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Issacson

Palatal springs

These springs are used for mesio-distal movement and buccal movement of teeth.They are usually the springs of choice for mesio-distal tooth movement because

they are protected by the base plate and so are less liable to damage.

Single cantilever spring (finger spring)

This spring may be used to move teeth labially

or in the line of the arch. It is normally

constructed from 0.5 mm hard stainless steelwire. Some operators prefer to use 0.6 mm wire

and activate the spring by a smaller amount

(50% of the deflection will give about the same

force). A coil is incorporated into the spring

close to its emergence from the baseplate. This increases the length of wire and

thus the flexibility of the spring. For maximal resilience, the coil should lie on the

opposite side of the spring from the tooth so that it 'unwinds' as the tooth moves(see Figure 3.1). For labial or buccal movement of teeth, a single cantilever spring

should be cranked (Figure 3.10) to keep it clear

of the other teeth. This also ensures that the

spring is protected by the baseplate even when

the tooth moves.

When a palatal cantilever spring isconstructed, the intended path of tooth movement

is determined and the required point of

contact of the spring with the tooth is markedon the model. A line is then drawn on

the model, at a right angle to the path of tooth movement and through the mid-

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crown width of the tooth. This indicates the correct position of the coil (Figure

3.11). The spring arm should be straight unless it has to be cranked to establish the

correct contact with the tooth (particularly when it is being used to retract a canine)

to ensure that the point of contact with the tooth is correct and that the tooth will

move in the right direction (see Figure 2.6, p. 10). The free end is finished neatlyafter the baseplate has been processed. A palatal spring is usually boxed to protect

it from damage, so that it lies

in the recess between the baseplate and the mucosa. A possible problem is that if

the spring catches between the teeth during removal it may be pulled away from

the baseplate. Readjustment

is difficult and will weaken the spring. If the spring is being used to carry out

movement along the line of the arch, such distortion can be prevented by incorporating aguard wire palatal to it (Figure 3.12) so that achannel is formed between the baseplate and guard. It is important that, during construction,the spring is blocked out adequately, so that itcan act freely, and is not impeded by the guard.

The use of a guard is a matter of personalpreference and some operators find that guardsare more trouble than they are worth. A guardcan help to prevent distortion of a spring -

which can be difficult to correct - but if thetechnician does not block out the springadequately before processing the baseplate, theguard itself may obstruct free movement.

AdjustmentA palatal spring is simple to adjust. Use adental mirror with the appliance in place tocheck that the spring contacts the tooth

correctly and lies close to the gingival margin(Figure 3.13). At the time of fitting slight activationof no more than 1-2 mm is advisable, butat subsequent visits an adjustment of 3 mm is

appropriate. The spring should not be bentwhere it emerges from the baseplate - this is asite of stress concentration and if the wire is further work-hardened by bending then fracturemay occur. The correct site of adjustment

is in the free arm of the spring as close to thecoil as possible. In cases where the direction ofthe spring has to be corrected to achieve the

intended direction of movement adjustmentmay be carried out further from the coil.

Double cant i lever spr ing ( 'Z' spr ing)

In some situations, particularly where anincisor is to be proclined, space for a singlecantilever spring is limited. In these circumstances,

a double cantilever, or 'Z' spring(0.5 mm wire) can be used (Figure 3.14). It is

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important that the limbs are as long as possible,otherwise these springs can be rather stiff. Ifthe limbs of the spring are short, the range of

adjustment is limited and the patient may findthe appliance difficult to insert. The springshould be perpendicular to the palatal surfaceof the tooth; otherwise it will tend to slide

incisally and to intrude the tooth. If a lateralincisor is to be proclined and space is very short

it may be permissible to construct a 'Z' springin 0.35 mm wire.The mechanical principles and general

features of the double cantilever spring aresimilar to those of the single cantilever spring.AdjustmentAdjustment can be carried out to the palatallimb first - close to the coil at the fixed end ofthe spring to establish the degree of activation- then at the other end of the limb to keep the

free limb perpendicular to the intended directionof movement of the tooth. It is often possibleto activate the spring in a single movement

by grasping its outer arm in the beaks of thepliers and pulling it gently forward and awayfrom the acrylic baseplate.

