BIOMECHANICS OF CANINE RETRACTION

IntroductionHistorical PerspectiveBiomechanicsClassification of canine retractionFriction mechanicsFrictionless mechanicsRecent AdvancesConclusion

Introduction

Retraction of the canines represents a fundamental stage in a considerable number of orthodontic treatments. Correct positioning of the canine after retraction, recognized to be of uppermost importance for function, stability, and esthetics, can be obtained either by uprighting after uncontrolled tipping or by means of biomechanically predetermined and controlled movement.

The principles for retraction currently used can be described as either (a) a "frictional" system in which the canine, through application of a force, is expected to slide distally along and is guided by a continuous arch wire or (b) a nonfrictional system with forces and couples built into the loops of an arch section.

Historical Perspective

Late in 19th Century

As extraction were done in the late 19th century finger springs or other methods were employed for the simple pushing back of canine teeth. This often resulted in tipping and elongation of teeth.

Tweed

He placed coil springs along the continuous .016 round arch and push cuspid from one side to the other meanwhile tying back the arch to the molar which were employed as anchors.

Ricketts

In 1980, Ricketts employed push coil springs in order to obtain Sectional Cuspid retraction.

Smith & Storey

Conducted a study to find the optimal force for distal movement of maxillary Canine. Lee (1966) showed optimal force for distal movement of Maxillary Canine with tipping was 15 to 260 cm.

Merrifield et al (1966)

Maxillary Canine was retracted in 0.017 x 0.022 ss wire with high pull J hook headgear. A distal offset bend mesial to second precuslar was given to maintain Canine in the alveolar trough.

Charles Burstone and HerberstA.Koenig (1976)

Studied the factor that affects the M/F ratio of canine retraction springs. They concluded that better control of the root during retraction was obtained by increasing Moment/Force ratio.

Wick Alexander et al (1978)

Showed in Vari Simplex Discipline, Maxillary Canine was retracted in 0.016 inch SS wire in 0.018 inch slot long Bracket, a Single bracket with flat rotational diamond form with two circular holes in wing is used. Power chain to exert force of 250 300 gms was used for Canine retraction.

Charles and Jones (1982)

Showed retraction of Canine using sectional mechanics does not need to bracket anterior teeth and allowed spontaneous aligning of crowded teeth.

Burstone (1982)

developed composite TMA spring for Canine retraction.

Poul Gjessing (1985)

Developed a sectional arch technique that produces optimal force system for controlled Canine retraction. PG (1994)

Yasoo Wartanahe and Keisuke Miyanoto (2002)

Developed a new Nickel Titanium Canine retraction spring, which was constructed from 0.016 x 0.022 Niti wire.

Sela et al (2004)

Conducted a study to evaluate the effect of rapid canine distraction on dentoalveolar tissue using semi rigid, individual tooth Borne distraction designed by Bengi.

Biomechanics

The principles for retraction currently used can be described as either (a) a "frictional" system in which the canine, through application of a force, is expected to slide distally along and is guided by a continuous arch wire or (b) a nonfrictional system with forces and couples built into the loops of an arch section.

Center of resistance

Every object or free body has one point on which it can be perfectly balanced. This point is known as the center of gravity.The movement of a free body depends upon the relationship of the line of action of the force to the center of gravityIn a restrained body, such as a tooth, a point analogous to the center of gravity is used; this is called the center of resistance. By definition, a force with a line of action passing through the center of resistance produces translation. The center of resistance of a single-rooted tooth is on the long axis of the tooth, probably between one third and one half of the root length apical to the alveolar crest. i.e. 66% of root length from the apex towards the alveolar crest

Two important points are evident from the definition of the center of resistance. First, the position of the center of resistance varies with root lengthA second important point is that the center of resistance varies with alveolar bone height. The movement of teeth in adults with alveolar bone loss will be different than in adolescents.

Center of rotation

It is the point about which the body appears to have rotated.It can be on or off the toothEg if center of rotation on the apex ,less root movementThe more nearly translational the moment the farther apically the centreof rotation would be located.Method to determine the center of rotation

Take two points on the tooth and connect before and after positions of each point with a line the intersection of perpendicular bisectors is the center of rotations

The movement of a tooth (or a set of teeth) can be described through the use of a center of rotation. The ratio between the net moment and net force on a tooth (M/F ratio) with reference to the center of resistance determines the center of rotation. since most forces are applied at the bracket, it is necessary to compute equivalent force systems at the center of resistance in order to predict tooth movement.Controlling the rotation gives precise control over the type of tooth movement (Marcotte)

Single force + single couple can produce any type of tooth movements.

Effects of forces

Translation: If the line of action of an applied force passes through the center of resistance of a tooth, the tooth will respond with pure bodily movement (translation) in the direction of the line of action of the applied force.Moment of a force. If the line of action of an applied force does not pass through the center of resistance, the force will produce some rotation. The potential for rotation is measured as a moment, and the magnitude of the moment is equal to the magnitude of the force multiplied by the perpendicular distance of the line of action of the force to the center of resistance

Types of tooth movement

Two important points are evident from the definition of the center of resistance. First, the position of the center of resistance varies with root lengthA second important point is that the center of resistance varies with alveolar bone height. The movement of teeth in adults with alveolar bone loss will be different than in adolescents.

Equivalent Systems

There are three simple rules that allow the calculation of equivalent force systems. Two force systems are equivalent if (1) the sums of forces in the x direction are identical, (2) the sums of forces in the y direction are identical, (3) the sums of moments about any point are identical.

Moment of a force with respect to center of resistance depends upon the perpendicular distance of its line of action to the center of resistance.

Therefore otherwise identical force placed different position will have different effects on tooth movement.

To determine how a tooth will move it is useful to evaluate the force system at bracket to determine equivalent force system at center resistance.

For bodily movement the force system at the bracket must be equivalent to a force with moment of couple at the center of resistance.

Equivalent force for bodily movement

The type of movement exhibited by a tooth is determined by the ratio between the magnitude of the couple (M) and the force (F) applied at the bracket.

In order to produce movement other than uncontrolled tipping that is to produce a controlled tipping or translation a single force is insufficient, a rotational tendency and also be applied to the bracket.

