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SPECIAL ARTICLE

On the management of extraction sites

Stanley Braun, DDS, MME, a Robert C. Sjursen, Jr., DDS, b and Harry L. Legan, DDS c Indianapolis, Ind.

Extraction sites may be needed to achieve specific orthodontic goats of positioning the dentition within the craniofacial complex. The fundamental reality that determines the final position of the dentition, however, is the control exercised by the clinician in closure of the extraction sites. A specific treatment objective may require the posterior teeth to remain in a constant position anteroposteriorly as well as vertically, while the anterior teeth occupy the entire extraction site. Another treatment objective may require the reverse, or any number of purposeful alternatives of extraction site closure. An appliance system developed over time, which provides this control, is described. The system takes advantage of aspects of continuous arch therapy that provides constant, positive orientation of the anterior and posterior groups of teeth to each other in three- dimensional space across an extraction site, combined with aspects of the segmented arch technique that permit definable and predictable force systems to be applied to these teeth. Consequently, the clinician has the ability to forecast treatment outcomes with confidence. (Am J Orthod Dentofac Orthop 1997;112:645-55.)

E x t r a c t i o n sites are created to achieve one or more objectives: to improve dental occlusion, to reconcile arch length deficiencies, to purposefully alter the facial profile, to optimize surgical-orthodontic correction of jaw discrepancies, and to improve function, v3 The management of any extraction site must therefore be under the control of the clinician to ensure that the teeth will ultimately reside in predetermined positions. Toward this end, Burst- one 4 has defined three types of controlled closure of extraction sites: Type A refers to those sites where the anterior teeth will occupy most or all of the extraction space; type B refers to equal occupation of the extraction site by the anterior and posterior teeth; and in type C, the extraction site is closed by the posterior teeth occupying most or all of the extraction site.

The types of closure may not necessarily be bilaterally identical in a given arch. For example, in asymmetric malocclusions, one extraction site may be identified as requiring a type C closure, whereas the contralateral site may require a type A closure. This may be necessary to achieve predetermined treatment goals of the dental centerline being coin- cident to the facial centerline, and the dentition to be properly positioned anteroposteriorly to support the facial soft tissues and to satisfy the clinician's concept of dental stability.

aClinical professor, Orthodontics, Vanderbilt University Medical Center and University of Illinois at Chicago. bAssistant professor, Orthodontics, Vanderbilt University Medical Center. ~Professor and Chairman, Orthodontics, Vanderhilt University Medical Center. Reprint requests to: Dr. Stanley Braun, 7940 Dean Rd., Indianapolis, IN 46240. Copyright © 1997 by the American Association of Orthodontists. 0889-5406/97/$5.00 + 0 8/1/75235

To achieve controlled extraction site closure, the appliance used must deliver definable force systems regulated by the clinician and not produce closure in some ambiguous, indeterminate way. Only when force systems are definable are the dental move- ments predictable and treatment outcomes fore- castable with confidence. In addition, the force systems should result in minimal to no tissue resorp- tion, move teeth with optimal velocity, cause minimal patient discomfort, and have an extended range of activation, while producing a relatively constant force system. This latter objective results in reducing the number of patient visits, while yielding tooth movement with a nearly constant center of rotation. The purpose of this article therefore is to describe an appliance system that has been developed over time and meets these goals.

GENERAL CONSIDERATIONS IN TOOTH MOVEMENT (Quantities in this manuscript are representative)

To translate a tooth or a group of teeth, it has been repeatedly shown that a moment-to-force ratio applied at a bracket should be equal to the perpendicular distance from the bracket to the center of resistance of an individ- ual tooth or group of teeth. 5-~ Further, investigators have recently demonstrated that translatory movement causes the least tissue damage because the periodontal stress is relatively uniformly distributed along the root surfaces. 9-12 In contrast, uncontrolled dental tipping results in the greatest periodontal stress located at both the dental apical and alveolar crestal regions. In root movement, the greatest periodontal stress is in the apical region, whereas in controlled tipping (center of rotation approximating the

