Page 90 - REMOVABLE ORTHODONTIC APPLIANCES

82 Removable Orthodontic Appliances
Appliance design can help to retain the appliance. Where two or
more incisors are to be moved over the bite, it
Molar capping may assist by disengaging the can be useful to choose an appliance with a
occlusion and eliminating any displacing activ- screw, which engages the labial segment. This
ity of the mandible on closure. This will usually has the advantage of providing additional
require only a very thin layer of acrylic on the retention at the front of the arch and further
posterior teeth. Indeed, the cusp tips of the resisting displacement.
molars may perforate the acrylic. An anterior
bite plane is usually contraindicated because it
may reduce the overbite. An increase in over- An appliance for lateral expansion and
bite is often advantageous for a stable result. correction of lateral incisor in crossbite
(Figure 9.6)
Springs to move the upper incisors over the
bite should avoid intrusive forces as far as Active components
possible. A 'T' spring inevitably has this effect Screw, cranked palatal finger spring.
so it is better to use a cranked cantilever spring.
A double coil 'Z' spring can also be used - espe- Anchorage
cially where space is limited - but is more diffi- The arch is to be expanded symmetrically so
cult to adjust. anchorage is reciprocal.
A unilateral buccal crossbite with a displace-
ment is often associated with a single lateral Baseplate
incisor in lingual occlusion on the same side. The baseplate is split in the mid-line to allow
Correction of the crossbite may be carried out for expansion. Posterior bite planes are
with a removable appliance incorporating a required: (a) to prevent occlusal interference
mid-line screw to expand the upper arch. Molar and eliminate the displacement; (b) to avoid
capping is desirable to disengage the occlusion secondary expansion of the lower arch by
and eliminate the displacement. A cranked occlusal forces; and (c) to help seat the appli-
palatal finger spring can be incorporated to ance, particularly after activation.
deal with the instanding incisor.
Retention must be adequate because the Points to note
forces used to move the upper incisors forwards The screw should be opened by the patient one-
will, through their action on the inclined palatal quarter turn per week. If the appliance is
surfaces of these teeth, cause a displacing force adjusted more frequently it may not seat fully
on the appliance. Retention is often poor in and will become difficult to wear. After the
young children where the molar eruption is expansion has been completed the molar
incomplete, but the presence of molar capping capping should be removed and retention is

Figure 9.6 An appliance to expand the upper
arch and correct a crossbite on 2|Adams'
clasps 64|46. cranked palatal finger spring
(0.6 mm). A mid-line screw and molar
capping.