Active spring components
Active spring components for tooth movement encircle the tooth in a linear fashion, exert spring pressure, and can be activated (Fig 10-37). The spring components of the active plate help to move sections of the dental arch or individual teeth.
The mechanical effect of spring components is different from that of screws. Screws exert a strong initial force that is tailored to the width of the periodontal space and is therefore biologically favorable. Springs are elastic and act with relatively weak but continuous pressure and with a thrust distance that is greater than the periodontal space.To exert effective pressure that will initiate remodeling processes, a specific spring force must be applied by a specific spring deflection. This means, however, that the continuously acting spring forces endanger the periodontal ligament, and hence the appliances can only be worn on an hourly basis.
Furthermore, if the spring forces are applied in a punctiform fashion and lead to tipping of teeth, excessive pressure intensity can easily arise, which stops the blood flow in the periodontal ligament. Spring forces often lie within the third level of biologic intensity and can result in damage.
The spring effect, depending on function, is punctiform for tooth rotation or linear throughout the tooth width for protrusion or extrusion. The springs are generally covered in acrylic but exposed in the effective area on the tooth.
Short, thick springs are hard; long, thin springs are soft. Springs with punctiform contact will tip or rotate the tooth; spring loops with linear or flat contact may in some circumstances move the tooth in multipoint application of force.
When technicians are choosing or designing their own spring shapes, they should check what tooth movement is to be achieved by spring pressure. The position and form of the force being applied and the usable abutments must be borne in mind, as well as an unimpeded movement path for the tooth. It is also important to ensure that the retentive components of the appliance can be used for tooth movements as passive abutments or bodily clasps.
The active components of an active plate can be varied to match the task in hand and permit numerous movement possibilities. Depending on function, there are three types of spring components:
- Springs as screw replacements
- Springs for stationary application of force
- Labial bows
Spring components as screw replacements are usually expansion springs. However, they may also be used as tension springs. In an omega shape, these are known as Coffin springs because they were being used as early as 1882 by S. H. Coffin. These Coffin springs take up less space than screws and allow variable movements of the appliance segments in different directions; ie, they can be used for any case that can be treated with screws.Their disadvantage is that patients cannot carry out activation themselves and the magnitude of force as a function of activation cannot be accurately measured. Continuous pressure in the third level of biologic intensity arises, which leads to damage.
A Coffin spring is bent out of 1.0- to 1.2-mm spring hard steel wire to form an 8- to 16-mm omega loop (Fig 10-38). It lies centrally in the maxilla and joins two sagittally sawn plate halves of an orthodontic monobloc or activator. Two small Coffin springs with 8-mm loop size opened up toward one another can be placed in a simple expansion plate to achieve torsional stiffness.
A Coffin spring can be activated at three places and offers the possibility of anterior, posterior, or total jaw widening. The larger the Coffin spring, the weaker the spring effect will be; the smaller the loop (or when two springs are mounted), the stronger the spring effect will be.The spring force cannot be measured as accurately as the expansion force of a screw.
A Coffin plate is an expansion plate whose edges lie palatally against the anterior and posterior teeth. It is sawn through in the middle, and the halves are pushed apart by a strong Coffin spring whose ends are fixed in the acrylic. By activation of the spring, the pressure can be evenly transferred to the alveolar processes or, if necessary, anteriorly increased.
Spring components for stationary force application
These active spring components act on individual teeth or groups of teeth. The spring forms can vary considerably in shape and size; differences can arise in terms of the effect and magnitude of force.
An intermediate or interdental spring for lateral, mesial, or distal tooth movements in the dental arch is bent out of spring hard round wire: 0.5 mm for anterior teeth, and up to 0.6 mm for canines and premolars. This kind of spring is basally let into the baseplate and can be activated; it is covered toward the tongue. It runs over the approximal surface in a vestibular direction; the springy part has an activation eyelet (loop or double loop). The end of the spring is bent round to form a loop and, coming from approximally, lies to a variable extent on the vestibular surface of the tooth being moved. When placed between two teeth in a closed form, it can be used to widen a gap, or it can be used with a U-shaped loop for readjusting or with crossed retentions for closing a diastema.
A finger spring is an interdentally acting intermediate spring for tooth movements in the sagittal and transverse directions that is fabricated from 0.6-mm spring hard wire. It has interdental/ approximal contact with the tooth being moved and is referred to as an interdental, intermediate, mesial, or distal spring.
A retracting spring is an intermediate spring that fits closely to the tooth on the vestibular side and helps align individual teeth (Fig 10-39). The springy part runs in a U or V shape, is fitted with an activating eyelet (loop), and can be anchored in the body of the appliance or soldered to the labial bow.
Protrusion springs arising from an appliance plate are the most commonly used springs in orthodontics. They are open or closed, single-tail or double-tail spring loops that can be crossed over each other several times. The loops of 0.4- and 0.7-mm spring hard wire are used for single-tooth movement, tooth rotations, and movement of groups of teeth. Frame loop, frame spring, paddle spring, or loop spring are possible synonyms.The protrusion spring is let into the baseplate so that a basal guide fan is formed in the acrylic base in which the spring is free moving and can be activated. Activation is achieved as the loops are bent open at the bending points or the specially bent activation eyelets.
An open protrusion spring, in the form of a compressed S, has at least three activation loops running parallel to each other at a distance of 1 to 2 mm (Figs 10-40 and 10-41). The loop width corresponds to the mesiodistal width of the tooth being moved.
A closed protrusion spring is fabricated from 0.4- to 0.6-mm spring hard wire; both ends of the wire are firmly anchored in the acrylic base, making this spring less elastic (Fig 10-42). It can have one or two activation loops and run over all the anterior teeth and canines to act as a support to a labial bow.
Spring loops (guide loops) are classified as closed protrusion springs and can contact individual teeth, run over several teeth, or run over the whole of the anterior region (Fig 10-43). However, the position of force application in the coronal area is significant for tooth movement. Depending on the degree of tooth movement, it must be possible to activate the spring evenly throughout the movement path.
A paddle spring is a closed protrusion spring for single-tooth movement in which both ends of the wire are anchored in the acrylic base (Fig 10-44). It can be used for maxillary anterior teeth (but it tends to cause tipping) or for buccal movement of posterior teeth.
Elastics are mostly fitted between maxillary and mandibular appliance plates to correct a displacement of occlusal position. To treat a re-trusive occlusion, a hook is mounted on the maxillary plate level with the canines and on the mandibular plate at the last molar. As a result, the mandible is pulled forward. If prognathism is to be corrected, the hook arrangement in the maxilla and mandible is transposed to pull the mandible backward. The retentive components of the active plate have to be particularly numerous and secure when using elastics for correcting occlusal displacement. Elastics can also be used for rotation of twisted teeth. A circular wire or metal band is placed around a severely twisted tooth, to which a hook is fitted that does not impede rotation of the tooth. An elastic from this hook to the plate exerts a pull to rotate the tooth.