| Abstract|| |
Aim: Compare the effect of three post designs on the fracture resistance and failure modes of composite core - fiber post - crownless tooth sets.
Materials and Methods: Ninety bovine incisors were selected and divided into nine groups of 10 specimens. The teeth were assigned to three groups based on the post design: Cylindrical, tapered, and double-tapered. Each group was subdivided into three subgroups in accordance with the diameter of the post: Small (No.1), medium (No.2), and large (No.3). The Panavia F system was used for post cementation. The specimens were mounted in acrylic resin blocks with a layer of silicone rubber covering the roots. A universal testing machine compressively loaded the specimens from the palatal side at a crosshead speed of 1 mm/min and at an angle of 135Ί to the long axis of the teeth, until failure occurred. The failure mode was determined by a stereomicroscope inspection of all the specimens. Data were analyzed by one-way ANOVA and the Tukey test (P < 0.05).
Results: The fracture resistance was affected by the type of post (P < 0.0001). A narrower diameter for all of the post systems allowed for higher resistance. The main failure mode in the large cylindrical group was catastrophic fractures, while the main failures in the other eight groups were favorable.
Conclusion: Narrower diameter posts showed higher fracture resistance. The dominant failure pattern was repairable fracture, except for those with large cylindrical groups.
Keywords: Fracture strength, fiber post, post diameter
|How to cite this article:|
Zogheib LV, Vasconcellos LO, Salvia AR, Balducci I, Pagani C, Bottino MA, Valandro LF. Fracture resistance of bovine incisors restored with different glass fiber posts: Effect of the diameter of fiber post. Indian J Dent Res 2012;23:623-7
Endodontically treated teeth frequently lose their coronal structures due to dental caries, previous restorations, trauma, and endodontic therapy.  When there is considerable loss of the crown, the use of root anchorage and a core buildup is necessary to bear the crown. ,,,, However, the use of posts does not strengthen the pulpless teeth. ,,
|How to cite this URL:|
Zogheib LV, Vasconcellos LO, Salvia AR, Balducci I, Pagani C, Bottino MA, Valandro LF. Fracture resistance of bovine incisors restored with different glass fiber posts: Effect of the diameter of fiber post. Indian J Dent Res [serial online] 2012 [cited 2019 Dec 8];23:623-7. Available from: http://www.ijdr.in/text.asp?2012/23/5/623/107379
Direct procedures using prefabricated posts have increasingly gained popularity,  as they allow a more sound structure to be preserved, , save time for the patient and professional, and they are less expensive. ,, With these goals in mind, prefabricated metal (titanium and stainless steel), fiber (carbon, quartz, and glass), and zircon posts are generally used. 
Fiber posts present a modulus of elasticity similar to dentin, which can reduce the risk of fracture ,, and improve stress distribution.  Moreover, if the endodontic treatment fails or the post fractures, it is faster, easier, and safer to remove a fiber post than a metal post, without substantial loss of tooth structure. 
In order to improve the fracture resistance of nonvital teeth restored with post systems, researches have investigated: Post materials, ,,,,,,,, design, ,, dimensions,  core material,  load direction,  amount of remaining dentin, , and location of the remaining coronal structure. 
Post designs can be classified in accordance with the shape and surface characteristics. Posts can be cylindrical, tapered or double-tapered, and of varying length and diameter. The surface characteristics of posts can be passive or active. 
The cylindrical design has been shown to increase post retention and produce uniform stress distribution along the post length.  However, the removal of a dental structure in the apical third of the root, and the presence of acute angles at the apex of the post, can result in stress concentration, especially in the apical third of narrow, tapered canals.  Tapered posts conform to the natural root shape and canal configuration, thus permitting optimal preservation of the tooth structure at the apex of the post. , However, this design is reported to have a lower retentive strength.  The double-tapered design endeavors to provide adequate stress distribution with the best adaptation, without extreme dentin removal at the root apex.  This design presents a slight cone in the apical portion; a bigger cone in the middle third; and a cylindrical design in the cervical region, offering better retention. 
