|Year : 2017 | Volume
| Issue : 1 | Page : 11-19
Fracture resistance of simulated immature teeth rehabilitated with different restorative materials: A three-dimensional finite element analysis
Akshata Chambanna Ron, J Karthik, Veena S Pai, B Vedavathi, Roopa R Nadig
Department Conservative Dentistry and Endodontics, Dayananda Sagar College of Dental Sciences, Bengaluru, Karnataka, India
|Date of Web Publication||25-May-2017|
Akshata Chambanna Ron
Department of Conservative Dentistry and Endodontics, Dayananda Sagar College of Dental Sciences, Bengaluru, Karnataka
Source of Support: None, Conflict of Interest: None
Aim: The aim of this study was to evaluate and compare the fracture resistance of simulated immature teeth rehabilitated with different restorative materials.
Materials and Methods: A three-dimensional mathematical finite element analysis model was generated using a simulated immature maxillary central incisor. Five different models were generated representing Model 1 (control group): an immature tooth model without any reinforcement material; Model 2: Mineral trioxide aggregate (MTA) as apical plug (4 mm) + dual cure composite resin (till the access cavity); Model 3: Biodentine as apical plug (4 mm) + dual cure composite resin (till the access cavity); Model 4: Biodentine filled in the entire root canal (8.5 mm) + dual cure composite resin (till the access cavity); and Model 5: MTA filled in the entire root canal (8.5 mm) + dual cure composite resin (till the access cavity). A force of 100N was applied at an angle of 130° to the palatal surface of the tooth. Stress distribution at cement-enamel junction was measured using the Von Mises stress criteria.
Results: The highest stress development was seen in the Model 1 (control group). Model 3 showed higher fracture resistance as the stresses developed (when a 4 mm of apical plug of Biodentine reinforced with dual cure resin) were less followed by Model 2, 4, and 5.
Conclusion: An apical plug of 4 mm Biodentine followed by intracanal rehabilitation with dual cure resin reinforces the immature tooth thereby increasing the fracture resistance.
Keywords: Biodentine; finite element analysis; immature teeth; intracanal rehabilitation; mineral trioxide aggregate.
|How to cite this article:|
Ron AC, Karthik J, Pai VS, Vedavathi B, Nadig RR. Fracture resistance of simulated immature teeth rehabilitated with different restorative materials: A three-dimensional finite element analysis. Endodontology 2017;29:11-9
|How to cite this URL:|
Ron AC, Karthik J, Pai VS, Vedavathi B, Nadig RR. Fracture resistance of simulated immature teeth rehabilitated with different restorative materials: A three-dimensional finite element analysis. Endodontology [serial online] 2017 [cited 2017 Dec 15];29:11-9. Available from: http://www.endodontologyonweb.org/text.asp?2017/29/1/11/207006
| Introduction|| |
Traumatic injuries of the dentition are a frequent occurrence following orofacial trauma in children. The most common site of dental injuries in the developing dentition is the maxillary anterior teeth. Injuries of such developing teeth result in catastrophic fracture often resulting in pulpal necrosis with incomplete root formation. The thin dentinal walls make them susceptible to fracture from mastication or minor trauma, a clinical situation representing a significant endodontic and restorative challenge. Hence, the goal of our present study was to investigate an alternative technique using the combination of mineral trioxide aggregate (MTA), Biodentine, and composite resin to rehabilitate the immature teeth using a finite element method.
| Materials and Methods|| |
Maxillary central incisor extracted for periodontal reasons was used to simulate an immature tooth, which was collected from the Department of Oral and Maxillofacial Surgery, Dayananda Sagar College of Dental Sciences.
The tooth length was kept as 21 mm. The root length was standardized to a length of 10.5 mm as measured from the apex to the facial cement-enamel junction (CEJ) by cutting off the root end with carborundum disk mounted on straight handpiece. Coronal access was made using no 4 round diamond bur using a high-speed handpiece (NSK) under water coolant. The canal was instrumented to the International Organization for Standardization no 80 K file (Mani). To simulate immature teeth, the canals were instrumented with Peeso reamers (no 1–6) (Mani) until a no. 6 Peeso (1.7 mm) could be passed 1 mm beyond the apex. This is according to Cvek's stage 3 of root development since at this stage, the root-to-canal ratio, in a mesiodistal dimension at the CEJ, is roughly 1:1.
The tooth was subjected for computed tomography scan. Cross-section of the tooth was obtained at equal intervals of 0.5 mm, in DICOM format. The data were then fed to the computer-based software MIMICS 9.0 imaging software for three-dimensional (3D) design and modeling through which the solid model was generated. This model was then imported to ANSYS 14.5 version for 3D geometric and finite elements mesh generations to incorporate element attributes and specifications.
