The Second Stage of Comprehensive Treatment

Palatal Anchorage: The relative stability of the anterior palate, both the soft tissue rugae and the cortical bone beneath them, is one possibility for obtaining this additional anchorage. Although removable appliances contact the palate, they are not effective in moving molars back, probably because they do not fit well enough. A fixed appliance that stabilizes the premolars and includes a plastic pad contacting the rugae is needed. Fortunately, most patients tolerate this with minimal problems, but contacting the palatal tissue has the potential to cause significant tissue irritation, to the point that the appliance has to be removed.

Once palatal anchorage has been established, there are several possibilities for generating the molar distalizing force. Austenite nickel–titanium (A-NiTi) coil springs compressed against the molars (from an anterior anchorage unit) produce an effective and nearly constant force system for the distal movement. Magnets in repulsion also can be used (Figure 15-3), but the amount of force changes markedly as tooth movement occurs. A-NiTi springs have the additional advantage of being less bulky and usually are a better choice. The pendulum appliance (Figure 15-4) uses beta-titanium (beta-Ti) springs that extend from the palatal acrylic and fit into lingual sheaths on the molar tube, which gives greater control of these teeth

In a small but well-characterized sample of patients who were treated to a super-Class I molar relationship with the pendulum appliance activated to produce 200 to 250 gm, Byloff et al found that molar movement averaged just over 1 mm/month (1.02 ± 0.68), with a considerable degree of distal tipping of the crown and an elevation of the molar (Figure 15-5, A).⁵ As one would expect, despite the contact of the appliance with the palate, the premolars and incisors were tipped anteriorly, but the molar moved distally 2 to 3 times as far as the anchor teeth. When the appliance was modified to minimize distal tipping of the molar, the distal movement of the molar crown was similar, but greater distal movement of the roots was obtained at the cost of increased treatment time and some additional forward movement of the incisors (Figure 15-5, B).⁶

However the molars were moved distally, they must be held there while the other teeth are then retracted to correct the overjet (see Figure 15-3). It is one thing to move molars back, and something else to maintain them in that position. Simply leaving the distalization appliance in place for 2 to 3 months leads to distal movement of the premolars by stretched gingival fibers, but as soon as the original premolar-based lingual arch and palatal pad are removed, a new lingual arch and pad from the distalized molars must be placed. Even so, especially if the molar tipped distally, it will tip mesially again as the space closes. Placing a tipback in the distalizing springs will keep the molar more upright and minimize relapse, but this increases the extrusive tendency, so as with headgear, the most successful molar distalization with the pendulum appliance occurs in patients who have vertical growth during their treatment. Even so, data show that on the average, much of the original distalization is lost during the second phase of treatment with a complete fixed appliance (Figure 15-6).

Headgear or Class II Elastics: The problem with headgear for this type of tooth movement always has been that force of moderate intensity with long duration is needed, while headgear tends to supply relatively high force with only medium duration even in cooperative patients. Unless second molars are extracted (see below), significant distal movement of first molars (>2 mm) with headgear occurs only when the molar is extruded simultaneously, which is acceptable in a patient with substantial growth in height of the ramus, but otherwise leads to unfavorable downward-backward rotation of the mandible. High-pull headgear is not very effective in distalizing molars.

In theory, force from Class II elastics also can be used to push the upper molars distally, by using a sliding jig to concentrate the force on the molars. This was a mainstay of the original Tweed technique. There is the risk of considerably more mesial movement of the lower teeth than distal movement of the upper teeth, however. In modern orthodontics, the major use of Class II elastics to a sliding jig would be to accentuate rotation of the upper molar as a component of correcting the molar relationship.

