1. A structure or structures used to provide support or resistance to unwanted tooth movement for an orthodontic appliance. Absolute anchorage describes the use of an implant or onplant as a specific aid to achieve anchorage during orthodontic treatment. Extra-oral anchorage is obtained from outside the mouth (e.g. by linking the appliance by means of hooks and elastics to a head cap). Intra-oral anchorage is obtained from within the mouth and may be simple, where there is active movement of one tooth versus several anchor teeth, compound, where teeth of greater resistance to movement are used as anchorage for the movement of teeth which have less resistance to movement, or reciprocal, where two groups of teeth are pitted against each other resulting in equal reciprocal movement of both groups of teeth. Intra-oral anchorage may also be described as intra-maxillary (anchorage obtained from teeth in the same jaw) or inter-maxillary (anchorage obtained from teeth in the opposing jaw). Mini-plate anchorage is obtained by securing small titanium plates with screws to the cortical bone of either jaw: they do not osseointegrate and therefore do not need to be exposed and can be loaded immediately after placement. Micro-screw anchorage is achieved by using titanium or surgical steel screws approximately 1–2mm in diameter and 4–9mm in length placed in alveolar bone; they are less invasive than implants, onplants, or mini-plates. Since they do not osseointegrate they can be loaded immediately after placement and can be removed by simply unscrewing.
2. The means of retention of a restoration.
3. The means of retention of a crown or bridge.
A review of research papers on orthodontic anchorage: Feldmann I., Bondemark L. Orthodontic anchorage: a systematic review. Angle Orthod 2006;76(3):493–501.