INTRODUCTION distribution in the remaining tooth structure [7]. The

INTRODUCTION

Restoration of endodontically treated teeth (ETT) is
necessary for restoring aesthetics and function and preserving the remaining
tooth structure 1. As a consequence of
root canal therapy (RCT), the water content of the tooth is decreased, leading to an increased brittleness, and
as a result, the fracture strength of the tooth declines by 69% 2,3.

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Due to structural
defects caused by caries, trauma, or previous restorations, many ETT need
reconstruction by post and core (P&C) in order to become reasonably functional.
The main reason for the use of a post in these teeth is to create a mechanical
retention for the core; however, this can lead to an increased risk of tooth fractures
4. Nonetheless, the type of restoration
is chosen based on the remaining tooth structure. P&C treatment is not
necessary when the loss of tooth structure is
minimal 4.

In general, P&C systems can be divided into two
categories: casting and prefabricated P&C. Prefabricated posts can be made
of metal or tooth-colored materials 3.

Cast metal P&C systems
are widely used due to their favorable physical properties, high strength, and
good retention. Cast
gold P&Cs are considered the gold
standard because of a high success rate. However, failure to achieve the desired
aesthetics is a major problem encountered upon using cast metal systems
5,6.

The use of tooth-colored posts, such as ceramic posts
and fiber-reinforced composite (FRC) posts, is common because of better aesthetic
results. Furthermore, ceramic posts demonstrate a high strength and hardness,
while fiberglass posts show a lower strength and a higher
elasticity 7. FRC posts have a higher
flexural strength than metal P&Cs and zirconia posts. The modulus of
elasticity (E) of these posts is close to that of the dentin; therefore, they have the ability to create a single-unit
bonding with the tooth 8, and
they absorb most of the stress leading to a limited stress distribution in the
remaining tooth structure 7.

The use of zirconia posts with cosmetic restorations
in ETT provides satisfactory results. These posts have favorable mechanical
properties; however, there is a risk of root fracture due to a high modulus of
elasticity (E) 9. Placing posts with
heterogeneous modulus of elasticity may cause stress concentration in the
dentin and at the post-dentin interface leading to root fracture 10.

Several studies have compared the fracture strength of
the teeth reconstructed with
prefabricated or casting P&Cs; however, the results are contradictory 11. In some studies, it has been concluded
that the teeth restored with fiber posts have a lower fracture strength in
comparison with those restored by using metal posts 12. In other studies, the fracture resistance of the teeth
restored with fiber posts was evaluated to be equal to or more than that of the
teeth restored with metal posts 13,14.

In respect to the fracture pattern, some studies have
shown that the fractures in the teeth restored with fiber posts are more favorable
than those in the teeth reconstructed
by metal posts 15,16,
while other studies have rendered contradictory results
in this respect 17,18.

To date, few studies have evaluated the fracture
resistance of the teeth restored by
the use of cast base metal P&Cs 19. Moreover, a limited number of studies exists with regard to
the simultaneous evaluation of the fracture
resistance and fracture patterns of more than four different P&C systems 20. Therefore, there is a need for further
studies to determine the most beneficial P&C system. The purpose of this in-vitro
study was to determine and compare the fracture resistance and fracture patterns
of endodontically treated maxillary central
incisors restored with different P&C systems. The null hypothesis of this
study was that there are no significant differences
among the fracture resistance values of different types of P&C systems.

MATERIALS AND METHODS

In this in-vitro study, 60 maxillary central incisors were
extracted from 30-50-year-old patients due to periodontal diseases and were
placed immediately in 5% thymol solution. Teeth with cracks,
caries, or fractured restorations were excluded. Afterwards, a digital caliper
(Mitutoyo Absolute
500-197-20, Aurora, IL, USA) was used to measure the diameter and height of the
teeth, and the teeth with an average length beyond 23±1 mm or less than 12 mm
(from the cementoenamel junction (CEJ) of the buccal
surface to the apex) were excluded.

With 10
samples per group, there was an 80% likelihood of a significant difference among
the groups in terms of the mean fracture
resistance (n=48, ?=0.05).

After preparing the access cavities, the root canals
were prepared according to the passive step-back technique up to #60 K-file (Dentsply/Maillefer, Ballaigues, Switzerland).
Root canal obturation was
performed according to the cold lateral condensation technique by using
gutta-percha points (Aria Dent, Asia Chemi Teb Co, Tehran, Iran) and AH26 endodontic sealer (Caulk/Dentsply,
Milford, DE, USA). After filling the access cavities with
a provisional restorative material (GC Caviton; GC Dental Products Corp.,
Tokyo, Japan), the teeth were stored at 37°C and 100% humidity for a week. Afterwards,
the coronal part of each tooth was
cut perpendicular to the long axis and at 1 mm coronal to the mesial CEJ by
using diamond discs (Ref. 070, D&Z, Berlin, Germany) mounted on a dental
lathe machine (KaVo Polishing Unit, EWL 80, Leutkrich, Germany) at a low speed under constant
water irrigation.

