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20 THE TECHNICAL PROCEDURES IN FILLING TEETH.

Outline Form is the form of the area of the tooth surface
to be included within the outline, or enamel margins, of the fin-
ished cavity; the laying out of and cutting to these lines should
be the first thing considered and accomplished. In all pit cavi-
ties the outline of the cavity will be found by cutting away all
enamel overhanging the decayed area, completely uncovering it,
and following out any sharp grooves connecting with the cavity
to such points as will enable a perfect finish to be given to the
margins of the filling when placed. This should always be done
before there is any attempt made to remove the decay from the
deeper parts of the cavity. As these cavities occur in surfaces
of the teeth that are habitually clean, except for lodgments in
the pits or fissures in which the decay begins, no further exten-
sion for prevention is required. In this class of cavities this
work is done generally with chisels and the enamel hatchets.
However, in the first opening of pits in which but little decay
has occurred, and in following out fissures and grooves, the bur
is often the best instrument. In smooth surface cavities, that is,
in proximate cavities, and buccal and lingual cavities, which do
not begin in pits or fissures, but which occur in the central por-
tion of an area of uncleanliness that is habitual, in which the
superficial injury to the enamel tends to spread, the laying out
of the outline form of cavities is done upon a different princi-
ple. In these it is not simply cutting away overhanging enamel
for the exposure of the dentin already decayed, but the object
should be to include within the outline of the cavity such por-
tions of the surface as are especially liable to decay in the future.
As decay is liable to occur upon surfaces habitually unclean, and
only upon the unclean areas, the whole of the habitually unclean
area should be included within the outline of the cavity. This
requires a careful study of the conditions surrounding each
smooth surface cavity and the extension of the cavity outlines to
include the area of the surface that may have suffered superficial
injury, or is in danger of decay in the future. This will often
require that sound enamel and dentin be cut away to obtain the
correct outline form, and is known as extension for the pre-
vention of the recurrence of the decay. The study of the
case should be made, the outline determined upon, and the cav-
ity cut to the outline form required as the first procedure.
Resistance Form is that shape given to a cavity intended