Segmented approach to simultaneous intrusion and space closure: Biomechanics of the three-piece base arch appliance

Bhavna Shroff, DDS, M Dent Sc, a Steven J. Lindauer, DMD, M Dent Sc, b Charles J. Burstone, DDS, MS, c and Jeffrey B. Leiss, DDS a Baltimore, Md., Richmond, Va., and Farmington, Conn.

Deep overbite correction and space closure in patients with flared incisors are mechanically difficult to achieve with conventional orthodontic treatment, The purpose of this article is to present an appliance design that allows simultaneous intrusion and retraction of anterior teeth as well as correction of their axial inclinations. A three-piece base arch was used to achieve simultaneous intrusion and space closure, Various clinical situations are discussed and analyzed from a biomechanical standpoint. Sequences of treatment, appliance design, and management of side effects are described in detail. The segmented approach to simultaneous intrusion and space closure is useful for achieving precise control of tooth movements in the anteroposterior and vertical dimensions. (AM J ORTHOD DENTOFAC ORTHOP 1995;107:136-43,)

Dur ing orthodontic therapy, correction of deep overbite in patients with flared incisors is a difficult biomechanical challenge, since uprighting of incisors often lengthens the crowns vertically and increases the overbite. Deep overbite may be ac- companied by intraarch spacing associated with flared incisors, or intraarch crowding requiring pre- molar extractions. In extraction cases, alignment of the anterior teeth does not correct their axial inclinations or the deep overbite. In either extrac- tion or nonextraction therapy, the deep overbite must be corrected to ensure that full space closure is possible when the canine relationship is Class I; therefore simultaneous intrusion and retraction of the anterior teeth may be desirable to achieve optimum treatment results. During intrusion of the anterior teeth, control of their labiolingual axiaI inclinations during retraction is critical for success- ful completion of treatment. This article describes a technique to intrude flared incisors, control their axial inclinations, and retract them simultaneously with good anchorage control, by using frictionless mechanics.

The orthodontic correction of deep overbite can be achieved with several mechanisms that will re- sult in true intrusion of anterior teeth, extrusion of posterior teeth, or a combination of both. For

aDepartment of Orthodontics, University of Maryland School of Den- tistry. bDepartment of Orthodontics, Medical College of Virginia, Virginia Commonwealth University. CDepartment of Orthodontics, University of Connecticut. Copyright 1995 by the American Association of Orthodontists. 0889-5406/95/$3.00 + 0 8/1/47903

example, leveling of a mandibular curve of Spee can be achieved by extrusion of molars and premo- lars. Extrusion of posterior teeth will result in increased lower facial height, steepening of the occlusal plane, and downward and backward rota- tion of the mandible with a worsening of the Class II skeletal relationship. The long-term stability of such a correction is questionable unless suitable growth occurs. Deep overbite correction by intru- sion of anterior teeth affords a number of advan- tages including simplifying control of the vertical dimension and allowing forward rotation of the mandible to aid in Class II correction. 1 Intrusion of anterior teeth to correct deep overbite may be indicated in patients with unesthetic excessive max- illary incisor showing at rest and a deep mandibular curve of Spee associated with a long lower facial height. By using segmented arch mechanics, genu- ine intrusion of the anterior teeth may be ob- tained. 2-4

During intrusion of anterior teeth, optimal mag- nitudes of force may be delivered constantly using low load deflection springs. Low forces are used during intrusion to minimize root resorption and decrease side effects on the reactive unit. It has been documented that the use of heavier forces will not increase the rate of intrusion. 5'6 The use of a point contact of force application is critical for obtaining true intrusion because the force may then be applied directly through the center of resistance of the anterior segment. This can be achieved by using an intrusion arch tied to the anterior segment to give a statically determinate force system? Al- ternative mechanisms such as a utility arch or a

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continuous arch wire with tip back bends located mesial to the first permanent molars may not achieve intrusion because the full engagement of the arch wire in the brackets of the anterior teeth produces an undesirable force system. In these cases, relative intrusion and flaring of the anterior teeth are achieved, resulting in a modification of the axial inclination of the anterior teeth that may or may not be desirable. 7

The selection of the point of application of the intrusive force with respect to the center of resis- tance of the anterior segment is important to pre- cisely define the type of tooth movement that will occur. True intrusion without axial inclination change is obtained by directing the intrusive force through the center of resistance of the anterior teeth. Since displacement of the intrusive force away from the center of resistance will result in either flaring or uprighting of the incisors, careful evaluation is necessary to monitor the axial incli- nation of the anterior teeth during intrusion. 8

