Lips and cheeks play an important role in masticatory functions. The purpose of this study was to analyze the lip movements of the dentulous subjects with the mandibular movements during mastication using 3-D Auto Tracking System. The subjects were 28 males and 22 females, and 8 types of test-foods (chewing gum, caramel, gummy-jelly, peanuts, cookie, carrot, two sizes of boiled fish paste) were used. The measuring point of the lips was placed on the mouth corner and that of mandibular movements was placed on the surface of the chin.
The length of the trace on each chewing cycle was evaluated as the index of the lip's activity. One sequence of mastication was divided into three (first, middle and last) stages. Time dependent changes of the lip's activity were evaluated statistically.
The results were as follows:
1. During tapping, the mouth corner moved to-and-fro between the rest position and the maximum opening position.
2. On the chewing side, the mouth corner was tear drop shaped, and on the non-chewing side, shuttled simply.
3. The lip's activity during mastication depended on the physical property of the test food.
4. As the mastication proceeded, the ratio of the lip's activity to the mandibular movements increased in all test foods except for gum.