Page 125 - My FlipBook

STRENGTH OF THE BITE. 121

catching a shot or a piece of bone upon the cusp of the tooth
and throwing it off. It is that which is caught upon the slope
toward the central portion of the tooth that crushes it off, not
that which is caught upon the slope toward the axial surface of
the tooth. This is more essential in the bicuspids than it is in
the molars, because they are less strong. In an ordinary mesio-
disto-occlusal cavity, where the pulp is still retained, the inclines
of the cusps may be left with perfect safety. All the strength of
the muscles will not break it. But when the length of the walls
is increased by extending the cavity to the floor of the pulp-
chamber the case is entirely different. The bicuspids have
sharper inclines than the molars, and food wedged into their sulci
has a greater splitting force, and in these, with a mesio-occluso-
distal cavity, and the pulp removed, it is necessary to protect the
cusps in every way possible.
In building out central incisors that have lost an angle there
is only about half the stress to contend with as upon the molars
;
and yet, in some instances there is as much as 200 pounds
brought upon them. It is not uncommon for persons to take a
bread crust between the teeth and put stress upon it, perhaps the
full strength of the muscles of the jaws, and then tear it off with
the hand, putting an additional force upon Now, that we
it.
have to build against, in building up these angles. We must pre-
pare for this in the anchorage of the fillings and depend upon the
strength of the gold when they are built. In this case it is nec-
essary that we have the welding property of the gold perfect in
every part ; any carelessness in the annealing of the gold will
tell in the breakage of fillings in incisal angles. A little portion
imperfecdy annealed will make a fault a little imperfection in
;
the malleting will make a fault, either of which will extend over
a portion of the area and cause a break. In the preparation of
gold for such building we should generally resort to annealing
upon a tray, allowing the gold to remain hot for a considerable
time in order that every particle may be well annealed. And
then pack the gold with a plugger point with a small condensing
area, and go over every part of it with great care and use enough
force.
The force required in making fillings, and the strength
required of fillings, will depend largely upon the strength of the
peridental membranes. If the peridental membranes are not