JCO INTERVIEWS

Previous articles in this series dealt with various stages of orthodontic treatment with preadjusted appliance systems.1-5 This final article will cover overjet reduction, a stage that is required in a large number of cases. We will not discuss surgical-orthodontic treatment, but will concentrate on mechanics proven effective in typical nonsurgical situations.

The four nonsurgical dental and skeletal changes responsible for overjet reduction are:

1. Mesial movement of the lower incisors

2. Distal movement of the upper incisors

3. Distalization of or limiting the forward growth of the maxilla

4. Mesial movement of the mandible due to:

a. Forward mandibular growth rotation, or

b. Limiting posterior dental and skeletal vertical development

Closely tied to overjet reduction is Class II molar correction, which involves the same dental and skeletal changes, except that the upper and lower molars are moved instead of the incisors. Molar correction must occur either prior to or simultaneous with overjet reduction.

Proper diagnosis of Class II cases, with or without severe overjet, involves the integration of these dental and skeletal changes. Of course, in non-growing patients the skeletal corrections must be carried out surgically. Each of these four types of movement will be reviewed in detail.

The cephalometric norms used in this article are derived from the Michigan growth studies6 and from recommendations by Steiner,7 McNamara,8 and Jacobson9 (Fig. 1).

Mesial Movement of the Lower Incisors

The end-of-treatment position of the lower incisors is important for several reasons. If they are left too far back, there will be a tendency toward a retrognathic profile and a long-term deepening of the overbite. If they are too far forward, the profile will be too full, and the alignment of the lower anterior segment may be unstable as the incisors drop back in response to lip pressure.

The APo line is useful in assessing treatment objectives, with a norm of +2mm. However, a rigid adherence to cephalometric norms is not in the best interest of many patients. It must be remembered that a norm is established primarily for Class I cases and is often modified when dental compensation is needed. In Class III cases the lower incisors may need to be more upright to accommodate the upper incisors, and in some Class II cases the lower incisors should be left slightly forward so the upper incisors do not need to be over-retracted.

It is conventional to aim for a treatment result where the lower incisors are at 90-95º to the mandibular plane,6 but the ideal angle decreases as the maxillary or palatal plane angle increases relative to the mandibular plane (MM), and will also vary as dental compensation is required.

The soft tissue is usually involved in the etiology of cases that require mesial movement of the lower incisors. For example, there may be a history of thumbsucking or hyperactive mentalis muscle activity such that the lower incisors have been held back and retroclined.

Class II, division 2 cases often involve a high lip line that results in both upper and lower incisors being retroclined. After the upper incisors have been moved forward to create a Class II, division 1 pattern with an overjet, the lower incisors are frequently back in the profile. It is then appropriate to tip the lower incisors forward to achieve a good anteroposterior position in the profile and a favorable interincisal angle (Fig. 2).

Distal Movement of the Upper Incisors

We consider the ideal position of the upper incisors to be APo +6mm, with an angulation of 110º to the maxillary plane. If the MM angle is high, then the angulation of the upper incisors to the maxillary plane and of the lower incisors to the mandibular plane may need to be lower than average, with an increased interincisal angle.

Distal movement of the upper incisors has traditionally been the primary method of correcting Class II, division 1 malocclusions. However, since true maxillary protrusion occurs in only about 20% of Class II cases,10 we prefer mesial movement of the chin to achieve an optimal facial profile. In adults, this requires orthognathic surgery.

When the initial upper incisor angulation is more than 115º, retraction is commonly begun by tipping the incisors until the normal angulation is reached, at which point bodily movement is attempted. In theory, therefore, round wires may be used during the early stages of this movement.

We recommend that rectangular wires be used instead, because if torque is lost and incisor angulation is not controlled, it is difficult to recover the lost torque. Our clinical experience in cases with severe initial overjet favors bodily retraction from the beginning, with light forces and good torque control (often involving the addition of torque to the incisor brackets of the preadjusted appliance).

