| Abstract|| |
Aim : The purpose of the study is to compare the regional micro-push-out bond strength of custom-made resin post and a prefabricated resin post luted using self-etch adhesive and/or etch and rinse adhesive system at various regions.
Materials and Methods : Forty freshly extracted human maxillary central incisors were selected for this study; 1.5-2-mm coronal to the cemento-enamel junction was removed with a diamond disk using a slow speed handpiece under cooling water. Working lengths were established, root canals were sequentially enlarged upto the apex until ISO size 50-K-file and obturated using gutta-percha and AH plus sealer by cold lateral compaction. Post spaces were prepared to a depth of 10 mm using paeso-reamer upto size #3. After preparation, each specimen was embedded in chemically cured acrylic resin. The roots were randomly assigned to four groups: Group I: Custom-made resin post+Self-etch adhesive, Group II: Glass FRC post+Self-etch adhesive, Group III: Custom-made resin post+Etch and Rinse adhesive, Group IV: Glass FRC post+Etch and Rinse adhesive. Fabrication of custom-made resin post was done. All the posts were cemented using a dual cure resin luting cement. Each root was sectioned perpendicular to the long axis using a microtome and subjected to micro-push-out bond strength.
Results: The micro-push-out bond strength in the coronal region, in Group I 13.5±1.66 MPa, Group II 12.08±0.8 MPa, Group III 11.15±1.06 MPa, Group IV 11.81±1.11 MPa. In the middle third region: Group I 11.43±0.740 MPa, Group II 10.584±0.504 MPa, Group III 10.0582±0.830 MPa, Group IV 10.35±0772 MPa. In the apical third: Group I 10.38±0.878 MPa, Group II 9.59±1.06 MPa, Group III 9.34±0.73o MPa, Group IV 8.77±1.02 MPa.
Conclusion : Within the limitations of the study, amongst all the four groups tested, custom-made resin post luted with self-etch system (CMPR-SE) showed higher micro-push-out bond strength values in all regions when compared to the other groups
Keywords: Custom-made resin post, etch and rinse adhesive, micro-push-out bond strength, prefabricated resin post, self-etch adhesive
|How to cite this article:|
Ranjithkumar S, Velmurugan N, Roy A, Hemamalathi S. Comparative evaluation of the regional micro-push-out bond strength of custom-made resin post system with a prefabricated resin post: An in vitro study. Indian J Dent Res 2012;23:484-9
Endodontically treated teeth are more susceptible to fracture  due to caries, access cavity preparation and removal of radicular dentin during cleaning and shaping procedure, thereby reducing the ability of the tooth to withstand masticatory load compared to vital tooth. Fiber posts in combination with resin luting cements have been widely used in restoration of endodontically weakened teeth.  FRC posts when luted along with resin luting cement claim to have elastic modulus and toughness similar to that of dentin, thus achieving better stress distribution. 
|How to cite this URL:|
Ranjithkumar S, Velmurugan N, Roy A, Hemamalathi S. Comparative evaluation of the regional micro-push-out bond strength of custom-made resin post system with a prefabricated resin post: An in vitro study. Indian J Dent Res [serial online] 2012 [cited 2019 Aug 22];23:484-9. Available from: http://www.ijdr.in/text.asp?2012/23/4/484/104954
These non-rigid posts when bonded to the root dentin create a functionally homogenous unit and could achieve a tooth-post-core monobloc.  The presence of high C-factor is a risk of debonding within the resin-dentin interface.  The difficulty in achieving adequate polymerization of the composite resins in the deeper areas of the root canal is also a concern due to the unpredictable depth of light transmission, thereby affecting the bond strength. 
The use of custom-made resin post and core as suggested by Velmurugan et al., may overcome some of these difficulties to a certain extent.  The custom-made resin post and core system is esthetically compatible and is bonded to the dentin resulting in a single monobloc. However, the bond strength of the custom-made resin post to the radicular dentin when luted with a resin luting cement has not been tested.
