/ L ~ Sivakumar et al

REVI EW Intra-arch Retraction Mechanics-A contemporary review. Arunachalam Sivakumar. MDS Assistant Professor

Ashima Valialhan. BDSIPb). DDS. MS tUSAI Professor and Head. Director of Postgradu;)te ludles. Department of Orthodontics and Denfofacial Orthopaedics, ManiJ)al College of DenIal Scienc.es, Manipal.

Abstract Extraction space closure involves carefu lly designed treatment strategy as to lose or not to lose the anchorage. Whether anterior retradion or poSlerior protraction. or a combination of both is used. the same basic principles of retraction mechanics apply. Retraction mechanics can be divided into two categories. fridion and frictionless. On

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A B

Figure 1: M o m ents of fo rce p laced at bracket leve l and not at center of resistance. A. C rown rotatio n . B. Crown tipping.

shou ld be. If the fo rce is too excessive, the posterior segrn e nt ca n inadve n e ntl y move anteriorly. The o ccurrence of bodi ly too th m ovem ent w ith sliding rnechani cs can be exp la ined as fo llows. The loolh experiences a moment of force in two planes of space. O ne moment ro tates the tooth m esia l-out , and the other causes di sta l tipping of the c row n (Figure 1 ).The m esia l o ut mome nt is a n undesirable side e ffect, b ut the dista l c rown moment contributes to the retractio n . Eventua lly, thi s di stal tipping causes binding of the arc hw ire, w hic h produces a m om ent of a couple Ihal results in di sta l root lo rque (Figure 2). As the tooth uprights, the momen t dec reases until no lo nger thewire binds. Then, the c rown slides a lo ng the a rc hw ire until the dista l c rown tipping again cau ses binding. The process is repeated until the

Figu re 2: This co uple produced by archw ire binding results in distal root torque

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J Incl Orlhod Soc 2006; 39:101 109

tooth is retrac ted o r the e lastic force is dissipated . A major adva ntage o f f r i ct io n m e c h a ni cs i s th at complicated w ire configurations are usually no t n eeded, m aking initi a l wi re p lacem ent less time co nsuming. A lso this can enhance patient comfo rt .} In fri c t io nl ess m ech anics, teeth are m oved w itho ut the brac ke l s sliding a lo n g the arch w i re. Retrac tio n is accompl ished with loops or springs, w hich offer more controlled tooth movem ent than sliding m ec h ani c s. Genera lly closing loop m echani cs is m o re complex due to the construc tion of the closing loop springs and the ir clini ca l m anagemen t. Whe n compared to slid ing m echa ni cs, closing loop m ech anics h ave the fo llowing advantages; ab sen ce of fri c ti o n b etween the bracket and the w ire, the force le ve ls a re easier to eva luate clinic a ll y and the M/F ratio of the c u spid and the posterior segm ent is predictable and contro llable during retractio n . A ccording to Bursto ne", a spring w ith closing loop s for retracting c u spids can b e described b y three princ ipal ch a rac te ri sti cs.

1. M o m ent to Force ratio (M/ F) app lied at the bracket w hose va lue determines the positio n o f the c ente r of ro tat ion during the orthodont ic movem ent;

2. Load/deflection rate (FlO) of the sp r ing (the level of force cau sed by the spring per activa tion).

3. Max imum strength (F max) that the spring is ab le to re lease w ithout perman ent deformation .

Fri ction M echanics

In sliding m ech ani cs, there is n o n eed to appl y a ba l anc ing m o m e nt for b o d il y m ovem e n t. T h e appro pri a te m o m ent is app li ed to the teeth via a continuous a rch w ire that passes through the brackets. Th e moments are delivered v ia couples, equa l and opposi te no ncollinear vertica l forces, at th e m esia l cmd di sta l bracket extremities. The c rown o f the canine w ill tip di stally until the d iagona lly opposite edges of the bracket slo t contac t and bind w ith the arch w ire. The wire then produces a couple to upright the root {Figure 3).The m agnitude of the m om ent is determined by the w idth of the bracket as well as characteri sti cs of the w ire su ch as a lloy, size and shape. The M/F changes as th e tooth moves, and the tooth responds, typi ca lly progreSSing from co ntro lled tipping to translatio n to root movement. Wire bracket fri c tion is a variab le factor as the m ovi ng teeth disp lace along the archw ire with this approach, m ak ing it difficult to accurate ly predict M/F.2

