Saudi Endodontic Journal

ORIGINAL ARTICLE
Year
: 2021  |  Volume : 11  |  Issue : 3  |  Page : 308--314

Evaluating effect of ferrule height, configuration, and post and core approaches on stress distribution of flared root canal: A three-dimensional finite element analysis


Nitin Gupta1, Sonali Taneja2, Vidhi Kiran Bhalla2,  
1 Consultant Endodontist, New Delhi, India
2 Department of Conservative Dentistry and Endodontics, ITS Centre for Dental Studies and Research, Ghaziabad, Uttar Pradesh, India

Correspondence Address:
Dr. Sonali Taneja
Department of Conservative Dentistry and Endodontics, ITS Centre for Dental Studies and Research, Ghaziabad, Uttar Pradesh
India

Abstract

Introduction: Radicular rehabilitation in cases of flared canals presents a challenge and is influenced by the type of post and the ferrule design. Use of anatomic posts has been advocated in such cases to allow for homogenous stress distribution. Hence, the aim of the present study was to evaluate and compare the effect of ferrule height, configuration, and post and core approaches on stress distribution of endodontically treated teeth with flared root canal by three-dimensional (3D) finite element analysis. Materials and Methods: Thirteen 3D models of single rooted maxillary second premolar were made using the solid works 2014 software. These were divided into four Groups I, II, III, and IV restored with no post, glass fiber, anatomic, and cast post and core, respectively. Group II, III, and IV were further subdivided into four subgroups a – no ferrule, b – 3 mm circumferential, c – incomplete ferrule of 2.5 mm on buccal and 1.5 mm on palatal side, and d – incomplete ferrule of 1.5 mm on buccal and 2.5 mm on palatal side. Load of 200 N at an angle of 45° was applied to buccal and palatal cusps. ANSYS/ABACUS standard solver with Microsoft Windows 10 Professional was used to analyze model data and perform stress analysis around various elements when subjected to occlusal loading in three dimensions. The maximum von Mises stresses were calculated within post, core, cervical, and radicular dentin and distribution at dentin and cement interface and cement and post interface. Results: The minimum stresses were seen in model restored with 3 mm circumferential ferrule with anatomic post and minimal stress was observed with no ferrule and rehabilitation with cast post. The presence of ferrule reduces the stress distribution in all the models. Incomplete ferrule design is associated with high stresses than partial ferrule. Conclusion: Ferrule allows for a uniform stress distribution and also reduces the stresses at the cervical region of the tooth. Palatal ferrule is more important to provide a fracture resistance to tooth as compared to buccal ferrule.



How to cite this article:
Gupta N, Taneja S, Bhalla VK. Evaluating effect of ferrule height, configuration, and post and core approaches on stress distribution of flared root canal: A three-dimensional finite element analysis.Saudi Endod J 2021;11:308-314


How to cite this URL:
Gupta N, Taneja S, Bhalla VK. Evaluating effect of ferrule height, configuration, and post and core approaches on stress distribution of flared root canal: A three-dimensional finite element analysis. Saudi Endod J [serial online] 2021 [cited 2021 Dec 1 ];11:308-314
Available from: https://www.saudiendodj.com/text.asp?2021/11/3/308/325406


Full Text

 Introduction



The rehabilitation of weakened root canal-treated teeth remains a challenge for restorative dentistry, and the survival rates for such teeth depend on factors such as the presence of ferrule, dentist's skill, type of post, and the final coronal restoration.[1],[2],[3] Intraradicular retention becomes essential with the use of post and core for the improved success of endodontically treated teeth.[4]

The correct choice of the post system used for the rehabilitation of weakened teeth and the ferrule effect is important for prognosis and longevity of the treatment of such grossly damaged tooth. Conventionally, flared teeth that were restored using cast metal posts are often unsuccessful because they lack in retention.[5] Thus, glass fiber posts can be relined with composite resin (indirect anatomic post) prior to cementation in flared root canals which will contribute to the reduced volume of luting cement and a better fit in the root canal. This technique of radicular reconstruction in endodontically treated tooth aims to improve the postadaptation, minimize cement thickness, improve retention, prevent adhesive failure, and increase fracture resistance.[6],[7]

