11.Stress Behavior of Cemented Fiber-reinforced Composite and Titanium Posts in the Upper Central Incisor According to the Post Length-Two-Dimensional Finite Element Analysis

8/12/2019 11.Stress Behavior of Cemented Fiber-reinforced Composite and Titanium Posts in the Upper Central Incisor Accor

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ORIGINAL ARTICLE

Stress behavior of cemented fiber-reinforcedcomposite and titanium posts in the upper centralincisor according to the post length:Two-dimensional finite element analysis

Ji-Hyun Jang a, Su-Jung Park b, Kyung-San Min b,Bin-Na Lee c,Hoon-Sang Chang c, Won-Mann Oh c, Hyunpil Lim d,Young-Tae Cho e,Jeong-Tae Koh f,h, Ho-Hyun Son g, Yun-Chan Hwang c,h*, In-Nam Hwang c*

a Department of Conservative Dentistry, College of Dentistry, Yonsei University, Seoul, Koreab Department of Conservative Dentistry, School of Dentistry, Wonkwang University, Iksan, Koreac Department of Conservative Dentistry, School of Dentistry, Dental Science Research Institute, Chonnam National

University, Gwangju, Koread Department of Prosthodontics, School of Dentistry, Dental Science Research Institute, Chonnam National University,

Gwangju, Koreae

Department of Manufacturing and Design Engineering, Jeonju University, Jeonju, Koreaf Department of Pharmacology and Dental Therapeutics, Gwangju, Koreag Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Seoul National University,

Seoul, Koreah Research Center for Biomineralization Disorders, Gwangju, Korea

Final revision received 29 November 2011; accepted 27 April 2012Available online 21 November 2012

KEYWORDSfiber-reinforced

composite post;finite elementanalysis;

stress distribution;titanium post

Abstract Background/purpose: This study examined the stress distribution in endodontically

treated maxillary central incisors restored with various lengths of either titanium or fiber-

reinforced composite (FRC) post-and-core systems, using two-dimensional finite element anal-ysis models.

Materials and methods:Eight models of the maxillary central incisor were formed, surrounded

by cortical bone, cancellous bone, and the periodontal ligament. Two different post-and-core

systems, titanium and FRC posts (D.T Light Post), were modeled. In each restorative system,four models were designed by changing the post lengths cemented to the root at 10 mm,

* Corresponding authors. Department of Conservative Dentistry, School of Dentistry, Chonnam National University, Yong-bong ro 77,Bukgu, Gwangju, Korea.

E-mail addresses: [email protected](Y.-C. Hwang), [email protected](I.-N. Hwang).

1991-7902/$36 Copyright 2012, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.http://dx.doi.org/10.1016/j.jds.2012.04.005

Available online at www.sciencedirect.com

j o u r n a l h o m e p a g e : w w w . e - j d s . c om

Journal of Dental Sciences (2012) 7, 384e389
mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.jds.2012.04.005http://www.sciencedirect.com/science/journal/19917902http://www.e-jds.com/http://dx.doi.org/10.1016/j.jds.2012.04.005http://dx.doi.org/10.1016/j.jds.2012.04.005http://dx.doi.org/10.1016/j.jds.2012.04.005http://dx.doi.org/10.1016/j.jds.2012.04.005http://dx.doi.org/10.1016/j.jds.2012.04.005http://dx.doi.org/10.1016/j.jds.2012.04.005http://www.e-jds.com/http://www.sciencedirect.com/science/journal/19917902http://dx.doi.org/10.1016/j.jds.2012.04.005mailto:[email protected]:[email protected]


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9 mm, 8 mm, and 7 mm. A 100-N load was applied at a 45 angle to the long axis of each model.

The stress distribution levels were calculated according to the Von Mises criteria.

Results:The titanium post showed stress concentrations toward the post when loaded, and its

stress concentration shifted toward the direction of the crown with a decreasing post length.

In contrast, there was no stress concentration in the FRC post when loaded and no shift of the

stress concentration toward the crown with a change in the post length.

Conclusion: Results suggest that the possibility of fracture of the FRC post is relatively low,

compared to the titanium post, even for a short post. The same criteria for installation of

a metal post should not be applied to an FRC post.Copyright 2012, Association for Dental Sciences of the Republic of China. Published by Else-vier Taiwan LLC. All rights reserved.

Introduction

A tooth can lose a large amount of structure due to a varietyof causes, including trauma and dental caries, and requiresrestoration and fabrication of an artificial crown afterendodontic treatment. However, in most cases, retention iscommonly enhanced when using a core with a post inside the

root canal, because pertinent retention and resistancecannot be maintained only with the residual tooth structure.A post is a device placed into the space formed in the rootcanal to maintain the core. Clinicians once believed thatrestoration of a lost tooth structure by a post reinforceda tooth that hadundergone endodontic treatment. However,that view has been refuted.1e3 Incontrast to Davy etal,3 whoreported that the stressin dentin decreases whenrestoredby a post, Travert et al4 and Kantor and Pines5 reporteda higher incidence of fracture when a post was applied toa tooth that had undergone endodontic treatment. Sorensenand Martinoff6 reported that a post in a tooth treatedendodontically did not have any reinforcing effect. Duret

et al7

suggested that the ideal post should have a modulus ofelasticity very similar to that of dentin, and proposed thefiber-reinforced composite (FRC) post as one that wouldmeet this requirement.

