Von Mises stresses on Mushroom-loop archwires for incisor retraction: a numerical study

ABSTRACT Objective: To perform a numerical simulation using FEM to study the von Mises stresses on Mushroom archwires. Methods: Mushroom archwires made of titanium-molybdenum alloy with 0.017 x 0.025-in cross-section were used in this study. A YS of 1240 MPa and a Young's modulus of 69 GPa were adopted. The archwire was modeled in Autodesk Inventor software and its behavior was simulated using the finite element code Ansys Workbench (Swanson Analysis Systems, Houston, Pennsylvania, USA). A large displacement simulation was used for non-linear analysis. The archwires were deformed in their extremities with 0° and 45°, and activated by their vertical extremities separated at 4.0 or 5.0 mm. Results: Tensions revealed a maximum of 1158 MPa at the whole part of the loop at 5.0mm of activation, except in a very small area situated at the top of the loop, in which a maximum of 1324 Mpa was found. Conclusions: Mushroom loops are capable to produce tension levels in an elastic range and could be safely activated up to 5.0mm.

RESUMO Objetivo: Realizar uma simulação numérica, por meio do Método dos Elementos Finitos (MEF), para obter as tensões de von Mises em arcos Mushroom. Métodos: Foram usados arcos com geometria Mushroom de titânio-molibdênio, com secção transversal 0,017" x 0,025". Adotou-se valores de tensão de escoamento (σesc) de 1.240 MPa e módulo de elasticidade (E) de 69 GPa. O arco foi modelado por meio do software Autodesk Inventor, e seu desempenho foi simulado utilizando-se o software de elementos finitos Ansys Workbench (Swanson Analysis System, Houston, Pennsylvania, EUA). Para a simulação, foi considerada a análise para grandes deslocamentos. O arco foi conformado em suas extremidades considerando-se planos de 0° e 45°, pré-ativado em 2,5mm e ativado por meio de suas extremidades verticais, separadas 4,0 mm ou 5,0 mm. Resultados: As tensões revelaram um valor máximo de 1.158 MPa na maior parte da alça, aos 5,0 mm de ativação, com exceção de uma área muito pequena, com valor de 1.324 MPa, situada no topo da alça. Conclusões: Os arcos Mushroom são capazes de produzir níveis de tensão situados dentro da região elástica e poderiam ser ativados com segurança até os 5,0 mm de ativação.

Three-dimensional analysis of an orthodontic delta spring

INTRODUCTION: The purpose of this study was to analyze the force system, moment-force ratios (M/F) and von Mises stresses in an orthodontic delta spring using a 3D finite element model. The M/F ratio produced by an orthodontic spring is related to the different types of tooth movement that are likely to occur in the sagittal and occlusal planes. METHODS: Analyses were performed using a 3D finite element model, and a data acquisition system was used to validate the numerical results. RESULTS: Reactive forces between 0.0 and 2.0 N were observed along the x-axis, while null values were observed along the y- and z-axes. The maximum activation that ensured geometric stability and mechanical stresses below the elastic limit of the material was 10.0 mm. CONCLUSION: The results indicate that a delta spring can provide (i) uncontrolled tipping for activation of less than 1.0 mm; (ii) controlled counterclockwise tipping for activation between 1.0 and 4.5 mm; (iii) translation for activation between 4.5 and 5.0 mm; and (iv) controlled clockwise tipping in the sagittal plane for activation between 5.0 and 10.0 mm. No tooth movement was observed in the occlusal plane for the M/F ratios observed.

