An ultrasound investigation of the lip levator musculature.

The aims of this study were to determine if there were any differences in the thickness of the lip levator musculature in men and women, and whether the height of the smile line in adults was related to the thickness of the lip levator musculature. Thirty Caucasian (13 males, 17 females), and 24 Asian (11 males, 13 females) undergraduate dental students participated in this study. The subjects were placed in high, medium, and low smile-line groups, according to the height of their upper lip while smiling. The thickness of the levator labii superioris and zygomaticus major muscles was measured on ultrasound scans of the relaxed muscles. Gender, right-left side, and ethnic differences in muscle thickness were determined, and the thicknesses of the lip levator musculature in subjects in the high, medium, and low smile-line groups were compared. There were significantly more women than men with high smile-lines and the zygomaticus major muscle was significantly thicker in the women, as compared with the men. There were, however, no statistically significant differences in muscle thickness in the three smile-line groups. While it appears that women have higher smile-lines and significantly thicker zygomaticus major muscles than men, the height of the smile line is not due to the thickness of either the levator labii superioris or zygomaticus major muscles.