The proportion of the rotational tendency (Moment) to the force applied at the bracket will determine the type of tooth movement produced.

Moment To Force Ratio At Bracket

TYPE OF TOOTH MOVEMENTM/FTranslation10/1Control Tipping (about apex)5/1Uncontrolled Tipping (about midroot)0/1Root Movement12/1

Force Constancy

The most desirable type of tooth movement is produced by a relatively constant force in an optimal range. It is important to design the active components of an appliance such that it has the following properties.

Low LDR

LDR depends on Wire Cross Section, length, material and configuration

Frictionless System

To achieve constant force and moment levels sliding frictional forces should be reduced, that impedes the movement of teeth, also these forces alter the predictability of the desired forces on the teeth during retraction process.

Scope of Retraction

Friction

ELASTIC MODLE WITH LIGATURE

ELASTOMERIC CHAINS OR POWER CHAIN

INTRA (OR) INTER MAXILLARY ELASTICS TO KOBAYASHI LIGATURES

COIL SRINGS

STAINLESS STEEL

NITI

J-HOOK HEADGEAR

SLIDING JIG AND TRACTION

MULLIGANS V BEND SLIDING MECHANICS

Frictionless

RICKETTS MAXILLARY & MANDIBULAR CUSPID RETRACTION SPRINGS

POUL GJESSING SPRING

BURSTONE T-LOOP RETRACTION AND ATTRACTION SPRINGS

MARCOTTE SPRING

MODIFIED LINGUAL LEVER ARM

Friction Mechanics

Mechanism of Action of Friction Mechanics

To move a tooth bodily, the force applied has to pass through the center of resistance of the tooth. However as the force is applied at the bracket level of the crown, the concerned tooth experience both force and moment. Moment of force is created in 2 planes of space. One moment tends to rotate the canine mesial out as the force application is buccal to the center of resistance and the other tends to cause distal tipping of the tooth as the point of force application is occlusal to the center of resistance.

The wire bracket interaction tends to counteract this moment by applying an opposite moment. As distal tipping of the crown takes place, the tooth slide along the archwire till binding occurs between the archwire and the bracket. This produces a couple at the bracket which results in distal root movement and hence uprighting of the tooth. As the tooth uprights, the moment decreases until the wire no longer binds, Then the canine retracts along the archwire till distal crown tipping again causes binding. This process is repeated until the tooth is retracted or the force gets depleted.

The variables affecting frictional resistance during tooth movement

Physical

1.Archwire.

a. Material

b. Cross-sectional shape/size

c. Surface texture

d. Stiffness

2. Ligation of archwire to bracket

a. Ligature wires

b. Elastomerics

c. Method of ligation:method of tying, bracket designs to limit force of ligation, self ligating brackets

3. Bracket

a. Material

b. Manufacturing process : cast or sintered stainless steel

c. Slot width and depth

d. Design of bracket : single or twin

e. First-order bend(in-out)

f. Seond-order bend(angulation)

g. Third-order bend(torque)

4. Orthodontic appliance

a. Interbracket appliance

b. Level of bracket slots between adjacent teeth

c. Forces applied for retraction

B. Biological

1. Saliva

2. Plaque

3. Acquired pellicle

4. Corrosion

Effect of Arch Wire

The effect of wire size - An increased wire size is associated with increased (wire friction) in general rectangular produce more friction than round wires. Stiffness is importance since the flexible wires during retraction will result in canine tipping lingually, distally and also in the incisor extrusion. The stiffer wires are less springy and deflect less for a given force. Changing the diameter or length of the wire greatly changes the stiffness. An adequate clearance should be provided between the bracket and the wire to prevent binding.

Arch Wire Material The stainless steel made of 16x22 inch and 17x25 inch wires required the least force to slide. The CO-CR wire required the least force than S.S. wire the nitinol wire required more force than S.S. wire and the beta titanium should be considered as more resistant to tooth movement in a sliding fashion.

Effect of Ligation Technique on Friction

Elastomeric modules tied in a figure of 8 pattern, exhibits more static frictional forces than elastomeric modules tied conventionally, S.S. ligatures, Teflon coated ligatures. Teflon coated ligatures exhibit the least static frictional force.

Self ligating brackets The Orthodontic brackets are now available with the self ligation bracket with highly resilient spring clip. These show lower levels of friction than others. Activa brackets and speed brackets show low arch wire friction.

EFFECT OF BRACKET

Sintered stainless steel brackets produce significantly lower friction than cast stainless steel brackets. Poly crystalline ceramic gives more friction than ceramic-reinforced composite with metal slot insert. Ceramic-reinforced composite without metal slot insert, exhibited very less friction.

Effect of Bracket width

The narrower the bracket the greater the length of the inter bracket wire therefore greater the flexibility of the wire in addition narrow brackets have less control over rotation and tipping.

Biological effect Saliva and saliva substitute serves as an excellent lubricant in the sliding of the bracket along a arch wire.

Retraction of Cuspids in one of the most common employed procedures in orthodontics. It is generally done for the following two purposes.

1.Anchorage Conservation

A common way to improve anchorage control is to pit the resistance of a group of teeth against the ovement of a single tooth, rather than divinding are arch into more or less equal segments. This offers the advantage that lower forces are used for the movement of asingle tooth and reaction forces one dissipated over a large PDL area in the anchor unit, thus discouraging loss of anchorage.

2. Crowding in the anterior segments

In case of moderate to severe crowding in the anterior segments. It is necessary to retract the canines to gain space for aligning the anteriors.

METHODS OF CANINE RETRACTION IN SLIDING MECHANICS

ELASTIC MODULEWITH LIGATURE

ELASTOMERIC CHANIS OR POWER CHAIN

INTRA (0R) INTER MAXILLARY ELASTICS TO KOBAYASHI LIGATURES

COIL SRINGS

STAINLESS STEEL

NITI

J-HOOK-HEADGEAR

SLIDING JIG AND TRACTION

MULLIGANS V BEND SLIDING MECHANICS

ELASTIC MODULE WITH LIGATURE

This method of retraction has been popularised by Bennett and Mclaughlin. A single elastic module of the type used to secure archwires to brackets is attached to the canine by ligature wires extending from the molar. These elastic tiebacks are activated 2-3 mm or to twice their original size to generate approximately 100-150 gms of force. Provided that the arches are properly levelled this light force allows for effective retraction with minimal tipping of teeth and maintenance of arch levelling. Bennett and McLaughlin found .019 x .025 rectangular wires in .022 slots to be most effective, providing maximum rigidity while allowing adequate freedom for sliding.