- Model 1 (control group): An immature tooth model without any reinforcement material
- Model 2: MTA as apical plug (4 mm) + dual cure composite resin (till the access cavity)
- Model 3: Biodentine as apical plug (4 mm) + dual cure composite resin (till the access cavity)
- Model 4: Biodentine filled in the entire root canal (8.5 mm) + dual cure composite resin (till the access cavity)
- Model 5: MTA filled in the entire root canal (8.5 mm) + dual cure composite resin (till the access cavity).
After the addition of material values [Table 1] into the software, static forces were applied at angle of 130° with longitudinal axis of tooth at palatal surface of the crown to measure the fracture resistance and stress distribution at the CEJ, using the Von Mises stress criteria. The values of force applied and fracture patterns were recorded for individual groups.
| Results|| |
The principal stresses in each model were studied. The results were presented in terms of Von Mises stress values [Table 2]. The values at the bottom of the diagram give range of the stress values.
| Discussion|| |
The occurrence of trauma in the permanent dentition has been reported to range from 2.6% to 35% with the greater incidence occurring between the ages of 7 and 15, when most permanent teeth are in an incomplete root development stage. Unfortunately, approximately 50% of the traumatized teeth are diagnosed with pulpal necrosis and incomplete root formation.
Management of pulpal complications at this time is a significant challenge because of thin root dentin walls and the wide-open apex, which both render instrumentation of the canal difficult and hinder formation of an adequate apical seal. Obturation techniques rely on apical seat against which an obturation material can be compacted which is difficult in open apices.
For such open apices, apexification involving induction of a calcific barrier at the apex, with calcium hydroxide has been tried. The root-end closure procedure using calcium hydroxide is standardized but time-consuming, requiring, on an average of 7–8 months for apical bridge formation. Associated with long treatment time is the uncertainty of patients returning back for follow-up visits, thus increasing the risk of failure of treatment  and also studies have shown that the long-term use of calcium hydroxide decreases the fracture resistance of root dentin.
MTA has a good root sealing ability, highly biocompatible, and been demonstrated to aid in the formation of apical hard tissue. MTA's suitability as a material for use in immature teeth in which root development was interrupted due to pulp necrosis has been investigated, and it has been reported that MTA promotes apical closure at a rate equal to that seen with calcium hydroxide.
Despite the high rate of success of apexification procedures using MTA, the tooth remains weakened due to the presence of thin root canal walls. Thus, restorative protocols should be followed to reinforce the remaining root dentin and also MTA was difficult to handle, had long setting time, and required 2 visits (Bortoluzzi et al. 2007, Wilkinson et al. 2007, Hemalatha et al. 2009, Dikbas et al. 2014). Calcium silicate-based cement, Biodentine, a recently introduced material has been used as an apical plug in the present study as there are limited studies available with the use of this promising material.
Williams et al. concluded that the cohesive strength and modulus of elasticity of gutta-percha were too low to reinforce the roots of an immature tooth. Following are the various techniques done to reinforce such teeth. Intracanal acid-etched bonded resin was thought to strengthen immature teeth and increased their resistance to fracture. Another alternative is the use of fiber posts (Schmoldt et al. 2011), which have an elastic modulus similar to dentin and bonded adhesively to dentin, reducing the failure of the restorative complex, and supporting tensile stress more effectively (Prisco et al. 2003, Santos-Filho et al. 2008, Soares et al. 2008 a, b).
It has been postulated that the apparent shortcomings of composite material may be attributable to the increased shrinkage seen due to the high configuration factor (C-factor) of root canals. These factors may cause the material to debond or pull away from the root dentin leading to debonding. The mismatch between the diameter of the prepared postspace and the fiber post selected is a problem, particularly in immature teeth with large root canals. The use of an auxiliary fiber post (Li et al. 2011) or fiber post relining (Faria-e-Silva et al. 2009, Macedo et al. 2010, 2013), as demonstrated in over-flared root canals, could overcome this difficulty. The increased contact between the relined fiber post and the canal wall increased the bond strength and improved the fracture resistance of teeth (Silva et al. 2011). However, limited information about fracture resistance and stress patterns is available when comparing composite resin, fiber posts, and relined fiber posts to restore immature teeth.