In patients with proclined incisors, a continuous intrusion arch tied at the midline cannot be used because the force system generated tends to worsen the axial inclination of the anterior teeth. This is because the intrusive force is applied ante- rior to the center of resistance of the incisors and the moment consequently produced tends to fur- ther flare the anterior teeth. One solution to this problem is the use of distal extensions to the anterior segment of wire where segmented intru- sion springs can be hooked at a point where the force acts at the estimated center of resistance of the anterior segment. 9

In many extraction cases the axial inclination of flared anterior teeth is corrected first by retraction of the incisors during initial space closure. When no further retraction is possible because of the presence of a deep bite and the reduction of the overjet, intrusion is initiated to open the bite and allow subsequent space closure. To achieve deep overbite correction and close extraction spaces si- multaneously, an appliance design needs to incor- porate a variable point of application of the intru- sive force, as well as a mechanism to direct the intrusive force along the long axis of the anterior teeth.

~NTRUSION- RETRACTION MECHANICS

The mechanism described here uses the prin- ciples of the segmented arch technique. ~ Seg- mented arch mechanics uses different wire cross- sections in a given arch rather than continuous wires? ~ The advantage of using such an approach is

that it is possible to develop a precise and predict- able force system between an anterior segment (incisors) and a posterior segment (premolar and molars) enabling pure intrusion of the anterior teeth and control of their axial inclinations. Move- ment of the posterior segment is also well con- trolled. The appliance described enables the mag- nitude of the moments and forces delivered to be well controlled. 12 Consequently, constant levels of force can be maintained, and the moment to force ratio (M/F) at the centers of resistance easily regu- lated to produce the desired tooth movements.

Sometimes, intrusive forces on the upper ante- rior teeth can be used to tip back the posterior teeth while partially or completely correcting a Class II buccal relationship. This article will em- phasize the use of intrusive forces for retraction of anterior teeth when intrusion is needed. The same mechanism with higher forces can be used to tip back buccal segments. If only anterior intrusion and retraction is indicated, the following proce- dures are generally followed.

After careful differential diagnosis and plan- ning, treatment is initiated by aligning the teeth included in the right and the left posterior seg- ments. After satisfactory alignment of the premo- lars and molars, passive segmented wires (0.017 x 0.025 stainless steel) are placed in the right and the left posterior teeth for stabilization. A precision stainless steel transpalatal arch (0.032 0.032) placed passively between the first maxillary molars consolidates the posterior unit now consisting of right and left posterior teeth. 13 Canines may be retracted separately and incorporated into the buc- cal segments 14'~5 or left at their initial positions. The anterior segment is aligned with a low stiffness arch wire. The next stage of treatment will involve the simultaneous intrusion and retraction of the incisor segment. To design the appliance optimally to obtain the desired force system, the position of the center of resistance of the anterior teeth may be estimated on a lateral cephalometric x-ray film. In clinical situations where incisors are proclined, the center of resistance of the anterior segment lies further lingual to the incisors crowns.

A three-piece base arch is used to intrude the anterior segment (Fig. 1). A heavy stainless steel segment (0.018 x 0.025 or larger) with distal exten- sions below the center of resistance of the anterior teeth is placed passively in the anterior brackets. The distal extensions end 2 to 3 mm distal to the center of resistance of the anterior segment. The intrusive force is applied with a 0.017 0.025 TMA tip-back spring (Ormco, Glendora, Calif.).

138 Shroff et aL American Journal of Orthodontics and Dentofacial Orthopedics February 1995

Fig, 1, Diagramatic representation of three-piece base arch. The anterior segment extends 2 to 3 mm distal to the center of resistance (CR) of the anterior teeth. Force acts through center of resistance.

Fig, 2. Diagram of three-piece base arch and Class I elastic stretched from maxillary first permanent molar to distal exten- sion of anterior segment. Class I elastics are needed to redirect force parallel to the long axis of the incisor.

(The point of application of the intrusive force on the distal extension of the anterior segment will be discussed later.) The overall force system obtained is an intrusive force anteriorly and an extrusive force posteriorly associated with the tip back mo- ment. The design of this appliance enables low- friction sliding to occur along the distal extension of the anterior segment during space closure (Fig. 2). The application of a light, distal force delivered by a Class I elastic to the anterior segment is used to alter the direction of the intrusive force on the anterior segment. This appliance design allows the application of the intrusive force to get true intru- sion of the incisors along their long axes.

Fig. 3. A, Intrusive force through CR will intrude incisor along line of action of this force. B, An intrusive force perpendicular to the distal extension and through CR will have the same effect as in A.

