|Year : 2015 | Volume
| Issue : 4 | Page : 296-301
Fracture resistance of endodontically treated permanent anterior teeth restored with three different esthetic post systems: An in vitro study
Ameet J Kurthukoti1, Jaya Paul2, Kapil Gandhi3, Divya B J Rao4
1 Department of Pedodontics and Preventive Dentistry, Jodhpur Dental College General Hospital, Jodhpur National University, Jodhpur, Rajasthan, India
2 Department of Pedodontics and Preventive Dentistry, Coorg Institute of Dental Sciences, Virajpet, India
3 Department of Pedodontics and Preventive Dentistry, Indra Prastha Dental College, Ghaziabad, Uttar Pradesh, India
4 Department of Hospital Administration, JSS University, Mysore, Karnataka, India
|Date of Web Publication||18-Sep-2015|
Dr. Ameet J Kurthukoti
Professor and Head, Department of Pedodontics and Preventive Dentistry, Jodhpur Dental College General Hospital, Narnadi, Jhanwar Road, Boranada, Jodhpur - 342 001, Rajasthan
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: Esthetic coronal reconstruction of fractured anterior teeth is often performed using intra radicular posts. Most of the commonly used commercially esthetic post systems do not exhibit similar physical properties as dentin resulting in failures. Aim: To evaluate and compare the fracture resistance and mode of failure of simulated traumatized permanent central incisors restored with three different post systems including biologic dentin posts. Materials and Methods: A total of 40 recently extracted human maxillary central incisors with similar dimensions were decoronated 2 mm above the cemento-enamel junction and endodontically treated. Ten specimens were randomly selected as the Group I - Control group (core built teeth without intraradicular posts). The remaining 30 teeth were equally divided and restored with zirconia (Group II, n = 10), fiber re-inforced composite (FRC) (Group III, n = 10) and biologic dentin posts (Group IV, n = 10) using resin bonded cement and their cores built-up. These samples were embedded in acrylic resin and then secured in a Universal Testing Machine and subjected to fracture resistance testing. The location of failure in the specimens was evaluated using a stereomicroscope. Results: Intergroup comparison revealed that the control group and zirconia post group (522 ± 110 N) demonstrated the least fracture resistance, while dentin post group (721 ± 127 N) the highest. There was no statistically significant difference between fiber post and dentin post groups. Fractures that were repairable were observed in fiber post and dentin post groups, whereas mostly unrestorable, catastrophic fractures were observed in the zirconia post group. Conclusion: Teeth restored with the biologic dentin post system demonstrated the highest fracture resistance and repairable fractures, closely followed by FRC post system. The least fracture resistance and most catastrophic fractures were demonstrated by the zirconia post system.
Keywords: Biologic dentin post, fiber re-inforced composite post, fracture resistance, location of failure, mode of failure, zirconia post
|How to cite this article:|
Kurthukoti AJ, Paul J, Gandhi K, Rao DB. Fracture resistance of endodontically treated permanent anterior teeth restored with three different esthetic post systems: An in vitro study. J Indian Soc Pedod Prev Dent 2015;33:296-301
|How to cite this URL:|
Kurthukoti AJ, Paul J, Gandhi K, Rao DB. Fracture resistance of endodontically treated permanent anterior teeth restored with three different esthetic post systems: An in vitro study. J Indian Soc Pedod Prev Dent [serial online] 2015 [cited 2019 Oct 23];33:296-301. Available from: http://www.jisppd.com/text.asp?2015/33/4/296/165675
| Introduction|| |
Endodontically treated teeth were known to present a higher risk of biomechanical failure compared to vital teeth. Posts are needed for restoring endodontically treated teeth with insufficient coronal tooth structure to retain a core for definitive restoration. The ideal post and core material should have physical properties such as modulus of elasticity, compressive strength and coefficient of thermal expansion similar to those of dentin. 
Various techniques and materials for post and core restorations have been advocated. They vary from custom made cast metal posts to prefabricated metal and nonmetal post, aesthetic posts and bonded posts, each with their own merits and demerits.
Despite great scientific and technological advances regarding restorative and adhesive materials, until date, no restorative material has come comparably close to the properties of the natural dental structures themselves.  Natural tooth fragments can be used to make dentin posts for strengthening the root canal in extensively damaged teeth.