Maxillary anterior teeth were selected for experimentation in the present study as they are more susceptible to trauma and external impacts owing to their localizations. Teeth with similar dimensions at the CEJ were included in the experimental procedure to ensure standardization. In this study, to simulate immature teeth, the canals were instrumented with Peeso reamers (1–6) until a size 6 Peeso can be passed 1 mm beyond the apex to simulate open apex. Earlier investigation revealed that the reinforcement of a canal with a diameter <1.5 mm seems not to be necessary while wall thickness of 2.63 mm is insufficient to weaken tooth structure. Therefore, preparation of the canal was further enlarged with 702 carbide burs to approximate Cvek's stage 3 root development. Cvek's stage 3 root development was selected for the present model; because at this stage, the root-to-canal ratio, in a mesiodistal dimension at the CEJ, is nearly 1:1. It has been indicated that in pulpless immature teeth, as root dentinogenesis is halted, depending upon the stage of root development, the thin root wall has incompletely developed peritubular and intertubular dentin with higher tubular density toward the cementum. When mature teeth are enlarged to resemble immature teeth, the outer parts of their roots would demonstrate lower tubular density and more intertubular dentin. Therefore, this could mean that although these teeth may morphologically mimic immature teeth, they are unable to do so in terms of tissue composition or physical characteristics.
In most in vitro studies, the variables that have not been considered during tooth selection are canal morphology, changes in dentin due to age, trauma, etc. These factors can affect dentin elasticity and thereby change the pattern of stress or fracture resistance of tooth. However, effect of a viable periodontal ligament and resilient alveolar bone is not confined.In vitro load testing, tooth in mounted in a resin block, which has limited resiliency.In vitro studies are encumbered with standard deviations that are relatively high. In addition, loading using an in vitro experiment may not be representative of the in vivo condition.
Other test methods include strain gauge method, use of universal testing machine, cyclic loading methods, and photoelastic analysis method unlike the other analytical methods gives a fairly accurate picture of stress distribution. Although it provides visual evidence of stress pattern within a model, it is, however, not applicable for objects consisting of more than one material such as dentin, periodontal ligament, and alveolar bone. This is the significant disadvantage of this technique.
Finite element analysis (FEA) allows material properties to be assigned to a structure. With the finite element method, one can easily set the conditions of each component such as Young's modulus and Poisson's ratio. This helps in simulation of the structure to be tested. The simulated model can be analyzed for stress distribution by applying force, simultaneously at many directions.
However, FEM is not without drawbacks. Results depend on its modeling methods and values assigned to the material properties. Furthermore, destructive strength cannot be estimated using this method as it is a nondestructive test method. Furthermore, the 2D FEA does not allow to correctly assess the spatial distribution of stresses and strains affecting a restorative system. Hence, 3D method was used in the present study.
The below [Table 3] denotes the tensile strength of each material and the maximum stresses that are developed in these materials. Factor of safety here is the maximum load (i.e., the number of times the applied load) that an individual model can withstand before it fractures.
In the present study, highest fracture resistance was recorded in Model 3 [Figure 1], followed by Model 2 [Figure 2], Model 4 [Figure 3], and Model 5 [Figure 4]. The highest stresses were recorded in Model 1 [Figure 5] with least resistance to fracture. Results of our study are in accordance with the study done by Brito-Júnior et al. an apical plug of 4 mm MTA followed by reinforcement with an adhesive material with a similar elastic modulus to dentin could improve the fracture resistance and stress distribution of immature teeth. (Lawley et al. 2004, Wilkinson et al. 2007) Kim et al. evaluated the fracture resistance between gutta-percha and composite resin and reported that placement of composite resin substantially increased the fracture resistance of thin walled, simulated immature teeth. Tanalp et al. found that immature teeth reinforcement with fiber post were significantly stronger than the unrestored teeth, but no stronger than those restored with gutta-percha. A similar study by Huseiyn et al. evaluated the fracture resistance between Biodentine, gutta–percha, and fiber postgroup in immature teeth and suggested that there is no significant difference between Biodentine and gutta-percha group. It is prudent to note that all the above studies were carried out with the radicular dentin thickness more than 2 mm where the reinforcement of radicular dentin is not really necessary. Thus, the validity and relevance of these studies are questionable.
In the present study, Model 3 [Figure 1] has shown highest fracture resistance (10 times the applied load) followed by Model 2 (7 times the applied load), in spite of the same radicular reinforcement material (i.e., composite resin). This indicates that some property of the apical plug material must have also influenced the fracture resistance. The modulus of elasticity of Biodentine and MTA being more or less similar, the variation in the strength of the set cement could have contributed to this difference. The compressive strength of Biodentine is 213.7 MPa and tensile strength is 16MPa as against MTA with compressive strength of 56.1 MPa and tensile strength of 9.5 MPa. Biodentine which has got higher tensile strength must have taken up more load with uniform stress distribution when used as an apical plug (Model 3) then in Model 2 where MTA was used.
