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218 PREPARATION OF CAVITIES.

])ain ii|)(>n tlie patient as possil)le. To this end, in cavities of con-
siderable extent, it is best, after the orifice lias been sutticiently enlarged,
to make a sweeping cut with an excavator around the cavity just below
Fk;. 167.
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Excavators.
the enamel line, thus freeing the decayed portion at that point. Follow-
ing this the remaining portion of carious dentin should be removed by
placing the blade of the excavator near the bottom of the cavity and
making draw-cuts toward the orifice. To cut in the reverse direction
would produce uncomfortable pressure upon the most tender portion of
the cavity, and possibly, by inadvertence, expose and wound the pulp.
AVhen burs are employed for the removal of caries it is safest to use
only such as are more or less rounded on their circumference, such as
the round or oval forms, for they more nearly conform to the natural
outline of the cavity, leave no angular grooves in the dentin difficult
or impossible to perfectly fill, and are not so likely to injure the healthy
subjacent dentin.
The varieties of bur known as the inverted-cone and tvheel, while very
useful for opening cavities, should not be used for the removal of caries
in deep cavities, because of the irregularities of surface which their
peripheral angles produce.
Rapidly revolving burs in an engine handpiece are very apt to cause
pain by the development of frictional heat. This may largely be pre-
vented by lifting the bur at short intervals and allowing it to run free
for a moment, which will prevent overheating the tooth and thus avoid
unnecessary pain.
Thorougli excavation of the cavity and the removal of all carious
dentin is absolutely essential to success. To allow any portion of it to
remain and trust to the employment of germicides for its sterilization
is running the risk of failure, for we can never be entirely sure of
disinfection. Besides this, there is no good reason for allowing cari-
ous dentin to remain.
By carious dentin is meant the remains or debris of the action of