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INVITED SUBMISSION
Year : 2018  |  Volume : 52  |  Issue : 6  |  Page : 83-92

Engineers' contribution to the development of orthodontics during its formative years (a historical review)


Former Professor and Head, Department of Orthodontics, SDM College of Dental Sciences, Dharwad, Karnataka, India

Date of Submission12-Sep-2018
Date of Acceptance20-Sep-2018
Date of Web Publication07-Dec-2018

Correspondence Address:
Dr. Vijay P Jayade
“Aditya”, Bhavani Nagar, Keshwapur, Hubballi - 580 023, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jios.jios_178_18

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  Abstract 


The contribution of numerous engineers to the field of orthodontics, when it was in its nascent stage, has not been well documented. Without taking into account their numerous inputs and the devices and materials which they developed, the story of the evolution of orthodontics would be incomplete. The present article aims at filling this lacuna in the documented history of orthodontics.

Keywords: Dental engineering, engineer and dentistry, historical review


How to cite this article:
Jayade VP. Engineers' contribution to the development of orthodontics during its formative years (a historical review). J Indian Orthod Soc 2018;52, Suppl S2:83-92

How to cite this URL:
Jayade VP. Engineers' contribution to the development of orthodontics during its formative years (a historical review). J Indian Orthod Soc [serial online] 2018 [cited 2018 Dec 16];52, Suppl S2:83-92. Available from: http://www.jios.in/text.asp?2018/52/6/83/247053




  Introduction Top


“The longer we are able to look back, the farther we can leap ahead.” This is one of those adages which have not lost their significance in spite of repeated usage. Some aspects of the history of orthodontics have been well documented.[1] We are aware of the immense struggle the pioneer orthodontists had to put up when they separated “Orthodontia” from mainstream dentistry and developed it into a specialty. In general, we also have some idea about the inputs during the early years from histologists (some of them were practicing orthodontists) who developed our understanding of the biological aspect of orthodontic practice. Few well-known names in this respect are Sandstedt (1905), Oppenheim (1911), Orban (1927), Ketcham (1929), Gottlieb (1931), and Schwarz (1932). However, very little is known about the engineers' contribution during the infancy of orthodontia. These engineers collaborated with the orthodontists in enhancing the efficiency of treatment by developing new gadgets and better materials for clinical use. Further, they emphasized that a theoretical understanding of orthodontic mechanics would help in turning the empirical orthodontic practice into a scientific specialty. The present article is aimed at reviewing some of the writings as were reported until 1930 in the older issues of various journals, particularly the International Journal of Orthodontia, International Journal of Orthodontia and Oral Surgery, and International Journal of Orthodontia, Oral Surgery, and Radiology. As the readers may be aware, these were the previous names under which the present American Journal of Orthodontics and Orthopedics was published during early successive periods of time.

Three major areas to which engineers contributed were as follows:

  1. The so-called “dental engineering” which attempted surveying the occlusion as also determining the correct arch form for a given case
  2. Scientific analysis of mechanics involved in orthodontic tooth movement
  3. Study of the then existing materials used in orthodontic practice and developing better materials.



  Dental Engineering Top


An engineer by name Mr. Rudolph L. Hanau [Figure 1] made a major contribution in developing what he termed “dental engineering.” He called himself a “consulting dental engineer” and prided in “solving such problems as other men in my profession did not care to attempt.” He defined Dental Engineering as “a branch of theoretical engineering that has allied itself with practical orthodontia.”[2] He wrote nine articles in the International Journal of Orthodontia-some outlining the scope and application of dental engineering to orthodontic practice, some defending it against the considerable criticism that this new idea met with, and some introducing the basics of applied engineering to the orthodontic profession. He also lectured on “dental engineering” in the orthodontists' meetings.
Figure 1: Rudolph L. Hanau

