Page 43 - REMOVABLE ORTHODONTIC APPLIANCES

Chapter 5

The baseplate

The acrylic baseplate constitutes the body of Acrylic
the removable appliance. It has three functions:
it provides a foundation, which supports other Orthodontic appliance baseplates are generally
components such as springs and clasps; it manufactured from cold-cured acrylic. It is
contributes to anchorage through its contact economical in terms of laboratory-time and
with the palatal vault and teeth that are not warpage is less than with heat-cured acrylic. It
being moved; and it may be built up into bite does, however, have more free monomer
planes to disengage the occlusion or produce present after curing and the appliance is less
overbite reduction. strong. The extra expense of a heat-cured appli-
ance may be justified where breakage is liable
to be a problem. Examples might be the pres-
ence of a deep overbite and very heavy occlusal
Design and construction
forces, the need for shallow but strong poste-
rior bite planes and the situation in which a
The baseplate needs to be thick enough to carry
the retentive and active components, but prosthetic tooth must be added to an ortho-
should be as thin as possible, compatible with dontic appliance.
strength. Ideally, this should be about as thick
as a sheet of modelling wax. Thicker appliances
may be tolerated but, particularly initially, may Anchorage considerations
be difficult for the patient to wear. The base-
plate should normally cover most of the hard In order to obtain maximum anchorage the
palate, finishing just distal to the first molars. It acrylic should cover most of the palatal mucosa,
should fit closely around the necks of teeth that finishing distal to the first molars, although it
are not being moved - otherwise food packing may include the second molars if they have
and gingival hyperplasia may occur. It should erupted fully. The acrylic should contact all the
be trimmed well clear of the teeth to be moved teeth in the arch with the exception of those
and care must be taken during manufacture to that will be moved. Where screws are to be
position wire tags in the acrylic so that trim- used, consideration must be given to the posi-
ming is possible. tion of the split of the appliance and its influ-
During appliance construction undercuts ence on the number of teeth to be moved.
rarely present a problem in young patients but, Where equal movement of groups of teeth is
for adults, undercuts may require blocking out required, for example upper arch expansion,
before the appliance is made. When lower the split should be along the mid-line with
appliances are used undercuts regularly require equal contact to similar groups of teeth. Where
blocking out. unilateral distal movement is necessary.