ELASTIC CHAINS

These were introduced into the dental profession in the 1960s and are used in many orthodontic practices for canine retraction, diastema closure, rotation correction and arch constriction.

Configurations

Elastomeric chains are available in 3 configurations:

Closed loop chain

Short filament chain

Long filament chain

Long filament chains generally deliver a lower initial force and exhibit a greater rate of force decay at the same extension.

Advantages

Inexpensive

Relatively hygienic

Easily applied without archwire removal

Not dependent on patient cooperation

Since molar anchorage is being reinforced, both palatal and buccal elastic traction may be applied.

In combination with the direct headgear system. The headgear wear for anchorage support does not as a rule need to be so intensive.

Disadvantages

When extended and exposed to the oral environment, they absorb water and saliva.

Permanent staining occurs after a few days in the oral cavity.

Stretching causes breakdown of internal bonds leading to permanent deformation.

Stress relocation leads to loss of force and hence, gradual loss of effectiveness.

It gives variable force.

The Element of friction and binding as the archwire flexes under force can occur, especially if the canine starts at an unfavorable angulation.

Prestretching

It is advisable to stretch or work the elastic module prior to activation to achieve the desired force levels, to overcome the problem or rapid force decay rate and provide for a more constant and consistent force delivery.

Environmental Factors

Tooth movement, pH and temperature changes, fluoride rinses, salivary enzymes and masticatory forces have all been associated with deformation, force degradation and relaxation behaviour of elastomeric chains.

Force Degradation

Most of the elastomeric chains generally loose 50% - 70% of their initial force during the 1st day of load application and at 3 weeks retain only 30 40% of their original force.

Some of the chains extended 100% of the original length produce force levels in excess of 450gms, leading researches to recommend and extension of 50% to 70%. Other chains when distracted 100% produce force levels of 300 grams. In view of the wide variation of initial force levels of different types of power chains, the prudent practitioner should employ a force gauge to determine the desired initial force. When just applied a power chain produce a force of approximately 250 300 gms.

4. INTRA OR INTERMAXILLARY ELASTICS TO KOBAYASHI LIGATURES

Kobayashi ligatures : are stainless steel ligatures incorporating a welded hook, for the inter or intra-maxillary elastics. They are loosely tied to the canine bracket and can be left as a hook pointing mesially (or) can be tied so that the hook faces distally, and is then bent forwards.

-Useful for applying light forces to tip the canine distally along a thin flexible archwire. Not for already upright canine, requiring further bodily movement.

Not an effective method of applying traction to slide the canine bodily along the arch wire, because the strong elastics required for bodily movement cause rotation and excessive binding of the canine.

5. COIL SPRINGS

Coil springs were introduced to the orthodontics world as early as 1931. During the manufacturing process, the material is subjected to winding that includes tensional and torsional components and hence spring properties may be slightly different from the wires made from the same materials. The various materials that have been used for making springs are;

i.Stainless Steel

ii.NiTi

i.Stainless Steel Coil Spring

Stainless steel coil springs are efficient methods of canine retraction. They apply more predictable levels of force compared to elastic based systems described before.

However, stainless steel springs have a relatively higher load deflection rate compared to some other material springs like NiTi springs. So, as the space starts to close, there is some force degradation due to lessening activation. The amount of activation has to be monitored to maintain ideal force levels.

Stainless Steel Coil spring normally used is made of 0.2 mm hard stainless steel wire wound on to an 0.8mm diameter wire mandrel.

Can be used in a variety of ways.

Coil spring threaded on to the archwire and compressed between the two canine bracket.

Coil spring compressed between a soldered stop on the arch wire and the canine bracket.

Coil spring compressed between an incisor bracket and the canine.

Coil spring compressed by a tie-back ligature.

NiTi Closed Coil Springs

Nickel titanium alloys were introduced to the dental profession by William P.Bleuer in the 1960s. He demonstrated the unique combination of properties of shape memory and super elasticity in addition to low modulus of elasticity, moderately high strength, high resilience and less corrosion. The concept of Nickel Titanium coil springs was introduced in 1979. The force degradation is very less due to the low load deflection rate. They deliver constant amount of force till they reach the terminal end of deactivation stage. They are available in lengths of 9mm and 11mm.

Advantages

Can be easily placed and removed without archwire removal

Do not need to be reactivated at each appoinment.

Patient co-operation not required.

NiTi springs generally achieve faster and more consistent space closure than elastomeric modules.

Disadvantage

1.Relatively unhygienic compared to elastic force systems.

Inhibitors to Canine sliding Retraction

Inadequate levelling resulting in archwire binding.

Damaged or crushed brackets causing archwire binding.

Soft tissue build up in extraction sites.

Cortical plate resistance (Narrowing of alveolar bone in retraction sites).

Excessive forces causing tipping and binding.

Occlusal interferences.

Insufficient or inconsistent force.

EXTRA-ORAL TRACTION

J-HOOK-HEADGEAR

During retraction of canines, the maintenance of posterior tooth position has always been a major concern for the orthodontists, mainly in those cases in which maximum anchorage is needed.

One of the methods for accomplishing distal movement of canines without loosing posterior anchorage is J-hook-headgear. It involves the use of headgear with J-hooks where the hooks attach along a continuous arch wire mesial to the canines and exert a force over them so that they will slide along the arch wire. Since it incorporates extra oral anchorage in canine retraction, it should be effective in maximum anchorage cases.

J-hook headgear, either of the straight pull or high pull type is cl8ipped on the archwire mesial to the canines to slide them distally.

