Active orthodontic appliances include the removable, self-activating plates with which artificial, continuously applied orthodontic forces bring about changes in tooth or mandibular position by means of springs, screws, or elastics (Fig 10-20).
An active plate can expand dental arches in a transverse direction and stretch them in a sagittal direction. Individual tooth movements, such as tipping, rotation, and lateral displacement, can be achieved with them; reverse horizontal overlaps (also known as overjets) and locally limited reverse articulations can also be corrected with active plates. Malocclusions that are not limited to the alveolar region but have a skeletal origin cannot be readily treated with active plates.
Active plates are removable appliances fabricated by dental technicians. They generally have the following advantages over fixed appliances:
- Damage to the teeth can be avoided because patients are able to remove the appliance themselves if they experience pain.
- The action of orthodontic forces can be interrupted, and tissue structures adapt more quickly.
- Appliances and teeth can be cleaned more easily and more thoroughly.
- An appliance can be removed if a patient is troubled by the esthetic effect.
Disadvantages of removable active plates include the following:
- Wearing times can only be monitored with the patient's cooperation.
- Clumsy handling by the patient can damage the appliance.
- Mechanical effects of the retentive and active components can damage dental enamel.
The elements of an active plate—the baseplate, the retentive components, and the active compo-nents—perform various functions. To be able to carry out movements with active plates, the baseplate parts or retentive components must not inhibit tooth movement, which means that the direction and the magnitude of movement must be properly planned. Eruption movements of the teeth also must not be impeded by baseplate margins or bite planes.
The baseplate is a rigid acrylic plate that houses the retentive and active components and fits closely to the palate or the inside of the inferior alveolar ridge. It can be divided in a variety of ways, enabling the individual segments of the baseplate to be moved differently. The baseplate is fixed to the teeth with numerous clasps. The edge of the baseplate lies against the teeth below the tooth equator and extends into the interdental spaces. The baseplate may be lengthened or widened by means of anterior or posterior bite planes.
Posterior bite planes on the posterior tooth surfaces are used to inhibit vertical migration of the posterior teeth and block that of the anterior teeth (Fig 10-21), so that enforced occlusal positions (eg, reverse articulation of individual incisors) are corrected.
Posterior and anterior bite planes can be used to correct displacements of occlusal position as the mandible is guided into the intended position during biting. Joint changes can be stimulated, mandibular guidance being enforced with additional elastics.
The action of the active plate arises when the existing active components are activated between the segments of the baseplate. The baseplate margins exert pressure on the teeth and induce a change in the dental arch when the screws or Coffin springs are activated (Fig 10-22). Expansion, retraction, or extension of the dental arch can take place, depending on how the baseplate is divided and how the individual components are moved.
The baseplate margin can avoid individual teeth so that these teeth are excluded from the active effect of the plate. If the baseplate margin only avoids specific contact areas selectively, the rotation of an individual tooth may also be enforced.
The clamping force applied when the appliance is inserted acts on the teeth. On insertion, the teeth tip and generate remodeling stimuli over areas of tension and compression. As well as the remodeling stimuli in the periodontal tissue, there is a compressive effect on the alveolar process, giving rise to changes in this area. In addition, the formation of new bone at the sutures may be initiated if tensile effects on this area exist. This leads to widening or extension of the palatal area.