Page 183 - My FlipBook

PORCELAIN INLAYS 167

bevelling of the margin that places the enamel
border at a different angle to the walls (as in
cavities prepared for gold filling) must be carefully
avoided. The floor of the cavity and the lines of
junction of the walls and the floor should be rounded,
and the whole cavity be so prepared that the matrix
or mould can be easily removed without distortion.
This usually means an entire absence of undercuts,
and a general sloping outwards of the cavity walls.
A smoothly cut, well-rounded undercut may, how-
ever, be made with advantage in certain cavities
where, owing either to absence of a wall, or much
sloping of an opposing wall, the matrix can be
cleanly removed without distortion. For instance,
if the cutting edge angle and much of the lateral
walls of an incisor, or canine, are missing, a good
deep undercut at the cervical wall is not only
desirable, but often essential, for the retainage of
the inlay. In a case of this description the matrix
can be Avithdrawn in the direction of the cutting
edge, and will perfectly retain its shape.
The edge of the cavity should be as clearly
defined and sharp as possible, for on the clear
distinctness of the margin depends the production
of a fine margin on the matrix, and on the clean
margin on the matrix depends the excellence of the
joint when the inlay is cemented to place. To

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