When a force is applied on to the tooth, the tooth is rotated . In order to do this clinicians rely on the ability of materials such as corrosion resistant alloys and to a limited extent rubber and rubber like materials to store energy. The energy is transferred to the tooth to do the work required. Lot of thought should be given in selection of the material and also to the design of the appliance in order to harness the stored energy to produce the forces of required magnitude and direction.
The two types of materials used, metals and rubber like substances have differing energy storing ability due to the vastly differing internal structure of them. Rubber consists of a three dimensional network of long chain polymer molecules and are capable of great extensibility. But the amount of energy that they can store per unit volume is low. Metals has a crystalline structure and have low reversible extensibility. But can store a greater amount of energy per unit volume.
For tooth movement a tooth requires 25-40gm of force ( 0.25-0.4 N) . It is a problem to make a spring of sufficient length within the confines of the mouth which applies a force of appropriate magnitude and direction.
The material should be corrosion resistant for use in the mouth. It should not get distorted with external forces. It should not be susceptible to fatigue fracture.
The ability of the appliance to resist distortion by the patient during routing use depend on
- Design –geometrical shape of the wire,length and the diameter of the wire used
- Inherent properties of the wire.
Basically an appliance is a device which utilizes the energy stored in the metal when it is activated. It should produce forces of desired magnitude and direction over a specific distance while being as robust and stable as possible.
Physical properties of the wire.
Physical properties of the wire which are important in clinical orthodontics.
- Flexural rigidity ( EI)
- Resistance to distortion
- susceptibility to fracture.
The flexural rigidity of a wire is a product of Young’s modulus of elasticity and a factor known as Second Mement of Inertia of the cross section of the wire. Young’s modulus of elasticity is an intrinsic property of a material which determines its resistance to elastic deformation in either tension, compression or flexure. The factor I depend on the shape and dimensions of the cross section of the wire and factor determines how stiff the wire is.
For circular cross section I = r4/4
R is the radius of the wire. The force generated for unit deflection of all removable appliances is directly proportional to EI. E of the wire does not very greatly. But I increases as cross section increases. I depend on the fourth power of the radius. By doubling the radius (holding everything is constant) the force applied by the wire increases 16 fold. If 0.6mm wire used instead of 0.5mm wire force generated will be doubled. If 0,7mm wire used instead of 0.5 mm. force increases four fold.
P/Y = 3EI/L3
P= force applied
Y = deflection of the free end of the wire.
Resistance of the wire to distortion
If load is applied to the free end of the cantilever spring the upper layer of the wire is extended and lower layers of the wire compressed. The maximum fiber stretch occurs in the outermost layers of the bent wire at any point may be calculated from following formula.
Retention of removable orthodontic appliance
In orthodontics the term retention has two meanings. One is the support given to the teeth in order to maintain the teeth in their new positions after orthodontic treatment. The other is the way the appliance is held in the mouth in order to make it effective. This chapter focuses on the retention of the removable appliance in the mouth.
Appliance retention is mainly provided by wires which are bent into different shapes to produce a clasp or bows. There have been many designs of the clasps used in the past with removable appliances. which have been attempted in the past. The arrowhead clasp was one of the most successful designs out of these though the construction and adjustment of arrowhead clasp is believed to be difficult. The design proposed by Adams ( 1955) of the modified arrowhead clasp at present referred to as Adam’ s Clasp is a major advance in removable orthodontics.
Depending on the region where retention is needed the clinician has to decide the appropriate design of the retention component of a given removable appliance.
Retention in posterior region.
Adams clasp is made of 0.7mm stainless steel wire or from blue Elgiloy for all permanent teeth except for permanent canines. For permanent canines and in cases where clasping of primary molar is necessary 0.6 mm wire is used
Adams clasp utilizes mesio buccal and disto buccal undercuts that can be found on crowns of almost all the teeth. If the tooth provide an undercut with 0.25mm depth the retention of the appliance will be satisfactory. In young children as the teeth are newly erupted anatomical crown may not be fully exposed. In such cases undercut will be just below the gingival margin. Therefore, it is necessary to trim the plaster model to reproduce the anatomical contour of the crown so that the maximum depth of the undercut may be utilized for construction of a well fitting clasp. It is important to trim by correct amount as inadequate trimming may not provide sufficient under cut and the appliance may not retain well and also excessive trimming will cause difficulty in fitting of the appliance. If adequate attention is not given at this stage of preparing the undercut for the clasp any adjustments made to the clasp at the time of fitting may not improve the retention of the appliance.
