Orthodontic Screws
3 Orthodontic ScrewsAn alternative method of providing a force is to use a screw as an integral part of the removable appliance ( Figure 16). The screw normally transmits its force by means of acrylic , which comes in contact with the teeth. many types of screw are commercially available for use in removable appliance (one dimension, two dimensions , fan shape, etc), the type of the screw is specific for each case and have to meat the requirements of an Adequate travel, Stability and minimal bulk.
Figure.16: Orthodontic Screws. A: 2sides; B: One Side.
Description:Screws are produce by a number of manufacturers and a wide range of sizes and shapes are available.
Atypical design has a central threaded screw, each end of which engages into a small metal or plastic block. One of this blocks caries two guide wires which lie parallel to the screw and pass through holes in the opposing block. The centre of the screw is enlarged into a small boss in which four radially positioned holes are visible. A small wire key is supplied and may be inserted into one of these holes and turned like a capstan through 90 degree until it touches the guide. For further activations the procedure must be repeated with the key inserted into the next hole.
Uses:
Screws are recommended only in those few situations where spring will be unsatisfactory. Nevertheless, there are certain situations in which screws are very useful, for example
Expansion
It is some times necessary to increase the width of the upper dental arch or to correct a unilateral cross bite .Symmetrical widening of the upper arch could be achieved by an removable appliance with suitable design containing four Adam's clasps on permanent molars and premolars, with a screw placed horizontally on the midline of the palate. In addition to a shallow posterior bite planes to prevent secondary lower arch widening (Figure 17).
Figure 17: Orthodontic Screw at the midline used for expansion
Distal movement:
Distal movement of molars could be achieved by a removable appliance carrying bilateral screws which will deliver force to the molars. Careful positioning of the screws will be necessary in accordance with the three dimensional orientations of the arch. A version with a screw only one side is also useful in a situation for example if one canine is buccally erupted(Figure 18).Positioning:
Although clasps and springs accept minor adjustment to correct faulty positioning, screws can only be corrected by cutting out the screw and re-cure. it is therefore most important to that the screw is accurately positioned in three dimensions during construction.Figure 18: Orthodontic Screw at one side of the arch used for distal movement.
Disadvantagesmost important is the bulky appliance that will cause discomfort to the patient.
over activation may cause difficulties in appliance incretion and will gradually lose its fitness
full cooperation of the patient is required since relapse is quick and may elongate treatment time.
Adjustments:
Activation of the screw is done by inserting the key wire into one of the activation holes and turn the screw to the direction that will open the until the key touches the guide screw ( the direction is usually marked by an arrow on the screw), this will make quarter turn. farther activation means to repeat the previous work. Activations are done by the patient once or twice a week, each quarter turn will provide a 0.2mm opining that means about 1-1.5 mm per month which is considered a reasonable amount of tooth movement.