'T' springWhere a premolar, or sometimes a canine, is to

be moved buccally, the patient may find insertion

of the appliance very difficult if a single ordouble cantilever spring is used. A 'T' spring

(see Figure 3.9) made from 0.5 mm wire, canbe much simpler to manage. The mechanicalprinciples are similar to those of a single

cantilever spring but, as both ends of the spring

are incorporated into the acrylic, the flexibilityis correspondingly reduced. Fortunately, alarge deflection is not indicated in this situation,

otherwise the patient may have problemsin inserting even this type of spring. It must berecognized that the force applied by the springhas a vertical, as well as a horizontal component.

If the tooth surface at the point of contactis nearly vertical (as is usually the case with anupper premolar) the intrusive component issmall. If this spring is applied to a sloping

surface, such as the cingulum plateau of anupper incisor, the vertical component will belarger and the labial component correspondinglysmaller. Even if the 'T' spring initially

contacts the more vertical incisal part of thepalatal surface of an incisor, it will come to reston the cingulum plateau as the tooth moves.This reduces the efficiency of the spring andthe tooth itself may be intruded. Intrusion is

usually unwanted when incisors are to beproclined because stability depends on a positive

overbite after treatment. The verticalcomponent also has a displacing effect on theappliance and retention may be a problem. For

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these reasons, 'T' springs are not usually usedfor labial movement of upper incisors.AdjustmentThe spring is adjusted by pulling it away from the

baseplate. Provided that it is only adjusted by asmall amount it should seat itself correctly when the patient inserts the appliance. Once the toothhas moved some distance, it may be necessary to

elongate the spring at the adjustment loops.

Buccal springsThese springs can be used for mesio-distaltooth movements, palatal tooth movement and,in conjunction with some form of bondedattachment to a tooth, occlusal movement,

rotation or buccal movement. Care needs to betaken during construction so that they do notintrude too deeply into the buccal sulcus.Instructing the patient on how to remove the

appliance should specifically address the easewith which the buccal springs may be distorted.Bucc al canine ret ractor

Abuccal spring is used where a buccally placed

canine has to be moved palatally as well asdistally - an operation for which a palatalspring is not satisfactory. Buccal springs are

liable to be uncomfortable (which can make them unpopular with patients). They are relativelyunstable in the vertical dimension (see

Figure 3.4) which can make them difficult toadjust, because if the spring acts on a slopingsurface, it will tend to slide along the incline.

An unsupported buccal spring has to beconstructed from 0.7 mm wire to provide stabilityand will be much less flexible than a palatal

spring, so that a small deflection generates alarge force.In view of these problems, particularly carefulattention has to be given to the design andconstruction of a buccal spring. The impression

must adequately reproduce the buccal sulcusand show the muscle attachments so that thespring can be kept clear of them. The springshould extend as far as the mucosal reflection

and lie just clear of the attached mucosa. Theoutline of the spring is drawn on the model. Thecoil lies just distal to the long axis of the tooth,while the anterior limb of the spring passesdown from the coil to the middle of the crown

and is carried round, in contact with the tooth,

to the mesial contact area. This design is morestable, making control of distal and palataltooth movements simpler than with the

conventional buccal retractor (Houston andWaters, 1977). The distal limb of the spring is

carried through to the baseplate, in contactwith the second premolar and above its contact

area.Adjustment

The spring should be activated by only 1 mm,otherwise the force will be excessive. Provided

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ScrewsAn alternative method of providing a force is to

use a screw as an integral part of the removableappliance. The screw normally transmits itsforce by means of the acrylic, which comes in

contact with the teeth. The patient usually activatesa screw once or twice a week. A fairlyhigh force is generated but is intermittent.Figure 3.27An extended labial bow (0.7 mm).

Nevertheless, there are certain situations inwhich screws are very useful.