Many Class II, division 1 cases have a narrow upper archform before treatment. As the archform becomes normal during leveling and aligning, the overjet will naturally be reduced, providing the molars are controlled and "lacebacks" are used in the early wires, with passive tiebacks in the first rectangular wires5 (Fig. 3).

Distal Movement or Limiting Forward Growth of the Maxilla

The skeletal position of the maxilla can be assessed with Steiner's SNA angle7 (norm 82º) or by relating A point to a perpendicular from nasion through Frankfort horizontal, as recommended by McNamara8 (norm 0mm, Fig. 4). It is important to note that the latter norm is based on "a sample of 111 young adults who, in the opinion of my co-workers and myself, have good facial configuration" and is therefore rather subjective.

If the increased overjet is due to a forward position of the maxilla, headgear or Class II elastics can be used to restrain maxillary growth in a growing patient. This is not required in a great number of cases, since most Class II malocclusions with increased overjets show normal to retrusive maxillary positions.

Actual distalization of the maxilla is difficult and requires cooperation with heavy orthopedic forces. Orthopedic forces usually will only limit the forward growth of the maxilla, which is normally 1mm per year in a growing child.6

Mesial Movement of the Mandible

McNamara recommends that the mandible ideally be 4mm behind the nasion perpendicular8 (Fig. 5). Steiner's SNB angle7 (norm 80º) can also be used as a horizontal reference.

Mesial movement of the mandible not only contributes to overjet reduction, but produces improved facial harmony in a large percentage of cases with severe overjet. If this can be achieved, then upper incisor retraction can be minimized for the best esthetic results. The first factor responsible for mesial movement of the mandible is forward mandibular growth rotation.

Mandibular growth rotation, as outlined by Bjork,10 can be divided into two categories, forward and backward (Fig. 6). Patients with forward growth rotation, which is more common, fall into three groups:

1. Those with the center of rotation at the temporomandibular joint. This occurs with a loss of teeth or with strong musculature and results in a deepening of the bite.

2. Those with the center of rotation at the incisal edges of the lower anterior teeth. This occurs with a long posterior face height and normal anterior face height and results in the posterior portion of the mandible rotating away from the maxilla.

3. Those with the center of rotation in the premolar area. This can occur when there is no anterior contact because of severe maxillary or mandibular overjet. The anterior face height decreases as the posterior face height increases, and the result is often a skeletal deep bite.

Backward growth rotation, fortunately, is less common. Such patients are in two categories:

1. Those with the center of rotation at the temporomandibular joint. This occurs when the bite is opened by orthodontic therapy and results in increased anterior face height.

2. Those with the center of rotation at the most distal occluding molar. This occurs with sagittal growth at the condyles and often results in a skeletal open bite.

Bjork found that the average rate of mandibular growth in males was 3mm per year (measured from the head of the condyle to pogonion), with a prepubertal minimum of 1.5mm per year at age 11.5 and a pubertal maximum of 5.5mm per year at age 14.5.10 The Michigan growth studies confirm these figures, with males from ages 6 to 16 showing an average increase in mandibular length of 3.1mm per year, and females from ages 6 to 16 an average of 2.3mm per year. 6

Although functional appliances have been claimed to "stimulate" mandibular growth beyond a patient's normal growth potential, clinical research to date has not substantiated this claim.11 For example, Harvold's studies indicated that the activator brought about an average increase in mandibular length of 1-1.5mm--clearly not enough to correct a significant overjet or Class II molar relationship.12

This is not to say that functional appliances cannot be effective in managing Class II cases with increased overjet, but that their true means of correction must be understood. Harvold said that the activator correction was primarily due to limiting the downward and forward eruption of the upper posterior dentition, while allowing upward and forward eruption of the lower posterior dentition, and that overjet reduction occurred mostly due to distal tipping of the upper incisors and mesial tipping of the lower incisors.12

In addition to forward growth rotation, the mandible can also be moved mesially by limiting posterior dental and skeletal vertical development. Geometrically, any procedure that reduces or maintains the MM angle allows pogonion to be moved forward in the profile (Fig. 7A). High-pull headgears, palatal bars, lingual arches, and posterior bite plates can allow such vertical control, which is also enhanced by the extraction of premolars.

