Page 17 - REMOVABLE ORTHODONTIC APPLIANCES

Biomechanics of tooth movement 9

Figure 2.3 When a tooth is tipped with a removable
appliance, the fulcrum of rotation is approximately
Figure 2.2 (a) When a force is applied to a curved 40% of the length of the root from the apex.
surface, the direction of the resultant movennent is at
right angles to the tangent at the point of contact, (b)
A partially erupted tooth will be intruded if a spring
is applied to the cuspal incline. in one direction, the apex moves in the opposite
direction (Figure 2.3). The exact level of the
fulcrum depends on a number of factors, which
Other parallel to it (Figure 2.2). Where the are not under control of the orthodontist: these
surface is curved, the force is resolved perpen- include root shape and the distribution of fibre
dicular and parallel to the tangent at the point bundles within the periodontal ligament.
of contact. If the force is applied at an angle to
the surface, tooth movement will be produced Bodily movements
by the perpendicular component. Thus, the
tooth will not move in the direction of the If a tooth is to be moved bodily, a force couple
applied force. must be applied to the crown in conjunction
Although the initial movement must be with the original force (Figure 2.4). This would
considered in three dimensions, it is convenient be necessary to allow precise control over the
to discuss it in the two planes which span the position of the fulcrum but is not normally a
space: first the plane through the long axis of practical undertaking with removable appli-
the tooth and in the direction of tooth move- ances. It is possible with fixed appliances and.
ment, and second, a plane of cross-section. to a very limited extent, with fixed components
used in conjunction with removable appliances.
Movements in the plane of the long Intrusion
axis
When a bite plane is incorporated in an appli-
When a force is applied to the crown of a tooth, ance, an intrusive force is applied to the teeth
movement is resisted by the periodontal liga- which contact it. The amount of true intrusion
ment. is small and overbite reduction with removable
appliances is largely the result of eruption of
the posterior teeth (Figure 2.5). Where an
Tipping movements incisor does not occlude perpendicular to an
A force applied at a single point on the crown anterior bite plane, it may be tipped labially.
will tip the tooth about a fulcrum. Although Intrusion of teeth may also be produced
many texts suggest that tipping takes place unintentionally by the incorrect application of a
about a fulcrum within the apical third of the spring. Where, for example, a spring to retract
root, it can be shown that the centre of rotation a canine is applied to the cuspal incline, the
is usually about 40% of the length of the root tooth will be intruded as well as retracted. This
from the apex (Christiansen and Burstone, most often happens when attempts are made to
1969). This means that while the crown moves retract a canine which has only partially