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170 THE TECHNICAX, PBOCEDURES IN FILLING TEETH.

may be built over and tlie form of the tooth restored, as seen in
Figure 216.
In the uppee molaks, the buccal wall is more often the weak
wall. This does not so often occur in case of occlusal cavities,
though it is not very infrequent. But oftener in mesial cavities
the mesio-buccal angle is found badly undermined by decay.
If the decay has sjjread along the dento-enamel junction until
it has reached the enamel under the cusp and left it unsup-
ported, the cusp should be cut away at once and be restored with
the filling material. Often in mesial ca^'ities, decay will have
extended far to the buccal without undermining the cusp, in
which case an extension of the enamel margin will not meet the
requirements without the removal of the cusp. In such cases
the enamel wall should be finished in the bucco-lingual plane.
Here the rule that, when the enamel must be cut near to the line
of a developmental groove, it should be cut to the groove or a
little beyond it, should be applied in case the buccal groove is
in question.
Undermining of the disto-lingual cusp of upper first
MOLARS. In upper first molars, with large disto-lingual cusps,
the contact i^oint is often toward the lingual, and caries of the
distal surface is therefore likely to begin in such position as
to undermine the disto-lingual cusp and weaken the lingual wall
of the cavity. Also the disto-lingual groove is usually deep and
sharply sulcate in these teeth, and forms a very weak line in the
enamel. Therefore, unless the lingual enamel wall is foimd well
supported by dentin, the cusp should be removed and the enamel
cut away to the disto-lingTial groove, and the cutting continued
gingivally until good strength is found.
This is illustrated in Figures 217-220, showing the upper
first and second molars, with the cavity in the distal surface
of the first molar disclosed bj^ the breaking in of the weakened
distal marginal ridge. As the teeth stood in the arch at the
time, the area of the contact and of near approach of surfaces
is very broad bucco-lingually. This is, in part, from the loss of
the prominence of the disto-lingual cusp by the breakage of the
contact point and in part by the twisting or rotation of the teeth
by the lingual part of the prominence of the distal cusp making
contact farther to the lingual after the first breakage, which
threw the pose of the teeth out of the balance with the forces
tending to hold the teeth solidly in contact. Attention was first
brought to this balance of pose of the contact points b.y the
results of Dr. Robert Arthur's method of cutting V-shaped