| Abstract|| |
Aim: The aim of this study was to evaluate the influence of different regions of dentin within the post space on the retention of fiber posts.
Materials and Methods: Ten human incisors with straight roots were selected for this study. Endodontic treatment of the specimens was done. The post spaces were created immediately after obturation and the posts were luted with dual-cure resin cement. Approximately 2.5-mm-thick sections were made from the coronal, middle, and apical thirds of the post space and thus we had three groups: Group I: Cervical, Group II: Middle, Group III: Apical.
The specimens were tested on a universal testing machine. Statistical analysis was done using the unpaired Student's t-test and one-way ANOVA test.
Result: The best push-out strength was obtained with the apical sections (14.69±0.298 MPa), followed by the middle (10.66±0.34 MPa) and cervical sections (9.73±0.42 MPa).
Conclusion: highest pust out strengths were obtained in apical sections followed by middle and coronal.
Keywords: Cementing media, dual-cure resin cement, fiberglass posts, post retention, push-out strength
|How to cite this article:|
Jha P, Jha M. Retention of fiber posts in different dentin regions: An in vitro study. Indian J Dent Res 2012;23:337-40
As early as 1746, Pierre Fauchard proposed the insertion of wooden dowels in the canals of teeth to support crown retention. Since then, many different materials have been proposed for 'reinforcement' and retention of the restorative core. 
|How to cite this URL:|
Jha P, Jha M. Retention of fiber posts in different dentin regions: An in vitro study. Indian J Dent Res [serial online] 2012 [cited 2019 Sep 18];23:337-40. Available from: http://www.ijdr.in/text.asp?2012/23/3/337/102219
Prefabricated, fiber-reinforced composite endodontic posts have been used since the 1990s, following the introduction of carbon fiber posts, which have an elastic modulus similar to dentin.  Other types of fiber-reinforced posts have been recently developed with the aim of obtaining a more esthetic treatment outcome and have resulted in the introduction of glass- and z fiber-reinforced composite resin posts. More recently, these posts were produced using translucent matrices so as to allow light propagation to enhance polymerization of photoactivated adhesive systems. 
A composite is defined as 'a solid formed from two or more distinct phases that have been combined to produce properties superior to or intermediate of those of the individual constituents.' 
In fiber posts, the fibers are embedded in a matrix of epoxy resin, and an interfacial agent such as silane is used to optimize the link between the two components.  Despite the advantages of fiber posts, the indications for their use in teeth was limited mainly by the difficulty in achieving adhesion to intracanal dentin. Using resin as a cementing agent along with polymer dentin bonding agents of similar flexibility will provide adhesive bonding within the root canal space.  Resin cements increase retention, tend to leak less than other cements, and provide strengthening of the root.  Bonding to intraradicular dentin is hampered by unfavorable conditions that are inherent within the root canals. Restricted access to the bonding substrate renders bonding procedures more technically demanding. Bond integrity is further challenged by the limited capacity to dissipate polymerization shrinkage stresses in long, narrow, post spaces that exhibit highly unfavorable cavity geometry. 
This study aims to evaluate the retention of fiber posts in various regions of the dentin of the post space.
| Materials and Methods|| |
Ten freshly extracted human maxillary incisors, extracted for periodontal reasons, with straight roots and root length more than 14 mm were selected for this study. Teeth having curved roots and root lengths less than 14 mm were excluded from the study. The teeth were stored in normal saline solution till they were used in the study.
The samples were cleaned with an ultrasonic scaler and disinfected with sodium hypochlorite solution. The crowns were sectioned at the cementoenamel junction using a double-faced diamond disk at low speed to facilitate instrumentation. A #30 K-file (Dentsply Maillefer, Ballaigues, Switzerland) was advanced in the canal until the tip of the instrument was visible at the apex, and a silicone stop was then placed at the coronal reference point, i.e., the cementoenamel junction. The length of the instrument in the canal was recorded (in millimeters) using an endo block (Dentsply Maillefer, Ballaigues, Switzerland), and the working length was determined by subtracting 1 mm from the previous length.
All the teeth were then instrumented with K-files (Dentsply/Maillefer, Ballaigues, Switzerland), with 5 ml of 5.2% sodium hypochlorite used to irrigate the preparation after each instrument. The apical preparation was done until the #55 file. After the apical preparation, the body of the canal was prepared by a step-back technique until the #80 K-file. At the change of each instrument, 5 ml of 5.2% sodium hypochlorite solution and 17% EDTA were used alternately for irrigation. Each instrument was used for 3 minutes, and thus the chemico-mechanical preparation totaled 30 minutes.
