Bodily retraction of maxillary incisors with round wires

American Journal of ORTHODONTICS Volume 66, Number

ORIGINAL

Bodily retraction of maxillary with round wires Anthony

A. Gianelly,

D.M.D.,

1 July,

1974

ARTICLES

incisors

Ph.D.*

Boston, Mass.

.T

he intent of this report is to describe a biomechanical system in which round wires are used for bodily retraction of the six maxillary incisors. Bodily movement, or translation, indicates that the teeth should move with no change in axial inclination. Bodily movement, like other forms of tooth movement, can be defined by means of the location of the center of rotation during the movement. Accordingly, during translation the center of rotation should be at infinity (that is, it is nonexistent), since only translatory movement will occur when there is no center of rotation. To position the center of rotation at infinity, a force system applied to the crowns generally must incorporate the simultaneous and coordinated use of a coupling mechanism and a second “tipping” force, such as that derived from an extraoral appliance or an elastic attached to the arch wire or to0th.l The coupling mechanism directs the root apex lingually (or distally for canines) while the “tipping” force moves the crown lingually or distally. When the ratio of the couple to the second force is adjusted properly, the center of rotation during movement can theoretically be positioned at infinity. Practically, this relationship is difficult, if not impossible, to achieve. Conceptually, however, a force system used to move teeth bodily generally must have these two component parts, a couple and a second “tipping” force. (It is presumably conceivable that two “tipping” forces, one directed superoposteriorly and one directed inferoposteriorly, can be placed so that their net effect is to move the tooth bodily.)2 In the technique outlined in this article the teeth are banded and wide Siamese brackets are placed on the incisors. The canine brackets have vertical slots. An 0.016 inch wire is formed so that two vertical extensions of the wire *Professor and Chairman, of Graduate Dentistry.

Department

of Orthodontics,

Boston

University

School 1

Am. J. Orthod. JUlV 1974

Fig. 1. A, Lateral view of 0.016 inch wire with tip-backs, unactivated torquing loops, and receptacles for elastic and/or gear placement. B, 0.016 inch wire. with activated torquing loops. C to E, Activated wire in place; to allow full action of the tip-back bands, the premolars are unbanded. F, Right lateral view during space closure. G, Occlusal view following full space closure. H, Left lateral view during space closure. Time required for space

closure:

13 months.

cross the midpoint of the brackets of the central incisors (Fig. 1). A tip-back is placed in the region of the second premolars, unless an anterior open-bite exists or molar protraction is necessary. The couples on the central incisors, directing the root apices lingually (i.e., third-order forces), are formed by angulating the vertical extensions of the wire lingually at least 15 degrees (Fig. 1, B). The coupling mechanisms on the canines, directing the root apices distally (i.e., second-order forces), are formed by the bracket n-ire engagement since the wire contacts the mesial-superior and distal-inferior aspects of the bracket. The function of the tip-back is to help stabilize the molar and enhance the second order force on the canine. When no tip-back is placed in the wire, there will be some freedom in the bracket-wire fit, since the wire is 0.016 inch and the bracket is 0.022 inch. For this reason, the canines may tip until the bracket and the wire make contact. Once the bracket and wire make contact on opposite corners, a type of coupling mechanism is formed. Further movement occurs as a result of both the coupling mechanism and a second retracting force. No coupling mechanisms are placed on the lateral incisors because the lateral incisors apparently move in the same

Bodily

Fig. 2. position

A, Pretreatment of incisors and

cephalogram of a canines. B, Cephalogram

girl

retraction

16 years 8 months

of maxillary

5 months after start

incisors

3

of age. Note axial of treatment. A small

space still exists distal to the canines. A straight-pull headgear attached to the incisors was the only retracting force. The gear was worn 18 hours per day. The force on the gear was 10 to 12 ounces per side. C, Maxillary superimposition of preand midtreatment cephalograms. The crowns and roots of the incisors (top] and canines [bottom) were retracted at the same time but not at the same rate. Since axial control was reasonably good, no adjustments were made in the system.

