King Saud University Journal of Dental Sciences (2012) 3, 33–38

King Saud University

King Saud University Journal of Dental Sciences

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Fracture resistance of three post and core systems

in endodontically treated teeth restored with all-ceramic

crowns

Tariq Abduljabbara,*, Haneef Sherfudhin

a, S.A. AlSaleh

a,

Abdulaziz A. Al-Helal b, Saleh S. Al-Orini b, Naif A. Al-Aql b

a Department of Prosthetic Dental Sciences, College of dentistry, King Saud University, Saudi Arabiab College of Dentistry, King Saud University, Saudi Arabia

Received 26 June 2011; accepted 5 October 2011

Available online 27 November 2011

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KEYWORDS

Fracture resistance;

Post and core;

Endodontically;

Teeth;

All-ceramic;

Crowns

Corresponding author. A

ollege of Dentistry, King Sau

545, Saudi Arabia. Tel.: +96

x: +966 1 465 6663.

-mail address: tajabbar@ya

10-8157 ª 2011 King Saud

sevier B.V. All rights reserve

er review under responsibilit

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ddress:

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Abstract The restoration of endodontically treated teeth requires the fabrication of a post and

core to provide retention and support for the final crowns. The objective of this study was to com-

pare the fracture resistance of endodontically treated teeth restored with glass fiber post and com-

posite resin cores, customized zirconia posts, and cast metal post and cores. A total of 40 human

extracted mandibular first premolars were used for the study. The teeth were randomly divided into

four groups. Group A represented a control group that did not receive any posts and was filled with

core material only; Group B comprised cast metal posts and cores; Group C comprised custom

milled zirconia posts and cores; and Group D comprised glass fiber posts. All groups were prepared

to receive all ceramic crowns. All samples were subjected to compressive testing with an Instron

machine (Universal Testing Machine) and fracture loads and failure patterns were analyzed. The

findings indicated a statistically significant difference between the failure loads in the groups stud-

ied. The mean load required to fracture the zirconia custom posts was higher (765.1 ± 48.5 N) than

the fiber posts and the cast posts and cores (P < 0.001). The fiber posts resisted a mean load of

Prosthetic Dental Sciences,

sity, P.O. Box 60169, Riyadh

325, mobile: +966 504413113;

(T. Abduljabbar).

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34 T. Abduljabbar et al.

561.4 ± 37.2 N which was higher than the cast posts and cores. The control group revealed the low-

est value of fracture resistance. Within the limitations of this study, it can be concluded that cus-

tomized zirconia posts and cores resisted a higher mean load (765.1 + 48.5) when compared

with other post and core systems. Failure patterns within this group revealed catastrophic failure,

but the failure loads were much higher than the average occlusal load.

ª 2011 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

Endodontically treated teeth with insufficient tooth structure

are often restored with crowns (Carter et al., 1983; SorensenandMartinoff, 1984). In teeth with substantial hard tissue lossresulting from cavities or trauma, posts are often necessary for

providing sufficient retention for the core material (Peroz etal., 2005). Although posts have been recommended tostrengthen the teeth, several investigators have cautioned that

posts with inadequate resistance to rotational forces can weak-en the teeth (Zhi-Yue and Yu-Xing, 2003).

The fracture susceptibility of teeth restored with posts maybe related to factors such as the amount of remaining tooth

structure, which provides resistance to the fracture of the tooth(Ng et al., 2006), as well as the characteristics of the post, suchas the material composition, modulus of elasticity, diameter,

and length (Fokkinga et al., 2006). A Root fracture is the mostserious type of failure in post-restored teeth (Testori et al.,1993; Wu et al., 2004). To avoid root fractures, a post having

a modulus of elasticity similar to that of dentin helps in distrib-uting the stress of occlusal load in a uniform pattern (Akkayanand Gulmez, 2002).

The cast gold post and core has been regarded as the ‘‘goldstandard’’ in post-and-core restorations because of its superiorsuccess rate (Bergman and Lundquist, 1989; Creugers et al.,1993). Alternatives to cast metal posts and cores have been

developed. The use of prefabricated posts and custom-madebuildups with amalgam or composite resin has simplified therestorative procedure because all steps can be completed chair-

side with acceptable clinical success (Linde, 1984). The choiceof an appropriate restoration for endodontically treated teethis guided by strength and esthetics. The restoration of teeth

with adhesively cemented internal restorations offers improvedmechanical stability over cemented restorations (Trope andTronstad, 1991).

