Page 121 - REMOVABLE ORTHODONTIC APPLIANCES

Laboratory procedures 113
stored in the patient's model box and will be Screws
available in the event of a breakage. Provided
that a broken appliance can still be fitted in the In the days when appliances were invariably
mouth it can be replaced on the model to allow heat-cured, screws had to be 'plastered out'.
clasp replacement or acrylic repair. That is to say that the exposed portion of the
screw needed to be covered with plaster so that
the wax (and therefore, later, the acrylic) did
Construction not flow into this part during construction and
obstruct turning. Nowadays, screws are usually
In the past, removable appliances were fully supplied with a tag of plastic, which protects the
constructed in wax. They were then flasked, exposed portion. To assist the accurate posi-
boiled out, packed and heat-cured using the tioning of the spring a small slot may be cut into
same method as for full dentures. Today, they the plaster model to seat the screw tag or the
are almost always made using cold-cured screw may be temporarily fixed in place with a
acrylic resins, which have been specially formu- dab of sticky wax. Alternatively, some techni-
lated for orthodontic use. cians will merely support the screw in place as
The model should initially be inspected and, the acrylic palate is built up. The plastic tag may
if necessary, trimmed to allow the arrowheads be removed later. Some screws incorporate a
of the clasps to engage undercuts. The palatal direction marker, which indicates to the patient
area will then be painted with a mould seal, the direction in which the screw must be turned.
which is allowed to dry. The wire components - Screws were originally made entirely of
clasps and springs - will be made, positioned on metal. Nowadays, some are made with plastic
the model and secured in place by flowing wax, blocks into which the screw thread engages.
melted with a wax-knife, around those parts The elasticity of the plastic allows the screw to
which will not be embedded in the baseplate. engage with slight friction so that the fit is less
The wax also serves to prevent the acrylic from likely to become floppy as treatment proceeds.
extending into unwanted areas. For example,
where posterior bite planes are to be incorpo-
rated into the appliance the acrylic should not Prosthetic teeth
enclose the clasp wires where they cross the
embrasures. From time to time it may be necessary to incor-
The acrylic for the baseplate is built up by porate a prosthetic tooth onto a removable
the alternate application of polymer powder appliance or a retainer. This represents an
and liquid monomer, generally using a small excellent way of maintaining a space, which will
plastic 'puffer' bottle for the powder and a later be restored with a bridge or some other
dropper bottle for the liquid. The resultant prosthesis. It also makes the appliance more
mixture has a firm gel-like consistency which acceptable to the patient. A particular technical
does not flow but may be trimmed with a problem is that the special cold-cured ortho-
knife. If the model is laid flat there will be a dontic resins are Bis GMA compounds which
tendency for acrylic to build up in the vault of do not bond well to prosthetic teeth and any
the palate during construction. This is tooth is likely to become detached from the
avoided by resting the model at an angle appliance. If the tooth is roughened and a small
during construction - turning it around as amount of conventional cold-cured acrylic is
each section is completed. As with a heat- incorporated between the tooth and the base-
cured appliance, the palate should be about as plate adhesion will be much improved.
thick as a single sheet of baseplate wax. The
acrylic can be built up to form anterior or
posterior bite planes to the required dimen- Clasp construction
sions.
The completed model is placed in hand- Accurate wire bending demands practice. It is
warm water in a pressure flask and left to cure. important that each bend is correct first time.
Subsequently, the appliance may be removed Any substantial correction, which must be made
from the model, trimmed, smoothed and to a bend, work-hardens the wire further and
polished. makes it more liable to break later, during use.