Variations in bracket placement in the preadjusted orthodontic appliance Nasib Balut, DDS, MS," Lewis Klapper, DMD, DScD, PhD, b James Sandrik, MS, PhD, = and Douglas Bowman, MS, PhD ' Me.rico City, Me.rico, am/Chicago, IlL This study was conducted to determine the accuracy of bracket placement with the direct bonded technique. Ten orthodontic faculty members bonded a preadjusted orthodontic appliance on models of five cases of malocclusion in a simulated clinical situation (mannequin). A total of 50 sets of models served as the population of the study. Photographs of the models were measured to determine vertical and angular discrepancies in position between adjacent bracket pairs from a constructed reference line. Variations are evaluated with respect to the classification of malocclusion, specific tooth type, and intra/inter operator differences. A mean of 0.34 mm for the vertical discrepancies and a mean of 5.54 ~ for the angular discrepancies are found in placement of the orthodontic brackets. (AMJ ORTHOD DENTOFACORTHOP 1992;102:62-7.)
T h e prcadjusted bracket system is the most widely used in orthodontic therapy today. The basic premise of the preadjusted system is that proper bracket position allows the teeth to be positioned with a straight wire into an ocelusal contact with excellent mesiodistal inclinations (tips) and excellent faciolingual inclination (torque). Most clinicians are aware that clinically the "straight wire" (preadjusted) appliance does not eliminate wire bending. Variations in tooth structure and malocclusions affect the final bracket positions. Variations from the straight wire appliance averages must be compensated by bends placed in the arch wires. The present study was designated to evaluate variations in placement in the vertical and angular bracket position from the ideal with a preadjusted orthodontic appliance ("A" Company, Johnson & Johnson, San Diego, Calif.). Variations in bracket position were evaluated with respect to the classification of maloeclusions, specific tooth type, and intra/inter operator differences. REVIEW OF THE LITERATURE Bracket placement and tooth structure
Originally Angle t taught that the best position of the band was where it fits better mechanically. Then, if possible, the bracket should be placed at the center of the labial surface of the tooth. Later, placement of 'Associate Professor, Universidad Intercontinental Mexico. bAssociate Professor, Loyola University of Chicago. 'Chairman of the Department of Dental Materials, Loyola Uniser~ity of Chicago. aAssc,ciate Professor of Biostatistics, Lo) ola University of Chicago. 8 / I / 27285
62
the anterior bands at the junction of the middle and the incisal thirds of the crown was recommended. 2 Ricketts 3 advocated the use of marginal ridges as guidelines for band and bracket vertical positioning. More recently, Andrews 47 introduced the bracketing technique of placing the straight guidelines of the bracket (vertical tie wings) parallel to the long axis of the clinical crown and then moving the bracket up or down until the middle of its slot base is at the same height as the midpoint of the clinical crown. Thurow 8 showed that two different vertical positions of a bracket on a tooth will cause two different buccolingual axial inclinations (torque). Meyer and Nelson 9 showed that an error of 3 mm vertically in bracket placement on a premolar can result in 15~ torque alteration and 0.04 mm alteration in the in/out adjustments. In orthodontics, what affects the design of the orthodontic appliances and their use is the inclination of the labial or buccal surface of the tooth crown to the long axis of either the entire tooth or the occlusal surface of the crown. Kraus, t~ Taylor, 2 and Dellinger tt found great variations existed in tooth structure that can affect treatment results. Improvements in bracket design were made incorporating tip, ~torque, ~2rotation, and differences in base thickness 47.13 beginning with Angle in 1928 and, more recently, with Andrews in 1970 who introduced the "straight wire concept" in preadjusted orthodontic appliance. Roth 14 in 1975 evaluated the preadjusted bracket system after he had used it for 5 years. In 1981 he
Volume 102
Nunzber I
Variations in bracket placement in preadjusted appliance
63
Fig. 2. Occlusal view of one =diagnostic set-up" showing occlusal contacts checked with articulating ribbon,
as a template for the final ideal tooth position desired for each case.
Fig. 1. Front view of one "diagnostic set-up" showing idea! angulation where teeth were positioned.
modified the appliance by altering the amount of preadjustment built into the brackets? 5 His objectives were to have the teeth in overcorrected positions at the end of treatment when unbent, full-sized wires were used. All modifications and improvements in orthodontic appliances were predicated on the ability of the clinician to accurately visualize and correctly place the bracket on the tooth. No one has actually studied how well this is accomplished clinically.
