[The effect of the hyper-erupted third molar on chewing movement]

To investigate the effect of the hyper-erupted third molar contacting closely to the distal surface of the opposing second molar on the chewing movement.

Twenty-three university student volunteers, 11 with hyper-erupted third molar teeth and 12 with normal occlusion, were enrolled in present study. There were totally 6 unilateral hyper-erupted third molars (3 upper and 3 lower) and 5 bilateral hyper-erupted third molars (1 bilateral upper and 4 bilateral lower). No other significantly abnormal occlusal phenomenon was observed. Five gum-chewing cycles masticated with an each side were separately recorded by means of K6-I mandibular kinesiograph (MKG) and the chewing cycles were analyzed. Both three-dimensional amplitudes and the related velocities were automatically documented with the instrument.

(1) The chewing graphs recorded on both frontal and horizontal planes in controls were typically smooth cycles with clearly occlusal phase tracings. On the sagital plane the opening and closing traces around intercuspal position (ICP) were usually overlapped with each other. (2) 63.6% frontal cycles of the subjects with hyper-erupted third molar had an obvious table (0.2 mm to 1.0 mm in width) around ICP, 50% horizontal cycles of the subjects had a 0.2 mm to 0.5 mm step near to ICP, 72.7% opening and closing traces on the sagital plane separated into anterior and posterior branches. The abnormal traces on the three different planes of the hyper-erupted third molar tooth group appeared to be related each other, implying that they might be caused by the same characteristics of occlusion, namely, the hyper-erupted third molar. (3) Subjects with hyper-erupted third molar masticated in more posterior direction, together with a higher chewing terminal velocity.

The hyper-erupted third molar had a significant effect on the chewing cycles around ICP, as well as on the amplitude and velocity of chewing movement.