven stress distribution between the post and
the intra-radicular dentin which may cause fracture
of the weaker part which is presented in the root
structure rendering the tooth non restorable and
indicated for extraction (3).
Hence, the shift to the use of different prefabri-
cated metal post systems that was favored by many
clinicians; these posts presented a fast, easy and
cheap technique; however, the esthetic outcome
was disappointing (2,4). Among the recently popu-
lar ready-made post systems used nowadays is the
glass ber post. These posts have gained their at-
tractiveness due to many advantages: very good
esthetic results, easy procedure, less clinical time,
easy retrievability and preservation of the remain-
ing tooth structure (5,6 ,7). On the top of that is the
main advantage of the glass ber post which is the
compatibility of its modulus of elasticity with that
of the dentin, giving it a unique property that aids in
protection of endodontically treated teeth from root
fractures (7,8). These posts are cemented to dentin
with adhesive resin cements, thus allowing this as-
sembly to become biomimicry, therefore more uni-
form stress distribution is allowed throughout the
radicular dentin and less susceptibility to unrestor-
able fractures is reached (8,9).
Nevertheless, the drawback that was detected
within the ber posts is its circular cross section
which render this treatment option to be perfect
in cases of root canals with circular cross section
only (9,10,11). Upon considering the oval-shaped and
ared canals in the dentition, it was considered to
have a high percentage due to widening of the root
canals during endodontic procedures, extensive
mean push out bond strength. Regarding the push out bond strength values for the root levels
regardless of the post techniques, there was a statistically signicant difference between mean
push out bond strength at different root levels where there was no statistically signicant difference
between coronal and middle root levels and both showed statistically signicantly higher mean
push out bond strength than apical root level for all post techniques.
Conclusions: The use of single circular post technique in oval or ared canals is not a good
option for long term clinical service. Decreasing the cement thickness space around the post
systems in oval canals will improve the push out bond strength of posts to dentinal walls of the root
canals. The introduction of oval posts for restoration of endodontically treated teeth with oval and
ared root canals gives promising results in push out bond strength tests.
KEYWORDS: Fiber post, Oval canals, ared canals, Oval ber post, Glass ber post, Self-
adhesive resin cement, Push-out bond strength, cement thickness.

ASSESSMENT OF PUSH OUT BOND STRENGTH AND CEMENT THICKNESS (3857)
carious lesions, old restorations and anatomical and
morphological reasons (9,12,13) and the circular ber
post systems are considered to be a poor treatment
option with these oval shaped canals.
Previous reports showed that 90% of the
mandibular rst premolars have oval shaped canals
starting at the CEJ and extending up to almost 2mm
from the anatomical apex (5,8,13). To be able to place
the circular ber post in the oval ared canals,
additional shaping of the root canal must be done
through sound intra-radicular dentin removal to
match the shape of a larger circular ber post to the
ared canals, this will compromise the tooth fracture
resistance (7) and increase the risk of root perforation
or fracture (3,14,15). Otherwise, cementation of the
mismatched circular ber post within the oval canals
associated with an excessive thick layer of resin
cements (16,17) leading to the presence of voids or
gaps from the luting process. This will generate high
stresses at the non-homogenous luting interphase
resulting in compromised bond strength of the post
(7,9). Furthermore, the polymerization shrinkage of
thick adhesive resin cement will further magnify
the stress concentration and failure in adhesive
bonding(7).
Many attempts have been tried to overcome this
shortcoming with oval canals. Among them are the
trial of relining the circular post with composite resin
to obtain adaptation to the post before cementation
(18), the use of several small posts or the use of
accessory post technique in which supplementary
posts are added along with the main ber post in an
attempt to improve adaptation and decrease luting
cement volume (7,19,20). Oval glass ber posts have
been introduced to overcome the clinical limitations
of circular ber posts and to extend the indications
of the ber posts to include all types and shapes of
root canals. Therefore, a better post t, adhesion and
retention strength can be achieved (17,21,22).