) L l-'S

FiGure 3 : Setlucnce of canine movement during retraction with sliding mec hilnics.

v - Bend Sliding M ech an ics

Originally developed by Thomas F Mulligan \ Ihis a pproach is particularly efficacious for closi ng space by moving individual teeth (i.e ca nine retraction or molar protraction). Mulligan introdu ced the concept of " Differential torque" as a means of effective Intraoral A nchorage."; Differential anchorage is obtained by the application of unequal alpha and beta moments. The higher moment is applied to the anc horage teeth. The differential moments are obtained by applying the concept of the off-center V-bend. An off center V-bend in a wire results in unequal m oments. The closer the v-bend to a tooth or set of teeth, the higher the applied moment. A si mplistic model (or envisioni ng this force system is to consider the length of wire from the posit ion of the V-bend apex to the brackets (Figure 4 ).The closer the V-bend apex is to a bracket, the shorter the wire; the further the distance of the V-apex to the bracket, the longer the w ire. A shorter w ire has a higher bending m oment than a longer wire. Therefore, a higher moment act o n th bracket closer to the V bend tha n the m ore distant brdcket. Depending o n the angle at which the w ire with an 0(( center bend crosses the bracket, and the length of the longer segm ent, the sma l ler moment can be clock wise, counter clockwise or non existent.

Sivakuma r el al

Figure 4 : Depending on the angle at which the wire with (In off center bend c rosses the bracket and the length of the longer segment. the smaller moment can be c lockwise. counterclockwise or non ex istent.

Can ine Ret raction via V -bend M ech anics

I( anc horage is required, then the V-benl is placed off center. The tooth located closest to the bend indicates the anchor side. The opposite is the non-anchor side. The an chor side requires a bodily type m ovement (or displacement, w hereas non anchor sid e tips somewhal due to the lesser moment (Figure 5). As the cuspids conti nue to move distally, the bend automaticall y "approach es" the center of the wire. until finally when the extraction sites are closed, the bend is centered. So, as the off center bend moves toward the center during space closure. the differentia l torque begins 10 gradually disappear, and becomes equal and opposite torq u e when th e bend is f inally centered. Root Parallel ism begins to effeo as the bend approaches center."

Figure 5: SchemOit ic representatio n or canine retractio n ViOi v -bend mechanics.

10)

/L ~ Friction Issue in Sliding M echanics

W h en two surfaces a re pressed togeth e r w ith a perpendic ul ar force, ano ther fo rce acting paral le l w ith these surfaces is required to cause o ne of the surfaces to move aga inst the res istance of the other. Th is is caused by the phenomeno n of frielian. Frictional fo rce is that force required to m ove a body aga inst th e resista nce caused by fri c tion. The amount of fri ctio nal force required depends o n the smoothness of the two surfaces a nd on the force that presses them together. Frict ion can occur because the archwire - bracket -ligature combination in some way produces a resistance 10 sliding from classical fri c tion, e last ic binding and nOlching. 7

For each archwi re - bracket combinat ion, a critica l contact angle of 2nd order angu latio n (ee) exists, at w hi ch classica l sliding fri ction g ives way to b inding. 'e' is 2"'1 order angul ation of an arch w ire re lative to a bracket. et: is c ritica l contact ang le or the second order ~ ngulation after w hich e lasti c binding (B I) occurs. ReSistance to slidi ng is pa rt iti o ned into classical friction (FR), e lasti c binding (BI) and phys ica l notching (NO), Both F~ ar:d BI are defined in te rms of norma l forces, N and kinetiC coefficients mk. 9z is second o rder angulation at w hich elasti c binding ends and phys ical notching (NO) begins. C lassica l fr ic ti o n occ urs b eca u se of the ligaLio n or normal force (N ) that eithe r presses the wi re into the slo t base o r slot F~a l1. In the p assive confi gu ration, FR is smaller in m agnitude but controls sliding becau se binding and notching do not exist at this juncture. In

9c 9z I',ls" ivp. con figUf

goa ls more readily than m ethods i n which fric ti o n plays a role.!