fashion as the central incisors. At one time coupling mechanisms were placed on the lateral incisors as they were placed on the central incisors, but clinical experience suggested that this was unnecessary. As indicated previously, if the ratio and the couple and the second force are well coordinated, movement will occur in a bodily fashion and the center of rotation during this movement will be positioned at infinity. Since this is an unlikely probability, pure bodily movement probably does not occur. Howcvcr, movement dots occur in a manner indicating that the center of rotation may approach infinity (Figs. 2 to 5). Thus, although the movement is not strictly translational, there is movement of the crown and the root in the same direction, and further adjustments in root position, once the teeth are retracted, are relatively uncomplicated. In addition, the extent of crown and/or root movement can be controlled. For example, if the central incisor tends to become more upright during retraction, one has only to increase the force of the couple or decrease the magnitude of the “tipping” force. Similarly, if the canine starts tipping, either the force of the couple has to be increased or the magnitude of the second force has to be reduced. A common method of increasing the force of the couple on the canine is to insert an appropriate uprighting spring in the slot of the canine bracket.

Am.

J. Orthnd. July 1974

Fig. 3. A, Pretreatment cephalogram of a boy 12 years 10 months of age. B, Cephalogram 5 months after retraction forces were applied. The force system consisted of Class II elastics (4 ounces per side) and a high-pull gear (approximately 10 ounces per side). The canines were unerupted. C, Maxillary superimposition of preand midtreatment cephalograms, The incisor was both retracted and apparently intruded. Although, the crown and root moved in the same direction, the inclination of the incisor changed because the crown moved faster than the root. The path of the unerupted canine is also shown. No modifications were made in the system because axial control was adequate.

The most, frequent occurrence is crown and root movement in the same direction but not at the same rate. The crown usually moves faster (Figs. 2 to 5). Accordingly, this situation can be rectified by increasing the magnitude of the coupling mechanism or by reducing the magnitude of the tipping force. However, both adjustments tend to decrease the rapidity of crown movement. For this reason, we generally prefer not to vary force magnitudes. In addition, there is an element of self-correction in this system because the magnitude of retraction is complete (since further the second force is reduced, once cro\rn crown movement is unnecessary). Reduction of the magnitude of the second force readjusts the ratio of the couple to the second force so that relatively more root movement (root torque) occurs. The reason is as follows: Root torque and bodily movement necessitate the use of comparable biomechanical systems, the simultaneous application of a coupling mechanism, and a second tipping force. The difference is the magnitudc of the second force which is less in root torque than in bodily movement. Let us consider as an example root torque of a central incisor. The couple rotates the incisor around the centroid of the tooth, directing the root apex lingually whi’le the crown tip moves labially. The function of the second tipping

Bodily

Fig. 4. A, Pretreatment after

treatment

started.

cephalogram The incisors

retraction

of a 23-year-old are retracted about

woman. 70 per

of

llary incisors

5

B, Cephalogram 8 months cent of the extraction site.

The retraction system consisted of Class II elastics (6 to 8 ounces) and a high-pull gear (12 ounces per side). The gear was worn approximately 10 to 12 hours per day. C, Maxillary superimposition of preand midtreatment cephalograms. Note the moderate intrusion and almost bodily retraction of the central incisor. The canines were not traced because of the inability to see the contour of the canine in the cephalograms.

force is to block the labial crown movement so that crown position is maintained lingually. while the root moves In bodily movement, the second force not only has to block the potential labial crown movement produced by the couple application but also must move the root lingually. Accordingly, the force magnitude has to be increased. When no root movement occurs during retraction, the magnitude of the coupling mechanism is increased, even though crown movement may be slow (Fig. 6). Essentially, the adjustments represent clinical decisions based on constant reappraisal. The control of axial inclination is monitored by progress cephalograms. The magnitude of the retraction force varies from 6 to 12 ounces per side, depending on the individual response. When Class II elastics are used, approximately 6 ounces of force is generally applied on each side in the initial stages. If the response is slow, the force is augmented by 2 to 4 ounces per side. Often, Class II elastics are used in conjunction with a high-pull gear attached to the incisor segment, particularly in deep-bite malocclusions. In this situation, the elastic force is approximately 4 to 6 ounces, and the gear force is 8 to 12

6

Am.

Giavlelly

J. Orthod. Julv 1974

Fig. 5. A, Pretreatment cephalogram of a girl 12 years 6 months of age. B, Cephalogram 8 months after start of treatment. Incisor retraction is essentially complete. The retraction force consisted only of Class II elastics (6 to 8 ounces). C, Maxillary superimposition. Both the crowns and roots of the incisors and canines were retracted. Although both the crowns and roots moved in the same direction, root movement was slower than crown

movement.

