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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; Correspondence: * corresponding author, goto{at}rad.dent.kyushu-u.ac.jp
Muscle cross-sectional area (CSA) is used as a measure for maximum muscle force. This CSA is commonly determined at one location within the muscle and for one jaw position. The purpose of this study was to establish a method to standardize the analysis of the CSA of the masticatory muscles in vivo, and to compare the CSAs along their entire length for two different jaw positions (opened and closed). The CSAs in the planes perpendicular to the long axes of the masseter, medial, and lateral pterygoid muscles were measured in ten normal young adult subjects by magnetic resonance imaging. Our results showed large differences among the muscles and a non-uniform change in CSA after jaw-opening. The method enables the CSA measurement to be standardized in vivo, and allows for a correct comparison of CSAs in different skull morphologies.
Key Words: human masticatory muscles • muscle cross-sectional area • magnetic resonance imaging • jaw-opening
The cross-sectional area (CSA) of the masticatory muscles is considered proportional to maximum muscle force (Weijs and Hillen, 1985a,b; van Eijden et al., 1997). Detailed data on the interindividual variation of muscle morphology, such as its length and physiological CSA and force indices, have been reported for the jaw-closed position in cadavers of older individuals (van Eijden et al., 1997), in whom it is known that the CSAs of the masticatory muscles are reduced with age (Newton et al., 1987, 1993). Investigations of the CSA in living human subjects can be performed by computed tomography (Weijs and Hillen, 1984b), ultrasonography (Emshoff et al., 1999; Bertram et al., 2003), or magnetic resonance imaging (MRI) (Hannam and Wood, 1989; Sasaki et al., 1989; van Spronsen et al., 1989). Although Weijs and Hillen (1984a) and van Spronsen et al.(1989) determined the CSA at several locations along the muscle’s length, the CSA is commonly determined at only one location, i.e., at the visually determined bulge of the muscle (more or less perpendicular to the muscle’s long axis), and for only one jaw position. Several jaw-closing muscles are known for their complex multi-pennate architecture, while the jaw-openers are much simpler in architecture (van Eijden et al., 1997). So far, it is largely unknown to what extent the CSA varies with the section location and jaw position and orientation. Because of the various muscle orientations, jaw-opening will have a differential impact on the CSA among muscles and, due to the complex internal architecture, possibly also on the CSA changes within muscles. The purposes of this study were to: (1) establish a method to standardize the analysis of the CSA of each masticatory muscle in vivo, (2) determine the CSAs along the entire length of masticatory muscles (masseter, medial, and lateral pterygoid muscles), and (3) examine the changes in these CSAs after a complete opening motion. We hypothesized that, because of the complex internal architecture of masseter and medial pterygoid muscles, the changes in CSA after jaw-opening would not occur uniformly from origin to insertion. In contrast, the change in CSA would be uniform in the less complex lateral pterygoid muscle.
Data Acquisition The subjects were ten normal Japanese adults, five females and five males, from 20 to 30 yrs of age. All subjects had normal skull shape, normal occlusion, well-arranged dentitions, and no signs of craniomandibular dysfunction. All experimental procedures were approved by the human experimentation committee of Kyushu University. All subjects gave informed consent. In each individual, the masseter, medial, and lateral pterygoid muscles were evaluated by MRI (1.5 Tesla; Magnetom Vision, Siemens AG, Erlangen, Germany; three-dimensional rapid gradient-echo sequence), allowing for the acquisition of a large quantity of three-dimensional high-quality data (voxel size: 0.9 x 0.9 x 1.25 mm). For the first collection of images, the subjects held their teeth loosely together in the intercuspal position. Next, a custom-made acrylic plastic block comfortably kept the jaw at the maximum open position. The MR data were converted to 168 continuous coronal slices at 1.25-mm intervals. After the pixel size on coronal images was interpolated from 0.9 x 0.9 mm to 0.45 x 0.45 mm, the contours of the muscles were traced on the coronal images with the use of image-processing software (NIH image 1.62, NIH, US Government, Bethesda, MD, USA). Each series of MR images was then reconstructed (Dr. View/LINUX, AJS Co., Ltd., Tokyo, Japan), resulting in three-dimensional data of the entire subject’s head, allowing for the visualization of high-quality images in any desired plane.
CSA Determination
For the lateral pterygoid muscle, we used the axial scan to determine the muscle’s axial angle (Fig. 1B-1  ). We then used reconstructed images parallel to this angle and perpendicular to the axial plane to estimate the muscle’s long axis. CSAs were estimated perpendicular to this muscle’s long axis (Fig. 1B-2). The CSA estimation of all muscles for the jaw-open position followed the same procedure.
Data Analysis Differences in relative position and the magnitude of the maximum CSA between the closed and open jaw positions were tested for their significance by the paired t test. The p < 0.05 was considered to indicate a significant difference.
A comparison of the right and left muscles at the closed-mouth position did not reveal any statistically significant difference in CSAs. Therefore, to simplify the presentation of the results, we show here only the data for the left muscles.