The development of tooth colored posts has improved the

esthetics of teeth restored with posts and cores (Sidoli et al.,1997; Mannocci et al., 1999; Rosentritt et al., 2000). Zirconiumdioxide and glass fiber-reinforced composite resins (FRC), in

particular, are the foundation of many modern post-and-coreconcepts (Qing et al., 2007). Several studies have investigatedthe fracture resistance of fiber posts since their introduction

and compared it with that of the metal posts. Unfortunately,the results have not been consistent, with some authors report-ing that endodontically treated teeth restored with fiber posts

showed lower fracture resistance than that of teeth restoredwith metal posts (Martinez-Insua et al., 1998; Newman et al.,2003). Other authors, however, indicated the fracture resis-tance of fiber post–restored teeth to be equal to or greater than

that of teeth restored with metal posts (Rosentritt et al., 2000;Raygot et al., 2001). Zirconia posts were first introduced byMeyenberg et al. (1995) who reported that the flexural

strengths (900�1200 MPa) of these posts was comparable tothat of cast gold or titanium, and that it is possible to have

the same post dimensions as high gold alloys or titanium. Zir-conia is a widely used material because of its good chemicalstability, high mechanical strength, high toughness, and aYoung’s modulus similar to that of stainless steel alloy (Piconi

and Maccauro, 1999). Apart from its favorable chemical andphysical properties, it has the esthetic advantage of having acolor similar to that of natural teeth (Ahmad, 1998; Vichi et

al., 2000). The high elastic modulus of elasticity of zirconiaposts at 200 GPa (Guazzato et al., 2004) causes stress to betransferred to the less rigid dentin, thereby resulting in root

fractures (Bateman et al., 2003). However, there is little con-sensus with regard to their mechanical behavior and reliabilityand other factors which would contribute to their optimal per-

formance. Therefore the objective of the study was to comparethe fracture resistance of endodontically treated teeth preparedwith a 2 mm ferrule restored with a cast post and core, a glassfiber post with a composite resin core, and a customized zirco-

nia post restored with an all-ceramic crown.

2. Materials and methods

2.1. Specimen preparation

A total of 40 sound, single-rooted mandibular first premolarswith similar size and shape were collected. Root lengths weremeasured from apex to tip of the cusps, together with the buc-

colingual and mesiodistal dimensions at the highest bulge witha digital caliper (Schnelltaster, Dentaurum, Germany). Thespecimens were stored in a solution of 0.9% saline at room

temperature. Selection criteria also included the absence ofroot caries, restorations, or previous endodontic treatment.The teeth were divided into 4 groups of 10 specimens each.All teeth underwent root canal treatment. The root canals were

instrumented manually in a step-back technique to an apicalsize of ISO 40. The canals were dried with absorbent paperpoints and obturated with gutta-percha (Roeko, Langenau,

Germany) and sealer (AH plus, Dentsply DeTrey, Konstanz,Germany) with cold lateral condensation. After endodontictreatment, each root was thinly covered with a silicone impres-

sion material to simulate the thickness of the periodontal liga-ment (Imprint II Garant light-body by 3M ESPE). All teethwere embedded in an acrylic resin cube (RAPID REPAIR,

Dentsply, Germany). The acrylic resin blocks were shaped tofit into a retentive device for fracture testing (Fig. 1).

2.2. Post space preparation

Post space preparation for all teeth was initiated after 7 daysfrom obturation by the use of a universal starter drill at a

Figure 1 Line diagram showing tooth preparation and embed-

ding in acrylic resin for testing in Instron machine.

Fracture resistance of three post and core systems 35

speed of 5000 rpm to a depth of 9 mm for the post space. Thepost space for all the groups was prepared with Gates Gliddensize 1 up to size 4 (1.1 mm) (and size 5 to enlarge the canal ori-

fice), keeping 4 to 5 mm as an apical seal (Fig. 1).

2.2.1. Teeth preparation before post cementation

All teeth were prepared to receive an all-ceramic crown with a

finish line of 0.5 mm above the CEJ. The axial reduction wasdone with an MRD gauged diamond (Lot-NR 1599, DFSDental and Technical Products, GmbH, Germany) which

was attached to the milling machine (K9 Milling Apparatus-990, Kavo, Germany) for every group. The MRD gaugeddiamond had a self-limiting tip, which produced a 1 mm deepchamfer, and the margins and the angle of convergence were

standardized. Each tooth after preparation had dentin supportof 2 mm. The buccolingual and mesiodistal widths were mea-sured with the calibrated gauge caliper to be approximately

in the same range (Table 1). The ferrule used in this studywas 2.0 mm. The occlusal surface of the prepared teeth wasflattened to ensure the accurate fit of the posts.

