Page 21 - My FlipBook

APPROXIMAL CAVITIES.
7
bluish color which is seen around an
occasionally apparently perfect,
If it is denied that a cannot be made
filling. filling water-tight, it is
still to be remembered that in this labial undercut, even thus
filling
imperfectly, the strain as in the case of the palatal aspect would prob-
ably crack the thin enamel. The subsequent loss of even a tiny
of enamel would
particle give sufficient entrance for fluids into this
lacuna. If, in addition to this, the excavation had
artificially-produced
not been thorough and just a little caries were left, it would require
no gray-beard to explain the blueness.
In Fig. 7 we have an approximal surface occupied by a filling which
thus indicates the outlines of the orifice of the 8 is
cavity. Fig.
placed in juxtaposition, and lines are drawn through the two for
comparison. The latter is a longitudinal section through the greatest
diameter of the filling, the approximal portion having been thus re-
moved. By comparing the lines a, a in the two figures, it is seen
that the undercut toward the cutting-edge is very slight. Any great
extension in this direction is not only unnecessary but endangers the
weakened
corner of the tooth, already by decay. Comparing the
lines b, b, it is seen that this of the is extended much
portion cavity
beyond the border. Also observe that this extension takes the form
of horns, conforming in with the line e, e, which
shape imaginary rep-
resents the gingival border of the enamel. By proceeding thus it is
that we leave the enamel at the border of
plain gingival equal strength,
whereas if with a rose bur we cut a groove connecting the horns f,f,
we gain nothing in retentive strength, while by thus undermining the
enamel unequally we leave it weak at\ In malleting gold against this
border the greatest strain will be against the weakest point, g, because
of unequal resistance, probably producing a crack or fracture, which
readily explains in one way the oft-noticed recurrence of caries at the
gingival borders. By comparing the lines c, c, the labial borders, it is
seen that a slight extension, or groove, exists along the full extent, de-
creasing toward the cutting-edge. The lines d, d, however, indicate that
at /i, where there is a decided
the lingual borders are identical, except
dip. Along the lingual portion of an approximal cavity (in the six
anterior teeth) it is usually safer to make no groove except at the
upper third, where the thickening of the tooth gives sufficient dentine
for a deep dip, which will add great rctentiveness to the cavity. Fig.
9 is important. It is a section through an incisor and filling, the
labial half being removed. At a glance it is seen that on this plane
the. cavity has no retentive shape whatever. This is because on this
plane, which is through the center of the pulp, extensive undercutting
would either the or produce weak in enamel.
approach pulp, points
The gingival border at g is seen to be strong, and shaped to resist
force which Were the
any necessary may be brought against it.