Enhancing retention is an important factor because thepurpose of a post is to retain the toothcrown by connectingthe remaining coreto the rootstructure.8e11 In a conventionalpost, the elements of contour, surface quality, thickness, andlength are important factors affecting retention.12e17 Thelength of the post is one of the most important concerns forcore retention.The more apically the post isplaced inthe rootcanal, the moreretentive it becomes,unless the apical sealofthe obturated root is damaged.11,13,14,18 The stress concen-tration in the layer between the post anddentin is affected by

the type of material, rather than any other factor; thus thematerial has become recognized as being advantageous forstress dispersion with a decreasing difference in the modulusof elasticity with dentin.7,19

This study examined the stress distribution in endodon-tically treated maxillary central incisors restored withvarious lengths of either titanium or FRC post-and-coresystems, using two-dimensional finite element analysis (2DFEA) models. The overall aim of the study was to determineif the length of commonly used titanium posts, which havea large difference in the modulus of elasticity from that ofdentin, is also applicable to FRC posts, which have a similarmodulus of elasticity to that of dentin.

Materials and methods

Eight 2D FEA models of maxillary central incisors werecreated surrounded by cortical bone, cancellous bone, andthe periodontal ligament, based on actual measurements ofthe extracted maxillary central incisor. Two different post-and-core systems, titanium and FRC posts (D.T Light Post;

Bisco, Schaumburg, IL, USA) and IPS-Empress-2 (IvoclarVivadent, Schaan, Liechtenstein) all-ceramic crowns wereused. The overall length of the tooth was 23 mm, and thediameter and overall length of the post were set to 1.4 mmand 13 mm, respectively. The length of the post exposed inthe upper part of the root was set to 3 mm. Four models ofeach post group were then designed by increasing thegutta-percha length to 4 mm, 5 mm, 6 mm, and 7 mm afterchanging the post lengths that had been cemented into theroot to 10 mm, 9 mm, 8 mm, and 7 mm, respectively. Theaverage thickness of the IPS-Empress-2 internal core andexternal layering porcelain were set to 0.8 mm and 1.2 mm,respectively (Fig. 1).

FEA software, ABAQUS Solver (Simulia Inc., Providence,RI, USA) a commercial FEA tool was used to generate themodel, create the mesh of the individual elements, andperform the analysis of the resulting models. The model ofthe incisor in cross-section was constructed usingmeasurements and geometries similar to other studies20e22

with isotropic material properties. All materials and inte-rior structures, such as post/core, alveolar bone, and theperiodontal ligament used in this study were taken fromprevious reports20e22 and were assumed to be homoge-neous, linearly elastic, and isotropic (Table 1).Fig. 2showsthe mesh generation, boundary conditions, and loadingdirection in the design model for the 2D FEA in this study.

A 100-N load was applied to the center of the lingual

plane of the crown at a 45 angle to the long axis of thetooth. The analysis was performed assuming that the postand core, and the post and dentin were in perfect adhesion.The stress distribution levels were calculated according toVon Mises criteria.20

Results

Compressive stress distributions are shown in Fig. 3. Themaximum compressive stress in the post under a load washigher in the titanium post at all cemented lengths than inthe FRC post; whereas in dentin, a higher stress was

Cemented fiber-reinforced composite and titanium posts 385


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observed with the FRC post. The maximum compressivestress with the FRC post decreased with a decreasinglength, but there was no significant change in the titaniumpost.

With all lengths of titanium posts, a higher compressive

stress was distributed in the post compared to that in thesurrounding dentin. In contrast, with FRC posts, a highercompressive stress was distributed on the facial side of theroot, where a compressive force was applied, compared tothe titanium post. In addition, with the titanium post,a high compressive stress was observed in the area wherethe protrusion of the root adhered to the core (Fig. 3A).

Regarding the stress distribution in the crown of thetitanium post when loaded, a higher compressive stress was

concentrated in the area from where the load was appliedto the post than in the surrounding area, and a stressconcentration toward the lower part of the core was alsoobserved. In contrast, the FRC post showed similar stressdistributions to that in the surrounding area (Fig. 3B).

Discussion

FEA is a research method widely used in basic dentistrystudies. During analysis of load distributions, FEAs examinestresses and deflections. FE modeling has several advan-tages over direct research methods performed in the lab. Itcan modify the diversity of each object and can substitutephysical properties of tissues, when biological tissue is

Table 1 Mechanical properties of the dental structure and restorative materials.

