Page 19 - My FlipBook

RETENTIVE SHAPING.

ically, except in pieces. If the material therefore were durable, the fill-
ing would be permanent as long as the opening was not enlarged by
decay or fracture. There are, however, other considerations which
may make it imprudent, or impossible, to follow this rule, as, for exam-
when such a course would cause the excavation to
ple, approach the
pulp too nearly. Dentistry is in many respects governed by mechan-
ical laws, but when we come to mechanics to tissues there
apply living
are frequently points at which the ordinary laws must be set aside and
reason allowed to hold In the case of the retention of a
sway. filling,
to
reason would set aside one law, however, only adopt another, which,
though not so general in its significance, would be indicated in a special
instance. Where it becomes unwise to to a
attempt enlarge cavity till
its orifice is its smallest diameter, there are at least two direc-
usually
tions in which extension may be made, which will sufficiently serve to
hold a solid Much may be gained at times
filling. by judicious rough-
ening of the surface of the cavity. Fig. i exemplifies such a case.
FIG. i. FIG. 2. FIG. 3. FIG. 4.

The crown of a molar has become denuded of enamel, and the dentine
is sensitive. A few dovetails made with a rose bur will
highly sharp
serve to retain a filling, though the orifice has the largest diameter.
Again, there may arise cases where the destruction leaves, not a cavity
proper with an orifice, but merely a loss of substance with no retentive
shape whatever. It is in these cases that the ingenuity of the dentist is
taxed. In many instances well-placed screws are of great advantage.
These will be described later.
In the simpler forms of cavities, those which may be described as
having surrounding walls and orifices, the rule first mentioned must
be but must be These cavities
usually applied, judgment employed.
are of three classes, approximal, crown, and surface, the latter in-
cluding palatal, labial, lingual, and festoon cavities.
shall consider because are the
I approximal cavities first, they
most difficult, and demand more skill and judgment. Fig. 2 is a
cross-section through an incisor which has been filled. A casual
glance demonstates the fact that the filling could not be dislodged,
because the greatest diameter, which is in the line a, a, is larger
than the opening, b, b. The cavity, however, though mechanically
correct, is made without due consideration of the fact that living