Page 28 - REMOVABLE ORTHODONTIC APPLIANCES

20 Removable Orthodontic Appliances
guard. It is important that, during construction, further work-hardened by bending then frac-
the spring is blocked out adequately, so that it ture may occur. The correct site of adjustment
can act freely, and is not impeded by the guard. is in the free arm of the spring as close to the
The use of a guard is a matter of personal coil as possible. In cases where the direction of
preference and some operators find that guards the spring has to be corrected to achieve the
are more trouble than they are worth. A guard intended direction of movement adjustment
can help to prevent distortion of a spring - may be carried out further from the coil.
which can be difficult to correct - but if the
technician does not block out the spring
adequately before processing the baseplate, the Double cantilever spring ('Z' spring)
guard itself may obstruct free movement.
In some situations, particularly where an
incisor is to be proclined, space for a single
Adjustment
cantilever spring is limited. In these circum-
A palatal spring is simple to adjust. Use a stances, a double cantilever, or 'Z' spring
dental mirror with the appliance in place to (0.5 mm wire) can be used (Figure 3.14). It is
check that the spring contacts the tooth important that the limbs are as long as possible,
correctly and lies close to the gingival margin otherwise these springs can be rather stiff. If
(Figure 3.13). At the time of fitting slight acti- the limbs of the spring are short, the range of
vation of no more than 1-2 mm is advisable, but adjustment is limited and the patient may find
at subsequent visits an adjustment of 3 mm is the appliance difficult to insert. The spring
appropriate. The spring should not be bent should be perpendicular to the palatal surface
where it emerges from the baseplate - this is a of the tooth; otherwise it will tend to slide
site of stress concentration and if the wire is incisally and to intrude the tooth. If a lateral
incisor is to be proclined and space is very short
it may be permissible to construct a 'Z' spring
in 0.35 mm wire.
The mechanical principles and general
features of the double cantilever spring are
similar to those of the single cantilever spring.

Adjustment
Adjustment can be carried out to the palatal
limb first - close to the coil at the fixed end of
the spring to establish the degree of activation
- then at the other end of the limb to keep the
free limb perpendicular to the intended direc-
Figure 3.13 (a) When a palatal canine spring is first tion of movement of the tooth. It is often possi-
fitted the spring may have to pass over the contact ble to activate the spring in a single movement
point between the teeth, (b) Once tooth movement by grasping its outer arm in the beaks of the
has commenced the spring should be re-contoured pliers and pulling it gently forward and away
closer to the gingival margin.
from the acrylic baseplate.

'T' spring
Where a premolar, or sometimes a canine, is to
be moved buccally, the patient may find inser-
tion of the appliance very difficult if a single or
double cantilever spring is used. A 'T' spring
(see Figure 3.9) made from 0.5 mm wire, can
be much simpler to manage. The mechanical
Figure 3.14 A double cantilever spring (0.5 mm
wire). principles are similar to those of a single