Contemporary removable appliances

8 Contemporary removable appliances

Removable appliances are not permanently attached to the teeth and can be taken out of the mouth by the patient. During the first half of the twentieth century, orthodontic practice in Europe was based largely on the use of removable appliances. However, over the past few decades there has been a significant decline in their use, primarily as a result of more efficient fixed appliances being available and an increase in numbers of orthodontic specialists able to use them. However, simple removable appliances retain a place in modern orthodontic practice, usually as an adjunct to fixed appliance therapy or for use in the retention phase of treatment. In particular, a group of predominantly removable functional appliances, used primarily in the management of class II malocclusion, have enjoyed a considerable resurgence in popularity in recent years. In addition, new treatment systems using vacuum-formed removable appliances, not only for retention, but also for active tooth movement have been developed.

Tooth movement with removable appliances

A variety of tooth movements can be achieved with removable appliances, either individually or on groups of teeth:

Removable appliances are also useful in maintaining tooth positions during retention.

Components of removable appliances

Removable appliances are composed of retentive and active components connected together by a baseplate. When designing a removable appliance, consideration also needs to be given towards anchorage, ensuring that the desired teeth will move under the active force applied by the appliance (Box 8.1).

Box 8.1 Principles of removable appliance design

The starting point for the design of any removable orthodontic appliance is deciding upon the desired tooth movements and how these will be achieved by the active components. Once these points have been addressed, consideration must be given to retention, anchorage and connecting all the components together using the baseplate. Retention is the mechanism by which the appliance stays in the mouth and is provided by passive components such as clasps and labial bows. Good retention is important to ensure that the active components of the appliance are correctly placed and therefore effective. A retentive appliance is also easier for the patient to wear and therefore optimizes patient compliance.

Anchorage for a removable appliance is provided from either an intra- or extraoral source. Intraoral anchorage comes primarily from the palate and dentition of the same dental arch (intramaxillary); whilst extraoral anchorage is from headgear attached to the appliance. In certain circumstances anchorage is reciprocal when the planned tooth movements for active and reactive components are equal. However, the aim is often for specific teeth to be moved by the appliance, with others remaining stationary. To prevent undesirable tooth movement and anchorage loss, active forces should be kept light and reactionary forces reduced by limiting the number of teeth being moved at any one time. This may mean only activating one spring at a time or providing more than one appliance in order to achieve the treatment aims.

Retentive components

The retentive components of a removable appliance are concerned primarily with seating it in the correct position, but they can also contribute towards anchorage.

Adams clasp

Adams clasps are constructed in 0.7-mm stainless steel wire and most commonly used on the first molars (Fig. 8.3), although they can be used on premolars and anterior teeth. The arrowheads of the clasp engage undercuts at the mesial and distal corners of the buccal tooth surface and can easily be adjusted at the chairside to increase retention. The bridge of an Adams clasp can also be used by the patient to remove the appliance from the mouth, whilst the orthodontist can use it to attach auxiliary springs or tubes for headgear.

Southend clasp

The Southend clasp is also constructed in 0.7-mm stainless steel wire, but is used for retention on the incisor teeth (Fig. 8.4). This clasp is activated by bending the U-loop towards the baseplate, which carries the clasp back into the labial undercut of the tooth.

Plint clasp

Plint clasps are useful when using a removable appliance in combination with a fixed appliance (Fig. 8.6). These clasps are constructed in 0.7-mm stainless steel and engage the undercuts on a maxillary molar band.

Active components

The active components of a removable appliance are responsible for producing the desired tooth movement. They can be categorized as springs, bows, screws and auxiliary elastics.

Springs

Mechanical principles should be considered when applying a force to any tooth with a spring:

The force (F) delivered by a spring is related to the length (L) and thickness or radius of the wire (R), as well as the deflection (D), such that

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Therefore, lighter forces can be delivered by increasing the length of the wire or reducing its diameter; however, this will make the spring more susceptible to distortion and breakage. This can be prevented to a degree by shielding the arm of the spring with the acrylic baseplate or sheathing it in steel tubing. Springs are usually constructed in stainless steel, either 0.5-mm in diameter, which are activated approximately 3-mm; or 0.7-mm, which are activated by 1-mm to give a similar force.

Palatal finger springs

Palatal finger springs are constructed in 0.5- or 0.6-mm stainless steel wire and used to move teeth mesially or distally along the dental arch (Fig. 8.8). The incorporation of a helix increases the length of the wire and allows the delivery of lighter forces whilst a guard wire will protect the spring from distortion. By convention, the helix is placed such that activation of the spring is achieved as it is tightened and it unwinds as tooth movement occurs; the spring should be positioned at right angles to the planned tooth movement.

Buccal canine retractor

Buccal canine retractors are constructed in 0.7-mm stainless steel, reduced to 0.5-mm if sheathed (Fig. 8.9). These springs can be used to retract buccally placed maxillary canines; however, when activated it is mechanically difficult to apply force directly to the mesial surface of the tooth.

Z-spring

The Z-spring is constructed in 0.5-mm stainless steel wire and generally used to move one or two teeth labially (Fig. 8.10). Activation is achieved by pulling the spring away from the baseplate at an angle of approximately 45°, which will tend to displace the appliance away from the palate; good anterior retention is therefore important.

T-spring

T-springs are constructed in 0.5-mm stainless steel wire and used to move individual teeth either labially or buccally (Fig. 8.11). Activation is again produced by pulling the spring away from the baseplate and therefore retention also needs to be good.

Coffin spring

A coffin spring provides a useful alternative to a screw for expansion (Fig. 8.12). This heavy spring is constructed in 1.25-mm wire and activated by pulling the two halves of the appliance apart manually or flattening the spring with pliers. Coffin springs deliver high forces that will tend to displace the appliance and good retention is important.

Screws

Screws can be embedded into the baseplate of an appliance and activated by the patient progressively turning a key (Fig. 8.14). Screws can be effective for expansion to correct a posterior dental crossbite, or for distal movement of the buccal segments, often supported by headgear. Each quarter turn of the screw activates it by approximately 0.2-mm and, therefore, should be done by the patient once or twice a week.

Removable appliance design and use

Comprehensive orthodontic treatment is no longer undertaken with removable appliances alone because the results are invariably inferior to those produced by fixed appliances. However, removable appliances are relatively simple to use (Table 8.1), generally well tolerated by patients and can be used very effectively to correct minor occlusal problems (such as crossbites) in the mixed dentition or provide a valuable adjunct to fixed appliance therapy.

Table 8.1 Clinical use of removable appliances

Bite plane

In a growing patient, the incorporation of a flat anterior bite plane in a removable appliance allows eruption of the posterior teeth and reduction of a deep overbite (see Fig. 11.15). It can also facilitate earlier placement of a lower fixed appliance without impinging on the occlusion. An inclined bite plane can be useful following functional appliance therapy, either as part of a retainer or as an adjunct during the transition from fixed to functional appliances to help to maintain sagittal correction.

Jan 1, 2015 | Posted by in Orthodontics | Comments Off on Contemporary removable appliances
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