Page 66 - REMOVABLE ORTHODONTIC APPLIANCES

58 Removable Orthodontic Appliances
Figure 7.13 A stainless steel connecting bar
(2 mm X 1 mm half round) can be used to
reduce the amount of lingual acrylic.

Retention
Lower appliances have poorer retention than
upper appliances. This is partly because of
absence of palatal support but also because the
shape of many of the lower teeth can be less
favourable for clasping. The buccal aspect of
the lower first molar presents a sloping surface
and little undercut is obtainable. The conven- Figure 7.14 A buccal spring (0.7 mm) to retracts3|.
tional Adams' clasp is more difficult to fit and
adjust satisfactorily than in the upper arch.
Therefore an additional clasp may be necessary
further forwards, perhaps on a first premolar or useful in the retraction of canines. The spring
canine. should be activated by curving the end of the
spring inward and cutting off a short piece of
wire (Figure 7.15). It is better not to adjust at
Design of springs the loop because this moves the active end of
the spring occlusally so that secondary adjust-
Lingual springs ment is required. The spring should be flexed
lingually to ensure that it engages on the canine
The size and shape of the lower baseplate is
such that it is difficult to incorporate springs on
the lingual side of an appliance for mesio-distal
movement of teeth. A limited amount of buccal
movement may be obtained with a lingual
spring particularly in the lower incisor region.

Buccal springs
The sulcus surrounding the lower arch is shal-
low and the design of a buccal spring has to take
this into consideration. Springs such as that Figure 7.15 The buccal canine spring is activated by
shown (Figure 7.14) are well tolerated and curving the end inwards and shortening the wire.