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  Screws Screw appliances consist of two parts connected    by a screw which has left- and right handed threads at opposite ends, the centre of the screw being formed into a boss or  plain section which is perforated so that the screw can be turned by means of a pin or wrench. In this way the two parts of the appliance are separated and when placed in the dental arch produce a pressure on the teeth which is accommodated by the slight natural mobility that occurs in teeth (  Fig. 1.3). In this way teeth are immediately moved or wedged apart by very small amounts and in the days subsequent to the  placing of the appliance, remodelling of the surrounding bone tissues takes place. Subsequent further screw adjustments are made to build up a total degree of movement such as may be required by the treatment plan.   Screws The doyen of screw devices was the Badcock screw which was a screw threaded into a tubular housing. The screw at its free end had a short collar which revolved on the head of the screw when rotated by means of a key or wrench, the two parts of the appliance in which the screw was embedded separated and  pressed against the teeth to which the appliance was clasped. e  parts of the appliance are separated, the only way in which the appHance can then be placed on the dental arch is by pressing  the teeth slightly apart due to the fact that the periodontal support allows this to happen. The movement of the teeth that takes place is due first of all to the fact that the periodontal membrane slings the tooth in the alveolar process and the membrane is compressed on one side and placed under tension on the other side. There is probably also a slight amount of flexibility in the cementum and the adjoining alveolar bone. The effect is that these elastic tissues store the pressure produced by the slight movement of the teeth. Teeth moved in this way must be allowed a period during which  bone remodelling can take place and, as bone remodels, slight additional movements are carried out by adjustment of the plate until the desired complete amount of movement has been achieved. Screws are sometimes used as a means of expanding the entire dental arch. It is, however, possible by means of suitable design of the baseplate to move quite small groups of teeth when required. This development has been due mainly to the manufacture of smaller sizes of screws. A further refinement is that modern screws are double-ended with left- and right-hand threads at opposite ends so that, as the screw rotates, the nuts or collars into which the threaded ends are placed move apart in equal and opposite directions. Today there are many types of screws being manufactured and these offer various possibilities for the reciprocal movement of segments of baseplate appliances {Fig. 2.6). Some screws have coil springs
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