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PREPARATION OP CAVITIES FOR PORCELAIN INLAYS 303

previous packing of gutta-percha or cotton saturated Avith chlora-
percha.
Outline Form should be the same as for other filling. The axial
wall should be the miniature of the tooth surface wherein the
cavity occurs. The gingival wall should be flat and meet the axial
at an acute angle. All other surrounding walls should meet the
axial at slightly obtuse angles. This gives a cavity with draw to
the labial allowing the incisal portion to swing out in advance,
the inlay going to place gingival first.
This hinge movement is slight but constitutes a valuable point

Fig. 167.—Class Five cavities for porcelain inlay.

in subsequent retention. Just before setting the inlaj- the axio-
incisal line angle should be sharpened to add retention form. In
eases where the decay resulting in a cavity is materially iiorseshoe
in form the cavity may be filled by two distinct operations.
This is accomplished by filling the cavity with cement and cut-
ting out one-half and filling with porcelain. This completed, the
other half is cut out and the operator then proceeds to fill that por-
tion. This results in two porcelain fillings with cement between.
One Point Must Be Observed. The first portion of porcelain
will necessarily slightly overlap a cement Avail. Before setting,
this portion of the inlay must be ground at the expense of the ex-
ternal surface of the filling to reverse the draw, or this portion of