BIOMECHANICS Anterior segment and direction of intrusive force

A number of different clinical situations may arise and they should be thoroughly analyzed from a biomechanical standpoint to determine the cor- rect force system necessary to achieve the treat- ment objectives.

An intrusive force perpendicular to the distal extension of the anterior segment and applied through the center of resistance of the anterior teeth will intrude the incisor segment (Fig. 3). It is possible to change the direction of the net intrusive force by applying a small distal force. The line of

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Fig. 4. A, Direction of net intrusive force through CR may be changed by application of a small distal force. The resulting intrusive force has a direction parallel to the long axis of the incisor and is distal to CR. B, The net force can be directed along the long axis of the incisor by applying the intrusive force more anteriorly.

action of the resultant force will be lingual to the center of resistance (Fig. 4, A) and a combination of intrusion and tip back of the anterior teeth will occur. Thus the line of action of the resultant force can be made parallel to the long axis of the anterior teeth if an appropriate distal force is combined with a given intrusive force. To obtain a line of action of the intrusive force through the center of resistance and parallel to the long axis of the incisors, the point of force application must be more anterior and as close to the distal of the lateral incisor bracket as possible (Fig. 4, B).

If the intrusive force is placed farther distally and an appropriate small distal force is applied (Fig. 5), intrusion and simultaneous retraction of the anterior teeth occurs because of the tip back (clockwise) moment created around the center of resistance of the anterior segment consisting of four incisors.

The distal force used in this intrusion retraction system is of very low magnitude and is used to redirect the line of action of the intrusive force. One advantage of this system is the low magni- tude of force applied on the reactive or anchorage unit.

CL IN ICAL APPL ICAT IONS OF THE INTRUSION RETRACTION MECHANICS

After treatment planning and developing treat- ment objectives, the desired force system should be determined with respect to the centers of resis-

Fig. 5. intrusive force can be directed along long axis of anterior teeth and applied lingual to CR. The farther lingual the force, the larger will be the moment acting to tip the incisors lingually.

tance of the anterior and posterior segments. The correct appliance design is chosen after careful analysis of the clinical situation as discussed above. Spacing or crowding among the incisors is usually addressed early in treatment. When intrusion- retraction mechanics are initiated, the anterior teeth will intrude and tip back with progressive space closure between the incisors and the canines. Distal movement of the canines may occur as the anterior segment contacts the canines. It is also possible to retract the canines indMdually and to include them in the buccal stabilizing segment of wire before the initiation of intrusion-retraction mechanics.

The force system generated on a molar is shown in Fig. 6, A. A tip back moment is created during intrusion of the anterior segment and will have a typical value of 900 gm-mm for an intrusive force of 30 g and an interbracket distance of 30 mm. In Fig. 6, B, the force is redirected to be parallel to the long axes of the incisors. Redirection and move- ment of the intrusive force distally reduces the moment on the buccal segment of teeth, and thus reduces the tendency for its natural plane of occlu- sion to steepen. Headgear is not usually required for anchorage control, since a net tip back moment is applied to the posterior segment. It is important to monitor the anterior and posterior segments and alter the force system if indicated. The resulting force system can be modified by changing the magnitudes and points of application of the intru- sive and distal forces with respect to the center of resistance of the anterior segment.

140 Shroff et aL American Journal of Orthodontics and Dentofacial Orthopedics February 1995

gl

ao turn - - - - - "~1

gr

B

Fig. 6. Comparison of force system developed on molar with identical 30 gm intrusive forces. A, Perpendicular to the occlusal plane. B, Parallel to the incisor long axis and lingual to CR. Note reduction of the moment on the molar in B.

CASE REPORT

A 10-year, 9-month-old black female patient pre- sented to the orthodontic clinic of UMAB Dental School for treatment. The extraoral examination of the patient showed good facial symmetry and a convex profile. Her upper and lower lips were significantly procumbent with respect to the soft tissue line Sn-Pg (subnasale-Pogo- nion), and her interlabial gap was 9 mm at rest. She presented with an acute nasolabial angle and a deep labiomental fold.

