Stress analysis of a bulk-filled Class V light-cured composite restoration

Influence of double application technique on the bonding effectiveness of self-etch adhesive systems

AIM

To evaluate and compare the effect of double-application of single-step self-etch adhesives using microleakage study and to analyze the dentin-adhesive interfacial micromorphology.

METHODS

In total, 72 extracted human premolars were divided into three groups for different self-etch adhesives (G Bond, GC [GB], Optibond, Kerr [OB], and Xeno V Plus, Dentsply [XV]). Class V cavities were prepared. Each group was further divided into two subgroups (n = 10) according to the placement technique of the adhesive, using the single-application [subgroup (a)] or double-application method [subgroup (b)]. Resin composite (Z 250, 3M ESPE, St. Paul, MN) was used to restore the cavities and light cured for 40 s. Twenty samples from each group were subjected to microleakage study. Two samples from both the subgroups of the three adhesives were used for scanning electron microscopic examination of the resin-dentin interfacial ultrastructure. Dye leakage scores were subjected to statistical analysis using Kruskal-Wallis and Mann-Whitney U-tests at significance level of P < 0.05.

RESULTS

GB depicted significantly more microleakage which was significantly greater than OB and XV. The double application led to significant decrease in microleakage of GB with no significant effect on the microleakage scores of other two all-in-one adhesives, that is OB and XV.

CONCLUSION

Double application of all-in-one self-etch adhesives improves the marginal sealing ability in dentin although it appears to be product dependent.

Analysis of factors associated with cracked teeth

INTRODUCTION

The purpose of this study was to analyze the characteristics, distribution, and associated factors of longitudinal fractured teeth according to the well-defined criteria of the American Association of Endodontists (AAE).

METHODS

One hundred seven teeth with longitudinal fracture from 103 patients were diagnosed and analyzed. The patients' signs, symptoms, age, and sex were noted as well as the tooth number, dental arch, filling materials, size/classification of restoration, crack direction, pulp vitality, whether the patient had undergone endodontic treatment, bite test results, percussion test results, wear facet, and periodontal pocket depth.

RESULTS

Eighty-seven teeth were diagnosed with a cracked tooth (81.3%), 14 were diagnosed with vertical root fracture (VRF, 13.1%), 4 had a split tooth (3.7%), and 2 had a fractured cusp (1.9%); 82.2% showed a sensitive reaction on the bite test. Longitudinal tooth fractures were observed most frequently in patient in their 40s. The upper first molar (28.0%) was most frequently cracked, followed by the lower first molar (25.2%), the lower second molar (20.6%), and the upper second molar (16.8%). Most longitudinal tooth fractures (72.0%) occurred mainly in restored teeth, whereas only 28.0% were found in intact teeth. Compared with resin (4.7%) or porcelain (0.9%), the use of nonbonded inlay restoration materials such as gold (20.5%) or amalgam (18.7%) increased the occurrence of longitudinal tooth fractures. Out of 107 of longitudinal fractured teeth, 33 (30.8%) were treated endodontically and 74 (69.2%) were not. VRF was associated with endodontic treatment.

CONCLUSIONS

The bite test is most reliable for reproducing symptoms. The combined use of various examination methods is recommended for detecting cracks and minutely inspecting all directions of a tooth.

Marginal permeability of one step self-etch adhesives: Effects of double application or the application of hydrophobic layer

Aim:

The purpose of this in vitro investigation was to evaluate the influence of double application and application of hydrophobic layer on marginal adaptation of four self-etch adhesive systems (XENOIII, ALLBONDSE, CLEARFIL SE TRI BOND, FUTURA BOND).

Materials and Methods:

One hundred and twenty class V cavities were prepared on intact, extracted human premolars and were divided into three groups of ten teeth each for all four adhesives. Group 1: Application of bonding agents as per manufacturer directions. Group 2: Double application of bonding agents. Group 3: Application of hydrophobic layer. The specimens were restored with composite and light cured. After thermocycling and immersion in 2% Basic Fuchsin dye solution, the teeth were sectioned and dye penetration was observed under a stereomicroscope at 20× magnification. All the samples were scored and results were analyzed using Kruskal-Wallis and Mann-Whitney tests.

Results:

Group 3, in which the adhesive systems were coated with hydrophobic layer, showed significantly decreased microleakage, followed by Group 1 and Group 2 for all the adhesive systems. And there is no significant different between Group 1 and Group 2.