Straight pull headgear allows swifter canine retraction than the high pull type. However, this may cause anterior extrusion (Perej et al., 1980; Hickham 1974) and unfavourable occlusal plane rotations (Bowden 1978). This might specially be a problem in high maxillomandibular angle cases.

High pull headgear may cause more bodily retraction and also aid in bite opening (intrusion). However, it is not as efficient for distal movement, needing prolonged periods of wear for modest results.

During the retraction, direction of force may be varied between straight pull and high pull according to the individual requirements of the case.

Advantages

Extremely conservative of anchorage. Additional molar support by head gear may be done.

Sympathetic over jet reduction might occur during canine retraction due to the distal force and binding of the archwire.

Can be applied to both upper and lower arches simultaneously if anchorage is at a premium by the use of Hickhams directional headgear system (1974).

Disadvantages

As force application is intermittent this is slower than other methods of canine retraction. This is specially true for high pull headgear, where the true retractive force on the canines may be quite modest.

Highly dependent on patient co-operation.

The molar and buccal segment correction is usually a later event in treatment compared to other systems.

Canine tipping and anterior extrusion can occur with th straight pull headgear.

7. SLIDING JIG AND TRACTION

A jig made of either (0.22 in.) round wire or (0.017 in. x 0.022 in.) rectangular wire, is slid on to the archwire in addition to a short piece of open coil spring of about 4mm in length. The coil spring lies in contact with the mesial surface of the canine bracket and the circle of the jig rests against the other end of the coil spring. Thus when traction is applied to the hook of the jig the coil spring is compressed against the canine, pushing it distally. The traction can be applied to the jig by either intra-or intermaxillary elastics, or by extra-oral traction. The diameter of the archwire should be close to the bracket size. Thin flexible archwires are not satisfactory for this technique.

Advantages

It has the advantage that the force exerted by the elastics is directed along the archwire and thus bodily movement of the canine is possible

Disadvantages

The jigs are quite difficult to lubricate and their length must be correct so that the canine is free to move back. They can rotate around the archwire and become caught under the bracket. They are fairly bulky and thus act as a food trap and an irritant to the cheek.

MULLIGANS V BEND SLIDING MECHANICS

V bend sliding mechanics was introduced by Mulligan in the 1970s. The basic principal was to apply differential moments to the teets via bends in the continuous archwire while force for retraction was applied by auxiliaries like elastic chain, coil spring etc.

This is a variation of the normal sliding of the canine along the continuous wire. In the .018 slot, .016 SS wire is used for retraction while in the .022 slot, .016, .018 or .020 wires may be used.

Lewis or other brackets with antirotation wings should be used for minimizing and subsequently correcting distal-in canine rotation. The archwire is not tied into incisor brackets during cuspid retraction. Engaging incisors reduces the distal root moment placed in the canine, resulting in excessive mesial root movement, and laos incisors flaring. So, either the incisors are not bonded or the archwire stepped gingival to the incisor brackets during cuspid retraction.

The wire is tied in for 4 to 6 weeks for alignment. The 450C V bends are added to the wire and 200 gms of force are applied between the canine and the molar. The V bends are placed without removing the archwire using calibrated optic pliers.

The purpose of the V bend is to allow differential mediodistal moments on the canines and molars. If the bend is placed off centre it creates a short and along segment. The shorter segment is more rigid and hence applies greater moments. So, if maximal canine retraction is required the bend is placed very close to molar and the bicuspids are not banded. This causes a strong distal crown moment on the molar which counteracts the auxiliary force tending to move the molar crown forward.

Thus this helps in reinforcing anchorage. On the other hand, the longer span of wire towards the canine, thought applying a moment to keep the canine upright, allows some tipping to occur as the moment is less. Thus the canine gets retracted by tipping and uprighting. As the canine retracts, the bend goes on becoming less off center and mesial crown uprighting moments on the canine increase. After closure of space, a bend may be placed just distal to the canine and the 2nd premolars banded. This allows equal and opposite moments on both the canine and the molar and thus allows root uprighting.

2nd premolars can be included at the start of retraction and a bend placed just mesial to them. This also allows the offset V bends to apply differential moments. However this is not as effective in anchorage conservation as placing the bend next to the molar because the amount of offset is less. In these cases the offset position is naturally eliminated as space closure occurs. When space closure is completed the V bend is centered which allows root parallelism.

FRICTIONLESS MECHANICS

Retraction is accomplished with loops (or) springs, which offer more controlled tooth movement than sliding mechanism. The force of a retraction spring is applied by pulling the distal and through the molar tube and cinching it back.

BIOMECHANICS OF FRICTIONLESS MECHANICS

The force system of an orthodontic appliance determine the type of tooth movement expressed. The forces act in all three planes of space (first, second, or third order) Most space closure concerns are second order, or the sagittal view. The components of any force system are

Alpha moment: This is the moment acting on the anterior teeth (often termed anterior torque)

Beta moment: These are the mesio-distal forces acting on the teeth. The distal forces acting on the anterior teeth always equal the mesial forces acting on the posterior teeth.

Vertical forces: These are intrusive-extrusive forces acting on the anterior or posterior teeth. These forces generally result from unequal alpha and beta moments. When the beta moments is greater than the beta moment, extrusive forces act on the anterior teeth while intrusive forces act on the posterior teeth. The magnitude of the vertical forces is dependant on the difference between the moments and interbracket distance (for equivalent alpha-beta moment difference).

Differential anchorage is obtained by the application of unequal alpha and beta moments. The higher moment is applied to the anchorage teeth. The differential moments are obtained by applying the concept of the off-centre V-bend. An off-centre V-bend in a wire result in unequal moments. The closure the V-bend to a tooth or set of teeth, the higher the applied moment. A simplistic model for envisioning this force system is to consider the V apex is to a bracket, the shorter the wire the further the distance of V-apex to the bracket, the longer the wire , a shorter wire has a higher banding moment than a longer wire. Therefore, a higher moment acts on the bracket closer to the V-bend than the more distant bracket.

When a retraction spring is used, two moments control gvertifcal and anchorage forces. The alpha moment produces distal root movement of the anterior teeth, while the beta moment produces mesial root movment of the posterior teeth.