On the other hand, cervical headgears, anterior bite plates, and intermaxillary elastics tend to open the MM angle and thus move pogonion backward (Fig. 7B). This effect increases geometrically in higher-angle patterns. It is also more difficult to maintain the MM angle in nonextraction treatment.

Mechanics of Overjet Reduction

Patients present with a variety of skeletal and dental pattern combinations,13 but there are only three major methods used to correct Class II molar relationships and reduce overjets: Class II elastics, headgear, and functional appliances. Fixed appliances serve primarily for individual tooth alignment, as well as for attaching Class II elastics and headgear.

The three primary methods, whether used separately or in combination, are the key to a successful end result. When planning the mechanics for a particular case, it is essential to have a clear idea of the vertical and horizontal positions of the maxilla and mandible, the posterior dental segments, and the upper and lower incisors. A progress cephalogram taken just before overjet reduction will be helpful in re-evaluating the initial treatment objectives.

A secondary method of overjet reduction involves extraction of the upper bicuspids only and retraction of the upper anterior segment.

The following extraction and nonextraction cases show various clinical situations prior to overjet reduction, along with the treatment needs of each.

Case 1

[show_img]293-jco-img-0.jpg[/show_img] In this extraction case, the lower arch appears finished, with the lower incisors well positioned in the facial profile. Upper incisor torque is correct, and overbite has been controlled. There remains 3mm of excess overjet.

The remaining 6mm of upper space can be closed reciprocally, using sliding mechanics, because the molar relationship is slightly Class III. The molars and premolars will move 3mm mesially, and the anterior teeth will move the same distance distally. Nighttime headgear support could be used if the molars move forward more rapidly than anticipated.

The rectangular wire will allow bodily control of the upper incisors, provided force levels are light. If the overjet were more severe, torque could be added to the incisor brackets.

Ideally, the second molars would be incorporated into the archwire, because the palatal cusps will tend to drop down as they follow the upper first molars mesially.

A passive tieback holds the lower spaces closed. An active tieback, with a module on the soldered archwire hook, delivers a space-closing force in the upper arch that simultaneously reduces the overjet.

Case 2

[show_img]293-jco-img-1.jpg[/show_img] This extraction case is similar to Case 1, with 3mm of excess overjet to reduce, but the molars are in a Class I relationship. This case therefore requires more anchorage.

Reciprocal space closure cannot be used because it would tend to produce a Class II molar relationship. Support from a nighttime headgear and/or a palatal bar will be necessary to keep the molars Class I. The upper second molars should be banded for proper torque control.

The rectangular archwire and tiebacks are set up as in Case 1.

Case 3

[show_img]293-jco-img-3.jpg[/show_img]

Here the space in the lower arch has been closed, but the lower incisors are 2mm back of the ideal of APo, + 2mm. Upper incisor torque is correct. The MM angle is an average 28º. The upper arch still has 4mm of space, and the overjet must be reduced by 4mm.

Reciprocal space closure can be used in the upper arch, with Class II elastics added to protect the molar relationship by bringing the lower arch forward to APo +2mm. In the upper arch, the posterior teeth will move mesially 2mm as the anterior teeth move distally 2mm. Again, a rectangular archwire with tiebacks is used.

The intermaxillary elastics would be contraindicated in higher-angle cases, where a high-pull headgear would be preferred for upper molar support. However, the lower incisors are seldom behind the APo line at this stage of high-angle cases.

