general dentistry

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radicular post to retain the restoration of the ... root. Increased post retention and fracture resis- ... tubules at different levels of the root canal ... and used within 2 weeks of extraction, were ... tooth). The root canals were rinsed with 17% ethylenediaminetetraacetic acid (EDTA) and ... a 22-gauge needle-irrigating tip (Ultradent).
Q U I N T E S S E N C E I N T E R N AT I O N A L

GENERAL DENTISTRY Effect of post diameter and cement thickness on bond strength of fiber posts Hesam Mirmohammadi, DDS, MSc, PhD1/Elie Gerges, DDS, MSc2/Ziad Salameh, DDS, PhD3/Paul Rudolf Wesselink, DDS, MSc, PhD4

Objective: This study evaluated the effects of different post diameter and oversized post spaces on the push-out bond strength of a fiber post to dentin. Method and Materials: Fifty extracted human maxillary central incisors and canines were divided into five groups and submitted to the push-out test (0.5 mm min-1). Groups 1, 2, and 3 were restored using a fiber post size that was identical to the drill size (sizes 1, 2, and 3, respectively), and groups 4 and 5 were both prepared with drill size 3, and restored using the size 2 and 1 fiber post, respectively. The fiber posts were cemented using self-adhesive dual-polymerized resin cement (RelyX Unicem). The data were analyzed using a one-way analysis of variance (ANOVA) and the Tukey test. Results: There were no significant differences in the mean values for push-out bond strength between groups with different post diameters (P > .05). However, the push-out bond strengths were significantly different between groups with different cement thicknesses, and group 4 yielded the highest bond strength (11.7 ± 0.4 MPa). For all groups, the apical third had the lowest bond strength value (P < .05). Conclusion: The highest push-out bond strength values were obtained when one incremental oversized post space was used. Clinically, fiber post space has to provide an optimum cement thickness (around 120 μm) for adequate cementation. (Quintessence Int 2013;10:801–810; doi: 10.3290/j.qi.a30179)

Key words: bond strength, cement thickness, fiber post, push-out, self-adhesive cement

Endodontically treated teeth that have

Adhesive resin composite cements,

reduced coronal tooth structure require a

which have an elastic modulus in the same

radicular post to retain the restoration of the

range as both the post and dentin, are rou-

tooth. Recently, the increasing demand for

tinely used to lute the post into the root.

esthetic posts and cores has led to the

Increased post retention and fracture resis-

development of metal-free post and core

tance and reduced microleakage have

systems, which serve as alternatives to cast

been reported for posts cemented with

post and core systems and metal posts.1-3

composite resin cements, as compared to conventional luting systems.4,5 However, the

1

Researcher, Department of Cariology Endodontology Pedo-

most common cause of failure in previous

dontology, Academic Centre for Dentistry Amsterdam (ACTA),

studies was due to pull-out of the cement-

Universiteit van Amsterdam and Vrije Universiteit, Amsterdam,

post-restoration assembly, which is caused

The Netherlands; and Assistant Professor, Department of Re-

by debonding between fiber post and resin

storative Dentistry, School of Dentistry, Esfahan University of Medical Sciences, Esfahan, Iran. 2

Assistant Professor, Department of Prosthodontics, School of Dentistry, Lebanese University, Beirut, Lebanon.

3

Associate Professor, Department of Prosthodontics, School of

cement and/or between resin cement and root canal dentin interface and inadequate bond strength.6,7 Thus, there is a need to investigate and improve the adhesion

Dentistry, Lebanese University, Beirut, Lebanon.

between the fiber post, resin cement, and

Em Professor, Department of Cariology Endodontology Pedo-

root canal dentin interfaces.

