In the area of tooth correction, there are many different methods for the correction of not only one's teeth, but also one's facial structure. The main method of tooth correction is orthodontics, the movement of teeth through pressure applied both from a wire attached to brackets on the teeth and also from the pressure applied on a tooth from the other teeth. There are a variety of reasons for getting the positioning of one's teeth corrected; for example, teeth which are not properly aligned (a condition referred to as malocclusion) can cause an individual problems due to poor ability to chew and poor dental hygiene, which could lead to infections and a compromised immune system.
   
    We will start out by first discussing the main form of orthodontic appliance: braces. There are numerous types of braces as well as numerous other devices which can be used along with braces in order to better correct a person's "bite" (the way in which their teeth meet when the jaw is closed). The following are some illustrations and explanations of various types of brackets and a couple of other orthodontic devices, as well as a brief explanation of the physics involved in each.
  
Braces

Dr. Peter Gold http://orthoforadults.com/toronto/tipsappliance.html#top

Pictured here is what we would call a regular bracket. It is the type which was most commonly used in the past, in which the wire (which spans between all teeth which we desire to be moved) fits into the grove in the center of the bracket. An elastic band is then twisted tightly about the bracket in order to hold the wire on. Regular brackets also come in clear and enamel-colored versions so that they are less noticeable if one so desires.

Dr. Peter Gold http://orthoforadults.com/toronto/tipsappliance.html#top

This is a new type of bracket that has begun to be used in the last few years and is called the "Damon System" type bracket. This type of bracket is generally preferred over the normal bracket for a few different reasons. This type has a built-in spring and low-friction system, thereby allowing for faster movement of teeth and therefore a shorter length of treatment for the patient. They are also smaller than normal brackets, making them both less noticeable and also more comfortable. It has also been shown that they are easier to clean than the normal brackets as they have no rubber band to trap in bacteria, therefore allowing for better oral hygiene during orthodontic treatment.

     The physics involved in a bracket-and-wire system is relatively simple: it is a matter of action and reaction forces. The brackets are firmly attached to the teeth which are out of alignment, and a wire is placed inside each bracket and firmly secured (with an elastic band in the case of normal braces or with the sliding door on the Damon brackets). Once secured to the unaligned teeth, the wire is no longer in is regular shape (these wires are generally "U" shaped), as an action force is applied on it by the bracket/tooth combination. The wire immediately begins to try to revert to its previous shape, thereby applying a "pulling", reaction force, on the teeth to which it is attached by the bracket. The wire not only applies a force on each individual tooth, but also causes the teeth to apply a force on one another as they all try to move into the shape of the wire.

Elastics & Palatal Expanders

Dr. Habib Zarifeh http://www.habibzarifeh.com/diagnois.htm	Elastics can be used for a variety of different purposes in the world of orthodontics, and are most commonly used in order to change the positioning of a particular tooth or the alignment of the top and bottom jaws. The elastics themselves are small rubber bands shaped in a loop. This loop can then be hooked on a tiny metal hook which is attached to the bracket and can span the top and bottom jaws, as seen in this photo to the right (you can also see that the brackets in this example are the normal ones with a colorful rubber band holding the wire in the bracket).
Courtesy of onedollardentist.com http://www.onedollardentist.com/bracesretainers.html	As described by their name, palatal expanders are used in the interest of expanding the palate in order to accomplish different goals. This device may be used in situations where the teeth are crowded in order to make the jaw wider so that there is more room for these teeth to come into alignment. It may also be used in situations where the top jaw is more narrow than the bottom, thereby correcting the bite so that the top and bottom teeth match up.
Photos courtesy of Advanced Dentistry of Spring http://www.whitersmiles.com/gallery03.php	I have provided here an example to illustrate what braces can accomplish. You can also see here, in these photographs, how the teeth eventually moved into the shape of the wire due to the pressure that was applied to them throughout treatment.

    The physics involved in the elastics is relatively similar to that of the braces themselves: it is mainly action-reaction forces which we observe here. In the illustration above, we can see that the elastic band is attached to the top and bottom brackets by little metal hooks on the brackets themselves. As we can see by the illustration, the metal hooks of the brackets are applying a force on the band, acting to stretch it. At the same time, the band is applying an equal force in the opposite direction from both metal hooks (the force applied to the upper hook is pointing along the band to the lower-right of the picture, while the force applied to the lower hook is pointing to the upper-left in the picture). Ultimately, the constant force applied by the band will alter the positions of teeth and also of the jaws. It is also important to note that, if the jaws are not aligned laterally (if, say, one is off to the side compared to the other), elastic bands can be used to apply a lateral force as well as a vertical one in order to somewhat correct the lateral positioning of the jaws.

    The final orthodontic device which I would like to discuss in relation to physics is called a palatal expander. This device works by applying a force to the teeth on either side of the jaw, causing those teeth (as well as the jaw bone in which the teeth are situated) to apply an equal but opposite force on the device, thereby causing the teeth to move outwards and the palate to widen. In the illustration, the force applied by the expander is represented by the blue arrows. This device is generally used as an alternative to tooth extraction in order to make room if teeth are crowded or in order to better align the top and bottom teeth if adjustments with braces are unable to accomplish this.