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.
Unlike fixed appliances, which can control the movement of a tooth in three dimensions, the force applied by a removable appliance is mediated by a spring, elastic or piece of acrylic, which can only make point contact with the tooth. As no reactionary force or couple is created, in these situations removable appliances are only capable of simple tooth tipping and apical or bodily movement is not possible. Tipping can be carried out in mesial, distal, buccal or lingual directions, with the rotation occurring about a fulcrum located close to the middle of the tooth root. For the retraction of teeth already mesially inclined, tipping can be an effective tooth movement; but it is inappropriate for teeth that are upright or distally inclined (Fig. 8.1).
Figure 8.1 Tipping of a tooth with a removable appliance is appropriate if it needs uprighting but inappropriate if the tooth is already angulated in the direction of the intended movement. In this case a mesially angulated canine can be uprighted with an activated spring (upper panels), but this is inappropriate if the tooth is already distally angulated as this would lead to excessive tipping (lower panels).
Incorporating an anterior bite plane on a removable appliance will increase the vertical dimension and allow differential eruption of the posterior teeth, which in a growing patient is an effective way to reduce a deep overbite.
If space is available, an anterior tooth in crossbite can be pushed over the bite using a removable appliance with an activated palatal spring or screw. Stability will depend upon achieving a positive overbite on the corrected teeth, to prevent them relapsing back into crossbite.
By incorporating a midline expansion screw or spring in an upper removable appliance, the maxillary arch can be widened. This is effective for the correction of posterior crossbites in the mixed dentition, but will produce only tipping of the buccal teeth; the crossbite should therefore be dental not skeletal in origin.
A whip-spring or elastic from a removable appliance can be used to extrude teeth by engaging a fixed attachment on a tooth to generate a vertical component of force. This can be useful for extrusion of an impacted central incisor in the mixed dentition (see Fig. 10.22).
An elastic run underneath a fixed attachment or bracket via a removable appliance can be used to intrude teeth (Fig. 8.2). Good retention is required, as the reaction to any intrusive force will tend to unseat a poorly retained appliance.
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).
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.
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.
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.
Ball-ended clasps engage into interproximal undercuts between the teeth (Fig. 8.5) and are activated by bending the ball towards the contact point.
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.
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.
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 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 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.
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-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.
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.
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.
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.
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.
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.
• Cribs adjusted for retention, springs activated and acrylic trimmed to allow planned tooth movement;
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).
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.
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.