Skeletal Anchorage: At this point, the advantage of skeletal anchorage for distalization of molars is so great that it is rapidly replacing the previous methods. With miniplates or a long bone screw at the base of the zygomatic arch (Figure 15-7)⁷ or bone screws in the palate to stabilize a lingual arch with springs for distalization,⁸ all the maxillary teeth can be moved back simultaneously (see Figure 18-47). Alveolar bone screws also are a possibility, but if they are between tooth roots, they block mesiodistal changes in tooth position, so using them for distalization requires changing screw position during treatment. With skeletal anchorage of any type, a fixed appliance with NiTi coil springs provides the best force system for distal movement.

Skeletal anchorage or not, two objects cannot occupy the same space at the same time, and there is only so much room at the rear of the maxillary arch. This means that in adolescents, extraction of the second molar (see below) may be necessary, and if second molars are distalized, early extraction of the third molar is indicated so that they do not end up impacted.

Spring Properties: The spring properties of a closing loop are determined almost totally by the wire material (at present, either steel or beta-Ti), the size of the wire, and the distance between points of attachment. This distance in turn is largely determined by the amount of wire incorporated into the loop but is affected also by the distance between brackets. Closing loops with equivalent properties can be produced from different types and sizes of wire by increasing the amount of wire incorporated into the loop as the size of the wire increases and vice versa. Wires of greater inherent springiness or smaller cross-sectional area allow the use of simpler loop designs.

Figure 15-14, taken from the work of Booth with steel wires,¹³ illustrates the effects on the spring characteristics of a steel closing loop from changing wire size, the design of the loop, and the interbracket span (the combination of these latter two parameters, of course, determines the amount of wire in the loop). Note that, as expected, changing the size of the wire produces the largest changes in characteristics, but the amount of wire incorporated in the loop is also important. The same relative effect would be observed with beta-Ti wire. For any size of wire or design of loop, beta-Ti would produce a significantly smaller force than steel.

Root-Paralleling Moments: To close an extraction space while producing bodily tooth movement, a closing loop must generate not only a closing force but also appropriate moments to bring the root apices together at the extraction site. As we have discussed in Chapter 9, for bodily movement, the moment of the force used to move the teeth must be balanced by the moment of a couple. If the center of resistance of the tooth is 10 mm from the bracket, a canine tooth being retracted with a 100 gm force must also receive a 1000 gm-mm moment if it is to move bodily. If the bracket is 1 mm wide, a vertical force of 1000 gm must be produced by the archwire at each side of the bracket.

This requirement to generate a movement limits the amount of wire that can be incorporated to make a closing loop springier because, if the loop becomes too flexible, it will be unable to generate the necessary moments even though the retraction force characteristics are satisfactory. Loop design is also affected. Placing some of the wire within the closing loop in a horizontal rather than vertical direction improves its ability to deliver the moments needed to prevent tipping. Because of this and because a vertically tall loop can impinge on soft tissue, a closing loop that is only 7 to 8 mm tall while incorporating 10 to 12 mm or more of wire (e.g., a delta-, L-, or T-shaped loop) is preferred.

If the legs of a closing loop were parallel before activation, opening the loop would place them at an angle that in itself would generate a moment in the desired direction. Calculations show that unacceptably tall loops would be required to generate appropriate moments in this manner,¹⁴ so additional moments must be generated by gable bends (or their equivalent) when the loop is placed in the mouth. An elegant solution to the design of a closing loop that would provide optimum and nearly constant moment-to-force ratios at variable activations was offered by Siatkowski in his Opus loop (Figure 15-15).¹⁵

Location of the Loop: A final engineering factor in the performance of a closing loop is its location along the span of wire between adjacent brackets. Because of its gable bends, the closing loop functions as a V-bend in the archwire, and the effect of a V-bend is quite sensitive to its position. Only if it is in the center of the span does a V-bend produce equal forces and couples on the adjacent teeth (see Figures 9-40 and 9-41). If it is one-third of the way between adjacent brackets, the tooth closer to the loop will be extruded and will feel a considerable moment to bring the root toward the V-bend, while the tooth farther away will receive an intrusive force but no moment.¹⁶ If the V-bend or loop is closer to one bracket than one-third of the distance, the more distant tooth will not be intruded but will receive a moment to move the root away from the V-bend (which almost never is desirable).