645

646 Braun, Sjursen, and Legan American Journal of Orthodontics and Dentofacial Orthopedics December 1997

a = molar auxiliary tube b = canine auxiliary tube

I d mb,._ I

I~ = Center of Resistance

Closure Force System Posterior Anterior F I

p Ma*""

Mp M a F ~ 12-13, "T - ~ 10, F'(d)=Mp-Ma Type 'A'

Type 'B' ~2.~Mp MF~-~ Mp=Ma, MP Ma - g - =W-- --10

Type 'C' ] F~__.F Mp Ma F ~ T F ' Mp Ma F = 10, --F-'--- = 12-13, F'(d)=Ma-Mp

Fig. 1. Force systems related to type A, B, and C extraction site closure.

root apex), the greatest stress is at the alveolar crestal region.

If an extraction site is closed with a translatory force system (M/F approximating 10) applied equally to the posterior teeth and anterior teeth, type B closure will occur, t3 (See Fig. 1.) In type A closure (posterior teeth remain essentially fixed anteroposteriorly), the clinician must increase the M/F ratio on the posterior teeth to approximately 12 to 13. This results in a periodontal stress distribution related to root movement, while the anterior teeth are simultaneously subjected to a M/F approximating 10, resulting in translation. This purposeful differential stress distribution between the posterior and anterior teeth takes advantage of the hierarchy of relative velocities of tooth movement, namely, root movement is a slower process than translatory movement. Because the system is in equilibrium, vertical forces occur (see Fig. 1) because a couple results (F' d), which is equal to the difference in magnitude between the posterior moment (Mp) and the anterior moment (Ma). These vertical forces are of concern for they have the potential of altering the occlusal plane. It is therefore important to control the differential between the posterior and anterior moments, so that the vertical forces are ostensibly bal- anced by occlusal forces.

To achieve type C closure, the previously mentioned force system is reversed, with the larger moment applied to the anterior teeth and the smaller moment applied to the posterior teeth as in Fig. 1. This results in a M/F ratio approximating translation applied to the posterior teeth

versus a M/F ratio approximating root movement applied to the anterior teeth. Once again, vertical forces are of concern. (The specifics of appliance activation for type A and type C extraction site closure will be addressed further in Treatment Procedures.) In the special case where the mandibular anterior teeth had been intruded earlier, one should consider the use of Class II elastics, as seen in Fig. 2, to eliminate the couple created by the larger anterior moment (Ma). In this case, the horizontal force (F) in the M/F may be reduced by half (F/2). The horizontal component of the Class II elastic is adjusted to be approximately equal to F/2. Consequently, the M/F ratio at the posterior teeth is approximately 10, causing translation, while the anterior teeth will experience root movement because of an increased M/F ratio related to the reduction of the force (F/2). In the maxilla, type C closure can be achieved through the use of protraction headgear (when the anterior teeth had been previously intruded) or alternatively, type C closure can also be obtained through the use of Class III elastics that will supply a horizontal component to the posterior group of teeth equal to F/2, and thus provide the moment-to-force ratio that will cause translation, similar to the system described for type C closure in the mandibular arch.

From the occlusal aspect, the forces acting on the buccal surfaces of the posterior teeth produce a moment as seen in Fig. 3. This moment tends to alter the arch form by decreasing the arch width in the premolar region. This moment may be negated by an activated lingual arch ~4,~5 or by dividing the mesiodistal force buccolingually. The

American Journal of Orthodontics and Dentofacial Orthopedics Braun, Sjursen, and Legan 647 Volume 112, No. 6

F / 2 1 7

I 7 E/2 ¢

t_

)

M p = M a E/2~-F/2

(__~__) Mp POSTERIOR - F/2 + E/2 ~ 10

Ma ANTERIOR - F/2 ~ 1 2 - 13

Fig. 2. Force system for type C closure in mandible with Class II elastics.

lingual force may be supplied by an elastic. The closure force on the anterior teeth does not have an effect on the arch width in the canine region.