2.3. Post and core fabrication

2.3.1. The control group

The gutta-percha was removed from the pulp chambers to 3-4 mm in depth. The dentine was etched for 15 seconds with

Table 1 Dimension of prepared tooth.

Buccolingual Mesiodistal

Mean

(mm)

Range

(mm)

Mean

(mm)

Range

(mm)

Control group (A) 3.38 3–3.7 5.2 5–5.4

Cast post-core (B) 4.3 3.7–4.81 5.46 4.75–5.87

Glass fiber post (C) 3.34 3.2–3.54 4.8 4.34–5.3

Zircon (D) 3.2 3.00–3.60 5.16 5-5.25

37 percent phosphoric acid gel (Total Etch). The surface wasrinsed with water and dried with paper points. The bondingagent EXCITE F (Ivoclar, vivadent) was applied and the mul-

ticore material was injected into the etched chambers. The corewas then built with the same material to the desired dimensions.

2.3.2. Glass fiber post group

The glass fiber posts (RelyX, Fiber 3 M ESPE) were cementedand the core was built up with MULTICORE FLOW system(Ivoclar Vivadent). The dentin was etched for 15 seconds with

37% phosphoric acid gel (Total Etch). The surface was rinsedwith water and dried with paper points. The bonding agentEXCITE F (Ivoclar, Vivadent) was applied and the multicore

material was injected into the etched roots. Each post wasplaced to its full depth, and the core was contoured to the de-sired dimensions.

2.3.3. Cast post and core

Cast posts and cores were fabricated with Duralay (RelianceDental Manufacturing, Worth, IL). Plastic Para-post systems

were covered with Duralay, and an impression of the canalwas made. The core was also built with the same material.All of the specimens were then prepared to final premeasured

dimensions. Posts and cores were invested in Beauty-Cast(Whip Mix Corp., Louisville, KY) without a ring liner. Aftercasting, each post and core was tried to verify the fit andcemented with zinc phosphate cement (Mizzy Inc., Cherry

Hill, NJ).

2.3.4. Zirconia post and core group

The same technique for fabricating the cast post and core was

used, except that the Duralay buildup of the post and coreswas scanned and the presintered Y-TZP Cercon Base blankswere milled with the Cercon brain unit (DeguDent, Hanau,

Germany) according to the manufacturer’s instructions. TheZirconia posts were cemented with glass ionomer cementGIC (GC Fuji I by GCAmerica) and seated for 7 min with

finger pressure.

2.4. Restorative procedures

1. Group A (Control). No post was placed. Only the core

material was placed and served as the control group forcomparison of the results.

2. Group B (Cast Post and Core).3. Group C (Fiber post).

4. Group D (Customized zirconium post)

2.5. Crown fabrication

After post and core cementation and finalization of the tooth

preparation, the distance from the finish line to the occlusalsurface of the preparation was 4 mm. Each tooth preparationfrom all the groups was measured with a digital caliper to en-

sure similar dimensions both mesiodistally and buccolingually.Final impressions were made for all the specimen with a poly-vinylsiloxane impression material (Elite by Zhermack),and themaster die was fabricated with die Stone (Heraeus Kulzer, Ha-

nau, Germany).

Figure 3 Photograph showing orientation of sample in Instron

machine.

Table 2 Failure pattern of study group.

Noncatastrophic

failure (restorable)

Catastrophic failure

(nonrestorable)

Group (N= 10)

10 0 A – Control Group

1 9 B – Cast Post and Core

7 3 C – Glass Fiber Post

1 9 D – Zirconium Post

36 T. Abduljabbar et al.

Zirconia copings were made by scanning the master die. Thepresintered Y-TZP Cercon base blanks were milled with theCercon brain unit (DeguDent, Hanau, Germany) according

to the manufacturer’s instructions. All the copings were steamcleaned, and porcelain veneering was done and measurementswere made after ceramic application to maintain the same

thickness of ceramic for all the specimens.The crowns were cleaned with ethanol, dried, and cemented

with glass ionomer cement (GC Fuji I by GC America). The

restorations were kept on the prepared samples under fingerpressure for 30s, and excess cement was removed with a sharpinstrument after 10 min. After the cementation procedures, thesamples were again maintained in a wet condition. Calibration

for each step and procedure in this study was done by the sameexaminer for each specimen.