En g ineering Princi ples in Loop d es ig n

The performance of a closing loop is determined b y 3 m ajor characteristics: I . Spring properties - the amount of force it de l ivers

and the w ay the force changes as the teeth m ove. 2. The m o ment it generates - Contro l of root position

loca tio n of loop - Vbend princ iple. 3. Additiona l design prInciples.

Spri ng p ro p e rties

These are determined al m ost to ta lly by the wi re m ateria l (stai nless steel or TMA). the size of the wire and the distance between the points of allachmenl. Wires of greater inherent spr ing iness o r sma lle r cross sect ional area allow the use of simpler loop designs.1l'

Root pa ralleli n g m o m en ts

A closing loop m ust generate not only a clos ing force but also appropriate moments to bring the root apices together at the extraction si tes. When a closing loop is activa ted, its ho rizontal legs attempt to rise at an angle to the plane of the arch w ire (F igure 7). The horizonta l legs are constrained by brackets and therefore deliver a m o m ent to those bracke ts. The m oment produced by a closing loop dur ing ac tivat ion is term ed act ivation or Inherent M oment. The aClivat io n m o m ent is dependent on the ch ange in angle that the horizontal a rms of the loop m ake w ith the bracket ... vhen a loop is pu lled apart. The ratio of these momeniS to the activation iorce is termed inherent M/F, a con stant fo r any g iven loo p geom etry. Inherent M / F increases as loop h eigh t increases. Because of int raora l anatomic limitations. loops cannot be m ade w ith enough he ight to achieve inherent M/F to translate individua l Leeth or groups of teeth. To achieve a hi gher I\lVF ral io. an angulation or

Figu re 7: Activ.ated vertical clOSIng loop s.

Sivakumar ~ al

.. 1 gabl e type b end mu st be put i n th e loop. The add itional moment prod uced by gabling in a Ip is term ed res idual moment. To achieve net translation, residua l m o m ents in the form of gable bends or anterior lingual root torque and posterior gable bend must be added. Ad ding these res idu a l momen ts h as several disadvantages:

,. The teeth m ust cycle through controlled l ipping to translat ion to rool m ovem ent to achi eve net translation .

2. Whenever residu al mom ents are added. the loop's neutra l position (zero activation position) becomes ill defi ned . m aking it difficul t to achieve proper acti va ti o n s.

3. The resulting eve rcha n g ing periodontal slress distr ibutio ns may no t yie ld the most rapid, least traumatic m ethod of space closure.

Two principles to remember in obtain ing a constant rvv F ralio are ( 1) use as high an activation moment and as Iowa res idua l moment as poss ible. and (2) lower the fo rce - defl ection and moment - deflection rates."

If a closing loop design capable of ach ieving inherent. constant M / F of 8 to 9 mm w ithout residual moments were available. en m asse space closure with uniform p eriodonta l stress distributions could be achi eved. I! "

Th e Gable bend an d N eutra l Posi tio n

It has been shown that the M/F ratios resulting from activating various loop conrigurations are insu fficient to prevent uncontrolled tipping un less Gable bends a re included.'" When Gable b ends a re p laced in the occlusal port ion of a vert ical loop configuratio n, an unintended m esiodista l fo rce i s introduced (F igure B).This force will a lte r the d esired m esiod istal force or iginally intended because of the cross over of the verti cal legs. T hi s cross o ver iore shortens the horizontal w ire length between the brackets. Burslone et al have

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Figure 8 : Vertica l loop containing Gable bends in position .. wi th subsequent vertical leg crossing over.