METHODS AND MATERIALS Pretreatment models of five orthodontic patients re'presenting different types of malocclusion were selected. One Class I, two Class II, Division 1, and two Class II, Division 2 malocclusion cases were taken from the Orthodontic Department at Loyola University School of Dentistry. The models were duplicated and a "diagnostic set-up" was fabricated from the duplicate models (Fig. 1). The teeth were positioned to "ideal" angulation (agreed ideal according to Andrew.'s six keys of occlusion, as modified by Roth) and occlusal contact and checked with articulating ribbon (Accu-Film, Parkell, Farmingdale, N.Y.) (Fig. 2). An acrylic occlusal registration key (upper and lower) was fabricated from each diagnostic set-up. The key served
Ten faculty members from the Orthodontic Department of Loyola University School of Dentistry were the subjects of this study. Each faculty member was asked to place preadjusted brackets ("A" Company) on five duplicated pretrcatment orthodontic patient models, from first molar to first molar, inclusively. The models were mounted on a mannequin to simulate the patient (Fig. 3). A total of 50 bonded cases of malocclusions served as the population for this study (10 faculty members • 5 cases). After the brackets were placed in the untreated models, the teeth Were sectioned from the base. The sectioned teeth were transferred to the occlusal registration made from the diagnostic set-up for that patient and secured in place with adhesive (cyanoacrylate). The sectioned teeth secured with adhesive to the occlusal registration were designated at the "transfer set-up." This represents the ideala relationship of teeth desired in the finished case (Fig. 4). Five photographs were taken of each transfer set-up oriented perpendicular to the crown of the teeth at a fixed distance with a 90 mm macro lens (Panagor) and Minolta 35 mm single lens reflex camera body. The lens was set on a 1:1 magnification ratio. The resulting photographs were digitized on a Houston Instrument Hi-Pad digitizer. The outer edges of each bracket slot were used as reference. The vertical and angular differences in brackets positions were measured between tooth pairs by digitizing the outer edges of each bracket with a program written for an IBM mainframe computer (IBM Corp., White Plains, N.Y.). The measurements were calculated from a reference line that was formed by connecting the outer edges of adjacent bracket pairs. The vertical measurements were calculated by measuring the difference between the perpendicular distance
64
Balut e t a / .
Am. J. Orthod. Dente~w. Onhvp. July 1992
Fig. 3. Duplicate model mounted on mannequin showing direct bonded brackets in place.
A
B Fig. 4. Transfer set-up. of the adjacent slot edges to the reference line (Fig. 5). The angular measurements were calculated from the difference between the arctangents of the slopes of the individual brackets to a reference line connecting the outer occlusal edges of the slots of the adjacent bracket pair (Fig. 5). If the four points ',,,'ere on the reference line, the linear and angular values would be "zero" for each tooth pair: (This would indicate ideal bracket placement on the models.) Any linear or angular differences observed represent errors in bracket placement. Two-way analysis of variance was used to determine whether differences exist, and multiple comparison procedures were made by a Tukey's HSD test to determine where the differences exist. The experimental error of this study was determined by repeating measurements of two facult2r members (10 cases) with a paired t test for the linear and angular measurements. A mean of 0.005 mm of linear difference ',,,'as found, and a mean of 0.087 ~ of angular difference was found, which was not statistically significant.
Fig. 5. Independence of vertical and angular relationships between adjacent bracket pairs. A, Bracket pair with both vertical (line A not equal to line B) and angular differences to the reference line. B, Bracket pair with vertical difference but no angular difference to the reference line. C, Bracket pair with angular difference but no vertical difference (line E equals line F) to the reference line.
RESULTS Discrepancies w e r e found in p l a c e m e n t o f the orthodontic brackets in height and angulation. A m e a n o f 0.34 m m for the linear m e a s u r e m e n t s and a mean o f 5.54 ~ for the angular m e a s u r e m e n t s were found (Table
Volume 102
Variations in bracket placement in preadjusted appliance
Number I
65
Table I. Mean, standard deviation, range, maximum values and minimum values of the linear and angular discrepancies
Standarddeviation (SD)
Range
0.34
0.29
5.54
4.32
Mean = X Linear discrepancies (mm) Angular discrepancies (degrees)
I
I
Maximum value
Minbnum value
1.80
1.80
0.00
29.10
29.10
0.00
Table II. Mean, standard deviation range of the vertical and angular discrepancies by faculty
Mean Faculty
N = 110
Linear
1 2
0.38 0.35
3 4 5 6
0.26 0.33 0.34 0.34
7 8 9 10
0.31 0.32 0.42 0.36
]
Range
SD Angular
Linear
6.02 4.87 5.77 5.12 6.76 4.48 5.21 5.40 6.33 5.48
0.28 0.30 0.22 0.26 0.29 0.28 0.27 0.29 0.34 0.31
I) between adjacent bracket pairs (all faculty members, all teeth). Table II shows the mean, standard deviation, and range of the linear and angular discrepancies by faculty, indicating statistically significant differences in angular bracket position among faculty no. 2, no. 5, and no. 6. For the linear measurements, statistically significant differences were found between faculty no. 3 and no. 9 (P --< 0.01). When the linear and angular discrepancies among the five models were analyzed, there was no significant difference among them in vertical discrepancy (P -> 0.05). Model no. 2 (Class II) showed significantly less angular discrepancy than Model no. 5 (Class II) (P -< 0.01). Tables III and IV indicate that the lower anterior teeth presented the least variation in bracket placement, in both angular and vertical measurements (P ~ O.OOl). The teeth that showed the most angular discrepancy were the upper anterior teeth and the upper and lower canines (P ~ O.OOl). The upper second premolars were the teeth which presented the most vertical discrepancy in bracket placement (P < O.OOl). DISCUSSION
Toward the end of the treatment, the teeth must be brought as close as possible to their final and functional
I
Angular
Linear
4.54 4.11 4.47 3.71 5.12 3.21 4.07 3.85 5.07 4.31
1.30 1.62 1.03 1.11 1.56 1.11 1.25 1.36 1.79 i.55
I
Angular 20.79 21.20 23.61 17.06 24.30 13.06 18.26 17.25 29.01 19.63
positions before debonding. This necessitates a perfect alignment of the marginal ridges, contact points, and roots of the teeth. Factors such as error in bracket placement, tooth irregularities, and variations in tooth structure make it difficult to achieve these goals accurately with the preadjusted orthodontic appliance. In this study, model no. 2 had significantly less angular discrepancy than model no. 5. The reason was that model no. 5 had a lower right first premolar severely malposed. The crowding and the size of the clinical crown of that specific tooth made it impossible to place the bracket correctly. In addition, model no. 2 had larger clinical crowns, not severely rotated teeth, making it less difficult for the operator to place the orthodontic brackets. There is no correlation between the type of malocclusion and the error in bracket placement. The error in placement seems to be more related to the skill of the operator, tooth structure, size of clinical crowns, and malposition of the tooth in the dental arch. In some cases, the operator has to compromise and place the bracket more gingivally because of interference with the long cuspal heights on opposing posterior teeth. Since any effect o f bracket/tooth interference was exchuted fronz this study, tile actual clinical accuracy of bracket positioning on the posterior teeth wouhl be much worse on a patient than was shown here. It appears that it was easier for the operators to
66
Balut el al.