Dentally, the patient displayed a Class II, Division 1 malocclusion in the late mixed dentition (Fig. 7). The occlusogram confirmed 11 mm of spacing in the maxillary arch. The anterior overjet was approximately 10 mm, and the overbite was 65%, with palatal impingement. A deep curve of Spee was present in the mandibular arch. The patient had a Class II skeletal relationship primarily because of a protrusive maxilla. The upper incisors were labially tipped with respect to Frankfort horizontal, and the lower incisors were in relatively normal position with respect to the mandibular plane. The treatment objec-

tives included a reduction of the maxillary protrusion both orthopedically and dentally, correction of the deep overbite, and achievement of maxillary space closure. Deep overbite was corrected by upper and lower incisor intrusion. In the maxillary arch, rotation of the first molars was achieved initially with a removable stainless steel transpalatal arch. High-pull headgear wear was initiated to correct the Class II occlusion and control the vertical dimension. Simultaneous intrusion and retrac- tion of the upper incisors was initiated after consolida- tion of spaces in the maxillary arch between the lateral and central incisors. Because of the proclination of the maxillary incisors, a three-piece base arch was selected to intrude them and a light distal force was applied to redirect the intrusive force along their long axes.

As intrusion occurred, the incisors tipped back and space closure was achieved simultaneously (Fig. 8). A continuous intrusion arch tied to the central incisors could not have been used in this situation because of the proclined position of the upper incisors. The application of an intrusive force anterior to the center of resistance

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Fig. 7. A, Intraoral view of occlusion: Frontal aspect. There is a 65% overbite with palatal impinge* ment and an anterior overjet of 10 mm. B, Intraoral views of the occlusion, maxillary occlusal view. The maxillary arch is symmetric with respect to the median Raphe and the soft tissue of the cheeks and lips. The maxillary arch has 11 mm of spacing confirmed by the occlusogram. C and D, Intraoral views of the right and left buccal occlusion showing a deep curve of Spee in the lower arch and a Class II, Division 1 type of malocclusion in the late mixed dentition. The maxillary anterior teeth are in tabioversion.

Fig. 8. A, Intraoral view of occlusion: Frontal aspect. After preliminary alignment of the molars and premolars and separate retraction of the canines, a three-piece base arch was used to simulta- neously intrude and retract the maxillary incisors. B and C, Intraoral views of the right and left buccal occlusion: The tip back spring is carefully positioned and activated. The chain elastic is redirecting the intrusive force along the long axes of the maxillary incisors.

142 Shroff et al. American Journal of Orthodontics and Dentofacial Orthopedics February 1995

Fig. 9. Intraoral views of finished occlusion: A, Frontal aspect. B, Maxillary occlusaq aspect. C, Mandibular occlusal aspect. D and E, Right and left buccal aspects.

BEFORE

. . . . . . AFTER

Fig. 10. Superimposition of maxillary cephalometric tracings before and after treatment showing movement of maxillary incisors and molars during treatment. The intrusive force applied on the maxillary incisors was redirected along their long axis and simultaneous intrusion and space closure was successfully achieved.

of the anterior segment would have flared the incisors farther. The upper arch was finished with a continuous arch wire (0.016 x 0.022 TMA). In the mandibular arch, a removable lingual arch was placed, and intrusion of the incisors was achieved with a continuous intrusion arch. After the leveling of the curve of Spee, a continuous arch wire (0.017 0.025 TMA) was used for finishing. The three-piece base arch allowed precise control of the delivered force system in the maxillary arch, since it was possible to direct the intrusive force along the long axes of the incisors and place it lingual to the center of resistance. Maxillary and mandibular Hawley retainers were delivered to the patient subsequent to debonding (Fig. 9). A superimposition of maxillary cephalometric tracings before and after treatment shows the movement of the maxillary incisors and molars during treatment (Fig. 10).

CONCLUSION

Deep overbite correction and space closure can be simultaneously achieved with the three-piece

American Journal of Orthodontics and Dentofacial Orthopedics Shroff et aL 143 Folume 107, No. 2

base arch intrusion mechanism in patients with flared incisors. The force system delivered on the anterior segment depends on the point of applica- tion of the intrusive force and its direction. This segmented approach to intrusion and retraction is clinically advantageous because it allows simulta- neous control of tooth movement in the vertical and anteroposterior planes. The low load deflec- tion rate of this appliance delivers a constant in- trusive force, and the levels of force can be kept low. The design of this appliance allows the clini- cian to deliver a well-controlled, statically determi- nate force system in which only minimal chairside adjustments are required.

We extend our thanks to Mrs. Jo-Ann Walker for preparing the manuscript.

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Reprint requests to: Dr. Bhavna Shroff Department of Orthodontics University of Maryland Dental School 666 West Baltimore St. Baltimore, MD 21201

AAO MEETING CALENDAR

1995 - - San Francisco, Calif., May 12 to 17, Moscone Convention Center (International Orthodontic Congress)

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