Tulimar Pereira Machado Cornacchia

Biomechanical evaluation of screw- and cement-retained implant-supported prostheses: A nonlinear finite element analysis

STATEMENT OF PROBLEM The mechanical stability of the prosthetic components in the implant-prosthesis complex is essential to the long-term success of the restorations. However, little is known about the differences in the biomechanical behavior of screw- and cement-retained prostheses. PURPOSE The purpose of this study was to compare the preload maintenance, stresses, and displacements of prosthetic components of screw- and cement-retained implant-supported prostheses by using the finite element method in a nonlinear analysis. MATERIAL AND METHODS Two 3-dimensional models were constructed: implant-supported fixed partial prostheses with 3 elements retained either by screws (SFP) or cement (CFP). After the simulation of screw tightening, the preload was calculated for both prostheses. Then vertical and oblique loads (100 N) were applied on the models. The preload was identified, the maximum von Mises equivalent stresses (SEQV) were obtained on the screws, and the displacement among the abutment, the implant, and screw was identified by observing the penetration and gap in the contact interfaces. RESULTS Under vertical load, there was a higher decrease in the preload and in the SEQV on the screw in the SFP. Under oblique load, the SEQV was 24% higher on the screw of the SFP. In the displacement analysis under vertical load, penetration was concentrated in the threads of the screw in the SFP and between the abutment and implant in the CFP. The gap was 118% greater for the SFP and was concentrated on the abutment extension. Under oblique load, the displacement pattern was similar for both prostheses, but with values 66% higher for penetration and 96% higher for gap for the SFP. CONCLUSIONS The SFP showed a higher biomechanical risk of failure than the CFP.

Abfraction and anisotropy-effects of prism orientation on stress distribution

This work discusses the effect of enamel anisotropy in the stress concentration at the cement-enamel junction (CEJ), a probable cause of fracture in enamel leading to abfraction. Usual simplifications when developing computer models in dentistry are to consider enamel isotropic, or that the direction of the prisms is orthogonal to either the dentine-enamel interface or the tooth outer surface. In this paper, a more refined model for the material behavior is described, based on laboratory observation and on the work of Fernandes and Chevitarese [1] . The material description is used in a two-dimensional (2D) finite element model of the first upper premolar, and the analysis is performed for two different situations: vertical loads, typical of normal mastication and horizontal loads, dominant in bruxism. The analyses were performed using a unit load, which under the hypothesis of linear response of the tooth, allows the combinations described in the text to simulate different functional and parafunctional loads. The results indicate that a realistic enamel description in terms of mechanical properties and spatial distribution of its prisms alters significantly the resulting stress distribution. For all cases included in this study, the detailed description of prism orientation and resulting anisotropy led to improved response in terms of stress distribution, even when loading was horizontal.

Determination of tangential and normal components of oral forces

Oral forces applied to human teeth during biting and mastication are normally described in the literature only in terms of their axial components. The purpose of this study was to fully determine the spatial characteristics of the oral resultant force – its normal and tangential components - for a given individual. A load cell was especially manufactured to measure oral force and was temporarily implanted as a prosthetic device in the dental arch of a volunteer, replacing his missing upper first molar. The mastication and occlusion tests were carried out in such a way the cell should withstand the loads applied to the molar, and its state of strain was recorded by strain gauges attached to it. Based on the results of these tests and using balance equations, normal and tangential components of the resultant oral force were determined. For direct occlusion, without interposition any obstacle between cusps, a peak normal force of 135 N was recorded simultaneously to a tangential force of 44 N. For mastication of biscuits, a peak normal force of 133 N and a tangential force of 39 N were obtained.

Standard of disocclusion in complete dentures supported by implants without free distal ends: analysis by the finite elements method

Objective The occlusal patterns are key requirements for the clinical success of oral rehabilitation supported by implants. This study compared the stresses generated by the disocclusion in the canine guide occlusion (CGO) and bilateral balanced occlusion (BBO) on the implants and metallic infrastructure of a complete Brånemark protocol-type denture modified with the inclusion of one posterior short implant on each side. Material and Methods A three-dimensional model simulated a mandible with seven titanium implants as pillars, five of them installed between the mental foramen and the two posterior implants, located at the midpoint of the occlusal surface of the first molar. A load of 15 N with an angle of 45º was applied to a tooth or distributed across three teeth to simulate the CGO or BBO, respectively. The commercial program ABAQUS® was used for the model development, before and after the processing of the data. The results were based on a linear static analysis and were used to compare the magnitude of the equivalent stress for each of the simulations. Results The results showed that the disocclusion in CGO generated higher stresses concentrated on the working side in the region of the short implant. In BBO, the stresses were less intense and more evenly distributed on the prosthesis. The maximum stress found in the simulation of the disocclusion in CGO was two times higher than that found in the simulation of the BBO. The point of maximum stress was located in the neck of the short implant on the working side. Conclusions Under the conditions of this study, it was concluded that the BBO pattern was more suitable than CGO for the lower complete denture supported by implants without free distal ends.

Stress distribution in a premolar 3D model with anisotropic and isotropic enamel

The aim of this study was to compare the areas of stress concentration in a three-dimensional (3D) premolar tooth model with anisotropic or isotropic enamel using the finite element method. A computed tomography was imported to an image processing program to create the tooth model which was exported to a 3D modeling program. The mechanical properties and loading conditions were prescribed in Abaqus. In order to evaluate stresses, axial and oblique loads were applied simulating realistic conditions. Compression stress was observed on the side of load application, and tensile stress was observed on the opposite side. Tensile stress was concentrated mainly in the cervical region and in the alveolar insertion bone. Although stress concentration analyses of the isotropic 3D models produced similar stress distribution results when compared to the anisotropic models, tensile stress values shown by anisotropic models were smaller than the isotropic models. Oblique loads resulted in higher values of tensile stresses, which concentrate mainly in the cervical area of the tooth and in the alveolar bone insertion. Anisotropic properties must be utilized in enamel stress evaluation in non-carious cervical lesions.

