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Orthodontics. Part 9: Anchorage control and distal movementD. Roberts-Harry1 and J. Sandy2
Anchorage is an important consideration when planning orthodontic tooth movement.Unwanted tooth movement known as loss of anchorage can have a detrimental effect onthe treatment outcome. Anchorage can be sourced from the teeth, the oral mucosa andunderlying bone, implants and extra orally. If extra-oral anchorage is used, particularly witha facebow then the use of at least two safety devices is mandatory.
1*Consultant Orthodontist, OrthodonticDepartment, Leeds Dental Institute,Clarendon Way, Leeds LS2 9LU; 2Professor of Orthodontics, Division ofChild Dental Health, University of BristolDental School, Lower Maudlin Street,Bristol BS1 2LY; *Correspondence to: D. Roberts-HarryE-mail: [email protected]
● Anchorage is the resistance to unwanted tooth movement● It can be obtained from a number of different sources● Loss of anchorage can have a detrimental effect on treatment● Safety is of prime importance when using extra-oral devices
I N B R I E F
Anchorage is defined as the resistance tounwanted tooth movement. Newton's third lawstates that every action has an equal and oppositereaction. This principle also applies to movingteeth. For example, if an upper canine is beingretracted, the force applied to the tooth must beresisted by an equal and opposite force in theother direction. This equal and opposite force isknown as anchorage.
Anchorage may be considered similar to a tugof war. Two equal sized people will pull eachother together by an equal amount. Conversely abig person will generally pull a small one withoutbeing moved. However, if two or more smallerpeople combine then their chances of pulling abig person will increase. Similarly, the moreteeth that are incorporated into an anchorageblock, the more likely it is that desirable asopposed to undesirable tooth movements willoccur. Undesirable movement of the anchorteeth is called loss of anchorage.
If an upper canine is to be retracted, with bod-ily movement using a fixed appliance, the forceapplied to the tooth will be approximately 100 g(Fig. 1a). Forces in the opposite direction varyingfrom 67 g on the first permanent molar to 33 gon the upper second premolar resist this. Lowlevels will produce negligible tooth movementand the effect of a light force of 100 g would beto retract the canine with minimal anteriorunwanted movement of the anchored teeth.However, if the force level is increased to say300 g (Fig. 1b), the force levels on the anchorteeth increase dramatically to the level whereunwanted tooth movements will occur.Although the canine may move a little distally,
the buccal teeth will also move mesially. Spacefor the canine retraction may be eliminated withinsufficient space left for alignment of the ante-rior teeth. Figure 1c compares the root area ofsome of the upper teeth. The combined root areaof the upper incisors and upper canines isaround the same as that of the first molar andpremolars. Therefore, if the upper labial segmentincluding the upper canines is retracted in ablock, there will be an equivalent mesial move-ment of the upper molar and upper premolar.These factors need to be very carefully consid-ered in planning anchorage requirements andtooth movement.
Anchorage may be derived from four sources:
• Teeth• Oral mucosa and underlying bone• Implants• Extra oral
TEETHThe anchorage supplied by the teeth can comefrom within the same arch as the teeth that arebeing moved (intra maxillary) or from theopposing arch (inter maxillary).
Intra maxillary anchorageThe anchorage provided by teeth depends on thesize of the teeth, ie the root area of the teeth. Fig.1c shows the root surface area of each of theteeth in the upper arch. The more teeth that areincorporated into an anchorage block the lesslikely unwanted tooth movement will occur. If a removable appliance is used, the base plateand retaining cribs should contact as many of
9
ORTHODONTICS1. Who needs
orthodontics?2. Patient assessment and
examination I3. Patient assessment and
examination II4. Treatment planning5. Appliance choices6. Risks in orthodontic
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the teeth as possible. Figure 2 illustrates thepoint. If upper canines are to be retracted with aremovable appliance, cribs on the first perma-nent molars and upper incisors will not onlyhelp with retention but also increase theanchorage considerably. In addition, the baseplate must contact the mesial surface of theupper second premolars and palatal to theupper incisors. If fixed appliances are to beused, the more teeth that are bracketed orbanded, the greater will be the anchorageresistance (Fig. 3).
