Bracket Placement with the Preadjusted Appliance - JCO-ONLINE.COM - Journal of Clinical Orthodontics
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Bracket Placement with the Preadjusted Appliance
VOLUME 29 : NUMBER 05 : PAGES (302-311) 1995
RICHARD P. McLAUGHLIN, DDS
JOHN C. BENNETT, LDS DOrth
With the standard edgewise appliance, brackets were usually placed by using a millimeter (Boone or Dougherty) gauge to measure
from the incisal or occlusal edge to the desired bracket position. These measurements worked, despite being the same for every
patient, because the orthodontist then added 1st-, 2nd-, and 3rd-order bends.
Andrews measured non-orthodontic normal models 1 to establish the tip, torque, and in-out values built into his Straight-Wire
Appliance. He found that the millimeter measurement system used with standard edgewise appliances would not allow consistent
reference points and hence a consistent expression of three-dimensional forces on each tooth (Fig. 1).
Andrews chose the center of the clinical crown as a horizontal reference point and the long axis of the clinical crown (the center of
the middle developmental lobe of each crown from central incisor to second bicuspid, or the buccal groove of the first or second
molar crown) as a vertical reference. 2 He recommended that the twin bracket wings be placed parallel to the vertical long axis,
with the center of the bracket slot on the center of the clinical crown. However, there are several ways in which bracket position
can deviate from the ideal:
1. Horizontal errors. Placing the bracket to the mesial or distal of the vertical long axis leads to undesirable tooth rotation (Fig. 2).
Such errors can be avoided by visualizing the vertical long axis--directly from the facial surface, or with a mouth mirror from the
incisal or occlusal aspect. Some orthodontists even draw a line on the tooth to indicate the correct vertical long axis.
2. Axial or paralleling errors. If the bracket wings are not parallel to the long axis, the result will be unwanted crown tipping (Fig. 3
). These errors can be avoided in the same way as horizontal errors.
3. Thickness errors. Leaving excess adhesive under a portion of the bracket base (Fig. 4) or failing to conform the base accurately
to the contour of the tooth can cause improper torque or rotation. This problem is overcome by expressing all excess adhesive
from beneath the bracket during placement and by more accurate contouring.
4. Vertical errors. Improper vertical placement can lead to extrusion or intrusion of teeth, as well as to torque and in-out errors (
Fig. 5).
All these errors are avoidable when dealing with fully erupted and anatomically normal teeth. As Andrews stated, the human eye is
quite accurate at bisecting and locating the center of a clearly visible object. However, in the following common clinical situations,
direct visualization is more difficult.
Gingival Concerns
1. Partially erupted teeth. Young patients frequently have partially erupted teeth on which it is difficult to locate the centers of the
clinical crowns (Fig. 6). Because the apparent clinical crown is foreshortened, the tendency is to place the bracket too incisally or
occlusally, especially with bicuspids and lower second molars.
2. Gingival inflammation. This also causes foreshortening of the clinical crown (Fig. 7), and the bracket again tends to be placed
too incisally or occlusally.
3. Teeth with palatally or lingually displaced roots. Gingival tissue covers a greater portion than normal of the clinical crowns of
such teeth, shortening the clinical crown (Fig. 8). The bracket would be placed too incisally or occlusally.
4. Teeth with facially displaced roots. This situation is often found with cuspids. The clinical crown appears longer, making the
bracket placement too gingival (Fig. 9).
Incisal or Occlusal Concerns
1. Incisal or occlusal crown fractures or tooth wear. With such teeth, the apparent clinical crown is shorter (Fig. 10). The problem
can be corrected by restoring the crown to its proper length or by estimating how long the crown was before the fracture or wear.
2. Crowns with long, tapered buccal cusps. Cuspids or bicuspids with long, tapered cusps often do not have adequate contact with
the opposing teeth. If the bracket is placed in the center of the clinical crown, the adjacent marginal ridges will not be aligned (Fig.
11). Improper placement can be avoided by selectively reducing the height of the cusp prior to bonding.
Crown Length Concerns
1. Disproportionately long clinical crowns. When a clinical crown is longer than the mean for that tooth, as is often seen with upper
central incisors, placing the bracket in the center of the clinical crown will lead to treatment errors. Esthetically, the crown will be
too long, and functionally, it will create an interference with the opposing dentition. The bracket must therefore be placed slightly
incisal to the center of the clinical crown. This adjustment may produce minor torque and thickness errors, but they can usually be
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corrected during the finishing stage with archwire bends.
