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142 THE TECHNICAL PBOCEDUEES IN FILLING TEETH.
far enough out into the embrasures toward the angles of the
teeth so that food passing over the margins of the filling will keep
them clean. This is called extension for prevention. In the
preparation of gingival third buccal or labial cavities, these
should be extended far enough toward the angles to effect a simi-
lar object.
Extension foe peevention is a definite requirement. The
phrase as first used meant the laying of cavity margins at points,
or along lines, that would be cleaned by the excursions of food
in chewing. It had the definite object of preventing the recur-
rence of decay at the margins of fillings where recurrence of
decay had been most often observed, by so lajdng the margins
as to obtain this cleaning effect. In proximal cavities the points
were those which, on account of near contact with proximating
tooth surfaces, were not well cleaned by excursions of food in
chewing, especially at the linguo-gingival and labio- or bucco-
gingival angles. This led to squaring out these angles, making
the gingival wall flat, or strictly in the horizontal plane, which
was found to be a decided gain. It was also found that in filling
buccal and labial gingival third cavities, there was rarely a recur-
rence of decay if the cavity margins were extended nearly to the
mesial and distal angles. Further study of conditions disclosed
the fact that so long as the teeth were actively used in mastica-
tion, no superficial beginnings of decay were found on those
angles of the teeth reaching from occlusal to the gingival ; i. e.,
the mesio-buccal, disto-buceal, mesio-lingual and disto-liugual
angles of the teeth. A strict examination of ten thousand per-
sons applying for dental operations in the clinic of Northwestern
University Dental School discovered but nine persons in whose
teeth decay had spread superficially, or on the surface of the
enamel, across these angles. While another count of the same
number of persons might give a different result, this is enough
to show that these angles are very generally immune areas. It
also seemed to demonstrate that decay never crossed these angles
in persons who were maldug the ordinary use of their teeth in
chewing food. In each ease discovered, the person could not
chew food because of sensitive cavities that prohibited the ordi-
nary use of the teeth, and each one had been in that condition
for from one to several years. This served to further establish
a fact long before demonstrated by ordinary clinical observation.
What extension foe peevention means. Extension for pre-
vention means only the laying of the margins of cavities so near
these angles as to obtain the benefit of cleaning in the excursions