In adults the clinical crown may be more than the anatomical crowns. If care is not taken in deciding the correct depth of the undercut the clasp will be too stiff as it engages a deep undercut. Therefore in adults the clasp should not be extended as far as the gingival margin.
Construction of Adams clasp
Advantages of Adams clasp
- bridge of the adams clasp provide a site to which the patient can applay pressure with the finger tips during the removal of the appliance.
- hooks can be bent during construction or can be soldered after construction to engage inter maxillary elastics.
- Auxiliary springs can be soldered to the bridge of the clasp.
- clasp cane be modified by winding a coil in the bridge or soldering a tubing onto the bridge of the clasp.
Adjustment of the Adam’s clasp
When the appliance is fitted into the mouth the clasp should be nearly passive. Active clasps can exert a palatally directed force on the tooth resulting in palatal tipping of the tooth if the appliance is ill fitting. This may reduce the undercut available and make the appliance ill fitting.
Lower molars do not have sufficient undercut on the buccal surface leading to poor retention of lower removable appliance. This problem may be overcome if the lingual undercuts are used.
Variation of Adam’s clasp
Adams clasps on canines
As permanent maxillary canines have a deep undercut. Therefore 0.6mm wire is used for clasps.
Clasps on primary molars.
The undercuts available in primary molars are not deep enough to provide adequate retention. In such cases additional undercuts can be utilized using soldered clasps. But construction and adjustment of them difficult. As an alternative a single arrowheads made of 0.8 mm wire can be used to enhance retention.
The Adams clasps have few limitations. If care is not taken during adjustment clasp may fracture due to work hardening. Method of repair depend on the site of fracture of the Adam’s Clasp. If the clasp is fractured at the arrowhead, it could be repaired by soldering. If the clasp fracture where it cross over the interproximal area or as it emerges from the base plate it is necessary to replace it with a new clasp. Some patients may complain of irritation of the cheek with the bridge if the bridge is too prominent especially if the tubes are soldered for extra oral anchorage.
C clasps or recurved claps are made on primary canines to improve retention of the appliance. C claps adapts along the gingival margin to provide retention.
Retention on anterior teeth.
Adams claps can be made on incisors if they are upright or very slightly proclined. It could be made on one incisor or on both incisors. When both incisors are included in the clasp it may be uncomfortable to the patient especially if they are very prominent. It can be improved by not extending the arrowheads too closer to the cervical margin and also curving the bridge to confirm with the curvature of the labial surface. Arrowheads also could be flattened to reduce the irritation to the upper lip.
Southend clasp the wire passes round the gingival margins of the incisors and utilizes the undercut between the incisors. It is easy to construct. And well tolerated by the patients it is the best retentive component if the central incisors are only slightly proclined. Breakage of Southend clasp is not very common. Fitted labial bow. When upper incisors are grossly proclined construction of adam’s clasps and southend clasps is not practicable, in such occasions fitted labial bows can be made to improve retention. It is made of 0.7mm wire and both central incisors or all four incisors could be included in the fitted bow.
Ball ended clasps
Ball ended clasps uses undercuts provided by the embrasure. When the embrasure is used there is a risk of gingival damage. Sometimes ball ended clasps can separate the adjoining teeth. these are useful on lower incisors and on deciduous teeth as they do not provide sufficient undercuts for other types of clasps.
Decision with regard to the type and position of the retentive components should be made after giving careful thought to the type of the active components used and the way the reactionary forces will resolve after activation of the springs. The number of clasps should be minimal so that the patient will find it easy to handle. Adjustment will be less complicated even in inexperienced hands. First molars usually provide sufficient retention. Depending on the nature of active components additional clasps should be used. For example a tooth closer to the incisors is clasped in cases with cross bite to counteract the reactionary forces which can dislodge the appliance.
The base plate
The main body of the appliance consist of the base plate. It has three main functions. It provides the base which put all the components of the appliance together. As the base plate is in contact with the palate and the other teeth it provide additional source of anchorage which is an added advantage in removable appliance. It also can be modified to make bite planes to reduce over bite or to disocclude the teeth in cases with cross bites.
Construction of base plate.