The purpose of the study is to compare the regional micro-push-out bond strength of custom-made resin post and a prefabricated resin post luted using, self-etch adhesive and/or etch and rinse adhesive system at various regions (cervical, middle and apical third) of the radicular dentin.
| Materials and Methods|| |
Forty freshly extracted human maxillary central incisors, with straight root canals measuring 15.5-16 mm, free of cracks, caries, fractures and fully developed apices were selected for this study. The teeth were stored in deionized water containing 0.1% thymol. The specimens were utilized for this study within 1 month of extraction.
Collection, storage, sterilization and handling of extracted teeth to be used in this study followed the Occupational Safety and Health Administration (OSHA) and center for Disease Control and Prevention (CDC) recommendation and guidelines. 
1.5-2 mm coronal to the cemento-enamel junction was removed with a diamond disk using a slow speed handpiece under cooling water to achieve uniform root lengths of 14 mm. The pulp tissue was removed with a barbed broach. Canal patency was determined by passing a size 10 K-file (Mani and Co) through the apical foramen. Working lengths were established by passing the instrument beyond the apex and shortening it by 1 mm. The root canals were sequentially enlarged up to the apex till ISO size 50 K-file (Mani and Co) followed by ISO size 60,70 and 80 using step back technique. The irrigants used were 5.25% NaOCl (HYPOSET, USP Hygienes, India) and 17% EDTA (Chenchems, India). The enlarged canals were rinsed with distilled water,  dried with paper points and obturated using gutta-percha and AH plus sealer by cold lateral compaction.
The post spaces were prepared 24 hrs after completing the endodontic procedures. The post spaces were prepared to a depth of 10 mm using paeso-reamer up to size #3, leaving behind 4 mm of intact gutta-percha to preserve the apical seal. Following post space preparation, the canals were irrigated with sterile water and dried with paper points. Presence of any residual gutta-percha in the walls of post space was checked using radiographs.
After preparation, each specimen was embedded in chemically cured acrylic resin. The roots were randomly assigned to four groups:
Fabrication of custom-made resin post
- Group I: Custom-made resin post (Filtek Z 250, 3M)+ Self-etch adhesive (Tetric-N-bond, Ivoclar Vivadent) (CMRP-SE).
- Group II: Glass FRC (fiber-reinforced composite) post (ESPE Rely X, 3M)+Self-etch adhesive (Tetric -N-bond, IvoclarVivadent) (FRC-SE).
- Group III: Custom-made resin post (Filtek Z 250, 3M)+Etch and Rinse adhesive (37% phosphoric acid (Scotchbond etchant gel 3M) and single bond 3M) (CMRP-E and R).
- Group IV: Glass FRC (fiber-reinforced composite) post (ESPE Rely X, 3M)+Etch and Rinse adhesive [37% phosphoric acid (Scotchbond etchant gel 3M) and single bond 3M] (FRC-E and R).
After preparing the post space, a conventional wax pattern of the prepared canal is made using inlay wax  [Figure 1]a. An elastomeric impression of this wax pattern was made [Figure 1]b. After removing the wax pattern, nano hybrid resin composite (Filtek Z 250, 3M) is packed into the mold space that is created and is light cured [Figure 1]c. Additional curing of the post is done after sectioning the impression [Figure 1]d, e. After which the trial fit of the post was carried out.
|Figure 1: Fabrication of custom-made resin post: (a) Wax pattern (b) Elastomeric of the wax pattern (c) Resin composite packed into the mold space and light cured (d) Sectioning the impression and additional curing (e) Custom-made resin post|
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The cementation of the posts for all the groups was performed in the following sequence. In self-etch adhesive group, adhesives were applied to the root dentin for 20 seconds, followed by light curing for 20 seconds. In etch and rinse group, root dentin was etched with 37% phosphoric acid for 15 seconds, followed by rinsing of canal with distilled water using a disposable syringe. The excess water was removed with absorbent paper points and the adhesive was applied and light cured for 40 seconds. All the posts were cemented using a dual cure resin luting cement (Rely X ARC resin cement).
The mixed cement was applied onto the surface of the posts and into the orifice of the root canals. The posts were inserted into the canal to the full depth by using finger pressure and excess was removed. Light curing was performed for 60 seconds through the posts.
