Crown tip and the vertical position of the teeth in relation to the lips are important elements of the esthetic results achieved by orthodontic treatment.1,2 Crown angulation, in turn, depends on proper placement of the brackets on the crowns from the beginning of treatment.
Tip is the angle between the occlusal plane and the crown long axis. When tip was first built into the bracket body by manufacturers, the slot was milled at an angle, making it difficult to place the bracket accurately in relationship to the occlusal plane and the crown long axis. With the rhomboidal bracket configuration, developed in 1983, the mesial and distal bracket wings could be used to align the bracket parallel to the long axis. In 1991, the trapezoidal Elite bracket related the mesial and distal bracket contours to the tip of the root.
Now a new bracket placement method, the Crown System (patent pending), has been developed for use with both Time and Crown Line brackets. This system is based on the prin-ciple of congruent surfaces--in other words, surfaces that have the same geometric form, but different sizes. For example, if you try to center a smaller coin on a larger one (Fig. 1), it is easy to recognize with the naked eye when the coins are off-center. Even if a segment of the larger coin is missing, the principle of congruent surfaces still works. The same concept can be applied to the relationship between a bracket and the crown of a tooth.
Analysis of Crown Forms
To determine the correct mean crown forms, intact teeth were photographed from thebuccal and the lingual and then enlarged 4.5times. The mesial and distal crown secants weredrawn for each tooth, starting at the intersectionbetween the incisal edge and the distal or mesialcrown crest, and ending at the cemento-enameljunction (Fig. 2). Since the cemento-enameljunction cannot be seen in a patient with healthygingivae or established with a probe, its locationwas estimated. The crown long axis and theocclusal plane were also constructed for eachtooth.
The start and endpoints of the secants weredigitized, and the angles formed by these lineswith the occlusal plane were calculated with agraphic software program, AutoCAD 12. Themeans, medians, and standard deviations of theangles were calculated using SPSS for Windows.
To test the accuracy and reproducibility ofthe estimated cemento-enamel junction locationsof the secant endpoints, a second group of teethwas evaluated. In this sample, the secants, crownlong axes, and occlusal planes were drawn onphotographs of plaster casts (Fig. 3).
Analysis of variance and an independent-samplest-test were carried out to evaluate thedifferences among the means, with statistical significanceestablished at the .05 level. No significant difference could be found between the two measurement methods (Tables 1 and 2).
Positioning Brackets with the Crown System
In bonding to teeth with normal crown forms, the brackets are positioned so they are parallel to the imaginary mesial and distal secants (Fig. 6). With incisor brackets, the incisal edge, which is normally parallel to the occlusal plane, can be used as another reference. On the cuspid bracket, the tip of the bracket base points toward the cusp tip for further assistance in alignment.
The vertical bracket positions should correspond to the LA points of Andrews.3 Our usual procedure, however, is to start by placing the mandibular second bicuspid bracket as close as possible to the gingival margin. All other brackets are then bonded at the same distance from the incisal edges. The same approach is used in the maxillary arch.
Although human crown forms have common anatomical characteristics, they still display a great deal of individual variation. A major advantage of the Crown System is the early recognition of abnormal crown forms. If either secant or the incisal edge is not parallel to the contours of the bracket base, it is easy to detect which crown edge deviates from the norm or has been altered--for instance, by abrasion. This helps determine which edges of the bracket should be parallel and aligned and which edges of the crown should eventually be built up or recontoured. Thus, the system can avoid the need for rebonding while allowing the brackets to achieve their preprogrammed tip and optimum esthetics.
Through the early diagnosis of crown-form discrepancies, the clinician can use the bracket positions to influence the course of treatment and the final result. In the theoretical example shown of two maxillary central incisors with abnormal crown forms (Fig. 7), the brackets can be placed in three different positions, all of which will have different effects on crown and root angulation. Bonding the brackets parallel to the mesial secants, as in Row A, might be appropriate if the incisal edges had been ground down or damaged by trauma, or in case of a tooth-size discrepancy between the maxillary and mandibular arches. Recontouring a central incisor that was already too small, as shown in Row B, would produce an even greater size discrepancy. If the incisors were wide enough, however, the approach of Row B would be preferable. The Crown System helps the clinician make the right decision at the beginning of treatment.
A male patient age 9 years, 8 months, presented with a Class I malocclusion, crowding in both arches, a midline deviation, and the maxillary right first permanent molar in crossbite (Fig. 8).