Page 33 - My FlipBook

FORMA TION OF CA VITY BORDERS. 1 9

in hand. To leave irregular borders in any cavity is to produce weak-
ness along the points of contact between the tooth and the filling.
Chipping of the margin will occur either during the placing of the
or later.
filling
Reference to a few special cases will demonstrate the point more
and
fully clearly. Fig. 31 is diagrammatic, but conveys the idea
clearly enough. It is supposed to represent the ragged outline of a
cavity in the approximal surface of a central incisor. In Fig. 32 we
observe the same, prepared and filled, according to methods too often
followed. There are three errors along the border. The cavity has
it is true, but
been formed on curved lines, they are weak lines and
not esthetic. The slight prominence at a has been left, thus produc-
an undulating border, instead of the more beautiful curve seen in
ing
Fig- 33- Moreover, if we study the cleavage of enamel we discover
that this prominence is weak, since it is unsupported, and must almost
certainly crack during the operation of filling. The force exerted
in packing gold against it would probably produce fracture along

FIG. 31. FIG. 32. FIG. 33.

the line of the enamel-rods, and if the disengaged piece did not
come away at once it would do so later, or else decay would be in-
vited here. For similar reasons the at b should not have
sharp point
been left. Even if no fracture were produced during the packing
of gold, the loss of a triangular segment would invariably follow upon
use in mastication. Again, the heavy undercut at c is an error, for
the strongest gingival border is one formed without undercut of any
kind, but having a flat surface against which force may be freely
exerted without fear of chipping. Fig. 33 is correctly formed and
more to the In
decidedly pleasing eye. Fig. 34 we have another
which is too often as seen in
cavity, incorrectly formed, Fig. 35.
As
This should have a border similar to Fig. 33. prepared in Fig. 35
the point a is weak. Gold is a soft, malleable metal, and where it is
brought to a thin edge, as seen, the impacting force of mastication
tends to flatten it out until it curls and breaks, leaving a ragged edge.
The point b is weak for reasons already given.