Human oral function: a review

Chewing is the first step in the process of digestion and is meant to prepare the food for swallowing and further processing in the digestive system. During chewing, the food bolus or food particles are reduced in size, saliva is produced to moisten the food and flavors are released. Taste and texture of the food are perceived and have their influence on the chewing process. There are several factors determining the chewing result. The teeth are important in the masticatory system. They form the occlusal area where the food particles are fragmented. This fragmentation depends on the total occlusal area and thus on the number of teeth. Another important factor in mastication is the bite force. The bite force depends on muscle volume, jaw muscle activity, and the coordination between the various chewing muscles. Also the movement of the jaw, and thus the neuromuscular control of chewing, plays an important role in the fragmentation of the food. Another aspect of chewing is how well the tongue and cheeks manipulate the food particles between the teeth. Finally, the production of sufficient saliva is indispensable for good chewing. Large differences in oral function exist among various groups of subjects, such as dentate subjects, partial and complete denture wearers, and subjects with implant-retained overdentures. Both maximum bite force and masticatory performance are significantly reduced, when dentures replace natural teeth. Oral function also is impaired in orthognathic patients, both before and after surgery, and in patients with a neuromuscular disease, myasthenia gravis. The muscle activity, needed to overcome the resistance of the food during mastication, consists of two contributions: an anticipating and a sensory-induced component. The anticipating muscle activity is generated only if food resistance is expected. The sensory-induced muscle activity immediately starts after food contact. About 85% of the muscle activity needed to crush the food is sensory induced, which indicates that jaw muscle activity is mainly of sensory origin.