American Journal of ORTHODONTICS Volume 72, Number 1, July, 1977 ORIGINAL ARTICLES Deep overbite correction by intrusion Charles R. Burstone Farmington, Conn. 0 ne of the major challenges of Class II treatment is the correction of deep overbite. Unfortunately, it is still common for the correction to be deter- mined by the system of mechanics that an orthodontist will employ, rather than the nature of the discrepancy. In most instances this correction is produced by the extrusion of posterior teeth, with the greatest success associated with patients who exhibit considerable mandibular growth. Differential treatment planning for the Class II patient requires that the relative amount of anterior intrusion and posterior extrusion be determined be- fore treatment and that differential mechanics be utilized to produce the desired correction. The amount of intrusion required will vary from patient to patient; however, some trends in treatment planning should be noted in the average Class II situation. Many Class II cases are characterized either by an A-B (apical base) discrepancy or by a greater-than-average vertical dimension. Lip length may be relatively short in relation to the vertical dimension. It is not desirable to increase the vertical dimension, since it would tend to make the A-B relationship more Class II and increase an abnormally large lower face. A great deal has been written about the undesirability of rotating a mandible open in the steep mandibular plane case ; the same precautions concerning rotation should also be employed in a patient with a large A-B discrepancy. Fig. 1 shows a patient in whom deep overbite was corrected by the extrusion of primarily lower premolars and molars associated with leveling the curve of Spee in the lower arch and the use of Class II elastics. The bony Class II relationship measured at points A and B haa become more severe as the mandible has swung downward and backward. The vertical dimen- sion has increased, creating an even longer lower face and potential instability in Department of Orthodontics, SchooI of DentaI Medicine, University of Connecticut Health Center. 1
Transcript
American Journal of ORTHODONTICS Volume 72, Number 1, July, 1977
ORIGINAL ARTICLES
Deep overbite correction by intrusion
Charles R. Burstone Farmington, Conn.
0 ne of the major challenges of Class II treatment is the correction of deep overbite. Unfortunately, it is still common for the correction to be deter- mined by the system of mechanics that an orthodontist will employ, rather than the nature of the discrepancy. In most instances this correction is produced by the extrusion of posterior teeth, with the greatest success associated with patients who exhibit considerable mandibular growth.
Differential treatment planning for the Class II patient requires that the relative amount of anterior intrusion and posterior extrusion be determined be- fore treatment and that differential mechanics be utilized to produce the desired correction. The amount of intrusion required will vary from patient to patient; however, some trends in treatment planning should be noted in the average Class II situation. Many Class II cases are characterized either by an A-B (apical base) discrepancy or by a greater-than-average vertical dimension.
Lip length may be relatively short in relation to the vertical dimension. It is not desirable to increase the vertical dimension, since it would tend to make the A-B relationship more Class II and increase an abnormally large lower face. A great deal has been written about the undesirability of rotating a mandible open in the steep mandibular plane case ; the same precautions concerning rotation should also be employed in a patient with a large A-B discrepancy. Fig. 1 shows a patient in whom deep overbite was corrected by the extrusion of primarily lower premolars and molars associated with leveling the curve of Spee in the lower arch and the use of Class II elastics.
The bony Class II relationship measured at points A and B haa become more severe as the mandible has swung downward and backward. The vertical dimen- sion has increased, creating an even longer lower face and potential instability in
Department of Orthodontics, SchooI of DentaI Medicine, University of Connecticut Health Center.
1
2 Bursto?le Am. J. Orthod. J&y 1977
f--- P
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. . \ ‘-‘. _- Fig. 1. Correction of deep overbite by extrusion of lower first molars and premolars as- sociated with Class II elastics and leveling of the lower curve of Spee. An undersirable increase in vertical and facial convexity has .occurred. (Solid line-before; dotted line- after treatment.)
the overbite correction. Extrusive mechanics has worsened the skeletal pattern since minimal mandibular growth has occurred during treatment. Patients J. Z. and M. H. were treated with the intrusion mechanics which are described in this article (Figs. 2 to 5). Even though these patients are characterized by minimal mandibular growth, it should be noted that the mandible has not exhibited a clockwise rotation during treatment; actually, the Y axis angle has been reduced.
