Page 63 - REMOVABLE ORTHODONTIC APPLIANCES

Class I malocclusions 55
When the appliance is fitted, the baseplate Such crossbites should be treated early (in the
must be trimmed away generously from the mixed dentition) to eliminate the displacement
palatal surface of the tooth. If this is not done and to allow the occlusion to develop with the
movements may be impeded and the gingivae mandible in a centric relationship. Occlusions
may proliferate in the gap between the tooth of this type are basically symmetrical and the
and the baseplate. Palatal movement of a apparent asymmetry is produced by the
premolar or canine rnay be undertaken with a mandibular displacement, which is due to
similar design of spring. occlusal interference. Symmetrical expansion
of the upper arch is required.
The presence of a unilateral crossbite with no
mandibular displacement suggests a true asym-
Bucco-palatal movement of molars metry, either of the maxillary or the mandibu-
lar arch (or both). Examination of the arch
Buccal movement
form and the face may indicate whether this is
Where an upper molar must be moved alveolar or skeletal in origin. In either case the
buccally, a 'T' spring may be successful lack of displacement indicates that there is no
provided that the palatal surface of the molar is underlying functional disharmony. Such an
reasonably vertical. The disadvantage of this occlusion may sometimes be accepted but if
spring is that it tends to displace the appliance treatment is to be undertaken it will be complex
and it may be difficult to obtain adequate reten- and is likely to involve fixed appliances.
tion. For this reason, a screw is sometimes A bilateral crossbite usually reflects an
preferred although it will inevitably make the underlying dental base discrepancy, but there is
appliance bulky. It is important to recognize rarely an associated mandibular displacement
that a molar crossbite may involve buccal and so treatment to correct the crossbite is not
displacement of the lower molar as well as mandatory. Indeed, correction of a bilateral
palatal displacement of the upper. If this is the crossbite is rarely stable and relapse is common.
case then correction of the upper tooth alone For these reasons, correction of a bilateral
will not correct the crossbite and it is usually crossbite with removable appliances should not
better to use a fixed appliance to correct the be attempted. In a few cases, the orthodontic
positions of both teeth. Specialist may correct a bilateral crossbite by
rapid expansion to separate the mid-palatal
suture. Even this treatment is prone to relapse.
Palatal movement
It is unusual to find a single upper molar in
buccal crossbite but where this does occur it An appliance for lateral expansion (Figure
should be corrected as early as possible because 7.10)
the tooth may over-erupt and create a func-
tional disturbance. If the first permanent molar Active component
is affected, it should be corrected before the A screw (or a coffin spring) is used.
second molar erupts and encroaches on the
space required. Retention
Good retention is essential. 6J6 and 414 should
be clasped. If the latter teeth have not erupted,
D|D or C|C may be clasped, but retention will
Upper arch expansion be less good.
Lateral arch expansion is indicated only in well-
defined circumstances. It is not a suitable Baseplate
procedure for the relief of crowding and if the The baseplate is split in the mid-line to allow
upper incisors are crowded; space must be for the expansion. Posterior biteplanes are
made by retraction of the canines. The princi- required for the following reasons:
pal indication for upper arch expansion is the • to eliminate the occlusal interference and
existence of a unilateral crossbite associated thus the displacement
with a lateral displacement of the mandible • to avoid secondary expansion of the lower
when the patient closes from rest to occlusion. arch by occlusal forces