2.6. Fracture strength test

The specimens were tested with a universal testing machine(Instron 8500 Plus, 100 Royal St. Canton, USA), set to deli-

ver an increasing load until fracture. The specimens wereplaced at an angle of 45 degree to the long axis of the tooth,with the application point midway between the lingual slope

of the buccal cusp, and between the central fissure and thecusp tip (Figs. 2 and 3). The crosshead speed of 0.01 cm/min was used, and the load was applied at an angle of 45degrees to the long axis of the tooth. The force applied was

recorded in newtons. After loading, the mode of failure wasrecorded for each specimen and classified as either a favor-able fracture above the cementoenamel junction (repairable)

or a catastrophic fracture of the root below the cemento-enamel junction (nonrepairable) (Table 2). These inspectionswere made with a stereomicroscope (Stereoscopic zoom

microscope, SMZ-1000, Nikon, Japan).

Figure 2 Illustration showing test specimen orientation in

Instron machine.

2.7. Statistical analysis

The fracture resistance (maximum load-to-failure) after load-

ing was compared among the 3 post types with a 1-way anal-ysis of variance (ANOVA). Comparisons among the pairs wereperformed with a Student-Newman-Keuls Multiple Compari-

sons Test exhibiting a P-value of <0.05, which was significant.

3. Results

3.1. Fracture strength test

The mean and standard deviations for failure loads were shownin Fig. 4. A statistically significant difference was observedamong the failure loads in the groups studied. The load requiredto fracture the zirconia custompostwas higher (765.1 ±48.5 N)

than the fiber post and cast post and core (P < 0.001). The fiberpost resisted a load of 561.4 ± 37.2 N, which was higher thanthe cast post and core (Fig. 4). However, the values were not

statistically significant within these 2 groups.

4. Discussion

The task of restoring endodontically treated teeth is encoun-tered almost daily in prosthodontic practice. Leempoel et al.(1987) evaluated a large sample of teeth with single crown

Figure 4 The mean fracture load among four groups studied.

Fracture resistance of three post and core systems 37

restorations and found that 39% were nonvital and had re-

ceived some type of post restoration. In an effort to improvethe fracture resistance of endodontically treated teeth restoredwith a post-and-core system, research has focused on post

materials, post designs, luting agents, and ferrule effect (Men-doza et al., 1997; Martinez-Insua et al., 1998; Akkayan, 2004).

In the current study, all specimens were restored and tested

with complete-coverage of crowns to ensure standardization.The placement of a crown during endodontic restoration testinghas been questioned, as this practice may obscure the effects of

different buildup techniques (Sorensen and Engelman, 1990;Libman and Nicholls, 1995). It is true that a crown creates aferrule effect and variation in the load distribution when placedover a core buildup if the margins encircle a sound dentin collar

(Butz et al., 2001). However, testing post and core preparationswithout placement of a crown would not have reflected clinicalpractice.

The current study attempted to compare the conventionalmetal post and core with fiber post and custom-milled zirconiaposts. It has been reported that more rigid reconstructions are

unable to absorb stress and are therefore susceptible to failure(Sidoli et al., 1997; Mannocci et al., 1999). The observations inthe present study may be attributed to the fact that zirconiaposts had the highest modulus of elasticity among the post

types tested. Higher modulus of elasticity results in less bend-ing of the post/core unit under load; consequently, less stress isexerted on the tooth (Butz et al., 2001). The failure pattern be-

tween the custom milled zirconia post and the conventionalcast post and core were similar (Table 2) and were classifiedas a nonrestorable fracture, which may be attributed to the

rigidity of the post.Available prefabricated fiber posts may be too wide for

some canals, especially mandibular incisors, and may exhibit

poor adaption to noncircular canals. Their use is limited whenthere is insufficient coronal tooth structure (ferrule less than2 mm) because of their lower modulus of elasticity, and they

may undergo flexure under functional stress and producemicromovement at the core, producing decementation of thecrown (Ng et al., 2006). The fracture resistance of this group

was comparable with that of the custom cast post and core(Fig. 2). With regard to the failure pattern, the fiber post groupexhibited more favorable fracture patterns (restorable) above

the cementoenamel junction than the cast post and core group(Table 2).

Fracture strength values from other studies were not com-parable with the results of the present study because of differ-

ences in research design. Moreover, in the present study, thezirconia posts were custom milled unlike other studies whichused prefabricated zirconia posts (Sorensen and Engelman,

1990; Butz et al., 2001). The results of this study suggest thatcustom milled zirconia posts and cores can be used whenesthetics are important, and when the anatomy of the root ca-

nal combined with the extensive loss of coronal tooth structurerequires the use of a custom post.

5. Conclusions

1. All 3 post and core systems performed favorably undercompressive testing.

2. The load required to fracture the zirconia custom post was

higher compared to the fiber post and cast post and core.3. The fiber post resisted a load higher than the cast post and

core.4. The fracture patterns of the teeth restored with fiber posts

were favorable (70%).5. Multiple cracks were seen with the zirconia post group

within the root body (90% nonrestorable).

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