Conclusion:

Marginal permeability of one-step adhesives can be minimized by the application of more hydrophobic resin layer, and the double application of one-step self-etch system can be safely performed without jeopardizing the performance of adhesives.

Finite element analysis of strength characteristics of various resin based restorative materials in Class V cavities

This study investigated the strength characteristics of various composites (Tetric Ceram, Tetric Flow, Prisma AP.H, Synergy Duo Shade, Synergy Compact, and Z 100) and compomers (Compoglass, Compoglass F, Dyract AP, F 2000) that were applied in a Class V cavity of a maxillary central incisor. The study was conducted by using a 3-dimensional finite element approach and in the study ansys package program was used. The tooth model had 294 elements and 420 nodes. The teeth considered were assumed to be subjected to an incisal load of 200 N acting at an angle of 26 degrees with the longitudinal axis of the tooth but the effects of different loading angles and different loads were also analysed. Hence, the loads of 100 and 400 N and the loading angles of 0 degrees representing bruxism and 90 degrees representing a traumatic load were also taken into consideration. The effects of the cavity preparation size were also studied. It was determined that any increase in the loading angle and/or the amount of the load resulted in a proportional increase in the stresses developed in the tooth. Furthermore it was also verified that, as a cavity weakens the tooth structure by creating a discontinuity in an intact tooth, the larger sized cavity preparations inevitably cause larger stresses to be developed in the tooth. Strictly from the mechanical point of view, the stresses developed in the restored teeth were determined to be inversely proportional with the modulus of elasticity of the restorative materials. Therefore within the scope of the study Z 100 was found to be superior to the other materials concerned.

The influence of system compliance and sample geometry on composite polymerization shrinkage stress

The objective of this study was to use finite-element analysis to model tensilometer tests of polymerizing dental composites. A typical sample in polymerization shrinkage stress tests is shaped like a flat disk, that is, has a high aspect ratio (ratio of diameter to height). In the experimental literature it is implied that the induced stress state in the flat disk composite samples is uniaxial. Three published tensilometer tests of curing dental composite samples with similar high aspect ratios (varying from 3 to 5) were modeled, but with test configurations having low, intermediate and high relative compliance (a tenfold variation). With the use of linear elastic finite element analysis, an instantaneous volumetric shrinkage of 1% was applied to the composite via the thermal analogy and the following questions were addressed: 1. Does the numerically predicted state of stress in composite samples tested in this fashion differ substantially from the uniaxial stress state assumed in the experiments?2. How do the numerically predicted stresses compare with the experimentally determined nominal stresses?3. Does compliance of the mountings influence the numerically predicted stress state? The finite-element results predicted a complex triaxial stress state that was strongly influenced by the compliance of the mountings. For the low and intermediate system compliance the model overpredicted the polymerization contraction stress, as would be anticipated due to the inability of the model to account for viscoplastic flow. For high system compliance, the numerical and experimental stress values were in better agreement, mainly because the linear elastic mountings accounted for most of the measured system compliance.

The influence of the restoration-tooth interface in light cured composite restorations: a finite element analysis

This study examines the early shrinkage behaviour of dental composite resins, and in particular the interfacial stresses around the margins of a composite restoration. The development of stresses at the restoration-tooth interface can have a detrimental effect on the longevity of a restoration. The influence of this interface on the stress system generated in the tooth was examined using finite element analysis. The restoration-tooth interface was simulated using spring elements of varying spring constants (k = 1, 10(2), 10(4), 10(10) N/mm). Interfacial stresses varied from -0.15 to 0.42 MPa for a spring constant of 1 N/mm, and from -19 to 68 MPa for a spring constant of 10(10) N/mm. Correlations between stiffness at the restoration-tooth interface and higher shrinkage stresses due to restricted shrinkage were found. Interfacial failure at the upper and lower regions of the interface, as well as cuspal movements of the order of 2 microm were predicated for the model of the highest spring constant, 10(10) N/mm. The restoration-tooth interface modelled by the spring elements was seen to have a conclusive effect on the ensuing stress system, as well as the longevity of the restoration.

Polymerization Contraction Stress of Resin Composite Restorations in a Model Class I Cavity Configuration Using Photoelastic Analysis

PURPOSE

An important factor that contributes to deterioration of resin composite restorations is contraction stress that occurs during polymerization. The purpose of this article is to familiarize the clinician with the characteristics of contraction stress by visualizing the stresses associated with this invisible and complex phenomenon.

