Tehran University of Medical Sciences Frontiers in Dentistry 2676-296X 12 4 2015 10 12 271 280 EN Allahyar Geramy Professor, Dental Research Center Dentistry Research Institute, Department of Orthodontics, Tehran University of Medical Sciences, Tehran, Iran Omid Mortezai Assistant Professor of orthodontics, Dental Caries Prevention Research Center, Qazvin University of Medical Sciences, Qazvin, Iran Masomeh Esmaily Assistant Professor, Department of Orthodontics, Lorestan University of Medical Sciences, Khorramabad, Iran Hojat Darvishpour Postgraduate Student, Orthodontics Department, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran 2015 10 12 2015 10 12 Objectives: Headgears are among the effective orthodontic appliances to achieve treatment goals. Unilateral molar distal movement is sometimes needed during an orthodontic treat- ment, which can be achieved by an asymmetric headgear. Different unilateral headgears have been introduced. The main goal of this study was to analyze the force system of uni- lateral expanded outer bow asymmetric headgears by the finite element method (FEM). Materials and Methods: Six 3D finite element models of a mesiodistal slice of the maxilla containing upper first molars, their periodontal ligaments (PDLs), cancellous bone, cortical bone, and a cervical headgear with expanded outer bow attached to maxillary first molars were designed in SolidWorks 2010 and meshed in ANSYS Workbench ver. 12.1. The mod- els were the same except for the degree of outer bow expansion. The outer bow ends were loaded with 2-Newton force. The distal driving force and the net moment were evaluated. Results: A decrease in the distalizing force in the normal side molar from 1.69 N to 1.37 N was shown by increasing the degree of unilateral expansion. At the same time, the force increased from 2.19 N to 2.49 N in the expanded side molar. A net moment increasing from 2.26 N.mm to 4.64 N.mm was also shown. Conclusion: Unilateral outer bow expansion can produce different distalizing forces in mo- lars, which increase by increasing the expansion. https://fid.tums.ac.ir/index.php/fid/article/view/299 https://fid.tums.ac.ir/index.php/fid/article/download/299/281