The centered T-loop-a new way of preactivation

Klaus D. Hoenigl, MD, DDS, a Josef Freudenthaler, MD, DDS, a Michael R. Marcotte, DDS, MS, and Hans-Peter Bantleon, MD, DDS, MS b Vienna, Austria, and Bristol, Conn.

The force system of a prefabricated and preactivated T-loop used for reciprocal space closure was determined by simultaneously measuring the horizontal and vertical forces, as well as the moments using a computer controlled measuring apparatus. Interbracket distances of 21, 24, 27, and 30 mm were used to mimic typical clinical situations. At a loop activation of 7 mm, the anterior and posterior segments first underwent controlled tipping, then translation, and finally, root uprighting as the moment-to-force ratio increased with deactivation. After the loop has been deactivated to 4 mm, however, it should be exchanged to avoid root abutment. (AM J ORTHOD DENTOFAC ORTHOP 1995; 108:149-53.)

I n the treatment of crowded dentitions, teeth are often removed to provide additional arch length. Satisfactory closure of the remaining space remains a difficult problem after teeth have been removed. With the forces and moments necessary for this controlled tooth movement, fixed appli- ances are necessary to transmit the necessary forces and moments to the teeth. Regardless of the fixed appliance technique used, however, extraction sites are closed either by pulling or pushing the teeth along an existing arch wire or by segmenting an arch and approximating the segments of teeth by means of frictionless springs.

The bracket of a tooth serves as means of applying a force or moment to a tooth. How a tooth moves, however, is determined by the force system at the center of resistance (Cres) of that tooth and, typically, this Cres is located 10 mm apically from the bracket. 1'2 This distance forms a lever arm, which, together with the force, forms the moment of a force (F d = M). This force system at the Cros from a single force acting at the bracket produces uncontrolled tipping (crown goes in the direction of the force, the root tip goes in the opposite direction) with a center of rotation (Crot) just slightly apical to the Cros.

When teeth are slid along an arch wire, the brackets change their angular orientation, attempt- ing to deform the arch wire. The arch wire, then, delivers a resistive moment to the tooth, the mag-

~Postgraduate student, Department of Orthodontics, School of Dental Medicine, University of Vienna, Austria. bProfessor and Chairman, Department of Orthodontics, School of Dental Medicine, University of Vienna, Austria. ~In private practice, Bristol, Conn. Copyright 1995 by the American Association of Orthodontists. 0889-5406/95/$3.00 + 0 8/1/52816

nitude of this moment depending on the size of the arch wire. With a round 0.016-inch wire in the bracket of a tooth, the turning bracket deforms the wire to a great extent, thus allowing considerable tipping of the tooth. A stiffer arch wire, e.g., 0.016 0.022-inch stainless steel, hardly deforms at all. However, considerable friction between the arch wire and the bracket is generated, making it difficult to control the remaining force on the tooth.

In the Segmented Arch Technique, developed by Burstone in 1962, 3 frictionless springs are used to attract the segments of teeth on either side of an attraction site. These springs are preactivated to deliver the required countermoments as they work-out (are deactivated). These countermoments should be the same magnitude as the tipping moments to achieve bodily tooth movement. The force systems from these springs can be tested on a measuring apparatus before using them clinically?

The 0.017 0.025-inch TMA T-loop, used for reciprocal space closure and described by Burstone, generates relatively high horizontal forces of ap- proximately 350 gm. Furthermore, moment preac- tivation bends must be placed at various points of the spring, depending on the interbracket dis- tance. 5 These moment preactivation bends, plus the horizontal force activation, allow the precise con- trol of the moment to force ratio (M/F), which, in turn, determines how a tooth will move.

Bantleon modified the length of the vertical loop arms from 7 mm to 8.5 mm and replaced the moment preactivation bends with a curvature of the proximal and distal legs. By increasing the height of the loop, the horizontal force is reduced to 250 gm.

The study was undertaken to examine the force

149

150 Hoenigl et aL American Journal of Orthodontics and Dentofacial Orthopedics August 1995

/

Fig. 3. 90 bend was made on anterior part of loop.

Fig. 1. Computer controlled measuring apparatus.

Fig. 2. Distance of 8 mm was marked on' both sides of loop (23 mm interbracket d is tance-7 mm loop activation = 16 ram; 16/2 = 8).

and moment character i s t i cs o f th is p re fabr ica ted ,

preact ivated centered T-loop.

MATERIALS AND METHODS

The measuring apparatus consisted to two units rep- resenting the active (anterior) and reactive (posterior)

segments of the teeth. To simulate the clinical situation, a Burstone vertical-tube bracket was placed on the an- terior segment (alpha unit), and a molar attachment with an auxiliary tube was placed on the posterior segment (beta unit).

