Page 260 - My FlipBook

258 FILLING MATERIALS.

Canada balsam in chloroform, or the solution of nitro-cellulose in
amyl acetate sold as '' kristaline " or " cavitine " are eifective
materials for this purpose. When the cavity is deep the layer of
cement should be brought to the inner line of the retentive grooves.
As soon as hardening takes place the metallic covering may bo given.
When the shallowness of the cavity will not permit a considerable
layer of the cement, a metal cap covering the pulp wall of the cavity
filled with the cement may be laid in ]>lace, the metal of the cap thus
sustaining the force.
These forms of cavity lining are of great utility, and should be
regarded as of importance.
Marginal Cavity Lining.—When cavities are situated on approxi-
mal surfaces of the teeth and extend high up on the cervical aspect so
as to place them beyond the probability of efficient service with metal
foils, and when the lateral walls of cavities are weak either by their
thinness or by instability from defects of structure, some form of
" lining " is necessary. In the one case, to ensure certainty of per-
formance at the cervix ; in the other, to prevent injury.
For the cervical part the choice is between (1) tin, (2) a combination
of tin and gold, and (3) amalgam.
Tin has the objection when superimposed above gold that it suffers
waste, in most instances by electrolysis, to which the mixture of tin and
gold is not liable. This latter combination—made by folding a layer
of the tin within the gold foil—appears to give the tin protection. This
combination is more plastic and more yielding than gold alone, and
permits adaptation and consolidation in places difficult of approach.
When used in connection with a matrix thorough consolidation may
be eifected without injury to the cervical margin when the tissues are
not dense.
When the color of a lining at the cervix will not be objectionable,
a quick-setting amalgam answers extremely well, and may at the same
sitting be followed by the completion of the operation with gold. In this
situation, whatever the lining material, close conformity with the lines
of the cervical form of the tooth must be assured. In many instances
the lining and the completion of this portion of the filling should be
effected before the rubber dam is placed, when the lining portion is for
the time being considered in its relations as a part of the tooth.
When it is necessary to use the mineral cements on approximal sur-
faces of the posterior teeth for temporary purposes, the cervical border
should be covered with a line of gutta-percha stopping, to protect this
vulnerable part of such fillings from the exposure of this border by
the solution to which they are there liable.
Lining Lateral Walls.—For this purpose choice should be made of