By varying the magnitude of these moments, differential movement of the posterior and anterior segments can be achieved. However, if the alpha and beta moments are unequal, vertical forces are also generated.

If the beta moment is greater than the alpha moment, anchorage is enhanced by the mesial root moment of the posterior segment, and there is a net intrusive force on the anterior teeth. If the alpha moment is greater, the anchorage of the anterior segment is increased, and there is a net extrusive force on the anterior segment. If the alpha and beta moments are equal in magnitude, no vertical forces are generated.

The distance that the anterior and posterior segments are to be moved depends on factors such as the degree of crowding, the soft-tissue profile, and the molar relationship. The amount of anterior retraction or posterior protraction needed should be determined before a loop is designed.

If only anterior retraction is necessary, the retraction loop should be placed closer to the canine than to the molar, and a globe bend should be added near the molar. A gable bend that is larger in the posterior dimension will produce a larger beta moment, thus increasing posterior anchorage.

For both retraction of the anterior segment and protraction of the posterior segment, the loop should be placed midway between the posterior and anterior segments. A gable bend of equal dimensions should be used, so that the alpha and beta moments are equal and reciprocal space closure occurs.

When only posterior protraction is desired, the loop should be located closer to the posterior segment, and an anterior gable bend should be placed with a greater alpha moment than beta moment, making the anterior teeth the anchorage segment.

Regardless of the initial magnitudes of the alpha and beta moments, changes in magnitude will occur during retraction. As the anterior teeth are retracted, the enhancing posterior anchorage. Also, as the beta moment becomes relatively greater, there is a greater intrusive force on the anterior teeth and a greater extrusive forces on the posterior teeth.

Concurrent with the decrease in both alpha and beta moments, there is an increase in the moment-to-force ratio of the retraction spring, resulting from the lower applied force produced by tooth movement. Since the M/F ratio increased as the spring deactivates, thee spring should not be reactivated too often. Frequent reactivation will not allow the spring to achieve a high enough M/F ration to produce translation.

METHODS OF CANINE RETRACTION IN FRICTIONLESS MECHANICS

RICKETTS MAXILLARY & MANDIBULAR CUSPID RETRACTION SPRINGS

POUL GJESSING SPRING

BURSTONE T-LOOP RETRACTION AND ATTRACTION SPRINGS

MARCOTTE SPRING

MODIFIED LINGUAL LEVER ARM

RICKETTS MAXILLARY & MANDIBULAR CUSPID RETRACTION SPRINGS

These retractors were made to be used in 0.018 bracket slot.

Maxillary Cuspid retractor

This is a combination of a double verticle closed helix & an extended crossed T, resulting in a loop design measuring 70mm of wire made of blue Elgiloy wire which delivered about 30-50gms of force per mm of activation.

Advantage

Rapid closure results from its use and only a few weeks of wearing is necessary.

Disadvantage

Bulky & irritating to soft tissues.

Mandibular Cuspid retractor

Since this large extended loop, was difficult to use in the lower arch, due to the fact, that it would extent into the chewing area.

The wire had to be altered at the site of the bridge so it was rolled and flattened, so that lower value could be delivered in that section more comparable to that of the upper. The mandibular cuspid retractor is a compound spring-a double vertical helical closing loop. The lower retractors can be used in upper, in case of patients with a very low sulcus but it delivers more than 50 gms per mm of activation.

It contains 60mm of wire 16x16 blue elgiloy and produces approximately 75 gms of force per mm of activation. A range of variation exists due to loop size and character of wire. Therefore 2 3mm of activation is required to produce the desired force.

Precaution to be used with these springs while activation

1.On initial placement an approximately 90o gable bend is essential so that both the canine and molar do not tip excessively.

Activation in the upper arch is 3 4 mm at each adjustment. This would mean pulling the wire through the tube and locking it with a simple bend.

In older adults, the activation is 1mm at the 1st inspection and subsequently 2 3mm in order to minimize the force.

In the lower arch the bone channel is too narrow for the cuspid root apex. So the initial force is towards the buccal and then slowly around the corner of the arch

The lower cuspid retractor has slightly more force per mm of activation than has the upper. So in adult patient initially 1mm, then 2-2.5mm of activation can be made.

For maximum anchorage cases, preparation in the form of Nance holding arch, lingual arch or utility arch is desired.

2. THE POUL GJESSING CANINE RETRACTION SPRING

The PG maxillary canine retraction spring was described by its originator Poul Gjessing in 1985. Essentially the spring consist of a double ovoid helix with a smaller occulsally placed helix, and is in the preformed version, available commercially constructed in 0.016 x 0.022 inch stainless steel wire..

Spring Design

The spring design made from 0.016 x 0.022 inch stainless steel wire. The predominant active element is the ovoid double helix loop extending 10mm apically and width 5.5 mm. It is included in order to reduce the load/deflection of the spring and is placed gingivally so that activation will cause a tipping of the short horizontal arm (attached to the canine) in a direction that will increase the couple acting on the tooth. Height is limited by practical considerations, so that a double loop is necessary to incorporate sufficient wire. The gently rounded form avoids the effect of sharp bends on load/deflection and, through the use of the greatest amount of wire in the vertical direction, reduction of horizontal load/deflection is maximized. At the same time, minimizing horizontal wire increases rigidity in the vertical plane. The smaller loop occlusally is incorporated to lower levels of activation on insertion in the brackets in the short arm (couple) and is formed so that activation further closes the loops.

The mesial and distal extensions of the looped wire segment are angulated both in the vertical and in the horizontal plane. When the spring is in place, but prior to activation of the driving force static antitip and antirotation couples will be exerted to the canine. The distal driving force is generated by pulling the distal, horizontal leg through the molar tube. A desirable force level of approximately 160 gm is obtained when the two sections of the double helix are separated 1mm. During the activation the force is matched by an additional couple (activation couple) arising from the double-helix loop which in theory, acts as four level arms.

Incorporation of a segment of a circle (sweep) in the distal leg of the spring in an adjustment with the purpose of eliminating undesirable beta moments acting at the second premolar bracket and tending to move the root apex too far mesially.