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The concept of dental engineering emerged when Hanau was consulted by an orthodontist Dr. F. L. Stanton [Figure 2] to design a dental surveying apparatus for mapping the occlusion in three dimensions (Stanton, incidentally, preferred the term “orthodontic engineering”[3]). Stanton felt the need for occlusal mapping because he thought that the description of occlusion as was proposed by Dr. Angle was unsatisfactory. Angle's description was based on the reference of “line of occlusion” which, Angle himself had admitted, was rather imprecise. Stanton quoted from Angle's book: “It is difficult to determine exactly what the form of this line should be in each given case.” Consequently, describing the positions of individual teeth as buccal/lingual, mesial/distal, superior/inferior, etc., about the line of occlusion, as advised by Angle, was not an accurate description. Stanton contented (and Hanau supported his view) that a three dimensional description of teeth positions with respect to three axes-two horizontal and one vertical, all at right angles to one another would be more precise. Bodily translation parallel to an axis, rotation around an axis or a combination of the two could also be specified more accurately. Hence, he embarked on this project of developing a method for “mapping the occlusion.”
Figure 2: Frederick L. Stanton

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The Dental surveying apparatus [Figure 3] that was made by Hanau could transfer, from an accurately made plaster model, all the points of interest located on the teeth, gums and palate onto the drawing paper. It employed the technique of “orthographic projections” in which the three dimensions of any object are projected separately using straight vertical parallel lines [Figure 4]. The occlusal view and the buccal view of both upper and lower arches were projected separately [Figure 5]. Stanton was convinced that from the data derived using the surveying device, the appropriate form of dental arches and their correct occlusion in three dimensions could be predetermined by mathematical calculations. He felt that the teeth would then “perform the functions or the mutual mechanical relations of the anthropologist.”
Figure 3: Surveying instrument

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Figure 4: Orthographic projections

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Figure 5: Occlusal and buccal “mappings” of a patient's dentition

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Apart from Angle's line of occlusion, the other popular method of deriving the appropriate arch form prevalent then was (Bonwill) Hawley arch form that was proposed in 1905.[4] This method also received attention from the protagonists of dental engineering. Hanau criticized it as irrational and unscientific and was quite severe in his condemnation. He enumerated several flaws in Hawley's method, the most serious being that the arch form was laid out only in two dimensions, whereas its third dimension, namely, the curve of Spee was ignored.[5] Second, the shape of arch forms developed by this method for different individuals was the same-varying only in size [Figure 6] which was illogical. The practice of using the dimensions of upper central incisor, lateral incisor, and cuspid as the radius for drawing a circle to form the anterior arc was absolutely erroneous in most cases. Further, arranging the posterior teeth on straight lines as in Hawley method was meaningless because they also should be arranged on a curvature. He contended that investigating the arch form was essentially an engineering problem and required an understanding of descriptive geometry, mechanics, and kinematics. “The dental arch form is a function of the tooth measurements (size and shape), of the relation of the teeth in each jaw as well as in opposite jaws, and of the kinematic and mechanical requirements.” Hence a new method based on engineering principles was promoted.
Figure 6: Hawley's arch forms

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Hanau used mathematical curves [Figure 7] to determine the most suitable arch form for the individual. He proposed a method that took into consideration the mesiodistal diameters of all the teeth (instead of only the upper anteriors as in the Hawley's method), overbite, buccolingual diameter of the posterior teeth as related to their buccolingual relations and mesio-distal relations of the two arches for predetermining their shapes.[5] In his method, the upper and lower dental arches consisted of five curves each – the anterior curve, two canine curves (which also included premolars) and two molar curves [Figure 8], (The upper and lower anterior and canine curves included slightly differing tooth materials in the respective arches). The steps were: mapping the occlusion, schematic representation of teeth and deriving the arch form [Figure 9]. The positions of teeth so derived on the drawing board indicated the amount and direction of tooth movements needed [Figure 10]. By superimposing the map of malocclusion over the corrected arch on transparent sheets, the least amount of tooth movement in both the horizontal and vertical planes could be planned.
Figure 7: Mathematical curves employed by Hanau

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Figure 8: Hanau's arch form

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Figure 9: From mapping the occlusion to derivation of the patient's arch form

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Figure 10: Amounts of tooth movements needed

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Stanton approached the Engineering Department of Columbia (University?) to have a second opinion on the accuracy of the method devised by Hanau. They endorsed the work. The mathematics involved, however, was too complicated and therefore impracticable for an orthodontist. Hence a much simpler method of predetermining the occlusion mechanically (in place of a method of trial by mathematical curves), was devised by Mr. Gilbert D. Fish. The teeth shapes and positions were registered on a paper 10 times magnified with the help of an instrument called pantograph. This enlarged view was named an occlusograph [Figure 11]. A new method for “kinematic linkage” was developed, which semi-automatically could generate the appropriate arch forms of any individual in a manner which would harmonize the two arches[6] [Figure 12]. The composite map of malocclusion and corrected occlusion were reduced to the natural scale by using the pantograph again.
Figure 11: “Occlusograph” of a patient