Inter maxillary anchorageTeeth in the opposite arch can provide very use-ful and important sites of anchorage control asFigs 4a,b illustrate. Good inter-digitation of thebuccal teeth can help prevent mesial movementof the buccal segment. Although there is onlyanecdotal evidence to support this view, manyclinicians feel this can be a useful source ofanchorage.
The second way that opposing teeth can beused is by means of elastics or springs runningfrom one arch to the other. Class II elastics(Fig. 4c) run from the lower molars to theupper incisor region, whereas Class III elastics(Fig. 4d) run from the upper molars to thelower incisor region.
Inter-maxillary elastic are invaluable inmany cases but do rely very heavily on goodpatient co-operation. The elastics need to bechanged every day and if they break (whichthey frequently do) they must be replacedimmediately. Class II elastics will also tend tohave unwanted effects on the occlusion. Theytend to tip the lower molars mesially and rollthem lingually. In addition, they can produceextrusion of the upper labial segment and thelower molars. Whilst extrusion of the lowermolars can help with overbite reduction,extrusion of the upper incisors is usually anunwanted side effect and has to be counter-acted by adding an upward curve to the upperarch-wire known as an increased curve ofSpee. Extrusion of the buccal teeth is undesir-able in patients with increased lower faceheight and therefore Class II elastics should beused sparingly in these cases. Similarly ClassIII elastics can extrude the upper molars, tipthem mesially and roll them palatally. Molarextrusion will decrease the overbite, which isusually undesirable in Class III cases. Elasticsalso tend to cant the occlusal plane and havebeen implicated in root resorption in the upperlabial segment, particularly if they are usedfor prolonged periods.
Functional appliances are another source ofintermaxillary anchorage. Whilst some clini-cians may believe these devices simply make themandible grow, this is not the case and whatevermandibular growth does take place, is accompa-nied by quite substantial movement of the denti-tion over the apical base. This means that mesialtipping of the lower and distal tipping of theupper teeth occurs.
67g67g
33g33g
100g100g
Fig. 1a A distalising force on the upper canine will produce a reciprocal force in theopposite direction on the anchor teeth. Provided the force level for bodily movement iskept low at about 100g then there will be minimal mesial movement of the anchor teeth
200g200g
100g100g
300g300g
Fig. 1b As the distalising force level increases the reciprocal forces also increase witha greater risk of loss of anchorage
2.22.21.81.8
2.72.7
2.32.3
4.64.6
6.76.7
6.96.9
Fig. 1c The combined root surface area of the anterior teeth is almost the same asthe molar and premolar. Attempting to move all the anterior teeth distallysimultaneously will result in an equal mesial movement of the posterior teeth
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ORAL MUCOSA AND UNDERLYING BONEContact between the appliance and the labialor lingual mucosa can increase anchorageconsiderably for either fixed or removableappliances. Contact between an orthodonticappliance and the vault of the palate providesresistance to mesial movement of the posteriorteeth. The anchorage provided by this meansis considerably greater if there is a high vault-ed palate as shown in Figure 5a, which willproduce a greater buttressing effect. A shallowvaulted palate (Fig. 5b) will provide much lessanchorage control because the appliance willsimply tend to slide down the inclined planeof the palate.
The mucosa and underlying bone can alsobe used when fixed appliances are used, forexample a Nance palatal arch (Fig. 5c). This isan acrylic button that lies on the most verticalpart of the palate behind the upper incisors andis added to a trans-palatal arch. These buttonsare again of more limited use if the palatalvault is shallow.
IMPLANTSOsseo-integrated implants can be used as avery secure source of anchorage. Implantsintegrate with bone and do not have a peri-odontal membrane. Because of this they donot move when a force is applied to them andin some cases they can provide an ideal sourceof anchorage. Recently small implants fororthodontic use have been specificallydesigned and can be used in the retro-molarregion to move teeth distally or anteriorly formesial movement. Short 4mm implants can be
Fig. 2 Incorporatingas many teeth aspossible in theappliance design andcovering the anteriorpalatal vault willincrease theanchorage
Fig. 3 When fixedappliances are used, as many teeth aspossible are banded toincrease the anchorage
Fig. 4a,b Inter-digitation of the buccal occlusion can helpincrease anchorage
Fig. 4c Intermaxillary elastics use teeth in the opposite arch as asource of anchorage. Class II traction is shown here
Fig. 4d Class III elastics
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paced in the anterior mid-line of the palate inthe thickest part of the nasal crest and a trans-palatal bar then connects the implants to theteeth (Fig. 6).