Measurements

Alpha bend: 15o

Beta bend: 12o for IInd Pre-molar and 30o for Ist Molar

Anti-Rotation bend: 35o

The ideal retraction spring should deliver a light continuous force of suitable duration and apply couples of suitable magnitude to prevent tilting and rotation occuring as the tooth retracts.

The PG retraction system has been designed to facilitate segmented treatment for extraction cases. The basic element of the system which is available in right and left versions is a prefabricated highly standardized, stainless steel retraction spring that is adjusted to fit both 0.018 and 0.022 edge wise appliance.

The PG universal retraction spring is designed for controlled retraction of either canines or upper incisors. No clinical alterations of the spring is needed and force system produced is independent of inter bracket distance. The spring is precalculated to deliver predictable M/F in three planes of space. The magnitude of the force delivered, which is kept within desirable physiological limits can be identified by reading the morphology of the spring during activation.

Clinical Application

Alignment of the Buccal Teeth

The spring is constructed to resist tendencies for tipping and rotation during canine retraction, not to correct existing rotations or extreme deviation in inclination. Therefore the buccal segment, including the canine, second premolar, first molar and eventually second molar must be leveled prior to the insertion of the spring.

Adjustment of Faciolingual loop inclination

The correct facio lingual position of the spring is obtained by adjusting the anterior and posterior extensions before insertion.

Bracket Engagement

The anterior extension of the spring is engaged in the canine bracket. The posterior extension must be engaged in both premolar and molar brackets to obtain optimum transverse control of the canine and alignment of the canine, premolar and molar. The anterior extension is pulled mesially until the small circular helix contacts the distal aspect of the canine brackets and the wire is secured by bending the anterior extension gigivally.

Activation

The spring is activated by pulling distal to the molar tube until the two loops separate. The wire is secured with the gingival bend in the posterior extension. Reactivation in the initial spring configuration should be done every 4 to 6 weeks.

This amount of activation produces the recommended initial load of 100gms. It is critical to avoid over activation of the spring, because a few mm of over activation can result in anchor loss. Since the average distance from the centers of the brackets to the CR are identical for the upper and lower canines, the PG retraction spring works equally well for canine retraction in either arch.

Deviations in anatomy or root inclination or improper clinical manipulation of the spring may result in steepening of the mesiodistal inclination of the retracted canine. This can be corrected by uprighting the tooth after retraction. The spring is modified in the mouth by placing a V bendin the buccal loop with the three prong plier, thus increasing the alpha moment to about 1500gms/mm which appears to be ideal for uprighting.

Minor rotations of the canine may also be noted in rare instances. They are easily corrected after retraction with lingual elastics.

Without additional anchorage support, the second premolar and the first molar can be expected to migrate mesially as the canine is retracted. Such mesial migration of the anchorage unit is oftern desirable, but in critical, cases it may be necessary to use a transpalatal arch and extraoral traction.

Clinical evaluation of the PG universal retraction spring has shown that the mesial movement of the anchorage unit takes place as a translation. The anchorage group as a whole is not affected by unwanted side effect such as extrusion and rotation. Therefore the magnitude of the couples and extrusive force generated by the posterior extension of the spring should be more than offset by neuromuscular forces of occlusion. Clinical evaluation of PG spring, in common with other retraction spring, is made difficult by individual patient variability.

Advantage

In general it has been found that the spring tends to tip the maxillary canine distally during retraction, unless the canine arm is gabled to approximately 45o. Although gabling by this amount should theoretically produce an excessively high N/P ratio during deactivation which would cause the canine to tip mesially. It has been found clinically that this does not occur because the spring distorts on insertion thereby reducing the gable angle. Excessive stress relaxation and undesirable large retraction forces can be produced by over retraction. For this reason activation should be confined to 1mm as recommended by Gjessing. Clinical observation, however, suggests that the recommended amount of lateral curvature helps to minimize this tendency.

Disadvantage

The PG spring is bulky, and the depth of the buccal sulcus limits the height of the pear. It should be noted that, as with other retraction components, a reduction in the heights of the spring will not only significantly decrease the flexibility but will also reduce the antitilt moment/force ratio.

BURSTONE T-LOOP RETRACTION & ATTRACTION SPRINGS

The Burstone T loop 0.017 x 0.025 T.M.A. composite retraction spring is used in Group A arches and the attraction spring is employed in Group B & C arches.

The difference lies in rotational control of the canine, which is achieved with a non-sliding mechanism. Antirotation bends are placed in the retraction assemblies to prevent the canine from rotation as it retracts.

It is also possible to use an arch wire to prevent rotation.

The Burstone composite T loop retraction spring is made from 0.017 x 0.025 inch TMA wire.

The basic element of the spring is a prefabricated highly standardized universal spring, which could be used on both the right and left sides. These prefabricated versions have to be preactivated as per a prescribed template.

Subsequent to pre-activation, the spring is precaliberated to deliver predictable moment to force ratios. The magnitude of the force delivered is identified by reading the horizontal separation of the vertical legs of the T loop.

Initially the M/F ratios are approximately 6-8 which produce controlled tipping. As the space closes and the spring deactivates, the force level delivered by the spring decreases at a much faster rate than the moments. This cause both Ma/F and Mb/F ratios to increase. These ratios soon become 10, where translation will occur.

Further deactivation increases the M/F ratio to 12 and teeth might undergo root movement.

As can be seen, overall translation is accomplished by combination of controlled tipping, translation and the root movement. Therefore, it is important that this attraction spring not be reactivated too soon. If reactivated too frequently the teeth would undergo only tipping. M/F ration must be allowed to increase sufficiently to allow root movement to occur.

Because there might be slight difference in the magnitude of the alpha and beta moments, vertical forces are exerted on the teeth, however, throughout the entire activation, the vertical forces produced are not clinically significant.

As previously stated, the M/F ratios must be allowed to increase sufficiently for translation to occur. Allowing the spring to deactivate approximately 2.5mm may give the proper M/F ratios to accomplish the desired movement.