Dentin posts have been used in the primary dentition with promising clinical and laboratory results. The use of a dentin post provides biocompatibility, a resilience that is comparable to the original tooth, excellent adhesion to the dental structure and composite resin, and low cost, as dentin posts are made from donated extracted natural teeth. Furthermore, the formation of a sole biomechanical functional system (monoblock) by means of an adhesion joining the dental structure, the cement agent, and the dentin post allows for a better distribution of stress along the root, minimizing the rate of adhesive and cohesive failure. 
Thus, the present in vitro study was undertaken to evaluate and compare the fracture resistance of simulated traumatized endodontically treated anterior teeth restored with two commercially available esthetic post systems and a customized biologic dentin post system.
| Materials and Methods|| |
After obtaining clearance from the Institutional Ethical Clearance Committee, freshly extracted human maxillary permanent central incisors were washed under running water and any soft tissue was scraped from the root surfaces, after which they were autoclaved for infection control and stored in airtight containers containing distilled water in accordance with International Standards Organisation guidelines. 
After examining the samples under a stereomicroscope, to check for any signs of fracture or craze lines they were then radiographed to ensure similar internal canal anatomy. Mesiodistal and buccolingual dimensions of all the teeth were measured using a caliper and samples presenting a difference of more than 20% from the mean were discarded leaving a total of standardized 40 samples. The exclusion criteria included teeth with incompletely closed apices, evidence of dental caries, craze lines or fracture lines on the root surfaces and any previous attempts of pulp therapy or failed root canal treatment.
The anatomic crowns of all the teeth were sectioned perpendicular to the long axis of the tooth with a water cooled diamond disc 2 mm above the cement-enamel junction (CEJ), so that the coronal surface was perpendicular to the long axis of the root, and the remaining root length was 13 ± 2 mm and the cut coronal surface was ground flat using an abrasive paper.
After obtaining straight line access using a round bur into the root canal, the pulp chamber and root canal were irrigated profusely with saline to remove any debris. A #10 sized K-file was then introduced into each root canal to determine the patency of the canal and upon visualization of the tip of the file at the apex, the final working length was established 1 mm short of this recorded length.
Pulp space preparation
The 40 maxillary central incisors were then instrumented with rotary K3 Endo ® tapered system (Sybron Endo, Orange, California, USA) in an torque controlled motor (VDW Silver ® , VDW GmbH, Munich, Germany), prepared using crown down technique to a master apical size of 40, 6% taper and obturated with Gutta-percha (Dentsply Maillifer, Tulsa Dental Corp., Tulsa, OK), using a resin-based sealer (Adseal ® , Meta Biomed, Korea) and radiographed to verify the quality of obturation in all the samples. Ten samples were randomly selected to constitute the control group.
Group I - Control group (10 samples)
In each sample, 2 mm of coronal Gutta-percha was removed using a No.5 Peeso reamer (Mani, Inc., Utsonmiya, Tochigi, Japan). The prepared spaces were etched with 37.5% phosphoric acid (Etchant Gel ® , Mission Dental, USA) for 15 s and washed for 20 s. The coronal root preparation was then restored with standardized cores using a resin core build-up material Multicore Flow ® (Ivoclar Vivadent AG, FL-9494 Schaan, Liechtenstein) with a dual cure dental adhesive (ExciTE-F ® DSC Ivoclar Vivadent), as per the manufacturers' instructions without any post-placement to yield an abutment height of 6 mm measured from the labial CEJ. The total abutment height comprised of 4 mm of the core material and a prepared dentin ferrule that measured 2 mm labially and 1 mm proximally [Figure 1].
|Figure 1: Control and experimental group specimen. Remaining obturated Gutta-percha and abutment height (2 mm ferrule + 4 mm core material)|
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Preparation of experimental groups
Gutta-percha from the remaining 30 samples removed from the root canals with a Peeso reamer, leaving only 3 mm Gutta-percha of apical Gutta-percha. Subsequently, a tapered No. 1 drill from Postec Plus ® Post Kit (Ivoclar Vivadent AG, FL-9494 Schaan, Liechtenstein) was used to enlarge the root canals in preparation for endodontic post placement.
The remaining 30 samples after selection of control group were randomly divided into the following three equal groups.
Group II - Zirconia post group (10 samples)
The teeth were restored with zirconia posts (Cosmo Post ® , Ivoclar Vivadent AG, FL-9494 Schaan, Liechtenstein). Similar cementation and core build-up procedures were employed in these samples [Figure 1].
Group III - Fiber re-inforced composite post group (10 samples)
The teeth were restored with fiber re-inforced composite (FRC) posts (Postec Plus ® Post Kit, Ivoclar Vivadent AG, FL-9494 Schaan, Liechtenstein). Similar cementation and core build-up procedures were employed in these samples [Figure 1].
