| Abstract|| |
Background: Remaining dentin wall thickness may influence the fracture resistance of tooth.
Aims: To investigate the effect of various coronal dentin wall widths on the fracture strength of root canal treated teeth.
Materials and Methods: Fifty recently extracted single canal mandibular premolars were used for the study. Ten unrestored teeth were used as control (Group 1); remaining teeth were root canal treated and divided into four groups (n = 10). The Groups 2a, 2b and 3a, 3b were having 2.5 mm, 1.5 mm remaining dentin with and without post, respectively. The samples fracture resistance was tested under the universal testing machine. The data were analyzed with one-way ANOVA and post-hoc Tukey test for comparative evaluation.
Results: The mean fracture strength observed in Group 1 was (29.75 Mpa) followed by Group 2a (28.97 Mpa), Group 2b (27.70 Mpa), Group 3a (23.39 Mpa), and Group 3b (16.38 Mpa). There was no statistically significant difference between control and Groups 2a and 2b with P > 0.05. The post contributed significantly for fracture resistance in Group 3a.
Conclusion: The endodontic post is not required in root canal treated teeth >2.5 mm coronal dentin wall width while the post is essential for a tooth with <1.5 mm dentin wall width to improve fracture resistance.
Keywords: Dentin; endodontically treated; post and core technique; tooth fractures
|How to cite this article:|
Haralur SB, Al-Qahtani AS, Al-Qarni MM, Al-Homrany RM, Aboalkhair AE. Influence of remaining dentin wall thickness on the fracture strength of endodontically treated tooth. J Conserv Dent 2016;19:63-7
|How to cite this URL:|
Haralur SB, Al-Qahtani AS, Al-Qarni MM, Al-Homrany RM, Aboalkhair AE. Influence of remaining dentin wall thickness on the fracture strength of endodontically treated tooth. J Conserv Dent [serial online] 2016 [cited 2020 Jul 2];19:63-7. Available from: http://www.jcd.org.in/text.asp?2016/19/1/63/173201
| Introduction|| |
Substantial coronal and radicular tooth structure is lost as a result of endodontic therapeutic procedures. Hence, endodontically treated teeth are known to present increased susceptibility for mechanical and biological failures than intact vital teeth. , The higher risk of fracture is also attributed to other factors such as effect of endodontic irrigants,  age changes in dentin,  and bacteria-dentine interaction.  In-vivo investigations have indicated compromised tactile sense to perceive the functional overload in root canal treated teeth.
The amount of remaining tooth structure is the most critical factor for the fracture resistance of endodontically treated teeth.  The rehabilitation of root canal treated tooth is complete only with the resumption of its shape and full function. The choice of restorative techniques for these teeth is mainly influenced by the extent of tooth structure destruction, type, and location of the tooth in the dental arch. The endodontic post is indicated for teeth with the substantial amount of coronal tooth structure destruction. The indiscriminate use of post is discouraged by the researchers due to its associated multiple risks. 
Frequently, clinician encounters a tooth with large central defect with normal height and peripheral dentin wall. The predicament of a restorative dentist to utilize the endodontic post in this clinical situation will be addressed by evaluating the fracture strength of the tooth with varying dentin wall thickness. Though the influence of remaining tooth structure height on fracture resistance is evaluated by many researchers,  studies on the influence of remaining dentin wall thickness on fracture resistance are few. The objective of this in-vitro study was to investigate the effect of coronal dentin wall thickness on the fracture strength of root canal treated teeth. It was also aimed to evaluate the influence of endodontic post on fracture resistance at various dentin wall widths.
| Materials and Methods|| |
Institutional Committee of Ethics approval was obtained for the study. Recently extracted 50 human mandibular premolars were used for the study with the mean root length of 15.83 ± 1.79 mm. The teeth included in the study were intact, caries-free teeth without previous endodontic treatment. They were examined under ×10 microscope to identify the micro cracks.
The teeth were divided into five groups of ten each (n = 10) depending on remaining coronal dentin wall thickness and post endodontic treatment [Figure 1]. Group 1 (n = 10) were considered as a control group. They were sound teeth, devoid of any endodontic treatment and restorations.
The remaining teeth samples were root canal treated; the canal cleaning and shaping was accomplished with stainless steel K-files. The root canals were enlarged up to 40 master file and obturated with Gutta-percha with cold lateral condensation technique. The coronal height of all the samples was maintained at 6 mm.