EXTRA-ORAL ANCHORAGEThis can be applied via a number of devicesand can be used in conjunction with eitherremovable or fixed appliances. Headgear isnot a recent invention and has been in use forover a century. Figure 7a is a picture of aKingsley headgear, which was in use as earlyas 1861.
The force from the headgear is usuallyapplied to the teeth via a face-bow (Klöen bow)as shown in Fig. 7b. This is fitted either to tubesattached to the appliance or integral with it asin the en masse appliance. The direction bywhich the force is applied can be varieddepending on the type of headgear that is fitted. Headgear can be applied to both themaxillary and mandibular dentition, and thereare a number of variations:
• Cervical• Occipital• Variable• Reverse
Cervical HeadgearThis is applied via an elastic strap or spring,which runs around the neck (Fig. 8a). It has theadvantage of being relatively unobtrusive andeasy to fit. However, it does tend to extrude theupper molars and tip them distally because ofthe downward and backward direction of force.This later effect can be counteracted to somedegree by adjusting the height and length of theouter bow. Cervical headgear should not beattached to removable appliances because it isprone to dislodge the appliance and propel it tothe back of the mouth.
OccipitalThis is also known as high pull headgear andis applied via an occipitally placed head-cap(Fig. 8b). It is easy to fit but is more obviousthan the neck strap and tends to roll off thehead unless carefully adjusted. Because theforce is in a more upward direction, there isgenerally less distal tipping of the upper molarand less extrusion, but also less distal move-ment than with cervical headgear. The tippingand extrusion effect again depend on thelength and height of the outer bow.
Variable This applies a force part way between cervicaland occipital (Fig. 8c) and is our preferredchoice. It takes slightly longer to fit thaneither cervical or occipital and is more obtru-sive. However it is secure and comfortable andthe vector of the force can be varied to pro-duce relatively less tipping and/or extrusion.
Whilst headgear is a very useful source ofanchorage, it has a number of disadvantages.These are as follows:
The most important of these problems is thefact that headgear can be dangerous and anumber of facial and serious eye injuries havebeen reported in the literature.1–3 The Stan-dards and Safety Committee of the BritishOrthodontic Society (BOS) have addressedthese concerns. An advice sheet produced bythe BOS is essential reading for anyone whowishes to use headgear.4
The main problems with headgear safetyrelate to the prongs at the end of the face-bowthat fit into the headgear tubes on the intra-oralappliance. It is possible for the bow to becomedislodged, either because it is pulled out of themouth or when the patient rolls over when theyare asleep. The recoil effect from the elastics candamage the teeth, oral mucosa, soft tissues of theface and most seriously, the eyes. In order tominimise these problem various safety deviceshave been suggested. These involve re-curvingthe distal end of the wire, using plastic coatedface bows and various locking springs.5,6 Inaddition a variety of snap-away face bows havebeen produced. If these are pulled beyond a preset distance, the neck strap comes apart and pre-vents any recoil injury. Another popular methodof preventing recoil is to fit a rigid safety strap,which prevents the bow from coming out of themouth if it disengages from the tubes. Someexamples of these safety devices are shown inFigures 8a-i.
The importance of headgear safety cannot beover emphasized and it is recommended that twosafety mechanisms are simultaneously used, forexample a locking spring and a snap away head-gear or a safety face-bow and rigid safety strap.
ReverseReverse or protraction headgear is useful formesial movement of the teeth, either to closespaces or help to correct a reverse overjet. It doesnot employ a face-bow, which is an advantagebut instead employs intra-oral hooks to whichelastics are applied (Fig. 9a,b).
LOSS OF ANCHORAGEThis is defined as the unplanned and unexpectedmovement of the anchor teeth during orthodon-tic treatment.