In patients exhibiting anterior arch length deficiencies, the canines often need to be initially translated part way into the extraction sites to align the incisors. In this case, it is important to divide the closure force (F) buccolingually to avoid first-order canine rotations, while simultaneously preventing first-order rotation of the posterior teeth (an active lingual arch need not be used in this case). As mentioned earlier, the lingual component of this divided force may be obtained through the use of lingual elastics. Problems related to patient compliance may be reduced through the use of lingual coil springs. Elasto- meric chains should be avoided because of their rapid force decay. 16 The issue of patient compliance is addressed further in the Treatment Procedures section.

TREATMENT PROCEDURES*

After initial tooth-to-tooth alignment of the posterior teeth, segments are established by using relatively stiff

" U s e of 0.018 slot brackets are assumed throughout. The 0.022 slot brackets may be used with a proport ional increase in wire sizes. Table 1 lists all brackets needed. Any 0.018 slot bracket may be used on teeth not

otherwise specified.

Center of Resistance

Rotating Moment = F- X

Fig, 3, Rotating moments caused by buccal forces during extraction site closure.

wire. A 0.016 × 0.022 stainless steel wire is used for this purpose. The segments are tied back to maintain their arch lengths (see Fig. 4). When the anterior teeth do not exhibit arch length deficiencies, the treatment occlusal plane is established by posterior dental eruption or by true anterior dental intrusion. 13,1s,17 If, however, the anterior teeth exhibit an arch length deficiency, the canines are engaged and steel-tied to a 0.016 × 0.016 stainless steel "track." The track originates from each auxiliary tube on the premolars bilaterally, and bypasses the incisors, as seen in Fig. 5. The wire track contains a step-up immedi- ately mesial to the premolar auxiliary tube, as well as a bend at the wire's exit. This establishes a molar-to-molar continuous arch of variable cross-section whose arch length is defined.

If the canines are malaligned axially, rotated, or require intrusion or extrusion, they should be corrected before establishing the track, is In all cases, the arch form should be preserved. During canine root movement, canine leveling, or correcting rotations, lingual arches should be present to enhance anchorage and to maintain symmetry of the posterior teeth.

After the canines are translated distally on the track, allowing access to the incisors, they are temporarily joined to the buccal teeth by placing extended 0.016 × 0.022 buccal segments, while the treatment plane of occlusion is established for the incisors at this time. Once this is accomplished, all the anterior teeth, including the canines, are consolidated on a stainless steel 0.016 × 0.016 arch wire molar-to-molar (the buccal segments are removed). Root alignment may be confirmed with an x-ray film at this time. Subsequently, the arch wire size is increased to 0.016 × 0.022. This allows the clinician to finalize arch form and accomplish any third-order incisor corrections needed.

Remaining space closure is accomplished en masse by placing a 0.017 × 0.025 anterior segment engaging the premolar tubes bilaterally, as seen in Fig. 4. The occlusal


Fig. 4. Buccal segments and anterior wire arrangement for en masse space closure.

Fig. 5. Track design for initial canine distal translation.

step in the wire distal to the canines bilaterally is needed to accommodate the vertical positional difference between the auxiliary tubes on the premolars and the slots of the canine brackets. The anterior teeth should be ligated together during en masse space closure. This relatively heavy anterior segment is used to fill the slots of the incisor brackets for torsional control during remaining extraction site closure. The previously used buccal segments are reseated into the buccal teeth, as seen in Fig. 4. Once again, a continuous arch of variable cross-section has been established (0.016 × 0.022 posteriorly, 0.017 × 0.025 anteriorly). All the teeth within each of the three segments are well-aligned to each other, and all that remains is en masse space closure. The arch form is maintained by the anterior wire engaging the buccal segments at the premolar auxiliary tubes.

ACHIEVING CANINE TRANSLATION AND SELECTED EN MASSE CLOSURE

To control the moment-to-force ratios and related centers of rotation, retraction springs should produce nominally predictable moments and forces. An acceptable spring currently available for this purpose is the 0.017 × 0.025 TMA T loop design by Burstone. 4 For its activations to result in predictable moments and forces, each dimension of the T loop must be specific. Fig. 6, A may be used as a template for this purpose. (This spring is also available prefabricated from the Ormco Corp.)

