Most functional movement of the mandible (as occurs during mastication and speech) takes place inside the physiologic limits established by the teeth, the temperomandibular joints, and the muscles and ligaments of mastication; therefore, these movements are rarely coincident with border movements.
Chewing. When incising food, adults open their mouth a comfortable distance and move the mandible forward until they incise, with the anterior teeth meeting approximately edge to edge. The food bolus is then transported to the center of the mouth as the mandible returns to its starting position, with the incisal edges of the mandibular anterior teeth tracking along the lingual concavities of the maxillary anterior teeth (Fig. 4-16). The mouth then opens slightly, the tongue pushes the food onto the occlusal table, and after moving sideways, the mandible closes into the food until the guiding teeth (typically the canines) contact.' The cycle is completed as the mandible returns to its starting position.9 This pattern repeats itself until the food bolus has been reduced to particles that are small enough to be swallowed, at which point the process can start over. The direction of the mandibular path of closure is influenced by the inclination of the occlusal plane with the teeth apart and by the occlusal guidance as the jaw approaches intercuspal position.'°
The chewing pattern observed in children differs from that found in adults. Until about age 10, children begin the chewing stroke with a lateral movement. After the age of 10, they start to chew increasingly like adults, with a more vertical stroke" (Fig. 4-17). Stimuli from the pressoreceptors play an important role in the development of functional chewing cycles. 12
Mastication is a learned process. At birth no oc-clusal plane exists, and only after the first teeth have erupted far enough to contact each other is a message sent from the receptors to the cerebral cortex, which controls the stimuli to the masticatory musculature. Stimuli from the tongue and cheeks, and perhaps from the musculature itself and from the periodontium, may influence this feedback pattern.
Speaking. The teeth, tongue, lips, floor of the mouth, and soft palate form the resonance chamber that affects pronunciation. During speech, the teeth are generally not in contact, although the anterior teeth may come very close together during "C," "CH," "S," and "Z" sounds, forming the "speaking space."" When pronouncing the fricative "F," the inner vermilion border of the lower lip traps air against the incisal edges of the maxillary incisors. Phonetics is a useful diagnostic guide for correcting vertical dimension and tooth position during fixed and removable prosthodontic treatment.14
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