Active components of removable orthodontic appliance
The force (F) delivered by a spring is expressed by the formula F adr4//3, where d is the deflection of the spring, r is the radius of the wire and / is the length of the spring. Radius and wire length, therefore, have most effect on wire stiffness.
Where the teeth needed for retention of the appliance are those to be moved, a screw rather than springs may be useful. However, screws are more expensive than springs and also make the appliance bulky. They also exert large intermittent forces, but this is tolerable because of the small (0.2 mm) activation with each quarter turn.
Elastic traction from intraoral elastics designed for orthodontic purposes should be avoided for overjet reduction with removable appliances as it tends to flatten the arch; in addition, the elastics may slide up the tooth surface and impinge on the gingivae. Elastic traction is best used with fixed appliances.