Page 22 - My FlipBook

J8 OPERATIVE DENTISTrV

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': one side only and is used by a pushing force applied in the diree-
(_ lion of the long axis of the shaft.
The chisel edge is made with a bevel at an angle calculated to
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— plane and cleave a substance possessed of a grain, and is so tem-
pered as to retain an edge Avhen working on hard substances.
The use of the chis,el is, therefore, the cleaving and planing of
enamel. The planing of dentine may be done M'ith a chisel or with
other instruments of a similar edge.
CJhisels are divided into sub-classes according to the shapes of
Z) their shanks, as straight, bin-angle, contra-angle, etc.
^ A hoe is that class of excavator with the cutting edge at a right
in angle Avith the shaft, sharpened on the distal side only and is used
by a pulling force applied parallel Avith the long axis of the shaft.
Hoes are divided into sub-classes according to the shape of their
shanks, as, mon-angle, bin-angle, contra-angle and triple-angle con-
tra-angle. The hoe is used mostly for cutting dentine.
A hatchet is that class of excavator Avith the line of the cutting
edge laid in the plane parallel Avith the long axis of the shaft.
Hatchets are divided into sub-classes the same as the hoes, ac-
cording to the shape of their shank, as, mon-angle, bin-angle and
triple-angle contra-angle. The hatchet form is indispensable for the
construction of flat Avails and internal surfaces, the straightening
of lines and the sharpening of angles.
A gingival marginal trimmer is a modified hatchet.
A spoon is that class of excavator AA^hich resembles in most re-
spects the hatchet, other than the cutting edge. This is sharpened
on one side only Avhich is rounded like the convex side of the boAvl
of a spoon from Avhich it derives its name. The cutting edge is
rounded and sharpened to a thin edge. Spoons are alAA^ays made
rights and lefts.
The use of a spoon is to remove foreign matter and softened
dentine from the tooth caAnty.
The angles betAveen the shank and the Avorking part are desig-
nated as mon-angle, bin-angles, and triple-angles, according to the
number of angles used being one, tAvo or three, respectively.
The contra-angle is the placing of such angles in the shank of
the instrument as to bring the cutting edge near the central line
of the shaft Avhich removes the tendency to tip or turn in the hand
during iise.
Bin-angles and triple-angles are properly made only Avhen con-
tra-angled, provided the cutting edge is more than three millimeters
from the central line of the shaft.