Page 168 - My FlipBook

136 PREPARATION OF CAVITIES

simple mortise form is now transformed into the dovetail
mortise form by cutting at the expense of the buccal and

lingual walls, rendering the cavity wider, bucco-lingually
at the junction of the buccal and lingual walls with the axial
wall (Fig. 145). In the compound mortise form, after ac-
centuating the gingival point angles, dovetail the lateral

walls of the auxiliary portion (the step), and slightly deepen
the two point angles in that por-
tion (Figs. 147, 148 and 163).
Technic for Retention Form.
Use the inverted cone bur,

chisel, hatchet and hoe. In deep-
ening the gingival point angles, do
so more at the expense of the axial,
labial, buccal and lingual walls,
than of the gingival wall. Place

the blade of the instrument in the
point angle, and after slightly deep-
ening it at the expense of the gin-
Fig. 168.—Accentuating the
gival wall, cut along the line angle
gingival point angle for conven-
ience and retention form. In most uniting the labial, buccal or lingual
instances an inverted cone bur ^
^
number one-half or number one is wall (aS the Case may be) with the
axial wall, for a distance of about
one-fourth the length of the wall, thus forming a slight groove,
which gradually disappears at its termination (Fig. 168).
For making the incisal retention form in proximal cavities

in incisors and cuspids, use the acute angle hatchet, placing its
blade in the point angle and cutting at the expense of the
labial and lingual walls, as well as deepening the angle
toward the incisal.
4. Bevel and Polish Enamel Margins.—What has been
said about resistance and retention forms applies entirely to

the dentin, these forms being provided for at the expense of

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