Page 23 - REMOVABLE ORTHODONTIC APPLIANCES

Chapter 3

Active components

The active components of modern removable
appliances comprise springs, bows, screws and
elastics. Springs, labial-bows and clasps, made
from hard-drawn stainless steel wire, are used
most commonly, although when the teeth to be
moved are also required for retention of the
appliance a screw is preferred (see Figure 9.9,
p. 84). Elastics may be used intraorally, or as
the active component of headgear.

Stainless steel
Figure 3.1 (a) A coil spring, which is activated by
When designing, constructing and adjusting being 'wound up', is more efficient than a coil spring
springs and bows, it is important to understand activated in the opposite direction (b).
some of the basic properties of the material.
Stainless steel wire is made by drawing the
metal through a series of dies of successively
smaller diameter. This process also causes the bent in a wire, it is differentially stretched so
work-hardening that gives the wire its spring that the outer surface becomes more work-
properties. At intervals during the drawing hardened and thus has better spring properties
process the wire must be heated to anneal it, than the inner surface. If the spring is deflected
otherwise it would become excessively work- in the same direction as the previous bending,
hardened and would fracture. The spring prop- its elastic recovery is better than if it is
erties depend on how much work-hardening deflected in the opposite direction (Figure 3.1).
has occurred in the final phase. For fixed appli- This is known as the Bauschinger effect.
ance archwires, high tensile wire may be used, Excessive bending will cause sufficient work-
but this is unsuitable for removable appliance hardening to fracture the wire. This is particu-
components because it is too liable to fracture larly liable to happen if the wire undergoes
on bending. Hard drawn wire is the most satis- reverse bending - for example, an incorrect
factory grade. bend which is straightened out is likely to frac-
Stainless steel will be further work-hardened ture during subsequent use.
by bending during the construction of compo- Surface damage to the wire during manufac-
nents. This can be advantageous in improving ture or, more often, during fabrication or
spring properties. For example, when a loop is adjustment of a component, also contributes to