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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 14  |  Issue : 2  |  Page : 96-103

Fracture resistance of rehabilitated flared root canals with anatomically adjustable fiber post


Department of Pedodontics, Faculty of Dentistry, Kafr El-Sheikh University, Kafr El-Sheikh, Egypt

Date of Submission23-Mar-2017
Date of Acceptance03-May-2017
Date of Web Publication30-May-2017

Correspondence Address:
Talat M Beltagy
Department of Pedodontics, Faculty of Dentistry, Kafr El-Sheikh University, El-Geish Street, Kafr El-Sheikh 33511
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tdj.tdj_16_17

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  Abstract 


Aim:
This study aimed to evaluate the fracture resistance of the rehabilitated overflared canals with anatomically adjustable everStick post.
Patients and methods:
Thirty extracted human maxillary central incisors with close similarity were selected. The specimens were coronally horizontally sectioned, endodontically treated and randomly divided into three groups (n = 10). Twenty of specimens were overflared, leaving 5 mm gutta percha seal. Group I: overflared specimens were rehabilitated with everStick post. Group II: overflared specimens were rehabilitated with EasyPost/composite. Group III (control group): specimens had normal unflared canals and were restored with EasyPost/core system. Composite resin was used to complete the core building-up for all groups. Fracture resistance of all specimens was measured using Instron testing machine. Statistical analyses were submitted to one-way analysis of variance and a post-hoc test. The level of significance was adopted atP value less than 0.05.
Results:
The results showed that the control group recorded the highest fracture resistance values, followed by everStick and EasyPost/composite group and the difference was significant among them (P < 0.05). Mode of failures showed 76.7% of specimens exhibited a repairable mode of failure while 23.3% displayed nonrepairable mode. Root fracture was recorded in one specimen of everStick group and in two specimens of EasyPost/composite group, however, four specimens in the control group displayed EasyPost fracture.
Conclusion:
The use of anatomically adjustable everStick post in rehabilitation of compromised teeth with flared canals seems to be significantly effective, promising in the improvement of fracture resistance, and increase the favorable mode of failure.

Keywords: fiber post, fracture resistance, rehabilitated flared root canals


How to cite this article:
Beltagy TM. Fracture resistance of rehabilitated flared root canals with anatomically adjustable fiber post. Tanta Dent J 2017;14:96-103

How to cite this URL:
Beltagy TM. Fracture resistance of rehabilitated flared root canals with anatomically adjustable fiber post. Tanta Dent J [serial online] 2017 [cited 2017 Aug 20];14:96-103. Available from: http://www.tmj.eg.net/text.asp?2017/14/2/96/207303




  Introduction Top


The restoration of endodontically treated teeth with extensive coronal destruction is still a great challenge to the dentist. These weekend teeth are more prone to fracture, especially at the cervical third. Therefore; there is a need to rehabilitate these teeth with techniques that will not compromise the integrity of their remaining thin-walled roots and the use of dowel-and-core systems to retain full and final crown restorations seems mandatory [1],[2],[3],[4].

Custom or prefabricated metal posts were widely used and considered the gold standard for decades due to their superior mechanical properties. Conversely, metal posts showed several disadvantages as the high incidence of catastrophic root fracture [5], corrosion, inflammatory reaction, discoloration and shadowing on the periodontium in the anterior esthetic region [6],[7]. Also, they need temporization that added time-consuming and increases the incidence of the root canal contamination [8].

The restoration of compromised nonvital teeth with metal-free, biocompatible, and homogenous materials with improved their optical and physical properties have become a major objective in dentistry [4]. The prefabricated fiber posts with favorable biomechanical properties were developed to overcome the disadvantages of the metallic posts. Their modulus of elasticity and dentin-like rigidity are almost similar to dentin that significantly reduces the incidence of root fracture and displayed long-term durability [9]. They are less expensive, easier and faster to fabricate, and they can be simply and quickly removed from the root canals in the case of retrievability [10],[11]. Furthermore, the adhesive and micromechanical bonding characteristics of fiber posts to the resin cement, dentin, and composite core give a natural hue improving the esthetics without much compromising the material strength [12],[13].

