Characteristic of Multidirectional Lip-closing Force Measured by a New Apparatus

Objectives: Lip closure is essential for such things as mastication, swallowing and articulation. Moreover, force closing the lip may be a potent factor for positioning the tooth and for inducing malocclusion. However, this force has not been evaluated in detail. Since the lip is closing in a multidirectional way, the directional distribution of the lip closing force must be assessed. The aim of this study is to develop a new apparatus for measuring the multidirectional lip-closing force. Methods: The apparatus consisted of a probe, pressure transducers and electric amplifiers. The probe is made of 8 silicon tubes(diam. 6mm)arranged either in a round shape or in an oval shape. The circumference of each probe was the same. Change of the internal pressure in each tube was detected by a pressure transducer. Adult healthy subjects with a natural dentition were instructed to hold the probe lightly with their lips, and then to close the lips with the maximum effort. The forces and electromyograms (EMG) of the unilateral upper and lower lip muscles were recorded simultaneously. Results: We measured the lip-closing force in 8 directions (upper, lower, right, left and the 4 directions in between). For the round-shape probe, the force in the vertical direction was large, whereas the force in the transverse direction was small. The force in the vertical direction was significantly smaller and the force in the transverse direction was significantly larger with the oval-shape probe than with the round-shape probe (p<0.05), although the total force and EMG activities of lip muscles were not significantly different. Conclusion: These results suggest that the shape of the probe does not influence the total lip-closing force, but the directional distribution of the force. Acknowledgment: This study was carried out in collaboration with “Proceed Inc.” which is supported by Shiojiri City.