The impact of ferrule design on the fracture resistance of endodontically treated teeth still remains a matter of debate. The ferrule of minimum 1.5–2 mm is recommended for the reconstruction of post retained endodontically treated teeth.[1] Nonetheless, sometimes, obtaining a regular circumferential ferrule becomes a difficult task due to an insufficient amount of the coronal tooth structure.[2]

In vitro studies cannot simulate many clinical variables such as elasticity and presence of periodontal ligament and supporting bone tissues. Finite element method (FEM) seems to be an appropriate tool for the analysis of dental materials and structures because of its capability to perform numeric analysis and transformation of the real problem on virtual offers several advantages such as simulation of natural condition, good reproducibility and accuracy, and reduced cost and time. Therefore, FEM may be preferred over other methods to understand the stress and strain behavior of the materials. Representation of stresses is done by the failure criteria by conversion of results in color maps in accordance to a predefined scale.[8],[9]

To our knowledge, no study till date has been conducted to evaluate the effect of ferrule height, configuration of different post and core materials on stress distribution in endodontically treated teeth with flared root canal. Therefore, the aim of the study was to evaluate and compare the effect of ferrule height, configuration, and post and core approaches on stress distribution of endodontically treated teeth with flared root canal by three-dimensional (3D) finite element analysis.

 Materials and Methods



The study was approved by the institutional ethical committee board of ITS Centre for Dental Studies and Research, Ghaziabad, under protocol number ITSCDSR/IIEC/2015-18/CONS/02. A sound maxillary second premolar with a single canal was selected. Teeth with caries, restoration, previous endodontic therapy, fractures, pulp stones, and root resorption were excluded. A prototype was obtained by surface scanning through computer-aided designing (CAD) machine (3M ESPE LAVA™ SCAN ST). The stereolithography (.stl) file generated was imported to a computer-aided software (Solid works 2014) and was converted into 13 Initial Graphics Exchange Specification (.iges) format files (neutral file format that allows 3D CAD packages to exchange data) to allow conversion of a hollow model into a solid model. Geometric models were prepared by selection of the elements and their incorporation in the framework model. Periodontal ligament (0.2 mm thickness) cortical and cancellous bone (1.5 mm thickness) was then modeled around all the models [Table 1].{Table 1}

Boolean operation was performed in all the models in Solid works network 2014, (Dassault Systemes, Paris). Root canal length was 19 mm and was considered endodontically treated in all the models. The working length was determined using the software and kept 0.5 mm short of the apex. Next instrumentation was simulated in the root canal up to #30 k file. The flared root canal space was simulated using the same software after leaving the apical 4 mm for the post space.

The models were divided into the following 4 groups:

Group I (Control group): No ferrule or post and restored with zirconia crownGroup II: Glass fiber post (15 mm length) was cemented after simulating the 4 mm apical seal, followed by zirconia crownGroup III: Anatomical post (Glass fiber post relined with composite resin layer) cemented after simulating the 4 mm apical seal, followed by zirconia crownGroup IV: Cast post was cemented after simulating the 4 mm apical seal, followed by zirconia crown.

All the three Groups (II, II, and IV) were further divided into four subgroups a, b, c, and d depending on the ferrule configuration as follows:

Sub Group a: No ferruleSub Group b: 3 mm circumferential ferruleSub Group c: Incomplete ferrule of 2.5 mm on buccal side, 1.5 mm in height on palatal side, and no ferrule on mesial and distal sideSub Group d: Incomplete ferrule of 1.5 mm on buccal, 2.5 mm in height on palatal side, and no ferrule on mesial and distal side.

A shoulder finish line of 1.2 mm in width was prepared around the 13 models [Table 1].

