Page 82 - REMOVABLE ORTHODONTIC APPLIANCES

74 Removable Orthodontic Appliances
Figure 8.14 The twin-screw appliance.
This appliance is particularly useful
when different amounts of movement
are required on each side. Molar
headgear tubes are soldered to the
Adams' clasps (1.15 mm ID).

correction. Soldered tubes will be required on An appliance to move buccal segments
the first molar clasps to take an extraoral face- distally: the 'en-masse' appliance
bow. (Figure 8.15)
This appliance derives its effect solely from
Retention extraoral force. Clasps are usually fitted on the
Clasps are required on first molars and possibly first molars and first premolars with expansion
also on first premolars. being achieved either with a coffin spring or a
mid-line screw. The extraoral force can be
applied directly to the appliance by incorporat-
Anchorage
Anchorage is provided by the contact of the ing an integral facebow into the acrylic. The
without
inactive
baseplate with the other teeth in the arch and appliance no is risk of producing the headgear so
increase
an
there
in
is
supported by extraoral anchorage.
overjet. Headgear needs to be worn for a mini-
mum of 14 hours each day to achieve satisfac-
Baseplate tory distal movement. If this is successful, it is
The baseplate is split in the premolar region. common to find reduction of the overjet taking
The acrylic may be trimmed well clear of the place at the same time. The distal movement
second premolars, or alternatively it may be left can be facilitated by the extraction of the upper
in contact to allow the second premolars to be second molars in those cases where third
moved back simultaneously as the screw is acti- molars are present and of a good size. A second
vated appliance to retract the canines and reduce the
overjet will usually be required.
Points to note
A disadvantage of this method is that if the Active components
headgear is not worn but the screw continues to The elastic of the headgear and a mid-line
be turned the reaction will produce a rapid screw or coffin spring to provide expansion.
increase in the overjet.
After the molars have been successfully Retention
corrected to a class I relationship, further appli- Clasps on the first premolars and first molars.
ances will be required to retract the first premo-
lars and canines and then to reduce the overjet, Anchorage
using similar appliances to those described for The anchorage is derived solely from the elas-
premolar extraction cases. Extraoral support tic or spring tension of the headgear. The face-
will invariably still be required. bow is normally integral to the appliance.