Page 33 - REMOVABLE ORTHODONTIC APPLIANCES

Active components 25
.The apron spring has good mechanical direction and so the stability ratio is poor.
properties but the Roberts' retractor is Minor movements of individual teeth can be
simpler to construct and more comfortable for obtained by incorporating bayonet bends at
the patient which makes it preferable. Some appropriate points (Figure 3.22). The major
orthodontists use the high labial arch with advantage of the labial bow with 'U' loops is
apron spring because repair is possible at the that if only minor overjet reduction or incisor
chairside but rewinding an apron spring can be alignment is required, it may be incorporated
tedious and, given the precautions outlined into an appliance with palatal springs for
above, the Roberts' retractor will give little canine retraction. When the canines are
trouble. retracted sufficiently, the bow may be
adjusted to retract the incisors. In view of the
heavy forces generated by very minor activa-
Adjustment tion of these bows, with the associated risk of
The bow is adjusted in the vertical limb as anchorage loss, it is better to modify the bow
described for the Roberts' retractor. for incisor retraction as described below. Note:
If the overjet is greater than 4 mm, it is usually
wiser to use a second appliance with a more
flexible bow.
Labial bow with 'U' loops
This is constructed on 0.7 mm wire. Flexibility
depends largely on the vertical height of the Adjustment
loops (Figure 3.21). Sulcus depth is limited, To reduce an overjet, the bow is adjusted at the
however, and because the wire is heavy these 'U' loops. This adjustment must be small. The
bows are very rigid in the horizontal plane. bow should be displaced palatally by only
Conversely, they are flexible in a vertical 1 mm. The flexibility can be greatly increased
by dividing the labial bow at the incisor retrac-
tion stage, so that there are two buccal arms
(Figure 3.23). This split bow is quite suitable for
incisor retraction but care must be taken during
adjustment to preserve the correct curve and
not to flatten off the arch anteriorly. This is
achieved by adjusting the bow at the 'U' loops,
rather than in the horizontal arms. Rotations or
Figure 3.21 A 'U' loop labial bow (0.7 mm) with minor individual tooth movements are difficult
passive stops mesial to the canines. to control with a split bow.

Figure 3.22 A bayonet bend for minor
adjustment of incisor position.

Figure 3.23 Flexibility of a 'U' loop labial
bow is greatly increased if it is divided.