The base plate should be of sufficient thickness in order to carry all the wire components. It should not be too thick so that the patient can wear it full time with minimum discomfort. It should cover most of the hard palate. Finishing just distal to the first molars. It should fit closer to the gingival margins of the teeth which are not intended to move to prevent food stagnation. It should be well clear of the teeth which are being moved. Before making it is necessary to block all the undercuts. In the upper arch in a young patient the undercut may not be a problem. But in the lower arch it is necessary to block the undercuts to make the insertion of the appliance easy.
Base plate is usually is made of acrylic either cold cure or heat cure. When cold cure acrylic is used it saves laboratory time and there is no warpage of the appliance during processing like in heat cure acrylic. But cold cure acrylic appliance have residual free monomer which could be toxic to the patient and also not very strong. Heat cure acrylic though time consuming, appliances are very strong therefore it is recommended in cases with deep over bites where heavy muscular forces could be anticipated
Anchorage and the base plate
Anchorage of the appliance could be improved by covering the maximum amount of the palatal mucosa with acrylic. It usually extend up to the distal of first molars. In cases with second molar erupted, palatal acrylic could be extended upto second molar. Acrylic should contact all the teeth except, which are intended to move.
Base plate could be modified by addition of acrylic to build up bite planes. If adequate instructions given to the technician, with regard to the construction of the bite plane chairside adjustments required could be minimized.
Anterior bite plane
Anterior bite plane is used for reduction of deep over bite before correction of over jet. Anterior bite plane changes the eruption pattern of teeth. It prevents eruption of lower incisors as lower incisors are in occlusion with the bite plane and allows eruption of posterior teeth if they are separated beyond freeway space. Over bite reduction using this method will be stable only in children who are actively growing with a favourable growth pattern. The bite plane allows eruption of posterior teeth which leads to increase of the vertical dimension. Muscles of the vertical sling experience increased tension. In children with favourable growth there is sufficient vertical growth to facilitate reattachment of the muscles releasing this tension. This adjustment of vertical dimension makes the corrected over bite stable. In actively growing children those who have average Maxillary Mandibular Plane aAngle grow favourably in vertical dimension. Those who have low FMPA do not have sufficient vertical growth, which facilitates this muscle reattachment. In cases with increased FMPA ( High angle) there is sufficient vertical growth to make the corrected over bite stable. As increase of vertical dimension may take the lower lip away from the upper incisors there won’t be sufficient control over the upper incisors from the lower lip at the end of treatment making the final over jet correction unstable. Therefore, in high angle cases over bite correction with anterior bite plane is not recommended in high angle cases. In adults irrespective of the vertical dimension, anterior bite planes should not be used for over bite correction as they do not have facial growth which makes the over bite stable.
It is important to give clear instructions to the technician with regard to the extent of the anterior bite plane both the thickness and the extent. The thickness of the anterior bite plane should be sufficient to disocclude the posterior teeth slightly beyond the free way space. In premolar region this separation should be about 2-3mm. If the separation is not sufficient, there won’t be eruption of posterior teeth as there is no stimulation for eruption. If the separation is too much the tongue will spread between the upper and lower posterior teeth and prevent eruption of posterior teeth. In some cases last standing tooth may erupt as the tongue does not interfere with the eruption of them resulting in open bite in premolar region which will be difficult to correct later.
Posterior limit of the bite plane should be extended sufficiently posteriorly to allow occlusion of the lower incisor teeth. It is important to inform the technician about the extent of the over jet as the bite plane should extend few millimeters beyond this to enable the occlusion with the lower incisors. Occlusal surface of the bite plane should be parallel to the bite plane this will ensure satisfactory contact of the lower incisors with th bite plane. Ideally contact angle of the lower incisor with the bite plane should be 90 degrees. If this contact is incorrect there will be either proclination or retroclination of lower incisors leading to an unstable lower labial segment.
Clinical management of the bite plane.
Anterior bite plane should be thick enough so that when the appliance is in place premolars are separated by 2-3mm. ideally anterior bite plane should be flat. But at the beginning if the incisors are irregular it may be necessary to adjust the bite plane so that at least three incisors are in contact. As the over bite improves and lower incisor irregularity corrects bite plane could be leveled with addition of cold cure acrylic. Improvement of over bite should be evident within first two months after fitting of the appliance. The thickness of the bite plane could be increased by addition of cold cure acrylic if further reduction of the over bite is necessary.