For routine use with fail-safe closing loops (as described later), the preferred location for a closing loop is at the spot that will be the center of the embrasure when the space is closed (Figure 15-16). This means that in a first premolar extraction situation the closing loop should be placed about 5 mm distal to the center of the canine tooth. The effect is to place the loop initially at the one-third position relative to the canine. The moment on the premolar increases as space closure proceeds. This is not ideal for maximum anchorage but is unavoidable with a loop in a continuous archwire. The design of the Opus loop calls for an off-center position with the loop 1.5 mm from the mesial (canine) bracket, which makes it more efficient than the simple loop design but more difficult to manage clinically.

Additional Design Principles: An important principle in closing loop design is that, to the greatest extent possible, the loop should “fail safe.” This means that, although a reasonable range of action is desired from each activation, tooth movement should stop after a prescribed range of movement, even if the patient does not return for a scheduled adjustment. Too long a range of action with too much flexibility could produce disastrous effects if a distorted spring were combined with a series of broken appointments. The ideal loop design therefore would deliver a continuous, controlled force designed to produce tooth movement at a rate of approximately 1 mm per month but would not include more than 2 mm of range, so that movement would stop if the patient missed a second consecutive monthly appointment.

It also is important that the design be as simple as possible because more complex configurations are less comfortable for patients, more difficult to fabricate clinically, and more prone to breakage or distortion. Engineering analysis, as the Opus loop demonstrates very nicely, shows that a relatively complex design is required to produce the best control of moment–force ratios. The possibilities of clinical problems from increased complexity always must be balanced against the potentially greater efficiency of the more complex design. The Opus loop has not been widely adopted because of concerns about its complexity and sturdiness. Clinical experience suggests that the average adolescent orthodontic patient can—and probably will—destroy almost any orthodontic device that is not remarkably resistant to being distorted.

A third design factor relates to whether a loop is activated by opening or closing. All else being equal, a loop is more effective when it is closed rather than opened during its activation. On the other hand, a loop designed to be opened can be made so that when it closes completely, the vertical legs come into contact, effectively preventing further movement and producing the desired fail-safe effect (Figure 15-17). A loop activated by closing, in contrast, must have its vertical legs overlap. This creates a transverse step, and the archwire does not develop the same rigidity when it is deactivated. The smaller and more flexible the wire from which a closing loop arch is made, the more important it is that the wire become rigid when the loop is deactivated.

Clinical Recommendations: These design considerations indicate that an excellent closing loop for 18-slot edgewise is a delta-shaped loop in 16 × 22 steel wire that is activated by opening, as shown in Figure 15-17. Such a wire fits tightly enough in an 18 × 25 mil bracket to give good control of root position. With 10 mm of wire in the loop, the force delivery is close to the optimum, and the mechanism fails safe because contact of the vertical legs when the loop is deactivated limits movement between adjustments and makes the archwire more rigid. It is important to activate the upper horizontal portion of a delta or T-loop so that the vertical legs are pressed lightly together when the loop is not activated (Figure 15-18). This also ensures that the loop will still be active until the legs come into contact.

With 16 × 22 wire and a loop of the delta design (so that the mechanism fails safe), with an activation of 1.0 to 1.5 mm, and with narrow 18-slot brackets, a gable bend of approximately 20 degrees on each side is needed to achieve an appropriate ratio between the moment of the couple and the moment of the force (M_C/M_F ratio) (Figure 15-19). With wider brackets, a smaller gable bend would generate the same moment. With the same loop in stiffer wire like 17 × 25, a gable bend of any given magnitude would produce a larger moment than in 16 × 22 wire. Remember, however, that the M_C/M_F ratio determines how teeth will move, so with a stiffer wire and the same activation a larger force would be generated and a larger moment would be needed. Optimum moment–force ratios can be achieved with several combinations of wire size, loop configuration, and gable angle and, as Siatkowski has shown, can be maintained over a variety of activations at the cost of design complexity.

