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INTRODUCTION

Cantilever springs are simple & efficient orthodonticappliances whose one end is inserted into a bracket

or a tube, or in the acrylic of a removable orthodonticappliance, while the other one is tied to another unitwith only a one point contact. Because a cantilever is asimple two-tooth appliance, its force system is staticallydeterminate, meaning that the forces and moments thatthe wire will apply to the teeth are easy to discernclinically.1 A moment and a force are created at the toothin which the wire is fully engaged, whereas only a singleforce (having equal magnitude but opposite direction)is developed at the other end of the cantilever—thesingle-point contact.2 The moment results from the couplecreated by the deflection of the wire in the edgewisebracket or tube. The value of the moment is equal to thelength of the cantilever multiplied by the force: M = F x d.

The forces and the moments can be readily measured,thus, no unknown forces are acting on the teeth. Adynamometer can be used to measure the force generatedas the wire is displaced to the point attachment. Byworking with measurable forces and moments, use ofthis type of appliance permits greater control by theorthodontist and improves the predictability ofmovement. This minimizes the potential for unexpectedtooth movement during orthodontic treatment.Cantilever Length and Load Deflection Ratio

Appliances producing the most well-defined anddramatic tooth movements are often those that are the

most simple biomechanically. Cantilevers should be aslong as possible, if their aim is to produce only a moment,while the force is less desirable. If, on the contrary, theeffect of the force is desirable and the moment lesswanted, the cantilever should be kept short and its crosssection dimension reduced, in order to keep the load/deflection rate low.

The load/deflection ratio delivered by a cantilevershould be as low as possible, leaving the force systemwith a high degree of constancy. This is another reasonfor generally keeping the cantilevers long.3,4 Cantileversprings can be fabricated from almost any orthodonticwire. Stainless steel and beta titanium wires are popularchoices because of their formability. Because of therelatively high stiffness of stainless steel (SS), helicesaid in reducing the force levels of SS springs while alsoincreasing the spring’s range of activation. In those caseswhere the cantilevers, for different reasons are short, theload/deflection rate can be lowered by the addition ofone or more loops, or by using a wire of smallerdimension. The latter does, however, also cause aproblem, as the play between wire and bracket (tube)may become unacceptable. A third and better solution isto choose a wire alloy with a lower stiffness. For thisreason and for its high formability, cantilevers areusually made out of (0.017x0.025) beta titanium. Nickel-titanium wires have also been used in cantileverapplications; however, these springs must beprefabricated by the manufacturer.5 Rectangular wiresare generally preferred for making cantilever springsbecause they resist rolling within the bracket or tube,thus, ensuring accurate control of the direction of forceapplication.

The relatively long interbracket span between points

*Professor and Head, Dept. of Orthodontics, Peoples DentalAcademy, Bhopal. **Professor, Dept. of Orthodontics, PeoplesCollege of Dental Sciences, Bhopal.

Application of Cantilever Mechanics in ContemporaryOrthodontics : Correction of Dental MidlineAsymmetry - A Case ReportAmitabh Kallury*, Upendra Jain**

AbstractCantilever springs are simple & efficient orthodontic appliances with several distinct clinical advantagesover continuous archwires. Knowing the force level and the distance between the two attachment sites, theorthodontist can quickly calculate all the forces and moments involved—not only the active forces, but thereactive forces as well. The relatively long interbracket span between points of attachment produces a lowload/deflection rate, which allows the delivery of well-defined, relatively constant forces and moments.Reactivations are needed less frequently. The cantilevers may be utilized in all the planes of space, bothbuccally and lingually to correct the malocclusion.Key Words : Cantilever mechanics, Orthodontics appliance, Load/deflection rate, Midline correction.

