Page 24 - REMOVABLE ORTHODONTIC APPLIANCES

16 Removable Orthodontic Appliances
the possibility of fracture. Surface defects act as ideal spring would have an almost flat
'stress rasors' and provide sites where cracks load/deflection curve so that the force applied
may start. Problems with wire fracture can be remained almost constant as the tooth moved.
greatly reduced by careful construction and For ease of management by the patient the
adjustment. spring should have a reasonably small deflec-
tion and should be resistant to displacement
and distortion. These criteria cannot all be
Annealing satisfied simultaneously.
A heated wire will eventually reach a tempera-
ture at which the grain structure is modified. Force
Stress-relief-annealing occurs between 450 and
500 degrees Celsius. This does not damage the For a single-rooted tooth, a spring should
spring properties of the wire (and can some- deliver a force in the range of 25-40 g (the
times improve them by dissipating induced lower forces being indicated for teeth with
stresses) but it is generally not considered to be short roots, such as lateral incisors).Very light
worthwhile for removable appliance compo- forces may lie below the threshold for a reason-
nents. Heating the wire above 900 degrees able rate of tooth movement. Excessive forces
Celsius will result in a complete reorganization delay tooth movement, overload anchorage
of the grain structure with loss of the spring (Figure 3.2) and may cause discomfort to the
properties and this cannot be restored, except patient.
by drawing the wire further. Such annealing is
liable to occur if the wire is overheated during
soldering or welding, so these techniques must
be employed carefully and are best used spar-
ingly.

Elgiloy

This is an alloy principally of cobalt and
chromium, which can be bent up in the soft-
ened state and hardened by heat treating after
the bending is completed. It has a greater resis-
tance to stress/strain factor and is used by
many operators for construction of Adams'
and other clasps, which reduces the likelihood
of fracture.

Mechanics of springs
Figure 3.2 The effects of different force values
Most orthodontic springs are variants of the during canine retraction, (a) The correct force
simple cantilever. For a round wire, the force produces maximum canine movement and minimum
movement of the other teeth, (b) An excessive force
generated by a small deflection within its elas- may give reduced canine movement and will result
tic limit depends on the deflection, the cross- in undesirable movement of the other teeth in the
sectional area of the wire and the length: arch. An increase in overjet is a sign of this.
F = d*c/l
The cross-sectional area of the wire is especially
significant. Doubling the wire diameter
increases the force 16 times and even using a In most situations, a spring activation of about
wire of 0.7 mm rather than 0.5 mm diameter 3 mm is satisfactory (Figure 3.3). If the spring is
doubles the force for a given deflection. The given greater activation the patient is more