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CHAPTER XLIII.
PREPARATION OF CAVITIES FOR PORCELAIN INLAYS

The filling of teeth with porcelain demands some change in the
usual and accepted form of cavity preparation for other materials.
Access Form. Access form reaches its maximum in porcelain
filling. Even greater access is required than for the gold inlay.
Hence preliminary separation should be practiced with all proximal
fillings, before forming the matrix, and generally mechanical
separation is of advantage when setting the filling.
Outline Form for Porcelain Inlays. Outlines must be extended
to regions of sound enamel. The obtaining of full length enamel
rods supported bj^ sound dentine is imperative. Extending to self-
cleansing margins is of additional advantage, yet not so impera-
tive as with gold filling, as secondary decay is not as liable to take
place about a porcelain filling.
The outline should not follow a developmental groove nor cross
a ridge at its extreme eminence. Sharp angles in outline should
be avoided. Extension for prevention as applied to the embrasures
is not as great as with metal fillings.
Extension for Resistance to Stress at margins is more essential
than with gold, due to the friability of porcelain margins.
Resistance Form for Porcelain Inlays. The rules for flat seats
for all fillings apply equally to porcelain fillings. The use of the
step in Class Four is essential to give added resistance to the tip-
ping strain. Margins should be extended to locations less fre-
quented by the crushing strain.
Retention Form for Porcelain Inlays. JMaximum retention form
is required in all directions except one, until the matrix has been
formed and the filling made ready for setting, when retention
should be added in the remaining direction.
Acute line and point angles should be avoided; all angles being
rounded angles until the matrix is formed.
Convenience Form for Porcelain Inlays. The filling of teeth
with porcelain requires more cutting for convenience form than
for any other method. This fact makes such fillings contraindi-
eated many times, due to the great loss of tooth substance neces-
Pre-
sary to properly form the matrix and introduce the filling.
vious separation will overcome this cutting to a large extent with
this as well as other fillings.
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