Am. J. Orthod. Dentofiw. Orthop. July 1992
Table III. Mean of angular discrepancies of each faculty by tooth pair in degrees Tooth pair (n = 5) Mttrillary arch i-I 1-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5 Mandibular arch 1-1 1-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5
5
10.30 7.14 5.87 6.97 6.61 6.24 8.20 4.76 6.03 3.95 2.90
13.29 2.96 5.45 4.76 5.54 4.14 4.95 2.21 5.87 5.52 3.72
7.42 6.40 4.32 7.72 7.09 3.95 5.49 3.64 5.27 3.84 8.34
6.46 3.56 4.04 3.13 4.08 4.87 5.58 2.62 5.36 3.86 6.80
7.16 7.10 4.93 8.33 8.89 13.17 7.05 6.23 8.31 3.24 9.93
4.80 4.52 6.91 7.41 1.96 5.95 5.53 3.25 3.18 3.56 3.99
2.95 4.12 4.02 6.60 6.96 5.58 6.52 1.91 7.69 3.94 3.32
6.27 3.32 4.92 6.01 9.43 4.44 7.93 5.23 7.30 6.05 7.57
7.76 3.51 5.32 6.88 3.73 7.12 5.39 5.89 5.56 5.48 8.50
8.69 3.61 6.16 4.32 6.14 11.61 5.98 5.64 4.13 7.69 6.49
7.56 4.62 5.19 6.21 6.04 6.90 6.26 4.13 5.87 4.71 6.15
4.11 5.33 5.92 5.22 6.56 5.72 11.16 4.29 4.24 4.15 4.80
3.37 2.98 3.56 5.58 5.55 3.00 7.01 3.82 2.28 7.99 3.54
3.72 2.27 4.28 3.68 4.79 5.99 9.32 8.44 8.39 6.77 5.34
4.74 3.09 2.32 7.77 5.38 9.70 6.28 7.02 5.20 8.46 2.24
3.11 3.21 7.12 4.31 6.08 6.63 6.97 7.00 7.56 4.72 7.65
2.14 4.13 6.44 4.05 2.48 4.66 5.55 4.11 4.21 3.86 5.90
2.93 4.66 2.10 5.78 5.83 7.52 7.77 7.89 4.45 7.73 4.44
3.85 4.52 3.29 7.24 4.25 3.65 5.14 4.39 4.82 6.19 2.92
5.94 7.12 5.66 11.63 9.30 9.10 7.83 7.59 3.26 4.47 2.32
2.35 2.74 4.35 4.62 3.48 6.61 6.10 5.69 5.63 9.95 3.51
3.62 4.00 4.50 5.98 5.37 6.25 7.31 6.02 5.00 5.93 4.26
Table IV. Mean of vertical discrepancies by tooth pair in millimeters Fact~ Tooth pair (n = 5)
Mttrillary arch 1-1 I-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5
0.27 0.57 0.67 0.54 0.66 0.52 0.33 0.37 0.24 0.62 0.55
0.24 0.44 0.57 0.18 0.63 0.39 0.66 0.45 0.30 0.75 0.30
0.27 0.24 0.60 0.16 0.27 0.28 0.40 0.19 0.24 0.30 0.28
0.11 0.44 0.51 0.23 0.30 0.44 0.19 0.35 0.34 0.61 0.36
0.14 0.46 0.31 0.26 0.17 0.42 0.47 0.48 0.28 0.63 0.63
0.33 0.40 0.39 0.33 0.13 0.46 0.19 0.44 0.31 0.87 0.65
0.06 0.21 0.16 0.23 0.33 0.50 0.31 0.52 0.35 0.82 0.32
0.18 0.21 0.36 0.18 0.12 0.45 0.37 0.55 0.69 0.82 0.57
0.13 0.28 0.48 0.58 0.41 0.54 0.21 0.33 0.44 0.63 0.38
0.21 0.55 0.33 0.29 0.30 0.49 0.20 0.36 0.23 0.99 0.28
0.19 0.37 0.44 0.30 0.33 0.44 0.33 0.40 0.34 0.70 0.43
Mandibular arch I-I I-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5
0.22 0.19 0.19 0.31 0.31 0.33 0.33 0.37 0.31 0.18 0.22
0.22 0.06 0.17 0.41 0.26 0.11 0.44 0.39 0.30 0.30 0.23
0.13 0.06 0.11 0.28 0.42 0.18 0.29 0.35 0.34 0.20 0.23
0.16 0.21 0.22 0.29 0.27 0.34 0.39 0.46 0.34 0.45 0.20
0.11 0.26 0.26 0.27 0.40 0.22 0.34 0.37 ().47 0.21 0.34
0.19 0.19 0.14 0.14 0.50 0.34 0.34 0.30 0.33 0.24 0.31
0.19 0.14 0.17 0.29 0.31 0.19 0.37 0.22 0.45 0.43 0.43
0.22 0.14 0.23 0.18 0.21 0.28 0.37 0.26 0.39 0.13 0.06
0.37 0.25 0.42 0.82 0.69 0.68 0.37 0.28 0.38 0.34 0.23
0.31 0.15 0.19 0.30 0.47 0.31 0.48 0.51 0.44 0.40 0.18
0.21 0.16 0.21 0.33 0.38 0.29 0.37 0.35 0.37 0.28 0.21
Volume 102 Number 1
visualize the long axes of the lower incisors and to place the bracket appropriately and at the correct height. The upper anterior teeth and the upper and lower canines showed the most angular discrepancy. It appears that the operators have difficulty in judging root angulation of these teeth. The teeth that presented the most difficulty in vertical bracket placement were the upper second premolars, possibly because of the length of their clinical crowns (which are sometimes short). This is exacerbated by the molar arch wire slot being gingivally positioned on the molar as a result of the headgear tube being occlusally positioned. The observed mean angular discrepancy of 5.54 ~ plus the standard deviation of 4.32 ~ indicates that a bracket error of 