Photoelastic analysis of stress generated by wires when conventional and self-ligating brackets are used: A pilot study

Articles Photoelastic analysis of stress generated by wires when conventional and self-ligating brackets are used: A pilot study Guilherme Caiado Sobral 1   Mário Vedovello Filho 2   Viviane Veroni Degan 2   Milton Santamaria Jr 3   1MSc in Orthodontics, School of Dentistry - University of Araras (UNIARARAS) 2Professor, Department of Orthodontics, UNIARARAS 3Professor, Postgraduate program in Orthodontics, UNIARARAS ABSTRACT OBJECTIVE: By means of a photoelastic model, this study analyzed the stress caused on conventional and self-ligating brackets with expanded arch wires. METHOD: Standard brackets were adhered to artificial teeth and a photoelastic model was prepared using the Interlandi 19/12 diagram as base. Successive activations were made with 0.014-in and 0.018-in rounded cross section Nickel-Titanium wires (NiTi) and 0.019 x 0.025-in rectangular stainless steel wires all of which made on 22/14 Interlandi diagram. The model was observed on a plane polariscope - in a dark field microscope configuration - and photographed at each exchange of wire. Then, they were replaced by self-ligating brackets and the process was repeated. Analysis was qualitative and observed stress location and pattern on both models analyzed. CONCLUSIONS: Results identified greater stress on the region of the apex of premolars in both analyzed models. Upon comparing the stress between models, a greater amount of stress was found in the model with conventional brackets in all of its wires. Therefore, the present pilot study revealed that alignment of wires in self-ligating brackets produced lower stress in periodontal tissues in expansive mechanics. Key words: Orthodontic brackets; Dental arch; Corrective orthodontics RESUMO OBJETIVO: o presente estudo analisou, por meio de um modelo fotoelástico, a distribuição das tensões geradas em braquetes convencionais e autoligáveis quando ativados com arcos expandidos. MÉTODOS: braquetes convencionais foram colados em dentes artificiais e, em seguida, foi confeccionado o modelo fotoelástico, utilizando como base o diagrama 19/12, de Interlandi. Foram feitas trocas sucessivas com fios de liga de níquel-titânio (NiTi) de secção circular 0,014" e 0,018" e de liga de aço de secção retangular 0,019" x 0,025", todos no diagrama 22/14 de Interlandi. A cada troca de fio, o modelo foi observado em polariscópio plano, na configuração de campo escuro, e fotografado. Foi feita a substituição por braquetes autoligáveis e repetido o experimento. A análise foi qualitativa, observando o local e o padrão da tensão das franjas nos dois modelos analisados. CONCLUSÕES: os resultados identificaram uma maior padrão de tensões das franjas na região do ápice de pré-molares em ambos os modelos analisados. Ao se comparar as tensões entre os modelos, observou-se uma maior quantidade de tensão nas franjas no modelo com braquetes convencionais em todos os fios utilizados no experimento. Portanto, o presente estudo mostrou que o alinhamento dos fios nos braquetes autoligáveis produz forças mais suaves nos tecidos periodontais nas mecânicas expansionistas. Palavras-Chave: Braquetes ortodônticos; Arcada dentária; Ortodontia corretiva INTRODUCTION Nowadays, orthodontists have many techniques and methods available for treatment planning. There is a great variety of brackets, with different prescriptions and forms that allow the orthodontist to individualize each case according to patient's needs.1 These needs make the scientific community endeavor to innovate in orthodontic appliances. Innovation, in turn, leads to better control of dental movement, given that one of the greatest challenges faced by the orthodontist is to come up with mechanical solutions to stimulate biological reactions of the periodontium without compromising treatment outcomes.2 Correct management of orthodontic forces depends on a series of factors, including friction generated between wires and brackets. In orthodontic sliding mechanics, friction poses clinical difficulties to the orthodontist. High levels of friction could decrease bracket efficiency, thereby reducing the speed of dental movement and hindering anchorage control.3 The concern of producing less friction, i.e., lower attrition between wires and brackets, contributed to the development of self-ligating brackets in which the tooth moves with the wires serving as a guide, since it does not involve the use of elastic ligatures which significantly increase friction between wires and the slot.4 The difference between conventional and self-ligating brackets system is the absence of elastic or metallic ligatures in the latter. In other words, brackets have a closing system that leaves the wire free inside the slot.5 One of the purposes of orthodontic mechanics is gaining space in the arch before alignment of crowded teeth. Including badly-positioned teeth in the wire without previous space gain leads to unwanted displacements of adjacent teeth.6 On the other hand, according to Damon,5 lower friction treatment provides transversal adaptation that prevents potential side-effects of alignment, thereby providing treatment of crowded teeth without previous mechanic space gain. In addition to treating crowding cases, this transversal adaptation might be used in favor of the orthodontist. For instance, in cases aiming at transversal expansion of one or both arches, Maltagliati6 showed that treatment with self-ligating brackets significantly increased the transversal dimensions. This unique behavior of the self-ligating system in comparison to the conventional one seems to derive from lower friction associated with heat activated nickel-titanium wires of small diameter acting as adjuvant in treatment results.6 One of the methods used to study the way forces manifest on bodies is by means of photoelasticity. The principle of photoelasticity is based on the fact that most materials turn birefringent (separation of light into two rays with different velocity and refraction indexes) when subjected to mechanical stress.7 , 8 Birefringence is manifested by colored fringes in areas of induced stress. Orthodontic material reproduces resilience of the periodontium.9 Monochromatic tones are used for analysis of force quantity, while colored fringes provide more information on stress direction and distribution.10 By means of photoelasticity, the present study analyzed the stress caused on conventional and self-ligating brackets when combined with nickel-titanium wires. MATERIAL AND METHODS Photoelastic model Only one photoelastic model was made. Initially, with conventional brackets (Kirium, Abzil Indústria e Comércio Ltda, São José do Rio Preto, Brazil) which were afterwards replaced by self-ligating brackets (Portia, Abzil Indústria e Comércio Ltda, São José do Rio Preto, Brazil) bonded with cyanoacrylate (Supe (...truncated)