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26 METHODS OF FILLING TFFTII.

and a neat, sharp marginal line ? The answer must be, that it is im-
The cross-motion of a instrument will not
possible! polishing always
expose the true edge even where the margin is a sharp right angle, and
it is less apt to do so along a bevel. If the disk or stone is used as has
been described to be the best method (revolving along the margin), the
first effect will be to produce a scalloped line, and if the operator then
out that line, he removes the edge of the gold,
attempts to straighten
so that a portion of the beveled enamel is uncovered, leaving a slight
groove along the length of the tooth next to the filling. Fig. 47
shows a section through a tooth and filling after such a mishap, the
exposed bevel resulting in a groove showing at a.
This idea of beveling is much more frequently advocated at the
gingival border, through a strange impression that the margin which
has gold lapping over it will better resist caries than where the
gold is brought against it in sharp contact, without overlapping.
Where such a beveled gingival margin is polished with either sand-
or the described is most
paper strips approximal trimmer, the groove
likely to occur. If the disk alone is depended on, the overlapping
gold will be left with a ragged edge in ninety per cent, of cases. If
the margin along the gingival border has been formed with a sharply-
defined angle, and the enamel is supported internally by dentine,
which has not been erroneously cut away for anchorage, then gold
may be packed solidly against and over it, and in polishing the ap-
proximal trimmer may be freely used until it no longer catches, when
a fine sand-paper strip will finish the marginal line as fine as a hair.
It has been wisely argued that the enamel margin should not assume
an acute angle, for the reason that the enamel at the extreme edge
being unsupported by dentine, will be likely to crumble during the
operation of filling, because of its tendency to cleave. For this rea-
son it has been advised that the enamel margin should assume the
direction of the enamel-rods at the
point operated upon.
Thus theory as well as practice, and mechanics, recommends that
the enamel should cut the surface of the tooth at a
margin right angle,
for it is usually true that the enamel-rods are arranged perpendicularly
around the dentine, so that they must assume almost if not quite right
angles to the surface.
If we examine the of
varying surfaces in the masticating portion
a molar or bicuspid, it becomes plain that to follow the rule here and
arrange the enamel margin so that at all points it shall be a right
angle, would be to undertake the impossible. Neither is it necessary,
for whereas in approximal and surface fillings we are compelled to
finish the gold down flush with the true margin of the cavity, in
molars and bicuspids the sulci being depressions which we wish to
eradicate, as well as to simply fill the cavity proper, we customarily