Effects of Screw- and Cement-Retained Implant-Supported Prostheses on Bone: A Nonlinear 3-D Finite Element Analysis.

Purpose:To compare the stresses and displacements on perimplant bone generated by screw- and cement-retained prostheses using the finite element method. Materials and Methods:Two models were constructed: partial fixed implant-supported prostheses with three elements retained by screws (SFP) or cement (CFP). Vertical and oblique loads of 100 N were applied on the models. Bone was analyzed by the principal stresses &sgr;1 and &sgr;3. The displacement between the implant and the bone was identified by the penetration and gap. Results:Results showed a similar pattern in the distribution of the principal stresses between both prostheses. Under the &sgr;1 stresses, the SFP showed similar values in the bone compared with the CFP. The analysis of the &sgr;3 showed stress peaks 28% higher in the SFP, considering vertical and oblique loads. Displacement analysis showed a similar pattern and similar values between the prostheses for penetration and gap under both loads. Conclusions:There were no important differences in the &sgr;1 analysis and the displacement between the SFP and CFP. The differences in marginal bone level reported between SFP and CFP in some clinical studies may not be related to a mechanical factor.

Evaluation of the effect of different enamel surface treatments and waiting times on the staining prevention after bleaching

Background Bleached dental enamel can be more susceptible to staining than the enamel that has never been bleached, especially right after tooth bleaching. The aim of this study is to evaluate the effect of surface treatments and waiting time prior to contact with dye on bleached enamel staining susceptibility. Material and Methods One hundred teeth were bleached with 35% hydrogen peroxide (Whiteness HP, FGM) and randomly assigned to G1 artificial saliva, G2 2% sodium fluoride (Flugel, Nova DFL), G3 casein phosphopeptide-amorphous calcium phosphate fluoride paste (CPP-ACPF, MI Paste Plus, GC America), G4 rinse for color maintenance after bleaching (Keep White Rinse, DMC) and G5 polishing with aluminum oxide-impregnated disks (Super Buff Disk, Shofu). Fifty specimens were immersed in red wine for 15 minutes, immediately after treatment, and the others one hour after. Color difference (∆E) was evaluated with a spectrophotometer (Vita EasyShade). Surface treatments and waiting time effects were analyzed with Kruskal-Wallis and Mann Whitney tests (p<0.05). Results Surface treatments (p>0.05) and waiting time (p>0.05) were not significant to decrease bleached enamel susceptibility to red wine staining. Conclusions Surface treatments were similar to artificial saliva for bleached enamel susceptibility to red wine staining. Immediate or one-hour-postponed contact with red wine did not affect bleached enamel color. Key words:Tooth bleaching, color, dental enamel, hydrogen peroxide, pigmentation.

Analysis of stresses during the polymerization shrinkage of self-curing resin cement in indirect restorations: A finite-element study

BACKGROUND Adhesive cementation is essential for the longevity of indirect esthetic restorations. However, polymerization shrinkage of resin cement generates stress, which may cause failures in the tooth-restoration interface. So, understanding of the biomechanics of resin cement is important for predicting the clinical behavior of an esthetic indirect restoration. AIMS To analyze the stresses generated during polymerization shrinkage of self-curing resin cement in ceramic and in indirect resin (IR) restorations, using the finite-element method (FEM). SETTINGS AND DESIGN Numerical study using the finite-element analysis. MATERIALS AND METHODS A three-dimensional (3D) model of a second molar restored with ceramic or IR onlay restoration was designed. The polymerization shrinkage of self-curing resin cement was simulated in FEM software using an analogy between the thermal stress and the resulting contraction of the resin cement. The localization and values of tensile stresses in the dental structure, cement, and adhesive layer were identified. RESULTS The location and value of the tensile stresses were similar for the two restorative materials. High tensile stresses were identified in the axiopulpal wall and angles of the tooth preparation, with the major stresses found in the cement located in the axiopulpal wall. CONCLUSIONS The high stresses values and their concentration in the angles of the prepared tooth emphasize the importance of round angles and the use of cements with lower rates of shrinkage.

Residual Stresses and Cracking in Dental Restorations due to Resin Contraction Considering In-Depth Young's Modulus Variation

Composite resins have been increasingly used as restorative material, both in anterior and posterior teeth, where they replace metal restorations. Its aesthetic characteristics, coupled with improved physical properties have made their use extend from just anterior teeth to also include posterior teeth. The use of such material in oral regions subjected to higher loading makes it important to account for the effect of residual stresses arising during polymerization, induced by resin contraction (Ausiello, Apicilla and Davidson 2002). Different reports can be found in the literature focused in this aspect and using different approaches, such as X-ray micro-computed tomography (Sun, Eidelman and Gibson 2009), 3D evaluation of the marginal adaptation (Kakaboura et al 2007) and 3 D deformation analysis from MCT images (Chiang et al 2010). These authors agree in the critical role played by resin contraction in restoration success. The Finite Element Method can therefore be a useful tool to investigate the cracking of interfaces, stress concentrations in the internal angles and effects of variations in the mechanical properties in the overall behavior of the resin-based dental restorations.