However, the fiber posts require shaping of the canal walls to fit the dowels, leading to dentin loss and increase the incidence of root cracks and fractures [14]. The use of a single fiber post in overflared canals may lead to poor adaptation, core instability, increase in cement thickness, and reduce the bonding and mechanical properties [15],[16].

The use of accessory fiber posts still not significantly decrease the cement thickness, moreover they increase the interfaces between the reinforcing system and the dentin that compromise the adhesive effect of the technique [17],[18].

Recently, the problems with the overflared canal reinforcement can be overcomed by a newly released novel, direct and anatomically adjustable glass–fiber reinforced everStick post. This post is a minimally invasive, soft, flexible, polymer of polymethyl methacrylate, and resin impregnated bisphenol A-glycidyl methacrylate uncured glass–fiber post. It can be customized and closely adapted to the morphology of the root canal giving the best choice for flared, oval and curved canal. Their flexural strength and elasticity are nearly similar to the dentin, thus an equal distribution of occlusal stresses along the root surface will evenly minimize the risk of root fracture [19].

Therefore, this study aimed to evaluate the fracture resistance of rehabilitated flared root canals using anatomically adjustable glass–fiber reinforced everStick post. The null hypothesis was that there are no differences among the fracture resistance of the different reinforcing systems.


  Materials and Methods Top


Fracture resistance of rehabilitated flared canals

A total of 30 extracted human upper permanent central incisors for periodontal reasons with nearly similar sizes and shapes with a close range of age (40–50 years) were selected [Figure 1]a. Teeth with caries, microfractures, cracks, previous restorations, and curved roots were discarded. All patients are informed about the purpose of the study and using of their extracted teeth according to Ethics Committee of Faculty of Dentistry, Tanta University. Informed consents from the patients were taken for the use of their extracted teeth in this in-vitro study. Immediately after the extraction, the teeth were rinsed and sterilized using 2% formaldehyde solution at room temperature, and pH 7.0 for 30 days. The calculus and soft tissues were removed mechanically and the teeth were cleaned using a rubber cup with pumice mounted on low-speed handpiece for 15 s.
Figure 1: Nearly similar sizes and shapes of extracted teeth (a).The crowns of specimens were sectioned perpendicular to the long axis of roots (b). Digital caliper was used to measure the specimen root length and dentine thickness (c and d). Specimens represent the three groups (e).

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Preparation of specimens

The anatomical crowns were sectioned perpendicular to the long axis of roots at 1 mm coronal to the cementoenamel junction of the proximal aspects using a diamond tip with ample water cooling, leaving 17 ± 1 mm root length [Figure 1]b and [Figure 1]c. Root canals of all specimens were instrumented according to the instruction of International Standardization Organization using K-file #55 apically. After irrigation with 2.5% sodium hypochlorite and 0.9% saline alternatively, the canals were dried and obturated with gutta-percha (Diadent, Korea) and eugenol-free root canal sealer (AH26; Dentsply Maillefer, Switzerland) using lateral condensation technique. The specimens were stored at 100% relative humidity and 37°C for 24 h.

Large tapered postspaces were uniformly prepared in 20 of the 30 specimens to a depth of 12 mm from the sectioned surface using a silicone stopper as a guide and leaving 5 mm intact apical seal. The post spaces of 20 specimens were overflared using different sizes of large tapered tungsten laboratory carbide burs (Zhejiang, China) that were mounted on special drilling machine under copious water cooling leaving about 1.5 mm dentin thickness labially as confirmed by a digital caliper (IOS, USA) [Figure 1]d The specimens of the three groups are represented in [Figure 1]e.

The smear layer was removed using 5 ml of 17% EDTA followed by 10 ml of sodium hypochlorite. The specimens were further irrigated with distilled water, dried, and then randomly divided according to the rehabilitating system used into two groups (n = 10). Group III: 10 specimens with normal unflared canals were selected and prepared.