10~ between bracket pairs, with the "A" Company appliance, would occur with the same frequency as a bracket pair placed in perfect alignment. This is somewhat surprising when one considers that few orthodontists request more than 10~ of tip be built into their preadjusted appliance prescriptions. Although 3 of the 10 faculty members differed statistically in their angular bracket placement measurements, their vertical measurements were statistically indistinguishable (means of 0.35 mm, 0.34 ram, and 0.34 mm). Those two faculty members who differed statistically in their vertical bracket placement showed no statistical differences in their angular bracket placement measurements (means of 5.77 ~ and 6.33~ No faculty member was statistically different from any of the other faculty members in both vertical and angular discrepancies in bracket placement. This indicates that there is no systematic difference among the faculty members in bracket placement. It would be reasonable to assume that the errors of the faculty represent what would be expected of orthodontic practitioners in general. These can be seen as independent variables in this study because of the method with which the measurements were referenced and the manner in which bracket positional variation may occur. Figs. 5, A to C illustrate the manner in which vertical and angular .discrepancies may occur independently of each other. The fact that the majority of the faculty members were so similar in their results seems to indicate a basic human limitation in direct placement of the brackets in the mouth. The clinical implications in the error of bracket
Variations bl bracket placenlent in preadjltsted appliance
67
placement in this study are unstable tooth positions, lack of root paralleling, food impaction because of marginal ridge discrepancies, and failure to establish the very specific occlusal scheme of canine rise or mutually protected occlusion. It should not be interpreted froth this study that achievement of acceptable orthodontic results is impossible with straight wire therapy. With proper wire bending or rebonding bracket positions, an excellent result can certainly be achieved; however, increased time and effort must be expended by the orthodontist and the patient to accomplish these goals. REFERENCES 1. Angle HE. The latest and best in orthodontic mechanism. Dental Cosmos 1928;70:il43; 1929;71:164. 2. Taylor RMS. Variations in morphology of teeth. New York: Charles C. Thomas, 1978. 3. Ricketts MR. Biopr~gressive therapy. Denver: Rocky Mountain 1979. 4. Andrews FL. The Straight Wire Appliance origin, controversy, commentary. J Clin Orthod 1976;10:99. 5. Andrews FL. Straight Wire Appliance, arch form, wire bending and experiment. J Clin Orthod 1976;10:8. 6. Andrews FL. The S.W.A. explained anad compared. J Clin Orthod 1976;10:174. 7. Andrews FL. The S.W.A. syllabus of philosophy and techniques. San Diego: LF Andrews Foundation for Orthodontic Education and Research, 1974. 8. Thurow CR. Edgewise orthodontics. 3rd. ed. St. Louis: CV Mosby, 1972. 9. Meyer M, Nelson G. Preadjusted edgewise appliance, theory and practice. AM J OR'nlOD 1978;73:485. 10. Kraus SB. Dental anatomy and occlusion. Chapter I. Baltimore: Williams and Wilkins, 1969. 11. Dellinger LE. A scientific assessment of the straaight wire appliance. AM J OR'IIIOD 1978;73:290. 12. Jarabak RL. Development of a treatment plan, in the light of one's concept of treatment objectives. AM J ORTHOD 1969;36:481. 13. Andrews FL. The six keys to normal occlusion. AM J OR.'RIOD 1972;62:296. 14. Roth HR. Five years clinical evaluation of the Andrews S.W.A. J Clin Orthod 1981;11:175. 15. Roth HR. Functional occlusion for the orthodontists, part i11. J Clin Orthod 1981;11:175.
