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EXCAVATION OF CAVITIES BY CLASSES. 71

future. This is called extension for prevention. It applies
to all smooth-surface cavities.
Case I.— A central incisor has shown sensitiveness near the
gum margin, and upon passing a pointed exploring instrument
over it, it catches at a number of points, and at some of these
the point evidently passes through the enamel into the dentin.
Examining the condition of the surface carefully, the appearance
of an area of habitual uncleanness is found to extend along the
gum line, occupying the middle three-fifths of the surface mesio-
distally, and about one-third of the surface gingivo-incisally.
The incisor teeth are now well cleaned and the rubber dam
adjusted over them, and the Hatch clamp applied to this tooth
and the gingivus forced well out of the way. The surface indi-
cated is well dried and examined. The enamel is found chalky,
whitish, with openings through its thickness in the central area
;
and, extending mesially and distally in a crescentic form, there
are whitish lines. In these lines an exploring instrument does
not catch when passed lightly, but when a little force is used it
does catch, showing a degree of softening of the enamel. Gin-
givally the line of injury ceases abruptly at the border of the
gums. That portion of enamel covered by the gum is smooth
and perfect.
Procedure,— One, or two, or three vigorous strokes of
chisel 15 breaks up and removes the injured and friable enamel
over the central area, leaving the dentin bare, and reveals the
fact that it is exceedingly sensitive. An inverted cone bur with
sharp blades, i ni millimeters in diameter, is placed with its end
perpendicular to the labial surface directly into the central decay
and quickly carried over the whole surface, extending the depth
a little into the dentin and somewhat enlarging the opening.
The effect of this first movement is to remove the hypersensitive
area almost, or quite, completely, and the rest of the excavation
can proceed with much less pain. The same bur is again intro-
duced and carried along the mark indicating the position of the
gum line until the mesial border of the surface is nearly reached
and all of the whitish line of the enamel removed. This may
possibly have required two or three cuts, but often may be done
with one. With the same bur the distal part is cut out the same
way and to the same extent. The result is a narrow, crescent-
shajjed cavity, no wider than the bur, following the gum line