One of the most common failures occurring
with the use of the ber posts is the post
debonding (9,10,23,24). Endodontic irrigants and
sealers (25), presence of thick smear layer, high
conguration of the root canal (c-factor), imperfect
moisture control (26), and presence of voids or gaps
associated with a thick layer of cement are reported
to be the main causes of decreased retention values
of the ber posts to the root canal (27,28). Debonding
with large cement lm thicknesses has been reported
by some retrospective clinical studies (29) which
highlighted the important value of reaching an ideal
reduced thickness of resin cement and obtaining a
proper t of ber post shape and size into the post
spaces inside root canals (10).
Several methods have been used to assess the
bond strength of ber posts to root dentin; micro
tensile, pull-out and push-out tests have been tried
for this purpose. Thus, the aim of the present study
was to evaluate the bond strength of ber posts
to oval-shaped root canals using different post
techniques and correlation of bond strength values
to different cement thicknesses at different post level
in different post techniques. The null hypothesis
was that different post techniques and different post
levels would have no effect on the push out bond
strength of the ber posts.
MATERIALS AND METHODS
Teeth selection and preparation
A total of thirty-two recently extracted human
mandibular rst premolars were selected and
stored in normal saline solution. Only single-rooted
premolar teeth with an oval shaped root canals were
included in this study; the teeth were described as
oval if the ratio of the long to short diameters was
≥ 2, measured at 5mm away from the apex. Other
inclusion criteria were: teeth with straight roots with
similar size and shape, mean root length of 14 mm
from buccal CEJ, completely formed apices and
intact clinical crowns. Teeth with open apex, caries,
cracks, fractures, resorption were excluded from the
study.

(3858) Khaled A. Elbanna, et al.E.D.J. Vol. 65, No. 4
Teeth were cleaned from deposits, stains
or calculus using ultrasonic scalers and were
disinfected in 0.5% sodium hypochlorite solution.
Then teeth were stored in distilled water at room
temperature until the experiment starts. The teeth
were horizontally sectioned 2 mm coronally to
the cementoenamel junction with a double-faced
diamond disk in a slow-speed handpiece with
copious coolant, so that length of the root was
standardized in all samples to be 14 mm.
Root canal preparation:
The teeth were embedded vertically in epoxy
resin blocks (Polypoxy 700, polymer, chemical
industries for construction Co., CIC, Egypt). so
that 1 mm apical to the cementoenamel junction
was kept above the epoxy resin blocks.
Conservative access cavity was done for all
teeth and the working length was adjusted to be 1
mm shorter than apical foramen. The root canals
were instrumented using protaper rotary instrument
(Dentsply Maillefer, Ballaigues, Switzerland).
Master apical le was selected to be F5. 3% NaOcl
was used as an irrigation solution between les.
Paper points were used to dry the root canals.
Then, the canals were obturated using ProTaper
gutta-percha cones and a sealer (AH plus, Dentsply
Maillefer, USA) (7). Root canal treatment for all
teeth was done by the same operator.
Post space preparation:
For all samples, gutta percha was removed using
size #2,3 gates glidden (Mani Inc., Tochigi, Japan)
to a depth of 10 mm leaving 4 mm apical seal. The
teeth were then randomly divided into four equal
groups (n=8) according to the post and core system
used.
Group I: Single circular post technique.
Group II: Relined post technique.
Group III: Double circular post technique.
Group IV: Oval post technique.
Regarding groups I, II, III, post space was
prepared, 10 mm in depth, using special drill size #
3 (Rely x ber post drill, 3M, ESPE). As for group
IV, the post space was prepared using nishing drill
size # 2 (Macro-lock oval post drill, RTD, France)
up to the same 10 mm in depth as the previously
prepared three groups. Afterwards the root canals
for all teeth were irrigated using saline solution and
dried with paper points.
