Several different orthodontic approaches have evolved for intrusion of the maxillary incisors.1-4 Whatever method is used, the control and stabilization of posterior anchorage is imperative, since incisor intrusion produces a reaction of extrusion and distal crown tipping of the posterior segments (Fig. 1).
A light, continuous force of about 25g per tooth appears to be the most efficient for intrusion.1,5,6 With the force concentrated over a small area at the apex, heavier forces should be avoided.
The anatomical position of incisor roots relative to the cortical plate appears to be another important factor.7,8 Maintaining the root well within cancellous bone and avoiding cortical impingement seems to improve the efficiency of intrusion and to diminish the potential for root resorption and the need for torquing.
If intrusion is not accomplished early in treatment, the maxillary incisors tend to extrude as their roots are torqued lingually (Fig. 2). Overintrusion of the incisor roots is recommended prior to initiation of any retraction or torquing (Fig. 3). The incisor roots then will not contact the labial cortical plate during minor crown tipping, thus avoiding extrusion.9,10
Such a procedure maintains a favorable occlusal plane and can eliminate a "gummy" smile (Fig. 4).
Torquing and Intrusion Auxiliary
I use the auxiliary shown in this article whenever intrusive and torquing forces are needed simultaneously to correct extruded or protrusive anterior teeth during retraction.
The spring auxiliary slides onto an anterior braided or solid rectangular archwire, either .0215" X .025" or .018" X .025". The spring is manufactured with lumens of both sizes, but the critical point is that the archwire must fill the entire slot for the spring to be effective (Fig. 5). The diameter of the spring-tempered wire is currently .012", but experiments are under way to produce an .010" wire that will enable increased torquing action with lighter forces.
The sectional archwire can be engaged in the central incisors alone, or in the central and lateral incisors. However, once the sectional archwire is parallel to the occlusal plane, it is advisable to disengage the lateral incisors and tie them passively to the intrusion arch to keep them from extruding below the occlusal plane.
The auxiliary hooks should extend gingivally and labially. A gingivally activated .018" intrusion arch, originating from the auxiliary .028" molar tube, is engaged in the labial hooks of the auxiliary after the sectional wire is tied in (Fig. 6). The intrusive arch should be broad enough to prevent impingement of the soft tissue overlying the cuspid roots when the wire is cinched.
The wire is activated enough to produce about 25g of force per tooth at the midline (Fig. 7). As this wire attempts to move gingivally, it applies an intrusive and torquing force through the auxiliary to the teeth that are tied into the sectional arch.
Elastic hooks can be incorporated in the intrusion arch so that light Class I elastics (1½-2oz) can be used to prevent labial crown tipping of the incisors (Fig. 8). This can also be accomplished by passively cinching the intrusion arch at the molars, or by using a helix instead of a tipback bend and ligating it passively to the molar tube (Fig. 9). Heavier forces or overcinching of the intrusion arch should be avoided, unless torquing force alone is desired, because this will reduce the intrusive effect by flattening the bite-opening bend.
When torquing incisors lingually on a continuous rectangular archwire, there is 1° of mesial convergence of the gingival portions of the central and lateral incisor crowns for every 4° of lingual crown torque.1,11 The same convergence would occur if the intrusion arch were engaged directly into the four incisor brackets (Fig. 10). However, this effect is eliminated with the torquing and intrusion auxiliary, because the intrusion arch is connected to the anterior segment by a one-point contact.
Applications of the Auxiliary
The torquing spring is especially effective in Class II, division 2 cases where the central incisors are lingually inclined (Figs. 11, 12, 13, and 14). By simultaneously torquing the roots as the incisors are intruded, the roots are kept safely away from the labial cortical plate and well within cancellous bone.
In a Class II, division 1 malocclusion, the .018" intrusion arch and torquing spring maintain proper root angulation as the anterior segment is retracted with light Class I forces (Figs. 15, 16, 17, and 18). This produces bodily distal movement with intrusion instead of the usual extrusive component seen with retraction.
In an extraction case, if intrusion and retraction progress rapidly, the roots of the lateral incisors may contact the mesial surfaces of the canine roots (Figs. 19AB and 19C-G). Once the canine roots are uprighted, retraction and intrusion can proceed.
Cervical headgears (in forward-rotating patients) or high-pull headgears (in backward-rotating patients) will greatly enhance the treatment results achieved with the auxiliary.
Once incisor retraction, intrusion, and torquing have been completed, a continuous .012" nickel titanium or .010" stainless steel archwire is engaged to intrude the canines while the intrusion system is still in place (Fig. 18C). After the canines have been tipped and intruded, a finishing arch of either .021"X.025" braided or .020" round wire is used to close any remaining spaces (Fig. 19F).
True intrusion can be verified by the appearance of a plateau, 5-10mm deep, directly lingual to the maxillary incisors (Fig. 20). This will remodel within six to 12 months after treatment.
Appliance Variations
One variation of the system is a torquing auxiliary that is engaged through the vertical slot of a bracket, thus allowing individual teeth to be torqued (Fig. 21).
Another variation is used when the upper incisors are flared labially and no lingual root torque is desired (Fig. 22). This is achieved by moving the point of force application perpendicular to the center of resistance of the incisors, thus producing no moment and no torque (Fig. 23). The point can be located by drawing an imaginary line that bisects the cemento-enamel junctions on the lingual surfaces of the central incisors. I strongly recommend that a lingual molar stabilizer such as a Nance button or transpalatal arch be used to minimize undesirable side-effects (Fig. 24).
Conclusion
The auxiliary shown in this article simultaneously torques and intrudes the incisors during retraction. By using light, continuous forces and maintaining the roots well within cancellous bone, root resorption and patient discomfort are minimized. Treatment time is significantly reduced because the incisors do not require torquing at the completion of retraction.
ACKNOWLEDGMENTS: The author wishes to acknowledge Lilah Brereton for her assistance in preparing this article and Dr. Norman Cetlin for his guidance and inspiration.