4

dontology, Academic Centre for Dentistry Amsterdam (ACTA),

The actual bond strength at the post-

Universiteit van Amsterdam and Vrije Universiteit, Amsterdam,

cement-root interface is influenced by many

The Netherlands. Correspondence: Dr S. Hesam Mirmohammadi, ACTA, Department of Cariology Endodontology Pedodontology, Gustav

factors, including the type of conditioning agent and the accompanying cement used,

Mahlerlaan 3004, 1081 LA Amsterdam, The Netherlands. Email:

the type of post, the cavity configuration of

[email protected]

the root canal, and the anatomic differences

VOLUME 44 • NUMBER 10 • NOVEMBER/DECEMBER 2013

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Q U I N T E S S E N C E I N T E R N AT I O N A L Mirmohammadi et al

in density and orientation of the dentinal

increase in retention when the diameter of

tubules at different levels of the root canal

the metallic posts was increased from

area.8-13 Furthermore, difficult moisture con-

0.5 mm to 1.1 mm at the apex. Moreover,

trol, polymerization stress inside of the root

when the length was increased from 9 mm

canal, and visualization difficulties can

to 15 mm, there was a 100% increase in

affect the bonding procedures as well.14,15

retention. Many in vitro studies suggested

Various adhesive systems and accom-

the influence of different resin cement thick-

panying resin cements have been used that

nesses on bond strength of fiber posts.27,29-

are able to remove or condition the smear

32

layer prior to luting fiber-reinforced compos-

offer a consensus as to the ideal thickness

However, to date, the literature does not

ite resin posts into root canals. Zicari et al16

of the resin cement or to the ideal fiber post

suggested that RelyX Unicem (RX) had a

diameter and length needed to improve

high tolerance for moisture because water

retention. As debonding is the most fre-

was formed during the neutralization reac-

quent cause of failed bonded fiber posts, a

tion of the phosphoric acid methacrylate,

better understanding of the factors that

basic fillers, and hydroxyapatite, which was

influence bond strength is required.

considered responsible for the greater

The aim of the present study was to

bond strength values in the apical sections

evaluate the effect of oversized post spaces

of the roots. Farina et al 17 also demon-

and the diameter of the posts on the push-

strated increased bond strength using RX

out bond strength of a fiber post. The null

for the cementation of glass fiber posts.

hypothesis was that post diameter and the

Pirani et al18 reported that the retentive

width of the gap between the root canal and

strength of a bonded post could be due to

post would have no influence on the push-

the combined result of micromechanical

out bond strength of the fiber posts.

interlocking, chemical bonding, and sliding friction. Recently, Dimitrouli et al19 suggested that the use of self-adhesive resin

METHOD AND MATERIALS

cement (SA) could provide bond strength values comparable to bond strength values

Fifty recently extracted human maxillary

of the etch-and-rinse systems. It has also

canines and central incisors (mean length

been shown that a modified application

23 ± 2 mm), which were stored in an aque-

procedure using SA in combination with

ous solution of 0.525% chloramine-T at 4°C

single-step self-etch dentin adhesives did

and used within 2 weeks of extraction, were

not improve the bond strength of the fiber

selected for this study. External debris was

post when compared to conventional SA.20

removed from the teeth using an ultrasonic

Although a significant body of informa-

scaler (Parkell Electronics Division). Radio-

tion has been published regarding the

graphs were used to eliminate specimens

retentive properties of prefabricated post

with irregular canals, pronounced canal

systems cemented with resin cements,21-24

curvatures, or evidence of internal resorp-

limited information is available regarding

tion. Teeth with roots shorter than 10 mm or

the effect of cement thickness and post

with defects or cracks were excluded. Teeth

diameter on the retention of fiber posts. On

were decoronated 2 mm coronally to the

the other hand, retrospective clinical stud-

most incisal point of the cementoenamel

ies have demonstrated that debonding

junction (CEJ) using a low-speed diamond

occurs

saw (Isomet 2000, Buehler) under copious

with

large

cement

film

thick-

nesses.25,26 One study also suggested that

water cooling.

resin cement film thickness influences the

Root canal preparation was performed at

pull-out strength of the fiber posts and

a working length of 0.5 mm from the apical

reported that the greatest bond strength

foramen (defined visually) using rotary

values were obtained when oversized post

instruments (FlexMaster, VDW) and a step-

spaces were used.27

back technique. Canals were prepared to

A previous study suggested that post

the ISO file size of 40 with a 6-degree taper.

