Types of Anchorage for Force Distribution
When a tooth or group of teeth is moved by springs and bands, the appliance has to be supported on other teeth to exert the movement thrust. In the process, the same movement thrust acts on the abutment, which means the teeth to which the appliance is anchored are loaded and may also be moved. Depending on the nature of the fixation of appliances, the distribution of forces in the masticatory system being treated can vary widely. In relation to distribution of forces for orthodontic tooth movements, a distinction is made between the following types of anchorage:
- Reciprocal anchorage with uniform distribution of forces
- Stationary anchorage with unequal distribution of forces
- Maxillomandibular anchorage with maxillomandibular distribution of forces
- Extraoral anchorage with extraoral distribution of forces
Reciprocal anchorage offers uniform distribution of forces because the orthodontically exerted force lies between two equally strong partners that put up equal resistance (Fig 10-15). The anchor tooth is moved just like the tooth being corrected. An example is an active plate with a midline expansion screw. The plate will engage on insertion and exert the same pressure on both halves of the jaw and rows of teeth if there are teeth of the same size on both sides. If the gap between two incisors is to be closed with the aid of elastics, reciprocal anchorage exists and the teeth are moved equally toward each other because the same regulating force is exerted on both teeth.
Stationary anchorage arises if teeth or groups of teeth of unequal size are exposed to regulating pressure. An unequal distribution of forces is achieved because two unequally sized partners offer different amounts of resistance (Fig 10-16). The stronger components are not moved by the absorption of force, which means the stimulus remains below the relevant threshold and tissue remodeling does not ensue. There are smooth crossovers with reciprocal anchorage, because stationary and reciprocal anchorage can be combined in a single appliance. An expansion plate acts reciprocally on both halves of the jaw, and with a mini-spring for single-tooth movement, the plate acts in a stationary fashion on the individual tooth; the spring force remains below-threshold for the teeth in the entire block. Using orthodontic implants or mini bone screws in the form of endosseous palatal implants results in anchorage for an orthodontic appliance that has full positional stability.
Maxillomandibular anchorage produces maxillomandibular distribution of forces between the mandibular and maxillary dental arches when, for example, disto-occlusion or mesiocclusion has to be rectified (Fig 10-17). An elastic module between a maxillary and a mandibular appliance is intended to pull the mandible in a distal direction. In this case, the maxilla is absolutely immovable so that a force from the maxilla is exerted on the mandible, enabling movement of the occlusal position. The most important maxillomandibular anchorage device is the activator, which as a passive appliance can produce maxillomandibular distribution of forces. Fixed hinges between the rows of teeth to remedy a disto-occlusion (Herbst appliance) are also classified as maxillomandibular anchorage.
Extraoral anchorage arises when orthodontic treatment is supported by bands or wire brackets fitted outside the mouth (Fig 10-18). Extraoral anchorage is usually intended to eliminate skeletal deformities, eg, to treat prognathism or in cases of maxillary atrophy.
The headgear is a device for extraoral anchorage in which a distinction is made between different directions of pull: cervical pull, horizontal pull, high pull, vertical pull, and frontal pull. It is an active device with fixed and removable components. There are three essential elements to a headgear: (1) head cap or neck pad with elastics for extraoral anchorage; (2) facebow for transferring forces (Fig 10-19); and (3) cemented bands with tubes to the maxillary molars, for intraoral application of force.
Headgear is used for the maxillary and mandibular dentition as well as the structures of the midface, with a dental and skeletal effect both in the distal movement of teeth or groups of teeth and in stabilizing molar anchorage in multiband treatment. It is also used for correcting inhibited sagittal development.