2. Disproportionately short clinical crowns. When a clinical crown is shorter than the mean, as sometimes occurs with upper lateral
incisors (Fig. 12), placing the bracket in the center of the clinical crown will likewise result in esthetic and functional errors. The
crown will be too short and will be out of contact with the opposing dentition. This situation is corrected by placing the bracket
slightly gingival to the center of the clinical crown.
This article presents an investigation and discussion of a bracket placement technique that can produce more accurate vertical
bracket placement in the clinical situations shown above.
Methodology
Four studies were made to pinpoint the centers of the clinical crowns:
1. Anatomic crown height. This has been defined as "that part of dentin covered by enamel". 3 Two published sources were used-one with data from more than 200 dentitions, 4 the other with an unspecified number of teeth. 5 Mean measurements of anatomic
crown height from these two studies were divided by two to obtain the distances from the incisal or occlusal edges to the centers
of the anatomic crowns.
2. Clinical crown height. This is "that part of enamel visibly present in the oral cavity". 3 The mean clinical crown height was
determined from 120 pretreatment study casts selected as follows:
Fully erupted clinical crowns throughout the dentition
No apparent proportionately large or small teeth
No evidence of significant lingual or palatal displacement of crowns
No significant gingival inflammation
No excessive crown wear or crown fractures
The measurements were divided by two to obtain the distances from the incisal or occlusal edges to the centers of the clinical
crowns.
3. Evaluation of cases treated according to Andrews's Six Keys. 1 Several nonextraction cases prepared for the American Board of
Orthodontics or the Angle Society were selected by one of the authors. The final casts were duplicated, the centers of the clinical
crowns were marked (Fig. 13), and any deviation from a straight line was recorded.
4. Bracket height of clinical cases at debonding. Bracket heights were measured in cases that had settled into a good "Six Keys"
occlusion before debonding. Bracket positions had to allow for placement of straight archwires, without step bends, in both arches
for the case to be selected (Fig. 14).
Results
1. Anatomic crown height. The greatest difference between the two published studies was .4mm for the upper lateral incisor (Table
1). Because of the similarity of the values, we felt it unnecessary to examine any additional sources.
2. Clinical crown height. The distance from the incisal or occlusal edge to the center of the clinical crown was only about .5mm less
than the distance to the center of the anatomic crown for every tooth (Table 2). Therefore, these averages were rounded to the
nearest .5mm to produce a "Theoretical Bracket Placement Chart" (Table 3). Values at +.5mm and +1.0mm were added for larger
teeth, and values at -0.5mm and -1.0mm for smaller teeth.
3. Evaluation of cases treated according to Andrews's Six Keys. Marks at the centers of the clinical crowns consistently deviated
from a straight line at the upper bicuspids and second molars (Fig. 15) and at the lower cuspids and first molars (Fig. 16).
4. Bracket height of clinical cases at debonding. As with the "Six Keys" cases, the aligned brackets were not located in the centers
of the clinical crowns of all teeth. In the upper arch, the bracket heights of the bicuspids were consistently .5mm greater than
those found in the Theoretical Bracket Placement Chart, and those of the second molars were .5-1mm less. In the lower arch, the
bracket heights of the cuspids and first molars were consistently .5mm less than those found in the chart. The lower second molar
bracket heights were either identical to those of the lower first molars, or in some cases .5mm more gingival.
The minor variations found in studies 3 and 4 above were incorporated into the Theoretical Bracket Placement Chart to produce a
"Recommended Bracket Placement Chart" (Table 4).
Use of Bracket Placement Chart
1. Measure the clinical crown heights of as many fully erupted teeth as possible from the patient's study casts, using dividers and a
millimeter ruler.
2. Divide each measurement in half and round to the nearest .5mm to obtain the distance from the incisal or occlusal edge to the
center of the clinical crown.
3. Select the row in the Bracket Placement Chart that contains the greatest number of equal values. Measurements for teeth that
are disproportionately large or small will differ from those in the selected row, but will thereby be automatically adjusted.
4. Position each bracket initially by visualizing the vertical long axis of the clinical crown (or the buccal groove of the molar) as a
vertical reference and the estimated center of the clinical crown as a horizontal reference.
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5. Use a bracket placement gauge to adjust the bracket heights to the exact values from the chart. A light-cured adhesive is
recommended for direct bonding because of its unlimited positioning time.
Discussion
Studies 3 and 4 provided clinical information needed to adjust for vertical crown relationships at the end of successful treatment.
