Comparison of two asymmetric headgear force systems: A finite element analysis

online article* Comparison of two asymmetric headgear force systems: A finite element analysis Samaneh Sadeghi1, Zohreh Hedayati2, Batoolalsadat Mousavi-Fard3 DOI: https://doi.org/10.1590/2177-6709.24.2.41.e1-6.onl Objective: The aim of this study was to evaluate the effect of displacement patterns of the molar teeth in response to different asymmetric headgear loading using 3D finite element method. Methods: A series of twenty-five facebow with different left vs. right outer bow length and different expansion of left vs. right were designed. The non-favored side (right side) was shortened at intervals of 10 mm, and favored side (left side) was expanded 10 degree greater than right side and 5 degree expansion were successively added. At the first phase, each side received 200-g load, implying the neck strap to displace toward shorter arm. At the second phase, a total of 400-g load was applied to the ends of the outer bow. Because of the neck strap displacement, the shorter arm received greater load than the left side, the magnitude of the applied force to each side depended on difference of left vs. right outer bow length and expansion. Results: All systems were effective in promoting asymmetric distal movement of the molars. However, the asymmetrical facebow with the 40 mm shortening and 25 degree expansion outer bow when unequal force applied could be used in asymmetric mechanics. Medial and occlusal displacing forces were observed in all systems. Conclusions: Both equal and unequal force application is effective for molar distalization. Expansion of the outer bow in the affected side and shortening of the outer bow in the normal side were effective to produced differential distal molar movement. Keywords: Asymmetric. Extraoral traction appliances. Finite element analysis. Objetivo: o objetivo desse estudo foi usar o método de elementos finitos 3D para avaliar o efeito no padrão de deslocamento dos molares em resposta ao uso de aparelhos extrabucais com diferentes forças assimétricas. Métodos: foram confeccionados 25 aparelhos extrabucais (AEB) com diferenças, entre os lados direito e esquerdo, quanto ao comprimento e ao grau de abertura do braço externo. O lado não favorecido (lado direito) foi encurtado em intervalos de 10 mm e o lado favorecido (lado esquerdo) foi aberto 10 graus a mais do que o lado direito, sendo adicionados 5 graus de abertura sucessivamente. Na primeira fase, cada lado recebeu carga de 200 g, causando o deslocamento da tala cervical em direção ao braço mais curto. Na segunda fase, foi aplicada carga total de 400 g às extremidades dos braços externos dos AEB. Devido ao deslocamento da tala cervical, o braço mais curto recebeu uma força maior do que o lado oposto; a magnitude da força aplicada em cada lado dependeu da diferença no comprimento e na abertura dos braços externos do AEB. Resultados: todos os sistemas foram efetivos em promover movimentação distal assimétrica dos molares. Porém, o AEB assimétrico com 40 mm de encurtamento e 25 graus de abertura do braço externo poderia ser usado na mecânica assimétrica, com aplicação diferenciada de força. Foram observadas forças mediais e oclusais de deslocamento em todos os sistemas. Conclusões: tanto a aplicação de forças simétricas quanto assimétricas são efetivas para a distalização dos molares. A expansão do braço externo do AEB no lado afetado e o encurtamento no lado normal foram efetivos na produção de movimento distal assimétrico dos molares. Palavras-chave: Assimétrico. Aparelhos de tração extrabucal. Análise de elementos finitos. How to cite: Sadeghi S, Hedayati Z, Mousavi-Fard B. Comparison of two asymmetric headgear force systems: A finite element analysis. Dental Press J Orthod. 2019 Mar-Apr;24(2):41.e1-6. DOI: https://doi.org/10.1590/2177-6709.24.2.41.e1-6.onl Kerman University of Medical Sciences, Kerman Dental School, Department of Orthodontics (Kerman, Iran). 2 Shiraz University of Medical Sciences, Shiraz Dental School, Department of Orthodontics (Shiraz, Iran). 3 Afzalipour Hospital, Kerman University of Medical Sciences (Kerman, Iran). 1 Submitted: February 09, 2018 - Revised and accepted: August 27, 2018 » The authors report no commercial, proprietary or financial interest in the products or companies described in this article. © 2019 Dental Press Journal of Orthodontics Contact address: Batoolalsadat Mousavi-Fard E-mail: 41.e1 Dental Press J Orthod. 2019 Mar-Apr;24(2):41.e1-6 online article Comparison of two asymmetric headgear force systems: A finite element analysis INTRODUCTION Headgear was introduced at the first time in early 1800s, until now, different modifications have been used.1 It could be used to restrain maxillary growth, retract maxillary molars, or hold the molars in place to reinforce the anchorage while retract canine and incisors2. However, to obtain successful results, extraoral traction requires considerable patient compliance. Different treatment modalities have been introduced to distalize maxillary molars to overcome patient compliance, such as palatal bar, repelling magnets, Nitinol coil spring, K-loops, superelastic wires, Wilson arches, Jones jig appliances, pendulum appliances, distal jet appliances and recently temporary anchorage devices (TAD).1 Compared with these appliances, headgear is the better choice because of restricting effect of maxillary growth and dental movement to correct Class II relationship.3 In some instances, there is unilateral Class II malocclusion (Class II subdivision), in which one side presents Class II molar relationship, while the other side is Class I. Such situation requires an asymmetric force system. Some changes in facebow convert symmetric headgear into asymmetric ones. These changes include asymmetric length of the right/left outer bow, which are referred as power arm facebow, asymmetric length of the right/left inner bow, different angulation right/left between inner and outer bow, swivel offset, hinged inner bow, different toe-in bend in the inner bow, or combination of them2. The most practical method to design asymmetric headgear is shortening one outer bow or elongating one inner bow2. Extensive clinical data have revealed the effectiveness of asymmetric headgear in unilateral distalization4. For better understanding of dental biomechanical behavior, the finite element analysis (FEA) was introduced in 1973 and is a useful method to quantify forces, moments and tensions, as well as other variables that allow appliance activations to be simulated for distal movement according to coordinates X, Y and Z. It is based on the separation of the analysis shape into subdomains through finite elements that could predict the mechanical behavior of the object under varied loading conditions.5,6 Despite the existence of a number of investigation on the biomechanics of unilateral facebows, © 2019 Dental Press Journal of Orthodontics 41.e2 there is still conflicting concepts regarding their effects. Nobel a (...truncated)