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Figure 12: Arch form derived from the occlusograph by “kinetic linkage”

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These methods of deriving an arch form, into which all malposed teeth needed to be adjusted to relieve malocclusion, involved variable amounts of arch expansion. Such expansion, in the light of present knowledge, leads to instability of correction. However, arch expansion was a well-accepted treatment method during those days.

Both Stanton and Hanau spoke in different Orthodontists' meetings in America. In a meeting of the American Association of Orthodontists at Pittsburg, there was a prolonged discussion on the paper read by Hanau.[5] Hawley lauded the efforts of Hanau. However, he raised the doubt as to how the arch form could be derived in a case in which the treatment was started before 12 years when all the permanent teeth had not erupted. Lischer enquired whether arch forms for deciduous, mixed and permanent dentition could be predetermined. He then pointed out that the dentition did not have a fixed relationship to the skull, but it kept on changing due to growth. He questioned, in the view of such tremendous change due to growth, what was the use of predetermining an arch for a patient of seven or 8 years who will be under treatment and observation until twelve or fourteen years? Others queried what happened to the form when tooth material on the two sides was unequal because of malformed teeth like peg-shaped lateral incisors, or when lower anterior tooth material was in excess in relation to the upper anterior. Hanau put forward his replies based on his understanding.

Stanton also gave a talk at the meeting of European Society of Orthodontia.[7] It generated a lot of interest. Dr. Schroeder praised it as the greatest development in orthodontics over many years. Some others felt that the occlusal plane that Stanton had selected as a horizontal reference plane was not a fixed one and hence unsuitable for comparing changes in occlusion. Others wondered about the accuracy of the center of mouth that Stanton had proposed for superimposing map of malocclusion on corrected occlusion.

Martin Devy, in his Editorial of the International Journal of Orthodontics, strongly defended the method of arch form predetermination as developed by Stanton, Hanau, and Fish.[8] He stated that the other method of judging the arch width in the molar area from the mesiodistal diameters of upper incisors was based on averages. It neither helped in average malocclusion nor did it apply to severe malocclusions. On the other hand, the dental engineering method was devised for every individual and therefore universally applicable.

However, the dental surveying method and its application in deriving individual arch form did not receive universal acceptance. Williams,[9] an orthodontist, proposed that the six upper anterior teeth form an arc of a circle whose center is midway between the two first molars and that the normal arch form is such that the ratio of intermolar distance to inter cuspid distance is 14:9. He based his observation on his studies of American subjects' dentitions as also of the collection of skulls in museums. In another article,[10] he criticized dental engineering stating that entities like dental arch form with infinite number of variations are governed by nature and therefore are not amenable to engineering concepts based on strict mathematical laws. “Dental Engineering is an unfortunate term to use in describing methods of determining arch form. It imposes upon the profession strict mathematical formulae impossible to follow, and contrary to nature's laws.” However, the validity of the ratio proposed by Williams and its usefulness was questioned by Hanau.[11] He rebutted the criticism of Williams point by point.[12]

Hellman[13] also criticized the method of developing an arch form based on his anthropological study. He contended that there is a definite relationship between the size of the teeth and the form of the dental arch. Further, using the dimensions of the teeth as a basis, a formula, diagram or plan of that form can be obtained in case of the malocclusion. Hanau[14],[15] replied saying that the shape of the teeth is more important than the size of the teeth in determining the arch form. Moreover, apart from these two factors, the kinematic, physiologic, mechanical, and other functions also play a role in establishing or maintaining an arch form. He explained the variations in the arch forms of apes and other animals by referring to the varied functions which the teeth play in these animals, for example, as weapons. Various shapes such as elliptical, parabolic, horseshoe shaped, and pear-shaped could all be explained using mathematical (geometric) curves.