On both units the horizontal and vertical forces were measured by means of four force recorders, and the moments were measured in their corresponding planes by means of two moment recorders. A computer-con- trolled step motor moved the beta unit in 0.2 mm steps toward the alpha unit. After each step, the analogue signals sent by the measuring amplifiers (CPM 16, Hot- tinger Baldwin, Germany) were digitized and read in a personal computer (Fig. 1). 6

Twenty prefabricated and preactivated centered T- loops (LPI Ormco, Vienna, Austria) were picked at random and assembled into groups of five each. Each group was measured for one of the interbracket distances of 21, 24, 27, and 30 mm: 7 mm of loop activation was subtracted from the interbracket distance and the re- maining distance divided by two, and this distance was marked on each side of the loop (Fig. 2). This halving of the remaining distance places the loop at the center of the two attachments. A 90 bend was made on the anterior part of the loop with Jarabak pliers in order that it can be inserted into the Burstone vertical tube bracket (Fig. 3).

To investigate the reproducibility of the loop appli- cation, each of three persons selected two T-loops that were then mounted at an interbracket distance of 24 mm on the measuring apparatus.

The data were used to evaluate arithmetic means, standard deviations, medians, and the first and third quartiles. One way analysis of variance was applied to compare the force systems produced by each of the three observers. RESULTS

At the four interbracket distances the arith- metic mean of the initial hor izontal force at a loop

American Journal of Orthodontics and Dentofacial Orthopedics Hoenigl et al. 151 Volume 108, No. 2

Table I. Arithmetic mean, standard deviation, minimum, maximum, median, first and third quartile of the horizontal force, and arithmetic mean of the moment-to-force ratio at 7 mm loop activation and after 3 mm loop deactivation

lnterbracket distance

30 mrn 27 rnm 24 mm 21 mm

Horizontal force 7 mm activation

Arithmetic mean 241 256 247 230 Standard deviation 21 19 21 9 Minimum 224 238 229 222 Maximum 275 287 275 245 1. Quartile 225 240 231 223 Median 239 253 235 227 3. Quartile 258 273 269 239

4 mm activation Arithmetic mean 132 138 132 110 Standard deviation 18 11 17 6 Minimum 111 124 117 103 Maximum 160 155 155 119 1. Quartile 118 129 118 105 Median 126 139 124 106 3. Quartile 148 147 151 115

M/F- ratio Arithmetic mean Alpha-position 7 mm activation 7.6 7.4 7.7 8 4 mm activation 12.3 11.5 11.7 13.2 Beta-position 7 mm activation 7.4 7.5 8.6 9.2 4 mm activation 12.8 12.8 14.4 14.8

activation of 7 mm was 230 to 256 gm. The lowest force measured was 222 gm, and the highest was 287 gm (Table I).

After 3 mm of deactivation, the average hori- zontal force decreased to 110 to 138 gm. The lowest force measured was 103 gm, whereas the highest was 160 gin. The mean horizontal force at all four interbracket distances is illustrated in Fig. 4.

The average alpha moment-to-force ratio was 7.4 to 8.0 at 7 mm loop activation and increased to 11.5 to 13.2 as the spring was deactivated to 4 mm (3 mm of deactivation). The beta moment-to-force ratio was 7.4 to 9.2 at 7 mm loop activation and increased to 12.8 to 14.8 as the spring was deacti- vated to 4 mm (also 3 mm of deactivation). The mean moment-to-force ratios at all four inter- bracket distances are illustrated in Fig. 5. The first and third quartile, as well as the arithmetic mean over the 3 mm of deactivation for the 27 mm interbracket distance, are shown in Fig. 6, to dem- onstrate the narrow range of the moment-to-force ratios.

For each of the measured intervals from 2 to 7 mm of loop activation, none of the tested variables alpha moment, beta moment, alpha

moment-to-force ratio, and beta moment-to-force ratio showed a significant difference among the three test persons.

DISCUSSION

When a force is applied to the bracket of a typical tooth, which might have a 10 mm distance between the bracket and the center of resistance, a tipping moment of a force is created with its magnitude equal to the magnitude of the force times the distance of 10 mm (M = F x d). To achieve bodily movement, an attraction spring must produce a countermoment of the same magnitude as the tipping moment.

At a loop activation of 7 mm, the moment-to- force ratio produced with the centered T-loop is 7.5 to 8. With this M/F ratio, the anterior segment is tipped distally with a center of rotation close to the apex of the central incisors (controlled tipping).

During the deactivation of the T-loop, the mo- ment-to-force ratio actually increases with the cen- ter of rotation moving from the apex outwardly to infinity (translation). First tooth movement is con- trolled tipping, but then changes to bodily move- ment or translation as the M/F approaches 10.