This control of vertical dimension ensures that one has not encroached on the interocclusal space during deep overbite correction and thus increases stability. It makes it easier for the patient to close his lips and improves the A-B relation- ship.l If our objective in a high percentage of Class II cases is to reduce or hold vertical dimension rather than to-increase it, correction of deep overbite becomes more difficult for it requires genuine intrusion of the anterior teeth. Although intrusion may complicate the mechanical treatment of the patient, it is necessary for the achieving of an optimal result.
The decision as to the proper cant and level of the occlusal plane should not be determined as an accident of mechanics but should be carefully evaluated at the beginning of treatment. The usual factors that should be considered are the natural plane of occlusion (the original axial inclinations and alignment of the posterior teeth), anterior esthetics (the relationship of the incisor to the upper lip), the amount of attached gingiva present in the mandibular incisor region, and the A-B discrepancy. If one were to generalize, most Class II patients require
Deep overbite correction 3
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Figs. 2 to 5. Correction of deep overbite by intrusion of incisors. Anterior cranial base superposition and separate maxillary and mandibular superpositions are shown. (Super- position of maxilla is on palatal plane at ANS; mandible is superposed at symphysis and anterior third of lower border.) Note genuine intrusion of maxillary incisors and re- duction in facial convexity. [Solid black teeth--after treatment.)
4 Bursto7le Am. J. Orthod. July 1977
a relatively flat occlusal plane that tends to coincide with the natural plane of occlusion of the posterior teeth. The plane should not allow more than 3 mm. of the incisors to show below the upper lip.
If WC accept this concept of an occlusal plane, it is apparent that more intru- sion of the upper incisor than of the lower incisor is required. Once again, this complicates treatment since it is much easier to intrude lower incisors because of their smaller root mass and the common presence of a curve of Spee in the lower arch. For optimal treatment, however, more intrusion is required in the upper arch than in the lower. It should be noted that in Patients J. Z. and M. H. genuine intrusion of the upper incisor is shown in the maxillary superposition which is greater than the intrusion shown in the mandibular arch.
Every patient with deep overbite requires a comprehensive treatment plan which establishes how the deep overbite should be corrected, either by extrusion of posterior teeth or inhibition and genuine intrusion of anterior teeth. This decision is based in part on where the clinician desires to place the occlusal plane, the amount of mandibular growth anticipated, and the vertica1 dimension desired at the end of treatment.
It is important to define intrusion, since the dental literature suggests am- biguity in its use. Intrusion refers to the apical movement of the geometric center of the root (centroid) in respect to the occlusal plane or a plane based on the long axis of the tooth. Labial tipping of an incisor around its centroid produces pseudo-intrusion. Although this pseudo-intrusion would help correct a deep over- bite in a Class II, Division 2 patient, it should not be confused with the genuine intrusion discussed in this article. Incisal edges should therefore not be used to evaluate intrusion, since they are easily affected by tipping movements of the incisors. Ideally, a point should be selected in the center of the root (centroid) and comparison should be based upon the movement of this point.
The basic intrusive mechanism
In the 1950’s I developed an approach to orthodontic therapy which did not use continuous arches.2 The technique, known as the segmented arch, used dif- ferent cross sections of wire within the same arch and wires that did not run continuously from one bracket to the adjacent bracket.39 4 Segmented arch pro- cedures have a number of advantages in space closure in extraction cases and in producing tooth alignment with minimum side effects.5 In particular, segmenta- tion allows for the genuine intrusive movement of the anterior teeth. One of the limitations of traditional continuous arch therapy has been its inability to pro- duce genuine intrusion.
The basic mechanism for intrusion consists of three parts: (1) a posterior anchorage unit, (2) an anterior segment, and (3) an intrusive arch spring (Fig. 6).
Early in treatment the posterior teeth are aligned and joined together with a buccal stabilizing segment, Once a buccal stabilizing segment of at least 0.018 by 0.018 inch (0.457 by 0.457 mm.), with or without loops, can be placed, intrusive mechanics can be begun. (The mechanics described are based upon a 0.022 inch (0.559 mm.) slot edgewise bracket. Although the cross sections of wire will differ
Deep oz~erbite correction 5
Fig. 6. Basic mechanism for intrusion; posterior anchorage unit, anterior segment in the
four incisors, and an intrusive arch. The intrusion arch is placed in the auxiliary tube on
the first molar attachment.
