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.

Figure 8.3 Adams clasp.

Adjustment at the arrowhead of the clasp (right arrow) will move it horizontally whilst adjustment at the point the wire emerges from the base plate (left arrow) will move the arrowhead vertically.

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.

Figure 8.4 Southend clasp.

Ball-ended clasp

Ball-ended clasps engage into interproximal undercuts between the teeth (Fig. 8.5) and are activated by bending the ball towards the contact point.

Figure 8.5 Ball-ended clasps.

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.

Figure 8.6 Plint clasp.

Labial bow

A labial bow is constructed from 0.7-mm stainless steel wire and can provide retention from the labial surface of the incisor teeth, which can be increased by contouring the wire around these teeth in a fitted labial bow or by placing an acrylic facing on the wire of the bow (Fig. 8.7). The labial bow is afforded flexibility by incorporating U-loops at each end, which allow activation by compression.

Figure 8.7 Standard (left), fitted (right upper) and acrylic-faced (right lower) labial bows.

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:

• It should be delivered at right angles to the long axis of the tooth and through a surface parallel to it; otherwise, a vertical force is introduced, which will tend to displace the appliance or intrude the tooth.

• It should pass as close to the centre of resistance as possible to reduce rotation.

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

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.

Figure 8.8 Palatal finger spring.

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.

Figure 8.9 Buccal canine retractor.

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.

Figure 8.10 Z-spring.

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.

Figure 8.11 T-spring.

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.

Figure 8.12 Coffin spring in a removable appliance with headgear tubes attached to the first molar cribs.

Active labial bows

An active labial bow can be used to reduce an increased overjet by tipping the teeth palatally if the upper labial segment is proclined and spaced. However, a normal labial bow will only allow a small range of activation and this can be improved either by increasing the amount of wire in the bow, as in a Mills bow, or by constructing it in a lighter wire, such as a Roberts retractor (Fig. 8.13). The Roberts retractor is constructed in 0.5-mm stainless steel with buccal arms sheathed in stainless steel tubing. Activation occurs by bending the vertical arms of the bow towards the palate and trimming the acrylic behind the upper incisors to allow palatal movement.

Figure 8.13 Roberts retractor.

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.

Figure 8.14 Upper removable appliance to correct a posterior crossbite in the mixed dentition. The design includes Adams cribs on the first permanent molars, ball-ended clasps between the deciduous molars, a midline expansion screw and posterior acrylic capping.

Elastics

Elastomeric forces can also be applied from a removable appliance and these can be useful in providing light force, which can be reactivated regularly by the patient. Intra-arch elastics can be used to retract the upper incisors as well as applying an intrusive force in patients with reduced periodontal support (see Fig. 8.2). Inter-arch application of elastics from removable appliances requires good retention to avoid displacement and is generally avoided.

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

• Impressions taken in alginate;

• Appliance fitted within 2 weeks of the impression to ensure good fit;

• Cribs adjusted for retention, springs activated and acrylic trimmed to allow planned tooth movement;

• Patient given instructions on wear and appliance care;

• Patient reviewed in one month; and

• Signs of good wear include evidence of tooth movement, loosening of the appliance, good speech with the appliance in place and the ability of the patient to insert and remove the appliance unaided.

Expansion

Removable appliances can provide an effective method for expanding the maxillary dental arch, particularly in the mixed dentition (Fig. 8.14):

• The patient activates the appliance by turning a midline screw until the crossbite is overcorrected (the palatal cusps of the maxillary buccal teeth occluding on the incline of the buccal cusps of the mandibular teeth); and

• Following expansion, the posterior capping can be removed and the appliance worn at night for a period of three to six months as a retainer.

Correction of anterior crossbite

Removable appliances are also effective at correcting an anterior crossbite (Fig. 8.15). Palatal Z- or T-springs can be used to correct one or two teeth in anterior crossbite, usually in conjunction with posterior acrylic capping to open the bite and allow movement of the teeth out of crossbite. Occasionally some anterior retention in the form of a Southend clasp may also be required (Fig. 8.16).

Figure 8.15 Correction of an anterior crossbite with an upper removable appliance.

Figure 8.16 Upper removable appliance to correct an anterior crossbite affecting the UR1 in the mixed dentition.

The design includes Adams cribs on the first permanent molars, ball-ended clasps between the deciduous molars, a modified Southend clasp on the UL1, a Z-spring for the UR1 and posterior acrylic capping.

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.