Roberts Harry1 and J Sandy2

Anchorage is an important consideration when planning orthodontic tooth movement. Unwanted tooth movement known as loss of anchorage can have a detrimental effect on the treatment outcome. Anchorage can be sourced from the teeth, the oral mucosa and underlying bone, implants and extra orally. If extra-oral anchorage is used, particularly with a facebow then the use of at least two safety devices is mandatory.

ORTHODONTICS

1.

Who needs

orthodontics?

2.

Patient assessment and

examination I

3.

Patient assessment and

examination II

4.

Treatment planning

5.

Appliance choices

6.

Risks in orthodontic

treatment

7.

Fact and fantasy in

orthodontics

8.

Extractions in

orthodontics

9.

Anchorage control and

distal movement

10. Impacted teeth

11.

Orthodontic tooth

movement

12. Combined orthodontic

treatment

^Consultant Orthodontist, Orthodontic Department, Leeds Dental Institute, Clarendon Way, Leeds LS2 9LU; 2Professor of Orthodontics, Division of Child Dental Health, University of Bristol Dental School, Lower Maudlin Street, Bristol BS1 2LY;

Correspondence to: D. Roberts-Harry E-mail: [email protected]

^Consultant Orthodontist, Orthodontic Department, Leeds Dental Institute, Clarendon Way, Leeds LS2 9LU; 2Professor of Orthodontics, Division of Child Dental Health, University of Bristol Dental School, Lower Maudlin Street, Bristol BS1 2LY;

Correspondence to: D. Roberts-Harry E-mail: [email protected]

Refereed Paper doi:10.1038/sj.bdj.4811031 © British Dental Journal 2004; 196: 255-263

Anchorage is defined as the resistance to unwanted tooth movement. Newton's third law states that every action has an equal and opposite reaction. This principle also applies to moving teeth. For example, if an upper canine is being retracted, the force applied to the tooth must be resisted by an equal and opposite force in the other direction. This equal and opposite force is known as anchorage.

Anchorage may be considered similar to a tug of war. Two equal sized people will pull each other together by an equal amount. Conversely a big person will generally pull a small one without being moved. However, if two or more smaller people combine then their chances of pulling a big person will increase. Similarly, the more teeth that are incorporated into an anchorage block, the more likely it is that desirable as opposed to undesirable tooth movements will occur. Undesirable movement of the anchor teeth is called loss of anchorage.

If an upper canine is to be retracted, with bodily movement using a fixed appliance, the force applied to the tooth will be approximately 100 g (Fig. 1a). Forces in the opposite direction varying from 67 g on the first permanent molar to 33 g on the upper second premolar resist this. Low levels will produce negligible tooth movement and the effect of a light force of 100 g would be to retract the canine with minimal anterior unwanted movement of the anchored teeth. However, if the force level is increased to say 300 g (Fig. 1b), the force levels on the anchor teeth increase dramatically to the level where unwanted tooth movements will occur. Although the canine may move a little distally, the buccal teeth will also move mesially. Space for the canine retraction may be eliminated with insufficient space left for alignment of the anterior teeth. Figure 1c compares the root area of some of the upper teeth. The combined root area of the upper incisors and upper canines is around the same as that of the first molar and premolars. Therefore, if the upper labial segment including the upper canines is retracted in a block, there will be an equivalent mesial movement of the upper molar and upper premolar. These factors need to be very carefully considered in planning anchorage requirements and tooth movement.

Anchorage may be derived from four sources:

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