diameter could affect retention of the metal

The root canals were irrigated between

posts. Nergiz et al 28 reported a 60%

analyses, and the canal spaces remained

802

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Q U I N T E S S E N C E I N T E R N AT I O N A L Mirmohammadi et al

filled with irrigation solution (5% NaOCl) dur-

a 22-gauge needle-irrigating tip (Ultradent)

ing analyses (approximately 10 ml per

and dried with sterile paper points. A self-

tooth). The root canals were rinsed with 17%

adhesive cement (RelyX UniCem Aplicap

ethylenediaminetetraacetic acid (EDTA) and

cement, 3M ESPE) was used for luting of the

final irrigation with 0.9% saline solution was

different posts. The capsule of the cement

performed. Root canals were dried with

was first activated for 2 to 4 seconds and

paper points (Coltene Whaledent), and

then mixed (Capmix, 3M ESPE) for 15 sec-

obturated with a thermoplasticized inject-

onds. The cement (2 to 3 mm) was dis-

able gutta-percha (Obtura II, Model 823-

pensed from an auto-mixing syringe that

600, Obtura) using AH Plus (Dentsply) as a

had been placed in the canal spaces with

canal sealer. The teeth were then stored in

an elongation tip (30 gauge), according to

water for 24 hours at 37°C. The gutta-per-

the manufacturer’s instructions.

cha was removed with a size 10 electrically

was introduced and held under digital pres-

heated endodontic plugger (Hu-Friedy).

sure. The excess cement was removed and

Three drill diameters (0.7 to 1.3 mm, 0.8 to

the specimen was left at room temperature

1.6 mm, and 0.9 to 1.9 mm) were randomly

for setting of the cement. The different post

combined with three fiber post sizes (RelyX

sizes used for each group were as follows:

fiber post, sizes 0.7 to 1.3 mm, 0.8 to



The post

Group 1: The canals for this group were

1.6 mm, and 0.9 to 1.9 mm; 3M ESPE) to

prepared with a size 1 low-speed post

yield five groups of 10 teeth each (Table 1).

drill (apical 0.7 mm, coronal 1.3 mm

Each drill was used to prepare the post

diameter) in combination with a size 1

space for 10 teeth and was then discarded.

(0.7 to 1.3 mm diameter) fiber post, as

To ensure standardization of post inser-

provided by the manufacturer (control

tion and removal, all post-space preparations were performed by the same oper-

group). •

Group 2: This group received the same

ator using a dental surveyor (Fedi 18

procedure as group 1, but the size 2 drill

Mariotti). The roots were transferred into an

(0.8 to 1.6 mm diameter) was used in

acrylic mount using an autopolymerizing

combination with size 2 (0.8 to 1.6 mm

resin (LD Caulk Division, Dentsply) that extended 1 mm below the buccal side of

diameter) fiber posts. •

Group 3: The group received the same

the CEJ. Immediately after the acrylic resin

procedure as group 1, but the size 3 drill

reached its doughy stage, the specimens

(0.9 to 1.9 mm diameter) was used in

were detached from the surveyor and

combination with size 3 (0.9 to 1.9 mm diameter) fiber posts.

placed in a cool water bath. Subsequently, the mounted specimens were secured in the



Group 4: This group received a similar

dental surveyor, and the coronal 9 mm of

procedure to group 3 (0.9 to 1.9 mm drill

each root canal was prepared with a drill of

diameter), but size 2 (0.8 to 1.6 mm

the same size to leave a minimum apical seal of 4 mm for the gutta-percha. Following

diameter) fiber posts were used. •

Group 5: The group received a similar

the preparation of the post space, the

procedure to group 3, but size 1 (0.7 to

canals were rinsed with saline solution using

1.3 mm diameter) fiber posts were used.