Unfortunately, some protagonists (perhaps egged by the manufacturers of surveying devices) started overemphasizing its use in orthodontic practice. This prompted the following criticism from Hellman[16] “No case of malocclusion of the teeth could be treated before it was thoroughly 'surveyed' by a competent and skilled engineer. Orthodontic surveying soon became quite fashionable. Almost every orthodontist became an engineer. It was really becoming so serious at one time as to give rise to the idea that to become a successful orthodontist one must take a course in engineering. Time, however, proved that this was not so. The engineers employed in this scheme have gone back to engineering, and the orthodontists who adopted it, though still surveying, contributed nothing to the advancement of orthodontia. You cannot become an engineer by knowing a few orthodontic tricks, and you cannot become an orthodontist by knowing a few engineering tricks.”

It is a mystery to me why a relatively more scientific method as developed by Hanau and Fish got abandoned within few years, whereas Hawley's empirical method that is purely based on assumptions continued to be followed for many years. Could it be because the former was introduced by persons from another specialty which was totally unfamiliar to orthodontists, while the latter was developed by one of their own brethren? One possible reason could have been that the arch form derived by it resulted into a significant amount of arch expansion in crowded dentitions, which must have entailed serious relapse. However, once the importance of maintaining the inter-canine arch width was realized, the engineers could have been approached again to suitably modify the method for its acceptance. Maybe, the extra work involved in using the surveying apparatus and occlusograph put off some individuals. The scathing criticism of influential leaders like Hellman could also have dissuaded others from employing the method.

Certain ideas from Dental engineering have resurfaced in relatively recent times. Occlusogram proposed by Burstone and Marcotte[17] is a two-dimensional projection of occlusion on the paper. Orthodontists are no longer averse to associate with engineers. Technologies used and developed by engineers have entered Orthodontics in a big way. Finite element method relates teeth and other structures to the three axes x, y, and z. Attempts to devise treatment plans based on three dimensional data obtained from digital imaging, and customizing the appliances to attain the results are being employed routinely. Dental engineering has finally arrived!


  Introducing Principles of Mechanics into Orthodontic Thinking Top


Both Hanau[18] and Fish[6] tried to convince the orthodontic community that they should be aware of the basic principles of mechanics since their practice involved movements of teeth. Hanau declared “The universal application of dental engineering principles in the practice of orthodontia will doubtless bring a new era in this science.” Fish urged the orthodontists to inform themselves of rudiments of mechanics “another science, older than your own and further developed, holds a greater store of information which you require for your further progress.” They defined in their writings terms such as kinematics, force (its direction, magnitude, resultant of forces), force couple, resistance, velocity, work, energy, power, moment, equilibrium, translation, and rotation [Figure 13].
Figure 13: Hanau's depiction of tooth intrusion and controlled movement using a force couple

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Hanau[18] elaborated on the practical application of the knowledge of engineering principles by calculating the appropriate force for assumed optimum movement. He argued that a given force would cause a certain amount of movement in a certain time. If it is observed that the movement is too great for the duration of treatment (i.e., too rapid), causing such ill effects as irritation of gums or loosening of the tooth, the correct amount of force for the right amount of movement could be precisely calculated. He admitted that the assumption of tooth movement being directly proportionate to the pressure applied, though mathematically rational, may not be correct because of biological considerations. However, he maintained that after many observations of carefully conducted experiments, practical constants in the equations could be determined.

Fish[6] introduced orthodontists to the application of laws of Newton in analyzing the functioning of appliances. He opined “since the relative motions of the parts of an appliance attached to the teeth are exceedingly slow, the appliance may be considered in equilibrium (therefore) the analysis of the forces involved falls under the head of statics instead of kinetics.” He even mentioned the center of resistance of the set of teeth. He further familiarized the orthodontists with the internal stresses and elastic deformations in the orthodontic wires.

Dewey, in another editorial[19] strongly recommended the introduction of knowledge of mechanics in the orthodontic practice. “We believe it is time for orthodontists to realize that they must have a greater knowledge of physics and mechanics that they have had in the past. It would be time well spent for the majority of the orthodontic profession to take up a study of engineering principles and apply them to their work to a greater extent than they have heretofore.”

