Saturday, October 29, 2011

A Note on Active Components of removable orthodontic appliances

Active components

The active components comprise of springs, bows,screws and elastics. Springs and bows are made of hard drawn stainless steel wire.  In situations where teeth to be moved are also included in retention and anchorage screws are used. Elastics are used intra orally in cases where traction of individual teeth required and extra orally when head gear is used for reinforcement of anchorage or for distal movement of teeth.

Spring design

Palatally approaching springs are made of 0.5mm stainless steel wire as wires lesser in thickness than this would get distorted when the child handle the appliance.
Palatal cantilever spring also should be as  long as possible to obtain maximum flexibility. However, dimensions of the oral cavity limits the maximum length that can be achieved. A coil of 0.3mm internal diameter could be incorporated to increase the effective length of the wire used for construction of the spring. It is better if the coil is wound in such a way it unwinds as the tooth moves. Because the elastic properties of the spring is superior if Bauschinger effect is used during activation.  A palatal spring may be unstable in the vertical direction so that it can get deflected vertically towards the palate causing difficulty in insertion of the appliance or causing trauma to the palate. The stability of the spring could be improved by incorporating a guard wire on the palatal surface of the spring and boxing the spring with acrylic on its oral side.

Buccaly approaching spring
If a canine is buccally placed it is difficult to approach the correct point of contact with a palatally approaching spring. In such cases, it is important to make a buccally approaching spring.  If a buccaly approaching spring is made of 0.5mm wire it will not be stable in the labial sulcus. Stability could be improved by if 0.5mm wire could be sheathed with stainless steel tubing which has 0.5mm internal diameter. The tubing is then incorporated into the base plate. This improves the stability of the spring. Such supported springs are used for canine retraction (supported buccal canine retractor) and for over jet reduction (Robert’s retractor). 

Buccaly approaching springs and bows can be made of 0.7mm wire they have adequate stability but they are extremely rigid. (Self supported buccal canine retractors and labial bows made of 0.7mm wire) so even a 1mm activation may apply a heavy force on the teeth.

Single cantilever spring ( finger spring)
Single cantilever spring ( finger spring)
Is made of 0.5mm wire. A coil of 3mm internal diameter is incorporated into the spring close to its point of emergence from the base plate. This increases the effective length of the wire improving its flexibility. 

When palatal cantilever spring is constructed it is necessary to determine the correct path of movement and then the required point of contact should be marked on the model. A line is drawn on the model at right angle to the path of tooth movement and through the mid crown width of the tooth. This will indicate the correct position of the coil. Arm of the spring should be either straight or cranked to make sure the tangent drawn at the point of contact is perpendicular to the desired path of movement of the tooth. The free end of the wire should be finished after the base pate has been processed. A palatal spring is boxed to protect it from damage so that it lies in the recess between the base plate and the mucosa.  If the spring catches between teeth during removal it may get distorted in such situations if the spring is used for movement of the tooth along the arch distortion can b prevented 
By incorporating a guard wire palatal to it so that a channel is made between the base plate and the guard wire.

Adjustment of palatal cantilever springs
It is necessary to check whether the spring contact the tooth correctly and lies closer to the gingival margin. The spring is then activated by 3mm.  The spring should not be adjusted at the point where it emerges from the base plate because it is appoint where there is stress concentration. If the wire is further work hardened it will break at this point. The correct site for adjustment is along the free arm of the spring as close to the coil as possible. After activation of the spring the point of contact should be checked and it is adjusted so that the tangent drawn at the point of contact is perpendicular to the intended direction of movement.  Once activate the appliance should be seated in the mouth and try to release the spring and see the amount of activation. In a case of a canine the spring should not go beyond the tip of the canine when it is in the passive status. The force generated with activation of the spring could be measured with a tension gauge as shown in the picture.
Single cantilever spring should be cranked when it is made on a tooth in the labial segment. This is to enable the spring to be embedded in the base plate without interfering with the other teeth.

Double cantilever spring ( Z spring)
Z spring
When a tooth which is in cross bite has to be moved by a considerable distance single cantilever is not satisfactory. In such cases it is necessary to modify the cantilever spring. Double cantilever or Z springs are made in such 0.5mm wire to suit the clinical situation. The arms of the springs should be as long as possible to reduce the stiffness of the spring. If the limbs are short the range of activation will be limited and also the child may find it difficult to insert the appliance.

The palatal surface has a slant therefore the spring should be made perpendicular to the palatal surface of the tooth to minimize the intrusive effect of the spring which is not satisfactory in cases with cross bites. The spring also will tend to slip incisally and become passive.   In cases where there is a doubt about the stability of the spring  a guard wire could be incorporated below the spring to prevent downward displacement of the spring. These guard wires could be made with a reservoir in cases with marked palatal displacement of the incisors.