Table 1

Group

Distribution of specimens according to drill diameter, post diameter, and cement thickness Drill diameter (mm)

Post diameter (mm)

Approx. cement thickness (μm) 20

Group 1

0.7–1.3

0.7–1.3

Group 2

0.8–1.6

0.8–1.6

20

Group 3

0.9–1.9

0.9–1.9

20

Group 4

0.9–1.9

0.8–1.6

120

Group 5

0.9–1.9

0.7–1.3

210

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Preparation of specimens for the push-out bond strength test

Failure mode analysis

After 24 hours of water storage at 37°C, the

assessed initially using a stereomicroscope

roots were cut into three 2-mm-thick slices

(Swift Stereo Eighty microscope, Swift

perpendicular to the long axis of the tooth

Instruments International) and then at a

using the Isomet saw under conditions of

higher magnification (500× magnification)

water cooling. The first cut was initiated

using scanning electron microscopy (SEM)

2 mm below the CEJ and was followed by

to determine the mode of failure, which was

two additional cuts that were 2 mm thick.

classified as follows:

Thus, the slices represented coronal, mid-



After the push-out test, the specimens were

dle, and apical sections of the post-space

post and the cement without visible

preparation. A 1-mm-thick slice was then removed from each sample using the same

cement around the post •

saw. These slices were fixed onto slides

Mode II: adhesive failures between the cement and the root dentin with cement covering the post surface

and polished up to 600 grit. The specimens were then subjected to a push-out test

Mode I: adhesive failures between the



Mode

III:

mixed

failures

(adhesive

using a universal testing machine (Accu-

between the cement and dentin and

force Elite Test Stand, Ametek, Mansfield &

cohesive in the resin cement) with

Green Division) and a cross-head speed of 0.5 mm/min. Due to the tapered design of the posts, three different sized punch pins

cement covering the post surface •

Mode IV: cohesive failures inside the dentin.

(0.5 mm, 0.8 mm, and 1.2 mm) were used

For SEM evaluation, the samples were ultra-

for the push-out testing.33

sonically cleaned using alcohol 90%, air-

The maximum stress was calculated by

dried, mounted on a metallic stub, gold

dividing the recorded peak load by the inter-

sputtered (JEOL, Fine Coat, Ion Sputter

facial area (SL) of the post fragment. To cal-

JFC-1100), and then observed under SEM

culate the exact bonding surface, the tapered

(XL 20; Philips).

design and the respective portions of the posts were considered. Therefore, each

Cement thickness evaluation

specimen was measured using a micrometer

Before push-out bond strength testing, two

screw (Ultra-Cal IV, Ted Pella), and the SL

sample specimens from the second apical

was calculated using the following formula:

slices of each group were evaluated by SEM

SL = π (R + r) [h2 + (R − r)2]0.5

(500× magnification) to compare the cement

where π = 3.14, R = coronal post radius,

thicknesses between groups. As the cement

r = apical post radius, and h = specimen

thickness surrounding the post was variable,

slice thickness.

the lowest thickness was reported as the cement thickness for each group (see Fig 1).

FP 210 μm

FP 40 μm

D

Rc

Rc 120 μm

FP

20 μm D a

Rc b

D

c

Figs 1a to c SEM images comparing the cement thicknesses across different groups (D, dentin; FP, fiber post; Rc, resin cement). (a) The image represents the cement thickness in groups 1, 2, and 3 showing the 20 μm thickness at the thinnest part of the resin cement. (b) The image indicates the cement thickness in group 4 (one oversized post space), which represents a cement thickness of 120 μm. (c) An image of the specimen from group 5 (two oversized post spaces) showing a cement thickness of 210 μm. The arrows indicate air bubbles, which were larger in this group.

804

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Q U I N T E S S E N C E I N T E R N AT I O N A L Mirmohammadi et al

Table 2

Mean values for push-out bond strength (MPa) by group

Bond strength (mean ± SD)

Group 1

Group 2

Group 3

Group 4

Group 5

9.2b ± 1.00

10b ± 0.7

9.5b ± 0.6

11.7a ± 0.4

9.5b ± 0.63

a,b

The same lower case letters indicate no significant difference between each group (P < .05).

Statistical analysis After performing the tests of normality of data

Table 3

distribution and homogeneity of group variances, a one-way ANOVA was performed to examine the effect of post diameter on interfacial strength for groups 1, 2, and 3 and to assess the effect of cement thickness on

Mean values for push-out bond strengths by root section (MPa) Root section

Group Group 1

post-hoc multiple comparisons were performed using the Tukey test (P < .05).