The procedure of remaining group preparation is described below:
- Group 2a: The teeth samples were root canal treated as explained earlier. The central dentin core was removed to keep the 2.5 mm surrounding dentin wall thickness with 6 mm height. The standardization of dentin wall thickness was done with a digital caliper by measuring at three points in each four dentinal walls. The post space was prepared with the sequential use of Gates-Glidden, peeso, and calibrated reamer. The post space length was standardized at 10 mm for all specimens. A minimum of 5 mm Gutta-percha apical seal was maintained during post space preparation. The post space was rinsed with copious distilled water and then gently air dried. The cementation of the epoxy resin fiber post (Easy post, Dentsply International, York, USA) was done by using self-adhesive resin luting cement (Breeze, Pentron Clinical, Orange, CA, USA) following the manufacturer's instruction. The access cavity and central defect were restored with posterior composite restorative material (Filtek p90, 3MESPE, St. Paul, USA.).
- Group 2b: Root canal treatment was completed as explained before. Gutta-percha was limited only to root canal. The remaining dentin wall thickness was maintained at 2.5 mm as described in Group 2a. The tooth was restored without endodontic post with posterior composite.
- Group 3a: The teeth samples were prepared similar to Group 2a. The remaining dentin wall thickness for this group was reduced to 1.5 mm. The teeth were restored with an epoxy fiber post and posterior composite core.
- Group 3b: The tooth samples were similar to Group 3a with remaining dentin wall thickness of 1.5 mm. The teeth samples were restored with posterior composite without endodontic post.
Preparation of testing samples for fracture resistance
The root surface area of all samples was covered with two layers of adhesive tape. The teeth were implanted vertically inside the identical polymethyl methacrylate acrylic mounting block with the help of twist drill (inside the canal) oriented in dental surveyor [Figure 2]. Teeth were mounted in an acrylic block by maintaining 1 mm above cementoenamel junction. Teeth were removed from the mounting block while acrylic was in a rubbery stage of polymerization. The samples were prepared with torpedo diamond bur for circumferential chamfer finish line of 0.75 mm. The preparation taper was standardized by a parallel milling machine. The analogous nickel-chromium metal coping were fabricated and were cemented on the teeth with type I glass-ionomer cement (Medicem, PROMEDICA Dental Material GmbH, Neumunster, Germany).
The adhesive tape which was applied before implanting them in acrylic block was removed from the root surface. The obtained space was filled with light body additional silicone impression material. The samples were mounted back onto the acrylic blocks. This provided the simulated cushioning effect of periodontal ligaments. The tooth samples were mounted on a universal testing machine. The load was applied on the buccal cusp at 30° angulation with crosshead speed of 2 mm/min until the failure [Figure 3]. The force at a failure point was recorded.
The data obtained were statistically analyzed using SPSS 19 (IBM Corporation, Armonk, New York, USA) for one-way ANOVA and post-hoc Tukey honestly significant difference (HSD), to find a statistically significant difference between the tested groups at 0.05 significance level.
| Results|| |
[Table 1] illustrates the mean and maximum compressive force values at which teeth failed for all the groups. Group 1 (control) displayed the highest mean fracture strength at 29.75 Mpa. It was followed by the Group 2a with 28.97 Mpa and Group 2b at 27.70 Mpa. Group 3a had an average fracture strength at 23.39 Mpa compared with 16.38 Mpa for Group 3b. There was statistically significant difference between the force required to fracture among Groups 1, 2, and 3 with P = 0.000 and F = 16.307.
|Table 1: The mean and maximum compressive fracture strength values for all groups and one-way ANOVA test|
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The Tukey HSD multiple comparison tests [Table 2] revealed no significant difference in fracture strength between the Groups 1 and 2a with P = 0.994. The comparative fracture strength of Groups 1 and 2b with a P = at 0.824 also showed no significant difference. The result shows that statistically significant difference between the Groups 1 and 3a (P = 0.016) and 3b (P = 0.000). The relative compressive fracture strength between Group 2a and Groups 3a, 3b was significant with the P values of 0.045, 0.000, respectively. The Groups 2a and 2b showed no statistically significant difference in maximum compressive load with P = 0.514. On the contrary, there was a statistically significant difference (P = 0.001) was noticed between the Groups 3a and 3b.