Both limbs of the double cantilever spring should be adjusted during activation. First, adjustment is carried out to the palatal limb close to the coil of the fixed end of the spring. This will establish the correct degree of activation. Then the second adjustment is made at the other end of the limb to make the free limb perpendicular to the intended path of movement.

T spring
T spring
 In cases where buccal movement of a premolar or a canine is needed if a single cantilever or a double cantilever spring is used patient may find it difficult to insert the spring. A T spring which is made of 0.5mm wire may be satisfactory for this purpose. The mechanical principles of a T spring is similar to single cantilever spring but both free ends are embedded in acrylic making it less flexible compared to it. The force applied by the spring has both vertical and horizontal components. If the tooth surface of the tooth at the point of contact is early vertical as in first premolar the intrusive component is small. If T springs are made on incisors which has a sloping surface such as the cingulum plateau of the upper incisor the intrusive component will be much larger. This has two disadvantages. One is the intrusive force may cause intrusion of the upper incisor which is unfavorable in cross bite cases and the reaction to this vertical component of the force may tend to dislodge the appliance.  Therefore t springs are not used in anterior teeth.

T spring is activated by pulling it away from the base plate if the spring over activate it may not seat properly; therefore extra care should be taken to avoid over activation. When the tooth is moving outwards the spring may not be in contact with the tooth. The spring should be elongated by adding wire from the reservoir.

Buccal springs

Buccaly approaching springs can be used for mesio-distal movement palatal movement or when an attachment is bonded on teeth for extrusion of the teeth or for rotation. As the sulcus depth is limited special care should be taken when the impression is taken for working model. Impression should be muscle trimmed to avoid encroachment of the spring into the muscle attachments. Bucccaly approaching springs may get distorted when the patient is trying to remove the appliance therefore every precaution should be undertaken to improve the strength and the stability of the spring.

Buccal canine retractor

 Depending on the thickness of the wire used buccal springs the flexibility and stability changes.

Self supported buccal canine retractor
Self supported buccal canine retractor
This is made of 0.7mm wire. It has a good strength and stability but even a slight activation may apply a great an excessive force even with 1 mm activation.

Supported buccal canine retractors
Supported buccal canine retractor
This is made of 0.5mm wire. Therefore the flexibility of the spring is satisfactory. As it is made on a sloping surface of the tooth it may be unstable in vertical palne so that the point of contact will change applying jiggling forces on the tooth. The stability of the spring in the vertical palne can be improved by supporting the wire with a tubing of a 0.5mm internal diameter. It is more than twice as flexible as the self supported retractor. This is not flexible as palatal retractor as the free end of the wire which is not supported is not long enough but has excellent mechanical properties when compared with a self supported canine retractors.

An activation of 2mm one quarter of the canine width is sufficient. Care should be taken not to bend the wire as it emerges from the tubing. As this a site of stress concentration it may fracture. Buccal retractor should be activated in two planes. Distal activation is carried out at the coil by bending the anterior limb over the round beak. Of a pair of spring forming pliers. Palatal activation is done along the anterior limb below the coil. Patient should be instructed to seat the spring carefully on the mesial surface of the tooth  as it may catch on the cuspal incline of the canine and remain ineffective or cuuse intrusion of the tooth.

Reverse loop buccal retractor

This retractor is satisfactory in the lower arch as there is a shallow sulcus. Flexibility of the retractor depends on the height of the vertical loop. Therefore it should be kept as long as possible. The main problem of this spring is it is stiff in the horizontal plane where flexibility is needed and unstable in the vertical plane.

As the spring is made of 0.7mm wire it should not be activated more than 1mm. this can be activated in two ways. One is by cutting 1mm wire from the end of the spring and recontourig the free end around the tooth or the other by opening the loop by 1 mm.

In labial bows both ends of the bow are embedded in acrylic. They could be either passive or active. Passive labial bows are used to improve retention of the appliance or as a retainer after correction of over jet. Active labial bows are used for incisor retraction.

Active labial bows
The choice of the labial bow mainly depend on the amount of incisor retraction needed. In cases with severe over jet a flexible labial bows such as Robert’s retractor is preferred. In cases with slight over jet or where minor irregularities need correction, a less flexible labial bow is preferred.