Fig. 7. Anterior view of intrusive arch. Arch lies gingivai to incisors. Canines and pre
molars are bypassed.
Fig. 8. Intrusive arch has been placed at the level of the incisors. A double rope tie pre-
vents arch from being displaced into the mucobuccal fold if a tie is accidentally lost.
Fig. 9. Forces acting on the teeth from an intrusive arch. The effect on the molar is ex-
trusion and a negative rotation (crown-distal-root-mesial). The moment (M) is equal to the
intrusive force (FA) times the distance (L) from the incisor to the center of resistance of the
molar.
with each segment for an 0.018 inch (0.457 mm.) slot, the same basic principlc)s can be used. The cross section and design of the intrusive springs are identical.) Right and left posterior segments are joined together across the arch by means of a transpalatal lingual arch in the maxilla. and a low lingual arch in the mandible.
When alignment is completed in the posterior segment, the buccal stabilizing segments and lingual arches remain in place and arc not continually adjusted as in continuous arch therapy. Conceptually, one should not think of the posterior teeth as a group of individual teeth but, rather, as a single multirooted tooth composed of all the teeth on the right and left sides of the arch in the posterior region.
To increase the stability of the posterior segment, wires that are 0.018 by 0.025 inch (0.457 by 0.635 mm.) or 0.021 by 0.025 inch (0.533 by 0.635 mm.) can be placed following initial alignment and thereafter maintained in place throughout treatment. A special bracket-tube or triple-tube combination is placed
Am J. Cbthod. July 1.977
Table I. Force values for intrusion’*
Teeth Force/Side Total force in midline
Moment/Side t (Gm.-mm.)
&w-r Central incisors Central and lateral incisors Central and lateral incisors and canine
Lower Central and lateral incisors Central and lateral incisors and canine
25 50 750 50 100 1,500
100 200 3,000$
20 40 600 80 160 2.4OOf
*Averages only. tMoment values based on 30 mm. distance from incisors to the center of resistance of posterior segment. tThis moment can efficiently tip back posterior teeth.
on the upper or lower first molar. The most lingual slot or tube is used for plaee- ment of the edgewise arch; the large round tube is used for headgear. An auxil- iary tube placed gingivally is the anchor point for intrusive springs. The auxiliary tube on the first molar is standardized for an 0.018 by 0.025 inch (0.457 by 0.635 mm.) wire, regardless of the slot dimension of the strap-up. It is important to reiterate that during t,he overbite correction adjustments are not made tooth to tooth in the buccal segment, except for purposes of minor tooth alignment, and that the only adjust,ment to be found is between the auxiliary tube on the first molar and the anterior segment.
The intrusive arch normally consists of an 0.018 by 0.022 inch (0.457 by 0.559 mm.) or 0.018 by 0.025 inch (0.457 by 0.635 mm.) edgewise wire with a 3 mm. helix wound 21/2 times placed mesial to the auxiliary tube. Curvature is placed in the intrusive arch, so that. the incisal portion lies gingival to the central incisors (Fig. 7). When the arch is tied to the level of the incisors, an intrusive force is developed (Fig. 8). In order that the arch does not increase its length during the activation, a gentle curvature should be placed with the amount of curvature increasing as one approaches the helix. In this way the activated arch wire will appear relatively straight, and as it works out during intrusion arch length will decrease and no anterior flaring is produced.
The intrusive spring is not directly tied into the incisor bracket. An anterior alignment arch or anterior segment is placed in the central incisors or the four incisors and the intrusive arch is tied either labially, incisally, or gingivally to that wire.
It is true that almost any intrusive bend placed on an arch of this type could produce dramatic leveling of the arch. However, where there is genuine intru- sion, greater control of the force system is needed. For this reason, the six major principles of intrusion will now be discussed.