Hence, it can be inferred from the results that the tooth samples with 2.5 mm remaining dentin thickness are considerably similar to the control group in their mean compressive fracture strength. The tooth samples with remaining 1.5 mm dentin thickness (3a, 3b) had less fracture strength compared to the control and Groups 2a, 2b.
| Discussion|| |
The endodontically treated teeth with significant loss of coronal tooth structure often require endodontic post to retain the core material before crown fabrication. Endodontic post failures are usually irreparable and invariably lead to the tooth extraction. Hence, the restorative dentist should take due care and make an informed decision while incorporating post in the treatment plan. The study included teeth samples with different dentin wall thickness with or without the post to analyze their influence on fracture resistance.
The results of the study illustrate the control group (Group 1) showed the highest compressive fracture strength (29.75 Mpa). It was followed by Groups 2a (28.97 Mpa), 2b (27.70 Mpa), and least fracture strength 16.38 Mpa was observed in Group 3b. As reported by Al-Wahadni et al.,  the intact tooth was more fracture resistant than the endodontically treated teeth. The researchers found the biomechanical stress distribution in post core restored teeth was distinctly dissimilar to an intact tooth. The endodontic post restored tooth flexes as a single unit while intact natural teeth exhibit compressive force on one side and tensile stress on the opposite side.  The difference between the flexural strength of post and remaining tooth structure leads to fracture of the tooth.
The important observation of the study was that there was no significant statistical difference in fracture resistance between the control (Group 1) and Groups 2a, 2b with P value of 0.994, 0.824, respectively. With the presence of sound coronal dentin more than 2.5 mm, the use of the post did not significantly change the fracture strength. The researchers  strongly suggest the self-supported coronal dentin improves the fracture resistance by favorable stress transmission to the root. Though the contribution of coronal dentin toward fracture resistance is controversial, the majority of researchers advocate the preservation of coronal tooth structure during endodontic-post restoration. The coronal dentin is reported to improve the retention and favorable stress distribution. The study results are in concurrence with the finding of Siso et al.  The coronal dentin improved the resistance to the tooth fracture. They suggest that the retained coronal dentin also provides the irregular contact surface with the core, helps in increasing retention and antirotational feature of dowel and core. Hence, the stress transmission to the root is reduced. The previous researchers were of the opinion that bridging by collagen fibers  and water content  in sound dentin walls also contributes to improved fracture resistance.
The study results revealed the Group 3a mean fracture strength significantly less than the Group 2a and control group. The mean compressive fracture strength of the samples with 1.5 mm remaining dentin wall thickness without post (Group 3b) was at 16.38 Mpa. The fracture resistance was significantly affected due to the remaining dentin wall thickness in Group 3a. According to Tjan et al.,  sound dentin around the post is critical for a better prognosis of endodontically treated tooth than the cervical metal collar. They reported that teeth with 2-3 mm retained dentin thickness were less prone to fracture under horizontal impact force. Stokes  also emphasized on the sound dentin preservation around the post for the favorable distribution of the functional load. It is suggested the fragile dentin wall lead to a significant elastic modulus difference between dentin wall and restorative material, predisposing it to fracture.  Henry,  conducted a photoelastic study and concluded that there is less concentration at the cervical finish line with sound coronal dentin.
The results showed a statistically significant improvement in the fracture strength from Group 3b (16.38 Mpa) to Group 3a (23.39 Mpa) with post inclusion. The result endorses the observation by Manning et al.  regarding the need for post and full coverage crown for the endodontically treated teeth with the substantial loss of coronal tooth structure. Zamin et al.  concluded that the filling materials alone are insufficient to restore the fracture resistance in the tooth with greater cervical preparation and thin remaining dentin wall thickness. Patel and Gutteridge  reported the fracture resistance was not significantly different between teeth without coronal dentin and teeth with partially retained dentin in a lingual or buccal wall. Hence, the presence of 360° coronal tooth structure enhances the fracture resistance of post restored teeth.
The main limitation of the study is that it is in vitro. The clinical situation is different in its force, angulation and surrounding supporting structure. Though the study was performed on sound extracted natural teeth, change in moisture content, presence of invisible microcracks, functional age changes, morphological changes of pulp and dentin are difficult to standardize.
| Conclusion|| |
Within the limitation of the study, we conclude the remaining tooth coronal tooth structure width contribute significantly toward the fracture strength of endodontically treated teeth. The endodontic post in a root canal treated tooth with 2.5 mm remaining dentin wall thickness does not contribute significantly toward the fracture strength. Endodontic post significantly reinforces root canal treated teeth with remaining dentin thickness <1.5 mm.
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Conflicts of interest
There are no conflicts of interest.
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Dr. Satheesh B Haralur
College of Dentistry, King Khalid University, Abha
Kingdom of Saudi Arabia
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]