Evidence

Several studies have used records from our practice to analyze the position of the IMPA. Glenn used a nonextraction approach to procline the mandibular incisors,¹ moving them forward 1.4 mm; Elms successfully used nonextraction to tip the IMPA 2 degrees in crowded arches²; Alexander used extraction to measure the difference in mandibular incisor movement between stable and unstable groups,³ the former moving only 2.8 mm while the latter moved 4.7 mm; Nevant studied the effectiveness of a lip bumper in tipping the incisors,⁴ citing a movement of 1.4 mm per year, or 2.9 degrees; and Papandreas reported an 8-degree annual uprighting of the mandibular incisors in extraction cases.⁵ In another study, Hansen measured the effects of the Herbst appliance on the mandibular incisors, reporting that the appliance’s anteriorly directed forces on the mandibular premolars causes the mandibular dentoalveolar segment to move forward, thereby resulting in proclination of the mandibular incisors.⁶

Mechanics

To control IMPA, attack the problem with exact bracket and archwire specifications:

• 0.018 slot
• –5-degree torque mandibular incisor brackets
• –6-degree tip on mandibular first molars
• Initial flexible rectangular archwire to give torque control
• And if needed:
  • –Interproximal enamel reduction (slenderizing) to gain space
  • –Early Class III elastics to upright molars

If crowding is so severe that a rectangular archwire cannot be engaged or if the treatment plan suggests that the incisors can be advanced, then a 0.016 nitinol (NiTi) mandibular archwire can be used.

A common mistake made by orthodontists is the use of open-coil springs between incisors to gain space, which causes flaring of the incisors. To reduce flaring, in addition to interproximal enamel reduction, Class III elastics (¼-inch 3-oz) can be worn for the first 3 days (72 hours) of treatment. This distalizing force can upright the mandibular first molars (which have a–6-degree angulation on the bracket) to gain space. Wearing the Class III elastics over a longer period can cause unwanted extrusion of the mandibular incisors, which is not advisable. At the next appointment, additional slenderizing will obtain better archwire engagement, and the patient can once more wear Class III elastics for 72 hours. This procedure can be used for up to 3 months.

As soon as the rotations have been reduced to the point where a 16 × 22 or 17 × 25 multistranded archwire can be engaged, the Class III elastics can be stopped. The torque of the mandibular anterior teeth is now controlled by the rectangular archwire and torque (–5 degrees) in the incisal brackets.

The trademark of the Alexander Discipline is the prescription of–5 degrees of torque in the mandibular central incisors. The torque values in the 0.018-inch slot brackets are designed to compensate for the 5 degrees of archwire freedom when a 17 × 25 stainless steel (SS) archwire is used. The resultant force is 0 degrees of torque expressed on these teeth. The–5 degrees of lingual crown–labial root torque compensates for the 0.001 inch of tolerance between the 17 × 25 archwire and the 0.018-inch bracket slot.

Exceptions

There are some exceptions to the 3-degree rule. For example, convex and concave profiles may require more changes. When convex profiles are treated with extractions, mandibular incisors will be uprighted much more than 3 degrees and will be very stable. In concave and deep bite cases, there is usually a need to advance the mandibular incisors more than 3 degrees to prevent a final concave profile. The stability in these cases is more suspect, so permanent retention is usually prescribed.

Evidence

In an unpublished Angle paper presented in 2009, Chuck Alexander found no relationship between maxillary incisor angulation and the long-term attrition of these teeth. As I study the long-term results on various patients, I occasionally find cases with good long-term stability but less than ideal Ls–Sn numbers. This leads me to conclude that other factors, such as IMPA, mandibular intercanine width, and leveled mandibular arch may be more important to long-term stability.

We must be careful not to overreact and say that this angulation is not important. Adequate torque in the maxillary incisors will functionally improve anterior guidance and overbite. It will also give a better esthetic smile.

Mechanics

• Use 0.018 bracket slot
• Pretorque (10-degree) brackets on maxillary anterior teeth
• Finish with 17 × 25 SS archwire

Exceptions

When IMPA is greater than normal, maxillary incisor torque will be decreased. It is not necessary to make any adjustments in the archwire or prescription because this happens automatically. When IMPA is smaller than normal, maxillary incisor torque will be increased when placing accentuated or reverse curve in the archwire. This will create the needed torque.

Evidence

Although much less study has focused on maxillary incisor torque, my own long-term studies show that the interincisal angle must be controlled (Alexander C, unpublished data, 2009).

Mechanics

Although I was taught to place torque in the archwire with 0.022 bracket slots to control each tooth, shortly after entering private practice, I changed to a 0.018 bracket slot with individual torques precut into each bracket. I found that I rarely needed to add or subtract torque in the archwire because the pretorqued appliance was working.

The same brackets respond differently to high-angle and low-angle patients. The only difference in the archwire is that opposite curves of Spee are placed in deep bite, low-angle cases and open bite, high-angle cases. The combination of change of torque in the archwire is caused by the curve placed in the wire and the different preexisting positions of the anterior teeth (upright in deep bite cases and flared in open bite cases). By creating accurate anterior torque control in both arches, an excellent interincisal angle is created.

0.018 versus 0.022

In volume 1 of this series, chapter 6 discusses the fundamental requirements for accurate torque control. For every 0.001 inch of “play” between the bracket slot and archwire size, 5 degrees of torque is lost.

Exceptions

When discussing the interincisal angle, always remember that this angle will have a direct relationship to the patient’s vertical skeletal angle (see chapter 5). In a patient with a low vertical pattern, the incisor angulation will be higher. A high vertical-angle pattern will have incisors with less angulation.