Evaluation of stress distribution in an endodontically treated tooth restored with four different post systems and two different crowns- A finite element analysis

Rajvi Nahar; Sunil Kumar Mishra; Ramesh Chowdhary

doi:10.1016/j.jobcr.2020.10.004

. 2020 Oct 9;10(4):719–726. doi: 10.1016/j.jobcr.2020.10.004

Evaluation of stress distribution in an endodontically treated tooth restored with four different post systems and two different crowns- A finite element analysis

Rajvi Nahar ^a, Sunil Kumar Mishra ^a,^∗, Ramesh Chowdhary ^b

PMCID: PMC7566941 PMID: 33088703

Abstract

Aim and objectives

The study evaluated the stress distribution in an endodontically treated tooth with four different post systems and restored with two different prosthetic crown materials using finite element analysis (FEA).

Material and methods

A total of eight 3D FEA models of maxillary central incisor were created and divided into two groups; group porcelain fused to metal (PFM) crown (n = 4) and group PEEK crown (n = 4). The endodontically treated tooth was restored using four different post materials such as fiber-reinforced composite (FRC), carbon fiber-reinforced polyetheretherketone (CFR-PEEK), glass fiber reinforced polyetheretherketone (GFR-PEEK) and polyetherketoneketone (PEKK) and composite resin core. A static load of 100 N was applied and von Mises stress was calculated. The stress ratio in dentin was analyzed by using the Mohr-Coulomb failure theory.

Results

In dentin, under the applied forces for both, PFM group and PEEK group, it was observed that CFR-PEEK post exhibited minimum von Mises stress while PEKK post exhibited maximum von Mises stress values. The result obtained showed that low stresses were detected using CFR-PEEK post than FRC, GFR-PEEK, and PEKK posts.

Conclusion

Stress observed in dentin, when CFR-PEEK post was used was lesser in comparison to the FRC post. It was noted that PEEK crown and the PFM crown showed similar effect in dentin in stress generation. Hence, PEEK crown can be used as an alternative to the PFM crown.

Keywords: Fiber post, Finite element Analysis, Glass fiber, Polyetheretherketone, Polyetherketoneketone

1. Introduction

An endodontically treated tooth is weaker than a natural tooth in terms of strength and fragility. In case of excessive crown destruction, clinicians often use post-core system to support the restorations that are to be made. The main purpose of an endodontic post is to retain the core material around it, which provides a base for the prosthetic crown over it.¹^,² The use of post and core systems have increased the chances of survival of natural tooth exponentially. The characteristics of post includes its material, elastic modulus, diameter and height that contributes greatly to the resistance to fracture of the restored tooth.³^,⁴^,⁵^,⁶

Metallic cast posts such as gold, nickel-chromium, titanium, and stainless steel are already in use. But due to low biocompatibility, chances of corrosion of these metallic cast posts and the negative effects on esthetics, clinicians have found non-metallic posts as alternative to it.⁷^,⁸^,⁹ Non-metallic posts such as fiber-reinforced composite (FRC), glass fiber-reinforced post and ceramic posts are commonly used now.¹⁰^,¹¹^,¹²^,¹³

Studies have shown that metallic posts induce lower stress in dentin whereas fiber-reinforced post and glass post causes the maximum stress in dentin. The cast post-core system showed a high stress level within the post-core system and transmitted low stress to supporting structures, whereas fiber post-core exhibited less stress within the post-core system but transmitted high stress to the supporting structures.¹⁴^,¹⁵^,¹⁶^,¹⁷^,¹⁸^,¹⁹

Researchers have suggested that post systems with elastic modulus near to dentin are known to reduce stress concentration.²⁰^,²¹ Recently, a newer material, Polyetheretherketone (PEEK), which is widely used in dentistry as implant superstructures, can be milled and pressed for fabrication of custom-made post.²⁰^,²¹^,²²^,²³^,²⁴^,²⁵ The modulus of elasticity of PEEK and its variants such as Carbon fiber-reinforced PEEK (CFR-PEEK), Glass fiber-reinforced PEEK (GFR-PEEK) and Polyetherketoneketone (PEKK) is equal to or near to dentin.²⁶^,²⁷^,²⁸ These materials can be tested for the fabrication of endodontic posts and to find whether they have enough potential to overcome the drawbacks of FRC post.²¹

The present study aimed to analyze and evaluate the distribution of stresses in a maxillary central incisor, which was treated endodontically and restored with four different post systems (FRC, CFR-PEEK, GFR-PEEK, and PEKK post) and with two different prosthetic crowns (porcelain fused to metal (PFM) and PEEK crown). The null hypothesis stated that there would be no difference in the distribution of stress in the maxillary central incisor restored with four different post materials and two different crowns.

