Numerical investigation of the factors affecting interfacial stresses in an MOD restored tooth by auto-meshed finite element method

3D finite element model based on CT images of tooth

Aim: This study aimed the description of a protocol to acquire a 3D finite element (FE) model of a human maxillary central incisor tooth restored with ceramic crowns with enhanced geometric detail through an easy-to-use and low-cost concept and validate it through finite element analysis (FEA). Methods: A human maxillary central incisor was digitalized using a Cone Beam Computer Tomography (CBCT) scanner. The resulted tooth CBCT DICOM files were imported into a free medical imaging software (Invesalius) for 3D surface/geometric reconstruction in stereolithographic file format (STL). The STL file was exported to a computer-aided-design (CAD) software (SolidWorks), converted into a 3D solid model and edited to simulate different materials for full crown restorations. The obtained model was exported into a FEA software to evaluate the influence of different core materials (zirconia - Zr, lithium disilicate - Ds or palladium/silver - Ps) on the mechanical behavior of the restorations under a 100 N applied to the palatal surface at 135 degrees to the long axis of the tooth, followed by a load of 25.5 N perpendicular to the incisal edge of the crown. The quantitative and qualitative analysis of maximum principal stress (ceramic veneer) and maximum principal strain (core) were obtained. Results: The Zr model presented lower stress and strain concentration in the ceramic veneer and core than Ds and Ps models. For all models, the stresses were concentrated in the external surface of the veneering ceramic and strains in the internal surface of core, both near to the loading area. Conclusion: The described procedure is a quick, inexpensive and feasible protocol to obtain a highly detailed 3D FE model, and thus could be considered for future 3D FE analysis. The results of numerical simulation confirm that stiffer core materials result in a reduced stress concentration in ceramic veneer.

A REVIEW OF FINITE ELEMENT APPLICATIONS IN ORAL AND MAXILLOFACIAL BIOMECHANICS

Finite element method (FEM) is preferred to carry out mechanical analyses for many complex biomechanical structures. For most of the biomechanical models such as oral and maxillofacial structures or patient-specific dental instruments, including nonlinearities, complicated geometries, complex material properties, or loading/boundary conditions, it is not possible to accomplish an analytical solution. The FEM is the most widely used numerical approach for such cases and found a wide range of application fields for investigating the biomechanical characteristics of oral and maxillofacial structures that are exposed to external forces or torques. The numerical results such as stress or strain distributions obtained from finite element analysis (FEA) enable dental researchers to evaluate the bone tissues subjected to the implant or prosthesis fixation from the viewpoint of (i) mechanical strength, (ii) material properties, (iii) geometry and dimensions, (iv) structural properties, (v) loading or boundary conditions, and (vi) quantity of implants or prostheses. This review paper evaluates the process of the FEA of the oral and maxillofacial structures step by step as followings: (i) a general perspective on the techniques for creating oral and maxillofacial models, (ii) definitions of material properties assigned to oral and maxillofacial tissues and related dental materials, (iii) definitions of contact types between tissue and dental instruments, (iv) details on loading and boundary conditions, and (v) meshing process.

Combined Implant and Tooth Support: An Up-to-Date Comprehensive Overview

Objectives. This article presents a review on the concerned topics and some considerations related to the concept of splinting teeth and implants in the rehabilitation of partial edentulism. Study Selection. An electronic PubMed/MEDLINE and manual search of identified articles and reviews as well as clinical, laboratory, and finite element studies was performed in this project. Due to the shortage in within-subject, long term, randomized, controlled clinical trials regarding the subject a meta-analysis was not possible. Results. Although surrounded with some controversy, joining teeth and implants during the rehabilitation of partial edentulism provides the clinicians with more treatment options where proprioception and bone volume are maintained and distal cantilevers and free end saddles are eliminated. It makes the treatment less complex, of less cost, and more acceptable for the patient. Conclusions. Whenever suitable and justified, combining implant and tooth support might be recommended as an alternative during rehabilitation of partial edentulism. Based on the literature, clinical tips and suggestions were recommended to increase the success of this treatment.