The assembled solid models were imported to ANSYS 15.0. The material properties of different elements such as modulus of elasticity and Poisson's ratio were assigned to respective elements [Table 2]. Thus, a 3D finite element models consisting of elements and nodes representing endodontically treated maxillary second premolar with flared root canals restored with different posts and zirconia crowns were constructed.{Table 2}

It was assumed that all materials used in this study were homogenous, isotropic, and linearly elastic except for glass fiber post reinforced with composite which is orthotropic. Solid 187 higher order 3D, 10 node tetrahedral element was used for meshing framework models to generate finite element models to perform the numerical simulations as it is well suited for modeling of irregular meshing. Each node of Solid 187 has 3° of freedom along X, Y, and Z axis. Fixed support was simulated at the base of the cortical bone. All the 13 models were subjected to a static load of 200 N to an angle of 45° to long axis of tooth at palatal cusp and buccal cusp each using the same software. ANSYS/ABACUS standard solver installed to a desktop computer with Microsoft Windows 10 Professional (64 bit, 16 GB RAM, 3.4 GHz) was used to analyze model data and perform stress analysis around various elements when subjected to occlusal loading in three dimensions.

The maximum von Mises stresses were calculated within post, core, cervical, and radicular dentin and distribution at dentin and cement interface and cement and postinterface as depicted in [Figure 1], [Figure 2], [Figure 3].{Figure 1}{Figure 2}{Figure 3}

The qualitative results were represented as stress maps (the hot color represents the highest stress values and the cold colors represent the lowest stress values), while the quantitative results were represented as various stress values.

 Results



The results are summarized in [Table 3].{Table 3}

The overall stresses in Group IV were higher than the Group I, II, and III irrespective of the ferrule configuration [Figure 3]Presence of ferrule reduces the stress distribution in all the models. Incomplete ferrule design is associated with high stresses than partial ferruleThe maximum core stress were seen in subgroup IVa (78.664 MPa) and the minimum stress was seen in subgroup IIb (8.1979 MPa).When the pattern of stress distribution at cervical, middle, and apical thirds of the glass fiber post, anatomic post, and cast post was analyzed, it was found that the stresses travelled from the cervical third toward the apical third with more stresses in the apical third as compared to cervical third irrespective of the ferrule in case of Group III and IV [Figure 2]When the stresses on cervical dentin were compared, the maximum stresses were observed in control group (22.727 MPa) and minimum stresses were seen in Group III (19.704)The maximum stresses in radicular dentin were seen in Group IV, when compared to the Group I, II, and III irrespective of the ferrule configuration [Figure 3]When the stress at the dentin cement and cement post interface was evaluated for Groups II, III, and IV, it was observed that the stress at the dentin-cement interface was more compared to cement post interface.

 Discussion



The correct choice of post system and the ferrule design are important for the prognosis and longevity of endodontically treated teeth.[10] Cast metal, glass fiber, and anatomic post have been investigated in the present study with different ferrule height and configuration in flared root canals.

In the present study, single-rooted maxillary premolars were scanned and analyzed because of their high susceptibility to fractures when subjected to occlusal loading after endodontic treatment. The periodontal ligament, cortical, and cancellous bone were modeled in all the 13 models to better simulate the oral conditions. The von Mises stresses are considered an accurate predictor of the fatigue failure and hence were evaluated in the present study.[11]

There is no sample size in FEM study. A total of 13 depicted sample models of endodontically treated maxillary second premolar were simulated based on different post designs and ferrule design. The samples are represented by the models. A standardized model for each group was generated using the software.

The results revealed that the overall stresses in cast post were higher than the control group (without post), fiber post, and anatomic post. The beneficial effect of fiber and anatomic post on the fracture resistance of the tooth has been emphasized in the study by Cagidiaco et al.[12]

The absence of ferrule increases the stresses irrespective of the post system used. Naumann et al.[13] also reported that preserving a ferrule, along with a stabilizing effect of a fiber post, can enhance the load-bearing capability of flared endodontically treated. However, a study by al-Hazaimeh and Gutteridge[14] demonstrated that the additional use of a ferrule (2 mm) preparation had no benefits in terms of fracture resistance of the prefabricated parapost system.