The micro-push-out bond strength testing was done at Central Institute for plastics engineering and technology. The coronal, middle and apical portion of each root was sectioned perpendicular to the long axis with a microtome to create 1±0.5-mm thick slices. Each slice was marked on the coronal side with an indelible marker. The push-out tests was performed at a cross head speed of 1 mm/min by using a universal testing machine. Care was taken to center the push-out pin on the center of the post surface, without stressing the surrounding post space walls. The load was applied on the apical side of the root slice to avoid any limitation of post movement due to post-space taper.
The force at the point of extrusion of the post segment from the thick slice was taken as point of bond failure and the value was recorded in Newton and converted to MPa. All the results were recorded and statistically analyzed.
One sample Kolmogorov-Smirnov test resulted that the samples were drawn from the Normal populations, therefore the parametric tests ANOVA and Tukey HSD post hoc tests were used to analyze the data.
| Results|| |
Mean Micro-push-out bond strength (MPa)±SD and test significance of mean values for the experimental groups [Figure 2], [Figure 3] and [Figure 4].
|Figure 2: Box-plot diagram representing the regional bond strength in the cervical-third of the root canal|
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|Figure 3: Box-plot diagram representing the regional bond strength in the middle-third of the root canall|
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|Figure 4: Box-plot diagram representing the regional bond strength in the apical third of the root canal|
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- Group I (CMRP-SE) had significantly higher bond strength values in the cervical region and middle third region when compared with Group III (CMRP-E and R) and Group IV (FRC-E and R). But no significant difference was observed between Group I (CMRP-SE) and Group II (FRC-SE).
- Even though Group II (FRC-SE) had higher bond strength values no significant difference was observed when compared with Group III (CMRP-E and R) and Group IV (FRC-E and R).
- In the apical region, Group I (CMRP-SE), Group II (FRC-SE) and Group III (CMRP-E and R) demonstrated higher bond strength values when compared with the Group IV (FRC-E and R). However, Group I (CMRP-SE) showed significantly higher bond strength values when compared with Group IV (FRC-E and R).
- Cervical region demonstrated significantly higher bond strength values when compared with middle and apical third in all the groups tested.
- In the middle and apical regions, Group I (CMRP-SE) and Group III (CMRP-E and R) showed no significant difference in bond strength values.
- Group II (FRC-SE) and Group IV (FRC-E and R) significant difference was observed between the middle third and the apical third.
| Discussion|| |
In endodontically treated teeth, most frequently encountered problem is the fracture of the tooth restored with post and core. This is due to the unequal stress concentrations that occur at various interfaces of the restorations.  Rigid posts that were traditionally designed to reinforce the tooth structure had high stress concentration at the apical termination of the posts inducing a wedging force on the root causing unsalvageable root fractures.  Dallari et al. In 1996 reported the concept of monobloc, wherein a carbon fiber post was bonded to the root dentin to achieve a tooth-post-core monobloc.  This concept of monobloc was said to produce uniform stress distribution patterns and decrease in the clinical failures of the post endodontically treated teeth. 
Several studies have proved that the successful clinical performance of resin post systems bonded with adhesive resin systems is due to their good retentive values and better distribution of mechanical stresses.  To achieve an effective bonding inside the root canal, various factors are influenced namely the effect of root canal irrigants  on the dentin collagen, hydration in root canal dentin as a result of pulp removal, type of conditioning agent and polymerization stress of the resin cements with unfavorable cavity configuration. ,
The custom-made resin post and core system is easier, economical and fabricated to fit the exact taper of the prepared post space. This acts as a single unit that eliminates the number of composite interfaces that occurs during layering technique and is being bonded to the root dentin resulting in a single monobloc that helps in better stress distribution. The high "C-factor" is said to be reduced due to the additional extra oral curing. 