By using a long flat-end water-cooled
fissure bur mounted on a high-speed handpiece, a finish line with a width of
1.2 mm was created in all the teeth. One- and 2-mm ferrule widths were formed on
the proximal and the buccal and lingual walls, respectively, with approximately
6 degrees of wall convergence. A 3-degree tapered diamond bur was used to
create this convergence angle similar to the method used in previous studies 21,22. The samples were randomly divided into
six groups (n=10).

 

In the first group, 11 mm of gutta-percha from each root canal was removed by #2-3
Peeso reamers. The root canals were prepared by using the black drill of the CosmoPost
ceramic post system kit (Empress Cosmo Ingot, Ivoclar Vivadent, Liechtenstein, Germany)
with the same length. A root canal impression was made by using a pattern resin
(Duralay, Reliance Dental Mfg. Co, Alsip, USA), and the core
was formed according to the
standard preparation of a maxillary central incisor so that the height of the resin
core in the buccal side was 4 mm above the tooth preparation. A silicone index
(Panasil®, Kettenbach
GmbH & Co. KG, Escheburg, Germany) was made from the formed core in order
to restore the other teeth. Afterwards, the posts were invested and cast by using a base metal alloy (Supercast,
Thermabond Alloy MFG, Los Angeles, CA, USA). All the P&Cs were cemented by
the use of Panavia F2.0 resin cement (Kuraray
Noritake Dental Inc., Osaka, Japan).

In the second group, the
preparation and molding were performed in the same way as in the first group,
except that a gold alloy (BEGO, Bremen, Germany) was used for casting.

In the third group, the teeth
were prepared in the same way as in the previous groups; zirconia posts of the CosmoPost
system with a diameter of 1.7 mm were tested in the root canal of each tooth. The
posts were placed inside the root canal of each tooth, and the core was formed
by wax (Dentsply
DeTrey, Surrey, England). After investing
and wax removal, glass ceramics reinforced with a special Lucite system (IPS
Empress® Cosmo Ingot C, Ivoclar Vivadent, Liechtenstein, Germany)
were pressed into the molds. Afterwards, ceramic P&Cs were air-abraded with 50-µm aluminum oxide particles and
were luted by the
use of Panavia F2.0 resin cement.

In the fourth group, the teeth preparation and selection
of zirconia posts were done in the same way as in the
previous groups. Afterwards, the posts were cut at 3 mm above the buccal
preparation by using a diamond bur (Meisinger, Dusseldorf, Germany) mounted on
a water-cooled high-speed handpiece. The posts were air-abraded with 50-µm aluminum oxide particles and were luted by Panavia F2.0
resin cement. Consequently, the cores were built
up by using a core build-up composite resin (Clearfil Photo Core, Kuraray Noritake Dental Inc., Tokyo, Japan) according to the manufacturer’s recommendations.

In the fifth group, the teeth were prepared according
to the method described above. The Svenska titanium post (No. L6, Svenska Dentorama,
Sweden) was used for reconstruction.

To ensure the compliance
of the end of these posts with the form of the root canals prepared by the #1.7 tapered-end drill of the CosmoPost system, the end of the posts were
milled to achieve the same tapered shape. The posts were air-abraded with 50-µm
aluminum oxide particles and were luted by Panavia F2.0 resin cement. The composite
cores were reconstructed in the same way as in the
fourth group.

In the sixth group, 11 mm of gutta-percha from each
root canal was removed by #2-3
Peeso reamers. The root canals were prepared by using the black drill of the Anthogyr
fiber post system (Fibio®, Anthogyr, Sallanches, France) with the
same length. The fiber posts were cut at 3 mm above the buccal preparations by using
a diamond bur (Meisinger, Dusseldorf, Germany) mounted on a high-speed
water-cooled handpiece. The posts were air-abraded with 50-µm aluminum oxide
particles and were luted by Panavia F2.0 resin cement. Next, the teeth
were etched with 35% phosphoric acid gel (Pegasus, England). The composite cores
were reconstructed in the same way as in the
fourth group.

At this stage, after correcting
the teeth preparations, impressions were made from the samples in the six
groups by an addition-curing silicone impression material (Panasil®;
Kettenbach GmbH & Co. KG, Escheburg, Germany).

After die preparation, a pattern
with the contour of a maxillary central incisor was formed on one of the samples by using a blue inlay wax (Kerr Co.,
Orange, CA, USA). A silicon index was prepared according to this pattern and
was used for the contouring of the other samples. Next, investing and casting were
performed, and full metal crowns were made for all the samples.

