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104 OPERATIVE DENTISTRY

with cohesive gold, we draAV it toward the mesio-distal plane along
the gingivo-axial line angle, alloAving it to fade out, after going
once or twice the width of the bur, taking the tooth substance
from the gingival wall. Treat both lower point angles in this man-
ner. In the step portion of the cavity follow the same procedure
in the two point angles, cutting all tooth substances at the expense
of the pulpal wall. This results in giving the cavity draAv to the
occlusal and giving your inlay four lugs, Avhicli key the filling to a
seating, particularly in the region of the gingivo-buccal and gin-
givo-lingual point angles. It also results in placing your retention
form high in vital cases and near the force of mastication, and in
a part of a vital tooth Avhich is well suited to stand the tipping
strain. (Fig. 58.)
In Non-Vital Cases the retention form should be placed low in
the tooth. In fact the major portion of it should be below the gin-
gival wall, and this is more frequently secured by the use of the
pin inlay. When the pin is not used, the pulp chamber is so shaped
that the Avax pattern will show a lug, which can be used for the
major portion of the retention.
Finishing of Enamel Walls. This part of the cavity prepara-
tion should be attended to with all of the care and detail that is
required when making a cohesive gold filling. In addition there-
to, after the planing has been done with a chisel, particularly
on the buccal and lingual outline, these margins should be pol-
ished with a veiy fine grit disk. This facilitates the travel of
the wax on these two surfaces when going to exit. A chisel fin-
ish on these surfaces results in a pattern that under the micro-
scope shows little fine projections, which have gone into the rough-
ened surface. In drawing the pattern these little projections have
])een bent and point gingivally. This results in an imperfect cast-
ing along these surfaces and interferes Avith the fit. Whereas if
the surfaces have been polished, a polished w'ax pattern results
nnd the completed inlay more nearly fits the margins.
When the cavity on account of decay is naturally retentive or
lias undercuts these are temporarily filled and overcome by cov-
ering the retentive portion of the cavity Avith some substance, as
temporary stopping or Avax of a different color than that used
in making the pattern.
Preparation of Cavities of Class Three.
The gold inlay is seldom indicated, in cavities of Class Three.