Professor Emeritus of Orthodontics Adj. Professor Mechanical Engineering Indiana University & Purdue University
History of the Development of Biomechanics
Fish (1917) theorized that orthodontics was applied biophysics, but inadequate knowledge of the tooth movement process precluded practical applications. Both functional and fixed appliances evolved as indeterminate mechanics because Newton’s Laws apply to only two abutments. Low force (physiologic) tooth movement was first advocated by Reitan (1947) based on histologic study of PDL necrosis and root resorption. Weinstein and Haack (1959, 1963) described the first modern concepts of biomechanics: practical clinical mechanics, equilibrium (minimal entropy), and material properties. This was the basis for demonstrating physiologic tooth movement due to cheek and tongue pressure. Simultaneously, Baldwin and Burstone developed fixed mechanics for predictable tooth movement of two abutments (units), i.e. teeth, segments or arches. Subsequently, Burstone, Fiorelli, Melsen, Roberts et al. applied determinate mechanics to dental segments, individual teeth, and implants. However, routine orthodontic treatment remained indeterminate for entire arches treated with fixed, functional and/or aligner appliances. Chang et al. (~2000) developed extra-alveolar bone screws as temporary anchorage for determinate mechanics to predictably move entire dental arches to resolve skeletal Class III malocclusion. Indeterminate mechanics was actually an advantage for aligners because detailed finishing required progress records for a new set of refining aligners. Treatment of millions of aligner patients produced massive databases for actual treatment effects that is currently used to predict statically indeterminate mechanics.
Finite Element Modeling: The Holy Grail of Determinate Mechanics
Finite Element Analysis (FEA) calculates stress within homogeneous or composite structures, based on the material properties for each component (finite element) within the defined boundaries. This computer-intensive method applies to all materials (tissues) living and dead for which the stiffness (Young’s modulus) and the compression/stretching coefficient (Poisson’s ratio) are known. FEA is viewed as the “holy grail” for determinate mechanics because the internal stress induced by an applied load can be calculated in 3D within the appliance, moving unit (tooth, segment or arch), and resisting tissue (periodontium). Bone resorption and formation along PDL surfaces are the principal adaptive phenomena resulting in tooth movement. PDL is a complaint layer (stress riser) between two relatively rigid tissues (tooth and bone). Its modulus is variable because internal vascularity has an erectile effect to resist short-term compressive loading; thus, teeth are only moved by sustained loads. FEA calculates instantaneous PDL displacement. If the applied load is within the necrotic threshold of the PDL, FEA iterations calculate the path of tooth movement and axis of rotation for the moving unit. Overall displacement over time is the rate of movement. These data suggest a two step approach for the most efficient comprehensive treatment: 1. align arches with light forces and laser-tempered CuNiTi archwires to produce a rigidly connected arch (multi-rooted unit), and 2. differential movement of entire arches with determinate mechanics.
W. Eugene Roberts DDS, PhD
Dr. Roberts received a DDS (Creighton University), PhD in Anatomy (University of Utah), and Certification in Orthodontics (University of Connecticut.) He is Professor Emeritus of Orthodontics and Adjunct Professor of Mechanical Engineering at Indiana University and Purdue University at Indianapolis.
Affiliations include: American Association of Orthodontists (AAO), American and International Colleges of Dentists, Diplomate American Board of Orthodontics (ABO); Midwest Component of the Angle Society; American Association of Orthodontists Foundation (AAOF).
Honors include: two NIH Postdoctoral Fellowships, National Science Foundation Senior Fellow in Space Biology (NASA), Docteur Honoris Causa (honorary medical degree) University of Lille, France; US Navy Commendation Medal with Combat V, Isaiah Lew Memorial Research Award (American Academy of Implant Dentistry Foundation); Jarabak Award for Orthodontic Education and Research (AAOF); Salzmann Lecture (AAO); Dr. Dale Wade Award for Excellence in Orthodontics (ABO); Mershon Lecture (AAO); Ketcham Award (ABO); and Career Achievement Award in Research (AAO).
He currently practices orthodontics, and well as serving as an editor, external appraiser, legal consultant, and research technology advisor.
Dr. Roberts has published 130 refereed articles, and 256 invited papers and case reports.