Page 29 - REMOVABLE ORTHODONTIC APPLIANCES

Active components 21
cantilever spring but, as both ends of the spring the patient inserts the appliance. Once the tooth
are incorporated into the acrylic, the flexibility has moved some distance, it may be necessary to
is correspondingly reduced. Fortunately, a elongate the spring at the adjustment loops.
large deflection is not indicated in this situa-
tion, otherwise the patient may have problems
in inserting even this type of spring. It must be Coffin spring
recognized that the force applied by the spring
has a vertical, as well as a horizontal compo- This is a strong spring made of a thick gauge
nent. If the tooth surface at the point of contact wire (1.25 mm) and is used for transverse arch
is nearly vertical (as is usually the case with an expansion, for example to treat a unilateral
upper premolar) the intrusive component is crossbite with lateral mandibular displacement
small. If this spring is applied to a sloping (Figure 3.15). It has the advantage over a screw
surface, such as the cingulum plateau of an that differential expansion can be obtained in
upper incisor, the vertical component will be the premolar and molar regions, but the appli-
larger and the labial component correspond- ance tends to be unstable unless it is expertly
ingly smaller. Even if the 'T' spring initially made and adjusted. For this reason, a screw
contacts the more vertical incisal part of the may be preferred unless differential expansion
palatal surface of an incisor, it will come to rest of the arch is required.
on the cingulum plateau as the tooth moves.
This reduces the efficiency of the spring and
the tooth itself may be intruded. Intrusion is Adjustment
usually unwanted when incisors are to be Before the spring is adjusted, it is useful to drill
proclined because stability depends on a posi- marking pits in the appliance. Divider readings
tive overbite after treatment. The vertical are taken of the transverse distance between
component also has a displacing effect on the these pits so that the amount of expansion can
appliance and retention may be a problem. For be controlled. Pliers should not be used to
these reasons, 'T' springs are not usually used adjust the spring because it is readily distorted.
for labial movement of upper incisors.
It is safer to expand the appliance by pulling the
sides apart manually, first in the premolar and
then in the molar regions. Care must be taken
Adjustment to maintain the two sides of the appliance in the
The spring is adjusted by pulling it away from the same plane during adjustment. If it is twisted
baseplate. Provided that it is only adjusted by a vertically, the appliance will not fit and this can
small amount it should seat itself correctly when be difficult to correct

Figure 3.15 A coffin spring (1.25 mm)
for transverse arch expansion.