Group I (everStick group)

The dentinal wall of the overflared canal was etched with 37% phosphoric acid gel and bonded with an adhesive (Adper Single Bond Plus; 3M ESPE, USA) according to the manufacturer's recommendations. A premeasured length of the soft unpolymerized everStick (GC Europe NV, Finland) post 1.5 mm diameter with the silicone strip were cut together using a sharp scissor, leaving 4 mm of the post extending above the sectioned root surface. The post was inserted inside the root canal by a tweezer moisten with light-curing enamel bonding agent StickRESIN (GC Europe NV) shaped by condensing to the prepared post space, and initially light-cured for 20 s using Demetron LC unit (Kerr, USA). The polymerized post was then removed from the canal and further light-cured from all sides for 40 s.

Additional posts 0.9 and/or 0.5 mm sizes were gently pressed to closely fit the main post using an endodontic plugger. The post surface was activated using a thin layer of StickRESIN, light protected for 3–5 min using a light shield, and then light-cured for 10 s inside the root canal.

The post was removed gently from the canal and light protected before cementation. Self-adhesive dual-cure resin cement (RelyX Unicem; 3M ESPE) was applied according to manufacturer's instruction, then the post was immediately inserted slowly inside the canal using locking tweezer. The excess cement was removed using a small brush and the post was light-cured for 40 s [Figure 2].
Figure 2: EverSstick post used in group I (a-e).

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Group II (EasyPost/composite group)

The etching and adhesive applications to the root dentinal surfaces of the overflared specimens were carried out as mentioned in group I. Nanofilled composite resin (Filtek Z350 XT; 3M ESPE) was injected into the root canal space using a delivery tip loaded in a compule tips gun. A Luminex vasolinized smooth light transilluminating plastic post #6 (Dentatus, USA) was centrally seated into the filled canal up to the apical root filling [Figure 3]a,[Figure 3]b. The composite was light-cured for 40 s and for additional 20 s following the Luminex post removal.
Figure 3: A Luminex smooth post #6 used for intracanal polymerization (a and b). EasyPost #4 was used to complete the rehabilitation technique (c). EasyPost was cut leaving 4mm above the sectioned specimen (d).

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EasyPost #4 (Dentsply Maillefer) was prepared according to manufacturer's instruction for insertion into the canal space [Figure 3]c. All EasyPosts were cut outside at a distance of 4 mm from the roots sectioned surface using water cooled diamond bur to retain core build-up [Figure 3]d.

The post was cemented with RelyX Unicem, the excess cement was removed with a small brush, and then light-cured for 20 s.

Group III (control group)

The specimens without flaring were used in this group. After conventional root canals obturation, gutta percha filling were removed by Gates Glidden drills (Dentsply Maillefer) leaving 5 mm apical seal. The canal spaces were prepared using dentatus classic reamers to receive EasyPosts #4. The etching, adhesive application, and post cementation were carried out as mentioned before.

Standard polyester crown forms (Anger GA, Poland) 6 mm in height that snugly fit the neck of all specimens were used to complete the core build-up from Filtek Z350 XT [Figure 4].
Figure 4: A standard polyester crown form was used for core building-up (a and b).

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All roots were individually checked by a radiograph [Figure 5] and were embedded vertically in a metallic mould filled with auto-cure acrylic resin (Acrostone, UK) leaving 3 mm of the natural root was exposed. The specimens were coded and thermocycled in distilled water for 500 cycles at 5–55°C with dwell time 30 s and transfer time 5 s, then stored at 100% relative humidity at 37°C for 72 h before testing [20]. The coded specimens were randomly chosen and subjected to fracture resistance test.
Figure 5: Pre and post rehabilitation radiographs of normal canal/easyPost group (a and b). Labial and proximal view of pre-rehabilitation radiograph of an overflared specimen (c and d). Post- rehabilitation radiographs of easyPost/composite group and everStick group (e and f) respectively.

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Fracture resistance (failure load) testing

All coded specimens were individually mounted in a 45° angulation custom-made jig that secured to the lower fixed compartment of Instron testing machine (Bucks HP12 3SY, UK) with a load cell of 5 kN at Faculty of Dentistry, Tanta University. The load was applied using a custom steel rod with round end 4 mm in diameter, placed palatally 3 mm below the incisal edge of the specimen [21],[22] [Figure 6].
Figure 6: Mounted specimen in Instron testing machine.

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A constantly increasing loading force was applied at a crosshead speed of 2 mm/min until failure occurred due to post displacement, post or core fracture, root fracture, or debonding the cement [22]. The load failure (in N) and the mode of failure were recorded [Figure 7].
Figure 7: Specimens with different mode of failures.