After polishing and adjusting each
crown on the teeth, the crowns were cemented by using a resin-modified glass ionomer
cement (GC Fuji Plus, GC Co., Tokyo, Japan) under a gentle pressure.

Each tooth was placed in a cylindrical
custom-made mold and was surrounded by a self-curing transparent acrylic resin (Acropars,
Marlic Medical Instruments CO., Tehran, Iran) at an angle of 130° relative to
the direction of force exertion.

In order to mount the samples,
each sample was attached vertically to the blade of a surveyor at the middle of
the incisal edge by the use of sticky wax (Kerr Co., Berlin, Germany). The
surveyor blade was brought down so that the CEJ of the tooth was sunken into
the acrylic resin.

The roots were covered with a
layer of wax (Dentsply DeTrey, Surrey, England), from 2mm below the CEJ to the
apex, to simulate the periodontal ligament (PDL) of natural teeth. The
remaining surfaces of the roots were covered with a 0.1-mm-thick thin foil. The
samples were removed during the warm acrylic phase, the foil around the roots was
removed, and the root space was filled with ImpregumTM (3M ESPE, Seefeld,
Germany). The samples were then put into place quickly.

In the next phase, thermocycling
(1500 cycles) was performed at 60-second intervals and at the temperature of 5-55°C in a water bath. Each sample was placed in the cylindrical
custom-made  mold. A 130° force was applied at a
crosshead speed of 1.5 mm/minute in a universal testing machine (TLCLO, Dartec
Ltd., Stourbridge, England) by a round-end
stainless steel pin to a point 3 mm below the
incisal edge in the palatal area of the crown. The fracture force (N) and fracture
patterns were recorded for each sample.

The samples with a fracture in
the upper one-third of the root were considered as restorable, while the fractures
at the lower two-thirds were considered as unrestorable.

The results were statistically
analyzed according to Kruskal-Wallis,
Mann-U-Whitney, and
Fisher’s exact tests in SPSS software program (Version 16.0, IBM Co., Chicago, IL, USA) at a significance level of 0.05.

 

RESULTS

The maximum mean fracture load was
observed in the first group (the teeth restored with a cast base metal alloy P&C),
while the minimum mean fracture load was observed in the sixth group (the teeth
restored with fiber posts and composite cores) (Table 1).