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of attachment produces a low load/deflection rate, whichallows the delivery of well-defined, relatively constantforces and moments. This, in turn, means thatreactivations are needed less frequently. A cantileverforce system can be directed exclusively toward thetreatment goal. Potential side effects can be identifiedand either minimized or negated by use of transpalatal

arch and lingual arch.6,7

Clinical Application of the Cantilever Principle

The basic force system produced by a simplecantilever spring is an ideal approach to deal withseveral orthodontic problems which are difficult to treaton continuous archwires. Some of the situations wherea cantilever can be used are :1. Correction of Dental Midline Asymmetry.2. Correction of Incisal Cant.3. Management of Impacted Teeth.4. Management of Vertical Problems like anterior

openbite and deep overbite5. Torquing of Anterior teeth with torquing springs.Midline Springs

Midline discrepancies are often symptoms of moreserious skeletal or dental asymmetries. Midline springsshould not on their own be expected to resolve suchcomplex asymmetries. In the context of a comprehensive

Fig. 6b : Intraoral frontal view showingcoinciding dental midlines.

Fig. 1 : Force system from cantilever springs.

Fig. 1a : Diagrammaticrepresentation of a cantileverspring with the posterior endinserted into a tube/bracket on

the molar.

Fig. 1b : Forces andmoments acting on the

teeth when the cantilever istied as a point contactgingival to the incisor

bracket.

Fig. 2 : Occlusal view of an anterior midline spring designed to shiftthe upper midline to the patient's right.

Fig. 2a : The passivesprint (blue) extends fromthe molar auxiliary tubeand has hook anteriorly

Fig. 2b : The spring is activated byattaching the hook to the base arch

wire (red). This creates a facial force onthe anterior, a lingual force on the

molar and moment on the molar torotate it mesiolingually. A transpalatalarch (green) is attached to counteract

the undesirable effect on the molar.

Fig. 3 : Pre treatment photographs.

Fig. 3a : Right buccal view showingclass I occlusion posteriorly.

Fig. 3b : Frontal view showing deviateddental midline.

Fig. 3c : Left buccal view showingclass I occlusion posteriorly.

Fig. 3d : Extra oralfrontal view showingmandibular deviationto the patient's left.

Fig. 4 : Cantilever spring to correct midline.

Fig. 4a : Right buccal viewwith lower cantilever spring

Fig. 4b : Frontal viewshowing improvement in

dental midline

Fig. 4c : Lect buccal viewwith upper cantilever

spring Fig. 5 : Anchorage reinforcement

Fig. 5a :Transpalatalarch

Fig. 5b : Lingual arch

Fig. 6 : Post treatment photographs.

Fig. 6a : Extraoralfrontal view showingmatching upper and

lower dental midlines.

402 JIDA, Vol. 5, No. 3, March 2011

treatment plan, however, they can often serve as a usefuladjunct to, or substitute for, other methods of midlinecorrection. These may include anterior interarch elasticsor skewed arch wires that are often associated withundesirable side effects that are difficult to control. Aprerequisite for aligning midlines in any orthodonticpatient is symmetric posterior segments to allow forattainment of Class I canine relationships bilaterally.

A spring can be designed to help shift anterior teethlaterally to the left or right when midline corrections arenecessary (Fig. 4). A wire extending from the appropriatemolar auxiliary tube is bent to move the midline to theright or left. A small hook bent into the spring allows itto be crimped over an existing arch wire or segment, or itmay be tied to the arch wire or individual teeth.Passively, the spring is lateral to the incisors in thedirection in which incisor movement is desired (Fig. 2 &Fig. 4). As the wire is activated, a second-order couple isdeveloped in the molar to rotate it mesiolingually (Fig.2B). The forces will be directed laterally at the incisorsand lingually at the molar. It is often desirable to have apassive transpalatal arch and lingual arch fabricatedfrom 0.036"SS (Fig. 5) in place to minimize any unwantedmolar movements.

