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80 OPERATIVE DENTISTRY
Avay between the gingival Avail and the incisal surface of the filling
"would be required to stand a strain just double the force at the in-
cisal, or place of impact. In diagram B, Fig. 37, the incisal point
angle is placed three-fourths of the way from the gingival to the in-
cisal and we then have
400
100 lbs. : X : : 3 : 4 z=: 3^ = 1331^ lbs. = x.
This shows a strain on the incisal point angle of one hundred and
thirty-three pounds. It will therefore be seen that the incisal point
rig. 38.—Drawings to illustrate the principle of the lever in the dislodgmcnt of fillings of the
fourth class, plans one and two.
angle should be laid as close to the incisal edge of the tooth as the
strength of the dentine protecting that angle will permit as it fol-
lows that: "TJic fartJier flie incisal angle is from tJie force of masti-
cation the greater will he tlie strain on hotJi dentine and filling at
tins angle."
With Fig. 38 we will consider the principles in a little more com-
plicated form. Let a represent the fulcrum, h and c the loads and d
the point of the application of the force. The radii of the arcs of
the circles represent a few of the directions from which force may
be received by the filling. With the light lines the force Avould be
80 OPERATIVE DENTISTRY
Avay between the gingival Avail and the incisal surface of the filling
"would be required to stand a strain just double the force at the in-
cisal, or place of impact. In diagram B, Fig. 37, the incisal point
angle is placed three-fourths of the way from the gingival to the in-
cisal and we then have
400
100 lbs. : X : : 3 : 4 z=: 3^ = 1331^ lbs. = x.
This shows a strain on the incisal point angle of one hundred and
thirty-three pounds. It will therefore be seen that the incisal point
rig. 38.—Drawings to illustrate the principle of the lever in the dislodgmcnt of fillings of the
fourth class, plans one and two.
angle should be laid as close to the incisal edge of the tooth as the
strength of the dentine protecting that angle will permit as it fol-
lows that: "TJic fartJier flie incisal angle is from tJie force of masti-
cation the greater will he tlie strain on hotJi dentine and filling at
tins angle."
With Fig. 38 we will consider the principles in a little more com-
plicated form. Let a represent the fulcrum, h and c the loads and d
the point of the application of the force. The radii of the arcs of
the circles represent a few of the directions from which force may
be received by the filling. With the light lines the force Avould be
