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INTENTIONAL EXTENSION OF CAVITIES. 9

the are weak (a, a) and liable to fracture during the of
edges process
This would be the case where an
filling. especially approximal
cavity either existed or should afterward occur. In forming such an
undercut it is very probable that the other cavity would be reached,
and thus the enamel separating the two would be very fragile.
In Figs. 15 and 16 we have longitudinal sections, the approximal
surfaces being removed. Fig. 15 shows a groove or undercut (a)
toward the cutting-edge, but none toward the gingiva. This latter
arrangement furnishes a solid surface against which to mallet gold,
while the one groove, terminating in the horns pictured in Fig. 13, is
all-sufficient for retention. Where a second is made toward
groove
the gingiva, as in Fig. 16, the enamel or border at a is weak, and
recurrence of more
decay probable.
INTENTIONAL EXTENSION OF CAVITIES.

In many cases it becomes necessary to extend the limits of cavities
beyond the line of carious destruction. Where this extension is
internal, and is done without enlargement of the orifice or cavity-
border, the course is pursued merely to obtain proper anchorage for
the There are, however, times when the borders of cavities
filling.
must be extended, and these cases we must consider.
It is not uncommon to find a mouth, especially among children, where
the point of a fine exploring instrument will readily detect a seemingly
in the cro\vn of a or a molar. the
tiny cavity bicuspid Suppose pa-
tient to be of cleanly habit, the mouth in a hygienic condition, the
teeth free from stains, and the caries unaccompanied by discoloration.
The operator selects a small rose bur, which slightly enlarges the
opening and then sinks into the dentine, throwing out a debris of
white decay ; to be conscientiously thorough, the case being a bicuspid,
he carries the bur along the sulcus to the opposite pit. The cavity is
and small of
then filled, fine-pointed pluggers pieces gold being used,
the whole when finished presenting a beautiful polished appearance.
The crown is since now it is
improved, jeweled ! Suppose that eight
or ten of such narrow streaks of be in different teeth
gold placed
about the mouth. to last a lifetime, one would think.
They ought
Then why is it that a year later some leak, while others show a bluish
discoloration about their borders?
The reason is that in his endeavor to save tooth-substance the
operator did not make the cavities large enough. He did not remove
allof the decay, and he could not do so, because, first, he did not suffi-
ciently enlarge the opening to the carious region, and second, he pre-
pared the cavities with a bur alone. It is frequently impossible to
detect all the ramifications of decay with an engine-bur, partly be-
cause the motion
rapid destroys the keen sense of touch, and more