Anterior Torque Control
“Success is the natural consequence of consistently applying the basic fundamentals.”
Incisor Mandibular Plane Angle Position
In the 1950s, Charles Tweed revolutionized orthodontic diagnosis with the Tweed Triangle. The hallmark of this triangle was the position of the mandibular incisor in relation to the mandibular plane, or the incisor mandibular plane angle (IMPA).
Always aware of the IMPA in my early cases, I soon realized that Tweed’s 90-degree angulation could not be accurate in all patients. I began to question the position of the IMPA while in graduate school, especially after I traced my own IMPA position and found that it was 100 degrees. Surely Dr Tweed would not have extracted on my straight profile to achieve an IMPA of 90 to 95 degrees. Unfortunately, several years ago I heard an orthodontist justify his patient’s final concave profile by saying that he satisfied the Tweed Triangle. But which is more important in the end: the triangle or the face? Therefore, I eventually adjusted my thinking from the 90-degree concept to a more realistic goal for the IMPA.
My goal for IMPA today is called the 3-degree rule. It makes sense that the most stable position for IMPA in every patient is the position in which the patient presents. And experience has taught us that we can upright the incisors considerably and retain stability. So the major problem for posttreatment stability of mandibular incisors becomes their excessive advancement. When analyzing several long-term studies, I concluded that, in most cases, we should limit mandibular incisor tipping to 3 degrees.
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
The difference in mandibular incisor proclination between skeletal Class II correction with a cervical facebow and functional appliances such as the Herbst stems from Newton’s third law of relativity: For every action, there is an equal and opposite reaction. With a facebow, the opposite reaction occurs at the back of the neck whereas the opposite force for any intraoral appliance affects the entire mandibular dentoalveolar segment. Although these appliances effectively address skeletal Class II problems, they are deficient in controlling IMPA.
In the Hansen study, IMPA shifted from 98.5 degrees to 109.5 degrees, meaning the mandibular incisors flared 11 degrees. Yet when the patient stopped retention 5 years after treatment, the incisors were uprighted to 101.5 degrees, a permanent proclination of 3 degrees from the original position (Fig 4-1).
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.
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.
Regardless of the original position of the anterior teeth, the incisors will be close to the ideal position after placement of the 17 × 25 SS archwire. This is the case no matter if the patient has bimaxillary protrusion being treated with extraction or a Class II, Division 2 deep bite being treated without extraction. For example, the cephalometric tracings shown in Figs 4-2 and 4-3 demonstrate how the–5 degrees of torque respond in two different types of malocclusion.
Proper maxillary incisor torque control is important for stability, esthetics, and function. The degree of torque in the maxillary incisors also affects the upper lip position. To have a good nasolabial angle of the upper lip requires proper angulation of the maxillary incisiors as well as their proper anteroposterior position. Yet the impossible-to-answer question is: What is proper? Although the normal line from the labrale superior (Ls) to the subnasale (Sn) is 101 to 105 degrees, this angulation, like IMPA, is not the same for all patients.
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.
- Use 0.018 bracket slot
- Pretorque (10-degree) brackets on maxillary anterior teeth
- Finish with 17 × 25 SS archwire
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.
Although the final torque of the maxillary incisors has nothing to do with their original position, the original position of the mandibular incisors affects their final torque and is critical for long-term stability. Therefore, the interincisal angle is an important consideration. The interincisal angle is created by the relationship of the maxillary and mandibular incisors. Functionally, this angle affects anterior guidance, which is critical to healthy temporomandibular joints.
Esthetically, the lips “drape” the anterior teeth, so a balanced interincisal angle is necessary for a balanced soft tissue profile. The normal measurement for this angle is 130 to 135 degrees. If the anterior teeth are placed excessively forward, the profile will be protrusive and the lips will not touch when relaxed. Conversely, if the teeth are placed too far lingually, a concave profile will result. When discussing the interincisal angle, therefore, the decision to extract or not extract is critical to the final result.
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).
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.
To obtain the required torque for the incisors to create the best interincisal angle for the patient, it is necessary to understand the effective torque that is delivered by the various bracket prescriptions (Figs 4-4 and 4-5).
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.
As Dr Tweed explained, controlling IMPA is the foundation for long-term stability. In my opinion, the biggest mistake made by orthodontists is early failure in controlling IMPA. This leads to advancing the anterior teeth, which can create convex profiles and, often, the need for lifetime retention.
The remarkable effect of the–5-degree torque built into the mandibular incisor bracket when placing the final 17 × 25 SS archwire is that it will beautifully control the incisors whether their original position is labial or lingual.
The decision to treat the patient with extraction or nonextraction will then determine the final position of the anterior teeth, thus creating a balanced soft tissue profile in addition to the greater possibility for stable results.
- Glenn G, Sinclair PM, Alexander RG. Nonextraction orthodontic therapy: Posttreatment dental and skeletal stability. Am J Orthod Dentofacial Orthop 1987;92:321–328.
- Elms TN, Buschang PH, Alexander RG. Long-term stability of Class II, Division 1, nonextraction cervical face-bow therapy: II. Cephalometric analysis. Am J Orthod Dentofacial Orthop 1996;109:386–392.
- Alexander JM. A Comparative Study of Orthodontic Stability in Class I Extraction Cases [thesis]. Baylor Department of Orthodontics, 1995.
- Nevant C, Buschang PH, Alexander RG, Steffen JM. Lip bumper therapy for gaining arch length. Am J Orthod Dentofacial Orthop 1991;100:330–336.
- Papandreas SG, Buschang PH, Alexander RG, Kennedy DB, Koyama I. Physiologic drift of the mandibular dentition following first premolar extractions. Angle Orthod 1993;63:127–134.
- Hansen K. Treatment and Posttreatment effects of the Herbst appliance on the dental arches and arch relationships. Semin Orthod 2003;9:67–73.
A 12-year-old girl presented with a severe Class II, Division 1 malocclusion with a 10-mm overjet and 6-mm overbite. This was a difficult case for me because at the time, I knew little about treating such difficult Class II problems.
A severe curve of Spee in the mandibular arch was offset with a reverse curve in the maxillary arch. However, there was no crowding in either arch.
Examination and diagnosis
With a deep bite and Class II, Division 1 malocclusion (ANB of 8 degrees), t/>