Controlling the force magnitude and constancy
It is important to use the lowest magnitude of force that is capable of intrud- ing incisors. If the magnitudes of force are too great, the rate of intrusion will
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Deep overbite comection 7
not increase and the rate of root resorption will increase. This has been demon- strated by Dellinger’s research on monkeys.‘j Even more significant is the reciPro- Cal effect on the posterior segments of too great a force. The posterior teeth will feel a vertical force which will tend to extrude the buccal segments and a moment or torque which in the upper arch will steepen the plane of occlusion and in the lower arch flatten it (Fig. 9). If only a single tooth, as a first molar, is attached to an intrusive spring, the undesirable side effect is seen primarily as a tip-back action, with the crown moving distally and the root mesially. Loss of anchorage during intrusion is primarily produced by the moment rather than by the force, since occlusal forces tend to negate the eruptive tendency. The moment is large because the distance from the ihcisors to the posterior teeth is great.’
The recommended forces for anterior intrusion are given in Table I. It should be noted that approximately 25 Gm. of force is delivered to an upper incisor and approximately half that amount to a lower incisor. A canine requires about 50 Gm. of force, on the average, for intrusion. Fig. 10 gives the load- deflection characteristics of typical 0.018 by 0.025 inch (0.0457 by 0.635 mm.) intrusive arches. The length of the arch is measured from the mesial aspect of the auxiliary tube on the molar to the midline of the dental arch parallel to the midsagittal plane. If the orthodontist desires to intrude four incisors, 100 Gm. of force midline would be required (25 Gm. per tooth). For a 30 mm. arch, 16.5 mm. of activation is required. (The intrusive arch is then bent so that its anterior portion lies 16.5 mm. below the level of incisor brackets.)
The suggested forces are averages based upon clinical experience. They can be modified if root circumference and length vary from the average. Care should be taken, however, not to increase the magnitudes significantly because of the possibility of upsetting the posterior anchorage. It is important to make sure not only that an optimal magnitude of force is employed but that the force operates relatively constantly.
Springs that deliver relatively constant force have low loa.d-deflection rates. An intrusive arch with a 30 mm. arm (perpendicular distance from the incisor to the first molar) has a load-deflection rate of 6 Gm. per millimeter. If this intrusive arch is activated 16.5 mm., 100 Gm. of force is produced in the midline, 50 Gm. per side. As the incisors intrude 1 mm., there is a change of force magni- tude of only 6 Gm.; hence, the delivery of force is relatively constant.
By contrast, high load-deflection mechanisms, such as some of the loops that are tried for intrusion, are activated only just a few millimeters; accordingly, the drop off of force is very dramatic for every millimeter of tooth movement. With a high load-deflection mechanism, it is not possible to deliver optimum forces since the activations required to produce the desired forces are in tenths of a millimeter and the orthodontist does not have the ability to carry out such minute activation.8 Furthermore, with a high load-deflection spring rate as the tooth moves, a rapid drop in force magnitude occurs, so that the optimal force may be only momentarily reached.
The clinician, therefore, learns that he must use greater than optimal forces to achieve any appreciable tooth movement. In short, in order to accomplish intrusion, it is necessary to deliver an optimal force constantly. A low load-
Am. J. Orthod. July 1977
INTRUSIVE BASE ARCHES
WIRE SIZE: ,018 x.025” DIAMETER OF HELIXz3mm. 21/z TURNS
320
1
L: 20mm
L-25mm
z
Lf30mm
i__L=35mm 0’ I I ! I I 1 I r I I I I 1 1
A 8 12 16 20 24 28
ACTIVAT!ON (mm)
Fig. 10 Load-deflection characteristics of the intrusive arch (1) is the perpendicular distance from the incisor bracket to the mesial aspect of the molar auxiliary tube. The total force is given on the vertical axis. Data are based on average arch forms. Activation can be
determined from this table, or a force gauge can be used in the mouth.
deflection rate spring makes it practical for the clinician to determine the magni- tude of the force, since activation is not so critical and assures that as intrusion proceeds there will not be a marked reduction in force magnitude.
