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In vivo Cross-sectional Area of Human Jaw Muscles Varies with Section Location and Jaw Position

¹ Department of Oral and Maxillofacial Radiology, Faculty of Dental Science, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan;
² Asahi Kasei Information Systems Co., Ltd., Tokyo, Japan;
³ Department of Radiology, Kyushu University Hospital, Fukuoka, Japan; and
⁴ Department of Functional Anatomy, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit Amsterdam, The Netherlands;

View larger version (88K): [in a new window]   Figure 1. Determination of the cross-sectional areas of the jaw muscles. (A) The determination of cross-sectional areas of the masseter and medial pterygoid muscles. (1) Frontal muscle angles (blue lines) are estimated with the use of coronal scans which cross the hypophysis (yellow line), as indicated by the top mid-sagittal image. (2) Reconstructed ‘lateral view’ images parallel to the frontal angles, and perpendicular to the coronal plane. The muscle’s long axis (green dotted line) was defined as the midline between the anteriormost and posteriormost margins of each muscle on these reconstructed images. The cross-sectional areas (red lines) were determined at 1-mm intervals perpendicular to this muscle’s long axis. (B) The determination of cross-sectional areas of the lateral pterygoid muscle. (1) The axial muscle angle (blue line) was estimated with the use of an axial scan, which showed the maximum area of the lateral pterygoid muscle. (2) Reconstructed images parallel to the axial angle and perpendicular to the axial plane. The muscle’s long axis (green dotted line) was defined as the midline between the superiormost and inferiormost margins of muscle. The cross-sectional areas (red lines) were determined perpendicular to the muscle’s long axis at 1-mm intervals. R, right side; L, left side. MS, masseter muscle; MP, medial pterygoid muscle; LP, lateral pterygoid muscle.

View larger version (37K): [in a new window]   Figure 2. The normalized graphs (for both maximum CSA and muscle length) of the cross-sectional areas in relation to muscle length. The data-points from the left sides of 10 subjects are shown in each graph. Cross-sectional areas at the closed and open jaw positions are shown, respectively, in the left- and right-side panels. The dispersion of data-points indicates the variation in muscle morphology among subjects. The crossbar shows the position at which the maximum CSA (mean) was located. *Significant difference between the closed and open jaw positions (P < 0.001).

View larger version (31K): [in a new window]   Figure 3. The non-normalized cross-sectional area (mean, n = 10) of masseter, medial, and lateral pterygoid muscles in relation to their total length and for the two examined jaw positions. Masseter, masseter muscle; Medial pterygoid, medial pterygoid muscle; Lateral pterygoid, lateral pterygoid muscle.