Niigata Dent. J. 39（1）：15 － 24,

2009

15

－ ORIGINAL ARTICLE －Effect of post type and loading condition on the failure resistance and primary
failure mode of flared canal teeth restored with fiber-reinforced or cast posts
Farhana Sharmin1）, Roxana Stegaroiu2）, Naoto Okada3）, Eriko Kitamura3）,
Kouichi Kurokawa2）, Minoru Yagi2）, Shuichi Nomura4）.
1)

Division of Oral Health in Aging and Fixed Prosthodontics, Niigata University Graduate School of Medical and Dental Sciences
2)

Master’s Program of Oral Health and Welfare Science, Niigata University Graduate School of Medical and Dental Sciences
3)
4)

Division of Bio-Prosthodontics, Niigata University Graduate School of Medical and Dental Sciences

Division of Comprehensive Prosthodontics, Niigata University Graduate School of Medical and Dental Sciences

ファイバーポストおよび鋳造ポストで修復した歯根の種類と荷重条件が破損強さと初期破損様式に及ぼす影響
Farhana Sharmin 1）, Roxana Stegaroiu 2）, 岡田直人 3）, 北村絵里子 3）,黒川孝一 2）, 八木1）

新潟大学大学院医歯学総合研究科
2）稔 2）, 野村修一 4）

口腔生命科学専攻

新潟大学大学院医歯学総合研究科

口腔健康科学講座

加齢歯科補綴学分野

口腔生命福祉学専攻3）

口腔生命科学専攻

口腔健康科学講座

生体歯科補綴学分野

4）

口腔生命科学専攻

顎顔面再建学講座

包括歯科補綴学分野

新潟大学大学院医歯学総合研究科
新潟大学大学院医歯学総合研究科

平成 20 年 12 月 17 日受付

平成 21 年３月９日受理

Key Words : cast post-and-core, failure mode, failure resistance, fiber-reinforced post, flared canal

Abstract
Objectives : This study investigated the influence of post type and loading condition on the failure resistance andprimary failure mode of flared canal roots.
Materials and methods : Flared post holes were prepared in 40 sound human lower premolar roots. Twenty roots
received glass fiber posts with resin core (fiber specimens) and 20 received cast post-and-core (cast specimens).
After cementation with resin cement, in 10 fiber and 10 cast specimens, a quasi-static bending test was performedby an Instron-type testing machine. In the other specimens, before the bending test, a bucco-lingual load wasapplied for 900,000 cycles using a lever-type device. Failure resistance and primary failure mode in relation withpost type and loading condition were statistically analyzed (significance level: 0.05).
Results : After cyclic loading, all specimens were intact; therefore, they underwent the bending test. Failure
resistance showed a significant difference only for the post type (p = 0.002) (two-way ANOVA). In the presence of
cyclic loading, the post type significantly influenced failure resistance (p = 0.010), whereas in its absence, it showed
only a tendency to influence failure resistance (p = 0.054) (Tukey test). In the cycled specimens, significantly fewer
root fractures and more frequent debonding without root fracture were found in the fiber than in the castspecimens (p = 0.02) (Fisher’s exact test).
Conclusions : Rather than cast post-and-cores, the use of glass fiber-reinforced posts and resin core for the
restoration of flared canal teeth may prevent root fracture, but debonding may occur under high loads.