The prepared canals were then blotted with #55 paper points (Dentsply/Maillefer, Ballaigues, Switzerland). Equal amounts of the two pastes of the AH Plus ® sealer (Dentsply/Maillefer, Ballaigues, Switzerland) were dispensed on to the mixing pad and were mixed with a cement spatula for 1 minute. The fit of the master cone, i.e., #55 gutta percha point, was checked. The canals were then obturated with gutta percha points (Dentsply/Maillefer, Ballaigues, Switzerland) and AH Plus ® sealer, using the cold lateral condensation technique.
The post space preparation was done immediately after obturation. Using the Gates Glidden drill #3 (Dentsply/Maillefer, Ballaigues, Switzerland) the gutta percha material was removed from inside the root canals, leaving 3-4 mm of gutta percha in the apical third of the specimen. Post space preparation was done with a #5 Peasoreamer (Dentsply/Maillefer, Ballaigues, Switzerland). During and after post space preparation, 5 ml of 5.2% sodium hypochlorite solution was used to irrigate the canals. In addition to sodium hypochlorite, 17% EDTA was used for the irrigation of the canals after post space preparation. Final rinsing was done with 5.2% sodium hypochlorite solution.
The fiberglass posts (Angelus, Brazil) were tested inside the post spaces for their fit. Then they were sectioned 2 mm above the specimen's coronal margin, using a double-faced diamond disk. The posts were then cleaned with 70% isopropyl alcohol. They were allowed to dry for 1 minute and then a gentle blast of air was applied with the help of three-way syringe. The post space was dried with multiple paper points.
Self-etching primer (3M ESPE Products, St. Paul, MN, USA) was applied with microbrushes (Dentsply/Maillefer, Ballaigues, Switzerland) and was left undisturbed for 10 seconds, and then a gentle blast of air was administered using a three-way syringe. The bonding agent was then applied on the root canal walls with a microbrush and light cured with an LED light-curing unit (SDI, Bayswater, Victoria, Australia) for 20 seconds with the aid of light transmitting fiberglass posts.
Equal amounts of the two pastes of RelyX™ (3M ESPE products, St. Paul, MN, US) were dispensed onto the mixing pad and were mixed with a cement spatula. The cement was then applied on to the root canal walls with the help of Lentulo ® spiral (Dentsply/Maillefer, Ballaigues, Switzerland). The cement was also applied onto the posts with the spatula. The posts were then seated in the post space with firm finger pressure. The excess cement was removed gently. The cement was light cured using an LED light-curing unit for 20 seconds from the coronal end of the posts.
All the specimens were stored in distilled water at 37°C for 24 hours in an incubator.
Each specimen was sectioned perpendicular to the long axis into approximately 2.5-mm thick slices using a double-faced diamond disk under water cooling. One slice each from the cervical, middle, and apical regions of the post space were obtained and thus we had three groups, as follows:
Group I: Cervical
Group II: Middle
Group III: Apical
To calculate the exact bonding area, the tapered design of the post with regard to the respective part of the post space ware considered. With the help of digital calipers (Yamayo, Japan) and a magnifying glass, the height of the slices and the internal diameters of the greatest and the smallest base were measured. The adhered area of each test specimen was calculated using the formula for the conical frustum, i.e., area = (R1 + R2)√ (R1−R2) 2 + H 2 , where
H = height of the slice
R1= internal radius of the greatest base
R2 = internal radius of the smallest base
A fitting device was fabricated to fit the upper and the lower arm of the universal testing machine. The upper fitting device had a tip of 0.8-mm diameter. The lower fitting device had a cylindrical hole of 3-mm diameter in the center to allow for the dislocation of the posts from the specimen. The lower fitting device also had three screws attached to it to stabilize the specimen slices.
The test specimen slices were fixed in the fitting device fitted to the universal testing machine (Zwick Z010, Germany). The 0.8-mm diameter tip was placed over the smallest base of the test specimen and a compression force was applied in an apical-coronal direction at a cross-head speed of 0.5 mm/minute until the post piece was dislocated. The peak value of the load required for dislocating the post from the specimen was recorded in Newtons. The push-out strength (in megaPascals) was calculated by dividing the recorded peak load by the calculated bonding surface area, i.e., push-out strength (MPa) = maximum load (N) / bonding surface area.
Statistical analysis was done using the unpaired Student's t-test at 1% level of significance.
| Result|| |
The results of this study show that the push-out strength of the apical region was significantly greater than that of the middle and cervical regions in all the three groups. [Table 1] shows that the highest mean values were obtained for the apical sections (14.69±0.298 MPa), followed by the middle (10.66±0.34 MPa) and cervical sections (9.73±0.42 MPa).
|Table 1: Mean (±standard deviation) push-out strength values of individual sections|
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| Discussion|| |
The purpose of making composite structures is to obtain better mechanical characteristics than can be obtained with single components. As far as fiber posts are concerned, the fibers are embedded in a matrix of epoxy-resin, and an interfacial agent such as silane is used to optimize the link between the two components. The elastic modulus of a fiber post is closer to dentin than that of metal posts. So, the use of prefabricated fiber posts can potentially reduce the incidence of root fractures. 