Reprint requests to: Dr. Nasib Balut Manuel E. Izaguirre 11-601-602 CTO Comercial Satelite EDO de Mexico City Mexico 53100
T h e prcadjusted bracket system is the most widely used in orthodontic therapy today. The basic premise of the preadjusted system is that proper bracket position allows the teeth to be positioned with a straight wire into an ocelusal contact with excellent mesiodistal inclinations (tips) and excellent faciolingual inclination (torque). Most clinicians are aware that clinically the "straight wire" (preadjusted) appliance does not eliminate wire bending. Variations in tooth structure and malocclusions affect the final bracket positions. Variations from the straight wire appliance averages must be compensated by bends placed in the arch wires. The present study was designated to evaluate variations in placement in the vertical and angular bracket position from the ideal with a preadjusted orthodontic appliance ("A" Company, Johnson & Johnson, San Diego, Calif.). Variations in bracket position were evaluated with respect to the classification of maloeclusions, specific tooth type, and intra/inter operator differences. REVIEW OF THE LITERATURE Bracket placement and tooth structure
Originally Angle t taught that the best position of the band was where it fits better mechanically. Then, if possible, the bracket should be placed at the center of the labial surface of the tooth. Later, placement of 'Associate Professor, Universidad Intercontinental Mexico. bAssociate Professor, Loyola University of Chicago. 'Chairman of the Department of Dental Materials, Loyola Uniser~ity of Chicago. aAssc,ciate Professor of Biostatistics, Lo) ola University of Chicago. 8 / I / 27285
62
the anterior bands at the junction of the middle and the incisal thirds of the crown was recommended. 2 Ricketts 3 advocated the use of marginal ridges as guidelines for band and bracket vertical positioning. More recently, Andrews 47 introduced the bracketing technique of placing the straight guidelines of the bracket (vertical tie wings) parallel to the long axis of the clinical crown and then moving the bracket up or down until the middle of its slot base is at the same height as the midpoint of the clinical crown. Thurow 8 showed that two different vertical positions of a bracket on a tooth will cause two different buccolingual axial inclinations (torque). Meyer and Nelson 9 showed that an error of 3 mm vertically in bracket placement on a premolar can result in 15~ torque alteration and 0.04 mm alteration in the in/out adjustments. In orthodontics, what affects the design of the orthodontic appliances and their use is the inclination of the labial or buccal surface of the tooth crown to the long axis of either the entire tooth or the occlusal surface of the crown. Kraus, t~ Taylor, 2 and Dellinger tt found great variations existed in tooth structure that can affect treatment results. Improvements in bracket design were made incorporating tip, ~torque, ~2rotation, and differences in base thickness 47.13 beginning with Angle in 1928 and, more recently, with Andrews in 1970 who introduced the "straight wire concept" in preadjusted orthodontic appliance. Roth 14 in 1975 evaluated the preadjusted bracket system after he had used it for 5 years. In 1981 he
Volume 102
Nunzber I
Variations in bracket placement in preadjusted appliance
63
Fig. 2. Occlusal view of one =diagnostic set-up" showing occlusal contacts checked with articulating ribbon,
as a template for the final ideal tooth position desired for each case.
Fig. 1. Front view of one "diagnostic set-up" showing idea! angulation where teeth were positioned.
modified the appliance by altering the amount of preadjustment built into the brackets? 5 His objectives were to have the teeth in overcorrected positions at the end of treatment when unbent, full-sized wires were used. All modifications and improvements in orthodontic appliances were predicated on the ability of the clinician to accurately visualize and correctly place the bracket on the tooth. No one has actually studied how well this is accomplished clinically.
METHODS AND MATERIALS Pretreatment models of five orthodontic patients re'presenting different types of malocclusion were selected. One Class I, two Class II, Division 1, and two Class II, Division 2 malocclusion cases were taken from the Orthodontic Department at Loyola University School of Dentistry. The models were duplicated and a "diagnostic set-up" was fabricated from the duplicate models (Fig. 1). The teeth were positioned to "ideal" angulation (agreed ideal according to Andrew.'s six keys of occlusion, as modified by Roth) and occlusal contact and checked with articulating ribbon (Accu-Film, Parkell, Farmingdale, N.Y.) (Fig. 2). An acrylic occlusal registration key (upper and lower) was fabricated from each diagnostic set-up. The key served
Ten faculty members from the Orthodontic Department of Loyola University School of Dentistry were the subjects of this study. Each faculty member was asked to place preadjusted brackets ("A" Company) on five duplicated pretrcatment orthodontic patient models, from first molar to first molar, inclusively. The models were mounted on a mannequin to simulate the patient (Fig. 3). A total of 50 bonded cases of malocclusions served as the population for this study (10 faculty members • 5 cases). After the brackets were placed in the untreated models, the teeth Were sectioned from the base. The sectioned teeth were transferred to the occlusal registration made from the diagnostic set-up for that patient and secured in place with adhesive (cyanoacrylate). The sectioned teeth secured with adhesive to the occlusal registration were designated at the "transfer set-up." This represents the ideala relationship of teeth desired in the finished case (Fig. 4). Five photographs were taken of each transfer set-up oriented perpendicular to the crown of the teeth at a fixed distance with a 90 mm macro lens (Panagor) and Minolta 35 mm single lens reflex camera body. The lens was set on a 1:1 magnification ratio. The resulting photographs were digitized on a Houston Instrument Hi-Pad digitizer. The outer edges of each bracket slot were used as reference. The vertical and angular differences in brackets positions were measured between tooth pairs by digitizing the outer edges of each bracket with a program written for an IBM mainframe computer (IBM Corp., White Plains, N.Y.). The measurements were calculated from a reference line that was formed by connecting the outer edges of adjacent bracket pairs. The vertical measurements were calculated by measuring the difference between the perpendicular distance
64
Balut e t a / .
