Origin and development of internal stresses during polymerization of composite resins

This study is focused on the development of polymerization stresses. These are non functional stresses and arise especially when the adhesive composite materials are cured in a cavity, in which the polymerization contraction is restrained by the cavity walls. During masticatory functioning these residual polymerization stresses elevate the overall stress levels. They exercise a cumulative effect, and therefore it is essential to evaluate the internal prestressed state of the tooth-composite complex. To study the stress conditions in these complexes, the right tools have to be employed. These consist of dynamic material properties during polymerization, and an appropriate equivalent stress expression for the strength differential effect. The origin of polymerization stresses is a combination of volumetric contraction and development of stiffness during the curing process. Two stages can be distinguished: pre-gel and post-gel. Because stiffness develops after the gel-point, only the post-gel phase plays a role in the development of polymerization stresses. An enhanced strain gage method is developed to determine the dynamic post-gel linear contraction. The development of elastic modulus cannot be measured directly, but has to be assessed indirectly through a polymerization stress evaluation. In an attempt to derive the modulus development, strain and stress data are combined. It turns out that the rate of strain and stress development is greatly dependent on the curing rate, and therefore on the test setup. Two temperature effects(exothermic heating and radiation of the curing light), which play an important role in the volumetric phenomena during the polymerization, are also investigated. Polymerization stress concentrations as a result of different filling techniques are analyzed in a finite element calculation. The numerical analysis suggests that an incremental filling technique raises stresses and cusp displacements. Furthermore, it shows that it is not necessary to model the timedependent behaviour of the development of mechanical parameters if the worsed case is sought

Effect of bone loss simulation and periodontal splinting on bone strain Periodontal splints and bone strain

AbstractObjectivesThe influence of bone loss and periodontal splinting on strains in supporting bone is still not well understood. The aim of this study was to analyse the effect of bone loss and periodontal splints on strains in an anterior mandible structure.MethodsTen anterior mandible models were fabricated using polystyrene resin. Eighty human teeth were divided in 10 groups (right first premolar to left premolar) and embedded in simulated periodontal ligament. Strain gauges were attached to the buccal and lingual mandible surfaces. The models were sequentially tested for 7 conditions: no bone alterations and no splinting; 5mm of bone loss between canine teeth; bone loss associated with resin splint between canine teeth; bone loss with wire splint; bone loss with wire/resin splint; bone loss with extracoronal fibre–glass/resin splint; and bone loss with intracoronal fibre–glass/resin splint. Oblique loads (50, 100, and 150N) were applied on the teeth. Data were analysed using 3-way ANOVA and Scheffe's test (α=.05).ResultsStrains on buccal surface were higher than on lingual surface. Bone loss resulted in strain increase at 100 and 150N loading. Dental splinting with resin resulted in strain values similar to the control levels.ConclusionsBone loss increased strain mainly in the buccal region. Dental splints with adhesive system and composite resin produced lower bone strains irrespective of occlusal load

Direct resin composite restoration of endodontically-treated permanent molars in adolescents: bite force and patient-specific finite element analysis

Objective: To evaluate the influence of three levels of dental structure loss on stress distribution and bite load in root canal-treated young molar teeth that were filled with bulk-fill resin composite, using finite element analysis (FEA) to predict clinical failure. Methodology: Three first mandibular molars with extensive caries lesions were selected in teenager patients. The habitual occlusion bite force was measured using gnathodynamometer before and after endodontic/restoration procedures. The recorded bite forces were used as input for patient-specific FEA models, generated from cone-beam computed tomographic (CT) scans of the teeth before and after treatment. Loads were simulated using the contact loading of the antagonist molars selected based on the CT scans and clinical evaluation. Pre and post treatment bite forces (N) in the 3 patients were 30.1/136.6, 34.3/133.4, and 47.9/124.1. Results: Bite force increased 260% (from 36.7±11.6 to 131.9±17.8 N) after endodontic and direct restoration. Before endodontic intervention, the stress concentration was located in coronal tooth structure; after rehabilitation, the stresses were located in root dentin, regardless of the level of tooth structure loss. The bite force used on molar teeth after pulp removal during endodontic treatment resulted in high stress concentrations in weakened tooth areas and at the furcation. Conclusion: Extensive caries negatively affected the bite force. After pulp removal and endodontic treatment, stress and strain concentrations were higher in the weakened dental structure. Root canal treatment associated with direct resin composite restorative procedure could restore the stress-strain conditions in permanent young molar teeth

Comparison of torsional stiffness of nickel-titanium rotary files with different geometric characteristics

Introduction: The aim of this study was to evaluate the theoretical effect from pitch and cross-sectional geometry on torsional stiffness of nickel-titanium (NiTi) instruments. Methods: Finite element models of NiTi rotary instruments with different cross-sectional geometries and different number of threads were made for comparison of torsional stiffness. Four cross-sectional shapes were tested: triangle, slender rectangle, rectangle, and square. Taper and external peripheral radius were the same for all models, whereas cross-sectional area and/or center core area were varied. Three pitch values (5, 10, and 15 threads) were tested for each type of cross-sectional geometry. The torsional stiffness of the 12 resulting finite element models was calculated by twisting the file shanks 20 degrees while holding the file tip at apical 4 mm. Results: The file models with larger pitch (fewer threads) had lower torsional stiffness. The models with the rectangular cross section had higher torsional stiffness than models with the triangular cross section, even when the cross-sectional areas were the same or the center core area was smaller. File models with larger cross-sectional area had higher torsional stiffness. Conclusions: Torsional deformation and/or fracture of NiTi rotary files might be reduced by reducing the pitch (increasing the number of threads) and increasing the cross-sectional areas rather than the center core area. (C) 2011 American Association of Endodontists.

