Biomechanics in orthodontics: Principles and practice

Abstract

The introductory chapter describes the physical laws of mechanics governing forces and motions in the context of orthodontics. Described are concepts such as moments and couples, centers of resistance, center of rotation, and moment-to-force ratio, all of which are essential to the understanding and control of orthodontic tooth movements. The second chapter, “Application of orthodontic force,” discusses the physical properties and the many variables involved in the variety of materials used in the components of an orthodontic appliance, such as brackets and the forces generated from archwires, coil springs, and elastic modules. Physical properties, such as stiffness, strength, working ranges, spring back, and formability, and the testing of these properties are described in detail. Chapter 3, “Analysis of two-tooth mechanics,” details the force systems created by straight and bent wires placed in the brackets of 2-tooth segments. In cases of crowding, an elastic straight wire is engaged in the brackets of teeth at different angles and levels. The teeth move in response to the elasticity of the wire; however, deciphering all the forces and moments in a complete dental arch is extremely complex because of the different anchorage values of the teeth, each having brackets and tubes of different widths and at various angulations and levels to each other. A wire engaged even in only 2 brackets affects the teeth in 3 planes of space (sagittal, transverse, and vertical). For simplicity, the authors described the 6 named geometry classes that can occur between 2 teeth, as originally described by Burstone and Koenig. Space closure mechanics can be achieved by using either a “Friction or frictionless system” (chapter 4). In the friction system, the teeth move by sliding a bracket along an archwire. The frictionless system entails moving teeth by means of loops. Whichever system is chosen, some unpredictable adverse mechanical effects during treatment are to be expected. These effects, together with the advantages and disadvantages of both systems, are fully explained. Anchorage, the resistance against undesired tooth movement, is comprehensively covered in chapter 5, “Anchorage control.” Various intraoral and extraoral methods of anchorage are described and shown. A brief section is devoted to the use, stability, biomechanical considerations, and limitations to microimplants. Chapters 6 and 7 are devoted to the biomechanics of correcting vertical and transverse arch discrepencies. Chapter 8 deals with the correction of anteroposterior arch discrepancies. Class II molar relationships can be corrected by distal movement of the maxillary molars, mesial movement of the mandibular molars, or a combination of both. Shown are the biomechanics of the various methods of achieving these end results. The ninth and final chapter relates to space closure after the extraction of teeth as a consequence of arch to tooth-size discrepancies or skeletal problems. Factors to consider include severity of crowding, vertical growth pattern, midline discrepancies, incisor-lip relationship, anchorage, differential mechanics, and general strategies to close spaces. The book represents a comprehensive, in-depth coverage of the biomechanical principles as they relate to orthodontics. The numerous illustrations are clear and almost self-explanatory. The text is informative and should serve as an excellent reference.