105

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/L shown that the neutral position of the loop configuratio n has been altered by the introduction of Gable be nds. 14 The Neutral Position can b e defined as the ho ri zonta l separation of the vertical legs of the spring before the int rodu ction of a hor izonta l or m esiodista l force. It has been show n that appropriat e ma g nitud es a nd occlusoging iva l locat io ns of the Gable bends are vital to maintain the neutral posi tion of the closing loop. Otherwise the clinician h as no m eaningful reference point from which to judge the spring's activation to obta in the force aspect in the M/F ralio (F igure 9). 15 Location of the loop

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..i -

I c/ ~o I i I I '----" -+-z.-0 ! ~ !

I i",(~--- I ---~"i

Figure 9: Occlusogingival positions or Gable bends.

Beca use of its gable bends, the closing loop fun ctions as a V-bend in the arch wire. With greater eccentric ity at one of the brackets, a larger M /F could be produced. Prac tically, thi s means that during canine retract ion, with the verti ca l loop placed closer to the ca nine, a higher ratio would be present on the canine wh ich cou ld better control the apex. The undesirab le effect is extrusion of the ca nine. 14

If o nly anterior retract io n is necessary, the retraction loop shou ld be placed closer to the ca nine th an to the molar, and a gable bend should be added nea r the m o lar. A gable bend that is larger in the posterior dimension w ill produce a larger beta moment, thus, inc reasing the posterior anchorage. For both re traction of the anterior segme nt and protra c tion of the po ste rior segment, the loop should b e placed mid way between the posterior and anterior segments. A gable bend of equal dimensions should be u sed, so that the a lpha and

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J Ind Orthod Soc 2006; 39: 1 0 I' 09

bela m o m ents are equal and reciproca l space closure occurs. When o nly poster ior protraction is desired , the loop shou ld be located closer to the posterior segm enl. 16

Regard less of the initial magnitudes of the a lfa and b eta moments, c hanges in magnitudes wi ll occur during retrac tion . As the anterior teeth are retra c ted , the magnitude of the a lfa mome nt decreases faster than that of th e b e l a mome nt , e nh an c in g po ste ri o r anchorage. 101 The alb moment differential o btained by eccentric posit ioning underscores the importance of careful clinica l placement of the positi o n o f loop placem ent.

Add itiona l design conside ration s

1. The loop should be #fail safe# - This means that, although a reasonable range of action is desired from each activation, tooth movement sho uld stop after a prescr ibed range of m ovem ent.

2. Design sho uld be as simple as possible. 3. Open verses closed retraction loops.

A loop is more effective when closed (lower load / defl ection rate) rather than opened during act iva tion. On the other hand, a loop deSigned to be opened can b e made so that w h en it closes complete ly, the verti c al legs com e into contact, effective ly preventing further movem ent and producing a desired fail safe effect.

Ricketts' m ax illary canine retractor

It is a combinat ion of a double closed heli x and an extended crossed 'T'. The retractor is fabrica ted u sing .016" .016" blue e lg il oy w ire(F igure 10A). In c riti ca l anchorage cases, 4 5 gable bends and 30 -50 glmm ac tivati o n i s recomme nd ed . 1 7 For lowe r ca nine retraction, double closed he lix is u sed. This de livers SOglmm of activation (Figure 10B). PG Spring

If offers excellent control of forces and momen ts and is the most effective c urrent design. This retrac tor is

A. Maxillary cuspid ret ractor. B. Mandibular c upid retractor

Figure 10: Ricketts re tractor.

fabrica ted using .O 16" .022"stainle steel wire (Figure II LThe design consi ts of double ovoid specia lized spring with a small loop occlusa lly in order to lower the level of activation in the brackets in the short arm. About 30 Sweep was incorporated into the distal leg and mesial leg was angu lated by I S ."