Many types of screw are commerciallyavailable for use in removable appliances.Desirable features in a screw are adequatetravel, stability and minimal bulk (Haynes and

Jackson, 1962). Screws can be used for manytooth movements, but they add to the expenseof an appliance while making it more bulky.Adjustment is normally carried out once or

twice weekly by the patient. We recommend

screws only in those few situations wheresprings would be unsatisfactory: for example,where the teeth to be moved are required for

retention of the appliance (see Figuie 9.9p. 84). Single and double guide pin screws areavailable (Figure 3.28). The latter are morestable but the former is useful where space islimited. Problems can occur with screws. Some

tend to turn back under load. If the applianceis left out it may not be possible for the patientto re-insert it and treatment may thus bedelayed. Screws apply intermittenl large

forces, which decrease as tooth movementoccurs. The large force is acceptable only

because the activation at any one time is small

(less than 0.2 mm). The tooth is thus movedwithin the limits of the periodontal ligamentand extensive hyalinization will not be

produced. Spring-loaded screws are available,in which a spring within the screw dissipates

the force over a period of time. Althoughthese offer theoretical advantages, they are

bulkier, more expensive and seem to offer fewclinical advantages.

AdjustmentThe patient is given the key to adjust the screw.It is worthwhile incorporating a marker in the

baseplate to indicate the direction in which thescrew is to be turned. An adjustment of one

quarter-turn each week will produce a rate oftooth movement of about 1 mm per month. The

patient must ensure that the appliance seatshome fully after adjustment. In some situations

it is possible to adjust the screw twice weekly,but this may lead to anchorage loss and theappliance may not seat home fully, so thatretention is less good. It is possible to monitor

the expansion achieved by the use of holes

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drilled into the acrylic in each half of applianceand to measure with dividers at each visit.Alternatively the screw may be turned back at

each visit and the number of turns recorded inthe notes.

Tooth movement with removable

SPRING DESIGNSSprings for Mesiodistal MovementThe Palatal Finger Spring

This is commonly used to retract a canine after the removal

of the first premolar. It can also be used to move any toothmesially or distally along the arch.

The simplest form of this spring is a straight piece of wireembedded at one end into the acrylic base plate. The usualwire diameter is 0.5 mm or occasionally 0.6 mm. In practice a coil is incorporated near the insertion into

the acrylic. Thisallows an increased length of wire to be accommodated

within the confined space and so makes possible the deliveryof a light force over a long distance. The coil should be madeas large as possible consistent with fitting it into the appliance,

say 3 to 4 mm. By convention it is placed so that whenactivated it is tightened as the appliance is inserted and

uncoils as the tooth moves (i.e. the coil is on the side of thewire away from the direction of tooth movement). This has

recently been shown to be of little importance.A coil spring of this design will usually be about 2 cm inlength from the point of application to its insertion into the

acrylic. The position of the coil is most important as itinfluences the direction in which the spring works. This will

be dealt with more fully in the section on applied design.Advantages. The spring provides a light pressure and is well tolerated.Several springs can be added if required to retract teeth successively.Disadvantages. The palatal finger spring cannot move teeth in a palataldirection. It is thus unsuitable for buccally placed teeth and its use insuch circumstances may move a tooth further buccally and tend toproduce rotation.

The base plate must not hinder movement of the springand this can be prevented in the following ways:

Open SpringThe spring is formed on the model and, except for thelocking tag, waxed in before the acrylic is added (in the caseof heat cured acrylic the spring would have to be plasteredin). The appliance is then constructed so that the spring is

left free to move and is not covered by acrylic. Its fulllength is visible in the mouth when the appliance is in place.Advantages. Cleaning is made very easy and binding of the springagainst the acrylic is eliminated. If the spring does become distorted

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it can be readily adjusted. Wire guards can be incorporated if necessaryto limit distortion.Disadvantages. If identical springs are placed bilaterally the applianceis weakened and the remaining isthmus of acrylic has to be thickenedin compensation.

Boxed Spring

This spring is waxed in during construction in a similar

manner to the open spring but the wax is kept shallow andthe acrylic is extended over it so that the spring is left freeto move in a recess on the fitting surface of the base plate.

At first sight it might seem sufficient to make this recess

only large enough to permit the spring to move from itsoriginal to its intended final position. It must be remembered,however, that during activation a spring must be flexed

beyond the position in which it will finally be passive. It istherefore necessary to extend the recess further distally in

order to allow activation of the spring during the final stageof tooth movement.Advantages. The acrylic cover over the spring strengthens the appliance

and gives a smoother surface for the tongue.Disadvantages. Unless waxing in is carried out carefully insufficientspace may be left for the spring to move between the acrylic and themucosa. Alternatively the thickness of the appliance may be undulyincreased.