Numerical Analysis of Dental Caries Effect on the Biomechanical Behavior of the Periodontal System

The aim of this study was to investigate the effect of dental caries on the stability of the periodontal system. This study presents a numerical analysis performed with three-dimensional (3D) finite element (FE) method to evaluate stresses in the bone surrounding the tooth with dynamic mastication combined loadings. In this work, we present a comparative study on infected and healthy periodontal systems. The infected tooth was modeled and a caries defect was introduced to the tooth coronal part. The infected tooth was evaluated and equivalent von Mises interface stress values were obtained for comparison with the ones exhibited by the healthy tooth. Our results by 3D FE analysis indicated that maximum stresses occurred primarily at the cervical level of root and alveolar bone. In the cortical bone, the stress value was greater in infected system (21.641 MPa) than in healthy system (15.752 MPa), i.e., a 37.4% increase. However, in the trabecular bone we observed only 1.6% increase in the equivalent stress values for the infected tooth model. Stress concentration at the cervical level may cause abnormal bone remodeling or bone loss, resulting loss of tooth attachment or bone damage. Our findings showed that decayed single-rooted teeth are more vulnerable to apical root resorption than healthy teeth. The numerical method presented in this study not only can aid the elucidation of the biomechanics of teeth infected by caries but also can be implemented to investigate the effectiveness of new advanced restorative materials and protocols.

Effect of Ceramic Thickness and Composite Bases on Stress Distribution of Inlays - A Finite Element Analysis

The purpose of this study was to determine the effect of cavity depth, ceramic thickness, and resin bases with different elastic modulus on von Mises stress patterns of ceramic inlays. Tridimensional geometric models were developed with SolidWorks image software. The differences between the models were: depth of pulpal wall, ceramic thickness, and presence of composite bases with different thickness and elastic modulus. The geometric models were constrained at the proximal surfaces and base of maxillary bone. A load of 100 N was applied. The stress distribution pattern was analyzed with von Mises stress diagrams. The maximum von Mises stress values ranged from 176 MPa to 263 MPa and varied among the 3D-models. The highest von Mises stress value was found on models with 1-mm-thick composite resin base and 1-mm-thick ceramic inlay. Intermediate values (249-250 MPa) occurred on models with 2-mm-thick composite resin base and 1-mm-thick ceramic inlay and 1-mm-thick composite resin base and 2-mm-thick ceramic inlay. The lowest values were observed on models restored exclusively with ceramic inlay (176 MPa to 182 MPa). It was found that thicker inlays distribute stress more favorably and bases with low elastic modulus increase stress concentrations on the internal surface of the ceramic inlay. The increase of ceramic thickness tends to present more favorable stress distribution, especially when bonded directly onto the cavity without the use of supporting materials. When the use of a composite base is required, composite resin with high elastic modulus and reduced thickness should be preferred.

Influence of the veneer-framework interface on the mechanical behavior of ceramic veneers: a nonlinear finite element analysis

STATEMENT OF PROBLEM

The chipping of ceramic veneers is a common problem for zirconia-based restorations and is due to the weak interface between both structures.

PURPOSE

The purpose of this study was to evaluate the mechanical behavior of ceramic veneers on zirconia and metal frameworks under 2 different bond-integrity conditions.

MATERIAL AND METHODS

The groups were created to simulate framework-veneer bond integrity with the crowns partially debonded (frictional coefficient, 0.3) or completely bonded as follows: crown with a silver-palladium framework cemented onto a natural tooth, ceramic crown with a zirconia framework cemented onto a natural tooth, crown with a silver-palladium framework cemented onto a Morse taper implant, and ceramic crown with a zirconia framework cemented onto a Morse taper implant. The test loads were 49 N applied to the palatal surface at 45 degrees to the long axis of the crown and 25.5 N applied perpendicular to the incisal edge of the crown. The maximum principal stress, shear stress, and deformation values were calculated for the ceramic veneer; and the von Mises stress was determined for the framework.

