General principles and indications for the use of removable mechanic appliances.

Although widely utilized in the past as the sole appliance to treat malocclusion, with the increasing availability and acceptance of fixed appliances, the limitation of removable appliances have become more apparent.

Removable appliances work by applying of a single force to the crown of a tooth. A removable appliance induces tipping of a tooth around a point known as the "centroid", "centre of resistance", which locates between 30 and 40% from the root apex of the hole length of a tooth. Tipping can be brought about in the mesio-distal or bucco-lingual direction. The key point to be considered is the position of the root apex before the beginning of treatment. Teeth that have their apexec at the correct position are ideally suited for treatment with removable appliances. In such cases the irregularity is usually due to the crowns being tipped from their correct position and, therefore, responds well to tipping tooth movements enabling the use of removable appliances (RA). RA are generally considered ineffective in bringing about bodily translation, derotation and uprighting of teeth due to the single point of contact of these appliances. Intrusion and extrusion are often induced, using forces of eruption and occlusion.

As a result the role of the RA is changing, it is becoming more widely used to transmit forces to blocks of teeth and as an adjunct to fixed appliance treatment. RA provide a useful means of applying extra-oral traction to segments of teeth, or an en­tire arch, to help achieve intrusive and/or distal movement. RA are also employed for arch expansion, which is another example of their usefulness in moving blocks of teeth. RA are particularly helpful where a flat anterior bite-plane or buccal capping is required to influence development to the buccal segment teeth and/or to free the oc­clusion with the lower arch.

RA are used passively as space maintainers following permanent tooth extrac­tions and also retaining appliances following fixed appliance treatment.

2. Advantages and disadvantages of removable appliances.
The advantages and disadvantages of RA are summarized in Table 1.

Table 1

3. Removable appliance design.

A design of a removable appliance suggests an active component, retention, anchorage and a baseplate.

Active componentis force sources required for desired movement. They are springs, bows, screws and elastics.

Fixationis required to retain RA in the mouth. A number fixation mechanisms are used. They are Adam's cribs, C-clasps, Southend clasps, fitted labial bows, reverse U-loop labial bows, Hawley bows.

Anchorageresists force generated by active component.

Baseplateholds all the elements together. Baseplate can not only hold other elements together, but also be active itself. Heat-cure acrylic is more robust than self-cure.

• Anterior bite-plane. Increasing the thickness of acrylic behind the upper inci­sors forms a bite-plane onto which the lower incisors occlude. A bite-plane is prescribed when the overbite needs to be reduced by eruption of the lower buc­cal segment teeth or reduce possible occlusion interferences to allow tooth movement. Anterior bite-planes are usually flat.

• Buccal capping frees the occlusion on the tooth being moved and allows fur­ther relative eruption of the incisors.

4. Active component and tooth movement.

The components used to apply force to the teeth are:

• Springs are usually made from 0.5, 0.6 or 0.7 mm-diameter stainless steel wire

• Bows are usually made from 0.5 or 0.7 mm-diameter stainless steel wire

• Screws opening 0.2 mm per ¹/₄ turn

• Elastics

Springs

Typical springs used on RA include:

• Z- spring - typically made from 0.5 mm stainless-steel wire on a single tooth and 0.6 mm on two adjacent teeth;

• T-spring - typically made from 0.5 mm stainless-steel wire;

• palatal canine retraction spring - made from 0.5 mm stainless-steel wire;

• finger spring - made from 0.5 mm stainless-steel wire;

• buccal canine retraction spring - made either from supported 0.5 or 0.7-mm un­supported stainless-steel wire;

• Coffin's spring - made from 1.2 mm stainless-steel wire. It is used to bring about slow dento-alveolar arch expansion where the upper arch is constricted or there is a unilateral crossbite. The Coffin's spring consists of a U- or omega-shaped wire placed in the mid-palatal region with the retentive arms incorpo­rated into baseplate.

Even small changes in the diameter or length of wire used in the construction of a spring will have a profound impact upon the force delivered. A commonly made mistake is to activate a spring in 0.7 mm wire as the same distance as one of 0.5 mm wire. In fact, for the same design of buccal spring, the 0.5 mm spring should be acti­vated about 3 mm, whereas the 0.7 mm spring requires only 1 mm activation to de­liver 25 - 50 g of force (light or physiological force). Incorporating a coil into the de­sign of a spring increases the length of wire and, therefore, results in the application of a smaller force for given deflection.

Palatal finger springs are the most commonly used active components for mesial or distal movement along the arch. They are fabricated from 0.5 mm wire and are easy to adjust and activate. If boxed out from title acrylic and made with a guard wire they are more stable in the vertical plane than a buccal spring.