Post cementation
The posts were cemented in the root canals
according to the different techniques as follows:
Group I: The post was cleaned with an alcohol
swab and then dried. Rely X Unicem 2 automix
self-adhesive resin cement (3M, ESPE) was applied
inside the root canal with the endo tip to allow void
free application of the cement inside the root canal,
then rely x ber post size # 3 (3M, ESPE) was placed
and centralized inside the root canal and manually
stabilized using steady nger pressure for 1 minute
(9). The excess cement was removed using a cotton
swab and the cement was light polymerized with
LED curing light blue phase (Ivoclar, Vivadent,
Schaan, Liechtenstein) for 40s through the post in
an occluso-apical direction.
Group II: Before cementation of the post, the
post was customized to the shape of the canal using
composite resin. The canals were lubricated with
water-soluble gel and composite resin Filtek Z 250
(3M, ESPE) was applied around the 3 M rely x ber
post size # 3 that was used in group I. the post with
the surrounding composite was placed inside the
canal and the post was allowed in and out the canal
to customize the shape of the post according to the
canal shape. The relined post was light polymerized
while still in position inside the root canal for 5s
then removed from the canal and polymerized for
60s on each side of the post. The root canals were
irrigated using saline and dried with paper points.
The cementation process was done as previously
mentioned within group I.

ASSESSMENT OF PUSH OUT BOND STRENGTH AND CEMENT THICKNESS (3859)
Group III: the same process was done as was
previously mentioned in group I, but the main post
was placed (3M Rely X ber post size # 3) up to the
post space and one accessory post (3M Rely X ber
post size # 1) was positioned beside the main one as
apical as possible without pressure.
Group IV: Macro-Lock Illusion post X-RO size #
2 was cemented inside the root canals following the
same cementation protocol as that used for group I.
Composite core fabrication:
The extended coronal parts of the posts were
sectioned leaving 3 mm above the remaining tooth
structure. To restore the coronal part, core formers
were used to standardize the dimensions of the
core build up. The dimensions of the core were 7
mm height and 4 mm in diameter. Filtek Bulkll
posterior composite (3MTM, ESPETM) was used to
build up all the coronal part of all the teeth. All the
teeth were prepared to receive full coverage crowns
with 1 mm thickness chamfer nish line and 2 mm
ferrule height. The taper of the prepared coronal part
was standardized using parallel milling machine
(Bravo, Mariotti, Forli FC, Italy).
Specimen preparation for push-out bond strength test
For each sample, the roots were cut horizontally,
perpendicular to the long axis of the root using low
speed diamond saw under water coolant to obtain
three 2 mm ± 0.1 thick root specimens representing
coronal, middle and apical third of the root. The
thickness of each specimen was measured using
digital caliper (Pachymeter, Electronic Digital
Instruments, China). The coronal slice representing
the coronal third of the root was adjusted to be 2
mm from the cemento-enamel junction, the middle
slice representing the middle third of the root was
adjusted to be 5 mm from the cemento-enamel
junction, and the apical slice representing the
apical third of the root was adjusted to be 7 mm
from the cemento-enamel junction. (Fig.1) Each
section was coded and photographed from apical
and coronal surfaces using a stereomicroscope
(SZ-PT; Olympus, Tokyo, Japan) at an original
magnication of 65x. Calibration was performed by
comparing an object of known length, a ruler in this
study, using the ‘‘Set Scale’’ tool generated by the
image analysis software (Image J; NIH, Bethesda,
MD). The diameter of the post was then measured,
and the radius was calculated to measure the surface
area for each section.
Cement thickness measurements Fig. (2)
The cervical, middle and apical slices of each
root for all samples were dipped in 90 % alcohol,
air dried, xed on a metallic stub. (30). The resin
cement thickness around each specimen obtained
after slicing were measured using scanning electron
microscope (SEM). SEM image was documented
for each slice at (x 200, x400, x800) magnication
and eight standardized points were recorded around
the circumference of each slice; between the canal
wall and the ber post perimeter. The measurements
were recorded using the SEM software (30).