Clinically by checking the position of the bracket slots to each other, one can determine whether it it time to reactivate the attraction spring. As soon as the bracket slots have the same mesiodistal angulation as at the start of the movement the spring can be reactivated.

The 0.017 x 0.025 symmetric T-Loop is available two heights a regular or long height is normally preferred while a short one is used when vestibular depth is a critical factor. This short spring is activated 3.0mm to 4.0mm. Like the 0.017 x 0.025 spring, the M/F ratio must be allowed to increase sufficiently for all necessary movements to occur to obtain translation. Again, position of bracket slots to each other and to the treatment occlusal plane is used to determine clinically when reactivation is necessary.

Spring Preactivation

Before the spring is inserted in the moth it needs to be preactivated as follows:

1.Curvature is bent in the occlusal part of the spring. This part of the spring may deform during activation and therefore needs to be over bent and followed by a trial activation. Then remove the excess curvature.

2.Open the ears in the gingival part of the T-spring and add some angle to the occlusal vertical arms so that the neutral position is correct. Then trial activate.

3.Recheck the T-spring on the template which is a guide for the required angulation.

To determine the amount of distal activation, the following formula is used to establish the spring length.

L = I.T.D. Activation.

Where,

I.T.D. is the inter tube distance from the mesial of the molar tube to the canine auxiliary tube.

Activation is the amount of the activation of the spring

L is the spring length, where B = L/2 i.e B is the distance from the center of the T-loop to the 90 degree band at the canine bracket position (alpha position).

Simultaneously applying a force from the lingual

By bonding a lingual button and use of elastics which is changed daily at a force level equal to that of a T-spring at half activation(3mm). So a toal distal force is applied, half from each buccal and lingual side. But patient compliance is an important consideration.

Placing antirotation bends in the attraction spring

Antirotation bends can be placed in the following areas the ears, the vertical arms, and to a small extent to the horizontal arms producing an angel of 120 degrees.

Using a optimally stiff buccal arch wire

Placing a cupid to cuspid stabilizing segment

Intrusive and extrusive forces result from the difference in the alpha and beta moments. As the force is applied from the buccal aspect, buccal movement of canine crown on intrusion and lingual movement on extrusion occurs. A cuspid to cuspid bypass wire effectively prevents rotation, actively derotates teeth when there is sufficient space, alters arch width and eliminate side effects from vertical forces.

Canine root movement

If the canines are retracted separately they may require a phase of root uprighting after space closure if controlled tipping has occurred. In deciding the type of root movement required the lateral, intraoral, 45 degree cephalograms are consulted.

Canine root movements are achieved by root springs, although the basic mechanism involved is the same for both enmass root movement of anterior segment and individual canine and incisor root movements.

One method is to use a bypass arch wire plus root spring. The bypass wire is a rigid continuous arch stepped occlusally to thecuspid and tied or cinched back distal to the last molar. A figure eight tie is a good precaution to prevent the spaces from opening. The root spring can be a cantilever made of 0.017 x 0.025 TMA wire. Another approach is to weld a 0.018 TMA loop onto the cuspid bypass.

Equal and opposite moments and horizontal forces should act on the canine and the anchorage unit. In case of a cantilever there will be a small vertical force extruding the canine and intruding the rest of the arch. As soon as the canine bracket hits the archwire, there will be equal and opposite moments. The horizontal forces can cause loss of anchorage and in critical situations head gears may have to be used. In addition a root spring moment has to be kept as low as possible to maintain anchorage.

Another method uses a posterior segment with a rectangular loop. It can be used if the anterior are not well aligned to fabricate a full arch wire with a bypass.

MARCOTTE SPRING

This is a type of minor cuspid retraction spring and is small, light 0.016 inch closing loop.

This spring extends from the auxiliary tube of the 1st molar bracket to the bracket on the cuspid, and is activated by being pulled throng the auxiliary tube and cinched. The buccal segment feels, then a protractive force and a positive moment, while the tooth is being walked back on the wire. Hence are used in Group B or C Arches. Activation should be limited to 1 2mm.

5.MODIFIED LINGUAL LEVER ARM TECHNIQUE

The adaptability of the periodontal tissue to orthodontic forces declines with age. Therefore, any adult patient should undergo periodontal examination prior to and during orthodontic treatment and adult mehanotherapy should involve pure bodily tooth movement to avoid overloading of the periodontal tissues.

A single force at the bracket slot level produces uncontrolled tipping a rotation around the center of resistance. This generates high periodontal stress levels at the apex and alveolar crest, which may result in apical root resorption or periodontal trauma. Rotation around the long axis of the tooth will also occur. To achieve direct translation, a single force directed at the center of resistance is needed.

Holographic examination has shown that the center of resistance of a single rooted tooth is located about 40 percent of the distance from the alveolar crest to the apex. The exact location is influenced by factors such as root length, marginal bone level, and characteristics of the periodontal ligament. With a normal bone level, the distance between the bracket slot and the center of resistance is about 10mm.

Appliance Design

The original lingual lever arms were complicated and expensive to construct, being made of cast precious metal extended across the palatal vault a simple alternative, is used now.

An .032 stainless steel spring wire is soldered to a bonding pad or a band. A hook is bent in the wire 20mm from thepad.

The lever arm is adapted to the palatal vault and bonded to the lingual surface of the tooth to be moved (usually a cuspid or premolar) at the same height as the bracket on the buccal side. Scotchbond should be used, in addition to the customary bonding adhesive, to resist the shearing forces that occur when loading the lever arm.

Two elastic chains or superelastic closed coil springs (Sentally Blue) are used as a power source; one is stretched buccally between the cuspid or premolar bracket and the molar tube at crown level and the other is stretched palatally from the lever arm to an extension soldered on a transpalatal bar. Sentalloy springs deliver constant forces of about 120 140g over a 4.5mm range.

The tip of the lever arm will thus be located at approximately the apical level of the bicuspid. In the vertical dimension, the distance between the hook of the lever arm and the center of resistance equals the distance between the buccal bracket and the center of resistance. The moments of the two equal force cancel each other out, producing a net force directed at the center of resistance.

In the anteroposterior dimension, there is a small difference between the distance from the tip of the lever arm to the center of resistance and the distance from the bonded bracket to the center of resistance.

