Midline Discrepancies

A relatively common problem at the finishing stage of treatment is a discrepancy in the midlines of the dental arches. This can result either from a preexisting midline discrepancy that was not completely resolved at an earlier stage of treatment or asymmetric closure of spaces within the arch. Minor midline discrepancies at the finishing stage are no great problem, but it is quite difficult to correct large discrepancies after extraction spaces have been closed and occlusal relationships have been nearly established.

As with any discrepancy at the finishing stage, it is important to establish as clearly as possible exactly where the discrepancy arises. Although coincident dental midlines are a component of functional occlusion—all other things being equal, a midline discrepancy will be reflected in how the posterior teeth fit together—it is undesirable esthetically to displace the maxillary midline, bringing it around to meet a displaced mandibular midline. If a dental midline discrepancy results from a skeletal asymmetry, it may be impossible to correct it orthodontically, and treatment decisions will have to be made in the light of camouflage versus surgical correction (see discussion in Chapter 7).

Fortunately, midline discrepancies in the finishing stage usually are not this severe and are caused only by lateral displacements of maxillary or mandibular teeth accompanied by a mild Class II or Class III relationship on one side. In this circumstance, the midline often can be corrected by using asymmetric Class II (or Class III) elastic force. As a general rule, it is more effective to use Class II or Class III elastics bilaterally with heavier force on one side than to place a unilateral elastic. However, if one side is totally corrected while the other is not, patients usually tolerate a unilateral elastic reasonably well. It is also possible to combine a Class II or Class III elastic on one side with a diagonal elastic anteriorly to bring the midlines together (Figure 16-2). This approach should be reserved for small discrepancies. Prolonged use of Class II or Class III elastics during the finishing stage of treatment should be avoided. Coordinated steps in the archwires also can be used to shift the teeth of one arch more than the other.²

An important consideration in dealing with midline discrepancies is the possibility of a mandibular shift contributing to the discrepancy. This can arise easily if a slight discrepancy in the transverse position of posterior teeth is present. For instance, a slightly narrow maxillary right posterior segment can lead to a shift of the mandible to the left on final closure, creating the midline discrepancy. The correction in this instance, obviously, must include some force system to alter the transverse arch relationships (usually careful coordination of the maxillary and mandibular archwires, perhaps reinforced by a posterior cross-elastic). Occasionally, the entire maxillary arch is slightly displaced transversely relative to the mandibular arch so that with the teeth in occlusion, relationships are excellent, but there is a lateral shift to reach that position. Correction again would involve posterior cross-elastics but in a parallel pattern (i.e., from maxillary lingual to mandibular buccal on one side and the reverse on the other side; see Figure 16-2).

Tooth Size Discrepancies

Tooth size discrepancy must be taken into account when treatment is planned initially, but many of the steps to deal with these problems are taken in the finishing stage of treatment. Reduction of interproximal enamel (stripping) is the usual strategy to compensate for discrepancies caused by excess tooth size. When the problem is tooth size deficiency, it is necessary to leave space between some teeth, which may or may not ultimately be closed by restorations. As a general guideline, a 2 mm tooth size discrepancy noted from Bolton analysis is the threshold for clinical significance³ (i.e., that large a discrepancy predicts that steps to deal with it will be required during treatment), but at the finishing stage, you get to see how accurate the prediction really was.

One of the advantages of a bonded appliance is that interproximal enamel can be removed at any time. When stripping of enamel is part of the original treatment plan, most of the enamel reduction should be done initially, but final stripping can be deferred until the finishing stage. This procedure allows direct observation of the occlusal relationships before the final tooth size adjustments are made. A topical fluoride treatment always is recommended immediately after stripping is done.

Tooth size problems often are caused by small maxillary lateral incisors. Leaving a small space distal to the lateral incisor can be esthetically and functionally acceptable, but a composite resin buildup usually is the best plan for small incisors (Figure 16-3). Precise finishing is easier if the buildup is done during the finishing stage of the orthodontic treatment. This can be accomplished simply by removing the bracket from the small tooth or teeth for a few hours while the restoration is done, then replacing the bracket and archwires (but bonding to a laminate may damage the surface, so a buildup to establish tooth size is okay, but a laminate should be delayed). If the restoration is delayed until the orthodontics is completed, it should be done as soon as possible after the patient is in retention. This requires an initial retainer to hold the space, and a new retainer immediately after the restoration is completed. The main reason for waiting until after the orthodontic appliance has been removed would be to allow any gingival inflammation to resolve itself.⁴

More generalized small deficiencies can be masked by altering incisor position in any of several ways. To a limited extent, torque of the upper incisors can be used to compensate: leaving the incisors slightly more upright makes them take up less room relative to the lower arch and can be used to mask large upper incisors, while slightly excessive torque can partially compensate for small upper incisors. These adjustments require third-order bends in the finishing archwires. It is also possible to compensate by slightly tipping teeth or by finishing the orthodontic treatment with mildly excessive overbite or overjet, depending on the individual circumstances.⁵