Guard WiresA guard wire may be incorporated to prevent displacementor distortion of a spring during wear. An open spring mayhave a guard wire placed on the tongue side or may occasionally

have wires on each side of it so that in action it slidesbetween the two. A boxed spring may carry a guard wire

between itself and the soft tissue of the palate to prevent thespring from becoming distorted away from the acrylic duringwear (such distortion, once it has occurred, may be difficultto correct).In either case the guard, to be of any use, must be placedwell down the spring from the coil, but it must not be

carried near enough to the path of tooth movement to impedethe necessary trimming of the acrylic. It is usually constructedin 0.5 mm wire and made with a slight curve so that it followsthe shape of the arch.Advantages.A carefully positioned guard wire can be a definite aidduring tooth movement and can limit distortion of the spring. If a broken finger spring has to be replaced the guard wire can holdthe

new spring in position while the acrylic is cured around its lockingtag.Disadvantages.A wrongly positioned guard wire may be useless ormay even hinder tooth movement. It is also easy to leave too l ittlespace for the spring between the guard wire and the acrylic.

The Buccal Canine RetractorThe overall shape of this spring can be seen in the figure.It comprises a posterior arm which passes across the line of

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the arch and up into the sulcus to support a coil from whichthe anterior arm descends to engage the canine. It is particularly

suitable in situations where the canine overlaps thelateral incisor labially. It is usually made in 0.7 mm wire butcan also be constructed in 0.5 mm wire with the supportingarm sheathed in 0.5 mm (i.d.) hard stainless steel tubing.

Because the spring curves up into the sulcus at over 90 toits insertion into the base plate it tends, during use, torotate within the acrylic rather like the action of turning adoor handle. To prevent this a definite crank must be madeto lock the spring into the base plate. It is not sufficient toturn the wire down on to the surface of the model as for an

Adams' clasp. Care must be taken to position this crank sothat it will not limit the later stages of canine retraction.

If the spring crosses mesial to the second premolar atgingival level it will prevent full retraction of the canine

and although this can be corrected at the chairside during thelater stages of retraction it is better if the spring is constructed

to cross the premolar at the level of the interstitialridge. For this reason it is helpful to present the technicianwith a model before extraction and to discourage the habit of

cutting the tooth to be extracted off the model. The coil should be of about 3 mm (i.d.) and placed so thatif its

position remained constant during movement the anterior

arm would swing like a pendulum. In reality the coil will notmaintain a constant position because the whole spring flexes

in use but nevertheless the above method will give the correctcoil position and allow the anterior arm to He more or less

parallel to the mesial surface of the canine andso deliver its

force at 90 to the long axis of the tooth. This will avoid

displacement of the spring. If the impression is well muscletrimmed it should help the technician to position the coilso that it is not likely to cause trauma to the sulcus and doesnot conflict with the muscle attachment in this area. It may

only be necessary to turn the end of the anterior arm througha right angle toward the mid-line and cut it off to leave afoot of about 2 mm which will engage the mesial edge of thecanine. When the spring is activated, however, this foot maytend to slip buccally from the tooth unless the whole spring

is swung a little in a palatal direction which may in turn havethe unwanted effect of causing trauma to the gingival margin.

A better design of foot is made by turning the end of the

spring into a loop of about 3 mm dia. which engages thecanine securely and rests against the gingival margin ratherthan sticking into it, see illustration of buccal canine retractor.If the canine is partially erupted a simple foot can be used

but can be improved if the end is annealed, flattened toresemble a golfing iron and curved to fit the surface of thecanine.Advantages. The spring offers good control of the canine during distalmovement. It prevents unwanted buccal movement and its end can beturned through 90 and used to tuck the canine palatally, if necessary,

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at the end of retraction. It generally displaces the appliance less than a palatal spring and is usually well accepted by adultsbecause it is frequentlyunnecessary to use clasping on the anterior teeth.

Disadvantages. The heavy gauge of the wire makes it easy to exert excess pressure on the canine. Unless the spring isproperly proportioned and constructed it may be unstable in the vertical plane and difficult to control. It can also causetrauma to the sulcus and is readily distorted.