MATERIALS AND METHODS

Internal residual stresses generated during polymerization of resin composite restorations were determined using micro-photoelastic analysis. Butt-joint preparations simulating Class I restorations (2.0 mm x 5.0 mm, 2.0 mm in depth) were prepared in three types of substrates (bovine teeth, posterior composite resin, and transparent composite resin) and were used to examine contraction stress in and around the preparations. Three types of composite materials (a posterior composite, a self-cured transparent composite, and a light-cured transparent composite) were used as the restorative materials. The self-cured composite is an experimental material, and the others are commercial products. After treatment of the preparation walls with a bonding system, the preparations were bulk-filled with composite. Specimens for photoelastic analysis were prepared by cutting sections perpendicular to the long axis of the preparation. Fringe patterns for directions and magnitudes of stresses were obtained using transmitted and reflected polarized light with polarizing microscopes. Then, the photoelastic analysis was performed to examine stresses in and around the preparations.

RESULTS

When cavity preparations in bovine teeth were filled with light-cured composite, a gap was formed between the dentinal wall and the composite restorative material, resulting in very low stress within the restoration. When cavity preparations in the posterior composite models were filled with either self-cured or light-cured composite, the stress distribution in the two composites was similar, but the magnitude of the stress was greater in the light-cured material. When preparations in the transparent composite models were filled with posterior composite and light-cured transparent composite material, significant stress was generated in the preparation models simulating tooth structure, owing to the contraction of both restorative materials.

CLINICAL SIGNIFICANCE

Polymerization contraction stress is an undesirable and inevitable characteristic of adhesive restorations encountered in clinical dentistry that may compromise restoration success. Clinicians must understand the concept of polymerization contraction stress and realize that the quality of composite resin restorations depends on successful management of these stresses.

Experimental validation of a finite element model of light-activated polymerization shrinkage

An important consideration in improving the longevity of dental resin composite restorations is how to minimize the stresses that develop between resin composites and the wall of the preparation as the resin shrinks during polymerization. If the stress is too great, failure of the bonded interface occurs and microleakage results, with possible margin failure, staining, and secondary caries. This present project was performed to validate a previously developed FEM approach for investigating polymerization shrinkage. Light-cured resin composite was condensed into a cylindrical cavity preparation, which contained a centrally located wire. A profilometer stylus was positioned on the end of the wire to detect the initial surface displacement as the wire moved with the superficial layer of composite resin. The plots of the experimentally derived data were compared to the data plot from the FEM. The initial segments of the experimental plots matched the calculated plot very closely in shape, thus validating the FEM approach.

Finite element analysis of thermo-debonding mechanism in dental composites

Finite element method (FEM) has been extensively used for evaluating interfacial status inside biomaterials. This study using FEM was designed to evaluate the thermal stress behavior of a filler-matrix interface. The results were then compared to those of a previous study obtained by a laser thermoacoustic technique (LTAT). The experimental systems (75/25 Bis-GMA/TEGDMA resin reinforced with 0, 25, 50, and 75 wt% 8-microm silanized/unsilanized BaSiO6) as used in the previous study were modeled in this study. The established finite element models were based on coefficient of thermal expansion (CTE) Mismatch Phenomenon. The mechanical properties of the silane coupling agent, such as elastic modulus and thermal expansion coefficient used in the silanized model, were assumed to have optimal heat flux transfer. A third (imaginary) material was proposed to block the transfer of thermal stress between the filler and matrix in the unsilanized model. The thermal load simulation was based on steady-state thermal analysis. The results showed that: (1) The strain energy and interfacial shearing stress calculated from FEM validate the results from the previous LTAT study. (2) Comparing the stress distribution of silanized and unsilanized FEM models, the acoustic signals in LTAT study are mainly derived from debonding of the filler-matrix interface of silanized specimens, and from the matrix area of unsilanized specimens. Based on results to date, we conclude that the finite element method may be a powerful tool for exploring thermoacoustic mechanisms of dental composites.

A simple method for the measurement of polymerization shrinkage in dental composites

OBJECTIVE

In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites.

METHODS

A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined.

RESULTS

Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%.

SIGNIFICANCE

This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.

Comparison of polymerization contraction stresses between self- and light-curing composites

OBJECTIVES

The objective of this study was to examine the distributions and the magnitudes of the internal stresses in self- and a light-curing composite restorations resulting from polymerization shrinkage.