As an example, to produce an anteroposterior force approximating 300 gm, the spring is activated as follows (refer to Fig. 1): The distance from the mesial aspect of the auxiliary tube of the first molar to the center of the vertical tube of the canine bracket (L) is measured. An

American Journal of Orthodontics and Dentofacial Orthopedics B r a u n , S jursen , a n d L e g a n 649 Volume 112, No. 6

T a b l e I. Required brackets and fittings

Maxilla

. ATTACHMENT TOOTH Part no. a

Left canine 347-3113 b Right canine 347-3013 b Canine-lingual any lingual button/hook 1st or 2nd premolars 394-5607 ° First molars 182-4518 b Lingual sheaths 671-3672 b

Mandible Left canine 347-3313 b Right canine 347-3213 b Canine-lingual as above 1st or 2nd premolars as above First molars 180-0002 b Lingual sheaths as above

a0.018 slots are listed -0 .022 substitution brackets are available. bOrmco Corp. CAmerican Orthodontics.

activation (A) of 8 mm will produce an approximate mesiodistal force of 300 gm (see Table II). This activation is subtracted from (L). The resultant difference (D) represents the sum of the anterior and posterior leg lengths of the T-loop springs. If this dimension D is divided by two, the posterior leg and the anterior leg of the T-loop spring are equal and are each (D/2). The T loop will thus be located in the center of the dimension L. It is advantageous to locate the T 1.5 mm anterior to this center position by having the posterior leg 1.5 mm longer than the anterior leg. The anterior leg is bent vertically at (D/2 - 11/2) to enter from the occlusal aspect of the canine vertical tube. (This bend should be overdone around the round beak of a plier and subsequently returned to the 90 ° position.) A small gingival bend (approximately 5 °) is made to identify the posterior leg length (D/2 + 11/2). As an example, if the posterior leg of the T loop spring is placed into the molar auxiliary tube and the vertical portion of the anterior leg into the tube on the canine bracket, and the posterior leg pulled distally through the molar auxiliary tube to the 5 ° bend, the distance between the vertical legs of the T will be 8 mm, resulting in a mesiodistal force of approximately 300 gm. The springs should next be contoured for patient comfort and to ensure buccolingual passivity, thereby preventing indeterminate, extraneous forces from occurring.

The moments in both the posterior and anterior portions of the spring are obtained by placing 30 ° to 40 ° bends in each of the six positions as shown in Fig. 6, B. These bends should be made around the round beak of a plier and be initially overdone, then returning to the 30 ° to 40 ° activation, as TMA wire has some "memory." By sharing these bends occlusogingivally, the moments will have the least effect on the mesiodistal force. A fully activated maxillary spring is shown in Fig. 7. Note the posterior and anterior moments are approximately equal.

'T'- Loop Template (Scale 1:1)

A

P O S T E R I O R A N T E R I O R

3 4 ~ 1 2 ' '

- - + 1 ½ 2 2

L= +,4) + + ACTIVATION

B Fig. 6. A, T-loop spring template (scale 1:1). Note: Spring is universal. There are no rights or lefts or alterations in design for maxillary or mandibular arches. B, Locations of moment producing bends in the T-loop spring.

The B type extraction site closure will occur from this spring configuration. To ensure the moment bends are properly distributed within the spring, each leg should be held with a plier and after orienting them in the same relationship as they would be intraorally (posterior leg horizontal, and anterior leg vertical), the vertical portions of the T loop should touch. If they do not, the 30 ° to 40 ° bends should be rechecked. It is important that the vertical legs do touch, for if they do not, the mesiodistal activation of 8 mm will be affected.

The forces and moments of the spring for various activations are representative and listed in Table II. Note the moment-to-force ratio will change from approximately 8 initially, to approximately 13 at the end of 4 mm of extraction site closure. This indicates that the centers of rotation of the anterior and posterior groups of teeth are somewhat variable. Although this is not ideal, it is the best available at this time. This spring should not be reactivated until at least half the activation has been reduced by extraction site closure. If extraction site closure on one side of the arch precedes the contralateral side, the springs should not be reactivated until both sides have closed 4 ram.

By locating the T spring 1.5 mm anteriorly, reactivation


Fig. 7. Activated T-loop spring (anterior and posterior moments are equal for type B extraction site closure).

