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RESISTANCE FORM 131

3. Cavities in Proximal Surfaces of Incisors and Cuspids
Involving the Angle.— (a) If the simple mortise form is
adopted the procedure is practically the same as that de-
scribed in the preceding paragraph, consisting in flattening the

gingival and axial walls and joining them at a definite line
angle, the gingivo-axial line angle. The resistance form is
afforded by the flat gingival wall.

Fig. 157. Fig. 158. Fig. 159.
Fig. 157.—Cavity on the proximal surface of a central incisor. The simple
mortise form.
Fig. 158.—Mesio-incisal cavity on an upper central incisor. Labial view. An
effort has been made to preserve as much of the labial enamel plate as conditions
will permit.
Fig. 159.—Lingual view of the cavity shown in Fig. 158. The flat gingival wall
is well shown; also the cutting of the incisal step more at the expense of the lingual
wall in order to preserve the labial enamel plate.

(b) Incisal Step Form.—With a carborundum stone re-
move the cutting edge over into the middle lobe of the tooth,
until the dentin is exposed. The labial and lingual margins

of the cavity should first have been extended out in close
proximity to the mesial or distal groove (depending on
whether the cavity is mesial or distal) and the angle cut away
the same distance. Now, with the inverted cone bur, cut a