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352 COMBINATION FILLINGS.
B. Cavities with large open'mgH. The rubber clam and matrix
having been adjusted, enough amalgam is packed
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against the matrix to form a shell of sufficient
strength to make the approximal wall of the
filling (see Fig. 331).
This will leave a hu'ge portion of the cavity
unfilled as sliown in tlie figure ; in this space is
placed cement, which is gently worked into the
a, Matrix; ^, amalgam packed soft amalgam, but with care not to carry it
against the matrix ; c, por- through to the matrix. Before the cement be-
tion of cavity to be nearly
filled with cement and fin- comes hard more amalgam is put in, the sur-
ished with amalgam.
plus cement is removed, and the whole finished
to look like an entire amalgam filling, while in reality it is only a shell
of amalgam, perfectly fitting the outside of the cavity, cemented into
place. If the walls of the tooth are frail, the cement Avill serve to
greatly strengthen them. If, as some claim, large metal fillings alter
sufficiently under changes of temperature to fracture frail walls, the
danger is by this method reduced to a minimum, as the amount of metal
is only just sufficient to give requisite strength.
There is another class of cavities which may be described in this
connection, presenting great difficulties in themselves,
Fig.
yet, with this simple matrix, they are often easily
filled. It is those cases where decay has reached the
alveolar border approximally, and extended on either
the buccal or lingual portion of the tooth, or both, in
such a manner that the dam cannot be made to stay
beyond the cervical border of the cavity. If a liga-
Alveolar line be-
ture is used, it will draw into the lateral grooves of yond which tlie liga-
ture cannot be made
decay and be of no use (Fig. 332).
to stav.
The mode of treatment is as already described,
with the exception that the matrix is adjusted before the rubber is put on.
After the matrix is in place, it is but the work of a moment to put a
Palmer clamp on to the tooth, and slip the rubber
dam over clamp, matrix, and tooth. If the matrix
Fig. 333.
has been carefully fitted there will be no trouble in
keeping the cavity dry long enough for any ordinary
operation.
There are certain buccal cavities, also, below
which it is difficult to retain the rubber dam. A
very narrow matrix, adjusted with ligature and
^l^n^P (Fig- 333), over which the rubber is placed,
Matrix and clamp
adjusted, ready for y^\\\ often greatly sim])lifv the operation. Modifica-
application of the
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