Case 4

[show_img]293-jco-img-5.jpg[/show_img]

In this case, the lower incisors are in good position, but the upper incisors are proclined to 122º, with 7mm of extraction space available and 7mm of overjet to reduce. The molars are Class I, contraindicating reciprocal space closure.

A round wire is shown because about 10º of tipping would be acceptable. However, a rectangular wire with light force levels would allow bodily control of the upper incisors.

A more severe overjet would definitely require a rectangular archwire, because torque would have to be added to the incisor brackets. Support from a nighttime headgear and/or palatal bar would also be needed. Most cases with proclined incisors require less anchorage in the early stages of overjet reduction, whether round or rectangular wires are used, because the initial movement is tipping.

Case 5

[show_img]293-jco-img-7.jpg[/show_img]

Both upper and lower extraction spaces have been closed in this case, but the lower incisors are set back in the profile, so that lower anchorage is available. Upper incisor torque is correct, the overbite has been controlled, and the molars are 2mm Class II. The excess overjet is 3mm.

Since the MM angle is an average 28º, the overjet can be reduced with Class II elastics. The upper arch, with a rectangular wire, will act as an anchorage unit. As the overjet decreases and the lower teeth move mesially, the molars will move toward a Class I relationship.

The lower incisors are at 89º and can therefore be allowed to tip forward as much as 6º. The tips of the lower incisors will thus move farther mesially than the lower molars (3mm to 2mm). A small amount of labial crown torque in the lower incisor region will assist this forward tipping.

The upper rectangular wire allows bodily control of the upper incisors, provided force levels are light. Passive tiebacks hold the upper and lower spaces closed. The upper second molars are banded for full torque control.

An .018" round wire could be used in the lower arch for this amount of overjet reduction. If the overjet were more severe, a rectangular wire would definitely be preferable to maximize lower anchorage potential. Later in treatment, a lower rectangular wire would be essential for finishing and detailing.

Case 6

[show_img]293-jco-img-9.jpg[/show_img]

This situation is frequently seen in moderate Class II, division 1 malocclusions after the extraction of four bicuspids. The lower incisors are well positioned, the overbite is properly controlled, and the molars are 3mm Class II. There is 3mm of lower space and 4mm of upper space to close, and 4mm of excess overjet to reduce.

This case shows the ease and sophistication of sliding mechanics and group tooth movement. Since the MM angle is average, the overjet can be reduced with Class II elastics. Nighttime headgear support will be required to the upper molars, or to the upper anterior archwire if there is a tendency toward a gummy smile.

Anchorage must be carefully balanced during overjet reduction, because the elastics will change the balance of lower space closure. The lower molars and premolars will move mesially, but the lower incisors will tend not to move distally. The headgear to the upper arch will ensure that the upper anterior segment moves the necessary 4mm distally, while the upper molars are prevented from moving mesially. Monthly monitoring and good patient cooperation will be essential.

The rectangular archwire allows bodily control of the upper incisors. Upper second molars should normally be banded.

Case 7

[show_img]293-jco-img-11.jpg[/show_img]

This case is typical of more difficult Class II, division 1 malocclusions treated with the extraction of four bicuspids. It is more challenging than Case 6 because the lower incisors are 2mm forward of an ideal position. Again, the excess overjet is 4mm, but in this case there is no available anchorage in the lower arch for Class II elastics. The molars are 4mm Class II.

Considerable headgear wear will be necessary to move the upper anterior segment 6mm distally. Lower space closure will need to be reciprocal, with the molars moving 2mm mesially and the incisors 2mm distally.

Treatment response would be better in a growing patient than in an adult, because a correction of this kind is difficult to achieve with tooth movement alone. Good cooperation will be required in wearing the headgear.

The rectangular archwire, as in Case 6, allows control of the upper incisors. Ideally, the upper second molars would have been incorporated into the archwire.

Case 8

[show_img]293-jco-img-13.jpg[/show_img]

The situation is similar to that of Case 5, but this is a nonextraction case. The lower arch is finished, but with the lower incisors set back in the profile, so that lower anchorage is available. The upper incisor torque is correct, the overbite has been controlled, and the molars are 2mm Class II. The excess overjet is 3mm.