Although the Recommended Bracket Placement Chart is thus slightly different from the average measurements taken from the
clinical crowns, we do not believe it will produce any clinically significant changes in torque or in-out.
The Bracket Placement Chart eliminates errors caused by gingival variations because the measurements are made from the
occlusal or incisal edges. This alone is a major advantage, since the majority of errors in vertical bracket placement are the result
of inadequate visualization of the gingival half of the clinical crown.
The chart also includes an adjustment for disproportionately large or small teeth, thus avoiding occlusal interferences, lack of
occlusal contact, and esthetic problems. The only difficult situations that cannot be accounted for are crowns with long, tapered
facial cusps or with incisal or occlusal fractures or wear. In these situations, appropriate adjustments must be made to the
suggested values.
No bracket placement system can completely eliminate occlusal interferences with lower brackets. When this occurs in the molar
region, it is usually due to a lingually inclined crown that elevates the brackets on the buccal surface. The choices are to allow the
interference until the crown is uprighted, to eliminate the interference by placing an upper anterior bite plane, to position the
affected bracket more gingivally and place a step bend so that the tooth is not extruded, or to position all brackets in the lower
arch proportionally more gingivally.
When interference occurs in the incisor region, it is normally because of a deep overbite. The clinician's choices then are to leave
brackets off the lower incisors until the bite opens sufficiently, to allow the interference until bite opening is achieved, or to place
an anterior bite plane.
Conclusion
We have tested the bracket placement method described in this article on a variety of cases for eight months. We find that it
improves treatment efficiency during leveling and aligning, with fewer cases requiring bracket repositioning due to vertical
placement errors. It has also been helpful in repositioning brackets on patients who were bonded before the development of the
Bracket Placement Chart.
Figures
Fig. 1 Brackets placed 5mm from incisal edges of upper central incisors have different torque and thickness expressions on large
vs. small teeth.
Fig. 2 Horizontal bracket placement errors can be avoided with careful technique.
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Fig. 3 Axial or paralleling errors can be avoided with careful placement technique.
Fig. 4 Excess adhesive beneath bracket base can cause thickness and rotational errors.
Fig. 5 Improper vertical placement can lead to extrusion or intrusion and to torque and in-out errors.
Fig. 6 Partial eruption makes it difficult to visualize centers of clinical crowns.
Fig. 7 A. Typical patient with healthy gingivae. B. Same case with gingival inflammation in upper right quadrant, reducing
apparent length of clinical crowns.
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Fig. 8 Lingually displaced root (right) can make clinical crown appear shorter.
Fig. 9 Facially displaced root (right) can make clinical crown appear longer.
Fig. 10 Incisal crown fracture or tooth wear makes it difficult to visualize center of clinical crown.
Fig. 11 With long, tapered buccal cusp, if bracket is placed in center of clinical crown, adjacent marginal ridges will not be
properly aligned.
Fig. 12 Patient with disproportionately large upper central incisors and small upper lateral incisors.
Fig. 13 American Board of Orthodontics case with marks at centers of clinical crowns.
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Fig. 14 Typical case selected for measurement before debonding.
Fig. 15 In ABO and Angle Society cases evaluated according to Andrews's Six Keys, centers of clinical crowns deviated from
straight line at upper bicuspids (usually .5mm more occlusal) and second molars (usually .5-1mm more gingival).
Fig. 16 In ABO and Angle Society cases evaluated according to Andrews's Six Keys, centers of clinical crowns deviated from
straight line at lower cuspids and first molars (both usually .5mm more gingival).
Tables
Fig. 1
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Fig. 2
Fig. 3
Fig. 4
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References
1. Andrews, L.F.: The six keys to normal occlusion, Am. J. Orthod. 62:296-308, 1972.
2. Andrews, L.F.: Straight-Wire--The Concept and the Appliance, Wells Co., Los Angeles, 1989.
3. Kraus, B.S.; Jordan, R.E.; and Abrams, L.: Dental Anatomy and Occlusion, Williams & Wilkins Co., Baltimore, 1969.
4. Zweiner, R.J.: Boucher's Clinical Dental Terminology, 4th ed., Mosby-Year Book, St. Louis, 1994.
5. Woefel, J.B.: Dental Anatomy: Its Relevance to Dentistry, 4th ed., Lea & Febiger, Philadelphia, 1990.
Footnotes
1. Straight-Wire Appliance, Registered trademark of "A"-Company, 11436 Sorrento Valley Road, San Diego, CA 92121.
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