The major factors affecting post retention are post dimensions (length, diameter), shape (conical, cylindrical), type of surface (serrated, screw, smooth), intracanal space preparation, type of cement, and operator skills. 
Gorracci et al, in 2004, showed more reproducible bond strength measurements using a conical version of the push-out design than with a microtensile technique. They concluded that when measuring the bond strength of luted fiber posts, the push-out test appears to be more dependable than the microtensile technique. , Therefore, this design was chosen for the present study. The latest studies in this field have highlighted the important contribution of sliding friction to the interfacial strength in composite materials. When a compressive load is applied on top of the fiber, friction occurs between the debonded portion of the fiber and the facing matrix, whereas shear stress continues to develop at the front of the propagating crack. From complete debonding to extrusion, only friction opposes fiber dislocation.  The retentive strength of a bonded post can be considered to be the combined result of micromechanical interlocking, chemical bonding, and sliding friction.
According to the results of this study, with regard to the dentin regions, the greatest retention means were observed for the apical third, followed by the middle third and the cervical third for all the three groups investigated. In contrast to our findings, Yoshiyama et al. (1998) observed less bond strength in the apical region. It is worth emphasizing that Yoshiyama et al. used dentin from the outside surface of the tooth and not intracanal dentin. Furthermore, the apical third studied was the apical third of the root, not the preparation for the post. As in the current study, Gastron et al. (2001) divided the roots into three parts with reference to the post space preparation destined for cementing the posts, and they too observed greater bond values in the apical third. , The difficulties in visualization and access when performing adhesive procedures inside the root canal increases the possibility of sealer or gutta percha residue on the dentin canal walls, especially in the apical third, reducing the surface area available for adhesion. Furthermore, the uncertainty of complete polymerization of the adhesive system and contact of the post with the filling material in the apical regions makes adhesion difficult in this third. 
Then how does one explain the greater retention values in the apical third that was found in this study?
During endodontic treatment, it is necessary to widen the canal in its initial thirds to facilitate penetration of the irrigating solutions; this results in a conical shape for the canal. Post preparation is performed with reamers that will wear the apical thirds more intensely. Therefore, the cervical region is more open to the influence of chemical substances and endodontic sealers, as less wear will not remove portions of the dentin that were in contact with the chemical substances, or which may still present cement residues. However, greater wear of the apical thirds eliminates portions of the dentin that were in contact with the chemical substances, or which may still present cement residues. Hence a clean dentin surface is present for the luting agent to interact with in the apical third.  Also, there is more intimate contact of the post with the dentin canal walls, mainly in the apical third, forming locking areas. Thus, retention of the post in this region will occur by adhesive bonding and mechanical overlapping, as observed in the clinical situation. The push-out test allows for an assessment of the mechanical overlapping. However, assessment of the bond strength only, as occurs in the microtensile bond strength test, is not possible. This makes it difficult to compare the results of studies that use different mechanical test methodologies; hence, the difference between our study and that of Yoshiyama et al. 
As opposed to the apical third, the cervical regions require greater volume of cementing material, increasing stress at the adhesive interface during polymerization shrinkage. This increased polymerization shrinkage may be critical for adhesion, taking into consideration the high C- factor of the root canal.  Also, as the cement is the weakest link between the post and the root canal dentin, the greater volume of cement may be the cause of the decreased push-out strength observed in the cervical third.
Based on the retention values found in the apical region, it is probable that the adhesive was effectively polymerized despite the distance from the light source. The post used contained glass fibers that, according to the manufacturer, conduct light energy, which may have helped in polymerizing the adhesive system in its most apical portions.  Furthermore, bond strengths to root canal dentin seems to be related more to the area of solid dentin than to the density of dentinal tubules.  As the tubule density decreases from the cervical to the apical thirds, this could be a reason for the increased push-out strength in the apical thirds. ,
| Conclusion|| |
Within the limitations of the present study, the following could be concluded:
- The best push-out strength was obtained with the apical sections, followed by middle sections and then coronal sections.
- Further studies are required to determine the mode of failure of cements used for cementing posts in the root canals.
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Department of Conservative Dentistry and Endodontics, Subharti Dental College, Meerut, Uttar Pradesh
Source of Support: None, Conflict of Interest: None