－ 15 －

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抄録：本研究は漏斗状に窩洞形成された歯根の修復物における破損強さと初期の破損様式に、ポストの種類と荷重条件が及ぼす影響を検討した。
漏斗状に根管形成した抜去下顎小臼歯 40 歯を用い、20 歯にはガラスファイバーポストとコンポジットレジンコア（ファイバーポスト群）、残りの 20 歯には鋳造ポストと鋳造コア（鋳造ポスト群）で支台築造を作製し、レジンセメ
ントで被験歯に合着した。各ポスト群 10 歯に、インストロン型万能試験機を用いてポスト－歯根複合体が破損するまで準静的曲げ試験を行った。残りの被験歯には、曲げ試験の前に、てこ型試験機を用いて 2-kgf の頬舌的荷重を 90万回加えた。反復荷重では全ての試料が破損しなかったので、引き続き準静的曲げ試験を行った。
破損強さの測定値には二元配置分散分析法で、ポストの種類に有意差が認められた（p ＝ 0.002）ものの、荷重条件要因および二要因間の交互作用には有意差が認められなかった。Tukey 法の多重比較検定結果から、ポストの種
類は反復荷重があると破損強さに有意差を示した（p ＝ 0.010）が、反復荷重がないと影響する傾向を示したものの有意には達しなかった（p ＝ 0.054）。破損様式に関しては、反復荷重を行った試料では Fisher's の直接確率計算法で分析すると、ファイバーポスト群が鋳造ポスト群よりも有意に歯根破折が少なく、歯根破折を伴わないポスト脱離が多かった（p ＝ 0.020）。90 万回作用させた疲労試験とそれに続く準静的曲げ試験の結果から、漏斗状に窩洞形成された歯根の修復物には、鋳造ポストよりもガラスファイバーポストとコア築成材料を用いる方が歯根破折を防ぐ可能性が示された。しかし、このガラスファイバーポストを用いた修復物には、大きな頬舌的荷重が作用すると偶発症として、ポスト脱離が高頻度に生じる可能性がある。

catastrophic root fracture. Although this supposition is

1. Introduction

supported by studies using only structurally soundroots,6,8） the clinical use of glass fiber-reinforced posts

Although many studies have pointed out thehas been also extrapolated to restore structurally

importance of preserving sound tooth structure apicalcompromised teeth whose post-hole preparation end

to the dowel-core margin, so that the crown can brace

up in flared canals. These are at higher risk forthe endodontically treated teeth and prevent fracture

fractures than the sound roots, because of the thin“ferrule(
effect”),1-4） clinicians are often faced with thewalls remaining14）; however, whether the selection ofneed of restoring teeth with extensive caries, in whichpost type can improve the failure resistance of roots

no ferrule can be maintained and the remaining dental

with flared canals has yet to be determined.

structure is rather fragile. For this purpose, custom

Clinical failures in fixed prosthodontics are often

cast post-and-core buildups or prefabricated metallic

caused by fatigue, as a result of long periods ofposts with resin cores have been traditionally used,function under occlusal forces (mastication, swallowing,

but a series of shortcomings has become evident inetc.); fatigue failures of the tooth-restoration complex

time. A serious complication that often leads to toothhave been clinically reported in connection with

2,5）

extraction is root fracture.

In vitro investigations ofendodontically treated teeth restored with posts anddifferent post systems have found that teeth restored

cores15）. However, when applications of new materials

with cast or prefabricated metallic posts demonstrated

or techniques are tested in vitro, failure resistance ofmost catastrophic failures by root fractures.6-8） Esthetic

teeth restored with posts and cores are investigatedhaveby quasi-static rather than fatigue tests. 6-8, 14, 16-18）

been also related to metallic posts. Therefore, non-Among the few studies that used fatigue loading tometallic posts have been added to the treatmentcompare fracture strength of fiber and metallic posts,

options of endodontically treated teeth. Among these,

one evaluated their torsion resistance19）, while anotherglass fiber-reinforced posts are esthetic and, according

study used high-stress cyclic loading in wet

to the manufacturer, their modulus of elasticity is 29.2

conditions20）. These studies bring valuable insight into

GPa, which is much closer to that of dentin (14.2

the fatigue failure of structurally sound roots restoredcomplications

GPa

11）

9）

and allergic reaction to metal

12,13）

10）). Recently,

with different posts. However, clinically, rather thanthe use of post materials with physical properties close

being exposed to torsion, teeth are compressed and4）to those of natural dentin has been recommended .

bent by the axial and lateral components of the

These are expected to decrease the stress in theocclusal load, respectively. Thus, the effects of thesedental structure, and thus reduce the incidence ofcomponents and especially that of the lateral) than dental alloys (90-100 GPa－ 16 －

Farhana Sharmin et al.

