Page 171 - My FlipBook

BEVELING ENAMEL MARGINS 139

axial surfaces, they begin to run more and more horizontally

to the long axis of the tooth; and (4), as the cervical line is
approached, their outer ends begin to radiate toward the
cervical, following the general law already mxcntioned, viz.,
that they run perpendicular to the surface of the dentin.
Practically the same conditions exist in the centrals, lat-

erals and cuspids, the same general laws being appKcable
(Figs. 171 and 172). (i) On incisal surfaces the rods are
perpendicular to the long axis of the tooth; (2) at the angles
the rods on the incisal and axial surfaces approach each other;
while on axial surfaces they lie more or less horizontal to
(3)
the long axis of the tooth; (4) radiating gingivally as the
cervix is approached.
In the manipulations incident to the fourth step of cavity
preparation, the established law is to bevel the enamel mar-

gins outward, following the direction of the rods in the loca-
tion in which we are working. The dentin is the natural
support of the enamel. A study of the accompanying illus-
trations will demonstrate why bevel of these margins is nec-
essary in order that no rods be left unsupported, the amount

of bevel depending on the direction of the rods. In pit and
fissure cavities very little bevel is necessary, the amount being
increased with the increase in size of the cavity, and the
approach of its margins toward the points of the cusps. In
proximal cavities a marked bevel is indicated at the gingival

margin (see illustration), while the amount on the incisal
margins will depend on their proximity to the angle and the
convexity of the surface, the rods deviating more at the angles
and on distal convex surfaces.
In centigrades the amount of bevel for cavity margins will

usually range between 2 and 7 degrees, depending on location
The tendency of beginners
and the filling material to be used .
will be to bevel too much.