Few laboratory studies have compared the biomechanical behavior of teeth restored with different post designs. , Some of these studies have evaluated the effect of these variables on retention  and stress distribution by means of finite element analysis.  However, there is still a lack of information concerning the biomechanical behavior of glass fiber posts under compressive loads when the post designs are varied. This current study is conducted to test the hypothesis that double-tapered glass fiber posts used in bovine roots offer higher fracture resistance values than the resistance provided by tapered and cylindrical posts, while also providing a larger number of repairable failures after being submitted to compressive load.
| Materials and Methods|| |
Ninety mandibular bovine incisors of similar shape and dimensions, free of cracks and fractures, were selected and stored in 0.9% saline solution at 4°C, when not being tested. After periodontal scaling, the anatomic crowns of the teeth were sectioned horizontally to the long axis, using diamond disks to standardize the root length at 15 mm. The buccolingual and mesiodistal dimensions were determined with an electronic caliper with a resolution of 0.01 mm (model Starrett 727, Starrett, Itu, Brazil) and the roots were assigned to three main groups, according to design, and nine subgroups in accordance with the diameter of the post system [Table 1].
Canal preparation was performed with drills corresponding to each post system at a length of 10 mm in all roots. During the preparation, the canals were irrigated with 0.5% sodium hypochlorite. All the roots were covered with a silicon film (Silica 30 Flexite ALBA G; Alba Adhesive Industry and Commerce Ltda, Boituva, SP, Brazil). Each root was centered inside a metal cylinder (25 mm in diameter and 30 mm high) using a drill corresponding to the caliber of the intra-radicular preparation. This bit was fixed to the vertical connecting rod of a surveyor and placed inside the canal. The tooth was then lowered into the metal cylinder, which contained acrylic autopolymerizing resin (Jet; Classic, São Paulo, SP, Brazil). To simulate the bone level and the biological distances, 3 mm of the root was kept outside the metal cylinder.
Each post was marked at a distance of 15 mm from its apex and sectioned horizontally at this level, using a water-cooled diamond bur (2200; KG Sorensen, São Paulo, SP, Brazil). The post was cleaned with ethanol, and a silane agent (Angelus, Londrina, PR, Brazil) was applied to its external surface. The root dentin was washed with water using disposable syringes. The excess water was removed with dry paper tips.
All posts were cemented with an autopolymerizing adhesive system (Ed Primer; Kuraray, Kurashiki City, Japan) and a dual-polymerizing adhesive resin cement (Panavia F; Kuraray, Kurashiki City, Japan) following the manufacture's guidelines. Drops of primer A and B of the adhesive system were mixed and two consecutive coats were applied to the canals, using microbrushes. The primer was allowed to dry for 60 seconds. The excess was removed, before drying with a soft jet of air. Pastes A and the B of the resin cement were measured and mixed until a homogeneous color was achieved. The adhesive cement was put into the canal, using a lentulo drill, and applied on the surface of the post. The post was inserted into the canal and kept in position. Excess cement was removed and the remainder was light-polymerized for 40 seconds (Ultralux-DabiAtlante, Ribeirao Preto, SP, Brazil) (Light intensity: 600 mW/cm).
One randomly selected specimen was used as a master die. The coronal portion was reconstructed using a micro-hybrid light-polymerized composite resin (W3D-Master, Wilcos, Sao Paulo, SP, Brazil); a preparation for a total crown, with a chamfer finishing line in the dentin, was made on the composite core. A palatal step design (0.3 mm deep and 1 mm wide), located at the center of the palatal surface and 2 mm below the incisal edge, was made with a high speed diamond bur. A condensation silicon impression was obtained and replicas of the preparation were made of acrylic resin (Duralay, Dental Reliance, Chicago, Illinois, U.S.A.). A transparent silicon matrix was formed on the replicas with 0.5 mm thick plates (Copyplast 0.5; BioArt, Sao Carlos, SP, Brazil) using a vacuum forming machine (Plastvac P7; BioArt, Sao Carlos, SP, Brazil). The matrix was filled with micro-hybrid light-polymerized composite resin (W3D-Master; Wilcos, Sao Paulo, SP, Brazil), seated on the root of another specimen along the long axis, and light-polymerized for 40 seconds on the vestibular and palatal surfaces. Every composite core was produced using the same procedures. All the composite cores were standardized to a height of 6 mm.
