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USE OF THE DENTAL ENGINE. 45

The sizes of burs are very important. With the dental
engines with which we were supplied a few years ago, no large
burs could be used to advantage, for the reason that the motion
was conveyed by a cable, which allowed large burs to jump and
chatter. Cord engines give to the bur a much smoother motion
and better cutting power. Practically none of them have suf-
ficient power to run a large bur to advantage. For our use,
then, burs for excavating that are over one and a half milli-
meters in diameter should not be used at all. The most useful
burs are one millimeter and less in diameter.
The round bur. The round bur is used only for the one
purpose of opening pit cavities in which decay has only just
begun. For this purpose round burs, from a little less than one
millimeter to one and a half millimeters in diameter, should be
used by placing them in the pit while in motion and swaying the
hand-piece to and fro so as to rotate the bur laterally while it
is rapidly turning upon its axis. This lateral motion of the
hand-piece will cause the bur to cut much more rapidly than when
simply held against the work. Use the smaller size first, and
when it has entered the pit, change it for a larger, using this
in a similar way, and follow this again with a larger size. Then
the roimd bur should be laid aside and the cavity finished, if
further enlargement is required, with other instruments. There
is no other use for a round bur in excavating cavities. It should
never be used for removing decayed dentin. If the pit cavity
requires no further extension, the round bur should be followed
by an inverted cone that will square out the pulpal or axial
wall of the cavity and make the angles with the surroimding
walls sharp and definite. A cavity with rounded angles is the
most difficult of all cavity forms to fill perfectly and should never
be made when any kind of metallic filling is to be used.
The inverted cone bur, the fissure bur. For other pur-
poses in excavating, it is a matter of choice in individual cases
between the use of the inverted cone and the fissure bur. These
burs should be used in several positions, which will be pointed
out.
In cutting seats or steps for anchorage in mesio- or disto-
occlusal cavities in the bicuspids and molars : After the mesial
or distal cavity has been well opened with cutting instruments,
and the cutting of a step in the occlusal surface is required,
choose a small inverted cone or fissure bur, never more than one
millimeter in diameter, and begin within the dentin close against
the dento-enamel junction, causing the bur to enter, and then