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94 Ot^ERATlVlS DENTISTRY
study of extension for prevention in this class of cavities. It ap-
pears that when the outline is carried quite to the angle that
secondary caries rarely occurs.
The Gingival Outline should be laid below the gum line for its
entire length until the angles are reached when it should emerge
from beneath the gum at a right angle to the free margin of the
gum.
The Occlusal or Incisal Outline should be carried to a region
of sound enamel. Where this extension does not carry this outline
farther than one millimeter from the free margin of the gum farth-
er extension should be made. AVith teeth surrounded by a heavy
gum, particularly if there seems to be a condition of hypertrophy

A B
Fig. 55.—Class Five filled. Cavities shov/n in Fig. 54.
present, the occlusal outline should be laid at least two millimeters
from the border of the gum.
Retention Form. Retention is secured by squaring out the four
point angles. The axial wall should generally be left as decay has
left it in the central portion. Any subsequent cutting should be
of such a nature as would tend to make it convex to the external,
or so to speak, the miniature of the tooth's surface in which it is
being cut. An effort to cut a flat axial Avail mesio-distally Avill
often endanger the pulp and is unnecessary as these cavities need
no resistance form.
In Large Buccal Decay often- the gum has so grown into and