Page 38 - REMOVABLE ORTHODONTIC APPLIANCES

Chapter 4

Appliance retention

The term 'retention' can have two completely rated by the action of the clasp. The design by
different meanings within orthodontics. It is Adams (1955) of the modified arrowhead clasp
used to describe the support that is given to the now known as the Adams' clasp represented a
teeth after a period of orthodontic treatment in major advance in removable appliance reten-
order to maintain their improved alignment tion.
and relationship. In the field of removable
appliances the same term is also used to
describe the resistance of the appliance to Posterior retention
displacement. This chapter is concerned with
the retention of the appliance in the mouth. The Adams' clasp (Figure 4.1)
Chapter 11 covers the use of removable appli-
ances as retainers. This clasp, like most others, is made from
Appliance retention is provided by wires, in 0.7 mm stainless steel wire or from Elgiloy
the form of clasps and bows. Good retention is (although for clasps on deciduous teeth 0.6 mm
essential if treatment is to proceed efficiently wire is more suitable). It engages the mesio-
and needs to be planned carefully. This is partic- buccal and disto-buccal undercuts that can be
ularly important if headgear is to be worn with found on the crown of almost every tooth. A
the appliance. Even where retention is good it is depth of undercut of about 0.25 mm is ideal.
prudent to reduce to a minimum any forces that In children, where the anatomical crowns of
tend to displace the appliance. Examples of the teeth may not be fully exposed, undercuts
such forces might be springs acting on cuspal may be difficult to locate, but they can generally
inclines, extraoral force applied in a downward be found at the mesio-buccal and disto-buccal
direction, or rocking of the appliance due to
contact of the lower incisors upon an excessively
undercut anterior bite plane. The patient will
find it harder to tolerate an appliance that is not
a firm fit and progress is likely to suffer.
When removable appliances were first used,
clasping was a particular problem. The arrow-
head clasp was one of the most successful
designs but was awkward to make and to Figure 4.1 An Adams' clasp (0.7 mm wire). The
and
adjust. The arrowhead engages the embrasure clasp engages undercuts on the mesio-buccal stands
tooth.
of the
aspects
bridge
The
disto-buccal
between teeth but, although this can offer good
clear of the buccal surface and the wire is closely
retention, it is easy to damage the gingival adapted to the tooth where it passes across the
papillae and the adjacent teeth can be sepa- contact area.