With an average MM angle, Class II elastics can be used to reduce the overjet. The lower incisors are at 89º and can be tipped forward by as much as 6º, as in Case 5. Passive tiebacks are placed to keep the upper and lower spaces closed, but they are not required in every such case.

The tips of the lower incisors can be expected to move farther mesially than the lower molars, since part of the incisor movement involves tipping. If the overjet were severe, brackets with additional torque would be required for upper incisor control.

Case 9

[show_img]293-jco-img-15.jpg[/show_img]

In this nonextraction case, the lower incisors can only be permitted to move mesially 1mm. The overbite has been properly controlled, which is especially important in this case, and the molars are 3mm Class II. There remains 3mm of overjet.

Only a limited amount of Class II elastic traction can be used, and therefore headgear support to the upper first molars will be essential. A regime of nighttime headgear and daytime elastics would be appropriate; this would produce a 24-hour distalizing force on the upper arch and only a 12-to-14-hour mesializing force on the lower arch.

The lower rectangular archwire has lingual crown torque added in the incisor region to resist forward tipping. The lower teeth should be ligated hard, so that the lower arch becomes an anchorage unit. The treatment response will generally be better in a growing patient than in an adult, as it is difficult to correct such a problem with tooth movement alone.

A patient like this or Case 8 will tend to posture the mandible into a forward position to obtain a convenient Class I occlusion and good intercuspation, but with the condyles forward in the fossae. Care must therefore be taken to ensure that the overjet is properly reduced.

Case 10

[show_img]293-jco-img-17.jpg[/show_img]

This is an actual case that required more care and management than either of the nonextraction examples shown above. A 12-year-old male presented with a Class II, division 1 malocclusion with a full Class II molar relationship and an 8mm overjet (Fig. 8).

The initial stage of treatment involved leveling and aligning the lower arch while beginning Class II molar correction with a combination high-pull/cervical headgear to the first molars. The upper incisors were bracketed for alignment and bite opening. The overbite was corrected by upper incisor control and lower arch leveling.

After leveling, the lower incisors were 1mm in front of APo, so they could be only slightly advanced during overjet reduction. The upper incisors showed adequate torque, and the overjet was still 8mm.

Because the patient did not wear the headgear consistently, the molars remained 6mm Class II. The first molars could have been distalized more easily if the second molars had been extracted and the third molars allowed to erupt into the second molar positions. A more favorable growth pattern would also have helped.

To complete the Class II correction, the patient was asked to wear the headgear to the upper arch about 14 hours per day. He was also asked to wear Class II elastics to an upper sliding jig about 10 hours per day. As Andrews has pointed out in his 10-hour force theory, this amount of elastic force applied to a stabilized and tied-back lower arch creates a minimal drain on lower arch anchorage.15

After six months of treatment, a Class I molar relationship had been achieved (Fig. 9). (We now prefer a closed coil spring to the metal tubing used for the sliding jig in this case, because it creates less friction.)

Overjet reduction could then be carried out by bracketing the upper cuspids and bicuspids and releveling the upper arch. The headgear continued to be worn 14 hours per day. The elastics were still worn 10 hours per day, but they were attached to hooks on the archwire rather than to the sliding jig (Fig. 10).

The overjet reduction was completed and upper spaces nearly closed in another 16 months (Fig. 11). In both these stages of treatment, a constant force was applied to the upper arch, first to complete molar correction and then to reduce the overjet. However, only an intermittent force was applied to the lower arch with Class II elastics.

About five months later, finishing and detailing were complete and appliances were removed (Fig. 12) .

ACKNOWLEDGMENTS: The authors wish to thank Miss Maria Brown and Mr. Maurice Berman for their help in the preparation of some of the drawings. [show_img]End.gif[/show_img]