Background: Lip closing functions have never been evaluated from the viewpoint of elastic properties.
Objectives: The purpose of the present study was to investigate the behavior of the lower orbicularis oris muscle during a button-pull exercise to measure lip closing force and quantitatively evaluate its elastic properties using sonographic elastography.
Methods: Appropriate compression loads for elastography were randomly measured on one of three types of acoustic couplers on three examiners. Compression tests were performed on three types of acoustic couplers within the appropriate compression load. Using the acoustic coupler with the lowest elastic modulus, the strain ratio of the lower orbicularis oris muscle during lip closing was measured, and elastography was performed on six males under tension loads of 0-8 N.
Results: The intraclass correlation coefficient (1, 3) for the tension load of 0 N was 0.81. Elastography showed that the strain ratio values increased significantly (p < 0.05) as the tension load increased.
Conclusions: Combining the data obtained from lip closing test devices and sonographic elastography enabled the muscle performance to be evaluated objectively and accurately.
Keywords: Lip closing function; elasticity; lower orbicularis oris muscle; sonographic elastography; strain ratio; ultrasound.