Page 25 - My FlipBook

CAVITY PREPARATION. 21

to afford such a seat for the fiUing as will best enable it to with-
stand the stress brought upon it in mastication. Its importance
stands in direct relation 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 floor or 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-occlusal cavities, in which the greatest possible support
is needed, the gingival wall of the proximate portion is cut flat,
and in the horizontal plane of the tooth, and extended from
buccal to lingual into fairly definite corners or angles. The step
is also given a flat horizontal seat. Generally this is easily
attained by the same instruments and by the same modes of
cutting used in gaining the outline form, if the two objects are
held strictly in view.
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 this provision is required in its more 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.
F"ormerly pits and grooves were much depended upon for this
purpose, but have proved delusive, so that latterly it has been
required that the form of the walls be such as to perform this
function.
Convenience Form, though secondary to other points in