Am. J. Orthod. Dente~w. Onhvp. July 1992
Fig. 3. Duplicate model mounted on mannequin showing direct bonded brackets in place.
A
B Fig. 4. Transfer set-up. of the adjacent slot edges to the reference line (Fig. 5). The angular measurements were calculated from the difference between the arctangents of the slopes of the individual brackets to a reference line connecting the outer occlusal edges of the slots of the adjacent bracket pair (Fig. 5). If the four points ',,,'ere on the reference line, the linear and angular values would be "zero" for each tooth pair: (This would indicate ideal bracket placement on the models.) Any linear or angular differences observed represent errors in bracket placement. Two-way analysis of variance was used to determine whether differences exist, and multiple comparison procedures were made by a Tukey's HSD test to determine where the differences exist. The experimental error of this study was determined by repeating measurements of two facult2r members (10 cases) with a paired t test for the linear and angular measurements. A mean of 0.005 mm of linear difference ',,,'as found, and a mean of 0.087 ~ of angular difference was found, which was not statistically significant.
Fig. 5. Independence of vertical and angular relationships between adjacent bracket pairs. A, Bracket pair with both vertical (line A not equal to line B) and angular differences to the reference line. B, Bracket pair with vertical difference but no angular difference to the reference line. C, Bracket pair with angular difference but no vertical difference (line E equals line F) to the reference line.
RESULTS Discrepancies w e r e found in p l a c e m e n t o f the orthodontic brackets in height and angulation. A m e a n o f 0.34 m m for the linear m e a s u r e m e n t s and a mean o f 5.54 ~ for the angular m e a s u r e m e n t s were found (Table
Volume 102
Variations in bracket placement in preadjusted appliance
Number I
65
Table I. Mean, standard deviation, range, maximum values and minimum values of the linear and angular discrepancies
Standarddeviation (SD)
Range
0.34
0.29
5.54
4.32
Mean = X Linear discrepancies (mm) Angular discrepancies (degrees)
I
I
Maximum value
Minbnum value
1.80
1.80
0.00
29.10
29.10
0.00
Table II. Mean, standard deviation range of the vertical and angular discrepancies by faculty
Mean Faculty
N = 110
Linear
1 2
0.38 0.35
3 4 5 6
0.26 0.33 0.34 0.34
7 8 9 10
0.31 0.32 0.42 0.36
]
Range
SD Angular
Linear
6.02 4.87 5.77 5.12 6.76 4.48 5.21 5.40 6.33 5.48
0.28 0.30 0.22 0.26 0.29 0.28 0.27 0.29 0.34 0.31
I) between adjacent bracket pairs (all faculty members, all teeth). Table II shows the mean, standard deviation, and range of the linear and angular discrepancies by faculty, indicating statistically significant differences in angular bracket position among faculty no. 2, no. 5, and no. 6. For the linear measurements, statistically significant differences were found between faculty no. 3 and no. 9 (P --< 0.01). When the linear and angular discrepancies among the five models were analyzed, there was no significant difference among them in vertical discrepancy (P -> 0.05). Model no. 2 (Class II) showed significantly less angular discrepancy than Model no. 5 (Class II) (P -< 0.01). Tables III and IV indicate that the lower anterior teeth presented the least variation in bracket placement, in both angular and vertical measurements (P ~ O.OOl). The teeth that showed the most angular discrepancy were the upper anterior teeth and the upper and lower canines (P ~ O.OOl). The upper second premolars were the teeth which presented the most vertical discrepancy in bracket placement (P < O.OOl). DISCUSSION
Toward the end of the treatment, the teeth must be brought as close as possible to their final and functional
I
Angular
Linear
4.54 4.11 4.47 3.71 5.12 3.21 4.07 3.85 5.07 4.31
1.30 1.62 1.03 1.11 1.56 1.11 1.25 1.36 1.79 i.55
I
Angular 20.79 21.20 23.61 17.06 24.30 13.06 18.26 17.25 29.01 19.63
positions before debonding. This necessitates a perfect alignment of the marginal ridges, contact points, and roots of the teeth. Factors such as error in bracket placement, tooth irregularities, and variations in tooth structure make it difficult to achieve these goals accurately with the preadjusted orthodontic appliance. In this study, model no. 2 had significantly less angular discrepancy than model no. 5. The reason was that model no. 5 had a lower right first premolar severely malposed. The crowding and the size of the clinical crown of that specific tooth made it impossible to place the bracket correctly. In addition, model no. 2 had larger clinical crowns, not severely rotated teeth, making it less difficult for the operator to place the orthodontic brackets. There is no correlation between the type of malocclusion and the error in bracket placement. The error in placement seems to be more related to the skill of the operator, tooth structure, size of clinical crowns, and malposition of the tooth in the dental arch. In some cases, the operator has to compromise and place the bracket more gingivally because of interference with the long cuspal heights on opposing posterior teeth. Since any effect o f bracket/tooth interference was exchuted fronz this study, tile actual clinical accuracy of bracket positioning on the posterior teeth wouhl be much worse on a patient than was shown here. It appears that it was easier for the operators to
66
Balut el al.