Drum spring

The spring consists of 4 parts namely a constant force spring, a drum, a spring box, and a central pin soldered

/

2Tm

I I

---------------------20Tm Figure t I : tdeal design of the PC spring.

to the molar band. It is activated by pu lling the end of Ihe spring. The force level is always 50g and doesn't vary with the amount of re traction. It prov ides a continuous and constant force resu lting in faster space closure compared to conventional closed coil springs. "

Nili canine retracti on spring

The spring is available in 016 x 022 NiTi wire with anli ti p and antirotation incorporated.Abil ity to deliver continuous forces and moments over a broad range of act ivalion.ln

T - loop

Segmented T- Ioop

These specialized sprin gs are engaged in the allachments only at thei r ends. These springs are tlrst bend into their passive shape in re lat ion to the allachments and then permanent ly deformed by incorporation suitable bends (preactivation bends) to

Sivakumar e( al

apply the required force system to the tooth or teeth to be moved

Burstone 017 x 025 TM A T-loop

This new shape is ca lled preactivation or deactivation shape. When the preactivated spring is engaged into the attachment it converts into the acti va tion shap . Differential anchorage is obtained by the application of unequal alfa and beta moments. The higher moment is app lied to the anchorage teeth. Previously, the approach described for achieving di iferentia l alb moments with segmented T-loops used asymmetric angu lation of the preactivation bends. The present trend is that the off center positioning with a symmetric shape is used to achieve a moment differential, and not spring shape."

The T- loop (0 17 x025 TMAI is designed for an activation oi up to 6 mm. At a full 6 mm activa tion , toolh movement occurs in 3 phases -t ipping, translation and root movement. The spring is positioned closer to the anchorage teeth. Clinica lly. the spring usually needs to be I to 2 mm closer to one side than to the other to obtain a moment differential.Other melhods to produce th e differential moments w ith segmented T loops, include composi te T-Ioop and use oi gable bends.

Composite relrac tion spring

T loop

ThisspringconsistsofO l8" TMA loop welded to 17 x 25 TMA.Thi s sp ring can be used for either en masse retraction of incisors or canine retraction.

T ilanium T loop Retract ion Spring (TILRS) TILRS is placed in a-position for maximum retraction of anterior segment and a 45 bend is placed in the posterior or b-posi tion.

Continuous arch T-Ioop

The T- loops, one on each side, are made distal to the cuspids. Desired alfa and beta moments are placed anter,or and posterior to the T.loop ve rtical legs. Recommended b-activation for A, B, C anchorages are 40 , 30 and 20 respecti vely."

Opus loop

This new design delivers a nonvarying target M/F within the range of 8.0 - 9.1 mm inherently, wi thout adding

t07

//

Il, S

\--- -\ l-

1~ ) t 'd - E . - ~II ....

Figure 12: Idea l design of the opus loop.

residual moments by twist or bends anywhere in the arch w ire or loop before insertion (Figure 12). The loop can be fabricated from 16 x 22 or 18 x 25 o r 17 x 25 TMA w ire. The design of the loop ca lls for an off center positio ning w ith the loop 1 .S mm from the m esia l (canine) bracket. It is acti vated by tightening it distally behind the m o lar tube and can be adjusted to produce maximum, moderate, or minimum incisor retraction. n.ll

Ka lra Simultane ous Intrusion Re traction Spring (K-SIR) K-SIR is a continuous 019 x 025 TMA arc h w ire wit h closed 7 mm x 2 mm U - loops at the extraction sites for en m asse retractio n .{Figure 13).A 90 bend p laced in the arch wire at the level o f U- loops creates 2 equa l and o pposite moment's, w hen centered in the extractio n space. A 60 V - bend located posterior to the center of

~ ~ ~ Figure 13: Kalra simultaneous intrusion and retraction spring

(K-SIR).

the inter brac ke t distance produ ces an inc reased clockwise m om ent o n the first molar.2l

