Page 65 - REMOVABLE ORTHODONTIC APPLIANCES

Class I malocclusions 57
the baseplate, without a palatal spring, may by breaking the plaster and can only be fitted in
be adequate. the mouth after considerable trimming. Since
the tags of molar clasps are embedded into this
Retention area they may become loose during such trim-
Clasps on 6|6. ming. Before the appliance is constructed, the
undercuts should be eliminated by waxing or
Anchorage plastering. Alternatively, the wire tags can be
Little force of reaction is generated and anchor- shaped so that they are largely out of the under-
age is not a problem. cut area which may then be trimmed away in
the laboratory at the end of construction
Baseplate (Figure 7.12).
This is trimmed well clear of the labially
displaced aspect of the tooth but should be left
in contact with the palatally positioned surface.
Points to note
Space may have to be created before the rota-
tion can be corrected. The labial bow is acti-
vated by tightening the bow at the 'U' loops. As
the tooth becomes aligned, it may be necessary
to incorporate a bayonet bend in the bow to
keep it clear of the other incisors (see Figure
8.13, p. 73). If one aspect of the tooth is
palatally positioned, a palatal spring should be
used in conjunction with the labial bow.
Figure 7.12 (a) Incorrect extension of the acrylic
into the lingual undercut can make the appliance
impossible to insert, (b) Correct finishing of the tag
Lower removable appliances on the Adams' clasp allows some lingual trimming of
the acrylic to facilitate fitting.
These have limited application and are not as
well tolerated as upper removable appliances.
The acrylic encroaches on the tongue-space
and as the buccal undercuts on the lower Physical limitations
permanent molars are small retention can be
poor. The space available for lingual springs is The form and situation of the lower alveolus
limited and patients may find buccal springs dictate that the acrylic baseplate will consist of
uncomfortable. a 'U'-shaped strip of plastic which runs parallel
Fortunately, in the majority of cases suitable to the roots of the teeth. This is very different
for treatment by the general practitioner, the from the situation in the upper arch and trim-
lower arch can be accepted without treatment, ming of the acrylic to allow movement of a
or with the spontaneous alignment of crowded tooth can easily weaken the appliance so that
lower teeth which can be expected to follow fracture occurs. In situations where subsequent
extractions. Where extensive lower arch treat- trimming may become necessary the appliance
ment is required, fixed appliances are neces- must be selectively thickened during construc-
sary. tion. Anchorage slip during the retraction of
teeth may allow slight labial movement of the
appliance, causing trauma to the lingual gingi-
vae of the lower incisors and to the mucosa
Undercuts
overlying the alveolus in this area. Some oper-
There are usually undercuts on the lingual side ators therefore prefer to replace the anterior
of the lower alveolus, especially in the molar part of the baseplate with a lingual bar, but this
and premolar region. If the acrylic is extended has the disadvantage of reducing the number of
into these areas during manufacture the appli- teeth and area of mucosa available for anchor-
ance can only be removed from the work model age support (Figure 7.13).