2. Materials and methods

2.1. Model generation

The study was conducted on a model of maxillary central incisor designed as per the specification by Wheeler.²⁹ Computer-aided design (CAD) was used to design a three-dimensional (3D) FEA models of intact maxillary human central incisor and its supporting structures of basic geometry. The models were exported as a Standard for Exchange of Product Data (STEP) files to the pre-processing software (ANSYS Workbench 15.0; Ansys, Inc.). The dimensions of various structures were derived from standard literatures.

In total, eight 3D FEA models were created. According to the type of restorative crowns given the models were divided into two groups, Group PFM crown (n = 4) and group PEEK crown (n = 4). Each group was subdivided according to the material used to make the post, these were fiber-reinforced composite (FRC), carbon fiber-reinforced polyetheretherketone (CFR-PEEK), glass fiber reinforced polyetheretherketone (GFR-PEEK) and polyetherketoneketone (PEKK). The core material used for all the post was composite resin. In each model, the post length was 9 mm with a diameter of 1.7 mm. The models had a composite resin core of 6.5 mm height and 4.7 mm width, built over the 2 mm of remaining dentin (ferrule). PFM crown with cobalt-chromium coping of thickness 0.5 mm, porcelain of thickness 1.2 mm at cervical margins and 1.5 mm at the incisal edge was modeled (Fig. 1A) and PEEK crown with a PEEK coping of thickness 0.5 mm, composite thickness of 1.2 mm at cervical margins and 1.5 mm at the incisal edge was modeled (Fig. 1A). A 0.67 μm thick layer of the resin cement was modeled between the core and the crown and at the post dentin interface. The structures like gutta-percha, gingiva, cortical bone, cancellous bone, cementum and periodontal ligament (PDL) were also simulated in the model.³⁰

Fig. 1 — A Model of post and core restored central incisor. B, Vertical force 100 N applied at incisal edge along long axis of tooth. C, Oblique force 100 N applied at angle of 45°.

2.2. Setting of materials properties

The Young modulus and Poisson's ratio of different materials used in the present study were taken from the quality articles and the respective properties of the materials were installed into the software (Table 1).¹⁹^,³¹^,³²^,³³^,³⁴^,³⁵^,³⁶^,³⁷ The mechanical properties of elements were presumed to be isotropic and homogenous except the orthotropic FRC post.³⁸ The boundary conditions were fixed at the alveolar bone level surrounding the root surface.

Table 1.

Mechanical properties of tissues and restorative material associated with finite element analysis.

Materials/Structures	Young Modulus (GPa)	Poisson Ratio
Cortical bone	13.7³²	0.30³²
Cancellous bone	1.37³²	0.30³²
Periodontal ligament	0.069³³	0.45³³
Dentin	18.6³⁴	0.31³⁴
Dual polymerized Resin cement	18.6³⁵	0.28³⁵
Gutta percha	0.00069¹⁹	0.45¹⁹
Composite Core	12³⁶	0.30³⁶
Composite (Adora)	10.7^a	0.30^a
FRC	45³³	0.28³³
CFR-PEEK	18³¹	0.39^a
GFR-PEEK	12³¹	0.40^a
PEKK	5.1^a	0.30^a
Cobalt chromium	21,8³²	0.33³²
Porcelain	69³⁷	0.30³⁷
PEEK	3.5^a	0.36^a

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Values provided by manufacturer.

2.3. Load and constraints

In this study, 100 N static load was applied in two directions: (a) 100 N vertical load was applied at the incisal edge parallel to the long axis of the tooth (Fig. 1B) (b) 100 N oblique load was applied at 45° angle relative to the long axis in an area of 1 mm², 2 mm lingual to the incisal edge (Fig. 1C).³ The oblique load was given to stimulate the occlusal centric contact with the opposite tooth.