Push-out bond strength testing:
Each root slice was mounted in a custom made
loading xture [metallic block with circular cavity at
the middle, this cavity for specimen housing having
a central hole to facilitate displacement of extruded
post, then subjected to compressive loading at
Fig. (1): Diagram showing specimen slices prepared for push-
out bond strength test.

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a crosshead speed of 1 mm/min via a computer
controlled materials testing machine (Model 3345;
Instron Industrial Products, Norwood, MA, USA).
Load applied by plunger of 1, 0.8, 0.5 mm
diameter corresponding to the radicular thirds
(coronal, middle and apical) to be tested. The
plunger tip was sized and positioned to touch only
the post, without stressing the surrounding dentin,
in apical coronal direction to push the post toward
the larger diameter, thus avoiding any limitation to
the post possibly owing to the canal taper. This way,
it was guaranteed that the overlaying dentin was
sufciently supported during the loading process.
The maximum failure load was recorded in Newton
and converted into MPa. The bond strength was
calculated from the recorded peak load divided
by the computed surface area as calculated by the
following formula as regards to groups I, II, III: (31)
[A = (3.14x r1X 3.14x r2) L], Where:
r1 apical radius, r2 cervical one,
L= [(r1-r2)2+h2]0.5 and h is the thickness of the
sample in millimetres].
Regarding group IV with oval post, the surface
area was calculated as was stated before in a
previous study (21).
Failure manifested by extrusion of the post and
conrmed by sudden drop along load-deection
curve recorded by Nexygen computer software. The
push-out bond strength was determined for each
root slice.
Statistical Analysis
Data were presented as mean and standard
deviation (SD) values. Repeated measures Analysis
of Variance (ANOVA) was used to study the effect
of post technique, root level and their interaction
on mean shear bond strength and cement thickness.
Bonferroni’s post-hoc test was used for pair-wise
Fig. (2). SEM images comparing the cement thickness across different groups: (A) Circular post technique. (B) Double circular
post technique. (C) Relined post technique. (D) Oval post technique.

ASSESSMENT OF PUSH OUT BOND STRENGTH AND CEMENT THICKNESS (3861)
comparisons when ANOVA test is signicant.
Pearson’s correlation coefcient was used to study
the correlation between cement thickness and shear
bond strength. The signicance level was set at
P ≤ 0.05. Statistical analysis was performed with
IBM SPSS Statistics for Windows, Version 23.0.
Armonk, NY: IBM Corp.
RESULTS
1. Push out bond strength
Repeated measures ANOVA results showed
that post technique (P-value <0.001, Effect size =
0.995) and root level (P-value <0.001, Effect size =
0.859) had a statistically signicant effect on mean
push out bond strength. The interaction between the
two variables had a statistically signicant effect
on mean push out bond strength indicating that the
variables are dependent upon each other. Table. (1)
The mean, standard deviation (SD) values
and results of repeated measures ANOVA test for
comparison between push out bond strength of the
different post techniques revealed that oval post
showed the statistically signicant highest mean
push out bond strength. Relined post technique
showed statistically signicantly lower mean
value followed by double circular technique.
Single circular technique showed the statistically
signicant lowest mean bond strength. (Fig. 3, table
2, 4).
Fig. (3). Bar chart representing mean and standard deviation
values for push out bond strength for different post
techniques.
TABLE (1) Repeated measures ANOVA results for the effect of different variables on mean push out bond
strength.