CASE REPORT

A 17 year old male patient (Fig. 3) reported to the clinicwith the chief complaint of unaesthetic smile. The patienthad previously undergone treatment with fixedorthodontic appliance after extraction of all the firstpremolars to correct dental proclination. His medicalhistory was not significant. Clinical examinationrevealed mandibular deviation to the left with lowerdental midline shifted to the left and upper midlinedeviated to the right. Patient had an asymmetric overjet(more on the right). Patient refused to undergo anysurgical procedure to correct the skeletal deformity. Asthe patient had good buccal occlusion and asymmetricoverjet, it was decided to correct the dental midline usingcontralateral cantilever mechanics. .022" slot MBTappliance was placed and anchorage was re-enforcedwith a passive 0.036" SS transpalatal arch and lingualarch (Fig. 5). Initial alignment was done on 0.016" NiTi.After progressing to 0.018" P+ SS archwire, all the teethwere consolidated with 0.009" ligature and cantileversprings were fabricated in 0.017x0.025 TMA to move themaxillary arch to the left and the mandibular arch to theright to coordinate the midline (Fig. 4). Adequate anterioroverjet on the patient’s right side was already presentbefore the midline was corrected. The objectives ofcorrecting the midline deviation and asymmetric overjetwere achieved and the appliance was removed after16months of active treatment (Fig. 6).

DISCUSSION

Midline correction with a cantilever spring providesa method of incisor movement with minimal side

effects.8,9 This technique may be used when either atipping movement or translation is the requiredmovement. When the incisors are tipped (Fig. 3B),frequently associated with ectopic eruptions or thepremature loss of primary teeth, a simple force is neededto upright the incisors and establish midline coincidence.In contrast, the use of an arch wire for alignment mayresult in future problems that would require additionalcorrection procedures. First, indiscriminant leveling maycreate a cant to the anterior occlusal plane. Also, thisapproach requires sliding brackets (teeth) along the archwire. The force of friction acts in the opposite directionof the intended movement, resisting movement of thedental midline in the desired direction. The teeth mayupright without obtaining the midline correction. Thecantilever provides an alternative mechanism thatavoids these potential difficulties. The point force maybe applied to the anterior teeth before placing an aligningwire. The cantilever can provide a pulling force to shiftthe midline. Each incisor needs to be independentlytipped to correct the midline. By tying the bracketstogether in a “Figure 8” and attaching the cantilever atthe level of the brackets, this simple tipping movementeasily corrects the midline discrepancy (Fig. 4).

CONCLUSION

Cantilever springs generate a predictable force systemthat is applicable to a wide variety of orthodonticproblems. Especially in situations in which a point forceis required, cantilever springs offer a simple option thatis easily tailored to an individual patient’s needs. Withtheir simple design, these springs may be used to providea creative solution in many orthodontic situations, whichwould otherwise have been difficult to treat on acontinuous archwire.

REFERENCES

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2. Burstone CJ, Koenig HA. Force systems from an ideal arch.American Journal of Orthodontics 1974; 65 : 270-89.

3. Burstone CJ, Baldwin JJ, Lawless DT. The application ofcontinuous forces to orthodontics. Angle Orthodontist 1961;31 : 1-14.

4. Burstone CJ. Rationale of the segmented arch. AmericanJournal of Orthodontics 1962; 48 : 805-22.

5. Nanda R, Marzban R, Kuhlberg A. The Connecticut IntrusionArch. Journal of Clinical Orthodontics 1998; 32 : 708-15.

6. Burstone CJ, Manhartsberger C. Precision lingual arches:Passive applications. Journal of Clinical Orthodontics 1988;22: 444-51.

7. Burstone C, van Steenbergen E, Hanley K. Modern EdgewiseMechanics and the Segmented Arch Technique. Glendora,CA: Ormco Corp, 1995.

8. Nanda R, Margolis MJ. Treatment strategies for midlinediscrepancies. Seminars in Orthodontics 1996; 2 : 84-89.

9. van Steenbergen E, Nanda R. Biomechanics of orthodonticcorrection of dental asymmetries. American Journal ofOrthodontics and Dentofacial Orthopedics 1995; 107 : 618-24.