17

component, which has a higher potential of damaging

the most apical level of the cementoenamel junction

teeth, have to be also investigated. Moreover, although

and the root section was flattened by a carborundum

testing under high-stress loading is time-efficient,disk to obtain a surface perpendicular to theapplying loads that are in the physiological range for alongitudinal tooth axis.

high number of cycles will more closely simulate the

All teeth were endodontically instrumented up to a

intraoral biomechanical environment that may finallysize 50 K-type file (MANI Inc., Ustunomiya, Tochigi,

lead to fatigue failure. Regarding the number of cycles,

Japan) with water irrigation between file sizes. After

Wiskott et al21） consider fatigue tests as conclusive forthe canals were dried, they were obturated using theclinical restorations if they are conducted up to alateral condensation technique, with gutta-percha

range of 104 - 107 cycles.

master points #50 (GC Co. Tokyo, Japan), auxiliary

Furthermore, in both static and fatigue studies oncones (GC Co.) and a calcium hydroxide root canalfailure of post-restored teeth, the outcome parametersealer (Sealapex, Kerr Manufacturing Co., Romulus, MI,

has been in many instances fracture strength/

USA). A small quantity of zinc phosphate cement

resistance.18,19） However, beside root/post fractures,(Shofu Inc, Kyoto, Japan) was filled on top of the gutta-

clinical failures of teeth restored with posts and corespercha and the specimens were stored in the saline forhave also included loss of retention and marginalat least 7 days before canal preparation.

22, 23）Thus, the present study was designed to

The canals were prepared to receive posts (Fig 1) torecord the first sign of failure (primary failure 17）):

a depth of 8 mm with #1 through #3 Peeso reamers

fracture of the tooth-restoration complex, debonding(MANI Inc., Ustunomiya, Tochigi, Japan) and #1(loss of post retention or presence of marginal gaps),

through #2 tapered reamers (RTP Reamer, Dentech,etc. Its purpose was to investigate the influence of post

Tokyo, Japan). On the lingual aspect, a keyway type

type and loading condition (presence or absence of aantirotational groove was also prepared with a

2-kgf (19.62 N) lateral load that was applied in 9 x 105

diamond bur. Thereafter, the canal was flared in thecycles), on the failure resistance and primary failure

cervical area by a conical carborundum point, tomode of roots with flared canals restored with posts

simulate the preparation required in gross carious

and cores. Two hypotheses were tested: 1) Post typedestruction (Fig 1). In all the specimens, the mesiodistal

influences failure resistance and primary failure mode

width of the remaining tooth structure in the cervicalof flared roots restored with a fiber-reinforced post

area was 1 mm on each side.

gaps.

and composite core or cast post-and-core and 2) the
loading condition influences the failure resistance andprimary failure mode of flared roots restored with theabovementioned posts and cores.

2. Materials and methods
2.1 Teeth preparation
Freshly extracted mandibular premolars weredisinfected with 6% sodium hypochlorite solution(Purelox, OYALOX Co., Ltd., Tokyo, Japan) andinspected under a stereomicroscope to check for caries,
crack and stain. Forty intact teeth of similardimensions were selected and stored in isotonic saline
solution. The mean dimensions (SD) of the teeth asmeasured at the most apical level of thecementoenamel junction were: 4.95 (0.43) mm

Fig 1. Photographs of specimens before mechanical testingand schematic diagrams of root canal preparation and post
& core design (dimensions in mm): a and b, with glass fiberreinforced composite resin post and glass fiber-reinforced
core build-up material; c and d, with cast post-and-core. (B =buccal, L = lingual).

2.2 Post and core fabrication and cementation
All the posts were fabricated by the indirect method.

mesiodistally, 7.25 (0.64) mm buccolingually, and 13.96

The impressions were taken with putty and injection(1.32) mm in root length.
Each tooth was decoronated with a diamond point at

types of vinyl polysiloxane impression material－ 17 －

18

新潟歯学会誌

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(Imprinsis, Tokuyama Dental Co., Tokyo, Japan). Fromin the central hole of a resin block with an orthodontic

these impressions, stone dies (Fuji rock, GC Co., Tokyo,

resin applied in layers (Dentsply International Inc.,

Japan) were fabricated. In half of the specimens, theYork, PA, USA) up to 2 mm below the root surface

dowel was fabricated from glass fiber-reinforced

level. To ensure the bond to both root and resin, acomposite resin posts (FibreKor, Jeneric/Pentron Inc.,

cyanon adhesive (Dental cyanon, Koatsu Gas Kogyou

Wallingford, CT, USA) and glass fiber-reinforced coreCo., Ltd., Chiba, Japan) was used on the inner side ofbuild-up material (Build-It, Jeneric/Pentron Inc.,

the Scotch tape during taping and on its outside with

Wallingford, CT, USA), to morphologically fit the tootheach of the orthodontic resin layers. During thispreparation. The same build-up material was used forprocedure, the preserved sprue of the cast core or thethe core. In all the specimens, core build-up caps for

