Page 18 - REMOVABLE ORTHODONTIC APPLIANCES

10 Removable Orthodontic Appliances
Figure 2.4 Bodily
movement of teeth, (a) A
force applied at a single point
on the crown results in
tipping, (b) To achieve apical
movement, or (c) bodily
movement, a force couple is
required. This can only be
effectively achieved by a
fixed appliance.

Figure 2.6 Application of force to the crown of a
Figure 2.5 An anterior bite plane allows vertical tooth, (a) The palatal cantilever spring is positioned
development of the posterior teeth. too far posteriorly, the tooth will be moved buccally.
The point of contact of the spring is also incorrect
resulting in unwanted rotation of the canine, (b)
Correct application of a palatal finger spring to a
canine.
erupted. For this reason, it is preferable not to
attempt to move a tooth until it has erupted
fully.
as distally. The unwanted buccal movement is
particularly difficult to avoid when the tooth is
Movements in the plane of the buccally positioned in the first place. In these
occlusion circumstances, a buccal spring, which can apply
a force at the required point, is essential.
The example selected in this discussion is the If the resultant force does not pass through
retraction of an upper canine, but the argument the long axis of the tooth, a rotation will be
applies equally to other situations. induced. Rotations of this sort may be particu-
The tooth will move in the direction of the larly difficult to avoid if the tooth is already
component of force perpendicular to its slightly rotated. Teeth in this position must be
surface. This is of particular importance when controlled by a spring which can apply the force
considering tooth movement in this plane. It is from a buccal direction.
common to find that the spring is positioned
too far posteriorly, so that the resultant force
does not lie in the required direction along the Rotations
line of the arch, but is directed buccally (Figure While rotations may inadvertently be intro-
2.6). The tooth will then move buccally as well duced as described above, the controlled rota-