Table II. R e p r e s e n t a t i v e force system values of T M A , 0.017 × 0.025 cen t e r ed T- loop spring

ActivaEon Force Moment M/F M/F Ratio (MM) (gram) (GrMM) RaEo with lingual eNs~cs

0 0 1350 - - - - 1 50 1550 31 10 2 100 1750 17.5 9 4 150 2000 13 8 6 250 2300 9 - - 8 300 2400 8 - -

can easily be accomplished by pulling the distal leg posteriorly until the vertical legs are 8 mm apart once again. The spring can be reactivated without removal. As en masse closure occurs, the projection of the 0.017 × 0.025 anterior wire will slide through the premolar brackets bilaterally. These projections should be clipped periodically, as they will strike the molars and interfere with closure if this is not done. This sliding continuous arch ensures the dental contacts of the teeth on each side of the extraction site will be properly positioned in all planes of space. In addition, the established arch form is maintained throughout closure.

The reader will recall that type A closure is achieved by increasing the moment-to-force ratio applied to the posterior teeth. This was illustrated through the use of Class II elastics as related to the maxillary arch, and Class I I I elastics in the mandibular arch. This approach to type A closure was obtained by increasing the M/F ratio through decreasing F. An alternative means of increasing the moment-to-force ratio at the posterior teeth would be to increase the moment (Mp). This can be done by locating the T loop 3.5 mm anterior to the center position. This lengthens the distal leg, allowing for an addit ional 30 ° to 40 ° bend to be placed in the posterior leg approximately 2 mm distal to position 1 in Fig. 6, B. Similarly, in type C closure, the

T loop is located 3.5 mm distal to the center position. This allows a similar addit ional 30 ° to 40 ° bend to be placed in the anterior leg, increasing the moment-to- force ratio applied to the anterior teeth.

In cases requiring initial canine retraction, the closure springs are activated in the same manner with the excep- tion that the spring activation is 4 mm. This provides an approximate mesiodistal force of 150 gm. A lingual force of approximately 150 gm is provided by placing an appro- priate elastic from the auxiliary lingual fitting on the first molars to a lingual button or equivalent on the canines. (See Fig. 8.) The presence of a passive lingual arch is optional. If a patient does not wear the lingual elastics, the canines will rotate a very small amount labiolingually and seize on the wire track, stopping all motion. This is an important fail-safe design. Failure of patient cooperation will be evident at the following appointment as normal canine translation occurs at a rate approximating 1 mm per month.

FRICTIONAL CONSIDERATIONS

It is important to consider that bracket/wire relative motion occurs in two instances: between the 0.016 × 0.016 track and the canine bracket when canines are individually


Fig, 8. Occlusal view of initial canine distal translation.

F

!,

,,.' Fig. 9. Couple at wire/bracket interface caused by dental tipping.

translated, and secondly, between the 0.017 × 0.025 anterior wire and the auxiliary tube of the premolar bracket during en masse space closure. Frictional effects must be considered at these two locations.

Because teeth undergoing orthodontics move at relatively low velocities, the law of static friction applies, F = b~N, where F represents the frictional force resisting motion, IX the coefficient of friction at adjoining bracket/ wire interfaces, and N the perpendicular force that exists at the bracket/wire interface in all planes. 19 Because all teeth are essentially supported by springs (the periodontal ligament), when they occlude, engage food, are touched by the tongue, they each move a minute amount. This motion is essentially random in all planes of space and results in the normal force N, becoming zero in excess of 590 times each day. 2°-22 Consequently, the frictional forces

Fig. 10. Pretreatment cephalogram tracing of patient with Class II malocclusion.

approach zero as well, thus not resisting motion at the wire/bracket interfaces. This has been shown in experi- ments conducted by Liew 23 and is supported by anecdotal evidence that similar extraction site closure velocities occur whether a wire joining the posterior and anterior teeth is present or not. It should be noted, however, when relative motion is attempted at the wire/bracket interfaces at two or more adjacent bracketed teeth, frictional forces cannot become zero for this requires that random dental motions occur in synchrony--a virtual impossibility. It is important to emphasize that there must be relative linear motion (related to translation) at the bracket/wire interface. If this is not the case, as in dental tipping, a couple

652 Braun, Sjursen, and Legan American Journal of Orthodontics and Dent@cial Orthopedics December 1997

Fig. 11. Appliances in place for type A maxillary closure.