top of the fiber post was fixed in the arm of apremolars (Build-it! Core Forms size 3, Jeneric/Pentron

surveyor, to align the post to the vertical axis. Prior toInc., Wallingford, CT, USA) were used to standardize

the cyclic loading, the artificial mobility of the restored

core dimensions. In the remaining specimens, wax

teeth was evaluated by measuring the buccolingual

patterns were shaped on the stone dies and a 10-mm

deflection of the loading point with an electric

long sprue was positioned on the top of the core as tomicrometer (Minicom, Tokyo Seimitsu Co., LTD.,be aligned to the post axis. After investing and wax

Tokyo, Japan) in the same setting as for theburnout, the mold was cast with a dental alloymechanical tests. Under 2 kgf (19.62N) load, the mean

(Castwell M.C., GC Co., Tokyo, Japan) that consisted( ∓ SD) was 35.7 ( ∓ 21.6) μ m.

primarily of 46% Ag, 20% Pd, 20% Cu, and 12% Au.
Before cementation, all posts were cleaned with an

2.3 Fatigue loading

alcohol wipe, air-particle abraded with a 50 μm

Half of the cast and half of the fiber specimens werealuminum oxide abrasive powder and a bonding agent

firmly mounted in a brass holder of a custom-made(Bond one, Jeneric/Pentron Inc., Wallingford, CT, USA)lever-type fatigue testing machine. 24） Serrate-type

was applied in one coat on the surfaces to be cemented

cyclic loading between 0 and 2 kgf (19.62N) was

and dried gently. In addition in the cast specimens, anapplied to the buccal face of the core, 4 mm from thealloy primer (Alloy primer, Kuraray Medical Inc.,

cemento-enamel junction (Fig. 2). The load was appliedTokyo, Japan) was applied before the bonding agent.

at a rate of 80 cycles/min (1.33 cycles/s) for 900,000

In all the specimens, the dentin surfaces to becycles in a wet environment at room temperature.

cemented were etched with phosphoric acid etching

During each cycle the specimens were loaded for 0.3 s.

agent (K-etchant Gel, Kuraray Medical Inc., Tokyo,

The machine was equipped with 2 types of shutoff

Japan) for 20 s and then thoroughly rinsed. Following

sensors to automatically discontinue loading in case of

manufacturer instructions, the excessive moisture wasspecimen failure.

removed, but the surface was left moist. Then, thesame bonding agent was applied in 2 consecutive coats
and the dentin surfaces were gently dried for 10 s,
before light-curing for 10 s. Cementation was
performed with resin cement (Universal C&B: CementIt, Jeneric/Pentron Inc., Wallingford, CT, USA),
following manufacturer’s instructions.
After cementation and between the experimental
steps that followed, each tooth was kept in saturated
humidity at 37℃ , using a hotting bath (Magnetic stirrer,
TOYO Chemical Laboratories Inc, Tokyo, Japan).
Twenty-four hours after cementation, the following
technique was designed in an attempt to simulate
tooth physiological mobility: the portion of the root tobe embedded was covered with one layer of Scotch
tape and each root with restoration in place was fixedFig 2. Fiber specimen mounted in the brass holder of alever-type fatigue testing machine. The wet environment
during the cyclic loading was created by wet gauze, which
emerged from a recipient filled with water and encircled
the cervical portion of the root.

－ 18 －

Farhana Sharmin et al.

2.4 Bending test

19Test) (p < 0.05), after the normal and homogeneous

In the specimens, in which no failure occurred after

distribution of the data was confirmed.

the cyclic loading, and in those that did not underwent

Specimen mode of failure was classified as either

cyclic loading, a single-cantilever bending test was

debonding (without root fracture) or root fracture. The

performed. The specimens were secured in an Instron-

frequencies of root fracture occurrence in each grouptype testing machine (AG-1000E, Shimadzu, Kyoto,were counted and compared by Fisher’s exact

Japan) and an increasing load (crosshead speed of 1

probability tests, to determine the probability of an

mm/min) with the same direction and location as in

association between post type and occurrence of root

the mechanical cycling was applied until failure wasfracture in each loading condition, as well as thatrecorded. For each specimen, the magnitude of the

between loading condition and occurrence of root

force causing failure was recorded in kgf.

fracture in each post type. The statistical analyses

The primary failure mode was defined from boththe force-deflection curve and specimen appearance at
the in-situ visual inspection. While it was recorded, theforce-deflection curve was continuously watched; when

were performed using a commercial computer
program (SigmaStat 2.03, SPSS, Chicago, IL, USA).

