Page 27 - REMOVABLE ORTHODONTIC APPLIANCES

Palatal springs
These springs are used for mesio-distal move-
ment and buccal movement of teeth. They are
usually the springs of choice for mesio-distal
tooth movement because they are protected by
the base plate and so are less liable to damage.

Figure 3.10 A cranked cantilever spring to procline
Single cantilever spring (finger spring) a central incisor.
This spring may be used to move teeth labially
or in the line of the arch. It is normally
constructed from 0.5 mm hard stainless steel
wire. Some operators prefer to use 0.6 mm wire
and activate the spring by a smaller amount
(50% of the deflection will give about the same
force). A coil is incorporated into the spring
close to its emergence from the baseplate. This
increases the length of wire and thus the flexi-
bility of the spring. For maximal resilience, the
coil should lie on the opposite side of the spring
from the tooth so that it 'unwinds' as the tooth
moves (see Figure 3.1). For labial or buccal
movement of teeth, a single cantilever spring
should be cranked (Figure 3.10) to keep it clear Figure 3.11 The coil of a palatal cantilever spring
of the other teeth. This also ensures that the should lie on a line from the mid-point of the crown
at 90 degrees to the direction of movement.
spring is protected by the baseplate even when
the tooth moves.
When a palatal cantilever spring is
constructed, the intended path of tooth move-
ment is determined and the required point of
contact of the spring with the tooth is marked
on the model. A line is then drawn on the
model, at a right angle to the path of tooth
movement and through the mid-crown width of
the tooth. This indicates the correct position of
the coil (Figure 3.11). The spring arm should be
straight unless it has to be cranked to establish
the correct contact with the tooth (particularly
when it is being used to retract a canine) to
ensure that the point of contact with the tooth
is correct and that the tooth will move in the
right direction (see Figure 2.6, p. 10). The free
end is finished neatly after the baseplate has
been processed. A palatal spring is usually
boxed to protect it from damage, so that it lies
in the recess between the baseplate and the
mucosa. A possible problem is that if the spring Figure 3.12 Adjustment of the guard wire to ensure
catches between the teeth during removal it free movement of spring.
may be pulled away from the baseplate. Re-
adjustment is difficult and will weaken the distortion can be prevented by incorporating a
spring. If the spring is being used to carry out guard wire palatal to it (Figure 3.12) so that a
movement along the line of the arch, such
channel is formed between the baseplate and