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lingual walls, which should be supported by dentine. It should be as
deep as the pulp will permit. The final step in the preparation of the
cavity should be the preparation of the margins. The enamel walls
should be properly beveled and polished. Enough of the enamel should
be removed in both the gingival and incisal thirds to prevent the forma-
tion of the frail margins of porcelain.

Fig. 17. Fig. 18. Fig. 19.

After the frail enamel has been broken down
with excavators or chisels, a fine-cut fissure bur of
Cb( Ccchniquc
of €avitv suitable diameter is used to prepare the labial
Tormation. and lingual margins. After the preparation of the
margins the triangular cavity is cut between the
enamel walls with a hoe excavator, the blade of which runs at a right
angle to the axis of the shank (Fig. 6 B). The blade should be short
and sharpened on both sides and the edge. This permits the cutting of
the dentine along the walls of the cavity. The incisal wall of the cavity
should be prepared with a small bur of such shape that it will form an
obtuse angle with the pulpal wall. The margins should be polished with
plug-finishing burs or Arkansas stone.
Fig. 18 shows a form of cavity preparation similar to Fig. 17, ex-
cept that it extends farther toward the incisal edge. This form of
preparation should only be used in those cases where the tooth is slightly
rotated in such direction that no stress will be applied to the incisal
ansfle.

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