3. Resultsa small, gradual drop-down in the force curve occurred,
the sample was visually inspected for gaps between

In the cycled specimens, no failure was recorded atcore and root. When any gap became visible, thethe end of the test; therefore, all the specimens weremachine was stopped to avoid further destruction.

then subjected to the quasi-static bending test in theThen, the roots were cut out of the embedment resin

Instron-type testing machine.

and the specimens were inspected by astereomicroscope to check for any other damage of the

3.1 Failure resistance

tooth or restoration. The presence of a gap between

Since the failure resistance data showed normal andcore and root without any fracture of the dental

homogeneous distribution, they were analyzed by two-

structure was defined as“debonding”. Conversely,

way ANOVA (2 x 2) (Table 1), which showed awhen no gradual drop occurred in the force-deflection

statistically significant difference for the factor post

curve and no gap was visible, the force was increased

type (p < 0.05). However, no statistically significant

almost linearly until a considerable, sharp drop-downdifference was detected for the factor load condition,

could be depicted. At that point, the testing machineor for the interaction between the 2 factors (p > 0.05).

was immediately stopped and the specimens were

The mean failure resistance and standard errors of theinspected, as described above. In these specimens, rootmeans for each group are illustrated graphically in Fig

fracture was always detected and the primary failure

3. The Tukey test showed that, in the presence ofmode was defined as“root fracture”.

cyclic loading, the post type resulted in a statisticallysignificant difference (p < 0.05) (Table 2), with higher

2.5 Statistical analysis

resistance of the cast specimens. In the absence of

The failure resistance (load-to failure) data werecyclic loading, post type showed a tendency toanalyzed statistically by two-way ANOVA (2 x 2) andinfluence failure resistance, but without reachingan all pairwise multiple comparison procedure (Tukey

significant difference (p = 0.054). Loading condition

Table 1 Results of the two-way ANOVA (2 x 2) of the failure resistance values (kgf)
Source of Variation

DF

SS

MS

F

P

Post type1

1056.578

1056.578

11.186

0.002

Load condition

1

5.837

5.837

0.0618

0.805

Post type x Load condition

1

25.53625.536

0.270

0.606

Residual

36

3400.518

94.459Total

394488.470

115.089

DF = degree of freedom, SS = sum of squares, MS = mean squares, F = variance ratio,
P = probability－ 19 －

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Table 2 Results of all the pairwise multiple comparisons of failure resistance values (kgf) (Tukey Test)
Comparison for

WithinDiff of Means

p

q

P

P<0.05

Load condition

Fiber

0.834

2

0.271

0.849

No

(Non-Cycled vs. Cycled)

Cast

2.362

2

0.769

0.590

No

Post type

Cycled11.877

2

3.864

0.010

Yes

(Fiber vs. Cast)

Non-Cycled

8.681

2

2.825

0.054

No

Diff of Means = Difference of Means, p = number of means spanned in the comparison, q = statistical datum derived in
Tukey Test, P = probability

Fig 3. Bar chart of the mean failure resistance values andstandard error of the mean (SEM) for each group (kgf): fiber
specimens (Fiber) submitted to cyclic loading (Cycled), castspecimens (Cast) submitted to cyclic loading, fiber
specimens not submitted to cyclic loading (Non-Cycled), andcast specimens not submitted to cyclic loading.

Fig 4. Stereoscopic aspects and force-deflection curves offailed specimens: a and b, fiber specimen failed bydebonding (arrow head); c and d, cast specimen failed byroot fracture (arrows). A slight gap of short span (arrowhead) is also visible at the tooth-core interface.

