|SECTION IV||FUNCTIONAL MOVEMENTS WHEN EATING: CHEWING AND SWALLOWING|
Functional movements of the jaws are the normal movements of the mandible during speech, eating (chewing), yawning, and swallowing. Tooth contacts during normal function (called functional occlusion) normally only occur during eating (chewing) and swallowing. You may think that your teeth need to touch when speaking, but go through the alphabet to see if teeth need to touch for any sound. You will find that, for most persons, teeth do not actually touch, although they must come close during sounds like “sss.” since if the incisors are moved very far apart, the result is more of an “sh” or whistling sound.
Eating involves the intake of food by placing it in the mouth, incising (bringing incisors together) to bite off a manageable size piece of food, chewing (also called mastication [mas ti KA shun]), and swallowing (also called deglutition [deg loo TISH un]). The following descriptions of incising, chewing, and swallowing apply to persons with ideal class I occlusion eating a piece of chicken.
Incising is the articulation of the anterior teeth performed to cut food into chewable pieces. Eating begins as the mandible drops downward to open the mouth, and the mandible is protruded as food is placed between the opposing anterior teeth. The mandible then closes in this protrusive position until the incisal edges of the anterior teeth meet the food. The mandible is then moved up and posteriorly with the mandibular incisors against lingual surfaces of the maxillary incisors, thus cutting off a small portion of the food.
Next, the tongue transfers food to the posterior teeth. It is held in position on the teeth of the working side by the cheek muscles and the action of the tongue. The teeth are brought together, engaging the food with the mandible in a slightly lateral position toward the side where food is located (working side). The upper buccal cusps are directly over the lower buccal cusps with the mandible in this lateral position. The closing motion slows as the mandible is forcibly closed31 while the canine overlap and inclines of posterior tooth cusps guide the mandible into maximal intercuspation of the posterior teeth for chewing. Tooth cusp slopes and triangular ridges act as cutting blades, whereas major and supplemental grooves serve as escape pathways (spillways or sluiceways) for crushed food to squeeze out through the buccal and lingual embrasures and over the tooth curvatures toward the cheek and onto the tongue. There, it can be tasted, mixed with saliva, placed back over the teeth, and chewed some more. This process significantly reduces lateral forces applied to the teeth that could be damaging to the teeth and their supporting bone. After the posterior teeth contact in MIP, there is a slight pauseQ before the mandible opens and moves laterally to commence the next chewing cycle. We usually chew like this on one side for several cycles and then switch the food over to the opposite side where a similar chewing cycle occurs. This process is called mastication.
Look in a mirror and move your jaw as far as possible in all directions (wide open and from right to left) to discover exactly how wide and long your total range of motion is. Then, chew some sugarless gum and notice that you use perhaps only half of this overall range of motion. Observe the pattern of movement of your mandible from the facial view during chewing to see if you move your mandible in a tear drop or circle shape similar to the pattern of chewing. Your side or sagittal view could also be viewed using a second mirror placed at 45°.