Force magnitude can be measured either from a force-deflection graph (Fig. 10) based on average arch form or by directly measuring the intrusive arch in the mouth with the use of a force gauge. The intrusive arch has certain character- istics which assure a low deflection rate and relative freedom from accidental permanent deformation under the forces of mastication. Although a wire of relatively large cross section is used, either 0.018 by 0.025 inch (0.457 by 0.635 mm.) or 0.018 by 0.022 inch (0.457 by 0.559 mm.), the load deflection is reduced by the long perpendicular length from the incisors to the auxiliary tube of the first molar. In addition, a helix 3 mm. in diameter is placed at the critical section immediately mesial to the auxiliary tube on the first molar in which 2.5 turns are placed (Fig. 11). This spring is an example of how a large cross-sectional wire can be used in proper design to deliver optimal and constant forces without being so flimsy that permanent deformation can occur under accidental loading.
Anterior single point contack
The intrusive arch is not placed directly into the brackets of the anterior teeth. The major reason why one avoids bracket engagement of the intrusive
Deep overbite correction 9
Fig. 11. A helix 3 mm. in diameter with 2.5 turns is placed immediately mesial to the auxiliary tube on the first molar. The helix lowers the force and delivers it more constantly without reducing the arch’s ability to withstand permanent deformation.
Fig. 12. Intrusive arch is placed incisal to brackets. A separate 0.010 by 0.020 inch (0.254 by 0.508 mm.] anterior segment joins the incisor.
Fig. 13. If the intrusive arch is placed in two incisors, it is necessary to round the wire SO that no torque is produced.
spring is that, inadvertently, anterior torque may be present in the arch. Even if no torque is present, as the intrusive arch works out, torque can be introduced. If, purposely or inadvertently, labial root torque is placed into the incisors, t,he intrusive forces are increased on the anterior teeth ; this added intrusive force is not needed and can produce anchorage loss of the posterior teeth.
On the other hand, if lingual root torque is present, it will have the effect of reducing the magnitude of intrusion on the incisors. In fact, if the lingual root torque is large enough, the direction of the force could reverse and the incisors could actually extrude.
The advantage of not tying an intrusive arch directly into the incisor brackets is that it allows the clinician t,o know more positively the force system delivered. By having a single point of force application on the incisors, one knows the full force system acting at both the incisor point and the buccal tubes. A system of this type is described as being statically determinant. Placing the intrusive arch into the brackets and
statically e x t r u d e e d system t h e e x t r u d e 2 t h i s o f force and is of
Am. J. Orthod. July 1977
desirable curvatures are formed in the wire during activation. This is particu- larly noticeable if small cross sections of wires are used for intrusion.
The anterior single-point contact allows for the placement of a series of anterior-alignment arches directly into the bracket. The anterior-alignment arches can include small-cross-section bundle or straight wires or wires with loops (Fig. 12). One can gradually work up to larger cross sections that can stabilize the anterior segment.
An exception in which the intrusive arch may be placed in the brackets of the incisors can be found in the example of central incisor intrusion alone (Fig. 13). If the intrusive arch is placed into the incisors, it is necessary to round the wire so that no torque is produced. Rounding the anterior segment of an intrusive arch going into four incisors may be a problem since torques can still be produced be- cause of the curvature in the anterior part of the arch.
Point of force ~plication
An intrusive force placed through the center of resistance of the incisors will intrude the center of resistance and not produce any labial or lingual rotation of the teeth. The center of resistance of an anterior segment can be estimated to be at the geometric center of the roots of the incisors to be intrudd
Deep overbite correctio7L 11
Fig. 14. As the intrusive force is applied more anteriorly to the center of resistance of the incisors, a positive moment is created which tends to move the root lingually, provided the incisor is restrained from flaring labially.
Fig. 15. A posterior extension has been placed on a lower 0.018 by 0.025 inch (0.457 by 0.635 mm.) anterior segment. Right and sectional intrusive springs are hooked on the extension.
Fig. 16. Force system of appliance shown in Fig. 15. Only forces on the teeth are shown. Note that the posterior extension allows force to be directed through the center of resistance of the incisor. No incisor tipping will occur.
Fig. 17. A long posterior extension is used to protrusive lower incisors to prevent flaring. The hook at the intrusive section is shown.
on the incisors is the major key to success. This control includes delivering optimal force magnitudes, delivering these forces constantly, delivering the intru- sive force at a single point contact, and controlling the point of force application with respect to the center of resistance of the anterior segment.