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The failure mode was observed using a magnifying lens. It was classified into repairable (would allow repair) and nonrepairable [23]. Fractured of the post or the root or both were considered the nonrepairable mode of failure. All data were collected, tabulated, and statistically analysis using SPSS (version 19; IBM, Illinois, Chicago, USA) and submitted to one-way analysis of variance. Least significance test was used as a post hoc test for the significant result for the test of analysis of variance. The level of significant was adopted at P value is less than 0.05.


  Results Top


The mean fracture resistance of the tested reinforcing systems is presented in [Table 1].
Table 1: Comparison of mean fracture resistance of the tested reinforcing systems among studied groups (N )

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The results of load failure test revealed that the normal canal/EasyPost group recorded the highest values 388.0 N, followed by everStick group 308.8 N (P < 0.05). EasyPost/composite group displayed the lowest values 254.9 N and the difference was significant among them (P < 0.05).

[Table 2] showed that 23 (76.7%) specimens exhibited a repairable mode of failure while seven (23.3%) specimens showed nonrepairable mode. Root fractures were observed in one specimen of everStick group and in two specimens of EasyPost/composite group, however, four specimens in normal canal/EasyPost group displayed post fractures.
Table 2: Mode of failures among the tested groups

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The repairable mode of failure was observed in six specimens of the normal canal/EasyPost group associated with a fracture of core build-up, dislodgment of EasyPost in two specimens of EasyPost/composite group, one specimen showed fracture of reinforcing system with post dislodgment, and five specimens showed a fracture of core build-up. EverStick group recorded dislodgment of two posts while seven specimens were observed with a fracture of core build-up [Figure 5].


  Discussion Top


Rehabilitation of compromised flared teeth has been a challenge to maintain esthetic, function, and durability of the weakened teeth [24]. Although the clinical study is more realistic, the in-vitro assessment can overcome clinical limitations as individual variations. By measuring the fracture resistance of reinforced compromised teeth, we may have an idea about the load bearing capacity of the reinforcing system and a base for in-vivo studies which are necessary for the cite definitive outcomes [25].

So, this in-vitro study was conducted to determine the fracture resistance and mode of failure of rehabilitated compromised flared permanent central incisors using the everStick post.

In this study, the natural human extracted teeth were preferred to simulate the clinical situations of rehabilitated teeth [26]. As the permanent maxillary central incisors are more susceptible to traumatic fracture at an early age and therefore they were chosen to simulate the clinical condition [27].

To standardize the specimens, the teeth of similar shapes, sizes and of close range of age group were selected. According to the International Standardization Organization recommendation; the specimens were stored moist in distilled water throughout the study, as the bonding can adversely be affected by the dehydration of specimen's hard dental tissue, especially dentin [28],[29].

The acrylic resin mould was used in this study as its modulus of elasticity is approximately similar to that of human bone [30] and the embedded specimens leave 3 mm of root exposed to simulate the level of healthy alveolar bone [31].

In this study, eugenol-free root canal sealer was used as eugenol interfere with the polymerization of resins, deteriorate the marginal seal, and had a significantly negative influence on bond strength and post retention [32].

StickRESIN light-cure was used for everStick post surface activation to form secondary interpenetrating polymer network IPN bonding and the post was light protected to prevent the activated resin from premature polymerization by the light [14].

All posts in this study were cemented with resin cement as it is recommended for post cementation in thin-walled roots due to its potential to increase the fracture resistance of reinforced teeth [33]. It increases the capacity of post adhesion, showed greater toughness and longevity, low solubility, and minimum microfiltration as compared to conventional cement [34],[35].

In the current study, the core foundation was formed using the standard polyester crown to unify standard cores for all specimens. To simulate the aging of intraoral conditions, it was important to expose the specimens to thermocycling and thermal fatigue [36].

The contact angle of loading force on the specimen can strongly affect fracture resistance. The more parallel the orientation of loading force to the long axis of the specimen, the more the fracture resistance [29]. Therefore, the loading force in this study was applied palatally at an angle of 135° to the horizontal to mimic as possible the clinical class I occlusion in the anterior region [37],[38].

