Page 25 - REMOVABLE ORTHODONTIC APPLIANCES

Active components 17
ciably less than 1 which means that the spring is
less flexible in a mesio-distal direction than it is
vertically. This makes them awkward for the
operator to adjust and for the patient to
manage (Figure 3.4).

Spring design
In order to have maximum flexibility a palatal
cantilever spring needs to be as long as possi-
ble, however, the dimensions of the oral cavity
Figure 3.3 Maximum activation of a 0.5 mm palatal determine the maximum length of spring. It is
canine retraction spring. usual to incorporate a coil, of about 3 mm

Figure 3.4 Stability
ratio of buccal canine
springs, (a) The standard
buccal canine spring is
more flexible vertically
than it is mesio-distally.
(b) Supported buccal
canine retractor has the
advantage that it is more
flexible mesio-distally
than vertically.

likely to insert it incorrectly. With a smaller internal diameter, in order to increase the
deflection the force applied will decrease effective length of the wire. Whenever possible
rapidly as the tooth moves so that intermittent the coil should be designed so that it 'unwinds'
movement will have to be accepted unless the as the tooth moves, because the elastic recovery
spring is activated more frequently. The will be better than that of a spring loaded in the
expected rate of tooth movement is between 1 opposite direction. A palatal spring constructed
and 2 mm a month, which means that monthly in this manner, which is boxed for protection
adjustments are sufficient if an activation of and made in 0.5 mm wire, has favourable char-
3 mm is used. The force delivered by palatal acteristics (Figure 3.5).
springs of 0.5 mm wire increases by about 15 g
for each millimetre of deflection, so that an
activation of about one-third of a tooth width
(3 mm) will deliver a force of about 45 g. If the
tooth moves 1 mm in a month, the residual
force will be about 30 g, which is still sufficient
to produce tooth movement.
Spring stability is also important - the ideal
spring should be flexible in the direction of its
action but be stiff in other directions, so that it
does not readily become displaced. A simple
cantilever is equally flexible in all directions
and has a stability ratio of 1. This can be
improved by support from the baseplate. Some
buccal springs have a stability ratio of appre- Figure 3.5 A palatal cantilever spring (0.5 mm)
boxed and guarded.