The human body has a natural resistance to displacement, and this resistance is called anchorage. Anchorage is based on Newton’s third law, which states that for every action there is an equal and opposite reaction.
In orthodontics the goal of anchorage is to resist unwanted tooth movement. Segments of teeth that resist movement and serve as “anchors” are used to pull against other segments that are proposed to be moved. Usually the anchor segment will contain more teeth, or teeth with greater root surface area, than the segment of teeth that is to be moved.
Numerous ways have been used to boost anchorage. These include auxiliary devices such as headgear, trans-palatal arches and other appliances. Many of these appliances are uncomfortable for patients, leading to less-than-desired levels of compliance and thus compromising treatment outcomes.
Dental implants have the ability to aid in anchorage, either directly or indirectly. Direct anchorage refers to any situation in which the forces that originate from the actual implant itself are used to augment anchorage. One example is a restored dental implant with an orthodontic bracket bonded to the restoration.
Indirect anchorage refers to a situation in which the implant stabilises multiple teeth, which then serve as an anchor unit. The most common method of achieving that is by placing an implant in the mid-palatal or retromolar regions (back area of the last lower molar teeth), and then linking the implant to the natural teeth by means of a wire or rigid fixation device, such as a trans-palatal arch.
This results in a stable anchorage unit composed of multiple teeth tethered together by means of a dental implant that serves as an additional anchorage. Several types of implant can be used to aid in orthodontic treatments, such as conventional titanium dental implants or titanium mini-screws (mini implants).
Conventional implants for orthodontic anchorage can be useful in cases in which the teeth are super-erupted after loss of the opposing teeth. In such cases intrusion is necessary in addition to the prosthetic replacement of the missing teeth.
The main advantage of this method is that the definitive restorations can also simplify orthodontic treatment.
Mini implants are an alternative approach that allows the placement of very small screws in areas where other implantable devices cannot be placed. These screws do not require osseointegration (functional attachment of the implant to the surrounding bone); they rely on mechanical retention, which makes their removal relatively simple and non-invasive.
Another advantage is that they can be loaded immediately. Additionally, local soft tissue irritation is reported to be minimal compared with irritation from other types of anchorage.