Group 2

For the variable regional bond strength, identical statistical tests were performed, although a Welch test was used for group 5

Group 3

because the variances were not homogenous. Therefore, a post-hoc analysis was

N

10.266 ± 0.931

20

Middle

9.179a ± 1.265

20

Apical

7.962b ± 0.927

13

Coronal

10.366a ± 0.619

20

Middle

10.685a ± 1.2

20

b

Apical

8.524 ± 0.975

14

Coronal

9.825a ± 0.839

20

Middle

9.826a ± 0.835

20

Apical

performed using Dunnett’s C test. The

a

Coronal

interfacial strength for groups 1, 4, and 5. For comparisons with significant differences,

Push-out bond strength (mean ± SD)

b

8.358 ± 0.570

12

Coronal

13.201a ± 0.673

20

region of each group was measured in

Middle

12.577b ± 0.601

20

order to find the mean push-out bond

Apical

9.431c ± 0.643

13

mean between coronal, middle, and apical

strength for each group. All statistical analy-

Group 4

Group 5

ses were performed using statistical soft-

Coronal

10.639a ± 0.547

20

Middle

9.832b ± 0.490

20

Apical

8.056c ± 0.839

13

ware (SPSS Windows Standard Version 15.0; SPSS). a,b

The same lower case letters indicate no significant dif-

ference within each group (P < .05).

RESULTS Push-out test

groups demonstrated significantly higher

The overall mean bond strength values and

bond

standard deviations for each group are pre-

(P < .05, Table 3). By optical microscopy

strengths

than

the

apical

third

sented in Table 2. The results indicated that

and SEM, most of the failures (approxi-

there were no significant differences in

mately 59% to 62%) were found to occur

mean push-out bond strength between

between the post and the resin cement

groups

diameters

without any visible cement around the post

(P < .05; group 1, 9.2 ± 1 MPa; group 2,

(mode I). Group 4 was the exception, as

10 ± 0.7 MPa; group 3, 9.5 ± 0.6 MPa).

this group demonstrated 45% mode I failure

However, the push-out bond strengths

and 39% mode III failure (Table 4).

with

different

post

between groups with different cement thicknesses (groups 3, 4, and 5) were found to

Microscopic observation

be significant, and group 4 yielded the

An analysis of the specimens under SEM

highest bond strength (11.7 ± 0.4 MPa).

revealed that the cement thicknesses were

Regarding the variable root regions, the

of the same order for groups 1, 2, and 3

coronal and middle thirds from each of the

(approximately 20 μm), whereas those of

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Q U I N T E S S E N C E I N T E R N AT I O N A L Mirmohammadi et al

Table 4

Distribution and percentage of failure modes following push-out test

Failure mode

Group 1

Group 2

Group 3

Group 4

Group 5

Mode I

33 (62%)

33 (61%)

31 (59%)

24 (45%)

32 (61%)

Mode II

6 (11%)

7 (13%)

6 (12%)

5 (10%)

7 (13%)

Mode III

10 (19%)

11 (20%)

12 (23%)

21 (39%)

10 (19%)

Mode IV

4 (8%)

3 (6%)

3 (6%)

3 (6%)

3 (6%)

53 (100%)

54 (100%)

52 (100%)

53 (100%)

52 (100%)

Total

groups 4 and 5 were significantly larger at

clinical success on the luting procedure. RX

approximately 120 μm and 210 μm, re-

is recommended for the cementation of the

spectively (Fig 1). Residual gutta-percha

fiber post; this newly developed self-adhe-

was found in some sections, which were

sive and dual-cure resin cement does not

associated with areas of debonding at the

require conditioning or bonding of the tooth

interface around the gutta-percha.

structure, which therefore reduces the highly technique-sensitive nature of multiple-step cementing procedures. RX con-

DISCUSSION

tains multifunctional hydrophilic monomers with phosphoric acid groups, which can

The aim of this study was to determine

react with hydroxyapatite and infiltrate and

whether

strengths

modify the smear layer. 39,40 It has been

between glass fiber posts and root canal

shown that light activation of the dual-cured

dentin depended on resin cement thickness

resin cement from the canal orifice offers

or post diameter. The tested null hypothesis

the advantage of establishing an immediate

must be partially rejected, as the push-out

coronal seal, although this technique also

the

push-out

bond

bond strength of the tested fiber post was

blocks the pathway for stress relief.8 There-

significantly influenced by the resin cement

fore, in the present study, RX was not light-

thickness; however, post diameter had no

cured. Compared to the results of this

significant influence.