Cyclic fatigue resistance tests of Nickel-Titanium rotary files using simulated canal and weight loading conditions

Objectives This study compared the cyclic fatigue resistance of nickel-titanium (NiTi) files obtained in a conventional test using a simulated canal with a newly developed method that allows the application of constant fatigue load conditions. Materials and Methods ProFile and K3 files of #25/.06, #30/.06, and #40/.04 were selected. Two types of testing devices were built to test their fatigue performance. The first (conventional) device prescribed curvature inside a simulated canal (C-test), the second new device exerted a constant load (L-test) whilst allowing any resulting curvature. Ten new instruments of each size and brand were tested with each device. The files were rotated until fracture and the number of cycles to failure (NCF) was determined. The NCF were subjected to one-way ANOVA and Duncan's post-hoc test for each method. Spearman's rank correlation coefficient was computed to examine any association between methods. Results Spearman's rank correlation coefficient (ρ = -0.905) showed a significant negative correlation between methods. Groups with significant difference after the L-test divided into 4 clusters, whilst the C-test gave just 2 clusters. From the L-test, considering the negative correlation of NCF, K3 gave a significantly lower fatigue resistance than ProFile as in the C-test. K3 #30/.06 showed a lower fatigue resistance than K3 #25/.06, which was not found by the C-test. Variation in fatigue test methodology resulted in different cyclic fatigue resistance rankings for various NiTi files. Conclusions The new methodology standardized the load during fatigue testing, allowing determination fatigue behavior under constant load conditions

Polymerization stress, shrinkage and elastic modulus of current low-shrinkage restorative composites

Objective. To compare currently available low-shrinkage composites with others regarding polymerization stress, volumetric shrinkage (total and post-gel), shrinkage rate and elastic modulus. Methods. Seven BisGMA-based composites (Durafill/DU, Filtek Z250/FZ, Heliomolar/HM, Aelite LS Posterior/AP, Point 4/P4, Filtek Supreme/SU, ELS/EL), a silorane-based (Filtek LS, LS), a urethane-based (Venus Diamond, VD) and one based on a dimethacrylate-derivative of dimer acid (N`Durance, ND) were tested. Polymerization stress was determined in 1-mm high specimens inserted between two PMMA rods attached to a universal testing machine. Total volumetric shrinkage was measured using a mercury dilatometer. Maximum shrinkage rate was used as a parameter of the reaction speed. Post-gel shrinkage was measured using strain-gages. Elastic modulus was obtained by three-point bending. Data were submitted to one-way ANOVA/Tukey test (p = 0.05), except for elastic modulus (Kruskal-Wallis). Results. Composites ranked differently for total and post-gel shrinkage. Among the materials considered as ""low-shrinkage"" by the respective manufacturers, LS, EL and VD presented low post-gel shrinkage, while AP and ND presented relatively high values. Polymerization stress showed a strong correlation with post-gel shrinkage except for LS, which presented high stress. Elastic modulus and shrinkage rate showed weak relationships with polymerization stress. Significance. Not all low-shrinkage composites demonstrated reduced polymerization shrinkage. Also, in order to effectively reduce polymerization stress, a low post-gel shrinkage must be associated to a relatively low elastic modulus. (C) 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.FAPESP[2008/54456-7]CAPES[3283-08-9]CNPq (Pibic

Polymerization shrinkage stress of composite resins and resin cements – What do we need to know?

Abstract Polymerization shrinkage stress of resin-based materials have been related to several unwanted clinical consequences, such as enamel crack propagation, cusp deflection, marginal and internal gaps, and decreased bond strength. Despite the absence of strong evidence relating polymerization shrinkage to secondary caries or fracture of posterior teeth, shrinkage stress has been associated with post-operative sensitivity and marginal stain. The latter is often erroneously used as a criterion for replacement of composite restorations. Therefore, an indirect correlation can emerge between shrinkage stress and the longevity of composite restorations or resin-bonded ceramic restorations. The relationship between shrinkage and stress can be best studied in laboratory experiments and a combination of various methodologies. The objective of this review article is to discuss the concept and consequences of polymerization shrinkage and shrinkage stress of composite resins and resin cements. Literature relating to polymerization shrinkage and shrinkage stress generation, research methodologies, and contributing factors are selected and reviewed. Clinical techniques that could reduce shrinkage stress and new developments on low-shrink dental materials are also discussed

How biomechanics can affect the endodontic treated teeth and their restorative procedures?

Abstract: Endodontic treatment is a common dental procedure used for treating teeth which the pulp tissue has become irreversibly inflamed or necrotic as a result of the carious process or dental trauma. This procedure which involves mechanical and chemical preparation of root canal may affect several mechanical and physical properties of the tooth structure. The endodontic treatment can also influence the longevity of the rehabilitation of endodontically treated teeth and biomechanics during the oral function. For restoring endodontically treated teeth several factor and clinical decisions should be observed. The decision of the fiberglass post usage and the restorative materials are related to several factors such as the quantity and quality of remaining dental structure, presence of ferrule, post cementation length and final coronal restoration. In this review, the authors will address the effect of the endodontic treatment procedures on canal shape and mechanical properties of a tooth, and also discuss the parameters and the biomechanical principles of root canal treated teeth