Page 39 - My FlipBook

USE OF THE DENTAL ENGINE. 35

laid aside and the cavity finished, if further enlargement is
required, with other instruments. There is no other use for a
round bur in excavating cavities. It should never be used for
removing decayed dentine. If the pit cavity requires no fur-
ther extension the round bur should be followed by an inverted
cone that will square out the pulpal or axial wall of the cavity
and make the angles with the surrounding walls sharp and defi-
*
nite. A cavity with rounded angles is the most difficult of all
"
cavity forms to fill perfectly. For other purposes in excavating
it is a matter of choice in individual cases between the use of the
inverted cone and the fissure burr. These burs should be used
in se\eral positions which will be pointed out.
Cutting seats or steps for anchorage in mesio or disto-
occlusal cavities in the bicuspids and molars.— After the
mesial or distal cavity has been well opened with cutting instru-
ments, and the cutting of a step in the occlusal surface is
required, choose a small inverted cone or fissure bur, never more
than one millimeter in diameter, and begin within the dentin
close against the dento-enamel junction, causing the bur to
enter, and then draw it to the surface of the enamel ; engage it
again in the same way and repeat the motion. With this move-
ment, using a small bur, a groove is readily cut through the
enamel into the center of the occlusal surface. In this cutting
the line of the mesial (or distal) groove should be followed,
because this is the weakest part of the enamel. If the bur
is too large, it will not cut so well. It is only by concen-
trating the force on a small bur that the enamel can
be cut to any advantage, and even then the cut should
be made from within outward. After this first cut has beea
made the groove or channel formed should be broadened by
chipping away the enamel with chisels, or the enamel hatchets,
and the inverted cone bur again used to undermine the enamel
upon either side, which is again chipped away. In this way a
seat or occlusal step of any required extent is readily formed,
the pulpal wall of which will have sharp and definite angles with
its surrounding walls.
Grooves that need to be cut out, merely for the purpose
of finding a position for finishing the filling, i. e. when there is
,
solid dentin beneat^h, are to be cut in the same manner, with
the inverted cone or fissure bur. In none of these cases should