RESULTS

Veneers with partial debonding to the framework (frictional coefficient, 0.3) had greater stress concentrations in all structures compared with the completely bonded veneers. The metal ceramic crowns experienced lower stress values than ceramic crowns in models that simulate a perfect bond between the ceramic and the framework. Frameworks cemented to a tooth exhibited greater stress values than frameworks cemented to implants, regardless of the material used.

CONCLUSION

Incomplete bonding between the ceramic veneer and the prosthetic framework affects the mechanical performance of the ceramic veneer, which makes it susceptible to failure, independent of the framework material or complete crown support.

Structural reinforcement and sealing ability of temporary fillings in premolar with class II mod cavities

AIM

To evaluate the capability to reinforce tooth structure and sealing ability of temporary filling materials in premolars with MOD cavities. The hypothesis is that temporary filling materials can concomitantly prevent microleakage and increase fracture resistance.

MATERIALS AND METHODS

Premolars received root canal treatment and MOD cavities. Cavities were restored with non- eugenol cement (CIM), glass ionomer cement (GIC) or light curable composite (BIO). Higid and without restoration were controls. Materials for flexual strength and teeth were tested for microleakage and compressive strength.

RESULTS

GIC and Higid presented similar compressive strength, higher than other groups. Bio and GIC presented similar flexural strength higher than BIO. CIM and BIO showed similar micro- leakage lower than GIC.

CONCLUSION

The hypothesis was rejected as filling materials tested failed to prevent microleakage and to increase fracture resistance concomitantly.

CLINICAL SIGNIFICANCE

GIC may be considered to restore weakened teeth subjected to occlusal loads. BIO and CIM are better choices to microleakage in teeth not subjected to mechanical stresses.

In Vitro Performance of Class I and II Composite Restorations: A Literature Review on Nondestructive Laboratory Trials—Part I

Posterior adhesive restorations are a basic procedure in general dental practices, but their application remains poorly standardized as a result of the number of available options. An abundant number of study hypotheses corresponding to almost unlimited combinations of preparation techniques, adhesive procedures, restorative options, and materials have been described in the literature and submitted to various evaluation protocols. A literature review was thus conducted on adhesive Class I and II restorations and nondestructive in vitro tests using the PubMed/Medline database for the 1995-2010 period. The first part of this review discusses the selected literature related to photoelasticity, finite element analysis (FEM), and microleakage protocols. Based on the aforementioned evaluation methods, the following parameters proved influential: cavity dimensions and design, activation mode (light or chemical), type of curing light, layering technique, and composite structure or physical characteristics. Photoelasticity has various limitations and has been largely (and advantageously) replaced by the FEM technique. The results of microleakage studies proved to be highly inconsistent, and the further use of this technique should be strictly limited. Other study protocols for adhesive Class II restorations were also reviewed and will be addressed in part II of this article, together with a tentative relevance hierarchy of selected in vitro methods.

[Three dimensional mathematical model of tooth for finite element analysis]

INTRODUCTION

The mathematical model of the abutment tooth is the starting point of the finite element analysis of stress and deformation of dental structures. The simplest and easiest way is to form a model according to the literature data of dimensions and morphological characteristics of teeth. Our method is based on forming 3D models using standard geometrical forms (objects) in programmes for solid modeling.

OBJECTIVE

Forming the mathematical model of abutment of the second upper premolar for finite element analysis of stress and deformation of dental structures.

METHODS

The abutment tooth has a form of a complex geometric object. It is suitable for modeling in programs for solid modeling SolidWorks. After analysing the literature data about the morphological characteristics of teeth, we started the modeling dividing the tooth (complex geometric body) into simple geometric bodies (cylinder, cone, pyramid,...). Connecting simple geometric bodies together or substricting bodies from the basic body, we formed complex geometric body, tooth. The model is then transferred into Abaqus, a computational programme for finite element analysis. Transferring the data was done by standard file format for transferring 3D models ACIS SAT.