Source of variation Type III Sum
of Squares Df Mean Square F-value P-value Effect size
(Partial eta squared)
Post technique 231.414 3 77.138 1143.840 <0.001* 0.995
Root level 21.540 2 10.770 97.586 <0.001* 0.859
Post technique x Root
level interaction 4.959 6 0.826 7.488 <0.001* 0.584
df: degrees of freedom = (n-1), *: Signicant at P ≤ 0.05
TABLE (2) The mean, standard deviation (SD) values and results of repeated measures ANOVA test for
comparison between push out bond strength of the different post techniques
Single circular Relined Double circular Oval P-value Effect size (Partial
eta squared)
Mean SD Mean SD Mean SD Mean SD
5.86 D0.55 8.8 B0.63 7.83 C0.67 11.32 A1.04 <0.001 0.995
*: Signicant at P ≤ 0.05, Different superscripts are statistically signicantly different

(3862) Khaled A. Elbanna, et al.E.D.J. Vol. 65, No. 4
The mean, standard deviation (SD) values
and results of repeated measures ANOVA test
for comparison between push out bond strength
at different root levels revealed that there was no
statistically signicant difference between coronal
and middle root levels; both showed statistically
signicantly higher mean bond strength than apical
root level. (Fig. 4, table 3,4).
2. Cement thickness
Repeated measures ANOVA results for cement
thickness showed that both post technique (P-value
<0.001, Effect size = 0.994) and root levels (P-value
<0.001, Effect size = 0.963) had a statistically
signicant effect on mean cement thickness.
The interaction between the two variables had
a statistically signicant effect on mean cement
thickness indicating that the variables are dependent
upon each other. Table (5)
The mean, standard deviation (SD) values
and results of repeated measures ANOVA test
for comparison between cement thickness of the
four post techniques revealed that single circular
technique showed the statistically signicantly
highest mean cement thickness. Double circular
TABLE (3) The mean, standard deviation (SD) values and results of repeated measures ANOVA test for
comparison between shear bond strength of different Root levels regardless of post technique.
Coronal Middle Apical
P-value Effect size
(Partial eta squared)
Mean SD Mean SD Mean SD
8.76 A2.35 8.99 A2 7.62 B1.78 <0.001* 0.859
*: Signicant at P ≤ 0.05, Different superscripts are statistically signicantly different
TABLE (4) The mean, standard deviation (SD) values and results of repeated measures ANOVA test for
comparison between push out bond strength values with different interactions of variables
Root level
Single circular Relined Double circular Oval
P-value
Effect
size
(Partial
Mean SD Mean SD Mean SD Mean SD
Coronal 6 DE 0.09 8.87 BF 0.37 7.88 CF 0.24 12.28 AE 0.2 <0.001* 0.991
Middle 6.3 DE 0.25 9.44 BE 0.36 8.53 CE 0.4 11.67 AF 0.24 <0.001* 0.978
Apical 5.28 DF 0.53 8.11 BG 0.15 7.07 CG 0.17 9.99 AG 0.39 <0.001* 0.967
P-value 0.003* <0.001* <0.001* <0.001*
Effect size (Partial
eta squared) 0.547 0.650 0.692 0.897
*: Signicant at P ≤ 0.05, A, B, C, D superscripts in the same row indicate statistically signicant difference between post
techniques,E, F, G superscripts in the same column indicate statistically signicant difference between root levels
Fig. (4) Bar chart representing mean and standard deviation
values for push out bond strength at different root
levels.

ASSESSMENT OF PUSH OUT BOND STRENGTH AND CEMENT THICKNESS (3863)
technique showed statistically signicantly lower
mean value followed by oval technique. Relined
technique showed the statistically signicantly
lowest mean cement thickness, (Fig.5, table 6).
The mean, standard deviation (SD) values
and results of repeated measures ANOVA test
for comparison between cement thicknesses of
different root levels revealed that coronal root level
showed the statistically signicant highest mean
cement thickness followed by the middle root level
Whereas the apical level showed the statistically
signicant lowest mean cement thickness. (Fig.6,
table 7,8).
Correlation between cement thickness and push
out bond strength whether at the coronal, middle or
apical levels showed that there was a statistically
signicant inverse correlation between cement
thickness and push out bond strength
Fig. (5). Bar chart representing mean and standard deviation
values for cement thickness of different post techniques
regardless of root level.
