Classification of Orthodontic Appliances

Forms of dysgnathia can be treated with mechanically or functionally effective, active or passive, rigid or elastic appliances. In the case of extreme malpositions, surgical measures may become necessary prior to orthodontic treatment (eg, extraction to eliminate crowding). A fundamental distinction is made in orthodontic treatment devices between removable and fixed appliances.

Removable appliances are divided into three groups:

1. Active appliances only act in the maxilla or in the mandible.
2. Functional orthodontic appliances for simultaneous treatment of both jaws are passively acting appliances.
3. Retainers are used to secure the treatment outcome.

There are smooth crossovers between active and passive appliances because many have both active and passive design elements, and they are often used alternately.

Active appliances produce and transfer forces actively to the teeth, periodontium, areas of the jaw, and other parts of the masticatory system by means of springs, wire ligatures, screws, and elastics. These are plate or framework appliances and create their effect continuously or in fixed periods of wear.

Removable active appliances are used in the primary dentition as well as the mixed and permanent dentitions. In the primary dentition, reverse articulations (also known as crossbites) and individual malpositions can be corrected; in the mixed dentition, comprehensive functional orthodontic treatments are prepared, and in the permanent dentition they serve as retainers. They are fabricated by the dental technician on a working model according to the dentist's instructions and comprise the following components:

• Baseplate made of colored acrylic
• Retentive components made of hard wire to anchor the appliance
• Active components in the form of hard wire springs or double-thread screws

Passive appliances act on the teeth, periodontal tissues, alveolar process, jawbones, and TMJs with the functional forces of the masticatory, lingual, lip, and cheek muscles and influence these muscles directly by specifically stimulating, inhibiting, or re-educating them in their activity. Such appliances are functional orthodontic devices, usually removable and fitting loosely in the mouth; the fixed inclined plane is one exception.

Functional orthodontic appliances are based on the idea that form and function are directly dependent on each other. They manage without continuous application of forces and make use of forces within the biologic range of tolerance.

Functional orthodontic appliances are used in the mixed dentition to correct the position of the jaws in relation to each other. The activator as a typical functional orthodontic appliance is employed in the treatment of displaced occlusal position.

Crozat appliances are removable devices made of archwires that are soldered together and highly elastic. They act as both active and functional orthodontic treatment appliances.

Retainers stabilize corrected tooth positions and do not transfer any additional forces. Retainers can be designed as removable or fixed. A retainer is simply constructed and contains no active components, only retentive components.

Advantages of removable appliances include the following:

• They allow simple, safe treatment without causing any harm to the patient's health.
• No damage to the teeth occurs as a result of removing the appliance from the mouth to activate it.
• Problem-free cleaning allows for minimal caries attack.
• The treatment appliance is easy to remove.
• Targeted fluoridation of the teeth is possible.

Disadvantages of removable appliances include the following:

• Bodily tooth movements are not possible.
• The treatment outcome is dependent on patient compliance.
• Continuous, regular wearing periods are necessary.
• There may be poor wearing comfort and restriction of speech.

Fixed orthodontic appliances are intraoral band-arch appliances, inclined planes, and palatal expansion appliances.