A closing loop archwire is activated by pulling the posterior part of the archwire distally through the molar tubes, which activates the closing loop the desired amount (1 to 1.5 mm) and then fastening the wire in that position. The wire slides through the brackets and tubes only when it is being activated. After that, as the closing loop returns to its original configuration, the teeth move with the archwire, not along it, so there is no resistance to sliding. There are two ways to hold the archwire in its activated position. The simplest is by bending the end of the archwire gingivally behind the last molar tube. The alternative is to place an attachment—usually a soldered tieback (see Figure 15-16) on the posterior part of the archwire, so that a ligature can be used to tie the wire in its activated position.

With a 16 × 22 closing loop, usually it is necessary to remove the archwire and reactivate the gable bends after 3 to 4 mm of space closure, but a quick reactivation is all that is needed at most appointments during space closure. As a general rule, if it is anticipated that a closing loop archwire will not have to be removed for adjustment (i.e., the distance to be closed is 4 mm or less), bending the posterior end of the wire is satisfactory. It can be quite difficult to remove an archwire that has been activated by bending over the end, however, and it saves time in the long run to use tiebacks for closing loop archwires that will have to be removed and readjusted.

Specific recommendations for closing loop archwires with the 18-slot appliance and narrow brackets are:

These recommendations certainly are not the only clinically effective possibilities. The principle should be that if a heavier wire (e.g., 17 × 25 mil) is used, the loop design should be altered to incorporate additional wire for better force-deflection characteristics. Also, the gable angulations should be adjusted according to both the springiness of the loop and the width of the brackets. With wide brackets on the canines, for instance, the reduced interbracket span would make any loop somewhat stiffer, and both this and the longer moment arm across the bracket would dictate a smaller gable angle, but the range of the loop would be reduced, which is why wide brackets are not recommended when closing loops are to be used.

1. The moments necessary for root paralleling are generated automatically by the twin brackets normally used with the 22-slot appliance. Unless the archwire itself bends, there is no danger that the teeth will tip excessively (Figure 15-21).

2. The rigid attachment of the canine to the continuous ideal archwire removes the danger that this tooth will be moved far outside its intended path if the patient does not return for scheduled adjustments. For this reason, a long range of action on the retraction springs is not dangerous, as long as the force is not excessive. The ideal force to slide a canine distally is 150 to 200 gm, since at least 50 to 100 gm will be used to overcome binding and friction (see Chapter 9). A-NiTi springs can produce this level of force over a wide enough range to close an extraction space with a single activation.

1. Add stabilizing lingual arches and proceed with en masse space closure. The resulting increase in posterior anchorage, though modest, will change the ratio of anterior retraction to posterior protraction to approximately 2 : 1.

2. Reinforce maxillary posterior anchorage with extraoral force and (if needed) use Class III elastics from high-pull headgear to supplement retraction force in the lower arch, while continuing the basic en masse closure approach. Depending on how well the patient cooperates, additional improvement of retraction, perhaps to a 3 : 1 or 4 : 1 ratio, can be achieved.

3. Retract the canines independently, preferably using a segmental closing loop, and then retract the incisors with a second closing loop archwire. Used with stabilizing lingual arches (which are needed to control the posterior segments in most patients), this technique will produce a 3 : 1 retraction ratio, but the added complexity and increased treatment time of this two-step method makes skeletal anchorage a more practical approach.

4. Use bone screws to stabilize the posterior segments. This is the ultimate reinforcement of anchorage, and it now is used to avoid two-step space closure.

Reinforcement with Stabilizing Lingual Arches: Stabilizing lingual arches must be rigid and should be made from 36 mil or 32 × 32 steel wire. These can be soldered to the molar bands, but it is convenient to be able to remove them, and Burstone’s designs (see Chapter 10) are preferred.