The specimens were positioned into a metal cylinder and stabilized in a specially developed device that allowed them to be positioned at an angulation of 135° to the long axis. The specimens were then placed in a universal load-testing machine (DL-1000; Emic, São José dos Pinhais, PR, Brazil), with the load head (dimensions: 1.22 mm × 6.04 mm) placed on the specially formed palatal step, and a compression load was applied (crosshead speed of 1 mm/min) until fracture occurred (failure value recorded in N).
The tested specimens were analyzed in a stereomicroscope and the fracture mode was classified as favorable (post displacement, core fracture, core fracture plus post fracture, and root fracture, up to the simulated bone level) or catastrophic (root fracture beyond the simulated bone level).
One-way ANOVA and the post-hoc Tukey test (5%) were used to analyze the fracture resistance data (P < 0.05). The nonparametric Chi-square and z-test were performed for the statistical evaluation of the mode of failure (P < 0.05) (STATISTIX, version 8.0, Analytical Software Incorporation, 2003).
| Results|| |
One-way ANOVA revealed that the fracture resistance was significantly affected by the type of post (P < 0.0001).
The Tukey test showed that narrower posts provided greater fracture resistance (N) of the specimens [Table 2]. The group with the narrow tapered diameter (Gr4) presented the highest resistance to fracture.
|Table 2: Means and standard deviations of fracture resistance (N) and Tukey's test results|
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With regard to the failure type in the specimens, the groups that showed the highest number of catastrophic fractures had a cylindrical design and large diameter posts. On the other hand, eight groups showed an increased number of favorable fractures after compressive testing [Table 3] and [Figure 1].
|Figure 1: Means and standard deviation of fracture resistance results (Kgf) (G1- cylindrical-thread #1; G2- cylindrical-thread #2; G3- cylindrical-thread #3; G4- tapered #1; G5- tapered #2; G6- tapered #3; G7- double-tapered #1; G8- double-tapered #2; G9- double-tapered #3)|
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|Table 3: Analysis of mode of failure with chi-square (χ2= 1.348, df = 2, P-value = 0.510>0.05) and z-test (CT vs T: z = 0.26; P-value = 0.794; CT vs DT: z = 0.83; P-value = 0.409; T vs DT: z = 1.09; P-value = 0.276)|
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| Discussion|| |
The present study compared the fracture resistance of restored teeth, with three different diameters (narrow, medium, and large), of different glass fiber posts. Through statistical analysis of the data, standard-deviations of 20 to 40% were observed in the median values and were in agreement with similar studies. , Additionally, the use of bovine teeth was shown to be valid for the evaluation of techniques and materials that would be clinically implemented. 
Some studies have attempted to simulate the periodontal ligament and the dental support structures during a compressive test. ,,, In the present study, all roots received a fine layer of silicone over their external surface before being placed inside the acrylic resin cylinders, to simulate the periodontal ligament. The root was placed in the acrylic resin cylinder with 3 mm of the root surface exposed to simulate biological distances. The rigidity of the acrylic resin was reduced by using silicone between the root and the acrylic resin.
The compressive strength test is usually used at an angle of 135° degrees in relation to the long axis of the root, in an endeavor to simulate the occlusal contact relationship found in Class I anterior maxillary and mandibular teeth. ,,,, In this current study, the loading tip used for the compressive testing of each specimen was located at the center of the palatal surface, 2 mm below the incisal edge on the standardized steps.
