Page 32 - REMOVABLE ORTHODONTIC APPLIANCES

24 Removable Orthodontic Appliances
Bows is very important because if the wire is bent
where it emerges from the supporting tube (a
Bows may be active or passive and will usually site of stress concentration) it will often fracture.
span a number of teeth. Both ends of the bow The bow is adjusted by bending it in the vertical
are incorporated in the acrylic. Active bows are limb below the coil. Provided that the spring has
used for incisor retraction. The bow selected been carefully made and adjusted correctly, frac-
will depend partly on the preference of the ture is rare. As the incisors move palatally, the
operator and partly on the amount of retraction bow will drop anteriorly and the level of the
required. Flexible bows, such as the Roberts' horizontal part will have to be adjusted.
retractor, are most suitable where a large over-
jet has to be reduced. When the amount of
retraction is small but minor irregularities need High labial bow with apron spring
to be corrected (and perhaps over-corrected) a
less flexible bow may be preferred because it is In concept, this is similar to the Roberts' retrac-
more precise in its action and needs only a tor. A heavy base arch of 0.9 mm wire extends
small amount of activation. into the buccal sulcus (Figure 3.20). The
impression must be muscle-trimmed so that the
buccal and labial fraenae can be avoided. There
Roberts' retractor is no need to extend the base arch to the full
depth of the sulcus because the apron spring is
This is a flexible bow that is constructed from very flexible. When the baseplate is to be cold-
0.5 mm diameter wire inserted into stainless cured, it is simplest to attach the apron spring
steel tubing to give support at either end of the before the appliance is processed, but where
bow (Figure 3.19). A coil is placed at the point the appliance has to be flasked for a heat-cured
of emergence of the wire from the tubing. The baseplate, the apron spring should be attached
tubing emerges from the baseplate distal to the after the baseplate is processed. The apron
canines. The horizontal section of the bow is spring (0.35-0.4 mm) is attached to the base
bent into the smooth curve to which the arch by wrapping a few turns round the vertical
incisors should conform. Even if the incisors step and the active spring is formed by two or
are irregular, the curve of the bow should be three turns around the horizontal part. After
smooth and they will be gathered in as they are the apron has been formed, its free end is
retracted. A common fault is to make this hori- attached to the base arch in the manner already
zontal section too short so that it fails to described for the fixed end. Practice is required
control the lateral incisors. The flexibility of before an apron spring can be neatly formed
this bow lies in the vertical limbs and coils that and it is necessary to use an adequate length of
should be of adequate size (at least 3 mm in wire and to maintain it under steady tension as
internal diameter). it is wound on to the base arch. After the appli-
ance has been finished, the base arch may be
made more comfortable for the patient by
Adjustment removing the central portion. This also makes
This bow is light and flexible. An adjustment of it easier to replace the apron spring should it
about 3 mm is suitable but the site of adjustment fracture during use.

Figure 3.20 A high labial bow (0.9 mm) with an
Figure 3.19 A Roberts' retractor made from 0.5 mm apron spring (0.4 mm). The central portion of the
wire supported in tubing of 0.5 mm internal high labial bow is removed after the baseplate has
diameter. been processed.