There are several causes of loss of anchorage.Some examples of these are:
Poor appliance designFailure to adequately retain the appliance, orincorporate as many teeth into the anchor blockas possible are common causes of anchorageloss. If fixed appliances are used, as many
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Fig. 5a A steep anterior palatal vault is a useful source ofanchorage due to the buttressing effect
Fig. 5b A shallow palatal vault provides less anchorage
Fig. 5c The palatal vault can be used for removable or fixedappliances. An example of a Nance button is shown here Fig. 6 An osseo-integrated implant with a bonded
palatal arch is being used to help close space in theupper arch without retroclining the upper incisors
Fig. 7a An early Kingsleyheadgear circa 1860
Fig. 7b,c A facebow(Klöen bow) is attachedto tubes welded tobands on the molars
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Fig. 8a A neck strap. Note the snapaway safety mechanism
Fig. 8b An occipital (high pull)headgear again with a snap away safetysystem
Fig. 8c A variable pull Interlandiiheadgear. A rigid plastic strip isemployed as a safety mechanism toprevent the facebow disengaging fromthe molar bands and coming out of the mouth
Fig. 8d,e The end of thefacebow can be re-curvedto improve safety
Fig. 8f,g A plastic coatedfacebow together with asafety neck-strap
Fig. 8h,i A Samuels lockingspring. This secures the facebow to the tube preventingaccidental disengagement.This should be used inconjunction with a safetyneck strap or snap awayheadgear
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anchor teeth as possible should be banded inorder to produce optimum anchorage. Remov-able appliances should have adequate retentionusing appropriate well-adjusted cribs or claspswith as much contact with the teeth and oralmucosa as possible.
Poor appliance adjustmentThe use of excessive force or trying to move toomany teeth at the same time may result inunwanted movement of the anchor teeth. Toavoid loss of anchorage, simultaneous multipleteeth movement should be avoided. If theappliance is poorly adjusted so that it doesn'tfit very well, or the force levels applied to theteeth are too high, then undesired tooth move-ment may occur. High force levels produced byover activation are one of the key reasons foranchorage loss.
The optimal force for movement of a singlerooted tooth is about 25–40 g for tipping andabout 75 g for bodily movement. If the force istoo low there will be very little movement,whereas too much force may result in loss ofanchorage. Excess force does not increase therate of tooth retraction as illustrated in Fig. 10.7
As the force levels rise the rate of tooth tippingalso increases up to about 40 g. Beyond this verylittle extra tooth movement occurs. Thusincreasing the force levels above about 40 g willnot increase the rate of tooth tipping.
The force levels that wires from fixed orremovable appliances exert on teeth usuallydepends on the following:
• The material the wire is made from• The amount it is deflected• The length of the wire• The thickness of the wire
Steel wire will exert a force that is directlyproportional to the amount the wire is deflectedup to its elastic limit. Figure 11 demonstrateshow decreasing the wire thickness and increas-ing the length (sometimes by adding loops) con-trols the force produced.
Modern alloys such as super elastic nickel tita-nium wires do not act in the same way as steel.These remarkable wires are capable of producing
Fig. 9a, b A reverse, or protraction headgear
Rate of canine retraction
0
0.5
1
1.5
0 10 20 30 40 50 60gm
(mm per month)
Fig. 10 The graph shows how increase force levels do not necessarily increase therate of tooth movement. The y axis shows the rate of movement in mm. The x axisis the amount of tipping force applied to the tooth. As the force level initially risesthe rate of tooth movement also increases. Above about 40 g the rate slows downand very little additional tooth movement occurs. There will however be a greaterrisk of loss of anchorage with increased force levels
0.5 mm0.5 mm
0.5 mm0.5 mm
0.6 mm0.6 mm
Fig. 11 A 0.5 mm diameter wire can bedeflected more than a 0.6 mm wirewithout increasing the force level. Thus agreater degree of activation is possibleand the appliance will require lessfrequent adjustments. Similarlyincreasing the length of the wire, forexample by incorporating loops allows agreater degree of wire deflection. Theforce characteristics may also be changed by altering the material the wire is made from
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a continuous level of force almost independent ofthe amount of deflection and have transformedthe use of fixed appliances in recent years. Heatactivated wire is now available that will increaseits force level as the temperature changes. Thesewires exhibit a so-called shape memory effect. Ifthe wire is cooled and tied into the teeth it deflectseasily into position. As the wire warms in themouth it gradually returns to its original shapemoving the teeth with it (Figs12a–c).
For optimal tooth movement it is importantthat continuous gentle forces are applied to theteeth. Fixed appliances are ideal for doing this.When removable appliances are worn, thepatient should wear them full-time except forcleaning and playing contact sports. Part-timewear produces intermittent forces on the teethand is likely to reduce the rate of movement.