Various techniques have been developed like the pull out, push out, the microtensile bond strength tests, to measure the adhesion of endodontic posts to root canal dentin. ,, Although pull-out tests is the most convenient technique for evaluating the post-cement-dentin bonds, but they have flaws and problems with precisely calculating the bonded surface areas. In case of microtensile bond strength tests, preparation of the samples has been shown to break the weak post-dentin bond which increases the risk of premature failure.  The push-out test provides a better estimation of the bonding strength than the conventional shear test, as the push-out fracture occurs parallel to the dentin- bonding interface which makes it a true shear test and stimulates the clinical condition more closely.  One of the disadvantages of the push-out bond strength is the nonuniform stress distribution when the test is performed on the whole post.  To overcome this problem, a modified push-out test which is more advantageous is the micro-push-out, were the specimen thickness is being reduced. ,
In Group I (CMRP-SE) significantly higher bond strength values were observed in the cervical third region and middle third region [Table 1] and [Table 2], [Figure 2] and [Figure 3] when compared with Group III (CMRP-E and R) and Group IV (FRC-E and R).But no significant difference was observed between Group I (CMRP-SE) and Group II (FRC-SE). Even though Group II (FRC-SE) had higher bond strength values [Table 1] and [Table 2], no significant difference was observed when compared with Group III (CMRP-E and R) and Group IV (FRC-E and R). Frankenberger et al., compared the bonding of self-etch and etch and rinse to the root dentin, there was no significant difference between etch and rinse or self-etch luting systems.  In contrary to the previous studies, results of the present study showed that the self-etch adhesives had significantly higher bond strength values in the cervical and the middle third region when compared with etch and rinse bonding system. This in part related to the difficulty in etching and removal of etchant, presence of residual moisture, and difficulty in applying the adhesive agent. 
In the apical region [Table 3], [Figure 4], Group I (CMRP-SE), Group II (FRC-SE) and Group III (CMRP-E and R) demonstrated higher bond strength values when compared to Group IV (FRC-E and R). However, Group I (CMRP-SE) showed significantly higher bond strength values when compared with Group IV (FRC-E and R).
Comparing the regional bond strength values [Table 4], the cervical region demonstrated significantly higher bond strength values when compared with middle and apical third in all the groups tested [Figure 5]. This was in agreement with the study done by Ferrari et al., were it was stated that the presence of higher resin tag density in the cervical third region could be the reason for the higher bond strength value when compared to the middle and apical thirds of the root canal.  Similar results were reported in previous studies, were it was concluded that irrespective of the post system used, highest interfacial bond strength was observed in the cervical region when compared to the middle and apical regions. ,
|Figure 5: Box-plot diagram representing the regional bond strength between the regions namely cervical, middle and apical in all the groups tested|
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Comparing middle and apical regions, Group I (CMRP-SE) and Group III (CMRP-E and R) showed no significant difference in bond strength values. This may be due to fabrication of the custom-made post, done to fit the exact taper of the prepared post space  thereby, reducing thickness of the resin cement interface.  The similar bond strength between the middle and apical region in CMRP may result in better stress distribution.
In Group II (FRC-SE) and Group IV (FRC-E and R) significant difference was observed between the middle third and the apical third. The apical third of the root had the least bond strength value. Reduction in bond strength in the middle and apical region may be related to the difficultly in distribution of the resin cement without void formation. 
Mallamann et al. evaluated  the microtensile bond strength in the root dentin and found that the cervical and the middle-third had higher bond strength values when compared with the apical third. Another reason for decreased bond strength in the apical region is that, the coronal portion of the canal is more accessible, easier to etch and bond than the apical regions and controlling moisture is difficult resulting in remnant water in the post space. 
In the present study, custom-made resin post luted with self-etch luting cement showed better bond strength values in the cervical and middle regions when compared with etch and rinse luting system. As discussed earlier, control of moisture after application and removal of etchant as well as, incomplete infiltration of the resin into the dentinal tubules could have resulted in lower bond strengths in etch and rinse luting system. 
Within the limitations of the study, The micro-push-out bond strength of the resin post bonded using self-etch adhesive was superior when compared to resin post bonded using etch and rinse adhesive. Hence, self-etch adhesives can be used for bonding of resin post inside the root canal.
Furthermore, the use of custom-made resin post will reduce the problems associated with the depth of cure and high C-factor. Similarity in bond strength between the middle and apical region in CMRP may result in better stress distribution.
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Department of Conservative Dentistry and Endodontics, Meenakshi Ammal Dental College and Hospital, Alapakkam Main Road, Maduravoyal, Chennai
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4]