Group IV - Biologic dentin post group (10 samples)
The teeth were restored with dentin posts.
The posts in all the above groups were cut to a length 3 mm coronal to the preparation.
Preparation of biologic dentin posts
For preparation of the biologic dentin posts, 10 extracted, donated permanent mandibular incisors were used following standard sterilization protocols.  Freshly extracted human mandibular central incisors were washed under running water, and all the soft tissue was scraped from their crown and root surface using a scaler. The teeth were then autoclaved at 121°C for 15 min for infection control. Using a diamond disc the crowns of the teeth were separated, and the roots sectioned mesio-distally along the long axis of the tooth. The irregular surfaces of these sectioned roots were smoothened using a diamond disc. These were then evenly shaped and passed through a ring gauge to obtain standardized "biologic dentin posts" that were approximately 1.7 mm diameter [Figure 2].
Preparation of the samples
The biologic dentin posts were cut to a length 3 mm coronal to the preparation. Previously used cementation and core build-up procedures were employed in these samples as well.
Periodontal ligament simulation
To reproduce the clinical situation and to provide a more accurate assessment of human root fracture strength, the methodology of this study included that the prepared tooth samples be embedded in polystyrene resin and periodontal ligament simulation  done using vinyl polysiloxane impression material (Express™ , 3M ESPE, Deutshland, Seefeld, Germany) [Figure 3].
|Figure 3: Polysiloxane elastometric impression material used for periodontal ligament simulation|
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Fracture strength testing
All the samples were positioned on a specifically designed inclined position device made of metal and subjected to a compressive load at a crosshead speed of 0.5 mm/min in a Universal Testing Machine until fracture. The load was applied on the palatal aspect of specimens at 135° to the long axis of the teeth [Figure 4]. Failure was defined as the first deflection on the "stress-strain chart" records.
|Figure 4: Fracture resistance test of specimens using Universal Testing Machine|
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The fracture strength values were recorded in newtons (N). The recorded values of fracture resistance were statistically analyzed using the independent t-test.
Evaluation of the mode of failure
After testing, the location of failure was classified under stereomicroscopic analysis (×10).
Location of failure was recorded as favorable and catastrophic. Core fracture and root fractures at the cervical third were classified as favorable (repairable) while fractures at the middle and apical thirds were classified as catastrophic (nonrepairable).
| Results|| |
The mean and standard deviations for failure loads were shown in [Table 1]. A statistically significant difference was observed among the failure loads in the groups studied [Table 2]. The load required to fracture the dentin post was higher than the control and zirconia post group (P < 0.001). The dentin post resisted a load of 721 ± 127.66 N, which was higher than the FRC post. However, the values were not statistically significant within these two groups.
|Table 1: Descriptive statistics showing the mean, SD and range of fracture strength values (Newtons) of the control and various experimental groups|
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|Table 2: Descriptive statistics showing the inter-group comparison of fracture strength values of control and various experimental groups|
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On evaluation of the mode of failure [Graph 1] all the 10 samples (100%) in Group IV demonstrated favorable fractures; while in Group II, 60% of the samples demonstrated favorable and 40% samples demonstrated catastrophic failures. In Group III, 9 samples (90%) showed favorable, and 1 sample (10%) showed catastrophic failure.
| Discussion|| |
Endodontic posts are required for supporting a core foundation when there is insufficient clinical crown remaining. The restoration of endodontically treated teeth with metal free, physiochemically homogenous material that have physical properties similar to those of dentin has become a major objective in dentistry. A few reported cases utilizing dentin as a post material have shown successful outcomes. , Hence, the probability of human dentin to serve as a post material needs to be investigated.
In this study, cores were not restored with crowns to exclude any external strengthening influence on the post and core and to eliminate parameters such as material structure, shape, length and thickness provided by crown restorations. In this manner, the structural integrity and fracture resistance of a post and core foundation were tested more precisely. ,
Each sample was positioned in a special apparatus that allowed it to be positioned at 45° to the buccal or lingual long axis, thus the angle, which formed between the long axis of the tooth to the long axis of the rod of the Universal Testing Machine was 135°. We used this angle to mimic a normal clinical situation, in which the angle formed by the long axis of the mandibular central incisor to maxillary central incisor is 135°. 
This study revealed that the teeth restored with zirconia posts showed lower fracture resistance compared to the dentin post and FRC post groups. The mean fracture resistance values of FRC post group were higher than zirconia post group but lower than the dentin post group.
