Page 186 - My FlipBook

94 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.

angles of fillings when judicious extension for its prevention is
neglected. (6.) Recurrence at the gingival margins of fillings
where contacts are of bad form, allowing food to leak into the
interproximal space. (7.) Hence the requirement that correct
forms be given contact points in the finishing of fillings.
The conical form of cavities in the dentin is a little different
in proximal cavities, where seen in sections cut mesio-distally,
from those in occlusal surfaces, because of the difference in the
trend of the dentinal tubules from the line of the dento-enamel
junction toward the pulp of the tooth. It is still a cone, how-
ever, with its base placed diagonal to its length, or in section it
is a triangle, having one of its basal angles obtuse. The tendency
to this form is best displayed in the decays in Figure 110. In
general, there is a greater tendency to rounding of the general
line of invasion of dentin than is seen in the decays beginning
in occlusal surfaces. There is the same tendency to wide soften-
ing of the dentin more rapidly than the decomposition of the
organic matrix when the opening in the enamel is small, as is
seen elsewhere; but this disappears when the enamel breaks
away, exposing the cavity to the occlusal surface. As the time
in relation to the progress of the decay at which this breakage
of the enamel occurs is very variable, extensive burrowing with
large amounts of softened dentin is liable to be found unex-
pectedly in that which seemed, upon superficial examination,
to be a small cavity. The large proximal decays in Figures 110
and 111 show something of the extent to which these cavities
may burrow before the marginal ridge breaks away, exposing
them to the occlusal surface.
A lower molar is shown, in Figure 107, with a mesial surface
decay and a distal surface decay, both of which are excellent
types of the early beginnings of caries in these surfaces. In
that in the mesial surface, the decay has just passed through
the enamel, no enamel rods having yet fallen away. In the distal
surface the enamel rods have fallen out and the extension of
caries along the dento-enamel junction is making progress. This
is seen best in the picture to the right. From this the hyaline
zone stretches away to the pulp chamber. This picture is a most
excellent study. It is well to note the small amount of dentin
between the occlusal surface and the pulp in this case, and also
the great extension of the mesial marginal ridge of the pulp.
The frequent extension of the mesio-buccal horn of the pulp in
both upper and lower first molars is a menace in cavity prepara-
tion that should be carefully guarded against when possible, by