The quantity of the remaining coronal structure, called the 'ferrule effect', and its location, have a significant influence on the fracture resistance of non-vital teeth restored with cast post core and prefabricated post systems. ,, However, this present study was conducted without a ferrule, even though the goal was to evaluate the fracture resistance when varying the diameter of the glass fiber post used to restore non-vital teeth.
The influence of covering the core with a complete crown, with margins, in a healthy tooth structure was examined in similar studies evaluating the fracture resistance of non-vital teeth. ,,,, In those studies, when the remaining dental structure had a ferrule length of 2 mm, the differences found between the post systems with different designs were considered insignificant, as this could alter the distribution of forces on the core and the post, thus making the function of the post less important. The load applied directly to the core, without the complete crowns and ferrule, resulted in significant differences between posts with different diameters. The absence of a crown has also been used in previous studies that evaluated fracture resistance , and stress distribution , in non-vital teeth restored with post systems. In the current study, the use of the crown was not performed, so as to exclude the influence of any kind of reinforcement on the post system, which might transmit some load directly onto the root and mask the effect of post dimensions. ,
Considering the tapered and cylindrical design groups, the teeth restored with narrow diameter posts obtained significantly higher fracture resistance values (404.03 and 360.88 N) than medium diameter posts, (276.54 and 255.95 N) and large diameter posts, (294.19 and 258.89 N), respectively. No significant differences were found in the group with the double-tapered design; however, the teeth restored with the narrow diameter posts (289.29 N) did obtain higher fracture resistance values than the medium (237.32 N) and large diameter posts (239.28 N). These results support the concept that post selection should be based on a configuration that preserves a greater quantity of tooth structure during post space preparation. ,,, This result suggests that posts with small diameters preserve more of the remaining root dentin, which is very important for the prevention of root fractures. , Further studies using mechanical fatigue dynamic testing in a humid environment should be conducted, ,, to analyze the long-term effect on different substrates and interfaces (fiber post, root dentin, composite core - fiber post, resin cement - fiber post and adhesive system - dentin interfaces).
One desirable property of dental posts is that if a restoration fails, the tooth can be preserved, allowing a new restoration to be made. The failures can occur on account of root fracture or fracture or dislodgment of the post. Obviously, fracture and dislodgment of the post allows for re-treatment, while root fracture would most probably lead to tooth extraction.  Group 3 (cylindrical, large) showed the highest number of catastrophic fractures (seven out of 10) when compared with the other groups. This design had a diameter of 1.5 mm, including the apical part. Therefore, the quantity of root dentin preparation in the apical third, during post space preparation, could weaken the roots and affect the fracture mode. ,
Comparison of the fracture resistance values of this current study with similar studies was not possible because of numerous variables, such as, tooth dimensions, minute variations in the morphology of the root canals, and variations in dentin caused by differences in the age of the specimens and direction of dentinal channels. , The limitation in the method of this present study must be recognized. Finally, the continuous increase of load applied on the tooth in the current study differs from what actually occurs in the oral environment. Thus, for more significant results, further studies should be conducted with methods that simulate clinical failure mechanisms more closely, such as, thermal cycling and cyclic loading.
| Conclusions|| |
Within the limitations of this study it was concluded that:
- Independent from the post design, narrower post diameters provided higher resistance to fracture of the specimens, perhaps due to a less invasive preparation of the post space.
- The group with large cylindrical posts showed a higher number of catastrophic fractures, potentially because the post space preparation was more invasive.
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Luiz Felipe Valandro
MSciD-PhD Graduate Program in Restorative Dentistry, Sao Jose dos Campos Dental School, São Paulo State University at São José dos Campos; MSciD-PhD Graduate Program in Oral Science (Prosthodontic Unit), Faculty of Odontology, Federal University of Santa Maria, Santa Maria
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3]