Selective intrusion
Indiscriminate leveling with a continuous arch or with sections can produce undesirable side effects in a patient with deep overbite. In F’ig. 18 a Class II, Division 2 maxillary arch is depicted. Commonly one would like to intrude the incisors to the level of the canines or perhaps produce some extrusion of the posterior teeth without altering the plane of occlusion (line A). A straight arch
12 Burstone Am. J. Orthod. July 1977
Fig. 18. A straight wire placed in brackets of a Class II, Division 2 case, instead of pro- ducing intrusion (line A), tends to steepen the plane of o9.lw 3nww July
A09 ( l i n 8 c - 0 . 0 1 3 6 1 A m . c - 0 . 0 9 9 8 8 T w 6 . 6 2 w i d t h T d ( i n ) T 0 8 7 7 straight224
14 Burstoae Am. J. Orthod. July 1977
Fig. 22. The extrusive force on the molar during incisor intrusion tends to tip the crown lingually. This can be prevented by using a lingual arch.
as a group of individual posterior teeth but as a single posterior tooth composed of all the individual teeth on the right and left sides of the arch. In segmented arch mechanics, individual adjustments are not normally made in the buccal segment of the lingual arch during treatment; adjustments are made between the auxiliary tube of the first molar and the anterior teeth.
Two basic side effects could be anticipated from intrusive mechanics. Looking from the intrusive mecha-
nisms that have been described. The forces are kept as low as possible. The largest number of teeth are present in the buccal segment; these teeth are relatively rigidly connected byw buccal 6 TD 3 Tr -0.2163 Tc0349e as
Deep overbite correction 15
Fig. 23. Patient R. A. before treatment. Deep overbite is characterized by overeruption of
the maxillary central incisors.
molars or change in width. The lingual arch also has the advantage that one does not have to be that precise in forming the widths of the intrusive arch since this is adequately controlled by the lingual arch. Lingual arches are not only helpful during the stage of intrusion, but they also help resist side effects at al- most any stage of treatment.
Avoiding extrusive mechanics
If one is to accomplish genuine intrusion in patients, it would be disappoint- ing to succeed and then to lose the intrusion by using eruptive mechanics on the incisors and molars. Extrusive mechanics on the posterior teeth should therefore be avoided. Examples of extrusive mechanics are the use of Class II and Class III intermaxillary elastics, cervical gear with outer bows placed high applied to the maxillary arch, and the placement of a reverse curve of Spee in the lower arch wire to extrude premolars.
One of the classic situations for inadvertently erupting incisors which have been intruded or are going to be intruded is placement of a continuous arch wire, with or without loops, through a canine which has a crown distal to the root. If the arch wire is placed into the canine bracket, it will lie occlusal and hence will produce eruption of the incisors. Incisors make very poor anchorage for distal root movement of a canine, since eruption occurs so much more easily than distal
Am. J. Orthod. July 1977
Fig. 24. Patient R. A. Intrusive arch in place intruding the central incisors.
Fig. 25. Patient R. A. Following intrusion of the incisors, all six incisors are aligned with a 0.010 by 0.020 inch (0.254 by 0.508 mm.] section.
root movement. It is preferable to bypass the canines during canine root move- ment, or in certain situations canine root movement should be completed before the incisors are joined to the rest of the arch.
The typical patient who requires intrusion also requires minimization of extrusion of the posterior teeth. An exception is found in some of the patients with flat mandibular planes who have well-developed musculature. Extrusive mechanics on posterior teeth can be used on some of these patients, provided the cant of the plane of occlusion is controlled. Intrusion could be the final result since the muscles of mastication may intrude the posterior teeth back to their original positions. In these patients, it is necessary to keep the arches in place until this intrusion has occurred. Inhibition of the eruption of posterior teeth in the growing patient can be accomplished using a number of procedures. Occipital headgear can be worn to the upper arch, cervical headgear with high outer bows to the lower arch. Chin caps can be useful in inhibiting the eruption of posterior teeth. Although there is not good documentation, it is possible that temporo- masseter exercises could further aid in this inhibition.
