Page 839 - My FlipBook

THE CONTOURING APPARATUS. 837

but sufficiently extensive to show on the model the bicuspids and
canines, for reasons that will become obvious.
After removal, replace the bands accurately in their positions in the
impression, and fill as before with Teague's or any good investing
material.
This material will give a model that will hold the bands in exact
relative position while they are being soldered, and one also that is suf-
ficiently extensive to enable the placing and soldering of the tubes in
proper position and direction—a thing of the utmost importance.
In selecting the tubes the smaller should loosely fit the threaded end
of No, 20 wire, which is the size to use for the fulcrum wire, f. The
size of the larger tube should be governed by the size of the power bar,
i. e. when the jaw is large with fully developed teeth, or when the dis-
tance is considerable from anchorage appliances to the upright bars on
the anterior teeth, the size of the power bar, p, should be No. 14. It
should rarely be smaller than No. 15, though when the operation is
attempted for very young children No. 16 will answer the purpose.
But the ordinary German-silver wire of the shops of these sizes will
not do. It must be specially prepared in order to withstand, without
bending, the great force exerted upon a bent bow or bar. All wire for
power bars should be drawn, without annealing, from No. 6, and be
nearly as rigid as tempered steel. In the selection of tubes the larger
should loosely fit the threaded end of the power bar, and be ^ to f of
an inch long.
An important feature is the position of the power-bar tubes. They
should be so placed and soldered to the anchorage bands that the power
bar—when placed in the tubes— will ex-
tend from it in a straight line to the ca- Fig. 894.
nines, where it bends over to engage with
the upright bars, c. (See Fig. 893.) If
this precaution be not taken, but instead the
power tubes are soldered in the ordinary
way, in contact with the buccal surfaces of
the bands, the power bow, in most instances,
will require to commence its encircling bend
immediately upon emerging from the tubes,
with a decided weakening of its rigidity and
possible fiiilure.
In order to obtain the proper position
it will often be advisable to rest the poste-
rior end of the larger tube upon that of
the smaller, as shown in Figs. 894 and 895. All projecting portions
that are liable to irritate the mouth should be rounded and polished.