B

A

Fig. 12. A, Posttreatment cephalogram tracing of patient with Class II malocclusion. B, Maxillary superimposition of treated patient with Class II malocclusion.

O

would result at the interface (see Fig. 9), and no amount of relative random bracket/wire motion would result in the normal forces of the couple approaching zero simultaneously. It is therefore important that translatory motion occur for efficient tooth movement.

CLINICAL EXAMPLES OF EXTRACTION SITE CLOSURE

The cephalogram tracing shown in Fig. 10 is of a patient with Class II malocclusion. No additional significant skeletal growth was anticipated. The treatment plan required removal of the maxillary first premolars with the anterior segment retracted fully into the extraction site (A

Fig. 13. Pretreatment cephalogram tracing of patient with Class Ill malocclusion.

anchorage). Posterior anchorage was planned through extraoral appliance wear during site closure. Mandibular extractions were not planned. It became obvious early in treatment that patient compliance, as had been promised, would not be forthcoming. Consequently, maxillary type A closure was introduced, using a moment-to-force ratio approximating 13 posteriorly, with a moment-to-force ratio of approximately 10 anteriorly. Fig. 11 illustrates the appliances in place, and Fig. 12, A is a cephalogram tracing after extraction site closure. Fig. 12, B is a maxil-


Fig. 14. Intraoral view of patient with Class III malocclusion during extraction site closure.

lary superimposition of this patient, indicating that type A closure was achieved.

The cephalogram tracing in Fig. 13 is of a patient with Class III malocclusion. No additional skeletal growth was anticipated. Correction of the malocclusion was planned through dental movement alone. Four first premolars were removed with type B extraction site closure in the mandible and type C extraction site closure in the maxilla. Fig. 14 illustrates the appliances in place during closure of the extraction sites. Fig. 15, A is a tracing of a cephalogram after closure, and Fig. 15, B is the related maxillary and mandibular superimpositions. The planned closure was readily achieved based on the predictability of the force systems (M/F ratios).

SECOND PREMOLAR EXTRACTION SITES

When there is a significant anterior arch length deficiency, requiring that canines and first premolars be initially translated distally, they are first joined together on a 0.016 × 0.022 wire as seen in Fig. 16, and translated as one large tooth, bilaterally. The incisors are bypassed with a vertical offset. To prevent the wire from striking the incisors as the canines and first premolars translate distally, the vertical offsets mesial to the canines may be altered symmetrically and bilaterally. Note the arch form and contact relation- ships across the extraction sites are continuously maintained in all planes of space.

The appliance configuration after consolidation with the incisors is shown in Fig. 17. The T-spring closure loop is activated in the same fashion as described earlier. Relative motion occurs between the anterior and posterior teeth at the premolar bracket slot. Excessive wire protruding mesially is bent gingivally to avoid striking the canines as the extraction site closes. It is important to ligate the first and second molars together. Failure to do this would allow the first

o¢ B

Fig. 15. A, Posttreatment cephalogram tracing of patient with Class Ill malocclusion. B, Maxillary and mandibular superpositions of treated patient with Class III malocclusions.

molars to translate anteriorly, because the T spring is at- tached to these teeth. If an active lingual arch is used, the T spring is activated 8 mm. Alternatively, if lingual elastics are used, the T spring is activated 4 mm. Type A, B, or C closure is achieved as described earlier.

CONCLUSION

It is important for treatment results to be under the control of the clinician. This can only be achieved by the application of predictable, controlled force systems. A fail-safe and user-friendly system for doing this has been described that has the added advantage of ensuring the

654 Braun, Sjursen, and Legan American Journal of Orthodontics and Dent@cial Orthopedics December 1997

Fig. 16. Preliminary retraction of canine and first premolar (second premolar extraction case).

