Treatment of Class II, Division 2 Malocclusion in Adults: Biomechanical Considerations

FLAVIO URIBE, DDS, MDS RAVINDRA NANDA, BDS, MDS, PHD

 ▲= Ligation Points

Fig. 3 Force system and ligation points of intru­sion arch in Class II, division 2 malocclusion.
 

A stainless steel base arch wire is used to slide the canines distally. To prevent the incisor bite from deepening due to the change in inclination of the canines (Fig. 4), an intrusion arch can be tied over the stainless steel wire (Fig. 5).The intrusion arch is ligated at the level of the lateral incisors and between the central incisors, delivering a distal crown tip back moment on the molars to effectively control the loss of distal anchorage often associated with sliding mechanics (Fig. 6). These mechanics are ideal for cases where anchorage is critical and, in adults, can eliminate the need for headgear or Class II elastics. With an intrusive force on the incisors and a moment on the molars, the base arch wire will not deflect too much, as is often seen in sliding mechanics due to the friction generated by canine retraction.

Fig. 8 .017" x .025" CNA mushroom-loop archwire after intraoral activation.

Mushroom-Loop Space-Closing Archwires

At this stage, in the majority of patients, the four incisors either need translation or controlled differential movement of their crowns and root apices. For translatory movement, a high, constant moment-to-force ratio of approximately 10:1 is recommended. If the ratio is too low, as is common with straightwire mechanics, the incisor crowns will move lingually, reducing the overjet and giving an erroneous impression of a tooth-size discrepancy when the spaces distal to the lateral incisors appear to be enormous. To correct this side effect, either the bite must be reopened or the incisors must be torqued, creating an unnecessary stress at the root apices as well as lengthening the treatment time.
       The preformed mushroom-loop space-closing archwire produces an ideal moment-to-force ratio. The mushroom shape of the loop will not interfere with the gingival tissue, and an activated loop will not become distorted, thus improving force delivery. For .022" brackets, an .017" x .025" CNA archwire comes preformed with the mushroom loops at standardized distances of 26­46mm in increments of 2mm (Fig. 7 A). This measurement represents the distance between the distal surfaces of the lateral incisors across the midline.
       Once the proper archwire has been selected, it is preactivated outside the mouth to achieve the proper moment-to-force ratio. First, the legs of both mushroom loops are carefully separated by about 3mm. Additional gable bends may be placed mesially as needed to increase the anterior moment (torque), and distal to the mushroom loop to increase the anchorage moment (Fig. 7B). Next, the torque on the distal legs is eliminated. The arch wire is then placed in the mouth and engaged across the arch from first molar to first molar (Fig. 8). Another lmm of activation is added, for a total of 4mm.

"

Fig. 9 A. Mushroom-loop archwire with spaces closed. Wire is left in place for another six weeks to allow residual moments to deliver proper axial root inclinations. B. Same patient with ideal axial inclinations.

The loop should not be reactivated until at least 3mm of space has been closed, thus maintaining a more constant moment-to-force ratio. After the spaces are completely closed, the wire should be left in the mouth for one or two additional visits, so that the residual moments can be used to correct the axial root inclinations of the anterior and posterior teeth (Fig. 9A). This completely eliminates the need for root uprighting and torquing springs and significantly shortens treatment time.

The finishing phase of treatment simply involves the use of co-ordinated .017" x .025" or .018" x .025" CNA wires. Minor bends can be placed in these beta titanium wires for detailing the alignment and occlusion. The finishing stage is usually short because of the correct positioning of the incisors after retraction (Fig. 9B).
       In adult patients, a maxillary modified Hawley wraparound retainer is ideal, because there is no interference in the occlusion. A lower bonded 3-3 retainer is recommended. It is important to emphasize that intrusion is a stable move­ment; minor overbite relapse should be expected, however, since its correction does involve some posterior buccal extrusion.

Case Report

A 26-year-old female presented with the chief complaint of "my teeth stick out" (Fig. lOA). She had a moderately convex hard and soft-tissue profile because of a retrusive mandible. A Class II, division 2 malocclusion was associated with a severe overjet and 100% deep bite due to moderately supraerupted upper incisors and excessively supraerupted lower incisors. The upper incisors were upright, and the lower incisors normally inclined. Both arches exhibited mild-to-moderate crowding.

The treatment objective in this case was to maintain the hard and soft-tissue profiles. In the vertical dimension, the goal was to intrude the maxillary incisors to improve the lip-to-incisor relationship and achieve a flat occlusal plane. The lower incisors needed to be intruded slightly, but extrusion of the posterior buccal segments was undesirable. In the anteroposterior dimension, the treatment objectives were to maintain the upper incisor crown positions and move the roots lingually. The lower incisors required minor intrusion as well as flaring. The molar positions, arch width, and midlines needed to be maintained.