Page 267 - My FlipBook

CAVITY PREPAEATION. 113

stand the stress brought upon it in mastication. Its importance
stands in direct rehition to the degree of the exposure of the
filling to the occlusion and to the strength of the closure of the
teeth. It is necessary to provide for a force of from one to two
hundred pounds, and, in some cases, more. The resistance form
consists in a flat seat for the filling, cut at right angles with the
direction of the stress of mastication, or usually, at right angles
with the long axis of the tooth. In occlusal cavities, for example,
the pulpal wall is to be cut flat and all of the surrounding walls
should be cut to definite angles with the pulpal wall. In proximo-
occlusai cavities, in which the greatest possible support is needed,
the gingival wall of the proximal portion is cut flat and in the
horizontal plane, with definite angles. The step is also given a
flat, horizontal seat. However, if in any case cutting to a flat
form to the full depth of decay in the central part will endanger
exposure of the pulp, the squaring-out should be in the form of a
shelf around the margins, as will be explained later.
Retention form is the provision for preventing the filling
from being displaced. A large part of this is provided for by
the resistance form. But it is further required that provision be
made that will prevent the filling from being thrown out of the
cavity by such lateral or tipping force as may be brought against
it. All cavities should be provided with ample retention form,
but thfs provision is required in its most perfect form in mesio-
or disto-occlusal cavities in the bicuspids and molars. In these
the provision is made in the form of a step cut into the occlusal
surface, which is more or less dovetailed. In most cavities the
retention form is made by so shaping certain of the opposing
walls that they will be strictly parallel or slightly undercut in
order that when the filling material is thoroughly packed between
them, it will be securely held in place. This is done variously in
different situations. It is to be looked to especially when the
outline and resistance form of the cavity has been developed.
Formerly, pits and grooves were much depended upon for this
purpose, but have proved delusive, so that laterally it has been
required that the foi'm of the walls be such as to perform this
function.
Convenience foem, though secondary to other points in the
formation of cavities, should not be neglected. "When the general
form of the cavity has been developed, modifications are to be
made that will render the form more convenient for placing the
filling material. Often by cutting a wall away to a certain
inclination, the plugger point will reach some portion of the
17