2.4. Meshing and contact characteristics

The mesh conversion was done to limit the approximate mesh refinement level. A controlled and connected element was used to mesh the volumes corresponding to each structure. The purpose of meshing was to divide the model generated into more discrete and smaller elements known as nodes. The total number of elements and nodes generated in this study were 136,808 and 191,365 respectively. The hypermeshing was done using hypermesh v10.0 software. The maximum and minimum von Mises stress was calculated using ANSYS software.

2.5. Mohr-Coulomb failure theory

The stress ratio in dentin was analyzed by using the Mohr-Coulomb failure theory. The Mohr-Coulomb stress failure theory states that if the maximum and minimum stress equals or exceeds the stress limits (tensile or compressive strengths) failure will occur.³⁹ The Mohr-Coulomb stress ratio (σ MC ratio) is expressed as:

σMCratio = σ max/UTS + σ min/UCS.Where UTS is the material's ultimate tensile strength and UCS is the ultimate compressive strength. If the σ MC ratio value exceeds 1, the material will fail. The dentin UTS and UCS were 96 MPa and 295 MPa, respectively.³⁴^,⁴⁰

3. Results

In the present study, the forces were applied in two directions in each model, and von Mises stress was calculated. The stress values for core, post, cement, and dentin for the PFM group and the PEEK group were presented in Table 2 and Fig. 2.

Table 2.

Von Mises stress (MPa) in dentin and surrounding elements in porcelain fused to metal crown and polyetheretherketone crowns.

		Vertical loading				Oblique loading
		FRC	CFR-PEEK	GFR-PEEK	PEKK	FRC		CFR-PEEK		GFR-PEEK	PEKK
Porcelain fused to metal crowns	Core	8.75	8.753	8.761	8.77	13.1	13.09		13.176		13.304
	Post	5.867	6.796	4.598	2.225	8.601	10.002		7.195		3.402
	Cement	33.922	34.122	34.687	35.053	46.514	46.514		47.103		47.168
	Dentin	20.206	20.12	20.257	20.486	31.578	31.491		31.678		31.977
Polyetheretherketone crowns	Core	30.501	30.373	30.654	30.989	51.391	51.271		51.544		51.851
	Post	19.479	22.029	16.115	8.397	30.567	34.201		25.367		13.845
	Cement	55.809	56.134	55.921	55.271	80.434	81.468		79.722		77.358
	Dentin	21.296	21.239	21.337	21.428	35.598	35.556		35.611		35.686

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CFR-PEEK, Carbon fiber reinforced polyetheretherketone; FRC, Fiber reinforced composite; GFR-PEEK, Glass fiber reinforced polyetheretherketone; PEEK, Polyetheretherketone; PEKK, Polyetherketoneketone.

Fig. 2 — Vertical loading and oblique loading PFM and PEEK groups.

Stress parameters were found to be below the tensile (135 MPa) and compressive (297.2 MPa) failure limit ofdentin in all the types of posts used in this study. The Mohr-Coulomb stress ratio was found to be well below the failure limit (Table 3).

Table 3.

Mohr–Coulomb stress ratio in dentin obtained for different posts.

Posts	σ MCratio (Porcelain fused to metal crown)	σ MCratio (Polyetheretherketone crown group)
FRC	0.333	0.375
CFR-PEEK	0.332	0.374
GFR-PEEK	0.333	0.375
PEKK	0.337	0.376

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When the stress generated in the core, under vertical loading in the PFM group was examined, the maximum stress was detected in core with PEKK post and minimum in core with FRC post. Under oblique loading, core with CFR-PEEK post exhibited a low stress value, while core with PEKK post exhibited a high stress value. In the PEEK group, the CFR-PEEK post exhibited lower stress values than FRC, GFR-PEEK, and PEKK posts both in vertical and oblique loading. The maximum von Mises stress was observed at the center of the cervical region of the core and minimum von Mises stress was observed at the incisal portion of the core in all the models in both vertical and oblique loading.