It is a pity that the orthodontic profession took more than 30 years to accept the scientific terms, concepts, and analytical methods introduced by Hanau and fish. Was it plain apathy, or was it a distrust of the engineers? We do come across some writings of few orthodontists in the years 1925 and 1930 on the mechanical aspects of orthodontic treatment. Henry[20] from England discussed some of the mechanical principles of orthodontic appliances. McKeag[21] from Ireland analyzed orthodontic spring designs using some of the physical laws. However, their attempts appear somewhat amateurish if strict yardsticks are applied. It was only during the 50s when a qualified engineer by name Egan W. Drenker took to orthodontic studies, and another engineer by name Donald Haack introduced a short course in mechanics in the Nebraska Dental school as a part of Master's program in orthodontics that the profession started taking a serious interest in engineering principles.[22] From the 60s onwards, Burstone and many others relentlessly pursued the rigorous application of engineering concepts to transform orthodontics from an empirical practice to the exact science, thus vindicating Hanau, Fish, and Dewey.


  Development of New Materials Top


If dental engineering and bio-engineering were rather abstract ideas for an ordinary orthodontist, improvement in materials was an aspect that touched the day to day work of every orthodontist, and therefore aroused greater interest. The materials commonly used then were the precious metal alloys (used for making band materials, solders, and wires) and a base metal alloy popularly named German silver or Nickel silver in the form of wires. Rubber (in the form of elastics), cements and brass (in the form of wires) were the minor materials used. The research was mainly directed to find substitutes for precious metal alloys and German silver.

The precious metal alloys were made from silver, gold, platinum, and iridium, and hence quite expensive. The cheaper base metal alloy German silver was a copper alloy with nickel, and it often contained zinc also. (The usual formulation was 60% copper, 20% nickel, and 20% zinc. It was sometimes considered a subset of brass.[23]) Although called Nickel silver for its silvery appearance, it contained no elemental silver unless plated. When used for making orthodontic appliances, it was coated with gold. However, the plating often peeled off in the mouth and the oxides leached on to the teeth causing their tarnishing. The black stains were quite unsightly and were difficult to remove. However, one advantage claimed for this alloy was its anti-septic (anti-caries) properties. The badly discolored teeth showed the very little occurrence of caries.[24]

The appliances used during the first two decades of the 20th century were the expansion arch (“E arch”), removable or semi-fixed labial and lingual arches with soldered springs for individual tooth movements, the pin and tube appliance and the ribbon arch appliance (Calvin case and Robinson introduced their own appliances, but they were not widely used). The qualities which orthodontists looked for in orthodontic wires for making appliances were elasticity (after doing the required tempering), esthetic appearance, cost and ease of soldering. German silver satisfied the first three of these but could only be soldered using soft (low melting) solders, since it lost its properties at high temperatures. Hence, the soldered joints were weak, and the springs often came out. Mershon insisted that only high-quality spring gold be used for his design of lingual arches.[25]

One trend, which had started setting in, was the use of smaller dimension wires to exert lighter forces on the teeth. While the expansion arches were made of as heavy wires as 14G (0.083” or 2.1 mm) Robinson[26] adopted the 20G wires (.035” or 0.89 mm) in the year 2011, and within few years went further down to 0.022” or 0.020” size– a reduction to almost one-fourth size (Of course, he used these wires as archwires and not for expansion). Others like Suggett[27] wholeheartedly supported the use of small dimension wires. While certain alloys like Platinum-iridium could not be drawn into such finer sizes, gold alloys drawn in smaller dimension wires lacked strength. Calvin Case[28] also advocated the use of small dimension wires– the lowest dimension he employed was 0.016” or 26G. He mentioned drawing small dimension Nickel silver wires cold from larger dimension wires (which were stored in contact with ice till they were taken out for use). He found them highly elastic and resilient. Apparently, these wires did not receive wider acceptance from the profession.

The Research Foundation of American (then called National) Dental Association launched a project to find alternatives for precious metal alloys, and a metallurgist by Fahrenwald headed the project. He published an exhaustive account of the research in 1916.[29] He enumerated following desirable properties for the alternatives: (1) The alloy should have a high melting point (above 1200°C). (2) It should be chemically stable and able to withstand soldering temperature without oxidizing. (3) It should be sufficiently strong, yet pliable enough to be given the desired shape. (4) Its coefficient of expansion should be low so that the desired dimension is obtained even when manipulated at a high temperature above 1000°C. (5) It should unite readily with gold, silver, and their solders. (6) Its cost of production should be low.