This produces a slight rotation, which is desirable if the tooth was rotated out distally before treatment.

An .016 x .022 stainless steel guiding archwire should be used. Because translation will occur, tooth movement can be accomplished without significant loss of force due to bracket friction.

COMPARATIVE STUDIES

Comparison of NiTi Coil springs Vs. elastics in Canine retraction Andrew L. Sonis, DMD J.C.O. 1994 volume XXVIII Numbers

The study was designed to compare the canine retraction rates of conventional elastics and nickel titanium coil springs, using a continuous archwire system.

The ideal force delivery system would meet the following criteria:

Provide optimal tooth-moving forces that elicit the desired effects.Be comfortable and hygienic for the patient.Require minimal operator manipulation and chairtime.Require minimal patient cooperation.Be economical.

Elastomeric auxiliaries are relatively consistent in producing tooth movements, but have several drawbacks. To compensate for the high and rapid decay rates of elastomeris, initial forces must often be greater than is desirable. In addition, patients must be seen frequently for change of elastomeric to ensure that the force remains adequate.

Nickel titanium coil springs have been shown to produce a constant force over varying lengths, with no decay. They may be able to meet all the above criteria for an ideal force delivery system.

Nickel titanium closed coil springs produced nearly twice as rapid a rate of tooth movement as conventional elastics along a stainless steel arch wire rated at about the same force level. This discrepancy is probably due to two factors: the ability of the springs to maintain a relatively constant force level compared to the elastics, and the elimination of the need for patient cooperation.

Invitro studies have shown that a nickel titanium closed coil spring can be stretched as much as 500% without permanent deformation, while still delivering a nearly constant force. On the other hand, latex elastics suffer significant and rapid force decay as the space closes and the length of the elastic decreases, the applied force decreases even more.

3. A comparison of the rate of space closure using a nickel titanium spring and an elastic module: A clinical study

R.H.A. Samuels, MD Sci., M.Orth., et al., AJO 1993 (464 467)

A study of the efficiency of space closure after premolar extraction was undertaken, comparing a nickel titanium closed coil spring and an elastic retraction module by using sliding mechanics along an 0.019 x 0.025 inch stainless steel arch wire in 0.022, inch preadjusted stainless steel brackets.

The particular property of super elastic nickel titanium in producinga light continuous force over a long range of action was compared with previously available materials.

Inference

1.The use of super-elastic nickel titanium coil springs resulted in a significantly greater and more consistent rate of space closure than elastic modules.

2.When examined clinically, there was no difference in tooth position produced by the two systems after space closure.

4. A clinical study of maxillary canine retraction with a retract spring and with sliding mechanics Peter Ziegler, DDS, et al., Am. J.orthod.DentoFac.orthop.1989;95:99-106

The efficiency of maxillary canine retraction by means of sliding mechanics along an 0.018 inch labial arch and an Alastil chain was compared with that using the canine retraction spring designed by Gjessing.

The rate of canine retraction and degree of tipping, and rotation of the canine were studied in 21 subjects by one of these two methods on either side of the dental arch. Measurements were made in the mouth and on photographs of dental casts. The canine was retracted faster and with less distal tipping with the spring than with the sliding mechanics. The canine retraction spring was not superior to the sliding mechanics in controlling canine rotation during the retraction.

RECENT ADVANCES

1.Canine retraction with rare earth magnets: An investigation into the validity of the constant force hypothesis (Am. J. Orthod Dentofac Orthop 1996; 109:489-95), John Daskalogiannakis, DDs, MSc, et al.

The objective of this study was to test the hypothesis that a prolonged constant force provides more effective tooth movement than an impulsive force of short duration. Six human subjects were selected, the mail criterion being a need for extraction of their upper first premolars. Canine retraction on these subjects was executed on one side with the application of a force rapidly declining in magnitude, produced by a vertical loop, and on the other side with the application of a relatively constant force. This type f force was achieved by a similar vertical loop which was constantly activated by three parylene coated neodymium-iron-boron (Nd2Fe14P) block magnets. The vertical loop on the control side was reactivated 6 weeks after the initial activation. No reactivation was necessary on the experimental side for the duration of the experiment. The rate of tooth movement on the two sides was compared over a period of 3 months.

Inference

On the basis of maxillary impression taken at frequent intervals during the course of the study, the canines retracted with a constant force moved statistically significantly more that the control canines during the experimental period. The average differences in the mean rates of tooth movement between the two sides were in the order of 2:1 in favour of the experimental side. There were no statistically significant differences in the changes of angulation (tipping) or rotation about they axis between the two sides. The duration of force application seems to be a critical factor in regulating rate of tooth movement. Conversely, magnitude of the applied force did not appear to be of primary significance.

2. Nickel Titanium Canine Retraction

A new nickel titanium retraction spring that incorporates a simple vertical closing loop with antitip and antirotation bends. The major advantage of this spring is the ability to use it without a preliminary leveling stage, because it can simultaneously retract the canines and level the posterior teeth. Its light, coninuous force allows an activation of a such as 10mm to complete canine retraction without reactivation of the closing loop.

Canine retraction spring made from .016 x .022 Titanium wire, with antirotation bends incorporated in closing loop.

Because it is impossible to maintain normal plier bends in a nickel titanium wire, the vertical closing loop and antitip and anti-rotation bends were momorized by heat-treating the wire in an electric oven. An .016 x .022 Titanium wire was contoured with a three-prong plier, embedded in a heat resistant plaster to maintai9n its shape, and heat treated for 15 minutes at 550oC, according to Ohuras method.

CONCLUSION

Depending upon the techniques employed a number of procedures are used for the retraction of canine, in the treatment of extraction cases.

Some mechanics employed may create compression or tension and use an arch wire, for control , allowing the canine to move. Unfortunately with such mechanics, high moments are produced which can inhibit its movements.

No single technique suits every situation because each technique has its limitation. Thus the individual operator must choose the method preference to treat malocclusion, which require, bodily movement or rotation of teeth with minimal time, to produce an aesthetic and functional and near ideal occlusion as possible.