T he evaluation of orthodontic attachment has often relied on relatively imprecise ex vivo experiments that measure only the weakest component in the bracket-cement-tooth system. Because the quality of attachment is primarily deter- mined by the stresses generated in response to the applied load, computer models are ideally suited to provide insight into the structural behavior of this system. The finite element method of stress analysis is a computer-aided mathematic technique for obtaining approximate numeric solutions to the abstract equations of calculus that predict the response of physical systems that are subjected to external influences. 1 The finite ele- ment method of stress analysis allows stress levels and distributions to be evaluated in systems with irregular geometry and often nonhomogeneous physical proper- ties. The technique has been applied with some success in orthodontic research 2-20 and has recently been applied to the evaluation of orthodontic attachment. Katona 21,22 and Katona and Moore 23 used a 2-dimensional finite ele- ment model of the bracket-tooth interface to assess stress distribution in the system when bracket-removing forces are applied. Similarly, Rossouw and Tereblanche 24 used a simplified 3-dimensional finite element model to eval- uate the stress distribution around orthodontic attach- ments during debonding. Katona 25 compared different methods of bracket removal and suggested that different loading methods resulted in significantly different stress patterns. In addition, peak stress concentrations were suggested to be responsible for attachment failure, which indicated that mean stress values were of little value in quantifying the quality of attachment. Funded by The Welsh Scheme for Development of Health and Social Research. a Senior Lecturer, Department of Dental Health and Development, University of Wales College of Medicine. b Research Fellow, Welsh Centre for Biomechanics, University College Swansea. c Senior Research Fellow, Department of Basic Dental Science, University of Wales College of Medicine. d Reader in Biomechanical Engineering, Department of Basic Dental Science, University of Wales College of Medicine. e Professor and Head of Department, Department Of Dental Health and Devel- opment, University of Wales College of Medicine. Reprint requests to: Jeremy Knox, Senior Lecturer, Department of Dental Health and Development, University of Wales College of Medicine, Dental School, Heath Park, Cardiff CF4 4XY,Wales, UK; e-mail, Knoxj@cardiff.ac.uk. Submitted, January 2000; revised and accepted, June 2000. Copyright © 2001 by the American Association of Orthodontists. 0889-5406/2001/$35.00 + 0 8/1/110987 doi:10.1067/mod.2001.110987 43 ORIGINAL ARTICLE An evaluation of the influence of orthodontic adhesive on the stresses generated in a bonded bracket finite element model Jeremy Knox, BDS, MScD, PhD, MOrth RCS, FDS (Orth), a Berislav Kralj, Dipl-Ing, PhD, b Pierre F. Hübsch, Dipl-Ing, PhD, c John Middleton, BSc, MSc, FRSA, d and Malcolm L. Jones, BDS, MScD, PhD, FDS, DOrth RCS e Wales, UK The objective of this study was to evaluate the stresses generated in the bracket-cement-tooth continuum by a tensile load case when the physical and geometric properties of cement are varied. A 2-stage approach was used. In the first stage, a validated 3-dimensional finite element model of the bracket-cement-tooth system was constructed that consisted of 15,324 nodes and 2971 finite elements. Bracket base geometry was held constant; the physical properties (elastic modulus; Poisson’s ratio) and geometry (lute thickness) of the cement varied. A simplified 2-dimensional model was then developed to investigate the localized effects of the cement lute thickness and the shape of the lute periphery on the stress distribution in the system. Small increases in stress were recorded under load within the cement as the rigidity of the cement increased. Similarly, Poisson’s ratio values above 0.4 appeared to have a small influence on the major principal stresses in the impregnated wire mesh layer when a tensile force was applied. Variation in lute thickness was shown to have an influence on the distribution of major principal stresses within the cement lute. Increased stresses were recorded at the lute periphery as the lute dimensions increased. The morphologic features of the lute periphery also appeared to have had a significant effect on the performance of an orthodontic adhesive. Acute cement-enamel angles resulted in an increased chance of singularity development and attachment failure. The physical properties and thickness of the cement lute and the shape of the cement lute periphery contribute to the stress distribution within the bracket-cement-tooth continuum and, therefore, the quality of orthodontic attachment provided. (Am J Orthod Dentofacial Orthop 2001;119:43-53)