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238 PREPARATION OF CAVPriES.

should 1)0 trimmed until solidity is obtainod, and also ho ronsidorably
bovoUod Ini' purposes of strou<2;tli.
Class *S'.— Orchtso-HiKjiKil cavities in the bicuspids and molars are
of rare occurrence except in the u])per first and second molars, where
they follow the line of the sulcus extendino; between the mesio-lingual
and disto-lingual lobes. Sometimes the cavity is nearly confined to the
oeclu.sal surface, running over on to the lingual surface l)ut slightly.
In such cases the cavity is easily prepared by simply cutting the occlusal
cavity through to the lingual surface, giving the cavity a relatively uni-
form depth at all points.
At other times the fissure on the lingual surface will extend farther
toward the cervical margin, and the cavity when prepared
Fig. 203.
will have the form of an L, the longer arm, a, represent-
ing the occlusal, and the shorter one, b, the lingual por-
tion of the cavity (see Fig. 203). Where the extent of
decay does not demand it, it would 1)0 a mistake to make
the floor level of the two portions of the cavity uniform,
as the extensive removal of sound dentin would greatly
weaken the disto-occluso-lingual cusp.
Where extensive decay has already weakened this cusp it is better to
amputate it below the level of the occlusal plane and extend the filling
over it.
Class T.—With the exception of those unusual cavities which
involve the greater portion of the crown of a tooth, the mesio-diMo-
occlusal cavities in bicuspids and molars are the largest in extent of any
met with. Being well exposed there is no lack of
Fig. 204.
either light or room in which to o])erate, and the only
difficidty associated with their preparation and filling
lies in their size and extent.
Their preparation is accomplished in the same man-
ner as those of classes P and Q, except that no special
retentive form need be given to the occlusal portion,
for with the filling once in place its general form will
secure it in position. Fig. 204 represents a typical cavity of this class
in a bicuspid tooth.