Controlled retraction of maxillary incisors Poul Gjessing, DDS* Aarhus. Denmark The force characteristics inherent in the prefabricated PG retraction spring have been shown to be efficient for controlled movement of canines. In the present investigation, this spring was analyzed with regard to its applicability for controlled retraction of the maxillary incisors. A description of the required system of moments and horizontal and vertical forces, acting at both the active (alpha) and reactive (beta) units, is presented. The three-dimensional force system generated by specific modifications of the spring was registered in a bench testing device mounted with strain gauges, and the resultant data presented in a graphic. It is concluded that the PG retraction spring can be used as a module for controlled retraction of both canines and incisors. The magnitude of horizontal and vertical forces is kept within the anticipated physiologic limits and can be identified by the shape of the activated spring. Variability of the distance between the anterior and the posterior points of force attack has no significant influence on the horizontal and vertical forces produced. Only minor clinical adjustments are needed to modify the retraction spring from canine-to-incisor retraction. (AMJ ORTHODDENTOFACORTHOP1992;101:120-31 .)

C o r r e c t positioning of the maxillary incisors is recognized to be essential for function, stability, and esthetics. Retraction of the maxillary incisors therefore represents a fundamental and often critical stage in orthodontic treatment. The resultant movement of the incisors depends on the tissue reaction produced by the applied biomechanical force system. A precondition for accomplishment of the desired incisor movement is remodeling of the premaxilla. Though implant studies t and laminagraphic investigation 2 have shown no evidence of permanent changes in the shape of the anterior palatal cortex that curves downward from a horizontal position to the more vertical alveolar process, torquing the root apex through this part of the palatal cortex resulted in relapse of the incisor inclination to a more vertical position and apical root resorption, followed by reestablishment of the pretreatment structure of the cortex. 2 Fig. 1 shows a typical result of maxillary incisor retraction in patients treated with the Begg technique and analyzed by the laminagraphic method. The overall treatment and the esthetic result are dominated by a dramatic extrusion of the incisors. This unwanted side effect is partially due to the uncontrolled apical tipping at the beginning of treatment and partially due to vertical forces generated as an adjunct to the moment induced for palatal root torque. 3 Similar findings are also

*Clinical Associate Professor, Department of Orthodontics, Royal Dental College. 811123137

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seen in the traditional type of edgewise mechanics, characterized by the use of continuous arch wires and by the limited control of vertical forces. To compensate for this unwanted side effect, clinicians have attempted to change the continuous wire into the shape in which one would expect intrusional forces to be induced to the anterior teeth. Unfortunately, this goal is difficult to achieve. The multiplicity of active and reactive forces developed,4 frequently influence the vertical position of the incisors in an unpredictable way. Buccal extrusions rather than the expected intrusion of the incisors occurs. Segmentation. The desired optimal distribution of forces between the anterior and the posterior parts of the upper arch can be obtained by dividing the arch into segments2 "6 Each segment is consolidated in a rigid entity by a section of heavy rectangular arch wire in tight contact with the bracket slots. One anterior segment, the active talil, contains the four incisors, whereas the two posterior segments include the buccal teeth of each side. The posterior segments are connected by a transpalatal arch to form one rigid multirooted entity, the reactive tlnil. Segmentation allows for the use of prefabricated springs that are precalibrated to produce the desired retraction, uprighting, and intrusion of the incisors. The physical characteristics of the retraction spring should be determined by the required balance between forces and couples produced at the points of force application during a certain amount of activation. Calculation of this balance is related to the position of the center of resistance (CR) of the segments.

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..j Fig. 1. Typical result of maxillary incisor retraction in patients treated with Begg technique and analyzed by laminagraphic technique. The anteroposterior repositioning of the incisors is impeded by the limited labiooral extent of the occlusal part of the premaxilla. This is commonly seen in Class II cases with overextruded maxillary front teeth. (From Ten Hoeve A, Mulie RM. J Clin Orthod 1976;10:804-22.)

Investigations on dry human skulls with the laser reflection technique and holographic interferometry7"8 have found the CR of the anterior segment to be localized between the canine and the first premolars at a horizontal distance of approximately 10 mm from the center of the lateral bracket. The vertical position was 9 to 10 mm above the level of the bracket slots. The use of a dry skull in this context has been shown to be of limited value as far as orthopedic forces are concemed. 9 It is not clear whether the dry skull is a useful model to evaluate initial orthodontic tooth movements. A recent study of initial tooth movements based on human autopsy material demonstrated a position of CR as shown in Fig. 2 measuring 7 mm distal and 9 to 10 mm gingival to center of the lateral bracket. ~° By activation of a retraction spring, a complex system of forces and couples is transmitted through the brackets of the segments (Fig. 2). There is one point of force application on each side of the incisor segment, the alpha position, and one point at each of the posterior segments, the beta position. The resultant direction and magnitude of force H acting at the alpha position is determinant for the movement of the incisors, whereas the resultant force at the beta position is also influenced by the force of occlusion. Analysis of the force system is facilitated by breaking down the resultant forces into horizontal vectors, vertical vectors, and moments. According to this scheme, the retraction spring is characterized by the (1) location of the points of force application, (2) magnitude of the horizontal force, (3) constancy of the horizontal force (load-to-deflection rate), and (4) magnitude and direction of moments acting at the alpha and beta positions.

iifi!ii: IOi!i!ii!iiiii!:iiii ili!iiiii ili!i:iiiiiii ! iiii : ::@.::~.......;'~:!:::i~...... i~ :~-i~:i~::i~~::~i~ ;!:: i~:: 4

Fig. 2. Theoretical force system generated by retraction spring monitored for controlled retraction of the maxillary incisor segment. Anterior point of attachment, lateral bracket; posterior point of attachment, molar tube.

Calculation of the theoretical force system required for simultaneous retraction uprighting, and intrusion of the anterior is exemplified in the following. (See Fig. 2.) It is considered that a retraction spring is calibrated to produced a moment-to-force (M/F) ratio of 9 by a horizontal retraction force of 100 gm. Accordingly the moment of the couple (solid arrow) delivered by the spring through the anterior point of attack (center of the lateral bracket) is 900 g m / m m . It is further anticipated that an intrusional force of 20 gm is directed through that point. Since the bracket center is located in a horizontal distance of 7 mm in front of CR, an additional moment of the vertical force (dotted arrow)

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is induced to the incisor segment. The magnitude of this moment is 7 × 20 g m / m m = 140 g m / m m . Thus the total alpha moment received by the incisor segment is an addition of the moment of the couple and the moment of the vertical force = 900 + 140 g m / m m = 1040 g m / m m . The total M / F ratio induced to each side of the incisor segment at this instant of activation is 1040:100 = 10. The concomitant force system acting at the beta position (center of the molar tube) is characterized by a mesially directed horizontal force of 100 gin, an occlusally directed force of 20 gm, and a moment produced by the posterior extension of the retraction spring. The magnitude of this beta moment (M beta) can be calculated from the formula Mt

Controlled retraction of maxillary incisors.

The force characteristics inherent in the prefabricated PG retraction spring have been shown to be efficient for controlled movement of canines. In th...
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