Table 3 shows the results of the Fisher’s exact

within both fiber and cast specimens did notprobability tests, in which the probability of an

significantly influence the failure resistance (p > 0.05).

association between post type and occurrence of root
fracture was analyzed for the cycled and non-cycled
specimens, respectively. In the cycled specimens,

3.2 Primary failure mode
The aspect of the force-deflection curves (Fig 4b andsignificantly fewer root fractures were found in thec), in combination with the visual and stereoscopic

fiber specimens than in the cast specimens (p < 0.05).

inspection of the failed specimens (Fig 4a and d),However, no significant difference was found in therevealed the following modes of failure: debonding of

non-cycled specimens (p > 0.05). Table 4 shows thethe post and core (13 specimens) and root fracture (27Fisher’s exact probability tests, in which thespecimens). In 22 of the root-fractured specimens, anprobability of an association between loading condition

oblique fracture line was visible in the cervical quarterand occurrence of root fracture was analyzed for theof the root. This crack started at the cervical surface

fiber and cast specimens, respectively. In the fiberof the root on one proximal aspect and descended

specimens, although a tendency of less root fractures

towards the lingual aspect, on which its most apicalwas found in the presence than in the absence oflevel was observed about 1 mm beneath thecyclic loading, this difference did not reach significanceembedment resin surface. In many specimens, the(p = 0.179). In the cast specimens, no significant

crack then rose on the opposite proximal aspectdifference was found in association with loading

towards the cervical surface of the root where it

condition (p > 0.05).

ended. In the other post-fractured specimens, 3 verticalroot fractures and 2 apical root fractures were

4. Discussion

recorded. Furthermore, in 10 of the root-fractured
specimens (5 cycled cast, 2 cycled fiber, and 3 non-

To have clinical relevance, an in vitro test should

cycled cast specimens) slight gaps were also observed

simulate intraoral conditions as accurately as possible.

microscopically at the core-dentin interface.

In this respect in the present study, the independent－ 20 －

Farhana Sharmin et al.

21

Table 3 Comparison of the primary mode of failure (occurrence/ non-occurrence of root fracture) between the cycled fiberand cast specimens, as well as between the non-cycled fiber and cast specimens (Fisher’s exact probability tests)
Load condition
CycledNon-Cycled

Post type

Root fracture

No fracture but debonding

Fiber

3

7

Cast

9

1

Fiber

7

3

Cast

8

2

P value
0.020
1

Table 4 Comparison of the mode of failure (occurrence/non-occurrence of root fracture) between the cycled and non-cycled
fiber specimens, as well as between cycled and non-cycled cast specimens (Fisher’s exact probability tests)
Post type
Fiber
Cast

Load condition

Root fracture

No fracture but debonding

Cycled3

7

Non-Cycled

73

Cycled91

Non-Cycled

82

P value
0.179
1variables that may remarkably influence the outcome

buccolingual deflection (SD) of 35.7 (21.6 μm), which iswere set as close as possible to those of the intraoral

within the physiological range (17 - 39 μm) reported

environment. Since dryness makes teeth more brittle,

for the same load direction and a similar location in theeach specimen was kept wet during both storage anddental arch.29）

the approximately 9-day long fatigue test.

The statistic analysis of the failure resistance data

To simulate the biomechanical behavior of toothby ANOVA (Table 1) showed that post type influenced

during function, applied load and tooth constraints are

the failure resistance of flared roots restored with aimportant. Graf & Grassl

25）

have reported that lateral

fiber-reinforced post and composite core or cast post-

force peaks between 0.5 and 2 kgf were observed inand-core. Moreover, the Tukey test (Tables 1 and 2)

the buccal direction of the molar teeth; during theshowed a significant difference for the post type infatigue test of this study, the tooth was slightly

association with cyclic loading, with lower failure

challenged by setting the load to the upper value (2resistance in the fiber than the cast specimens (Fig. 3).

kgf). During each cycle, the specimens were loaded for

Although the factor loading condition was not found to

0.3 s, which approximately corresponds to the duration

significantly influence the failure resistance (Table 2),26-28）

The cyclic

it showed a tendency of affecting the failure mode inloading was carried out at 80 strokes/min, which isthe fiber specimens (Table 4). Putting all these facts

within the range of the average human chewing

together the following explanation could be given.

frequency (70 to 80 cycles/min).26,27） The duration of

Fatigue loading in wet environment, in association

cyclic loading (900,000 strokes) may be equivalent to 3

with the use of composite resin added cervically

years of clinical function, because it has been estimatedaround the fiber-reinforced post to fit the flared canal,that application of fatigue-type stress during

increased the likelihood of debonding in the fiber

mastication may amount to approximately 300,000

specimens. This supposition is supported by the factof tooth contact in one chewing cycle.