Material property of the post has been shown to affect the stress distribution. It is more favorable when two substances of equivalent or almost near modulus of elasticity approximate each other. Thus, considering that the modulus of elasticity for dentin is ~20 GPa, the glass fiber post (~54 GPa) would be considered a more favorable commercial post in terms of stress distribution as compared to stainless steel (~220 GPa) or zirconia (~200 GPa). When stress is applied to any post system, a very rigid post (with high modulus of elasticity) will no longer follow the elastic deformation but will create localized stress peak inside the root, eventually leading to system failure (root fracture). Since the fiber post has characteristics simulating natural dentinal structure and it acts as a shock-absorber, dissipating most of the stresses on the final restoration with only a small fraction being transmitted to the dentinal walls, it demonstrates favorable fractures. 
In the present study, teeth restored with solid dentin posts (Group IV) exhibited the highest fracture resistance (721 N). This can probably be explained on the basis of the following factors:
- Physiomechanical properties of a dentin post
- Uniform stress distribution
- Shock-absorbing potential of a dentin post. 
Dentin has a complex microstructure and a modulus of elasticity of ~20 GPa, varying in different locations and directions, which may provide a mechanism that inhibits crack propagation in dentin.  The dentin post closely resembles root dentin in all the physical properties like modulus of elasticity, viscoelastic behavior,  compressive strength, thermal expansion, etc. Furthermore, the fracture toughness of dentin has been found to be better than most of the contemporary restorative materials. A dentin post forms a micromechanical homogenous unit with the root dentin that results in uniform stress distribution.  The similarity in the elasticity of a dentin post to root dentin may allow post flexion to mimic tooth flexion so that the post acts as a shock absorber, transmitting only a fraction of the stresses placed on the tooth to the dentinal walls. 
Currently, very few studies have been evaluated the fracture resistance of dentin posts. A recent study has reported higher fracture resistance value of dentin posts in comparison to fiber posts.  However in our study, there was no statistically significant difference between FRC post and dentin post. The notable differences could be attributed to the differences in the commercially used brands of FRC posts used in both studies and variations in the methodology.
Ambica et al.  have reported similar results in their study comparing the fracture resistance of two commercial post systems with human dentin post systems. The human dentin post samples demonstrated the highest fracture resistance, followed by FRC posts and carbon fiber post groups.
The results of our study showed that, 100% of the samples in the dentin post group showed favorable fractures, whereas 80% of the samples in the FRC post group and 40% of the samples in the zirconia group showed favorable factures. These results suggested that when dentin post and fiber post systems were used, less damage to the tooth structure occurred at failure load.
There exists a definite correlation between the post material and fracture of roots. Posts with a modulus of elasticity significantly greater than that of dentin might create stresses at the tooth/cement/post interface, with the possibility of post-separation and failure. On evaluation of the mode of failures, the dentin post and fiber post groups showed more favorable (repairable) fractures, compared to the zirconia post group.
The zirconia post group showed the highest number of catastrophic fractures. Fracture resistance is of greater importance than retention because a post can always be recemented if dislodged from the tooth. However, if the root fractures catastrophically, then the tooth is invariably lost. 
A major limitation in our in vitro test model is the multidirectional characteristics of masticatory forces, which cannot be duplicated in a Universal Testing Machine; wherein only a single unidirectional load is applied. It is evident that this type of in vitro loading may not always truly simulate an in vivo situation.
The other factors that affect the fracture resistance of post restored endodontically treated teeth are post diameter, length, design and adaptability, amount of remaining root dentin, cement and method of cementation, core material and design, crown design and biocompatibility of post material.  Hence, one should be cautious while directly extrapolating the findings of our present study into clinical practice.
We recommend further in vitro studies evaluating the above-mentioned parameters using human dentin posts before they can be used as viable alternatives to the currently available commercial post systems.
| Conclusions|| |
Within the limitations of the present study, the following conclusions can be drawn:
- Teeth restored with the biologic dentin post system demonstrated the highest fracture resistance, closely followed by FRC post system. The least fracture resistance was demonstrated by the zirconia post system.
- Teeth restored with the biologic dentin post system and the FRC post system demonstrated more favorable fractures rendering them amenable to retreatment, whereas teeth restored with the zirconia post system demonstrated more catastrophic fractures making them difficult for retreatment.
The dentin posts hold great promise as a new alternative to the currently available commercial esthetic post systems. Tooth banks can be established that can store these posts in advance and kept ready for use. Depending on the clinical requirement, the desired post may be selected, autoclaved and used.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]