Am. J. Orthod. Dentofiw. Orthop. July 1992
Table III. Mean of angular discrepancies of each faculty by tooth pair in degrees Tooth pair (n = 5) Mttrillary arch i-I 1-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5 Mandibular arch 1-1 1-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5
5
10.30 7.14 5.87 6.97 6.61 6.24 8.20 4.76 6.03 3.95 2.90
13.29 2.96 5.45 4.76 5.54 4.14 4.95 2.21 5.87 5.52 3.72
7.42 6.40 4.32 7.72 7.09 3.95 5.49 3.64 5.27 3.84 8.34
6.46 3.56 4.04 3.13 4.08 4.87 5.58 2.62 5.36 3.86 6.80
7.16 7.10 4.93 8.33 8.89 13.17 7.05 6.23 8.31 3.24 9.93
4.80 4.52 6.91 7.41 1.96 5.95 5.53 3.25 3.18 3.56 3.99
2.95 4.12 4.02 6.60 6.96 5.58 6.52 1.91 7.69 3.94 3.32
6.27 3.32 4.92 6.01 9.43 4.44 7.93 5.23 7.30 6.05 7.57
7.76 3.51 5.32 6.88 3.73 7.12 5.39 5.89 5.56 5.48 8.50
8.69 3.61 6.16 4.32 6.14 11.61 5.98 5.64 4.13 7.69 6.49
7.56 4.62 5.19 6.21 6.04 6.90 6.26 4.13 5.87 4.71 6.15
4.11 5.33 5.92 5.22 6.56 5.72 11.16 4.29 4.24 4.15 4.80
3.37 2.98 3.56 5.58 5.55 3.00 7.01 3.82 2.28 7.99 3.54
3.72 2.27 4.28 3.68 4.79 5.99 9.32 8.44 8.39 6.77 5.34
4.74 3.09 2.32 7.77 5.38 9.70 6.28 7.02 5.20 8.46 2.24
3.11 3.21 7.12 4.31 6.08 6.63 6.97 7.00 7.56 4.72 7.65
2.14 4.13 6.44 4.05 2.48 4.66 5.55 4.11 4.21 3.86 5.90
2.93 4.66 2.10 5.78 5.83 7.52 7.77 7.89 4.45 7.73 4.44
3.85 4.52 3.29 7.24 4.25 3.65 5.14 4.39 4.82 6.19 2.92
5.94 7.12 5.66 11.63 9.30 9.10 7.83 7.59 3.26 4.47 2.32
2.35 2.74 4.35 4.62 3.48 6.61 6.10 5.69 5.63 9.95 3.51
3.62 4.00 4.50 5.98 5.37 6.25 7.31 6.02 5.00 5.93 4.26
Table IV. Mean of vertical discrepancies by tooth pair in millimeters Fact~ Tooth pair (n = 5)
Mttrillary arch 1-1 I-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5
0.27 0.57 0.67 0.54 0.66 0.52 0.33 0.37 0.24 0.62 0.55
0.24 0.44 0.57 0.18 0.63 0.39 0.66 0.45 0.30 0.75 0.30
0.27 0.24 0.60 0.16 0.27 0.28 0.40 0.19 0.24 0.30 0.28
0.11 0.44 0.51 0.23 0.30 0.44 0.19 0.35 0.34 0.61 0.36
0.14 0.46 0.31 0.26 0.17 0.42 0.47 0.48 0.28 0.63 0.63
0.33 0.40 0.39 0.33 0.13 0.46 0.19 0.44 0.31 0.87 0.65
0.06 0.21 0.16 0.23 0.33 0.50 0.31 0.52 0.35 0.82 0.32
0.18 0.21 0.36 0.18 0.12 0.45 0.37 0.55 0.69 0.82 0.57
0.13 0.28 0.48 0.58 0.41 0.54 0.21 0.33 0.44 0.63 0.38
0.21 0.55 0.33 0.29 0.30 0.49 0.20 0.36 0.23 0.99 0.28
0.19 0.37 0.44 0.30 0.33 0.44 0.33 0.40 0.34 0.70 0.43
Mandibular arch I-I I-2 2-1 2-3 3-2 3-4 4-3 4-5 5-4 5-6 6-5
0.22 0.19 0.19 0.31 0.31 0.33 0.33 0.37 0.31 0.18 0.22
0.22 0.06 0.17 0.41 0.26 0.11 0.44 0.39 0.30 0.30 0.23
0.13 0.06 0.11 0.28 0.42 0.18 0.29 0.35 0.34 0.20 0.23
0.16 0.21 0.22 0.29 0.27 0.34 0.39 0.46 0.34 0.45 0.20
0.11 0.26 0.26 0.27 0.40 0.22 0.34 0.37 ().47 0.21 0.34
0.19 0.19 0.14 0.14 0.50 0.34 0.34 0.30 0.33 0.24 0.31
0.19 0.14 0.17 0.29 0.31 0.19 0.37 0.22 0.45 0.43 0.43
0.22 0.14 0.23 0.18 0.21 0.28 0.37 0.26 0.39 0.13 0.06
0.37 0.25 0.42 0.82 0.69 0.68 0.37 0.28 0.38 0.34 0.23
0.31 0.15 0.19 0.30 0.47 0.31 0.48 0.51 0.44 0.40 0.18
0.21 0.16 0.21 0.33 0.38 0.29 0.37 0.35 0.37 0.28 0.21
Volume 102 Number 1
visualize the long axes of the lower incisors and to place the bracket appropriately and at the correct height. The upper anterior teeth and the upper and lower canines showed the most angular discrepancy. It appears that the operators have difficulty in judging root angulation of these teeth. The teeth that presented the most difficulty in vertical bracket placement were the upper second premolars, possibly because of the length of their clinical crowns (which are sometimes short). This is exacerbated by the molar arch wire slot being gingivally positioned on the molar as a result of the headgear tube being occlusally positioned. The observed mean angular discrepancy of 5.54 ~ plus the standard deviation of 4.32 ~ indicates that a bracket error of 10~ between bracket pairs, with the "A" Company appliance, would occur