After experimenting with 11 elements, selected on the basis of the periodic law of atomic weights, singly and in combinations to form alloys, it was found that tungsten and molybdenum met four of the requisites. The drawback noticed was that they oxidize at red heat and that they do not solder with gold and its alloys easily. Their brittleness was also a concern.

The first two of these problems were solved by coating them with gold in a molten bath. For overcoming brittleness, a meticulous procedure of heat treatment and forging was developed. Fahrenwald claimed that the gold coated tungsten and molybdenum were far superior to platinum or its alloys.

Pollock[30] mentioned that Dr. Price and Mr. Fahrenwald were the first to advocate the use of tungsten and molybdenum for constructing orthodontic appliances. Pollock and Robinson[31] both used tungsten wires in their practice and reported their high degree of satisfaction. Special pliers had to be developed for bending the wires since they were very brittle [Figure 14]. Robinson was the first orthodontist to introduce loops in the archwire to increase its flexibility [Figure 15].
Figure 14: Pliers developed for manipulating molybdenum and tungsten wires

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Figure 15: Loops in the arch wires introduced by Robinson

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While the use of tungsten as an archwire material was subsequently given up, molybdenum in its alloy form has become an integral part of present orthodontic practice.

Williams,[32],[33] another metallurgist, presented his research on spring wires and orthodontia alloys in two Annual meetings of the American Society of Orthodontists (1922 and 1924). He had become interested in the orthodontic wires because he had been an orthodontic patient himself when he was young. His work was almost totally restricted to precious metal alloys. His above-mentioned talks were like a mini crash course in metallurgy. In his two papers, he explained the terms (tensile) strength, elasticity, elastic limit, yield point, springiness, hardness, etc., used in engineering to describe the physical properties of alloys. He elaborated on the methods of forming alloys, drawing the wires and the instruments developed to test the various properties of the wires. He also discussed the factors which cause defects in the archwire and some clinical tips to improve the wire properties. Many of the delegates attending the meetings commended Mr. Williams on his presentation. One of them pointed out the difference in the usage of the term “torsion” in engineering and orthodontia– in the former it meant bending while in the later it meant twisting.

Dr. Mershon enquired the effect of oral secretions on the commonly used alloys. To investigate this aspect, Williams took help from Dr. Ellis to place sections of wires in patients' mouths (adjacent to the lingual arches) for periods ranging from 2 to 8 months, then studied them under the microscope and reported in his second paper that the wires made from precious metal alloys did not get affected by oral secretions. He also studied the soldered joints under the microscope to determine if any particular composition of solder gave stronger joints. His findings were that higher the karat (gold content) in the solder, better was the joint. Similarly, the best joint was obtained with a minimum of heat.

Subsequently, Williams was invited again to the annual meeting of the American Association of Orthodontists in 1928. He strongly advocated[34] the use of electrical appliances for welding, soldering, and annealing, since the heat produced would be under absolute control. He emphasized close cooperation between the orthodontist and the metallurgist.

Brumfield[35] read a paper in the meeting of the American Society of Orthodontists in 1930 on “Structural features related to Orthodontic materials and appliances.” He covered aspects such as the strength, flexibility, impact resistance, and fatigue failure of the materials. He discussed how a comparative table of wrought wires prepared to relate strength and flexibility to dimensional changes could be employed in designing appliances. He also gave suggestions regarding heat treatment and soldering. He then presented a quantitative analysis of some of the springs.


  Conclusion Top


Orthodontics today prides itself on its biomechanical nature of practice. While the biologic contributions in its development have been duly recognized for a long time, a similar acknowledgment of engineers' contribution does not appear to have been made. Although some of these contributions did not have a long-lasting impact, they are important from a historical perspective. Other contributions formed the basis for much of the later progress. The present article is an attempt to pay tribute to those forgotten individuals who sowed the seeds of mechanics and metallurgy in transforming orthodontics from an empirical practice into a thriving science.

Acknowledgment

The author wishes to gratefully acknowledge the help received from the Management, Principal, Librarian, and staff and P. G. students of the Department of Orthodontics and Dento-facial Orthopedics of the Rural Dental College, Pravara Nagar-Loni; and in particular from Dr. N. G. Toshniwal (H. O. D).

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15]



 

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Abstract
Introduction
Dental Engineering
Introducing Prin...
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