It is important for a lower stabilizing lingual arch to lie behind and below the lower incisors, so that it does not interfere with their retraction. If 36 mil round wire is used, the lower lingual arch is more conveniently inserted from the distal than from the mesial of the molar tube. The maxillary stabilizing lingual arch is a straight transpalatal design. Because maximum rigidity is desired for anchorage reinforcement, an expansion loop in the palatal section of this wire is not recommended unless a specific indication exists for including it.

If lingual arches are needed for anchorage control, they should be present during the first and second stages of treatment but can and should be removed after space closure is complete. Their presence during the finishing stage of treatment, after extraction spaces have been closed, is not helpful and may interfere with final settling of the occlusion.

Reinforcement with Headgear and Interarch Elastics: Extraoral force against the maxillary posterior segments is an obvious and direct method for anchorage reinforcement. It is also possible to place extraoral force against the mandibular posterior segments but is usually more practical to use Class III elastics to transfer the extraoral force from the upper to the lower arch.

Interarch elastics for anchorage reinforcement were a prominent part of the original Tweed method for maximum retraction of protruding anterior teeth. In the Tweed approach to bimaxillary protrusion, “anchorage preparation,” achieved by tipping the molars and premolars distally, was done before space closure. As anchorage was being prepared in the lower arch, Class III elastics were used to maintain lower incisor position while this is done. After the lower incisors were retracted, sliding the canine initially and then using a closing loop, the lower arch was stabilized and Class II elastics were used to prepare anchorage by tipping back the upper molars, before the upper incisors were retracted.

Although the original Tweed approach can be used with the contemporary 18-slot appliance, it is rarely indicated. Prolonged use of Class II and Class III elastics is extrusive and requires good vertical growth for acceptable results. Distally tipping the molars augments their anchorage value primarily by first moving these teeth distally, then mesially.

Segmented Retraction of the Canines: Individualized retraction of the canines with segmented closing loops is an attractive method for reducing the strain on posterior anchorage and is a readily available approach with the modern 18-slot appliance. It is also possible to retract the canines by sliding them on the archwire, but the narrow brackets usually used with the 18-slot appliance and the tight clearance and relatively low strength of a 17 × 25 archwire produce less-than-optimum sliding.

For use of segmented closing loops, an auxiliary tube on the molar is needed. An auxiliary tube on the canine is unnecessary because the retraction spring can fit directly into the canine bracket. The PG spring designed by Gjessing is an efficient current design (Figure 15-23),¹⁸ but is somewhat complex to fabricate and activate. After the canine retraction, closing loops, either in a continuous archwire or with a segmented arch approach, are then used for the second stage of retraction of the incisors.

Segmented canine retraction of this type presents two problems. The first is that it is difficult to control the position of the canine in all three planes of space as it is retracted. If the canine is pulled distally from an attachment on its buccal surface, the point of attachment is not only some distance occlusal but is also buccal to the center of resistance. This means that without appropriate moments, the tooth will tip distally and rotate mesiobuccally. Both a root-paralleling moment and an antirotation moment must be obtained by placing two different gable bends in the same spring. Control of the vertical position of the canine, particularly after the gable bends in two planes of space have been placed, can be a significant problem.

Second, much more than with en masse retraction using segmented mechanics, segmental retraction of canines does not fail safe. The canine is free to move in three-dimensional space, and there are no stops to prevent excessive movement in the wrong direction if a spring is improperly adjusted or becomes distorted. Loss of vertical control is particularly likely. A missed appointment and a distorted spring can lead to the development of a considerable problem, and patients must be monitored carefully.

Retraction with Skeletal Anchorage: Skeletal anchorage for retraction of protruding incisors is most easily accomplished by placing a bone screw in the dental alveolus between the second premolar and first molar (see Figure 10-48). Using the bone screw to stabilize the posterior segment (indirect anchorage) while closing the extraction space with loop mechanics is preferred, especially if the 18-slot appliance is used. The closing loop would be designed and activated as described above.