Fig. (6). Bar chart representing mean and standard deviation
values for cement thickness of different root levels
regardless of post technique
TABLE (6) The mean, standard deviation (SD) values and results of repeated measures ANOVA test for
comparison between cement thickness of the four post techniques.
Single circular Relined Double circular Oval
P-value Effect size (Partial
eta squared)
Mean SD Mean SD Mean SD Mean SD
268.3 A105.2 16.28 D2.45 156.67 B48.11 80.97 C9.63 <0.001* 0.994
*: Signicant at P ≤ 0.05, Different superscripts are statistically signicantly different
TABLE (5) Repeated measures ANOVA results for the effect of different variables on mean cement thickness.
Source of variation Type III Sum of
Squares Df Mean Square F-value P-value Effect size (Partial
eta squared)
Post technique 527602.130 3 175867.377 879.506 <0.001* 0.994
Root level 87422.914 2 43711.457 413.170 <0.001* 0.963
Post technique x Root
level interaction 94590.639 6 15765.107 149.015 <0.001* 0.965
df: degrees of freedom = (n-1), *: Signicant at P ≤ 0.05

(3864) Khaled A. Elbanna, et al.E.D.J. Vol. 65, No. 4
TABLE (7). The mean, standard deviation (SD) values and results of repeated measures ANOVA test for
comparison between cement thicknesses of different Root levels regardless of post technique.
Coronal Middle Apical
P-value Effect size (Partial
eta squared)
Mean SD Mean SD Mean SD
181.69 A147.6 119.97 B92.22 90 C53.04 <0.001* 0.963
*Signicant at P ≤ 0.05, Different superscripts are statistically signicantly different
TABLE (8) The mean, standard deviation (SD) values and results of repeated measures ANOVA test for
comparison between cement thickness values with different interactions of variables.
Root level Single circular Relined Double circular Oval P-value Effect size
(Partial
Mean SD Mean SD Mean SD Mean SD
Coronal 395.13 AE 31.34 17.49 D1.62 220.94
BE 8.8 93.19 CE 4.03 <0.001* 0.990
Middle 285.54 AF 19.03 17.72 D0.77 130.77
BF 5.93 72.85 CF 1.99 <0.001* 0.990
Apical 151.23 AG 1.21 13.62 D2.12 118.3 BG 12.07 76.85 CF 3.77 <0.001* 0.988
P-value <0.001* 0.734 <0.001* 0.007*
Effect size (Partial
eta squared) 0.982 0.040 0.941 0.486
A, B, C, D superscripts in the same row indicate statistically signicant difference between post techniques,
E, F, G superscripts in the same column indicate statistically signicant difference between root levels
TABLE (9) Results of Pearson’s correlation coefcient for the correlation between cement thickness and
push out bond strength at different root levels.
Coronal Middle Apical
Correlation
coefcient (r) P-value Correlation
coefcient (r) P-value Correlation
coefcient (r) P-value
-0.724 <0.001* -0.785 <0.001* -0.637 0.003*
*: Signicant at P ≤ 0.05

ASSESSMENT OF PUSH OUT BOND STRENGTH AND CEMENT THICKNESS (3865)
(r = -0.724, P-value <0.001), (r = -0.785,
P-value <0.001) and (r = -0.637, P-value = 0.003),
respectively. An increase in cement thickness is
associated with a decrease in push out bond strength
and vice versa. Table (9)
DISCUSSION
Over the years, the increased use of ber
reinforced posts has been well documented, owing
to the unique property of matched elastic modulus
to the human dentin. This property along with the
use of the resin luting cements for cementation
provided a perfect bond between dentin and bre
post allowing uniform stress distribution throughout
the root dentin and therefore reducing the incidence
of root fractures. (32,33,34,35)
Commonly, the ber posts are rounded in cross
section, however, the oval root canals have been
stated as a common conguration with a high
percentage in some teeth. Oval ber posts have been
introduced in the market so that better adaptation
and improved bond strength to the dentin canal
walls can be achieved within the oval and ared
canal shapes. (36)
The aim of this study was to evaluate the effect
of different post techniques used with oval canals
on the push-out bond strength between the ber post
and root canal dentin at different root canal levels.
