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176 THE TECHNICAL PEOCEDURES IN FILLING TEETH.

when the decay is uncovered by the removal of the undermined
enamel, the cavity extends through from mesial to distal. When
the pulp has been removed and the cavities trimmed to form, it
is found that both the buccal and lingual portions remaining
will be frail from the loss of tooth material. Therefore, the prep-
aration must be on special lines in order to gain the greatest
strength possible. The point of especial danger is in the break-
ing of the remaining parts of the cusps by the wedging of food
between their central inclines in chewing. If we look at Figures
239, 240, and remember that the strong cusp of the lower first
bicuspid strikes between the cusps of this tooth and that the
crush of food material between these teeth tends to force these
cusps apart and to break them, we will realize the danger. There-
fore, when the facts are discovered, this tooth is separated well
from both its neighbors to limit the cutting to the buccal and to
the lingual as much as possible, the gingival walls are made flat
to afford the best possible seat, or resistance form, the slopes of
the cusps forming the sulcus are removed sufiSciently for a strong
stratum of filling material to be built over them, as shown in
Figures 241, 242. Just so much of each cusp is removed and
replaced with filling material as will catch the splitting stress
that comes in the crushing of the food, placing it entirely upon
the filling material, which is solid from buccal to lingual. This
removes the splitting stress from the weakened cusps. The man-
ner of doing this is clearly seen in the two Figures 241, 242, which
explain this method of conserving the strength of the tooth much
more perfectly than can be done in words. Figure 243 shows
the completed case in occlusal view.
Many of these bicuspids with mesio-occluso-distal cavities
have been filled with the preparation shown in Figure 244. This
shows much more of the tooth retained, but leaves the central
inclines of the cusps in position so that the wedging comes upon
the cusps instead of the filling. Sooner or later a powerful split-
ting force is exerted between the inclines of these cusps and the
inevitable result is depicted in Figure 245. In time, one or the
other of the cusps is almost sure to be split off; sometimes it
will be the buccal, as shown, and generally in almost the exact
place and form shown, and sometimes it will be the lingual cusp.
Therefore, whenever the pulp is removed, forming a comjilete
separation of the cusps in case of both mesial and distal surfaces
being cut through, whether the tooth is bicuspid or molar, all
splitting stress between the cusps must be removed l)y cutting
over the crest of the cusps sufficiently far to place all of the