Models with incomplete ferrule were associated with higher stress in comparison to complete ferrule models. With the presence of ferrule, the resistance moment arm is increased leading to a reduction in lever arm, thus reducing the stresses.[15] Naumann et al.[16] reported that the incomplete crown ferrule in case of fiber post was associated with greater variation in load capacity after chewing simulation. In contrast, the study by Dikbas et al.[17] and de Oliveira et al.[18] observed no effect of different ferrule configuration in teeth restored with fiber post.

The maximum stresses in the core were observed in Group IV (cast post) which was in accordance with study by Garhnayak et al.[19] The stresses in subgroup c with greater ferrule on a buccal side were more as compared to subgroup d with greater ferrule on palatal side. This finding is in accordance to the study by Ng et al.[20] which concluded that palatal axial wall was more effective than the labial axial wall in increasing the fracture resistance of endodontically treated teeth.

Finite element model analysis revealed that stress on the anatomic and cast metal post was distributed more in the apical third, with higher stress in case of cast post. This finding is in accordance with the study by Cailleteau et al.,[21] Hsu et al.,[22] and Kumar and Rao.[23] This was because the cast metal post has a high modulus of elasticity and is more rigid, resulting in more stress concentration on the apical third of root canal which might be the potential cause of root fracture in future.

The maximum stresses in cervical dentin were observed in the Group I (control) and the minimum stresses were seen in the Group III (anatomical post). The results revealed that the postinsertion decreases the stress distribution at cervical dentin. The study by Belli et al.[24] concluded that anatomic postplacement reduces stress levels in flared canals, which is similar to our study. The stresses in the case of cast post were less at cervical dentin because of its higher modulus of elasticity which transfers the stress to the apical third of the inner dentin.

In the case of Group IV (Cast post), the maximum stress concentration was seen at the inner radicular dentin due to its high modulus of elasticity, while in Group II (fiber post) and III (anatomical post), it was seen at the outer dentin. The similar modulus of elasticity of the glass fiber post as well as anatomic post and dentin allowed homogenous stress distribution of forces throughout the entire radicular dentin.[25],[26] The modulus of elasticity of the post affected the intracanal stress distribution and has been emphasized in study by Al-Omiri et al.[27]

Limitations

The static load was applied in this study while the fractures of the root generally occur as a result dynamic loading.[14] Furthermore, the interfaces were assumed to be perfectly bonded that does not represent the clinical situation.[28] Another limitation was that the materials were assumed to be isotropic, homogeneous, and linearly elastic, with the exception of the glass fiber post which was orthotropic. As the biologic tissues are anisotropic and heterogeneous, therefore, this virtual simulation did not reproduce the heterogeneity of the actual clinical condition.[4] Furthermore the cement layer in case of glass fiber post was assumed to be perfectly air bubble free and consistently present throughout the assembly, which is different from the clinical situation, in which there may be the poor adhesion of the post due to formation of air bubble and salivary contamination which may lead to the increase in stresses.[29] The actual stresses may differ from the clinical scenarios; hence, further studies are warranted in this aspect to validate the results of the present study.

 Conclusion



Within the limitations of the present finite element analysis study, the following conclusions may be drawn:

Use of a rigid post system is associated with high stress concentration than a nonrigid postIncorporating the ferrule showed reduction in stress distribution irrespective of the type of postPalatal ferrule provided superior fracture resistance compared to buccal ferruleModulus of elasticity of the post influenced the intracanal stress distributionAnatomic post reduced stress levels when used in flared root canalsThe stresses at the dentin-cement interface were more as compare to the cement-post interface irrespective of the post system used.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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