Material Youngs modulus (E; MPa) Poissons ratio (y) Reference

Crown

IPS-Empress-2 layering ingot 100,000 0.25 15IPS-Empress-2 layering ceramic 65,000 0.19 15

Core Composite 12,000 0.30 18

Dentin 18,600 0.31 18Post

Titanium 112,000 0.33 18

FRC (D.T Lght Post, Bisco) 15,000 0.29 Manufacturers

informationPeriodontal ligament 68.9 0.45 20

Cortical bone 13,700 0.30 20

Gutta percha 0.69 0.45 20

Cancellous bone 1370 0.30 20

Figure 1 Dimensions of the investigated model of teeth

restored with a cylindrical post and IPS Empress 2 crown. Figure 2 Finite element model for the analysis.

386 J.-H. Jang et al


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required. Maximum standardization is also available.23 FEAsare performed with 2D and 3D models; a 2D FEA was chosenin this study because it can obtain mathematical resultsmore easily than the 3D method; 2D FEA modeling easilyrepresents stress differences without using unnecessarilycomplex geometries.24e26 This study assumed isotropicproperties for materials and dentinal structures. Further-more, with the maxillary central incisor used in this study,the 2D FEA analysis represented similar locations of peakdentinal stresses with the 3D FEA, despite simplification of

the 2D model.27e29 Nonetheless, 2D models limit the anal-ysis, as the stress distribution along the Z direction cannotbe evaluated. The isotonic loading method, which maydifferin vivo, is one of the limitations of the present study.A 3D model may demonstrate varying amounts of stress inall planes in clinical situations.

In the current study, a 100-N load was applied to thecenter of the lingual plane of the crown at a 45 angle tothe long axis of the tooth. This experimental 100-N load wassuggested to represent a normal chewing force as a third of

Figure 3 Distribution of compressive stresses when loaded (MPa). (A) Distribution in the root dentin and post according to the

post length. (B) Distribution in the crown portion.



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the maximum biting force.30 The oblique force to the longaxis of the tooth simulated the protrusive force that isthemost frequently applied force to the maxillary incisor.20,31

To simulate a clinical situation, IPS-Empress-2 andcomposite resin cores were used. The preparations weredesigned to produce a ferrule effect for reducing directforces to the post-and-core system21 and the all-ceramiccrown was supported by the ferrule.

Currently, clinically available posts are fabricated froma range of materials, such as gold alloy, titanium, Co-Cr,ceramics, and FRC. In this study, titanium and FRC withdifferent elastic moduli of restorative materials were chosento compare the stress distributions of various post lengths.Under loading, the ferrule specimen with the titanium postshowed a higher stress concentration in the post than didteeth restored with the FRCpost, which is consistent withresults from previous studies.20,21 The results demonstratedthat the FRC post resisted oblique occlusive stresses betterthan the titanium post, due to a better modulus of elasticity.In the complete bonding model used in the present study,stress to the FRC posts was distributed more evenly in thedentin, rather than in the core area and near the root. In

contrast, with the titanium post,the stress was concentratedat thepost andwas notdispersed to thedentin. This behaviorseems to be related to thebetter fracture resistance of teethrestored with FRC posts which had a similar rigidity to dentincompared to metal posts.

The present results suggest that the FRC post can resiststress concentration, even when placed only at half thedepth of the root, leading to a decrease inthe risk of fractureof the tooth. If the depth of placement was shortened, boththe FRC and titanium posts showed stress concentration atthe posts. In contrast, the maximum stress generated in theFRC post placed 7 mm deep, which corresponded to half thelength of the root, was less than that of the stress generated

in the titanium post placed 10 mm deep. Other studies sug-gested that excessive preparation to obtain a longer postspace is not essential for better fracture resistance of post-restored teeth.32,33 Similarly, Cecchin et al reported thatposts that were just above half of the root length weresufficient to improve the root fracture resistance and weresimilar to posts that were two-thirds of the root length.34

Adhesive fixation of FRC posts and ferrule incorporationmight decrease the effect of post length on the fractureresistance of dowel-restored teeth. Clinically, placement ofan FRC post at half the depth of a root is contentious.Nevertheless, new guidelines for FRC posts, which differfrom those for metal posts, can be established if correlationsbetween the length of the FRC post and retention, fracture

resistance, and fracture characteristics can be evaluated.In conclusion, the possibility of fracture of FRC posts is

relatively low compared to titanium posts, even for shortposts. Adhesive cement, FRC posts, and a full coronalrestoration with a ferrule may reduce the effect of postlength on tooth fracture resistance.

Acknowledgments

This study was supported by the National Research Foun-dation of Korea (NRF) grant (no. 2011-0030759) funded bythe Korean government (MEST).

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11.Stress Behavior of Cemented Fiber-reinforced Composite and Titanium Posts in the Upper Central Incisor According to the Post Length-Two-Dimensional Finite Element Analysis