An Alternative Buccal Retraction Spring

An alternative design is shown here. The authors rarely usethis spring but it can be particularly useful for buccally

placed canines which erupt fairly high up on the buccal

surface of the alveolus with a mesial inclination. It is alsoof particular use with lower appliances. As is the case with

the normal buccal spring this spring is constructed in 0.7 mmwire but instead of a coil it merely has a large loop in thesulcus from which the mesial arm of the spring engages ontothe canine. The spring can be designed with or without areversed loop as desired. In either case it is important that

the loop should be made sufficiently large to give goodflexibility to the spring and that the spring itself should beso shaped that the arm which engages the tooth lies atapproximately right angles to the mesial surface.Advantages. The spring is useful for buccally placed canines and is lesslikely to traumatize the sulcus. This makes it especially applicable tolower removable appliances.Disadvantages. Once again the spring tends to be fairly rigid and activationcan be difficult to control.

The Double Cantilever or 'Z' Spring 0.5 mm

This is a variant of the palatal finger spring and is the commonesttype in this series. The name is self-explanatory as thespring is bent into the shape of a 'Z' with two coils. It should

be constructed with the spring compressed and the presenceof two coils makes it possible for the end of the spring

delivering the force to be activated in a straight line ratherthan to move through the arc of a circle.Advantages. The spring is compact and may be incorporated on a smalltooth such as a lateral incisor even when there is clasping on the centralsand a spring on the canine. A larger spring may be constructed to acton two neighbouring teeth. The force is delivered in a straight line and

the spring may be given a slight upward activation which will alloweasier insertion and will not trap the spring between the acrylic andthe enamel.Disadvantages. It is less suitable for posterior teeth because, as is thecase with the cranked palatal finger spring, it is readily trapped on theocclusal surfaces of the teeth during insertion. If the spring is madevery small it can easily produce an excessive force during activation.If the spring is used for more than a slight amount of movement theremay be problems with its vertical stability. The reactionary force tothis spring from the incisal cingulum tends to displace the anteriorpart of the appliance. Good anterior retention is therefore essential.

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The 'T Spring 0.5 mmAgain the shape is self-explanatory. Both ends of the wire

are embedded into the base plate and the cross piece restson the palatal surface of the tooth to be moved. The additionof extra bends half way up the spring increases its flexibilityand provides spare wire for its extension during tooth movement.

Advantages. The spring is particularly suitable to provide buccal movementof buccal teeth. Its action means that it is less likely to catch on

the occlusal surface of such a tooth during insertion. The provision ofextra loops extends its range of activation. It occupies 'only a smallspace and can be used on a premolar concurrently with other movementssuch as canine retraction.Disadvantages. If this spring is used to procline an incisor it willinevitably deliver a strong upward component of force through thecingulum.

All these springs are generally boxed in under the acrylicof the base plate. Skilful construction is necessary if thesprings are not to make the acrylic excessively bulky.Springs for Lingual Movement

Various designs are available, all emerging from the acrylic,crossing the embrasure and passing up towards the sulcus.The type of end is determined by the relationship of anavailable embrasure and also by the activation and pressurerequired. These springs must be robust and 0,7 mm wire isgenerally used.

Springs for the Reduction of Overjet and Alignment

of IncisorsA wide variety of appliances and springs have been designedto carry out these movements. This is no doubt because of

the lack of an ideal spring. Each type has its advantages anddisadvantages which may be more or less important to differentsituations and operators.

Heavy Wire:The Labial Bow with Small Loops0.7 mm Wire

This is unsuitable for the reduction of anything but thesmallest overjet and for squeezing irregular incisors into Hne.Larger overjets would take a long time to reduce. A verysmall activation of the bow will produce an excessive forceand such force may lead to anchorage loss or to discomfort

which results in non-wear of the appliance. This type of

bow is still widely used, however, and is often found incorporatedinto an appliance bearing palatal finger springs toretract the canines. No doubt this is done in an attempt toreduce the number of appliances required for a given case.In the authors' opinion such an approach is often a falseeconomy and this type of bow should be used with caution.Advantages. The placing of offsets into the bow can, in combinationwith selective trimnning of the acrylic, squeeze irregular incisors intoline. The rigidity of the bow makes it suitable for modification as aretainer following active treatment.

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Disadvantages. Even slight activation of the wire can readily produceexcessive pressure and because of this the bow is unsuitable for thereduction of any but the slightest overjet.