Nickel-titanium alloys: a systematic review

OBJECTIVE: A systematic review on nickel-titanium wires was performed. The strategy was focused on Entrez-PubMed-OLDMEDLINE, Scopus and BioMed Central from 1963 to 2008. METHODS: Papers in English and French describing the behavior of these wires and laboratorial methods to identify crystalline transformation were considered. A total of 29 papers were selected. RESULTS: Nickel-titanium wires show exceptional features in terms of elasticity and shape memory effects. However, clinical applications request a deeper knowledge of these properties in order to allow the professional to use them in a rational manner. In addition, the necessary information regarding each alloy often does not correspond to the information given by the manufacturer. Many alloys called "superelastic" do not present this effect; they just behave as less stiff alloys, with a larger springback if compared to the stainless steel wires. CONCLUSIONS: Laboratory tests are the only means to observe the real behavior of these materials, including temperature transition range (TTR) and applied tensions. However, it is also possible to determine in which TTR these alloys change the crystalline structure.

Influence of voids in the hybrid layer based on self-etching adhesive systems: a 3-D FE analysis

The presence of porosities at the dentin/adhesive interface has been observed with the use of new generation dentin bonding systems. These porosities tend to contradict the concept that etching and hybridization processes occur equally and simultaneously. Therefore, the aim of this study was to evaluate the micromechanical behavior of the hybrid layer (HL) with voids based on a self-etching adhesive system using 3-D finite element (FE) analysis. MATERIAL AND METHODS: Three FE models (Mr) were built: Mr, dentin specimen (41x41x82 μm) with a regular and perfect (i.e. pore-free) HL based on a self-etching adhesive system, restored with composite resin; Mp, similar to M, but containing 25 percent (v/v) voids in the HL; Mpp, similar to Mr, but containing 50 percent (v/v) voids in the HL. A tensile load (0.03N) was applied on top of the composite resin. The stress field was obtained by using Ansys Workbench 10.0. The nodes of the base of the specimen were constrained in the x, y and z axes. The maximum principal stress (σmax) was obtained for all structures at the dentin/adhesive interface. RESULTS: The Mpp showed the highest peak of σmax in the HL (32.2 MPa), followed by Mp (30 MPa) and Mr (28.4 MPa). The stress concentration in the peritubular dentin was high in all models (120 MPa). All other structures positioned far from voids showed similar increase of stress. CONCLUSION: Voids incorporated into the HL raised the σmax in this region by 13.5 percent. This behavior might be responsible for lower bond strengths of self-etching and single-bottle adhesives, as reported in the literature.

Alguns aspectos da mecânica das alças de retração ortodôntica / Some aspects of the mechanics of the orthodontic retraction springs

As alças de retração ortodôntica são comumente empregadas em sistema Edgewise por meio de uma abordagem seccional (ex.: retração de caninos) ou por meio de uma abordagem segmentada (ex.: retração dos dentes anteriores em conjunto). Estas alças podem modular a demanda de ancoragem, dependendo dos objetivos impostos pelo plano de tratamento. Neste trabalho são discutidas as propriedades mecânicas relacionadas ao projeto de alças de retração ortodôntica, o sistema de forças decorrente de sua conformação geométrica e ativação, e os métodos empregados para testá-las. Dentre as técnicas utilizadas na análise de uma alça, o Método dos Elementos Finitos (MEF) merece destaque. Embora seja baseado em modelos matemáticos, é uma ferramenta eficaz e não onerosa para a simulação computacional de um protótipo, podendo-se assim avaliá-lo antes de sua construção física. Já os métodos experimentais se aproximam mais da condição real, porém com custo mais elevado. A utilização em diferentes estágios de ambos os métodos é aconselhável para um melhor desenvolvimento e caracterização dos protótipos.

Retraction springs are commonly used in Edgewise mechanics in a sectional (e.g. canine retraction) or in a segmental approach (e.g. "en masse" anterior retraction). These springs can modulate the anchorage needs depending on the treatment plan objectives. In this work it is discussed the mechanical properties related to the spring's design, the force system originated by their geometric shape and activation, and the methods used to test them. Among the techniques used to analyze a retraction spring, it can be pointed out the Finite Element Method (FEM). Nevertheless the FEM is based on mathematical models, it is an efficient non-onerous way for computational simulation of a prototype, so it can be evaluated before its physical construction. On the other hand, the experimental methods are closer to real conditions, but they are more costly. The use of both methods in different stages is recommended for a better development and characterization of prototypes.