Relation between Upper and Lower Molars during Functional Mandibular Movement

Objectives: Most researches about the mastication were investigated in the later stage prior to deglutition, for the mandibular movements of this stage were stable compared with those of early stage. However, in the early stage of the mastication, much masticatory force to comminute foods will be required and this stage plays a very important role in the mastication process. This study investigated the relation between upper and lower molars in the main occluding area during the first stroke of mastication. Methods: The subjects were five adults with the normal dentition. Study models were fabricated and occlusal surface of them were measured with the three-dimensional shape measurement system in each subject prior to the experiment. Five trial of detecting main occluding area were performed with 4.0mm length temporary filling materials in each subject. During these trials, six-degree-of-freedom mandibular movements were measured simultaneously.In the molar region, the contact area at the intercuspal position within main occluding area and corresponding "A" and "B" contact area were selected. The reference points UA and UB were selected for the surface of upper casts, and LA and LB belong to the surface of lower casts. The occlusal surface of the cast model were manipulated on the personal computer according to the data of mandibular movement measurement and the distance between reference point UA and LA, UB and LB were calculated (|A|,|B|). In every 0.5 mm, from the intercuspal position to the 3.0 mm away from this position (at the edge of lower left incisor) during biting, the ratio of |A|/|B| were calculated. Results: The mean values of each subject showed 1.003 ±0.007. Conclusions: These results reflected that the occlusal contacts of upper and lower molars come into contact approximately vertically each other during comminuting foods in the first stroke of mastication.