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SILICATE IN THE MAKING OP A FILLING 153
the cavity outline in areas subject to as little stress as possible.
In locations subject to great liability to stress, it is necessary to ex-
tend the outline until full-length enamel rods, supported by sound
dentine, have been reached and then beyond that to a location not
subject to the travel of the cusps of opposing teeth in the process
of articulation. It is not necessary to pay much attention to devel-
opmental grooves, for when these grooves are normally formed they
are fully as strong as the material in hand. It is most important
that all enamel eminences be avoided, as the material is quite friable
and offers very little support to the cavo-surface angle.
Retention Form. Provision against the tipping strain is the
same as for other fillings and is more like that for amalgam. This

Fig. 89. Fig. 90.
Fig. 89.—A large Class Three cavity properly prepared for a silicate filling. Note the
small amount of dentine yet remaining near the incisal angle. While this angle can properly
remain when using a silicate filling, it would be entirely out of the question when using co-
hesive gold.
Fig. 90.—Two extensive Class Three cavities properly prepared for silicate fillings. In both
of these cavities the dentine has been practically all repioved at the incisal angles. Cases like
these may be filled with silicate but should be regarded as temporary in a large majority of the
cases. The retention of these angles after filling will depend entirely upon the amount of
force to which they were subjected. They would be comparatively permanent in cases of ir-
legularity when that condition placed these angles in a position removed from stress in oc-
clusion and articulation.
material only reaches its maximum strength to resist dissolution and
the crushing strain when it has been so thickly mixed that it has
lost practically all of its adhesive qualities. Therefore, the rules
which apply to cavity preparation in reference to retention form
would be the same as in the use of amalgam. "We must have flat
walls excepting the axial, flat seats of generous proportions and def-
inite angles.