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Deep overbite correction 17
Fig. 26. Patient R. A. at completion of treatment.
Treahnent sequence
Unless the incisors are protruded, intrusion is started during or following the initial alignment of the posterior segments. Patient R. A. (Fig. 23) exhibited deep overbite with excessive extrusion of the central incisors. During the first appointment an 0.018 by 0.018 inch (0.457 by 0.457 mm.) alignment arch was placed in the buccal segments and an 0.018 by 0.022 inch (0.457 by 0.559 mm.) intrusive arch was inserted to the central incisor only, bypassing the canines and lateral incisors (Fig. 24). When the central incisors were intruded to the level of the lateral incisors and canines, a 0.010 by 0.020 inch (0.254 by 0.508 mm.) anterior arch segment was placed from canine to canine to align the anterior teeth and to hold the incisor intrusion (Fig. 25). Simultaneously, in the lower arch, tip-back segments were used to remove the lower curve of Spee and to re- tract the lower posterior segments. Headgear to the upper arch was used to restrain the buccal segments in order to correct the Class II occlusion produced by retraction of the lower buccal segments. The occlusion at the time of deband- ing is shown in Fig. 26. The molar bands have been left in place so that head- gear worn 6 to 8 hours a day could be used as a retentive appliance.
At the beginning of treatment Patient M. M. showed 100 per cent overbite and a lateral open-bite tendency (Fig. 27). Since the central incisors were posi- tioned occlusal to the lateral incisors, an 0.018 by 0.018 inch (0.457 by 0.457 mm.) connector was fabricated joining the central incisors together, bypassing the canines and lateral incisors (Fig. 28). Intensive arches were used until the
Fig. 27. Patient M. M. before treatment. Deep overbite with lateral open-bite. To avoid increase of vertical and eruptive incisor relationship to upper lip, no leveling of mandib- ular curve of Spee was planned.
Fig. 28. Patient M. M. Incisor connector in place before placement of the intrusive arch.
Fig. 29. Patient M. M. Following incisor intrusion and alignment of the six anterior teeth, the anterior segment is retracted en masse with 0.010 by 0.020 inch (0.254 by 0.508 mm.) retraction spring.
.
Fig. 30. Patient M. M. at completion of treatment. Curve of Spee remains in lower arch. Deep overbite was corrected by upper incisor intrusion.
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Deep overbite correction 19
Fig. 31. Patient M. M. Cranial base superposition (Dotted line-After treatment]. No man- dibular rotation or increase in vertical dimension has occurred during treatment.
Fig. 32. Patient M. M. Maxillary and mandibular superpositions. Upper incisors have been intruded 7 mm., measured at their apices.
central incisors reached the level of the lateral incisors, and then all four incisors were intruded as a unit. En masse space closure of the six anterior teeth was effected by an 0.010 by 0.020 inch (0.254 by 0.508 mm.) anterior retraction as- sembly (Fig. 29). The finished result is shown in Fig. 30.
Because of the short upper lip, the curve of Spee was maintained in the lower arch, with all of the intrusion occurring in the upper incisor region (Figs. 31 and 32). The skeletal pattern with its large vertical height anteriorly, facial convexity, and steep mandibular plane required treatment that would maintain the vertical dimension and correct the deep overbite by intrusion. If the Iower curve of Spee had been leveled, the result would have been undesirable ; the vertical dimension would have been increased, so that the patient would not close her lips and too much upper incisor would have shown below the upper lip. Indiscriminate leveling of the lower arch should not be attempted in this type of case.
Canine inrtrusion
It is usually not possible to intrude all six anterior teeth at one time without producing undesirable axial inclination change in the posterior segment. TJsing
20 Burstone Am. J. Orthod. Jzcly 1977
Fig. 33. An 0.018 by 0.025 inch (0.457 by 0.635 mm.) canine-intrusion spring. Passive position.
Fig. 34. Canine-intrusion spring is activated by placing its anterior end into the vertical tube of the canine.
Fig. 35. An 0.018 by 0.018 inch (0.457 by 0.457 mm.) intrusive spring attached to the auxiliary tube on the second premolar. This spring is used if molar auxiliary tube is not available.