Mandibular human premolars with oval canals were
selected in this study to simulate clinical conditions,
(34) however, the human teeth represented a relatively
large variation in morphology size and mechanical
properties of the specimens.(37,38) This limitation
has been overcome through standardization of
the dimensions of the roots for the selected teeth,
endodontic treatment and post space preparation
with similar dimensions throughout all the selected
teeth for all the tested groups.
Inadequate bonding strength resulted in
debonding between post and root canal dentin. The
push out bond strength test was used in this study
to determine the bond strength between different
post techniques in oval root canals. This test
gives rise to more uniform stress distribution than
conventional shear or tensile tests. (23) The fracture
takes place parallel to the dentin adhesive interface
within the push out test, which makes it a true shear
test. (39) The shear stresses arise with the push out
test is comparable to the actual clinical conditions,
thus representing the true interfacial bond strength
between dentin canal walls and the ber post.(10,23)
Among the different research methods used, the
push out test examined the bond strength of ber
posts to root with less data distribution,variability,
more uniform values and less premature failures. (40)
The null hypothesis of this study was rejected
as the push out bond strength of the ber post was
inuenced by both the different post techniques
and different post levels along the canal length.
The results of this study revealed that there was a
statistically signicant difference between mean
push out bond strength values of different post
techniques regardless of root levels. The oval post
technique showed statistically signicant highest
mean push out bond strength values. This could be
attributed to the increased adaptation of the oval
post to the oval canal shape resulting in decreased
thickness of surrounding luting cement. This is
conrmed by the results of the cement thickness
measurements done using the scanning electron
microscope, which revealed lower cement thickness
readings than single circular and double circular
post techniques.
Up to date, in the literature, the studies correlating
the push out bond strength of the post with the
shape of the post and cement thickness along the
root are controversial. Some researchers proved that
a thinner cement thickness resulted in greater bond
strength values, others conrmed (41,42) that a thick
cement layer between canal walls and ber post
causes adhesive post failures and debonding. On
the contrary,(28,43) some authors did not support these

(3866) Khaled A. Elbanna, et al.E.D.J. Vol. 65, No. 4
results and showed that a thicker luting cement
improved the retention of bre posts. (44,45) While,
other in vitro studies concluded that alterations in
cement thickness layer do not signicantly inuence
the retentive strength of the cement with the bre
post. (46,47)
Regarding the single circular post technique, the
authors point of view was assured through analysing
the results of this group which showed least push
out bond strength values and largest cement
thickness layer. The excessively thick cement layer
might have resulted in low bond strength values
and higher frequencies of post debonding. It is
worth mentioning that this thick layer of cement
that lls the space between the oval canal wall and
the ill-tting circular post has led to the presence
of bubbles or voids within the cement layer. This
represented areas of weakness within the whole
assembly. Additionally, the large cement thickness
might be an inuencing factor in induced shrinkage
stresses caused by polymerization shrinkage of the
luting cement which is known as C-factor. This
might have intensied the stress concentration and
subsequent failure in bonding. This gives another
explanation to the causes of the weak bond strength
showed within the single circular post technique
group. This analysis has been well documented
previously in several previous studies. (28,48)
As regards to the results of the double circular
post technique group which revealed lower statisti-
cally signicant bond strength values than oval post
technique but higher statistically signicant bond
strength values than single circular post technique.