Because the groups did not meet the assumption of
homogeneity of variances (Kruskal-Wallis test, P<0.001), the relationship between the studied groups regarding the fracture resistance was evaluated by Mann-U-Whitney test (Table 2). This table shows that the groups with casting P&Cs (groups 1 and 2) and group 3 (zirconia post and casting core) have significant differences with the other two groups (groups 5 and 6) restored by non-casting cores. The obtained results with regard to the fracture pattern in the studied groups are shown in Table 3. The Fisher's exact test revealed that there were no significant statistical differences among the studied groups in terms of the relative frequency of the fracture patterns (P=0.998). DISCUSSION In the present study, the fracture strength and fracture patterns of endodontically treated maxillary central incisors reconstructed by six various types of P systems were evaluated. According to the results of the current study, the null hypothesis was rejected. Regardless of the limitations of this study, the findings showed the superiority of the mean fracture strength of maxillary central incisors reconstructed by cast P and zirconia post and casting core (Table 2). These findings concur with those of other studies which stated that a higher rigidity of posts might lead to a better stress distribution and a higher fracture resistance 23. Similarly, the teeth reconstructed with cast metal P were reported to have a higher fracture strength 19. This could be due to the fact that cast P exhibit a better adjustment with root canal walls leading to a more uniform stress distribution 14. On the contrary, a higher fracture strength was reported in the teeth constructed by the use of post fibers in comparison with metal posts 13. This could be associated with the close modulus of elasticity of fiber post and dentin 6,8. According to the findings of the present study, the teeth restored by zirconia post and casting ceramic core system showed a significantly higher fracture resistance when compared to other aesthetic post systems and titanium posts (Table 2). These findings were in agreement with those reported by Heydecke et al 9. In addition, Butz et al 24 described that the fracture strength of the teeth restored by zirconia post and composite core was significantly lower than that of the teeth reconstructed by zirconia post and ceramic core. Therefore, it is suggested that zirconia posts and ceramic cores can be used as an alternative to cast P in the frontal aesthetic zone, as was recommended previously 9. In addition, Heydecke et al 9 reported a higher fracture strength for zirconia post and composite core compared to titanium post and composite core system; we also found similar results. However, no significant differences were found in the two studies in this regard. These findings were in contrast to the results of the study by Toksavul et al 25. Although zirconia post and composite core seem to be beneficial when early restoration is needed in the aesthetic zone, a major disadvantage of such ceramic posts is the difficulty of removal from the root canal 26. The lowest fracture strength was found in the group of teeth reconstructed by means of fiber post and composite core system. The fracture strength was significantly lower in this group in comparison with groups 1, 2, and 3, as was the case in some other studies 19. The low fracture strength of the teeth restored by fiber posts could be due to the low modulus of elasticity (E) of such posts which leads to an increased bending of the P unit under loading; consequently, more stress is exerted on the tooth 2. On the other hand, in some studies, the lower modulus of elasticity (E) of fiber posts was considered favorable, and the teeth restored by fiber posts demonstrated a higher fracture strength compared to metal posts 8,13. It is worth mentioning that although the mean fracture strength of fiber posts (416.5 N) was found to be poor in our study, it was higher than the value reported in other studies (200 N) 26, 27. Thus, it could be concluded that these posts are resistant to regular occlusal forces. In the present study, there was no significant difference among the groups in terms of the fracture type. Nevertheless, the greatest number of unrestorable fractures was seen in the group of zirconia post and cast ceramic core system. Such findings are in accordance with the results of the study of Akkayan and Gulmez 26. It was declared that because of the high modulus of elasticity (E) of zirconia posts, forces are transferred to the tooth-post interface 29M1  leading to severe tooth fractures 26. However, the findings of Toksavul et al 25 were in contrast to the mentioned finding as they stated that the teeth reconstructed by zirconia post and cast ceramic core showed the lowest frequency of unrepairable fractures. In addition, Heydecke et al 9 found fewer unrepairable fractures when using zirconia posts. In the present study, we tried to choose the natural teeth with a close age range. In addition, the teeth with similar lengths and widths were selected and were randomly divided into six groups to decrease the effect of various tooth diameters. One of the disadvantages of natural extracted teeth is that even if they have similar diameters, they may differ in terms of the contour, dentin thickness, moisture content, and shape of root canals; these factors could influence the stress distribution in the remaining tooth structure 30. On the other hand, the use of plastic teeth does not simulate the modulus of elasticity (E) and bonding characteristics of natural teeth 25. In addition, Strub et al 31 reported the higher fracture strength of natural teeth compared to artificial teeth. The environment in which the teeth are kept influences the changes in hard dental tissues, particularly the dentin 32. In previous studies, either normal saline or thymol solution was utilized 33. It was described that normal saline could negatively affect the bond strength between post and dentin 34. Hence, we used thymol solution for preservation of the teeth. The teeth were collected within the last 6 months, as suggested by Naumann et al 33. In some studies, gutta-percha and sealers were not used as they might decrease the adhesion of the cement to dentin 35. Although the application of eugenol-based sealers can affect the properties and bonding of the resin cement, root canal obturation itself has little effect on the root strength 36. Moreover, by eliminating gutta-percha and sealer, the real clinical setting cannot be replicated, and the results could not be translated to in-vivo situations 33. Therefore, in the current study, the root canals were obturated by using gutta-percha points and AH26 endodontic sealer. A great number of studies up until 2009 evaluated the fracture strength of the teeth restored without placement of veneers 37. Although this method eliminates the effect of some variables including the quality, contour, and thickness of the veneer, it does not simulate the clinical practice repercussions. Moreover, the impact of ferrule on the final treatment outcome could not be determined 33. In the current work, full metal veneers were applied in order to reinforce the teeth. However, such veneers are not used in the frontal aesthetic zone, and their fracture characteristics might be different from those of porcelain-fused-to-metal or full ceramic veneers 33. Various appropriate ferrule widths have been reported in the literature 9,25. In the present study, this width was set at 2 mm in buccal and lingual areas, and at 1 mm in the proximal area, similar to previous studies 9,25. We subjected the teeth to a static load applied to the palatal aspect of the crown at a 130° angle. Such an angle can simulate the forces applied to maxillary central incisors in an Angle class I dentition 25. However, simultaneous application of dynamic and static loads may reproduce a more realistic oral condition 9. In order to simulate the clinical settings, the physiological mobility of teeth should be reproduced 33. In the present study, the PDL was imitated by using a thin layer of ImpregumTM to simulate physiological tooth movements. Materials such as polyether, silicone, polyvinyl siloxane, and artificial PDL have been used in previous studies 33. Acrylic polymerization is a heat-releasing process. The removal of the experimental teeth during the primary stages of polymerization, as performed in the current study, may prevent damage to dental structures which would indirectly affect the fracture strength 38. However, materials such as plaster or acrylic resin 39 could not accurately mimic the characteristics of maxillary or mandibular bones, and the influences of such materials on the outcomes of previous studies should be considered 33.   CONCLUSION In the present laboratory study, a higher fracture strength was detected for cast metal P compared to the other evaluated systems, which might be due to a better stress distribution. The lowest fracture resistance was found in the teeth restored by fiber post and composite core. Zirconia post and ceramic core may be a proper and aesthetically appealing substitute for cast metal P.  M1Reference 28 has not been cited in the text. Please add reference 28 to the text.