When a force is applied to a tooth, there is aninitial period of movement as the periodontal
membrane is compressed (Fig. 13). No toothmovement occurs for a few days after this, ascells are recruited in order to remodel the socketas well as the periodontal membrane. This cellrecruitment takes a few days and is known as thelag effect. Part-time wear of appliances will notallow efficient cell recruitment and the lag phasewill not be passed which may result in poortooth movement. This is another reason whyfixed appliances, which cannot be left out of themouth by patients, are much more effective thanremovable appliances at achieving a satisfactorytreatment outcome.
RETRIEVAL AND PRESERVATION OFANCHORAGEExtra-oral devices can be used for distal move-ment as well as anchorage reinforcement. Foranchorage control wearing the headgear atnight-time only is usually enough. In order toproduce distal movement, the patient shouldwear the appliance in excess of 12 hours usu-ally for the evenings as well as at nighttime.While some practitioners increase the forcelevels for distal movement purposes, it is ourexperience that this is not necessary and aforce of approximately 250–300 g per side isadequate for both distal movement andanchorage control.
Many devices have been described to reduceor eliminate the need for headgear. These arehowever of limited use and can only produce avery small amount of extra space. If these gadg-ets are used without anchorage re-enforcementunwanted mesial movement of the anchor teethcould occur. Figures 14a–c shows one exampleknown as a Jones jig. To produce distal move-ment of the molars the anchorage is reinforcedwith an anterior trans-palatal arch. A jig incor-
Fig. 12a-c Super elastic heatactivated wires produce a lightcontinuous force almost regardlessof the amount of deflection. Whencooled they become very flexible(12a) but return to their originalshape as they warm in the mouth(12b,c)
a
b c
Lag period
0
0.5
1.0
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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Days
mmFig. 13 Tooth movementrequires light continuousforces. In this graph toothmovement in mm is shownon the y-axis and time indays on the x-axis. If aforce is applied to a tooththe periodontal membraneis compressed and there isa small amount of initialmovement. Movementthen stops as bone cells arerecruited and the socketstarts to be remodeled.After about 14 dayssufficient recruitment andremodeling has occurred toallow the tooth to move
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porating a nickel-titanium coil spring is insertedinto molar tubes and tied into the premolarbands. The molars are distalised using the ante-rior teeth from premolar to premolar as theanchorage block. It is important to note the lossof anchorage that is occurring as demonstratedby the simultaneous mesial movement of thefirst premolars. Once distal movement of themolars has been achieved the anchorage rein-forcement can be transferred to the molars(palatal arch or Nance button) and the premo-lars, canines and incisors retracted. True anchor-age re-enforcement with these devices is diffi-cult to achieve and headgear, or implants muststill be considered the mainstay of producingeffective distal movement.
Thanks to Mr. R Cousley for figure 6 and Mr. J Kinelan forfigures 14a-c
2. Samuels R H A, M Willner F, Knox J, Jones M L. A nationalsurvey of orthodontic face bow injuries in the UK and Eire. Br J Orthod 1996; 23: 11-20.
3. Samuels R H A, Jones M L. Orthodontic face bow injuries andsafety equipment. Eur J Orthod 1994; 16: 385-394.
4. British Orthodontic Society, 291 Grays Inn Road, LondonWC1X 8QJ.
5. Postlethwaite K. The range and effectiveness of safetyheadgear products. Eur J Orthod I988; 11: 228-234.
6. Samuels R H A, Evans S M, Wigglesworth S W. Safety catchfor Kloen face bow. J Clin Orthod 1993; 27: 138-141.
7. Crabb J J, Wilson H J. The relation between orthodonticspring force and space closure. Dent Pract Dent Res 1972;22: 233-240.
Fig. 14a-c A Jones jig for distal movement of the molars (14a).A palatal arch is fitted to the first premolars to increase theanterior anchorage. A jig is then inserted into the buccal archwire and headgear tubes. An open nickel titanium coil spring isthen slid over the shaft of the jig and compressed by sliding acollar onto the shaft and tying it to the premolar (14b). Thisthen uses the upper premolars and palatal vault to distalise the molars (14c). Note the simultaneous mesial movement of the first premolars which is a sign of anchorage loss
a
b c
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