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5G OPERATIVE DENTISTRY
Extension for Prevention will come in for consideration Avhen
the outline has for other causes been brought near to the free
margin of the gum. A full application of the rule ''Extension for
prevention" would demand that the gingival outline be carried
under the free margin of the gum when the gum has already been
approached to within one millimeter. A failure to extend the out-
line is permissible in mouths kept scrupulously clean.
Retention Form. This step is very simple when the cavity does
not involve the occlusal surface and is fully obtained when the in-
ternal line angles have been well squared. However, when the
cavity reaches the occlusal surface, the filling is subjected to the
greatest amount of tipping strain in mastication. These will then

A B
Fig. 22.—Class One filled. Cavities shown in Fig. 21.

demand a flat gingival wall, and in some cases of a vital tooth, a
flat pulpal wall placed parallel to the gingival wall, and the line
angles surrounding these walls well defined. The four point angles
should be slightly acute.
Finish of Enamel Walls. In the management of these axial sur-
face pit and fissure cavities the varying slant of the enamel rods
should not be lost sight of. This should be noted Avhen outlining
the cavity with the chisel. The rods will generally be found to
incline towards the pit, from every direction close to the defect,
Avhile a little way out they will be found at right angles to the
surface.
Going farther toward both the occlusal surface and gingival line.