Lingual Orthodontics: A Status Report, Part 2: Research and Development
In Part 1 of this series (JCO, April 1982), a brief overview of lingual orthodontics was presented. In this installment, we would like to share some of the details in the research and development of an edgewise lingual appliance and to provide an historical perspective.
Lingual History
While the various current bonded lingual appliances are a direct result of recent bonding technology, lingual mechanics is nothing new.1 Reference to a "lingual removable arch" was published in 1889 by John Farrar. In 1918, Dr. John Mershon published a paper entitled "The Removable Lingual Arch as an Appliance for the Treatment of Malocclusion of the Teeth". In 1922, at the first annual meeting of the newly formed Southern Society of Orthodontists, Mershon's presentation on labial and lingual arches with finger springs was reported as being a highlight of the meeting. In March 1942, at a PanAmerican Congress in New Orleans, Dr. Oren Oliver gave a clinic on a labiolingual appliance. In the mid-'50s, Dr. William Wilson demonstrated a labio-loop-lingual appliance that was a forerunner of the Wilson modular appliance system.
Lingually placed arches of many types, as well as lingual cleats and buttons, have been used for many years. However, their use has been as a supplement to labial mechanics, with no cosmetic incentive. Credit for the developments in the current "invisible braces" belongs to many. While this series presents the work of the authors, Ormco, and the Lingual Task Force, others deserve recognition for their creativity, ingenuity, perseverance, and pioneer spirit. Dr. Kinya Fujita's work on his "mushroom arch" lingual approach shows great promise, and his published findings have undoubtedly done much to stimulate the work of others.2,3 Dr. Vincent Kelly has contributed to our knowledge and provided yet another approach to lingual mechanotherapy.4 Dr. Stephen Paige, using conventional light wire brackets on the lingual, has shown that there are many ways to achieve excellent lingual results.5 A great deal of knowledge resulted from Dr. A.J. Wildman's early work on a totally lingual base arch and locking springs. Several orthodontists have struggled with conventional brackets placed lingually, including Dr. Michael Diamond.
Current Development
Current development of lingual orthodontics began in earnest by 1975, when it became apparent that bonding of brackets was a viable procedure, and that "esthetic" plastic brackets were a compromise. The author (Kurz), having an exclusively adult practice with many patients who were entertainers, fashion models, and other cosmetically conscious professionals, felt a need for a truly esthetic mode of orthodontic care delivery. Working in conjunction with engineers at Ormco, Kurz initiated designs and studies on a near-conventional edgewise approach to lingual mechanics.
The initial criteria were:
1. to ultimately offer the same degree of control as is obtainable with conventional fixed appliances,
2. to develop a smooth, low-profile appliance with minimal interference with soft tissue, for patient comfort, and
3. to develop a lingual appliance with the least deviation from familiar, well-established labial edgewise principles, if possible with a straight-wire approach.
Early designs of a bracket with an incisal/occlusal slot were discarded, since they would increase the bracket profile and compromise tipping control. Siamese bracket design was also discarded in favor of reduced mesiodistal bracket width, greater patient comfort afforded by a single width bracket, and the belief that some of the benefits of a twin bracket design, such as rotational control, could be compensated for through the use of the newer, more resilient wires.
With the basic bracket design and the desired criteria in mind, it was necessary to determine if lingual tooth anatomy and intertooth relationships were amenable to a lingual edgewise straight-wire approach. To confirm the feasibility of such an approach, topographical contour maps of lingual anatomy were made from models of several finished orthodontic cases (Fig. 1). It became apparent that, with the exception of short clinical crowns, lingual morphology presented no great obstacles to establishing a straight archwire plane in both maxillary and mandibular arches. It was also observed that this plane was parallel to the occlusal plane in most cases.
Next came the task of converting the desired bracket designs to configurations that would fit within the projected archwire plane, and determining the necessary bracket torques, thicknesses, and angulations. It appeared, initially, that the most direct method to accomplish these determinations was to measure the lingual surfaces of "ideal" occlusions in a study similar to that conducted by Andrews on the labial. However, the method selected for the measurement of lingual bracket torques and thicknesses was to relate the lingual determinants to labial tooth anatomy. This was done to establish a data base from which simple mathematical computations could produce a lingual appliance of any selection of torques relative to established labial appliances.
To accomplish this study, about 400 teeth closely resembling the anatomical form published by Wheeler6 were separated from study models, sectioned sagittally, mounted, and projected onto a screen, using an optical comparator a 10x magnification. Tracings of the labial and lingual profiles were made, and a line was drawn through the "LA-point" (the long axis midpoint of Andrews), representing the plane of the archwire, to define labial torque (Fig. 2). Lingual equivalent torque values were then calculated, studied statistically, and reduced to a set of average lingual torques.