study, previous studies reported lower bond

Regarding the method and materials,

strength values when using RX for the

the main disadvantage of using human

cementation of fiber posts.16,41 Goracci et

teeth was the relatively large variation in

al41 reported a mean bond strength value of

morphology, size, and mechanical proper-

5.1 MPa, and speculated that the self-adhe-

ties of the specimens.34,35 Previous studies

sive resin cement was ineffective for etch-

have reported that intact natural central

ing through the thick smear layer produced

incisors and canines are optimal for clini-

during the post-space preparation. How-

cally simulating the treatment of endodonti-

ever, Dimitrouli et al19 found greater bond

cally treated teeth with an endodontic post

strength values (11.3 ± 8.8 MPa) by strictly

system.36-38 The post system used in this

following the manufacturer’s recommenda-

study, which consisted of the RelyX fiber

tions for luting the fiber posts. This differ-

post, involves a parallel, tapered-end glass

ence may be explained by the non-uniform

fiber post that offers the potential advan-

resin-cement interface formation, by the

tages of reduced stress concentration at

application of cement only to the post sur-

the apex and the preservation of tooth

face (in the study of Goracci et al41), or by

structure. This system also allows for a uni-

variation in the resilience of the testing

form progressive increase in cement thick-

machines at different laboratories.

ness and post diameter when used with

The stress distribution for the push-out

standardized incremental intervals. As

test is expected to be more uniform than for

these are passively inserted inside of the

conventional shear or tensile tests. This dis-

root canals, the dislocation resistance

tribution leads to a shear stress that is com-

mainly depends on the luting agent and the

parable to the stress created under clinical

806

VOLUME 44 • NUMBER 10 • NOVEMBER/DECEMBER 2013

Q U I N T E S S E N C E I N T E R N AT I O N A L Mirmohammadi et al

conditions.42 The fracture occurs parallel to

3, and approximately 210 μm for group 5).

the dentin-adhesive interface, which makes

In spite of a theoretical decrease in the

it a true shear test; therefore, the test mea-

C-factor for group 5, one may expect relax-

surements truly express the interfacial bond

ation to be more efficient as the unbonded

strength between the dental tissue and the

area becomes thicker. However, when the

material.43

cement thickness is too great, more air is

In this study, no significant differences

incorporated into the surface area, and this

were found when the matched drill and post

may inhibit the polymerization of the resin

sets were used (groups 1, 2, and 3). More-

around the minuscule air bubbles.47 The

over, the post diameter had no significant

incorporation of air bubbles may also

influence on the push-out bond strength,

weaken the resin cement substantially,

which is represented by the ratio of the

which perhaps explains the relatively low

debonding force (F) to the surface (SL). As

bond

the radius increased from group 1 to group

(Fig 1c).48

strength

recorded

for

group

5

3, there was a proportional increase in the

Regarding the root level, specimens

debonding force, which led to similar bond

from the apical third of the roots showed

strengths in group 1, group 2, and group 3.

significantly lower bond strengths, which is

bond

in agreement with previous studies.14,35,49

strength were significant when an oversized

This can be explained by a lower number of

post space was used (group 4). These find-

dentinal tubules and the more sclerotic

ings are in agreement with previous studies

structure of dentin in the apical part of the

that reported greater bond strengths for

roots. It has been recently reported that the

oversized post spaces.27,30 The intimate fit

higher C-factor in the apical part can nega-

that was produced with the matching drill

tively influence cohesive strength of resin

and post sets may not have provided ade-

cement, which could be another explana-

quate space for the resin cement to

tion for lower push-out bond strength in the

develop its maximum strength. Moreover,

apical part.50

The

differences

in

push-out

the relative lack of oxygen in such a narrow

Approximately 62% of the specimens in

space may have quickened the setting of

groups 1, 2, 3, and 5 showed a mode I fail-

the resin and prevented the complete inser-

ure at the post-cement interface (Table 4).

