Page 46 - My FlipBook

lingual stress. The ste[) should be cut across the incisal edge, extending
from one and one-half to three millimeters ; it should be from one and
one-half to two millimeters deep on the labial surface (Fig. 28 C).
Fig. 29 shows a form of cavity preparation similar to Fig. 26, except
that the step involves both labial and lingual plates. The labial plate
should form a reverse curve with the axial margin of the cavity. Its
margin should be so prepared that it will form a right angle with the
inciso-gingival curve of the surface. The" gingival and middle thirds of
the step should be prepared similar to the preparation of the step for
Fig. 26. The lingual plate of the axial wall of the step should be cut
further laterally than the labial, or a shallow groove should be made be-
tween the two plates with a fissure bar. This breaks the straight surface
formed in a labio-lingual direction, and thus assists in preventing the
cement from washing from the joint for the entire width.
Figs. 30, 31 and 32 show methods of step preparation, which, in the
author's opinion, give the best results in most cases in which there has
been extensive loss of tooth structure. The labial and lingual margins
of the step should form right angles with the gingivo-incisal curves of
these surfaces. They should form reverse curves with the axial margins
of the cavity. In those teeth with thick incisal edges, a shallow cavity
may be cut between the labial and lingual plates when these plates are
approximately the same length (Fig. 30). In teeth with their incisal
edges, the lingual plate should be cut almost half the thickness of the
incisal end, and should extend farther gingivally than the labial plate
(Figs. 31 and 32). It should be so formed that its gingival wall will be at
right angles to the concavity of the lingual surface. The axial-lingual
plate of enamel of the step should be cut further laterally than the labial,
or a shallow groove should be cut between the labial and lingual plates
of the step.
The preparation of the cavities for Figs. 25, 30, 31 and 32 is as
follows The gingival wall should be concave labio-lingually, and it should
:
run at right angles to the pulpal wall, and its margin should unite with
the margins of the labial and lingual surfaces in such manner that it will
be concave mesio-distally. Both the labial and lingual walls should be
parallel. They should slightly converge toward the center of -the tooth
as they approach the incisal edge, and their margins should run at right
angles with the curve of the surface. A triangular cavity should be cut
between these walls, gradually diminishing in depth toward the incisal
edge (Fig. 30). The labial and lingual walls of the cavity should unite
with the pulpal walls in such a manner as to form slightly obtuse angles.
Lower incisors receive stress in a vertical and labial direction. In
the preparation of step cavities in these teeth, where the step involves only

42