Root Paralleling

In the Begg technique (see Figure 16-1), the moments necessary for root positioning were generated by adding auxiliary springs into the vertical slot of the Begg (ribbon arch) bracket. In most instances, a heavier (20 mil) archwire replaced the 16 mil archwire used as a base arch up to that point to provide greater stability. Root paralleling was accomplished by placing an uprighting spring in the vertical slot and hooking it beneath the archwire. Since root-paralleling forces are also crown-separating forces, it was important to tie the crowns together across extraction sites. In the modified Begg technique using the Tip-Edge appliance, root paralleling is accomplished with uprighting springs, very much as it was with traditional Begg treatment. The rectangular wire is used primarily for torque (faciolingual root movement), not the mesiodistal root movement needed for root paralleling after teeth were allowed to tip during space closure.

During space closure with the edgewise appliance, it is almost always a goal of treatment to produce bodily tooth movement during space closure, preventing the crowns from tipping toward each other. If proper moment-to-force ratios have been used, little if any root paralleling will be necessary as a finishing procedure. On the other hand, it is likely that at least a small amount of tipping will occur in some patients and therefore some degree of root paralleling at extraction sites often will be necessary. If brackets were not oriented correctly on maxillary lateral incisors and premolars in both arches (the usual problem areas), root separation or paralleling may be needed in nonextraction cases. It is wise to obtain a panoramic radiograph toward the end of the second stage of typical treatment to check for both root positioning errors and root resorption that might dictate ending treatment early.

Exactly the same approach used for root positioning in Begg technique can be employed with the edgewise appliance if it includes a vertical slot behind the edgewise bracket that allows an uprighting spring to be inserted and hooked beneath a base archwire. When only steel archwires were available, this approach often was used, but in contemporary edgewise practice, it has been almost totally abandoned in favor of angulated bracket slots that produce proper root paralleling when a flexible full-dimension rectangular wire is placed.

With the 18-slot appliance, the typical finishing archwire is either 17 × 22 or 17 × 25 steel. These wires are flexible enough to engage narrow brackets even if mild tipping has occurred, and the archwire will generate the necessary root paralleling moments. If a greater degree of tipping has occurred, a more flexible full-dimension rectangular archwire is needed. To correct more severe tipping, a 17 × 25 beta-Ti (TMA) or even a 17 × 25 nickel–titanium (M-NiTi, not superelastic NiTi [A-NiTi]) wire might be needed initially, with a steel archwire used for final expression of torque.

With wider 22-slot brackets on the canines and premolars and with the use of sliding rather than loop mechanics to close extraction sites, there is usually even less need for root paralleling as a finishing procedure. But with 22-slot brackets, if teeth have tipped even slightly into an extraction space or if other root-positioning is needed, steel archwires (19 × 25 steel, for instance) are much too stiff. A 21 × 25 beta-Ti wire is the best choice for a finishing archwire under most circumstances, and if significant root positioning is needed, 21 × 25 M-NiTi should be used first.

Although superelastic NiTi (A-NiTi) wires perform much better than elastic NiTi (M-NiTi) wires in alignment, this is not true of their performance as rectangular finishing wires. The great advantage of A-NiTi is its very flat load-deflection curve, which gives it a large range. In the finishing stage, however, appropriate stiffness at relatively small deflections, rather than range, is the primary consideration. A-NiTi wires usually deliver less force than their M-NiTi counterparts (this will depend on how the manufacturer manipulated the wire [see Chapter 9]), and if rectangular A-NiTi is used in the finishing stage, the wire’s properties in both bending and torsion must be considered in the choice of the wire. M-NiTi almost always is the better choice for rectangular nickel–titanium wires. Occasionally, a severely tipped tooth will be encountered (almost always because of a bracket positioning error), and a longer range of action is needed. This may indicate using a rectangular A-NiTi wire initially, then M-NiTi. An alternative, usually practical only if the edgewise brackets have a vertical slot or tube, is an auxiliary root-uprighting spring (Figure 16-4).

A root-paralleling moment is a crown-separating moment in edgewise technique just as it is in Begg or any other technique. It is important to remember this effect. Either the teeth must be tied together or the entire archwire must be tied back against the molars (Figure 16-5) to prevent spaces from opening. Not only extraction sites but also maxillary incisors must be protected against this complication. When a full-dimension rectangular wire is placed in the maxillary arch, spaces are likely to open between the incisors in nonextraction as well as extraction cases. Tying the maxillary incisors together, which can be done conveniently with a segment of elastomeric chain from the mesial bracket of one upper lateral incisor across to the mesial bracket of the other, is necessary during finishing.

Torque