Heavy Wire:

The Labial Bow with Large Loops0.7 mm Wire

Loops may be of varying size up to the type incorporated inthe bow described by Mills. Although such a bow will give alight pressure over a long distance it is difficult to manufacture

and also to adjust. Loops of a less extreme size permiteasier adjustment and yet give a light controllable force.Advantages. It is still possible to put pressure on individual teeth and

the bow is useful in retaining the corrected position after tooth

movement.Disadvantages. Skill is required to control the activation of the bowand at the same time prevent the wire from traumatizing the sulcus.The wire tends to slide gingivally up proclined teeth and must thereforebe activated occlusally. Sometimes it is difficult to maintain the wire

at the right position on the teeth..

Light Wire:

The Roberts' RetractorThe retractor described by Roberts is shown. It is constructed

in 0.5 mm wire but the buccal arms are sheathed from thecoil into the acrylic with 0.5 mm (i.d.) hard stainless steel

tubing to provide added strength.Advantage. This spring produces a light force and is easily adjustable.Because it swings downwards and backwards during use it does nottend to slide gingivally up proclined incisors and it is well tolerated.Disadvantages. If the supporting arms are not correctly positioned thesulcus may be traumatized. In the event of breakage major reconstructionwill be necessary.

Principles of Design: Screws

When an appliance is inserted into the mouth any spring

which is active must flex in order to seat in the correctposition against the tooth which is to be moved. Once in

place it will deliver a continuous light force which willdecrease gradually over several weeks as the tooth moves.The action of an orthodontic screw is very different.

Various types of small screws have been used to moveindividual teeth buccally but they do not seem to offer any

advantages over a suitable design of spring. An orthodonticscrew does not usually contact the teeth and is embedded at

both ends into the acrylic which is subsequently split with a

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saw. Activation is achieved by turning the screw so that thetwo sections of the acrylic are moved apart and the appliance,

which is still rigid, will not fit completely passively. As it ispushed into position the acrylic or the wirework will deliverforce to the teeth. A slight amount of adaptive movementcan be accommodated by the periodontal membrane and

bony adaptation subsequently occurs. If the screw is overactivated the appliance cannot be fully seated.

SCREW DESIGNS

Screws are produced by a number of manufacturers and awide range of sizes and types is available.A typical design has a central threaded screw, each end of

which engages into a small metal or plastic block. One ofthese blocks carries two guide wires which lie parallel to the

screw and pass through holes in the opposing block. Thecentre of the screw is enlarged into a small boss in which four

radially positioned holes are visible. A small wire key issupplied and may be inserted into one of these holes and

turned like a capstan through 90 until it touches the guide.

For further activation the procedure must be repeated withthe key inserted into the next hole.

Until recently it was necessary for the technician to boxin the central portion of the screw with plaster (or with waxif a cold cure system was being used) so that the acrylic did

not encroach on to the moving parts and prevent activation.Most modern patterns of screw are supplied with a soft

plastic tag covering the central portion. The tag provides aconvenient way of holding the screw in place during applianceconstruction and is torn off after processing. Screws are

available in two forms. In one type the threaded pin andguides project beyond the blocks when the screw is closed.

In the other the metal or plastic coverage is extended so thatno projections are visible. Either type can be cured in theclosed position and then opened but an attempt to cure the

former sort in the open position and close it subsequentlywill be unsuccessful. The acrylic behind the blocks will not

permit the screw to project beyond the metal as it is turned.A number of points must be borne in mind when consideringwhether to use a screw or spring for a particular task.

BulkDespite the decreasing size of modern orthodontic screws an

appliance carrying a screw still has to be considerably thickerthan one without. Unless particular care is taken it is easy

during the construction to thicken the entire vault of thepalate even when the screw is situated on one side.Patient Co-operation

We have pointed out that unlike a spring a screw can only beactivated to give a small amount of movement over the

course of a few days. For this reason it is not adequate forthe operator to adjust the appliance monthly. Regular very

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slight activation must be provided and the screw must usuallybe turned by the patient through a quarter of a revolution

once or twice weekly. If it is turned less frequently progresswill be very slow. If it is turned more frequently then the fitmay gradually deteriorate. More reliance has to be placed onthe patient and it is doubly important to make sure that he

or she knows exactly what is to be done and can demonstratehow to turn the screw successfully before leaving the surgery.