Under vertical and oblique loading, in both PFM and PEEK group, it was found that the stress was maximum the CFR-PEEK post whereas minimum the PEKK post. The stress values found in CFR-PEEK post with PFM crown were 6.796 MPa under vertical loading (Fig. 3A) and 10.002 MPa under oblique loading (Fig. 3B). The stress values found in PEKK posts with PFM crown were 2.225 MPa under vertical loading and 3.402 MPa in oblique loading. In CFR-PEEK post with PEEK crown, the stress values generated were 22.029 MPa under vertical loading and 34.201 MPa under oblique loading, while in PEKK posts with PEEK crown, the values were 8.397 MPa under vertical loading (Figs. 3C) and 13.845 MPa in oblique loading (Fig. 3D).

Fig. 3 — A, CFR-PEEK post with PFM crown in vertical loading. B, CFR-PEEK post with PFM crown in oblique loading. C, PEKK post with PEEK crown in vertical loading. D, PEKK post with PEEK crown in oblique loading.

When the stress values detected in dentin were examined for vertical loading in the PFM group, maximum stress was observed in PEKK Post (20.486 MPa) (Fig. 4A) and minimum in CFR-PEEK Post (20.12 MPa) (Fig. 4B). Even under oblique loading, PEKK post (31.977 MPa) exhibited a high stress value (Fig. 4C) and CFR-PEEK post (31.491 MPa) exhibited a low stress (Fig. 4D) value. In the PEEK group under vertical loading and oblique loading, the CFR-PEEK post (21.239 MPa and 35.556 MPa respectively) exhibited low stress values (Fig. 5A and B) and PEKK post exhibited maximum stresses (21.428 MPa and 35.686 MPa respectively) (Fig. 5C and D). The maximum von Mises stress was seen at the center of the cervical region in dentin in all the models in both vertical and oblique loading in both types of crowns.

Fig. 4 — A, Stress in dentin by PEKK post with PFM crown in vertical loading. B, Stress in dentin by CFR-PEKK post with PFM crown in vertical loading. C, Stress in dentin by PEKK post with PFM crown in oblique loading. D, Stress in dentin by CFR-PEEK post with PFM crown in oblique loading.

Fig. 5 — A, Stress in dentin by CFR-PEEK post with PEEK crown in vertical loading. B, Stress in dentin by CFR-PEEK post with PEEK crown in oblique loading. C, Stress in dentin by PEKK post with PEEK crown in vertical loading. D, Stress in dentin by PEKK post with PEEK crown in oblique loading.

The stress seen in both PFM and PEEK groups, at post and core interface, were almost similar. It was observed that the stresses were higher in oblique loading when compared to vertical loading in both the groups. As per the maximum von Mises stress criteria, all the groups showed similar stress concentration areas. There was high stress concentration in the buccal cervical region in dentin. In core, stress concentration was seen in the buccal aspect of the cervical region.

4. Discussion

The present study analyzed the null hypothesis that no difference would be found in stress distribution in maxillary central incisor when restored with four different post materials and two different crowns. Based on the results obtained, the tested null hypothesis was rejected.

FEA study helps us find the areas of maximum stress concentration and allows us to identify the areas that could lead to failure of the restoration. The mechanical properties and elastic modulus have an impact on the transmission of forces on the structures.⁴¹^,⁴² In this analysis of force transmission, a comparison was made of the stresses generated on the core, post and dentin with the use of newer materials such as CFR-PEEK, GFR-PEEK and PEKK with the commonly used FRC posts with two different crown materials (PFM and PEEK).

During post and core treatment to restore a compromised endodontically treated tooth, the space is filled with a material that has a definite stiffness, unlike the pulp tissue and this creates an unnatural stress distribution within the tooth. The post and core material and the cement used to lute it to dentin exhibits different properties which creates different fatigue behavior in different components.⁴³^,⁴⁴

Metal post and core systems that were used routinely in practice had shown greater instances of vertical root fracture, thus leading to the extraction of the tooth.¹⁵^,⁴⁵ High stress generated at the dentin-post interface leads to cracks in dentin and even sometimes causes fracture of dentin.¹⁷^,⁴⁶ Fiberglass post with resin core come in existence to overcome this drawback of metal post since fiberglass has a low modulus of elasticity but had similar strength with favorable stress distributions within the root.⁴⁷^,⁴⁸