Fig. 36. Active state of spring shown in Fig. 35. Helices lower load-deflection rate and reduce unwanted negative moments on the canine.
the suggested force values, typically 100 Gm. of force on a side is required to intrude the incisors and the canines. Table I shows that 100 Gm. would produce a moment of 3,000 Gm.-mm. to the posterior segment if the perpendicular distance from the incisors to the center of resistance of the posterior segment was 30 mm. Since moments of this magnitude are most effective, tipping of the posterior teeth will occur more rapidly than the intrusion, and since this tipping is not required, intrusion mechanics will not be successful. If the posterior segment were backed up with an occipital headgear in the maxillary arch, it is possible to eliminate this undesirable moment as well as the eruptive force on the posterior teeth. Without excellent cooperation from the patient in the wearing of headgear, intrusion of six anterior teeth simultaneously should not be attempted.
Two types of situation require separate canine intrusion. In the first the canine lies bilaterally occlusal to the premolar and the canine must be intruded separately following anterior intrusion. In the second, the canines have not erupted symmetrically and canine intrusion is required on only one side. In
Deep overbite correction 21
Fig. 37. Canine root spring. During canine root movement intrusion can be carried out simultaneously.
patients with deep overbite it is usually a mistake to level and extrude infra- erupted canines. Many of these canines should be left in their original position and the incisors should be intruded to their level.
Figs. 33 and 34 show a canine-intrusion spring which is activated to produce 50 to 75 Gm. of force. It is fabricated from 0.018 by 0.025 inch wire inserted into the auxiliary tube of the first molar and into the vertical tube of a canine bracket. Since the intrusive force lies lateral to the center of resistance of the canine, it is necessary to place a slight constrictive force in the spring to keep the canine from flaring labially. To minimize the chance of producing an undesirable moment in the canine tube, it is a good idea to round the wire in the portion of the spring that is placed in the vertical tube of a canine. If the incisors have already been intruded, it is necessary to join them to the posterior segments by an anterior wire inserted in the auxiliary tubes of the premolars stepped either occlusally or gingivally around the canine. This wire holds the incisors in place and adds further anchorage for the intrusion of the canines.
If the auxiliary tube in the first molar is not available for an intrusive spring, an 0.018 by 0.018 inch (0.457 by 0.457 mm.) spring can be constructed which inserts into the auxiliary tube of the most anterior premolar (Figs. 35 and 36). If no auxiliary tubes are available, a continuous segment from molar forward to canine can be constructed of this design. The 0.018 by 0.018 inch (0.457 by 0.457 mm.) intrusive spring is a modified rectangular loop with helices placed mesial to the brackets. This design reduces the load-deflection rate and, more important, assures that as the spring works out a vertical force will be delivered without an undesirable moment being produced on the canine. If a canine is flared, a moment is produced which flares the canine more; hence, it is necessary to tie the canine back on both the buccal and lingual aspects. A buccal tie alone could cause the canine to rotate with its distal aspect toward the lingual.
In addition to specialized intrusive springs, separate canine intrusion can be produced by canine-retraction assemblies or root springs, In Fig, 37 a root spring is being used to simultaneously retract the root and intrude the canine.
Ant. J. Ol’thod. July 1977
Summary
t h e anterior teeth, while others require pri- marily extrusion. This article has discussed the principles of incisor and canine intrusion and has demonstrated the use of intrusion springs that are capable of intruding incisors with minimal side effects on the posterior teeth.
Six principles must be considered in incisor or canine intrusion: (1) the use of optimal magnitudes of force and the delivery of this force constantly with low- load-deflection springs; (2) the use of a single point contact in the anterior region ; (3) the careful selection of the point of force application with respect to the center of resistance of the teeth to be intruded; (4) selective intrusion based on anterior tooth geometry; (5) control over the reactive units by formation of a posterior anchorage unit ; and (6) inhibition of eruption of the posterior teeth and avoidance of undesirable eruptive mechanicu6.4.86.187 48.201mechanics.3 Tr -0.024ET BT 0.75 Tj0j0j1 84 45 Tr 55m Tw (m/F0 6 Tr 55f ) T36r