152 Hoenigl et al. American Journal of Orthodontics and Dentofacial Orthopedics August 1995

Horizontal Force in g

300

250

200

150

100

50

0 I I I I I I I 7 6,5 6 5,5 5 4,5 4 3,5

Activation in mm

30 mm , 27 mm ~, 24 mm ~ 21 mm

Fig. 4. Mean horizontal force at four interbracket distances.

16

14

12

10

8

6

4

2

0

a lpha- M/F J

I I I I I I I

6,5 6 5,5 5 4,5 4 3,5

Activation in mm

18

16 14

12

I0

8

6

4

2

0

beta - MIF

I I

6,5 6

30 mm *

I I I I I

5,5 5 4,5 4 3,5

Activation in mm

27 mm " 24 mm ~ 21 mm t I

Fig. 5. Mean alpha and beta moment-to-force ratios at four interbracket distances.

After a deactivation of 3 mm, the alpha moment- to-force ratio is 11.5 to 13.2, which means more distal root movement is occurring than distal crown movement. The roots of the anterior segment are

16

14,

12

10

8

16

14

12

10

8

6

4

2

Alpha Moment-to-Force Ratio

I I f I I I I

6,5 6 5,5 5 4,5 4 3,5

Activation in mm

I I I i I I I

6,5 6 5,5 5 4,5 4 3,5

Activation in mm

mean . . . . Q25 . . . . Q75 I

Fig. 6. Arithmetic mean, first and third quartile of alpha and beta moment-to-force ratios at interbracket distance of 27 mm.

now made upright again with an ultimate center of rotation at or close to the brackets of the anterior teeth (root uprighting).

The average moment-to-force ratio in the pos- terior segment is slightly higher, thus inducing light vertical forces. The posterior segment undergoes, just like the anterior segment, controlled tipped, translation, and then root uprighting.

After a deactivation of 3 mm the T-loop should be exchanged to avoid root abutment. Differences in the horizontal force and moment-to-force ratios between the four interbracket distances are of minimal clinical relevance. Therefore only one size of the preactivated centered T-loop is needed for reciprocal space closure, and this can be adjusted according to the interbracket distance. The op- erator has to merely measure the "interbracket" distance between the mesial surface of the molar attachment to the distal surface of the vertical slot of the Burstone bracket or, alternatively, of a washer soldered on the anterior stabilizing seg- ment. Seven millimeters of loop activation are subtracted from this interbracket distance; the

American Journal of Orthodontics and Dentofacial Orthopedics Hoenigl et al. 153 Volume 108, No. 2

Fig. 7. T-loop is more opened in alpha position, thus indicating lower moment-to-force ratio there. Activation of this loop has to be corrected.

remaining interbracket distance is divided by two, and this distance is marked on each side of the loop by means of the accompanying pattern. To obtain a 90 bend at the mesial mark, it is im- portant that the wire is "overbent" and adjusted by bending back to 90 . By doing so, one can be sure that, after insertion in the mouth, the bend is in fact of 90 and that the alpha moment has a sufficient magnitude.

The great advantage of the centered T-loop can be seen in the possibility to control clinically the force system in the patient's mouth. Both horizon- tal arms must be opened to an equal amount; if one arm is opened more than the other, the moment there is less and the activation must be adjusted (Fig. 7).

CONCLUSIONS

With the one-way analysis of variance, no sig- nificant difference between the subjects was found. It can be concluded, then, that the centered T-loop can produce the necessary force systems for a net translation of teeth and that these force systems can be duplicated by different operators.

REFERENCES

1. Burstone CJ, Pryputniewicz J. Holographic determination of centers of rotation produced by orthodontic forces. AM J ORTHOD 1980;77:121-32.

2. Vanden Bulcke MM, Burstone CJ, Sachdeva RCL, Dermaut LR. Location of the centers of resistance for anterior teeth during retraction using the laser reflection technique. AM J ORTHOD DENTOFAC ORTHOP 1987;91:375-84.

3. Burstone CJ. The rationale of the segmented arch. AM J ORTHOD 1962;48:805-21.

4. Burstone CJ, Koenig HA. Optimizing anterior and canine retraction. AM J ORTHOD 1976;70:1-20.

5. Burstone CA. The segmented arch approach to space closure. AM J ORTHOD 1982;82:361-78.

6. Bantleon HP. Die Simulation von Zahnbewegungen in Ab- h~ingigkeit von Drehmoment und Kraft unter Anwendung einer daftir entwickelten elektronischen Me[3apparatur. Medizinische Habilitationschrift. Graz 1989.

Reprint requests to: Dr. Hans-Peter Bantleon Universit~itsklinik fiir ZMK Abteilung fiir Kieferorthop~idie W~ihringerstr. 25a A-1090 Wien, Austria