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EXCAVATION OF CAVITIES BY CLASSES. —
Before filling, the operator may, if he thinks it will be more con-
venient for starting the filling, cut two convenience points, one
in the axio-mesio-gingival angle and one in the axio-disto-
gingival angle. This should be done with the square end of an
inverted cone bur, about A or A millimeters in diameter, and be
about half the depth of the head of the bur. After forcing the bur
into the dentin to this depth it should be drawn a little away along
the axio-gingival line angle extending the pit into a slight groove
that diminishes and runs out at a length of about three diameters
of the bur. This little extension will increase the hold of the
gold and render the starting easier and more certain.
We now have a crescent-shaped cavity, with somewhat blunt
points, with a perfectly flat axial wall that includes the area of
the surface most liable to decay.
Case 2. Left upper central incisor with labial decay.
Cavity open and occupying the gingival third of the surface
inciso-gingivally, and the middle three-fifths mesio-distally.
The incisors are cleaned, the rubber dam adjusted, the gingivus
pushed well back with the Hatch clamp, and the tooth dried.
It is now seen that a line of injury to the enamel runs from the
mesio-gingival angle of the cavity to a filling in the mesial
surface, and another such line connects with the distal surface, in
which there is a small open cavity. The filling in the mesial
surface is good and sufficient except at its labio-gingival angle,
where slight decay has begun. These lines of injury show as a
w^hitening of the enamel only, there being as yet no apparent
loss of substance. The patient is twenty years old and suffer-
ing badly from caries.
Procedure.—All overhanging enamel is chipped away from
the incisal portion with chisel 20, using the thumb and palm
grasp, and the dentin wall squared up to a sufficient depth.
Then with chisel 15 the gingival wall is cut away to sound
dentin, again using the thumb and palm grasp and cutting from
mesial to distal. This wall is found solid and clean very close to
the gingival line, only a very slight band of thin enamel remain-
ing, but it is sufficient to prevent actual injury to the attachment
of the peridental membrane. This is favorable and regarded as
important, because it insures a healthy free margin of gum after
the operation. The angles are also cut to sound dentin with the
chisel. At the angles the superficial injury to the enamel is