Removable Orthodontic Appliances: Design, Components, Indications and Mechanics

Removable orthodontic appliances are devices that can be inserted into and removed from the mouth by the patient (Proffit, 2018). The orthodontic force produced by a removable appliance depends on three main factors: the rate of force application, the intensity of the force, and the resistance of the tissues to which the force is applied (Romanec et al., 2018).

Removable appliances are broadly classified into active and passive appliances (Romanec et al., 2018).

Active appliances contain mechanical components that generate force to produce dentoalveolar changes. These components include springs and screws (Proffit, 2018). In contrast, passive removable appliances do not contain mechanical components intended to actively move teeth. Their primary function is to maintain or retain the achieved position of the teeth following orthodontic treatment (Proffit, 2018; Romanec et al., 2018). Examples include habit breakers, space maintainers, and Hawley retainers (Proffit, 2018).

Indications and contraindications

Removable appliances are mainly indicated for dentoalveolar malocclusions where simple tipping movements are sufficient to correct the malocclusion (Cain, 1986). They may also be used for overbite reduction through anterior intrusion and posterior extrusion (Cain, 1986). Mildly narrow arches with limited crowding can be managed using simple expansion appliances (Cain, 1986). A unilateral crossbite involving a single malpositioned tooth may also be corrected using tipping movements (Proffit, 2018).

Removable habit-breaking appliances can be used to prevent or intercept the effects of abnormal oral habits (Gawlik et al., 1995). In addition, removable appliances can be used for retention to maintain corrected tooth positions following fixed orthodontic treatment (Proffit, 2018).

However, removable appliances have limitations. They are generally unsuitable when significant skeletal discrepancies, severe crowding, or bodily tooth movement are present (Proffit, 2018). Other contraindications include severe vertical discrepancies, severe deep overbite, apical malposition, and severe tooth rotations, as these movements are difficult to achieve predictably with removable appliances (Proffit, 2018).

Advantages and disadvantages

Figure 1 compares the major advantages and disadvantages associated with the use of removable orthodontic appliances (Proffit, 2018).

Components of removable orthodontic appliances

Although removable appliances can vary considerably in design, their main components consist of an active component, a retentive component, and an acrylic base plate (Proffit, 2018).

The active component produces the force required for tooth movement. Examples include bows, springs, screws, and elastics (Cain, 1986).

The retentive component maintains the appliance in position and prevents unwanted displacement during function. Common examples include clasps and bows (Cain, 1986).

The acrylic base plate forms the main body of the appliance and provides the framework that incorporates and connects the active and retentive components into a single functioning appliance (Proffit, 2018).

Mechanics of removable orthodontic appliances

The mechanics of removable appliances depend on the type of tooth movement required. Common movements include tipping, extrusion, intrusion, expansion, and distalization (Proffit, 2018).

When designing a removable appliance, the wire dimension must be selected according to the amount of force required to produce the desired tooth movement (Cain, 1986). The flexibility of an orthodontic wire is influenced mainly by its length and diameter (Proffit, 2018).

The relationship between wire diameter, length, force, and deflection can be expressed as:

D ∝ PL³/T⁴

Where:

• D = amount of deflection

• P = amount of pressure

• L = length of the spring

• T = thickness/diameter of the spring

The force generated by the wire is strongly influenced by its diameter and length (Proffit, 2018). Increasing the diameter substantially reduces flexibility, whereas increasing the length increases flexibility and the range of activation. For example, doubling the diameter can decrease deflection by approximately 16 times, while doubling the length can increase flexibility by approximately eight times (Proffit, 2018).

For a single-rooted tooth, the recommended orthodontic force is approximately 25–40 g (Proffit, 2018). Excessive force may delay tooth movement, overload the anchorage, and cause patient discomfort (Cain, 1986). The direction of tooth movement is determined by the point at which the spring contacts the tooth (Lohakare, 2008).

References

• Cain, S.B., Cain Steve B, 1986. Active removable orthodontic appliance and method of straightening teeth. U.S. Patent 4,609,349.

• Gawlik, J.A., Ott, N.W. and Mathieu, G.P., 1995. Modifications of the palatal crib habit-breaker appliance to prevent palatal soft tissue embedment. ASDC journal of dentistry for children, 62, pp.409-411.

• Lohakare, S.S., 2008. Orthodontic removable appliances. JAYPEE BROTHERS PUBLISHERS.

• Romanec, C., Dragomir, B. and Bica, C., 2018. The prophylactic orthodontic treatment with removable appliances in children. Rev. Chim.(Bucharest), 69(3), pp.693-696.