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220 PREPARATION OF CAVITIIJS.
Shaping the Cavity.
This is one of the most important of all operations associated with
the stopping of a cavity, for according as it is properly or improperly
performed will sncccss or failnre resnlt. Too much stress cannot be
laid upon its importance, nor too great care be exercised in its accom-
plishment.
Inasmuch as a filling is retained in place mechanically it follows that
the cavity must be of such shape as to favor retention. To this end it
should be larger within (at least at certain points) than at the orifice.
An exception to this rule lies in such cavities as are of small diameter
and of more than moderate depth. In cavities of this character,
parallel walls will suffice, because lateral-surface contact is so great in
proportion to the mass to be held in place that disi)lacemcnt could not
occur. In larger cavities of moderate depth, however, the reverse is
the case, and they will require the assistance of internal enlargement
for the retention of the filling. To govern each of the conditions two
rules may be formulated :
1. When the depth of the cavity is greater than the diameter of the
orifice, parallel lateral walls will prove retentive.
2. When the diameter of the orifice is greater than the dejoth of the
cavity, the latter wall have to be somewhat enlarged internally to retain
the filling.
Examples of the first class are found in the narrow but rather deep
cavities which occur on the lingual surfaces of the upper incisors
near the cervix ; in the pit cavities on the buccal surfaces of molars
and in the small cavities found on either side of the enamel ridge on
the occlusal surfaces of the lower first bicuspids.
Examples of the second class are found in numberless places on any
of the crown surfaces.
In some cases cavities will be found of such form that when the
decay has been removed they will have a naturally retentive shape, but
in the great majority of cases more or less sound tissue will have to be
removed in order to give them the required form. To give a cavity a
retentive form it is not necessary that its interior be enlarged throughout
its whole extent, but it must be larger at two or more points, and these
points must be opposite one another. Frequently it will be easier to
enlarge the cavity at all points, and to this no objection can be urged
provided too much sound tissue be not removed or the pulp be not too
nearly approached. Too great enlargement tends to weaken the cavity
walls and therefore should be guarded against.
In shaping the cavity internally instruments should be employed
that will leave the surface free from angles, for the filling material can-
220 PREPARATION OF CAVITIIJS.
Shaping the Cavity.
This is one of the most important of all operations associated with
the stopping of a cavity, for according as it is properly or improperly
performed will sncccss or failnre resnlt. Too much stress cannot be
laid upon its importance, nor too great care be exercised in its accom-
plishment.
Inasmuch as a filling is retained in place mechanically it follows that
the cavity must be of such shape as to favor retention. To this end it
should be larger within (at least at certain points) than at the orifice.
An exception to this rule lies in such cavities as are of small diameter
and of more than moderate depth. In cavities of this character,
parallel walls will suffice, because lateral-surface contact is so great in
proportion to the mass to be held in place that disi)lacemcnt could not
occur. In larger cavities of moderate depth, however, the reverse is
the case, and they will require the assistance of internal enlargement
for the retention of the filling. To govern each of the conditions two
rules may be formulated :
1. When the depth of the cavity is greater than the diameter of the
orifice, parallel lateral walls will prove retentive.
2. When the diameter of the orifice is greater than the dejoth of the
cavity, the latter wall have to be somewhat enlarged internally to retain
the filling.
Examples of the first class are found in the narrow but rather deep
cavities which occur on the lingual surfaces of the upper incisors
near the cervix ; in the pit cavities on the buccal surfaces of molars
and in the small cavities found on either side of the enamel ridge on
the occlusal surfaces of the lower first bicuspids.
Examples of the second class are found in numberless places on any
of the crown surfaces.
In some cases cavities will be found of such form that when the
decay has been removed they will have a naturally retentive shape, but
in the great majority of cases more or less sound tissue will have to be
removed in order to give them the required form. To give a cavity a
retentive form it is not necessary that its interior be enlarged throughout
its whole extent, but it must be larger at two or more points, and these
points must be opposite one another. Frequently it will be easier to
enlarge the cavity at all points, and to this no objection can be urged
provided too much sound tissue be not removed or the pulp be not too
nearly approached. Too great enlargement tends to weaken the cavity
walls and therefore should be guarded against.
In shaping the cavity internally instruments should be employed
that will leave the surface free from angles, for the filling material can-
