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EXTENSION FOR PREVENTION II9

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Rule 7. In proximal cavities extend the labial , buccal and
lingual walls well out into the embrasures, into areas where they

will be less likely to decay; the incisal or occlusal margins be-
yond the contact point; and the gingival margins under the free
margin of the gum. The extension of these margins, as laid
down in this rule, follows the law known as '' extension for
prevention," which requires a brief explanation at this time.

Extension for Prevention

Proximal surfaces of the teeth are much more subject to
decay than other portions, because food and germs accumu-

late more readily here, and are less easily removed. This
being the case, if the margins of cavities are left in these
vulnerable areas, they will be much harder to keep clean,
and hence decay is likely to recur readily. Recognizing this
fact, it is the accepted practice to extend the margins of cavi-
ties, when they occur in these areas, by cutting away enough

of the tooth substance to carry them out to locations where the
food is not so liable to deposit, and where they may be more
readily cleansed. The food does not accumulate under the

gum margin so much as it does a short distance occlusally to
this point, hence the reason for extension of the gingival
margins. Food accumulates very readily and frequently
remains just gingivally to the contact points of two approxi-
mating teeth, and in order to obviate the necessity of leaving

the incisal or occlusal margins within this vulnerable area,
these margins are carried beyond this point down toward the
incisal, or occlusal surface, where they can be more readily
cleansed. Buccal and lingual margins, for the same reasons,

are extended out into the embrasures for a certain distance,*
depending on the convexity of the tooth surface and the char-
acter of approximation.