Trimming of the bite plane
When over bite is reduced sufficiently, over jet is reduced either with the same appliance or with a new appliance. If the same appliance is used, it is necessary to wait until all posterior teeth regain contact. If a new appliance is made, the bite plane should be of sufficient thickness to prevent separation of the posterior teeth. This is important to prevent rocking of the appliance with undermining of acrylic during trimming of the bite plane before activation of labial bows. Trimming should be done gradually to allow retraction of incisors. It is helpful to mark the occlusion of the lower incisors with the bite plane using an articulating paper so that over trimming of the bite plane could be avoided. Bite plane should be trimmed just enough to allow posterior movement of the upper incisors which is anticipated with one activation. If acrylic is over trimmed child may posture the mandible forward and bite on incisors which will prevent retraction of upper incisors. Lower incisors may also erupt leading to relapse of the over bite. trimming of the bite plane is first done vertically to clear the palatal surface of the upper incisors by about two mm and then undermining trimming should be carried out towards the palatal mucosa. This undermining trimming is necessary as the palatal mucosa wrinkles with palatal movement of the anterior teeth. This bunching up of mucosa takes time for reorganization. During this period gingival is irritated by the base palte resulting in gingival hyperplasia especially if the patient does not give meticulous attention to oral hygiene.
Posterior bite plane
Posterior bite planes are used to eliminate anterior or lateral displacement of the mandible. As teeth which are in premature contact in the centric path of closure fitting of an appliance with a posterior bite plane may facilitate spontaneous movement of the teeth which are in premature contact. In cases where correction of cross bite is necessary, posterior bite plane is necessary to disocclude the teeth. this will enable the mandible to a centric relation.
Thickness of the posterior bite plane should be adjusted carefully to allow even occlusion of all posterior teeth on both sides. As the inter maxillary space is of wedge shape, posterior bite plane should be thinner posteriorly than anteriorly. Accurate construction of posterior bite plane demands mounting of models on an articulator. Considerable amount of chairside time could be saved if posterior bite plane is constructed accurately.
If adjustment of the posterior bite plane is necessary, it should be checked with an articulating paper ensuring that the patient closes in centric so that the occlusal surface of the bite plane could be faceted to receive the lower teeth in centric occlusion.
In cases with anterior cross bites the thickness should be just sufficient to disocclude the anterior teeth. This adjustment may lead to perforation of the posterior bite plane. Once the cross bite is corrected posterior bite plane could be trimmed off completely. In cases where there is a risk of child posturing the mandible to the previous position leading to relapse posterior bite plane could be trimmed over two or three consecutive appointments.
Anterior inclined planes
Removable appliances should be fitted within two weeks of impression taking. If teeth have not been moved during this period the appliance should fit without any adjustment. If any trimming is necessary it should be done with great care because if a gap left between the plate and the teeth it will encourage food stagnation. If the appliance is not fitting well it is wise to bend the clasps away from the teeth to ensure whether the base plate or the wires responsible for ill fitting appliance. If there are under cuts which prevent seating of the appliance it should be trimmed without cutting the polished surface of the appliance so that the base plate maintains contact with the teeth. once the appliance is fully seated in the mouth, it should be examined to ensure path of movement of the tooth intended to move is clear of acrylic
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.
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.
A palatal spring boxed and guarded.
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) an d 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)
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 workhardened 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 interfereing with the other teeth.
Double cantilever 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.
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 p;oint 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 unfavourable 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.
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
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.
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
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.
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 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 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.
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.
Orthodontic Study models
Orthodontic study models are accurate reproduction of teeth, alveolar arches and surrounding soft tissues. They are made of plaster and are considered as an essential diagnostic records in orthodontics. Study models visualize the teeth and their relationship to each other in three dimension.
Uses of study models.
- they are essential in treatment planning as they visualize teeth in three dimension
- occlusion of the teeth can be seen on the lingual aspects which is not possible on a patient.
- they provide a record of occlusion at the beginning of treatment which can be used for monitoring of progress during treatment
- They are helpful to educate and motivate the patient about treatment progress.
- if it is necessary to transfer the case study models provide a record of the occlusion at the beginning of treatment.
- they are useful as a good medico legal record.
- they are useful for space analysis.
Construction of study models.
- As the first step lower model is trimmed. First it is important to determine the occlusal plane of the lower arch.
- Then draw line parallel to the occlusal plane on the base of the lower model.