A statically D eterminate Retraction System [SDRSJ T hi s nove l syste m co n si sted of a sing le - fo rce c ant ilever arm m ade o f 017 x 025 TMA for acti ve ret ractio n and a passive ri g id stabili z ing unit. Since the act ive component for space closure is a cantilever, it is sim ple to m easure the force system of the spring

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J Ind Orthod Soc 2006; 39:101 - 109

w ith a fo rce Gauge. A turn of he lix is p laced in fro nt of the aux iliary tube for the molar and ended w ith a hook at its anterio r end. A 90 bend is placed in the midd le of the spring. The spring is activated 90 at the he l ix as well .24

The W ave Spring

Th is spring takes the sh ape of a wave w hen extended. A superelastic nic ke l titanium alloy de livers a re latively la rge amount of activatio n- about 90g of force- from an extrem e ly compact spring- o n ly 6mm lo ng i n its resting state . The wave spring can be u sed in any situation where a a closed-coil spring wou ld be appropriate for relraclion.2s

Con c lus ion

Frictional bi nding and the swi ng effect are the main pro bl ems assoc i ated with sl iding mec h anics. Theoretically, these can be overcome by the u se of a frictio n less system, which includes a loop as the source of the applied force. H owever, the f ri ctio nless system fai ls to produce better resu lts in practice because of the complexity of loop forming and the presence of unknown factors. In addit io n, minor errors can result in m ajor differences in tooth m ovem ent, and some patients find the loop .uncomfo rtable .

Predictable force system is a b ig advantage of retraction with closing loop mechanics. M/F appl ied at the b racket is the most important factor th at contro ls the type of tooth movement induced by a retraction spring. And, load d e fl ecti o n r ate is con side red as a principle c h aracteri stic to describe a spring fo r closing loop mechanics. A recent study o n comparing the effects of friction and friction less mechanics reported a superiority of fri c tio n mechanics over fri c tio n less system in term s of rotational contro l and dimension al m a inten ance of the arch . Frictionless mechanics were shown to be more effect ive a t reduc in g tipping an d extru sio n . No d i fference were found in an c h o rage c ontro l and concluded th at both m ech an ics perform simil arly. 2f Therefore, a work i ng knowledge of both friction and fric tionless m echan ics is m andatory for a clinician.

Acknowledgeme n ts

We express o ur sincere thanks to Mr.Umesh A ch arya for his wonderful artwork.

J LL'S he Communicat ions

Ashima Valialhan, 8DS(Pbl, DDS. MS (USA) Professor and Head. Director of Poslgradlldle Studies. Department oi Orthodontics and Dentofadal Onnopdedics Manillal College oi Denial Sciences. Manipal, Manipa l- 576104 e-rnall: Jvalialhan@)'ahoo.com

References

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2. Nand.l RS. Ghosh J. Biomechanical considerations in sliding mechanics. In: Nanda R (ed). Biomechanics in clinical orlhodontics. w.e. Saunders Compdo)', Phi ladelphia 1997. p.191.

3. Staggers IA. (linicdl considerations in the use of relrdc lion mechanics. J Clin Orthod 1991; 25:3&4-3&7.

4. Burstone C. Orthodontics as a science: the role of biomechanics. Am J Orthod Oenlofadal Orthop. 2000; I 17{S):598-6oo.

5. Mulliga n TF. Common sense mechanics Part II. J Clin Orthod 1980;14:481-488.

6. Siatkowski RE. Force system ana lysis of Vbend sliding mechan ics. I Clin Orthod. 1994 ;28(9):539 4&.

7. Kusy RP, Whitley JQ. Assessment oi second-order clearances betwee n orthodontic archwires and bracket slots via the critical contact angle for binding. Angle On hod. 1999 ;69(1):7 1-80 ..

8. Kusy RP, Whilley IQ. Friction between different wire-bracket configuralions and materials. Semin Onhod. 1997 ;3131:1&&-77.

9. Hansen 10, Kusy RP, Saunders CR. Archwire damage from ce ramic brackets via notching. Orthod Rev 1997: 11 : 27-31.

10. Burstone CI . Applic