Since

axial

control

was

adequate,

no

modifications

were

made.

ounces. However, the gear is worn only 12 to 14 hours a day. When a gear alone is used, we apply 10 to 14 ounces of force. A reasonably rapid response is usually produced when 6 to IO ounces of force is used on each side. These forces are approximately three to four times as large as forces used per side). Since efficient reto tip teeth relatiye1.v quickly (2 to 3 onnccs modeling of the attachment apparatus is apparently stimulated when the force is suhthreshold or nonper unit area is neither too little nor too great (that necrotizing), the success obtained with the use of light 2 to 3 ounce forces probably reflects the fact that relatively small pressure-tension zones are created tipping ; accortlingly. light forces can hc adequately distributed during crown over the involved areas. The larger force magnitudes used with this system to obtain bodily movement might indicate that large pressure-tension zones must be created in order to stimulate the appropriate remodeling changes for bodily movement in the attachment apparatus. Since more root surface area is involved, the magnitude of force must be increased over the tipping force level to establish a comparable force per unit area relationship. An average of 8 to 12 OL~CCS for bodily movement was selected because the results of an experiment which compared the vascular reactions of “tipping” and “bodily movement” forces indicated that the magnitude of the bodily movement force had to be increased by a factor of four to elicit a similar response

Bodily

Fig. 6. A, Pretreatment 2 months

after

start

of

cephalogram treatment;

of the

retra’ction

of maxillary

a girl 12 years 2 months retraction system consisted

of

incisors

7

age. B, Cephalogram of Class II elastics

(4

ounces per side) and a high-pull gear (10 ounces per side). C, Maxillary superimposition of preand midtreatment cephalogram. The incisors tipped lingually with a center of rotation close to the apex. The canines tipped with a center of rotation in the apical third. Since the roots did not move in the same direction as the crown, the biomechanical system was modified by increasing the magnitude of the coupling mechanism.

as a tipping force3 Presumably, the increase indicates that more surface area is involved in the bodily movement reactions. An 0.016 inch wire is used because it can often be introduced as the initial leveling wire, and the torquing loops are easily incorporated into the wire. Thus, a conventional leveling wire may not be necessary with 0.022 inch brackets. Finally, it is apparently tolerated quite well by patients. Although larger wires have not been tested, they obviously can be used. They would produce more efficient couples on the canine. Since torquing loop incorporation may be difficult in larger wires, a torquing auxiliary could be used for the incisor coupling mechanism. The speed of this system appears to be adequate for clinical use. With reasonably constant retraction forces, incisors are generally retracted within 8 to 14 months. Another prevalent method of retracting incisors is a two-phase process in which the crowns are first tipped lingually; the roots are then uprighted or torqued. The tipping movement, as indicated, can be accomplished rapidly and efficiently with light forces. Clinically, large protrusions can be reduced within 3 to 4 months. However, the necessary root torque phase requires the application of larger force magnitudes as well as the simultaneous application

Alti.

J. Orthod. JuW 1974

of a cm~plc ant1 il sccontl force. In acldition, the uprighting phase can last from 6 to 1-I months. Thus, incisor retraction in the two-phase procedure can last from 8 to 16 months. The bodily retraction hiomechanical system described ia this report compares favorably ill terms of both time for retraction and force magllitnelcs. P’nrthrrniorc~, it is rclirti\7tly u~~com~~liratecl to apply. REFERENCES

1. Burstone, C!. J. : Mechaniw; tllc‘ lGomrxhxnics Keidel, I<. A. (editors) : T’istas in Ortho(loutics,

of

Tooth Movemeut. In Iiraus, B. S., and Plriladt~lphia, 1962, Lea 8: Febiger, pp.

19i-213.

2. Ackerman, 454,

.J. A.:

A controlled

light

continuous

force

technique,

Ahf

J. ORTHOD.

56:

443-

1969.

3. Parks, TV. H., ant1 Sclln&ler, 8. A.; CI~:tugw in thcb vnscularity during tipping and bodily movements of varying drgrees of University S~~hool of Gr:lduatc~ Ihtist r,v, 197 I.

of the periodontal ligament forrc, M.Sc.D. thesis, Boston

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