Page 49 - REMOVABLE ORTHODONTIC APPLIANCES

Anchorage 41
to provide a point of attachment for intraoral
elastics, which deliver a force to the anterior
aspect of the upper fixed appliance. The recip-
rocal force is distributed evenly to the lower
arch. The method offers the advantage that a
removable appliance can be fitted quickly and
long before it would be possible to fit a lower
fixed appliance and progress to sufficiently
rigid archwires to accept class II traction.
In a class III occlusion an upper appliance
can be used to deliver class III traction. Such an
appliance can also be used to expand the upper
arch or to procline incisors. Again, the reten-
tion must be good and hooks provided at the
back of the appliance for attachment of elastics.

Anchorage consideration in
appliance design

The anchorage provided by a tooth will be
determined by the type of tooth movement
permitted and the surface area of the root.
Bodily movement, for example, offers more
resistance than does tipping. With removable
appliances it is not possible to prevent tipping
of the anchorage teeth, but good adaptation of
clasps and bows will minimize it.
When designing an appliance, it is important
to incorporate as many teeth as possible into
the anchorage, thus increasing the total root
area of the teeth involved and consequently the
resistance of the anchorage unit (Figure 6.4).
The teeth in the anchorage unit should be
well clasped and there should be close contact
of the acrylic to the palatal aspect of the teeth.
This will help to minimize unwanted movement
of the anchorage teeth.
Teeth should only be moved in small groups,
except where reciprocal anchorage applies or
extraoral traction is being used. As a general
rule only one buccal tooth on each side should
be moved at a time and canines should be
moved separately from incisors. An attempt to
retract the incisors together with the canines is
likely to result in at least as much forward
movement of the posterior anchor teeth.
Figure 6.4 Anchorage considerations m relation to
tooth movement, (a) Moving a single tooth, the other
teeth in the arch and the palate provide favourable
anchorage, (b) When retracting 3|3 the reaction to
Anchorage assessment
this movement results in forward movement of the
anchorage teeth, (c) When 43|34 are retracted, there
Anchorage may appear adequate when the are fewer teeth available for anchorage and a greater
appliance is designed but its stability needs to be number of teeth to be moved. The anchorage
assessed at each visit so that it can be reinforced balance is less favourable.