In the present study, Model 5 has shown least fracture resistance followed by Model 4, which is in accordance with an in vitro study done by Topçuoglu et al. Based on the manufacturer's claims of Biodentine as a dentin substitute, it was expected to have similar fracture resistance values as that of dentin, which is not same with the results of the present study. Possible reason for this could be explained based on the evaluation of the stress patterns. The high level of stress concentration at the cervical level was found in Model 5 followed by Model 4, Model 2, and Model 3. The Von Mises stress is a failure criterion that demonstrates energy transmission in a structure, that is, where there is a higher concentration of energy while the maximum principal stress discriminates the tensile and compressive stress fields. Some dental materials (Biodentine) and structures have high compressive strength (213.7 MPa) but are brittle when subjected to tensile stresses (16 MPa). Thus, the maximum principal stress shows the locations within the structure that are more prone to failure by tensile stress (Chai et al. 2009), which is reflected in our study also. In Model 5, backfilling was done with MTA which has tensile strength of 9.5 MPa and in Model 4, backfilling was done with Biodentine which has tensile strength of 16 MPa; whereas in Model 2 and 3, ParaCore having higher tensile strength of 41.07 MPa was used. This explains the higher stress values and consequent reduction of fracture resistance in Model 5 and 4 as compared to Model 3 and 2.
As explained above, even though the stress values at the apical area differ between the models tested in this study on evaluating the overall stress pattern of all models [Table 2], it appears that there is no significant difference. This may be explained by the limitation of the finite element simulation relates to the assumed properties for materials and tissues, namely, that all materials used are linear, isotropic, and homogeneous. In reality, most dental materials and tooth tissues are anisotropic and nonhomogeneous. In addition, the loading scenarios investigated lack the complexity that occurs during functional load in the patient, which results in nonlinearity of the load application and its effects.
Another limitation of FEA is the interface between the dental materials and dentin was simulated as a perfect bond during all load application processes, which is not realistic. Tay et al. reported that the degradation patterns of resin-dentin interface inside the root canal created by self-etching adhesive systems due to water sorption and induced collagenolytic activity adversely affects the longevity of adhesively bonded post. Dantas et al. reported that the self-etching systems due to their hydrophilic characteristics tend to present potential degradation in the course of time as water plays a key role in polymer degradation and concluded that the tested etch-and-rinse adhesive systems had a better performance in terms of bond durability over time than the self-etching adhesive systems. Moreover, thus Monoblock effect is questionable over a period. However, our study was of 3 months duration and the longevity of the bond with extended time periods needs evaluation.
Leiendecker et al. who evaluated that whether prolonged contact of mineralized dentin with Biodentine and MTA Plus adversely affects dentin collagen matrix integrity. The findings of their study showed that prolonged contact of mineralized dentin with Biodentine and MTA Plus has an adverse effect on the integrity of the dentin collagen matrix. Therefore, caution is advised when calcium silicate-based materials are used to obturate root canals with thin dentinal walls and in filling the full length of the canal to prevent collagen degradation that might lead to root fracture.
In a recent study, Sawyer et al. examined whether prolonged contact of dentin with two recently introduced calcium silicate-based materials, Biodentine and MTA Plus, adversely affects flexural properties. They stated that dentin flexural strength exposed to Biodentine decreased significantly after 2 and 3 months, whereas that exposed to MTA Plus decreased significantly after 3 months of aging. Furthermore, they stated that the fracture resistance of roots will probably not adversely affect when these calcium silicate-based materials are used as apical plug material. However, the practice of completely obturating root canals with these new calcium silicate-based materials may decrease the fracture resistance of teeth. Bowen et al. and Aliana et al. reported that highly alkaline calcium hydroxide induces a caustic degradation effect on exposed collagen and this is mediated by the breakdown of intermolecular bonds in collagen fibrils, increasing their water absorption leading to swelling.
Thus, the results of our study along with those found in literature suggest that obturation of the entire canal with MTA or Biodentine is not a suitable method of reinforcing an immature teeth with thin dentinal walls. However, further in vitro and in vivo studies are required to evaluate the effect of these materials on the longevity of the bond as well as structural changes of radicular dentin.
| Conclusion|| |
Within the limits of this study, it can be concluded that:
- Simulated immature teeth with open apices and thin dentinal walls filled with Biodentine as an apical plug and intraradicular reinforcement with dual cure resin showed highest fracture resistance followed by MTA apical plug reinforced with dual cure resin and the least was in the model in which complete canal was reinforced with either MTA or Biodentine
- On comparing the composite reinforcement groups (Model 2 and 3), teeth with apical plug of Biodentine showed marginally higher resistance to fracture than in teeth where MTA was used as an apical plug
- It is better to restore an immature teeth with an apical plug of Biodentine or MTA followed by intraradicular reinforcement with dual cure resin rather than restoring the entire root canal with MTA or Biodentine
- Biodentine can be considered as an alternative to time-tested MTA as an apical plug on account of its improved handling characteristics and shorter setting time.
However, further in vitro and in vivo studies with regard to longevity of the bond as well as structural changes these materials can produce on radicular dentin needs to be evaluated before extrapolating the results of the present study to a clinical scenario.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]