This technique has been tried as an attempt to im-
prove adaptation of the post to oval canal walls and
decrease luting cement thickness. However, the re-
sults were not promising, this might be contributed
to the air entrapment within the cement layer upon
insertion of the second post, as was observed within
the scanning electron microscope photos with most
specimens within this group. Air entrapment occurs
most frequently between the two posts resulting in
non-uniform cement layer and unstable stress dis-
tribution throughout the canal walls which compro-
mised the bond strength of the post. (7,49) The prac-
ticality of using multiple posts with oval or ared
canals has been discussed in some former studies al-
though the results were disputing and not clear. (8,50)
The fabrication of the anatomic post through
customization of the circular bre post with resin
composite all around the post to take the exact
shape of the root canals gives also encouraging
results. This might be attributed to the least cement
layer thickness obtained within this post technique.
In previous studies, the authors interpreted the
high bond strength values of the relined group by
the increased frictional retention, highlighting the
effective retention of the relined bre posts to the
dentin through sliding friction against dentinal
walls of root canals rather than true adhesion.(26,51)
It is notable that even though the post technique
showed lower cement thickness values than oval
post technique, the latter revealed better bond
strength values than relined post technique. This
point needs further investigations on whether the
resin composite surrounding the post had led to
any changes in the stress distribution within the
multiphase assembly of the post systems.
Concerning the results of the push out bond
strength at different root levels regardless of
post techniques, it has been concluded that the
coronal and middle root levels showed statistically
signicant higher mean push out bond strength
values than apical root level for all groups except
the relined group where the coronal and middle
were higher than the apical but with no statistical
signicant difference between the three root levels.
This may be correlated to the decreased number of
the dentinal tubules in the apical third resulting in
reduced intratubular diffusion of composite resin
luting cement at this area. This has been discussed in
previous studies earlier in the literature.(34, 51) Some

ASSESSMENT OF PUSH OUT BOND STRENGTH AND CEMENT THICKNESS (3867)
authors stated that the apical bonding is critical
as well due to difcult accessibility and cleaning
within these narrow diameters, correspondingly
the presence of gutta percha remnants hinder the
bonding capability at this region. (52,53) Besides,
incomplete polymerization of resin luting cements
may be a factor in reduction of the push out bond
strength in deeper apical areas. (54)
It is important to mention that dual cure self-
adhesive resin cement was used in this study for
cementation of all post techniques. This was chosen
due to its less technique sensitivity. Dual curing
cement was selected as it is supposed to adequately
polymerize in deeper areas of the post space that
cannot be reached by light. However, it was stated
that sometimes with the absence of light, some dual
curing cements may not reach an adequate degree
of polymerization.(54,55) It is worth mentioning that
Rely X Unicem cement was introduced into the post
space using the cement long narrow tip to reduce
the tendency of air entrapment and introduction of
bubbles or gaps conrming a high-quality void free
cement lm, although voids in the cement layer
were noticed in some specimens especially with
single and double circular post techniques.
It is of signicance to state that the oval ber
post used in this study is made of quartz ber and
the circular posts are made of glass ber. This might
have presented a limitation to the present study.
Similarily,Uzun et al (36) compared the oval quartz
ber with the circular glass bers and reported
higher bond strength to the oval posts. Other studies
compared quartz ber post with quartz circular post
and revealed similar bond strengths in oval shaped
canals.(10,21) On contrary,Some authors concluded
insignicant differences among different ber post
types .(56)
The addition of thermal and mechanical loading
as well as aging process might have given more
clinical simulation to the present study which is
considered one of the limitations of this study.
As the bre posts have been popularly used
nowadays, further studies should be conducted to
assess the bond strength of different post techniques
after articial aging which might give more realistic
results. The de-bonded posts must also be examined
under magnication to determine the mode of
failure of different post techniques.
CONCLUSIONS
Within the limitations of this in vitro study, the
following may be concluded:
- The use of single circular post technique in oval
or ared canals is not a good option for long term
clinical service.
- Decreasing the cement thickness space around
the post systems in oval canals will improve the
push out bond strength of posts to dentinal walls of
the root canals.
- The introduction of oval posts for restoration
of endodontically treated teeth with oval and ared
root canals gives promising results in push out bond
strength tests.
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