flexures per year.

that significantly more failures by debonding without

12,26）

Regarding tooth constraints, a too rigid constraint ofroot fracture were found in the fiber than the cast

the root during the mechanical testing will lead to a

specimens in the presence, but not in the absence ofmore stressful biomechanical environment than thecyclic loading (Table 3). Another argument in favor of

intraoral fixation of the tooth in the alveolar socket,

this explanation is that, in the fiber specimens, awhereas a too loose constraint will have an opposite

tendency of more debonding without root fracture was

effect. To ensure artificial tooth mobility similar to thefound in the presence than in the absence of cyclic

physiological behavior, one layer of Scotch tape wasloading; however, this was not the case for the castwrapped and bonded around each root, before fixation

specimens (Table 4), in which instead of composite

in orthodontic resin. This resulted in a mean

resin, a dental alloy that is more resistant to cyclic－ 21 －

22

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loading and humidity ensured the post fit into thefiber post specimens18） or glass fiber-reinforced posts

flared canal. In the fiber specimens, the adhesive resin

when tested without crown coverage.8） In anothercement, matrix resin of post, and composite resin

study that investigated failure resistance and primarymight have been deteriorated by the fatigue test in

failure mode of post-and-core submitted to quasi-static

the wet environment and this may have allowed for acompressive load, a tendency of increased failure

greater deflection and thus a larger deformation of theresistance, but no significant difference, was reported

more flexible fiber-reinforced post. This put a higher

for teeth restored with cast post-and-cores inburden on the weakest tooth-adhesive cement-

comparison with those restored with carbon-fiber

composite resin interfaces, which therefore failed atposts.17） Although the difference in the direction oflower loads than the weakest link of the tooth-force application does not allow for direct comparison,

restoration complex in the cast specimens. In contrast,

the findings of the abovementioned study are in

the more frequently encountered root fracture andagreement with the results of the cast and fiber

resistance to higher loads in the cycled cast specimens

specimens not subjected to cyclic loading in thecan be explained by the higher stiffness of these posts,

present study.

as compared with the glass-fiber reinforced posts andFrom the point of view of re-treatment options, inthe cervically added composite resin. Higher loads

vitro studies on the failure resistance/mode of post-

were needed to deflect them, and thus a higher burden

and-core restored teeth often classify modes of failure

was concentrated in the cervical region of theinto “favorable /unfavorable”17） or “retrievable/

surrounding tooth structure. This ended up in more

irretrievable”,8） while clinical studies similarly defineroot fractures. Once a fracture occurred, a higher“reversible/irreversible”complications.33） Similarly, theburden was transferred to the tooth-cement interface

failure modes recorded in the present study could be

and slight debonding (gaps) followed immediately inclassified as either “favorable”“reversible”() orhalf of the cycled cast specimens. On the other hand in“unfavorable”“irreversible”(). Debonding without

the absence of cyclic loading, the difference in stiffness

fracture fell in the first category, since re-cementationof the 2 posts could explain the tendency of higherof the restoration could be a possible re-treatment

failure resistance in the cast than the fiber specimens;

option, while root fracture was classified into the latter,

however, in the absence of the fatigue test in wet

because the fracture line continued well below theenvironment, the tendency did not reach significancesimulated bone level, and thus would most probably(Fig 3).

imply tooth extraction. From this point of view, theIn the present study, root fractures and debonding

use of glass-fiber reinforced posts, which led to(gaps at the core-dentin interface) were recorded as

significantly less root fractures after cyclic loading,modes of failure. The later were most probably caused

may prevent root fracture to a greater extent thanby the debonding at the post-dentin interface.

cast post-and-cores, but debonding, a reversible

Clinically, the following mechanical failures have been

complication, may occur under high buccolingual load.

reported in teeth restored with posts and cores: root

However, an 18-year retrospective survival study thator restoration fracture, restoration loss of retention,

included full crowns with posts warns that themarginal gaps, etc.

Among these, root fractures

occurrence of a previously reversible complication is aand debonding of the post and core (post loosening)

predicting factor for an irreversible complication.33）

have been reportedly the most common post and core

Therefore, any complication should be carefullycomplications clinically encountered.32） Thus, it may beanalyzed and its cause should be removed, to avoid a

extrapolated that in a mechanical test, rather thanmore serious complication that can end up with toothrecording only the specimen fracture, the investigation

extraction.