Labial bow with U loops
U loop labial bow
This labial  bow is constructed with 0.7mm wire. Flexibility of the bow depend on the height of the loop. The depth of the sulcus limits the height of the loop. The bow is rigid in horizontal plane  as the wire is thick but flexible in the vertical plane making the stability ratio poor. Only advantage of this labial bow is in cases with palatal springs for canine retraction this labial bow could be incorporated in the same appliance and once the canines have been retracted the labial bow could be activated for over jet correction. As the forces generated by this labial bow is very high there is a risk of anchorage loss.

Activation of u loop labial bow. The bow is adjusted at ‘U’ loops. The activation should be very slight. Once activated labial bow should be displaced palatally only by 1mm.

Split labial bow
Split labial bow
Flexibility of U loop labial bow could be improved by splitting it in the middle. But the activation will be difficult. During activation extra care should be taken to prevent flattening of the labial segment. Labial bow should be activated at the U loops .

Labial bow with reverse loops.
This is also made of 0.7mm wire. The flexibility of this labial bow is much superior to U loop labial bow as the reverse loops incorporate more wire into the labial bow. But during construction extra care should be taken to keep the loops clear of the Adam’s clasps on first molars. If this bow is incorporate with canine retractors the labial bow will have added control on distally moving canines. As this bow also comparatively rigid it should be activated only by 1mm.

Activation of the labial bow with reverse loop is carried out in two stages. The vertical loop is first opened by compressing t at the loop as shown in the figure. This lowers the labial bow in the incisor region. A compensating bend should be made at the base of the loop to make the contact of the labial bow with the upper incisor correct.

Extended labial bow
Extended/Flexible labial bow
Extended labial bow also made of 0.7mm wire. But the flexibility is very much improved by incorporationg large loops when compared with U loop labial bow. In units where facilities are not available  for construction of Roberts Retractors this is a useful alternative. As the loops are comparatively large patients find it uncomfortable.

Activation the extended labial bow should be activated with special care as the loops can get distorted leading to trauma either tot the gum or lip.  Loop forming pliers with groves on one beak is used for the activation.  Adjustments are carried out on tow places. First the on the upper border of the loops marked A  to make the activation effective and then at the  point B B’  to make the level of the bow on incisor teeth. as this labial bow is also made of 0.7mm wire only 1 mm of activation is recommended.

Roberts retractor
Roberts retractor
Roberts Retractor is a flexible labial bow made of 0.5 mm wire. The strength and stability of the bow is improved by inserting the wire into a tubing with 0.5mm internal diameter. Horizontal section of the bow is made into a smooth curve and extends from distal of lateral incisor on one side to the other. The vertical section of the bow and the coil  gives the  flexibility to the bow. Coil is made as the wire emerges from the tubing the tubing with the wire is inserted into the base plate distal to the retracted canines.  Coil should be of adequate size, at least 0.3 mm internal diameter.

This bow is very flexible and activation of 3mm is sufficient. Activation should be carried out with great care. If wire is bent at  the point where it emerges from the tube it is liable to fracture. This a point of stress concentration. The activation is carried out by bending the vertical limb inwards below the coil. As the incisors are moving back the bow drops down therefore the horizontal part should be lifted up to maintain the correct point of application  of force on incisors.

Labial bow with Apron Spring.
This is similer to the Roberts retractor with regard to the flexibility. Apron spring is made of either 0.4 mm wire or 0.35mm wire. It has a horizontal part which is smoothly curved to take the shape of the labial surface and vertical limbs which is wound onto the heavy base arch of 0.9mm wire.

Labial bow with apron spring is activated by bending the wire inwards along the vertical limbs. This is liable to fracture. But a new apron spring could be wound chairside.

Expansion screw
A screw can be incorporated in to the base plate to use a source of force generation. The screw transmits the force through the base plate which come into contact with it. The screw applies an intermittent force

Many types of screws are commercially available. The screw should have adequate travel  good stability and should be of minimal bulk which are desirable properties. Number of guide pins determines the stability of the screw. Screws with  double guide pins are more stable. But in cases with restricted space screws with single guide pins have an added advantage. Activation is carried by the patient once or twice weekly.   Screws sometimes may tend to turn back when there is resistance to tooth movement. Screws apply an intermittent heavy force. This is not a big problem as at any given time the screw activation in less than or equal to 0.2mm. therefore the movement of the tooth is still within the limits of the periodontium so that extensive hyalinization do not occur. Therefore spring loaded screws which dissipate the force slowly  over a long period of time so that a continuous force is applied on the teeth. but spring loaded screws are very bulky making a limitation for their use.

The direction in which the screw should be indicated on the base plate using a marker embedded in the base plate. The patient is given a key to open the screw. A quarter turn opening of the screw per week will move the tooth about a 1mm per month.


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