Page 246 - My FlipBook

244 EXCLUSION OF MOISTURE.

done to the teeth on both sides of an upproxinuil cavity. It is neces-
sary here to place the cervical margin of the cavity in full view and to
make certain the exclusion of moisture, which otherwise might pass the
valve by capillary attraction.
The ligature should usually be passed but once around the tooth and
then be tied with a surgeon's knot, the place of the knot being on the
outside. When there is much strain the thread may be passed twice
around the tooth, but this should be avoided as being more painful and
as increasing the bulk of the ligature.
To prevent the rubber from displacement by the movement of the
cheeks on the posterior teeth when they are long, if after drying the
surface a little sandarac or damar varnish is applied at the last inter-
stice, the rubber becomes fixed.
In cavities extending above the cervix where a ligature cannot be
placed above the cervical border of the cavity, other means have to be
adopted to obstruct the entrance of fluids. Here the strait of rubber
between the holes should be much wider than usual ; the abundant fold
may then be forced beyond this margin with a matrix, when, by drying
the parts and by the deft introduction of alcohol varnish and suitable
wedges, drvness of the parts is attained. In the most extreme cases of
this nature the part beneath the normal gum line may be filled w'ith
a permanent plastic substance, as described in the section on Lining
Cavities (see Chapter X,, p. 258).
The Securement of the Dam from Displacement.—AVhen the
teeth are short from incomplete development or when their form is
tapering from the gum toward the occlusal aspect there is always ten-
dency of the rubber to escape, and the contraction of the commissure
of the lips tends to the displacement of the dam at the posterior teeth,
the latter movement often being sufficient to overcome the friction of
the ligatures. When these difficulties arise a clamp is required.
The Clamp.—This is an instrument of much value not only as a
means of securement of the rubber, but as an adjunct to prevent the

Fk;. 214. Fig. 215.

Dr. Southwiek's clamps. Dr. Huey's clamps.

rubber from obstructing the view. Clamps are more especially needed
to detain the rubber on the molars and are rarely required for the bicus-
pids or the anterior teeth, since, if the foregoing directions are followed,.
the necessity for their use will but seldom be presented.