“We build too many walls and not enough bridges.”
— Isaac Newton
Dentofacial changes are primarily achieved by the orthodontist applying forces to teeth, the periodontium, and bone. Hence, the scientific basis of orthodontics is physics and Newtonian mechanics applied to a biologic system. The modern clinician can no longer practice or learn orthodontics as a trade or a technique. He or she must understand forces and how to manipulate them to optimize active tooth movement and anchorage. Communication with fellow clinicians and other colleagues in other fields requires a common scientific terminology and not a narrow “jargon.” There is no such thing as a unique “orthodontics physics” divorced from the rest of the scientific community. New appliances and treatment modalities will need a sound biomechanical foundation for their development and most efficient use.
Every profession has its trade tools. The carpenter uses a hammer and a saw. The medical doctor may prescribe medication and is therefore a student of proper drug selection and dosage. Traditionally, the orthodontist is identified with brackets, wires, and other appliances. Such hardware is only a means to an end point: tooth alignment, bone remodeling, and growth modification. The orthodontist achieves these goals by manipulating forces. This force control within dentofacial orthopedics is analogous to the doctor’s dosages. An “orthodontic dosage” includes such quantities as force magnitude, force direction, point of force application (moment-to-force ratios), and force continuity.
Historically, because the end point for treatment is the proper force system, one might expect the development and usage of orthodontic appliances to be based on concepts and principles from physics and engineering. On the contrary, however, most appliances have been developed empirically and by trial and error. For that reason, treatment may not be efficient. Many times undesirable side effects are produced. If appliances “work,” at a basic minimum the forces must be correct, which is independent of the appliance, wires, or brackets. Conversely, when bad things happen, there is a good possibility that the force system is incorrect.
These empirically developed appliances rarely discuss or consider forces. Forces are not measured or included in the treatment plan. How is it possible to use such mechanisms for individualized treatment? The answer is that they are shape driven rather than force driven. Different shapes and configurations are taught and used to produce the desired tooth movement. This approach is not unreasonable because controlled shapes can lead to defined wire deflections that relate to the produced forces. Unfortunately, there is so much anatomical variation among different patients that using a standard shape for a bracket or a wire or even modifying that shape will not always produce the desired results predictably.
An example of a shape-driven orthodontic appliance is what E. H. Angle called the ideal arch. In a typical application of this ideal arch, an archwire is formed with a shape so that if crooked teeth (brackets) are tied into the arch, the deflected wire will return to its original shape and will correctly align the teeth. Today, wires have been improved to deflect greater distances without permanent deformation, and brackets may have compensations to correct anatomical variation in crown morphology. The principle is the same as Angle’s ideal arch, but this approach is now called straight wire. Straightwire appliances can efficiently align teeth but can also lead to adverse effects in other situations. The final tooth alignment may be correct, but the occlusal plane may be canted or the arch widths disturbed. Intermediate secondary malocclusions can also occur. An understanding of biomechanical principles can improve orthodontic treatment even with shape-driven appliances by identifying possible undesirable side effects before any hardware is placed. Aligners also use the shape-driven principle of an ideal shape.
All orthodontic treatment modalities, including different brackets, wires, and techniques, can be improved by applying sound biomechanics, yet much of clinical orthodontics today is delivered without consideration of forces or force systems. This suggests that many clinicians believe that a fundamental knowledge and application of biomechanics has little relevance for daily patient care.
There are many principles and definitions used in physics that are universally accepted by the scientific community. At one extreme, there is classical physics—concepts developed by giants like Newton, Galileo, and Hooke. There are also other scientific disciplines, such as quantum mechanics. What the authors find disturbing is the hubris of what they call pseudo-biomechanics—new physical principles developed by orthodontists that are separate and at odds with classical mechanics. Orthodontists’ journal articles and lecture presentations are filled with figures and calculations that do not follow the principles of classical mechanics. Orthodontists may be intelligent, but we should not think we can compete with the likes of Newton.
There is another major advantage in adopting scientific or classical mechanics. The methodology, terminology, and guiding principles allow us to communicate with our scientific colleagues and set the stage for collaborative research. Imprecise words can confuse. We speak of “power arms,” but power has a different meaning to an engineer than it does to a politician or a clinician. Force diagrams in orthodontic journals are difficult to decipher and may not be in equilibrium. The concepts, symbols, and terminology presented in this book are not trade jargon but will be widely recognized in all scientific disciplines.
Note that the theme of this book is orthodontic biomechanics. The “bio” implies the union of biologic concepts with scientific mechanics principles. Let us now consider some specific reasons why the modern orthodontist needs a solid background in biomechanics and the practical ways in which this background will enhance treatment efficacy.
Optimization of Tooth Movement and Anchorage
The application of correct forces and moments is necessary for full control during tooth movement, influencing the rates of movement, potential tissue damage, and pain response. Furthermore, different axes of rotation are required that are determined by moment-to-force ratios applied at the bracket. For example, if an incisor is to be tipped lingually around an axis of rotation near the center of the root, a lingual force is applied at the bracket. If the axis of rotation is at the incisor apex, the force system must change. A lingual force and lingual root torque with a proper ratio must then be applied. These biomechanical principles are relevant to all orthodontic therapy and appliances—headgears, functional appliances, sliding mechanics, loops, continuous arches, segments, and maxillomandibular elastics (a...