tion of the posts. Another factor that

Similar findings were reported by Le Bell et

strongly interferes with the development of

al,51 Perez et al,29 and Dimitrouli et al. 19

high bond strength is the cavity configura-

RelyX fiber posts contain epoxy resin (3M

tion, or C-factor.44 One method that is used

ESPE, technical information); therefore,

to estimate the C-factor is to divide the free

lower interfacial strength is expected due to

surface area by the total bonded area, as

the absence of a chemical bond between

described

by

Bouillaguet

et

al 45

and

the methacrylate-based RX luting agent

Jongsma et al,46 who reported that the

and the epoxy resin, a highly cross-linked

C-factor is greater than 200 for post cemen-

matrix of fiber posts that does not have

tation. In the present study, groups 1, 2,

functional groups available for reactions.51

and 3 exhibited small unbonded surfaces

Furthermore, during the RX setting reaction,

due to the thin layer of cement. Thus, there

the negatively charged phosphoric acid

could have been insufficient stress relief by

groups of the methacrylate monomers bond

flow and a high probability that one or more

to calcium ions in the tooth structure, there-

of the bonded areas would pull off or

fore shrinkage stress causes debonding of

debond. In groups 4 and 5, the resin

the cement from the post surface. Simulta-

cement may have had the opportunity to

neously, the setting of the cement takes

flow due to the oversized post space, which

place through a radical polymerization

may have resulted in the relaxation of poly-

reaction of the methacrylate monomers that

merization stresses. On the other hand, no

is started by the initiator system to provide

significant differences were found between

a highly cross-linked, three-dimensional

group 5 and the first three groups, although

network,

the cement thicknesses were different

remaining functional groups to react with

(approximately 20 μm for groups 1, 2, and

the resin of the fiber post. In addition, the

and

this

VOLUME 44 • NUMBER 10 • NOVEMBER/DECEMBER 2013

process

leaves

no

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Q U I N T E S S E N C E I N T E R N AT I O N A L Mirmohammadi et al

a

b

c

d

e

f

Figs 2a to f Example showing the clinical implications of one incremental oversized post space. The maxillary right second premolar was obturated using a warm-vertical technique. (a) The palatal canal was prepared for fiber post placement using RelyX drill size 2 (red, 0.8 to 1.6 mm), immediately after down packing of the gutta-percha. The post space was irrigated with normal saline, applying EDTA for 2 min, and then 5 ml of distilled water. (b) The RelyX fiber post (yellow, 0.7 to 1.3 mm) was examined for length fit. (c) RelyX Unicem cement was applied in the prepared space from the bottom to the top. (d) The tested fiber post was placed and the tooth restored using dental composite resin. (e) The final radiographic image (parallel). (f) The final radiographic image with mesial angulation.

intimate fit between the post and the dentin

CONCLUSION

walls may have resulted in the complete escape of the resin cement from the post

The greatest push-out bond strength values

space during post insertion. However, when

were obtained when one incremental over-

the resin cement was placed in an over-

sized post space was used.

sized space, as in group 4, sufficient func-

The apical third of the root canals pre-

tional groups became available for reaction

sented the lowest bond strength potential,

at the post-cement and dentin-cement

suggesting that the length of the post should

interfaces. This, in addition to less shrink-

be limited to the coronal half of the root.

age stress, may explain the higher percentage of mode III failures in this group. Traditionally, post spaces are prepared

Post diameter had no influence on the push-out bond strength of the tested fiber posts.

after finishing the root canal obturation. This may result in residual gutta-percha around

CLINICAL SIGNIFICANCE

the post space, which will decrease the

Fiber post space has to provide an opti-

bond strength. In the present study this was

mum cement thickness (around 120 μm) for

confirmed under SEM evaluation. It can be

adequate cementation (Fig 2). For the

suggested to place the fiber post immedi-

RelyX fiber post system, this can be

ately after the down packing of gutta-per-

reached by using a post one size smaller

cha in the apical part. Hence, residual

than the size of the post drill to create the

sealer on the root canal walls needs to be

post space. Bond strength of fiber post to

removed by rinsing and drying.

root canal dentin will be lowered by too thick or too thin cement thickness.

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Q U I N T E S S E N C E I N T E R N AT I O N A L Mirmohammadi et al

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