22,23,30-32）

Finally, some limitations of this study needs to beof the initial sign of failure may lead to an outcome ofacknowledged. A study that compared failure

clinical relevance.
This study is also in agreement with in vitro studies

resistance/modes of various post systems at different

on fiber posts, in which significantly higher failure

stages of tooth restoration showed different results forloads, but more tooth fractures, were reported for castteeth restored with post-and-cores only and thosepost-and-core specimens in comparison with carbon-

additionally covered by crowns. 8） This implies the－ 22 －

Farhana Sharmin et al.

23

necessity of further testing of post-and-cores under

R and Brackett SE: Fundamentals of Fixedcrown coverage to increase clinical relevance of theProsthodontics. 3rd ed, p 181-209, Quintessence,

results. Moreover in the present study, cyclic loading

Chicago, 1997.

tended to influence failure mode in the fiber specimens

２）Creugers NH, Mentink AG, Fokkinga WA and

only, and it did not significantly affect failure

Kreulen CM: Five-year follow-up of a

resistance. Prolonged cyclic loading needs to beprospective clinical study on various types of

performed to elucidate the effect of loading condition

core restorations. Int J Prosthodont, 18: 34-39,

on the failure resistance/mode of restored teeth.

2005.
３）Tan PL, Aquilino SA, Gratton DG., Stanford CM,

5. Conclusions

Tan SC, Johnson WT and Dawson D: In vitro
fracture resistance of endodontically treated

Within the limitations of this in vitro study, thecentral incisors with varying ferrule heights andconfigurations. J Prosthet Dent, 93: 331-336, 2005.

following conclusions were made:１）Rather than cast post-and-cores, the use of glass-４）Dietschi D, Duc O, Krejci I and Sadan A:

fiber reinforced posts and core build-up material

Biomechanical considerations for the restoration

for the restoration of flared canal teeth mayof endodontically treated teeth: A systematic

prevent root fracture, as found in a fatigue test

review of the literature - Part 1. Composition

followed by a quasi-static bending test. However,

and micro- and macrostructure alterations.

debonding, a reversible complication, may more

Quintessence Int, 38: 733-743, 2007.

frequently occur in these fiber-post teeth under

５）Morgano SM and Milot P: Clinical success ofcast metal posts and cores. J Prosthet Dent, 70:

high buccolingual loads.
２）After 900,000 load cycles, flared canal teeth

11-16, 1993.

restored with cast post-and-core resisted to６）Akkayan B and Gulmez T: Resistance tosignificantly higher loads than those restored

fracture of endodontically treated teeth restoredwith fiber posts, but when they failed,with different post systems. J Prosthet Dent, 87:

significantly more unfavorable failures (root431-437, 2002.
７）Sirimai S, Riis DN and Morgano SM: An in vitro

fractures) were recorded.
３）The application of 900,000 cycles of load tended

study of the fracture resistance and theto influence primary failure mode in the fiber

incidence of vertical root fracture of pulplessspecimens only, and it did not significantly affectteeth restored with six post-and-core systems. J

failure resistance. Prolonged cyclic loading

Prosthet Dent, 81: 262-269, 1999.

needs to be performed to elucidate the effect of８）Cormier CJ, Burns DR and Moon P: In vitroloading condition on the failure resistance/mode

comparison of the fracture resistance and failureof restored teeth.

mode of fiber, ceramic, and conventional post
systems at various stages of restoration. J

Acknowledgements

Prosthodont, 10: 26-36, 2001.
９）Rosentritt M, Sikora M, Behr M and Handel G:

This study was supported in part by a Grant forIn vitro fracture resistance and marginal

Promotion of Niigata University Research. We express

adaptation of metallic and tooth-colored post

our gratitude to Prof. Emeritus Osamu Miyakawa,systems. J Oral Rehabil 31: 675-681, 2004.

Niigata University for his invaluable guidance throughout10）Al-Hiyasat AS, Bashabsheh OM and Darmani H:

the study. We are also grateful to Mr. Syuji Nakano for

An investigation of the cytotoxic effects oftechnical assistance. We also thank Pentron Japan Inc.

dental casting alloys. Int J Prosthodont, 6: 597601, 2003.

for providing the resin cement and bonding agent.

11）Stanford JW, Weigel KV, Paffenberg GC andReferences

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