RESULTS

Using the programme for solid modeling SolidWorks, we developed three models of abutment of the second maxillary premolar: the model of the intact abutment, the model of the endodontically treated tooth with two remaining cavity walls and the model of the endodontically treated tooth with two remaining walls and inserted post.

CONCLUSION

Mathematical models of the abutment made according to the literature data are very similar with the real abutment and the simplifications are minimal. These models enable calculations of stress and deformation of the dental structures. The finite element analysis provides useful information in understanding biomechanical problems and gives guidance for clinical research.

Strength estimation of different designs of ceramic inlays and onlays in molars based on the Tsai-Wu failure criterion

STATEMENT OF PROBLEM

Successful restoration of large molar defects is a serious clinical problem. Studies on the strength of teeth restored with ceramic restorations of various designs have provided conflicting results.

PURPOSE

The purpose of this study was to determine the shapes of large MOD ceramic restorations in molars most likely to prevent failure and to produce a favorable distribution of contact stresses between the cement and teeth during mastication.

MATERIAL AND METHODS

The study was performed using a finite element analysis with contact elements. Eight 2-dimensional models of mandibular first molars with the following designs of MOD ceramic restorations were created: an inlay with a butt joint margin, an inlay with a beveled margin, an onlay with a butt joint margin, and an onlay with a rounded shoulder margin. The restorations had 3-mm or 5-mm isthmus widths. Models of opposing maxillary crowns were also developed. Computational simulation of mastication of boluses in the frontal plane was conducted, during which the stresses occurring in the ceramic restorations, cement, and tooth structure were calculated. The Tsai-Wu failure criterion was used to evaluate the strength of the materials. Contact stresses at the adhesive interface between the tooth structure and resin cement around these restorations were analyzed.

RESULTS

According to the Tsai-Wu failure criterion, the margin of the beveled inlay and the surrounding tissue could be damaged during masticatory simulation. At the junction of the butt joint margin inlay and enamel, contact tensile stresses appeared. The lowest inverse of the Tsai-Wu strength ratio index appeared in the onlay with a rounded shoulder margin. At the adhesive interfaces around margins of large onlays, compressive contact stresses occurred.

CONCLUSIONS

For the large molar MOD ceramic restorations tested, the lowest values of the inverse of the Tsai-Wu strength ratio index and a favorable distribution of contact stresses between restoration and tissues appeared in the onlay with a rounded shoulder margin.

Efficient 3D finite element analysis of dental restorative procedures using micro-CT data

OBJECTIVES

This investigation describes a rapid method for the generation of finite element models of dental structures and restorations.

METHODS

An intact mandibular molar was digitized with a micro-CT scanner. Surface contours of enamel and dentin were fitted following tooth segmentation based on pixel density using an interactive medical image control system. Stereolithography (STL) files of enamel and dentin surfaces were then remeshed to reduce mesh density and imported in a rapid prototyping software, where Boolean operations were used to assure the interfacial mesh congruence (dentinoenamel junction) and simulate different cavity preparations (MO/MOD preparations, endodontic access) and restorations (feldspathic porcelain and composite resin inlays). The different tooth parts were then imported in a finite element software package to create 3D solid models. The potential use of the model was demonstrated using nonlinear contact analysis to simulate occlusal loading. Cuspal deformation was measured at different restorative steps and correlated with existing experimental data for model validation and optimization.

RESULTS

Five different models were validated by existing experimental data. Cuspal widening (between mesial cusps) at 100 N load ranged from 0.4 microm for the unrestored tooth, 9-12 microm for MO, MOD cavities, to 12-21 microm for endodontic access cavities. Placement of an MOD adhesive restoration in porcelain resulted in 100% cuspal stiffness recovery (0.4 microm of cuspal widening at 100 N) while the composite resin inlay allowed for a partial recuperation of cusp stabilization (1.3 microm of cuspal widening at 100 N).

