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CHAPTER VII.

RETENTION FORM.
Definition. Retention form is that part of the procedure in cav-
ity preparation which deals with the provisions for preventing the
filing from being displaced by the tipping strain. Force which
results in tipping the filling bodily from the cavity, is one of the
greatest enemies to permanency in tooth filling, second only to re-
current caries.
Partially Provided For in Resistance Form. Retention form is
partially provided for in the previous step of resistance form, but
it is further necessary that provision be made to resist the force of
mastication in order to prevent the filling as a whole from being
moved from its seat.
Maximum Retention Form is required in cavities in the proximal
surfaces as the missing proximal wall renders these fillings particu-
larly exposed to injury by the tipping force, during the movements
of the mandible.
Flat seats for fillings are imperative in retention form. Seats
should be cut in a plane at right angles to the stress of nlastica-
tion, which is usually at right angles to the long axis of the tooth.
The Step as a Part of Retention Form. The addition of the step
in cavities of Class Two and Class Four is for the purpose of giving
added retention form. By this procedure in proximal cavities in
bicuspids and molars, the stress upon buccal and lingual walls of
the cavity proper is transferred to those portions of the same walls
which are a part of the step, a location much better situated to
withstand the tipping strain. In cavities of Class Four, the addi-
tion of the step on the incisal or lingual, or both, will give added
retention form, avoiding heavy cutting at the angle, Avhich weakens
the remaining tooth substance at the angle, to say nothing of the
dangers of crossing the retractive tract of the pulp in this location.
Maximum Retention Form is not required when making simple
cavities, as they are protected from the dangers of lateral strain
by the presence of surrounding external walls. This will be found
to be the case in cavities of Classes One, Three and Five when oc-
clusion is normal. While in cavities of Classes Two, Four and Six,
nuieli additional cutting is sometimes necessary to give ample re-
tention form.
Acute Angles Required. IMuch of the retention form required
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