Lecture 11

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   Orthodontic springs on removable appliances today are almost universally made of 18/8 stainless steel or nickel/chrome wire. Before the introduction of stainless steel wires for orthodontic purposes, platinized gold wires were used but this material had rather a small elastic limit so that the range of action of the springs made from it was fairly short. Stainless steel is more elastic and this makes possible the construction of springs with an adequately long range of action. The degree of flexibility and the amount of pressure exerted are determined by the length and thickness of wire that is used, whether part of the length is formed into coils or not. The finer gauges of wire appear more flexible and the thicker gauges less flexible. The length of a spring that can be used in an orthodontic appliance is restricted by the dimensions of the dental arch and the limits imposed by the size of the oral cavity. Geometry: Size and Shape Changes related to size and shape are independent of the material. In other words, decreasing the diameter of a steel wire by 50%would reduce its strength to a specific percentage of what it had been previously. When a round wire is used as a finger spring, doubling the diameter of the wire increases its strength eight times (i.e., the larger wire can resist eight times as much force before  permanently deforming or can deliver eight times as much force). Doubling the diameter, however, decreases springiness by a factor of 16 and decreases range by a factor of two. More generally, for a round cantilever wire, the strength of the wire changes as the third power of the ratio of the larger to the smaller wire; springiness changes as the fourth power of the ratio of the smaller to the larger; and range changes directly as the ratio of the smaller to the larger (Figure 9-12). FIGURE 9-12 Changing the diameter (d) of a wire, no matter how it is supported, greatly affects its properties. As the figures below the drawing indicate, doubling the diameter of a cantilever wire makes it 8 times as strong, but it is then only as springy and has half the range. More generally, when wires of any type made from two sizes of wire are compared, strength changes as a cubic function of the ratio  of the two cross-sections; springiness changes as the fourth power of the ratios; range changes as a direct proportion (but the precise ratios are different from those for cantilever wires). Geometry: Length and Attachment Changing the length of a wire, whatever its size or the material from which it is made, also dramatically affects its properties (Figure 9-13). If the length of a cantilever wire is doubled, its  bending strength is cut in half, but its springiness increases eight times and its range four times. More generally, when the length of a cantilever wire increases, its strength decreases  proportionately, while its springiness increases as the cubic function of the ratio of the length and its range increases as the square of the ratio of the length. Changing from a cantilever to a supported wire, though it complicates the mathematics, does not affect the big picture: as wire length increases, there are proportional decreases in strength  but exponential increases in springiness and range. FIGURE 9-13 Changing either the length of a wire or the way in which it is attached dramatically affects its properties. Doubling the length of a cantilever wire cuts its strength in half but makes it 8 times as springy and gives it 4 times the range. More generally, strength varies inversely with length, whereas springiness varies as a cubic function of the length ratios and range as a second power function. Supporting a wire on both ends makes it much stronger but also much less springy than supporting it on only one end. Note that if a wire is rigidly attached on both ends, it is twice as strong but only one-fourth as springy as a wire of the same material and length that can slide over the abutments. For this reason, the elastic properties of an orthodontic archwire are affected by whether it is tied tightly or held loosely in a bracket. A removable appliance incorporating a cantilever spring for initial tipping of a maxillary canine toward a premolar extraction site. Note that a helix has been bent into the base of the cantilever spring, effectively increasing its length to obtain more desirable mechanical properties.    As a guide to the amount of pressure that springs made from round stainless steel wire exert. Table 2.1 lists the results of measurement of the deflection produced by a 20-g pressure on a number of springs that are routinely used in removable orthodontic appliances. Fig. 2.2 shows the use of gauges to measure spring pressure. fig 2-2  Fig. 2.2. The use of pressure gauges.  A, A plunger type of gauge calibrated in ounces. At the opposite end there is a hook which makes it possible to measure elastic tensions.  B, Measuring the pressure of a cantilever spring. C, The Correx pressure gauge calibrated in grammes.  D, Measuring the pressure of an apron spring.  The design and placement of a spring determine the direction in which it will apply its force. Three principles are important with all types of spring: 1. The force should be delivered at right angles to the long axis of the tooth. When this principle is met all the force applied to the tooth is used to achieve movement. When it is not met a vertical component of force is produced which will tend to displace the spring. Examples of such displacement may be seen during retraction of a canine with a poorly  positioned buccal spring or when a labial bow is activated palatally on proclined incisors. 2. as far as possible the force should be applied through a surface which is parallel to the long axis of the tooth. Failure to comply with this will not only cause displacement of the spring but can sometimes  produce unwanted intrusion of the tooth. The displacement may be corrected by altering the direction of spring activation so that it is more nearly at right angles to the surface to which it is applied, but this will increase the tendency to intrude the tooth and will produce an increased displacing force on the appliance. An example of this may be seen when an attempt is made to retract a partially erupted canine by activating the spring on the sloping mesial surface of the cusp. It also occurs when an incisor is proclined by spring activation on the sloping surface of the cingulum.
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