The post material having a Young modulus closer to dentin usually generates high stresses in the post and low-stress concentration in the weakened root and at the post-dentin interface.⁴⁴ In the present study, a similar result was obtained with CFR-PEEK post which had the highest stress concentration within the post and low stresses at the post-dentin interface. The reason for this could be CFR-PEEK has a Young modulus of 18 GPa which is very close to that of dentin (18.6 GPa). PEKK posts generated the least stress within the post but it was noted that in dentin the stress concentration was slightly higher than all other groups. It was also noted that when CFR-PEEK post was used with PFM crown the stresses were lower than when used with PEEK crown.

Studies have suggested that high stresses generated on the facial aspect of the coronal third of the root indicated that there was an increased chance of failure of the post and core in that tooth.⁴⁴^,⁴⁹^,⁵⁰ Every effort should be made to reduce the stress in the coronal third of the root. In the present study, the stress concentration was more on the facial aspect of the coronal third of the root in all the models, but compared to all the posts, CFR-PEEK post generated less stress. CFR-PEEK generated low intensity stress due to its rigid properties and other post were more flexible, thus generated more stress.

Boschian et al. stated that post with a higher modulus of elasticity transfers nonhomogeneous and destructive stresses into the tooth and their associated structures.⁴¹ Santos-Filho et al. stated that there should be a similarity between the mechanical properties of the restorative material and dental tissues.⁴⁸ In the present study, CFR-PEEK had shown good results as it had a high modulus of elasticity as compared to the cast posts which areused routinely.

Loss of adhesion of the post or the crown is a common phenomenon that occurs due to stress concentrations which exceed the failure limit of the cement interface. This causes microleakage of bacterial and other fluids leading to the development of periapical lesions and failure of the restoration or loss of the endodontic treated tooth.⁵¹ Ichim et al. and Savychal et al. found in their study that the presence of a ferrule increases the stresses in dentin. They also stated that ferrule reduces the stresses at the luting interfaces and decreases the chances of displacement of the restoration by providing mechanical resistance.⁴²^,⁵² However, in the present study, the stresses detected in the ferrule area were low and were slightly higher at the luting interface when compared to dentin.

The post investigated in the present study can be of clinical use as all the tested post exhibited stresses well below the failure limit of dentin and had the Mohr-Coulomb stress ratio less than 1. The present study had certain limitations like the structures were considered homogenous and isotropic, thus the materials are needed to be tested in the clinical environment and the results of this FEA study and clinical outcomes should be interpreted with caution. Further laboratory and clinical studies to be done to simulate the post-dentin bonding degradation by contamination with saliva, water, and blood. The novel PEEK materials used in the present study are less known about as posts, so further study is required to know about its bonding efficiency with luting agent.

5. Conclusion

Taking into account, the limitations of the present study, the following conclusion was drawn: CFR-PEEK is a better material for the fabrication of endodontic post than FRC as it had generated less stress in the remaining dentin. GFR-PEEK and PEKK materials can also be used as materials to fabricate endodontic posts as they had very similar stress distribution compared to FRC posts. PEEK crowns can also be used instead of PFM crowns as it produces similar stress indentin.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Ethical approval

Not applicable.

Declaration of competing interest

The authors have declared no conflict of interest.

Acknowledgement

Nil.

Biographies

Rajvi Nahar She had worked for the concept of the work and acquisition of the data required. She had drafted the paper and is responsible for the final approval of the version of the article to be published. She agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any parts of the work were appropriately investigated and resolved.

Sunil Kumar Mishra He had designed and interpreted the data for the work and drafted the paper along with Rajvi Nahar. He is also responsible for the final approval of the version of the article to be published. He agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any parts of the work were appropriately investigated and resolved.

Ramesh Chowdhary He had designed and drafted the article along with and revised it critically. He is responsible for the final approval of the version of the article to be published. He agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any parts of the work were appropriately investigated and resolved.

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PERMALINK

Evaluation of stress distribution in an endodontically treated tooth restored with four different post systems and two different crowns- A finite element analysis

Rajvi Nahar

Sunil Kumar Mishra

Ramesh Chowdhary