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Il8 PREPARATION OF CAVITIES

STEPS IN CAVITY PREPARATION

1. Establish the outhne form.
2. Remove the decay.

(a) Convenience form.
3. Give the cavity proper form (b) Resistance form.
(c) Retention form.
4. Bevel and polish the enamel margins.
5. Perform the toilet of the cavity.
I . Establish the Outline Form.—Carry the marginal out-

line to the location which it will occupy on the completion of
the cavity. This procedure, and not the removal of the decay,
should be the first thought to enter the reader's mind, for
reasons which will be understood better as he progresses.
—
Rule I. Extend the margins until solid tooth structure, free
from decay, is reached.
—
Rule 2. Leave no overhanging enamel margins unsupported
by dentin. The enamel of the tooth is extremely brittle, owing
to its histological structure, and depends for its strength on
the support which '*t receives from the underlying dentin.
—
Rule 3. In fissure cavities, extend the margins to include the
entire fissure. Pits and fissures are extremely liable to decay,
and if this rule is not followed decay is liable to occur in that
part of the fissure not included in the cavity, with a conse-

quent undermining of the filling.
— Extend the margins of cavities approaching closely
Rule 4.
to deep developmental grooves, to include the grooves for reasons \

of strength, as well as for the reason given in Rule 3.
—
Rule 5. Unite two cavities approaching closely to each other,
to prevent the leaving of a weak ridge of tooth structure

between them.
—
Rule 6. Extend the gingival margins of cavities occurring in
the gingival third of labial, buccal and lingual surfaces under the
free margin of the gum, where they will be less liable to decay.