Page 43 - My FlipBook

RESISTANCE FORM 39

Resistance Form as Applied to Filling Material. We are forced
to consider the properties of the filling material to be used in each
individual cavity. In preparing the cavity we consider the resist-
ing power of the enamel margin we are able to obtain. We also
take into account the resistance of the filling material used, to the
crushing strain, as this property varies greatly. Amalgam, even
under the most favorable manipulation, is subject to flow and more
or less spheroiding, which often results in a slight exposure of
the cavo-surface angle. Again, amalgam is not ductal, hence these
edges of this filling are easily fractured at the margins under
stress. This liability to fracture at the margin is also true of our
cement and silicate fillings and great care should be exercised in
placing the margins of these fillings. Cohesive gold, especially
Avhen alloyed with platinum, is our best filling material to resist
the crushing strain at the margins, and when the edges are not too
thin, the repeated blows from the opposing teeth only tend to drive
this material in closer adaptation to the margins. When using the
gold inlay, it is quite necessary to exercise great care at the mar-
gins to resist the crushing strain, not of the gold, but of the en-
amel margin and the intervening cement, for unless the gold in-
lay fits better than the average gold inlay, there is a line of ce-
ment which is subsequently dissolved. This leaves the last rods at
the cavo-surface angle unprotected, and very liable to injury.
It therefore folloAvs that the amount of marginal extension for
resistance form is less for cohesive gold and gold inlays than other
fillings. The greater the edge strength of the filling material, the
more protection it gives the cavity margins. Yet resistance form
should receive careful consideration with fillings of maximum edge
strength.