Physics and clinical orthodontics: 100 years ago and today

It is appropriate to recognize our forebearers who 100 years ago practiced and developed a new specialty of orthodontics. We often think of these pioneers as relatively uninformed clinicians working with primitive appliances and lacking the broad scope of understanding of our modern orthodontists. We can imagine them compensating for knowledge inadequacies with highly developed technical skills such as wire bending and the soldering of intricate components. Certainly, the journals at that time would not be publishing significant and erudite articles on the applications of physics and engineering to clinical orthodontics.

To the contrary, an article that appeared in the International Journal of Orthodontia in June1917 by Gilbert Dudley Fish demonstrates advanced thinking in the application of biomechanics. In fact, many of the concepts discussed by Dr Fish not only are valid, but also might be ignored or poorly understood by some orthodontists today. Let us consider some quotes from this 1917 article.

Yet it must be admitted, that the only known means of correcting deformities of the dental arch, are fundamentally mechanical. It is axiomatic, that teeth, the surrounding tissues, and any appliance attached to the teeth, are all governed in common by the immutable laws of the exact physical sciences. It can be demonstrated, that the analytical problems in mathematics, physics, and animal mechanics, underlying the orthopedic treatment of malocclusion, cannot be rationally handled, or even intelligently studied, by men unversed in technology. Wherefore I urge you to consider well the question of informing yourselves as to the rudiments of mechanics. Provide the students in your schools of orthodontia with instruction in the subject.

Dr Fish’s thought—that the primary means of orthodontic treatment with the exception of orthognathic surgery is the application of forces—is still true today. Other biologic approaches are discussed but not yet available. Therefore, we should not attempt treatment without a fundamental understanding of the physics that are involved. Even today, not all orthodontic programs offer quality courses in physics and biomechanics. The goal is to graduate students who can apply sound Newtonian physics to clinical practice. This goal currently has not been universally reached.

I admit, that it will not be easy to give the dentist, trained in subjects far removed from technology, a grasp of the fundamentals of kinematics and dynamics; but I insist, that the necessity has arisen and must be faced.

This is still a problem. Dentists and orthodontists may be frightened by mathematics and physics. Clinicians are people-oriented and may consider physics a topic suitable for nerds. Fortunately, much of orthodontic biomechanics need not be complicated.

I wish to plant in your minds the idea, that another science, older than your own and further developed, holds a great store of information which you require for your further progress.

Mechanics is not the trade of an artisan; it is a major division of physics. Mechanics bears about the same relation to the shop-work of the so-called “mechanic,” as engineering bears to the work of the locomotive engineer.

Even now, the vast knowledge from physics, engineering, and material science to help solve orthodontic problems is not fully appreciated. “Mechanics” and its derivative “biomechanics” are basic or fundamental science and should not be confused with a “mechanic” such as the fellow who fixes your car.

Editor’s note: On February 11, 2015, the AJO-DO learned that Dr Charles J. Burstone suffered a heart attack and passed away in Seoul, Korea. It was a very sad day for his family, friends, colleagues, and for all of orthodontics. We thank him for all he has done for all of us, including this editorial, which he submitted in December.

When an appliance presses against a tooth, the tooth presses back against the appliance with equal intensity.

Bodies at rest are in equilibrium; i.e., are not acted upon by any unbalanced force or couple.

Inasmuch as the relative motions of the parts of an appliance attached to the teeth are exceedingly slow, the appliance may be considered in equilibrium. This principle is a valuable one in appliance design, because it enables the designer to apply the laws of equilibrium and so analyze the reactions between appliance and teeth. In fact, because the appliance may be regarded as stationary, the analysis of the forces involved falls under the head of statics instead of kinetics .

Dr Fish has it correct. Newton’s First Law describes bodies in equilibrium. Appliances are in equilibrium. This important principle is the basis of understanding and developing orthodontic appliances. Some orthodontists will say that an elastic acting on the anterior teeth will produce an equal mesial force on the posterior teeth based on Newton’s Third Law. This is true, but it is based on Newton’s First Law. The tooth pushes on the wire, and the wire pushes on the tooth with equal force, as described by Newton’s Third Law.

If the material, sectional dimensions and form of an elastic wire appliance be known, and also its elastic deformations when in place, and in addition the exact nature of each attachment or connection to band or crown, then it is possible to calculate the direction and magnitude of every reaction between tooth and appliance.

By Hooke’s law, deformation, or strain, is directly proportional to the stress accompanying it, within the limit of elasticity of the material. By the theory of flexure of elastic materials, the condition of stress at any point in an elastically deformed wire of any shape, can be calculated mathematically…

Here, the principles that are used to determine the forces and moments from an orthodontic appliance are clearly developed. This includes an understanding of the mechanics of materials and beam theory. This anticipates later research in orthodontic appliances by using beam theory and finite element techniques. It may not be so simple, since wire force-deflection might not be linear. Nonlinear behavior is seen at large wire deflections, with superelastic nickel-titanium wires, and with friction effects. Still, Dr Fish had the vision to recognize this basic contribution to clinical orthodontics.

The location of a hinge anterior to the center of resistance of the set of teeth attached to a yoke, provides for greater movement of the bicuspids or temporary molars than of the posterior teeth. If the hinge be placed at the center of resistance, the side unit moves out without rotation.

Many of us think we invented the concept of “center of resistance” to describe the relationship of force to tooth movement. Actually, it is an old concept: Leonardo da Vinci used the term. This is certainly an early use of the phrase as applied to orthodontics.

…the tendency of a buccal wire to press against the front teeth or to bury itself in the gum, may be eliminated by using heavy gauge wire in front and light wire, ending in long flexible extensions, on the sides; the great stiffness of the front as compared with the sides of the arch, causes most of the deformation to occur in the sides.

Dr Fish suggested a very modern idea, where wire stiffness is not constant at every point along an archwire, to eliminate an undesirable problem.

This seminal article shows that today, like 100 years ago, orthodontics continues to move from an “art” to a “science.” The basic biologic sciences are well recognized by the profession. As Dr Fish pointed out, we orthodontic clinicians still do not fully take advantage of the useful knowledge and concepts from physics and engineering and their applications to biomechanics and treatment. True, while we have developed much high technology in orthodontics, it is more important to understand the principles behind both new and older appliances. Beginning with our graduate students, clinicians should be trained to think creatively using scientific biomechanical concepts based on modern physics. This is what we clinicians must do every day when we treat our patients.

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Apr 6, 2017 | Posted by in Orthodontics | Comments Off on Physics and clinical orthodontics: 100 years ago and today
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