Similar studies were conducted to define lingual pad profile and contours, lingual molar bracket torques, rotations, base curvatures, and in-out relationships. Bracket angulation determinations were made in a similar fashion, with lingual bracket tip being related to the labial. Rudimentary cusps or enlarged cingulae on maxillary incisors add greater variance to the already highly variable lingual morphology. It was found necessary to reduce rudimentary cusps to permit adaptation of the bonding bases and to pevent significant changes in torque.
From this preliminary data, recognizing the limitations of the relatively small data base, high standard deviations, and greater torque changes that result from minor variations in height placement on the sloping lingual surface, initial brackets were fabricated.
Initial Brackets
An .018" slot size was selected for conservation of incisal-ingival bracket dimension and for compatibility with existing archwires. Brackets were machined, using a hardened stainless steel alloy to further conserve on the over-all bracket dimensions. However, in spite of these efforts to minimize the incisal-gingival bracket dimension, it was apparent that, with the physical constraints of a durable bracket and the limitations in bracket placement, avoidance of occlusion on the maxillary incisor brackets was virtually impossible for the majority of malocclusions. It was therefore decided that the incisal wing of the maxillary incisor brackets would incorporate a bite plane. The bite plane was to serve the dual purpose of assisting in opening deep bites and redirecting the forces of occlusion to prevent shearing of the bond (Fig. 3).
At this point in the appliance development, one of the greatest clinical problems was the difficulty in ligating the brackets. Patient acceptance appeared to be I better than expected, with tongue irritation and speech problems at a minimum. Starting in late 1979, all cases were bonded indirectly, and bracket retention was at least 90%. Initial cases were entirely lingual, including the second molars (Fig. 4). By December 1979, 40 patients were in treatment with this approach.
Later Modifications
During 1979 and 1980, a number of modifications were made. With difficulties in maintaining control of the posterior segments and the encroachment of the second molar tube on tongue space, it was decided to produce a terminal tube for the first molars and to treat the posterior segments with conventional buccal segmental mechanics. Lingual brackets were placed on bi-cuspids when possible, and the lingual archwire was terminated at the first molars.
Once the transition was made from a continuous lingual arch to a combined lingual and buccal segmental approach, a number of options developed. For example, lower bicuspids, which frequently present extremely short lingual crown length, could be bypassed on the lingual in favor of a buccal segment, bicuspids through molars. This presents no great esthetic compromise on most patients (Fig. 5).
Modification was made in the bite plane on the maxillary cuspids, from a flat plane to a bi-beveled plane, in order to minimize bracket-cuspid interference in the final Class I cuspid relationship (Fig. 6). Bracket torques were increased as additional laboratory and clinical data became available. In 1979, a series of specially contoured lingual bonding bases was produced, following a topographical mapping study of lingual crown morphology (Fig. 7).
By the spring of 1981, 110 cases were in treatment with a number of the earlier cases reaching completion. It was apparent that the earlier, more subjective means of determining bracket height and angulation in the indiect setup were not sufficiently accurate. A number of modifications in the laboratory procedures were implemented. These will be described in detail in the next article in this series.
The Lingual Task Force was established in December 1980 to provide additional input on design considerations and to expand the treatment modalities. Ball hooks were added to all lingual brackets at this time. Ball hooks, while aiding greatly in placing elastic ligatures and elastics (Fig. 8), can cause gingival hyperplasia.
By September 1982, 2865 patients were undergoing lingual orthodontic treatment by over 650 specialists evaluating this mode of therapy. Only a few cases have been completed, and these with earlier appliance designs. Of the 2865 cases, 2154 have been in treatment for less than six months, 573 are six to twelve months along, and the remaining 138 cases have been in lingual treatment for over a year. By the first part of 1983, it is expected that some 50 cases will be nearing completion.
Conclusion
A great deal has been learned and much remains to be done. Details of case selection, appliance pros and cons, the diagnosis and treatment planning of the lingual case, and several case histories will be presented in the remaining articles in this series.
Research by Ormco and by several universities is ongoing, to develop and refine our knowledge in:
1. Etiologic factors and the intensity and duration of soft tissue changes.
2. Lingual versus labial tooth morphology.
3. Patient personality profiles.
4. Influences of bite opening.
5. Speech changes.
6. Labial versus lingual differential force distribution.
7. Influences on TMJ.
8. Design of instruments and auxiliary devices. 