METHODS

Butt-joint box-shaped cavities (5.0 x 2.0 mm2, 2.0 mm in depth) prepared in composite molds were filled with either a self- or light-curing transparent resin composite. The restorations were cross-sectioned perpendicular to the longitudinal axes of the cavities and observed using polarizing microscopes. The principal stresses in the restorations, normal and shear stresses at the cavity wall were evaluated by photoelastic analysis.

RESULTS

The distributions of the principal stresses and the stresses generated at the cavity wall in both the self- and the light-curing composite restorations were similar. The maximum stress generated at the cavity wall in the light-curing composite restorations was twice as large as that seen in the self-curing restorations.

CONCLUSIONS

The results of this study indicated that the difference in the magnitude of the internal stresses between self- and light-curing composites was not related to the distribution of the stresses. The velocity of polymerization appeared to be the most important factor contributing to the magnitude of the internal stresses generated in the composite restorations in this study.

Three-dimensional finite element analysis of thermal shock in a premolar with a composite resin MOD restoration

A three-dimensional finite element model of a human premolar with a composite resin Class II MOD has been used to investigate the temperature changes and induced thermal stresses associated with the imbibing of a hot liquid. Regions of high tensile stress were revealed and their possible clinical significance with respect to microleakage and failure in the approximal region of the restoration are discussed.

Modeling the mechanical behavior of the jaws and their related structures by finite element (FE) analysis

In this paper, we provide a review of mechanical finite element analyses applied to the maxillary and/or mandibular bone with their associated natural and restored structures. It includes a description of the principles and the relevant variables involved, and their critical application to published finite element models ranging from three-dimensional reconstructions of the jaws to detailed investigations on the behavior of natural and restored teeth, as well as basic materials science. The survey revealed that many outstanding FE approaches related to natural and restored dental structures had already been done 10-20 years ago. Several three-dimensional mandibular models are currently available, but a more realistic correlation with physiological chewing and biting tasks is needed. Many FE models lack experimentally derived material properties, sensitivity analyses, or validation attempts, and yield too much significance to their predictive, quantitative outcome. A combination of direct validation and, most importantly, the complete assessment of methodical changes in all relevant variables involved in the modeled system probably indicates a good FE modeling approach. A numerical method for addressing mechanical problems is a powerful contemporary research tool. FE analyses can provide precise insight into the complex mechanical behavior of natural and restored craniofacial structures affected by three-dimensional stress fields which are still very difficult to assess otherwise.

Stress analysis of a bulk-filled Class-V chemical-cured dental composite restoration

Clinical problems associated with the polymerization shrinkage of dental composite restorations include tooth sensitivity, crown fracture, discoloration, recurrent decay, and loss of restoration. Our goal was to determine whether these complications could be attributed to the transient stresses developed during contraction. Thus, a finite element model was used to calculate the transitory deformations and composite-tooth interface stresses produced during the shrinkage of a chemical-cured ideal Class-V composite restoration. It was found that the interface stress peaks moved with the polymerization front, and that in some instances, their intermediate magnitudes were higher than the final, fully cured, values. Therefore, the results indicate that clinical failure may be related to these transitory changes during the polymerization process.

Finite element stress analysis of three filling techniques for class V light-cured composite restorations

An important disadvantage of current dental resin composites is polymerization shrinkage. This shrinkage has clinical repercussions such as sensitivity, marginal discoloration, and secondary caries. The objective of this study was to compare three filling techniques in terms of the transient stresses induced at the resin composite/tooth interface during polymerization. The techniques were: bulk filling (B), three horizontal increments (HI), and three wedge increments (WI). A simple Class V cavity preparation was modeled in finite element analysis. Polymerization shrinkage was simulated by a thermal stress analogy, thereby causing 1% shrinkage due to an arbitrary coefficient of thermal expansion. Interface normal and shear stresses were calculated at nine steps during polymerization, proceeding from 0% to 100% volume of cured resin. The importance of the interface transient stresses was revealed by the finding that, in most cases, their peak values exceeded the final or residual stress. Also, the WI and B techniques consistently exhibited the highest and lowest maximum transient stresses, respectively. These results from the simple model of a Class V restoration suggest that bulk filling of light-cured resin composites should be used in restorations which are sufficiently shallow to be cured to their full depth.