The results of the current study revealed that the fracture resistance and the failure mode were different among the tested groups, thus the null hypothesis is rejected. The results reported that the control group recorded the highest mean fracture resistance values among all groups and the difference was significant, while EasyPost/composite group recorded the lowest resistance and the difference was significant. This finding agrees with the opinion which stated that the fracture resistance of rehabilitated thin-walled roots is proportionally directed to the thickness of the remaining dentinal wall [39],[40].

The everStick group showed high fracture resistance than EasyPost/composite group. This may be attributed to the close elastic modulus of the everStick post to dentin that flexes together under loading force. Also, the dentin-like behavior of the post facilitates better stress distribution and yielding high fracture strength values [41].

In addition, several factors might influence the mechanical properties of FRC posts as the type of polymer matrix and length, diameter, number, and fiber-orientation of embedded fibers [42]. Thus, the presence of a high molecular weight polymethyl methacrylate chains in the everStick post act as stress-breaker via plasticize the stiffness of highly cross-linked bisphenol A-glycidyl methacrylate matrix, decrease stress concentration at the interface of fiber–matrix during deflection, and absorption of emerging stresses through the matrix [43].

Also, during manufacture of FRC posts, the rehabilitating effect of unidirectional impregnated fibers can be created. These impregnated fibers are soaked with resin matrix in a prestressed tension that released after curing causing fibers compression which can absorb the tensile stresses under flexural forces [44]. Moreover, these fibers facilitate stress dissipation, supports the fillers of composite layers, and act as a crack stopper [44],[45]. The more increase of fibers in the matrix, the more increase of the post resistance to microcracking [46]. The silanized fiber of everStick is another essential method for improving the fiber/matrix interface strength [47].

The everStick group exhibited 308.8 N, this finding disagrees with those of Kıvanç et al.[48] and Bolay et al.[49] who recorded 938.4 and 705.5 N, respectively, while Maccari et al.[50] reported 136.3 N with the use of glass–fiber reinforced resin post. These conflicting results may be due to variations in methodology, sample sizes, the biochemical composition of human extracted teeth, canal morphology, and physical and chemical properties of the reinforcing materials used in this study [51].

Ferrario et al.[52] reported that the maximum biting forces in healthy young adults ranging from 75 to 190 N in the anterior region. The minimum fracture resistance values obtained for all tested groups were 230, 285, and 328 N for EasyPost/composite, everStick, and control group respectively exceeding the maximum incisal biting forces.

Different failure patterns were reported in the tested groups of this study; fracture core build-up, fracture EasyPost, root fracture, everStick post displacement, fracture of the reinforcing system with EasyPost displacement, and complete displacement of the reinforcing system.

Post displacement of a specimen was shown in the everStick group. A possible explanation may be related to the low elastic modulus of the post resulting in higher bending under loading stresses, leading to debonding of the cement and complete displacement of the everStick post [49],[53]. Also this group showed no post or root fracture, this was probably due to the close similarity between the accepted range of elastic modulus of dentin [13],[14],[15],[16] GPa [54],[55] and the everStick post [13],[14],[15],[16] GPa [7] resulted in a biomechanical homogeneity unit that dissipating the forces overall the root without root fracture, so, they might act as stresses absorbers [13],[26].

The nonrepairable root fractures observed in EasyPost/composite group may be due to the rigidity of Filtek Z350 XT composite with high micro-hardness and elastic modulus of 30.1 GPa [56] which will not deform under stresses. This may lead to stress concentration with the development of a slowly growing crack and catastrophic root fracture, especially with thin-walled roots. The more the filler content, the more the elastic modulus and the more the resistance to deformation [57],[58].

Limitations of this study

  • The manipulation of the everStick post used in this study was technically sensitive
  • It was difficult to collect periodontally affected maxillary permanent central incisors at an early age.



  Conclusion Top


The use of anatomically adjustable everStick fiber-reinforced composite post in the rehabilitation of compromised teeth with flared canals seems to be effective and promising in the improvement of fracture resistance, and increase the favorable mode of failure.

Recommendation

Further long-term clinical assessment of the everStick post is desirable and recommended to support these in-vitro results.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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