with the same frequency as a bracket pair placed in perfect alignment. This is somewhat surprising when one considers that few orthodontists request more than 10~ of tip be built into their preadjusted appliance prescriptions. Although 3 of the 10 faculty members differed statistically in their angular bracket placement measurements, their vertical measurements were statistically indistinguishable (means of 0.35 mm, 0.34 ram, and 0.34 mm). Those two faculty members who differed statistically in their vertical bracket placement showed no statistical differences in their angular bracket placement measurements (means of 5.77 ~ and 6.33~ No faculty member was statistically different from any of the other faculty members in both vertical and angular discrepancies in bracket placement. This indicates that there is no systematic difference among the faculty members in bracket placement. It would be reasonable to assume that the errors of the faculty represent what would be expected of orthodontic practitioners in general. These can be seen as independent variables in this study because of the method with which the measurements were referenced and the manner in which bracket positional variation may occur. Figs. 5, A to C illustrate the manner in which vertical and angular .discrepancies may occur independently of each other. The fact that the majority of the faculty members were so similar in their results seems to indicate a basic human limitation in direct placement of the brackets in the mouth. The clinical implications in the error of bracket
Variations bl bracket placenlent in preadjltsted appliance
67
placement in this study are unstable tooth positions, lack of root paralleling, food impaction because of marginal ridge discrepancies, and failure to establish the very specific occlusal scheme of canine rise or mutually protected occlusion. It should not be interpreted froth this study that achievement of acceptable orthodontic results is impossible with straight wire therapy. With proper wire bending or rebonding bracket positions, an excellent result can certainly be achieved; however, increased time and effort must be expended by the orthodontist and the patient to accomplish these goals. REFERENCES 1. Angle HE. The latest and best in orthodontic mechanism. Dental Cosmos 1928;70:il43; 1929;71:164. 2. Taylor RMS. Variations in morphology of teeth. New York: Charles C. Thomas, 1978. 3. Ricketts MR. Biopr~gressive therapy. Denver: Rocky Mountain 1979. 4. Andrews FL. The Straight Wire Appliance origin, controversy, commentary. J Clin Orthod 1976;10:99. 5. Andrews FL. Straight Wire Appliance, arch form, wire bending and experiment. J Clin Orthod 1976;10:8. 6. Andrews FL. The S.W.A. explained anad compared. J Clin Orthod 1976;10:174. 7. Andrews FL. The S.W.A. syllabus of philosophy and techniques. San Diego: LF Andrews Foundation for Orthodontic Education and Research, 1974. 8. Thurow CR. Edgewise orthodontics. 3rd. ed. St. Louis: CV Mosby, 1972. 9. Meyer M, Nelson G. Preadjusted edgewise appliance, theory and practice. AM J OR'nlOD 1978;73:485. 10. Kraus SB. Dental anatomy and occlusion. Chapter I. Baltimore: Williams and Wilkins, 1969. 11. Dellinger LE. A scientific assessment of the straaight wire appliance. AM J OR'IIIOD 1978;73:290. 12. Jarabak RL. Development of a treatment plan, in the light of one's concept of treatment objectives. AM J ORTHOD 1969;36:481. 13. Andrews FL. The six keys to normal occlusion. AM J OR.'RIOD 1972;62:296. 14. Roth HR. Five years clinical evaluation of the Andrews S.W.A. J Clin Orthod 1981;11:175. 15. Roth HR. Functional occlusion for the orthodontists, part i11. J Clin Orthod 1981;11:175.
Reprint requests to: Dr. Nasib Balut Manuel E. Izaguirre 11-601-602 CTO Comercial Satelite EDO de Mexico City Mexico 53100
