The use of miniscrews for reinforcement oforthodontic anchorage has become increas-

ingly popular in recent years, especially in adultpatients who do not want to wear extraoral appli-ances. Miniscrews are convenient, save time, andproduce good treatment results with no need forpatient cooperation. In some patients treated withminiscrews, however, mechanical factors can pro-duce unusual changes or side effects. The presentarticle explains these mechanical variations and pro-vides tips for solving the problems they create inthe sagittal plane. Subsequent articles will coverside effects in the horizontal and transverse planesand present relevant clinical cases.

Anchorage reinforcement is most common-ly needed in patients with severe protrusion. In con-ventional retraction with sliding mechanics afterfirst premolar extractions, the molars typicallymove forward 3.6-3.8mm.1-3 Anchorage reinforce -ment can allow more retraction of the incisorswhile reducing forward movement of the molars.

Anterior retraction with sliding mechanics isusually accomplished by placing elastomeric chainor nickel titanium springs between hooks on theanterior teeth and the second molars. The anteriorand posterior segments rotate around the center ofrotation, which causes bowing of the archwire. Aprecurved archwire can be used to prevent thisside effect (Fig. 1).

VOLUME XLII NUMBER 2 © 2008 JCO, Inc. 79

Biomechanical Considerations inTreatment with Miniscrew AnchoragePart 1 The Sagittal PlaneMIN-HO JUNG, DDS, MSD, PHDTAE-WOO KIM, DDS, MSD, PHD

Dr. Jung is a Clinical Professor, Department of Orthodontics, Collegeof Dentistry, Seoul National University, and Department of Ortho -dontics, Samsung Medical Dental Center, Seoul, and in the privatepractice of orthodontics at SNU Orthodontic Clinic, 3rd Fl., Tae-namB/D, 72-3 Cham-won Dong, Seo-cho Gu, Seoul, South Korea; e-mail:fortit@chol.com. Dr. Kim is Professor and Chairman, Department ofOrthodontics, Dental Research Institute and College of Dentistry,Seoul National University.

Dr. Jung Dr. Kim

Fig. 2 Retraction force with miniscrew anchorageproduces rotation of entire arch around center ofrotation (blue dot).

Fig. 1 Usual changes during anterior teeth retrac-tion with sliding mechanics. Anterior and posteriorsegment rotates around center of rotation of eachsegment (red dots); archwire is forced to bendnear center of rotation of entire arch (blue dot).These changes can easily be prevented with pre-curved archwires (bottom).

©2008 JCO, Inc. May not be distributed without permission. www.jco-online.com

The use of miniscrews for anchorage rein-forcement produces somewhat different mechan-ics. Because the force used during retraction is notreciprocal, either the entire arch4,5 (Fig. 2) or theanterior segment (Fig. 3) will rotate around the cen-ter of rotation. In cases of severe protrusion, whereabsolute anchorage is required in both arches,these mechanics can produce posterior open biteand deep overbite (Fig. 4). The use of precurvedarchwires will result in an even stronger intrusiveforce on the posterior segment. Following are sev-eral possible solutions to these problems.

Redirecting the Retraction Force

One approach is to lengthen the archwire

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Biomechanical Considerations in Treatment with Miniscrew Anchorage

Fig. 3 Rotation of anterior segment around centerof rotation (red dot) in absence of friction in poste-rior segment.

Fig. 5 Long archwire hook and high miniscrewplacement produce retraction force that passesdirectly through center of rotation.

Fig. 6 Higher line of force promotes irritation ofsoft tissue by elastomeric chain or coil springs be -cause of archwire curvature (red circle).

Fig. 4 A. Simultaneous rotation of both archesduring anterior retraction with miniscrew anchor-age. B. Resulting posterior open bite. C. Posterioropen bite after space closure with miniscrewanchorage.

A

B

C

hook and raise the miniscrew insertion point to re -direct the vector of retraction force, so that it pass-es through the center of rotation of the anteriorsegment. This is located between the lateral incisorand canine roots, 6.76mm above the cervical area,6or at the level of the root tip7 (Fig. 5). Melsen andcolleagues recommended that the archwire hookextend 10mm from the main archwire,8 but anatom-ical limitations usually make this impractical.

Elastomeric chain or coil springs positionedabove the bracket level may impinge on the soft tis-sue because of archwire curvature (Fig. 6). Fur -thermore, in many cases, it is difficult to place theminiscrew high enough (Fig. 7). Insertion in themobile mucosa increases the risk of inflamma-tion around the miniscrew and may lead to failure.9Therefore, it is almost impossible to use a retrac-tion force that passes through the center of rotation,and other methods should be considered.

Posterior Intermaxillary Elastics

Placing intermaxillary elastics between theposterior teeth can be a solution. Light 3/16" inter-maxillary elastics, worn only at night, can preventposterior open bite (Fig. 8). Because such elasticscan extrude the posterior teeth, however, they arenot recommended for patients with vertical skele-tal patterns.