Fig. 17. En masse second premolar extraction site closure.

teeth will maintain alignment throughout extraction site closure. The patient need only be seen each 6 weeks because of the long ranges of activations. In the event of patient noncompliance, the system seizes and essentially stops without deleterious side effects. The clinician may easily discover noncompliance at the next appointment. After extraction site closure, minimal finishing procedures are required because the alignment of dental contacts and axial inclinations are maintained throughout treatment and extraction site closure (A, B, or C) has been controlled so that the targeted occlusion is attained at com- pletion of extraction site closure.

REFERENCES

1. Peck S, Peck H. Frequency of tooth extraction in orthodontic treatment. Am J Orthod 1979;76:491-6.

2. Shields TE, Little RM, Chapko MK. Stability and relapse of mandibular anterior alignment: a cephalometric appraisal of first premolar extraction cases treated by traditional edgewise orthodontics. Am J Orthod 1985;87:27-38.

3. deCastro N. Second premolar extraction in clinical practice. Am J Orthod 1974;65:115-37.

4. Burstone CJ. The segmented approach to space closure. Am J Orthod 1982;82: 361-78.

5. Smith RJ, Burstone CJ. Mechanics of tooth movement. Am J Orthod 1985;82:294- 307.

5. Tanne K, Sakuda M, Burstone CJ. Three dimensional finite element analysis for stress in the periodontal tissue by orthodontic forces. Am J Orthod Dentofae Orthop 1987;92:499-505.


7. Burstone CJ. The biophysics of bone remodeling during orthodontics--optimal force considerations In: Norton LA, Burstone CJ. The biology of tooth movement. Boca Raton: CRC Press, 1989.

8. Braun S, Winzler J, Johnson BE. An analysis of orthodontic force systems applied to the dentition with diminished alveolar support. Eur J Orthod 1993; 15:73-7.

9. Wehrbein H, Fuhrmann RAW, Diedrich PR. Human histologic tissue response after long-term orthodontic tooth movement. Am J Orthod Dentofac Orthop 1995;107:360-71.

10. Wehrbein H, Furhmann RAW, Diedrich PR. Periodontal conditions after facial root tipping and palatal root torque of incisors. Am J Orthod Dentofac Orthop 1994;106:455-62.

11. Reitan I,:2 Biomechanical principles and reactions. In: Graber TM, Swain BF, editors. Orthodontics, current principles and techniques. St Louis: CV Mosby, 1985.

12. Reita~ K. Some factors determining the evaluation of forces in orthodontics. Am J Orthod 1957;43:32-45.

13. Marcotte MR. Biomechanics in orthodontics. Philadelphia: BC Decker, 1990. 14. Burstone CJ. Rationale of the segmented arch. Am J Orthod 1962;48:805-22.

15. Bursmne CJ, van Stienbergen E, Hanley KI. Modern edgewise mechanics and the segmented arch technique. Glendora (CA): Ormco Corp., 1995.

16. Baty D, Storie D, von Fraunhofer J. Synthetic elastomeric chains: a literature review. Am J Orthod Dentofac Orthop 1994;105:536-42.

17. Burstone CJ. Deep overbite correction by intrusion. Am J Orthod 1977;72:1-22. 18. Burstone CJ, Goldberg AJ. Beta titanium wire: a new orthodontic alloy. Am J

Orthod 1980;77:121-32. i9. Genzer I, Youngner P. Physics. Morristown (NJ): General Learning Corp., 1973. 20. Okeson JP. Management of temporomandibular disorders and occlusion. 2rid ed.

St Louis: CV Mosby, 1989. 21. Flanagan JB, Clement SC, Moorees CFA. The 24 hour pattern of swallowing in

man. J Dent Res 1963;42:abstract 165, 1072. 22. Schneyer LH, Pigmon W, Hanahan L, Gilmore RW. Rate of flow of human parotid

sublingual and submaxinary secretions during sleep. J Dent Res 1956;35:109. 23. Liew CF. Reduction of sliding friction between an orthodontic bracket and

archwire by repeated vertical disturbances. Unpublished dissertation. Am J Orthod Dentofac Orthop 1995;106:669.

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entire extraction site

posterior teeth

anterior teeth

extraction space type

contralateral site

type c closure

extrac tion site

posterior groups of