SIGNIFICANCE

The described method can generate detailed and valid three dimensional finite element models of a molar tooth with different cavities and restorative materials. This method is rapid and can readily be used for other medical (and dental) applications.

Evaluation of a reinforced slot design for CEREC system to restore extensively compromised premolars

OBJECTIVES

The structural stability and adhesive strength of a large-cavity premolar with a CEREC restoration is a frequent problem in long-term use. This study sought to determine whether an additional reinforced slot could increase tooth/ceramic retention using finite element (FE) analysis and fracture testing.

METHODS

The cavity was designed in a typical MODL restoration failure shape when the lingual cusp has been lost. Two FE restored-tooth models with different cavity designs were created using image processing, contour stacking, and mesh generation. Interfacial (normal and shear) stresses were then calculated with and without the slot design for restored teeth under lateral and axial forces and different interfacial conditions (bonded and de-bonding). For validation, a fracture experiment was performed with and without reinforced slot designs for large ceramic CEREC restorations.

RESULTS

The maximum stresses at the buccal wall increased when a lateral occlusal force acted on the restored tooth with a slot design. Conversely, the interfacial stresses decreased when the restored tooth received a uniform axial occlusal force. After de-bonding on the buccal tooth/ceramic interface, the stresses increased by an average factor of three over those obtained with a bonded interface. The fracture forces were consistent with the tendencies predicted in FE analyses.

CONCLUSIONS

An additional reinforced slot for the CEREC restoration of a large cavity could increase retention when a restored tooth receives an axial occlusal load. However, the benefits of a slot seem to be doubtful for a premolar often subjected to a lateral load.

Towards automated 3D finite element modeling of direct fiber reinforced composite dental bridge

An automated 3D finite element (FE) modeling procedure for direct fiber reinforced dental bridge is established on the basis of computer tomography (CT) scan data. The model presented herein represents a two-unit anterior cantilever bridge that includes a maxillary right incisor as an abutment and a maxillary left incisor as a cantilever pontic bonded by adhesive and reinforced fibers. The study aims at gathering fundamental knowledge for design optimization of this type of innovative composite dental bridges. To promote the automatic level of numerical analysis and computational design of new dental biomaterials, this report pays particular attention to the mathematical modeling, mesh generation, and validation of numerical models. To assess the numerical accuracy and to validate the model established, a convergence test and experimental verification are also presented.

Finite element analysis of stresses in molars during clenching and mastication

STATEMENT OF PROBLEM

During physiological functions of the masticatory system such as swallowing and chewing, teeth are subjected to variations in force application. Most in vitro analyses of stress have not analyzed the combined forces acting on teeth.

PURPOSE

The purpose of this study was to analyze the stresses induced in a mandibular molar during clenching and chewing of morsels with various elastic moduli.

MATERIALS AND METHODS

The investigation was performed by means of finite element analysis with the use of contact elements. Two-dimensional models of the mandibular first molar and the crown of the opposing maxillary molar were created. The computerized simulation evaluated the clenching and chewing of 4 morsels with different elastic moduli (similar to hard gum, tough meat, bone, and combination of hard gum and bone). The movement of the studied teeth was simulated in the frontal plane. Teeth models crushed morsels and closed into the maximal intercuspation position. The values of stresses in the mandibular molar were calculated during these situations.

RESULTS

The study revealed that clenching of molars and chewing morsels of high elastic moduli resulted in maximal equivalent stresses within occlusal enamel. During mastication of morsels of low elastic moduli the stress concentration was located in the cervical region of the lingual side of the mandibular molar. Masticating a low-elasticity morsel containing a fragment of bone caused the highest equivalent stresses in the lingual wall and high tensile stresses in enamel near the central intercuspal fissure of the tooth studied.

CONCLUSION

During mastication of various morsels, maximal equivalent stresses occurred in occlusal enamel and in the cervical region of the lingual wall of the first mandibular molar. The more unfavorable and highest stresses were exerted during mastication of nonhomogeneous morsels.