Vertical Retraction Forces

In patients with gummy smiles or other fac-tors favoring intrusion of an entire arch, more ver-tical retraction forces can be used to preventocclusal plane rotation (Fig. 9). Occlusally direct-ed archwire hooks should be placed posterior to thecanines. This method can also be used to controloverbite during retraction in cases of deep overbite.

Anterior Biteplanes

Occlusal plane rotation due to forces of occlu-sion can be prevented by bonding anterior bite -planes to the lingual surfaces of the anterior teethat the beginning of retraction (Fig. 10). Biteplanesare also helpful in preventing attrition from ceram-

Fig. 8 Force produced by posterior intermaxillaryelastics (red arrows) prevents posterior open bite.

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Fig. 9 Apically positioned miniscrews and occlus -ally oriented hooks produce more vertical retrac-tion force (blue arrow) for intrusion of entire arch.

Fig. 7 Limited width of gingival tissue precludeshigh placement of miniscrew. White circle indi-cates optimal miniscrew location for translationalmovement.

Jung and Kim

Fig. 11 A. Biteplanes built up directly with com-posite resin.* B. Prefabricated acrylic biteplanesbonded to central incisors.

Fig. 12 Silicone putty trays used to make bite planesfor patients with different amounts of overjet.

Fig. 13 Additional anterior miniscrews create ver-tical force to counteract occlusal plane rotationand maintain torque.

*Transbond Plus, trademark of 3M Unitek, 2724 S. Peck Road,Monrovia, CA 91016; www.3MUnitek.com.

Biomechanical Considerations in Treatment with Miniscrew Anchorage

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A

B

Fig. 14 Anterior miniscrews positioned betweenroots of lateral incisors and canines.

Fig. 15 Protraction force moves entire arch anteri-orly, producing arch rotation around center of rota-tion (blue dot).

Fig. 10 Anterior biteplane (red triangle) preventsocclusal plane rotation due to forces of occlusion(red arrow).

ic bracket wear. They can be built up directly onthe teeth with composite resin,* or prefabricatedacrylic biteplanes can be used (Fig. 11). The pre-fabricated biteplanes can be easily fabricated fromsilicone putty trays, saving chairtime (Fig. 12).

To minimize extrusive effects and related pa -tient discomfort, anterior biteplanes should be placedat the level of the pretreatment overbite. If they areused after the development of posterior open bite orplaced at a level that can produce posterior open bitein a vertical skeletal pattern, clockwise mandibularrotation will occur. Longer bite planes can be usedin cases of significant overjet.

Additional Anterior Miniscrews

In a patient with a gummy smile or over -erupted upper incisors, additional miniscrews canbe placed in the upper anterior region to producea vector of force that counteracts occlusal planerotation and preserves anterior torque (Fig. 13).These miniscrews can be placed between either thecentral and lateral incisors or the lateral incisors andcanines (Fig. 14). Some orthodontists prefer thearea between the central incisors, but to avoid thefrenum, the miniscrew must be placed submu -cosally with wire extensions.

If the molars are being protracted, the occlusalplane will rotate in the opposite direction, andanterior open bite can occur (Fig. 15).

REFERENCES

1. Kim, T.K.; Kim, J.T.; Mah, J.; Yang, W.S.; and Baek, S.H.: Firstor second premolar extraction effects on facial vertical dimen-sion, Angle Orthod. 75:177-182, 2005.

2. Ong, H.B. and Woods, M.G.: An occlusal and cephalometricanalysis of maxillary first and second premolar extractioneffects, Angle Orthod. 71:90-102, 2001.

3. Kocadereli, I.: The effect of first premolar extraction on verticaldimension, Am. J. Orthod. 116:41-45, 1999.

4. Nanda, R.: Biomechanical and clinical considerations of a mod-ified protraction headgear, Am. J. Orthod. 78:125-139, 1980.

5. Tanne, K.; Miyasaka, J.; Yamagata, Y.; Sachdeva, R.; Tsutsumi,S.; and Sakuda, M.: Three-dimensional model of the humancraniofacial skeleton: Method and preliminary results usingfinite element analysis, J. Biomed. Eng. 10:246-252, 1988.

6. Lee, H.K. and Chung, K.R.: The vertical location of the centerof resistance for maxillary six anterior teeth during retractionusing three dimensional finite element analysis, Kor. J. Orthod.31:425-438, 2001.

7. Vanden Bulcke, M.M.; Dermaut, L.R.; Sachdeva, R.C.; and Bur -stone, C.J.: The center of resistance of anterior teeth duringintrusion using the laser reflection technique and holographicinterferometry, Am. J. Orthod. 90:211-220, 1986.

8. Melsen, B.; Fotis, V.; and Burstone, C.J.: Vertical force consid-erations in differential space closure, J. Clin. Orthod. 24:678-683, 1990.

9. Park, H.S.; Jeong, S.H.; and Kwon, O.W.: Factors affecting theclinical success of screw implants used as orthodontic anchor-age, Am. J. Orthod. 130:18-25, 2006.

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