For vestibular movement of incisors, Z- springs (for 1 tooth in 0.5mm, for > 1 tooth in 0.6) are used and T- springs are used for premolars (0.5 mm) and molars (0.6). They are all activated only 1-2 mm by pulling them away at 45° from baseplate in the desired direction of movement.

Bows.These types of components are used for both overjet reduction and for providing anterior fixation. An example of a bow used for overjet reduction is the Robert's retractor made from 0.5 mm stainless-steel wire with supporting buccal sleevings. Anterior fixation can also be provided by Hawley bow (made from 0.7 mm wire).

Screws

Basically there are two types: first, the spring-loaded piston (e.g. Landin) with activation by turning with a special screwdriver in the direction of movement, it is used for proclining one incisor; second, the screw type (e.g. Glenross) which is opened or closed by means of a key, a quarter turn separating the two halves by 0.2^ mm. The screws can be used to move teeth:

1) transversely - for the correction of posterior crossbites;

2) anteroposteriorly - for moving teeth over the bite, especially if more than one in­cisor needs to be moved. Ideal tooth movement is achieved by turning the screw a quarter of a turn every 3-7 days. Rapid maxillary expansion is a variation of this type of expansion and normally relates to fixed appliances. However, it is possible to use a removable appliance to deliver this type of force if the appliance is well constructed and the patient is very cooperative, and in these cases, the midline screw is turned twice a week.

Elastics.Using of elastics to reduce an overjet is popular with patients as they are less visible than a metal spring. To reduce an overjet, the design appliance can be made by con­verting a labial bow, cutting it and bending the ends into hooks adjacent to upper ca­nines. Latex elastics are stretched between hooks and lie over the incisors. For most patients a 3.5 oz. 5/8 inch elastic is required. But elastics have a tendency to slide up proclined incisors and retract the gingivae instead.

5. Fixation (retention). Main types of fixation mechanisms.

A number fixation mechanisms are used to retain RA in the mouth. They include:

• Adams' cribs 0.7 mm-diameter SS

• C -clasps (circumferential) 0.7 mm-diameter SS

• Jackson's clasps 0.7 mm-diameter SS

• Southend clasps 0.7 mm-diameter SS

• Triangular clasps, ball-end clasps, Schwarz clasps

• Fitted labial bows 0.7 mm-diameter SS

• Reverse U-loop labial bow 0.7 mm-diameter SS

• Hawley bow 0.7 mm- or 0.8 mm-diameter SS

Clasps act by engaging certain constricted areas of the teeth that are called undercuts. There are two types of undercuts that are found in natural dentition.

a) Buccal and lingual cervical undercuts

b) Mesial and distal proximal undercuts.

Buccal/Lingual cervical undercuts: The buccal and lingual surfaces of molars have a distinct undercut at the cervical margin. Examples of clasps that engage these buccal and lingual cervical undercuts are the circumferential and Jackson's clasps. These undercuts are available for clasp fabrication only in those teeth that are fully erupted. Mesial and distal proximal undercuts: The molars are widest mesio-distally at the contact point and gradually taper towards the cervical margin. Examples of clasps that engage these undercuts are the Adams' clasp and Crozat clasp.

The Adams' claspis made of the following parts:

a) two arrowheads

b) bridge

c) two retentive arms.

The two arrowheads engage the mesial and distal proximal undercuts. The arrow­heads are connected to each other by a bridge which is at 45° to the long axis of the tooth. They can be used on partially or fully erupted teeth, on molars, premolars and on incisors. The clasp can be modified in a number of ways (Adams with J hook, with additional arrowhead, with soldered buccal tube - it permits use extra-oral an­chorage using face bow and so on).

6. Anchorage and its types.

Anchorage is defined as the source of resistance to the reaction from the active com­ponents) in an appliance. In practice, it is a balance between the applied force and the available space. E.g., in a case where 3 | 3 are being retracted following extraction of 4 | 4, an equal, but the opposite force will also be acting on 65 | 56. The amount of forward movement of these anchor teeth will depend upon their root surface area and the force used.

Anchorage loss can be minimized by limiting the number of teeth being moved at any one time, applying the correct force for the movement required and increasing the resistance of the anchor teeth (e.g. by permitting only bodily movement). In some situations, movement of the anchor teeth is desirable. However, it is important to as­sess the anchorage requirements of a particular malocclusion before embarking on treatment, and if indicated reinforce from the start.

Date: 2016-04-22; view: 890