The normalized individual CSA determinations along the entire muscle’s length among all subjects are presented (Fig. 2
After jaw-opening, the relative position of maximum CSA of only the lateral pterygoid muscle changed significantly (from 0.42 to 0.54, p < 0.001). The interindividual variation in the position of the maximum CSA was comparable with that of the jaw-closed position (for masseter, medial, and lateral pterygoid muscles, 26.1%, 7.1%, and 10.4%, respectively).
The mean non-normalized CSAs of all 3 muscles along the entire muscle length for both jaw positions are shown (Fig. 3
After jaw-opening, the masseter muscle’s CSAs were decreased inconsistently, mainly in the plateau portion, and its length increased, on average, by 6.1 mm (from 79.2 to 85.3 mm). Note that the calculation of the average CSAs (Fig. 3 ) generates different muscle lengths (92 and 96 mm) that point to the length measurements in the individual with the longest masseter muscle. The medial pterygoid muscle showed a decrease of CSA, mainly at its maximum CSA, and increased its length from 64.0 to 66.4 mm. The lateral pterygoid muscle increased its CSA after jaw-opening, and shortened from 41.9 to 34.0 mm.
The maximum CSA at the jaw-closed position of the masseter muscle was the largest (5.2 cm2), followed by the lateral (3.8 cm2) and medial (3.0 cm2) pterygoid muscles (Table
Using MRI, we developed a method to estimate the CSA along the entire length of the muscles. This method ensures the standardized determination of the maximum CSA, taking into account individual differences in muscle orientation. It enables these CSAs to be compared between and among various muscles and individuals, and allows for the influence of jaw position to be examined. To our knowledge, this is the first study quantifying the CSAs of masticatory muscles, taking into account the orientation of individual muscles and estimating the changes in the CSA after jaw-opening in living humans. Although some interindividual variation is present, the general shape of these muscles, expressed by the subsequent estimated CSAs, was remarkably constant. In the jaw-closed position, the maximum CSAs of masseter muscles reported in the present study were comparable with those reported in other studies (Weijs and Hillen, 1985a; Newton et al., 1987, 1993; Gionhaku and Lowe, 1989; Hannam and Wood, 1989; Sasaki et al., 1989; van Spronsen et al., 1989; Xu et al., 1994; Gan et al., 2000). The medial and lateral pterygoid muscles showed CSAs of 3.0 and 3.8 cm2, which are smaller than the previously reported ranges of 3.1 to 3.8 and 4.2 to 4.7 cm2, respectively. A possible explanation is that we analyzed the CSAs of sections perpendicular to the long axes of the muscles, while, in most other studies, the sections were taken parallel to anatomical planes (e.g., Frankfort horizontal, coronal plane, etc.). This might lead to an overestimation, especially for the medial and lateral pterygoid muscles, which have large medio-lateral components in their orientations. In a previous publication, we presented the CSA parallel or vertical to the Frankfort horizontal plane from the same subjects (Goto et al., 2002). These CSAs were indeed larger than those of the current study. Additional reasons for this discrepancy may be related to race, sex, and face morphology. However, previous studies of the CSA of jaw muscles do not reveal a consistent difference between populations. From this, it can be suggested that variations due to population differences are of a lesser magnitude than those due to methodological differences. It is difficult to clarify possible population differences without using standardized methods. The maximum CSAs of masseter, medial, and lateral pterygoid muscles in the current study were closer to the muscle’s insertion than found in the study of Weijs and Hillen (1984a). Moreover, in the study of Weijs and Hillen, the medial pterygoid muscle showed a nearly constant CSA for a large portion of the muscle, while the muscle was fusiform in our study. Comparison with the results of van Spronsen et al.(1989) suggests a difference in the form and length of both the masseter and medial pterygoid muscles. This might be related to the inter-slice interval of 6 mm in that study, making the determination of muscle length difficult. Our results show that there is variation in muscle form among subjects, among muscles, and within the muscles. In addition, the results indicate that the position at which the maximum CSA is determined is crucial in the medial and lateral pterygoid muscles, whereas, in the masseter muscle, this is inconsequential, due to its broad muscle portion, showing near-maximum CSAs. Nevertheless, the determination of the CSA at only one location of the muscle may not reveal the true maximum CSA. Moreover, in previous studies, scans were simultaneously made for both sides of the masticatory system, and their orientations were according to the estimation within one individual, which was further copied to all examined subjects. The variations in the previously published CSAs reveal that this parameter can differ substantially, depending on the methods and subjects, indicating that a standardized method, taking into account individual differences, as used in the present study, might improve the CSA estimation. Koolstra et al.(1989) published a method that estimated the line of action of muscles as a regression line through the centroids of mathematical slices of muscles. Our method is comparable with that method, since we used a series of midlines to determine the muscle’s action line (long axis).
After jaw-opening, the variation of the normalized muscle contours tends to become larger for the masseter and medial pterygoid muscles at the origin (Fig. 2
The relative length at which the maximum CSA was found did not change during jaw-opening for the masseter and medial pterygoid muscles. The lateral pterygoid muscle, however, significantly changed its maximum CSA position from 0.42 to 0.54 during opening (Fig. 2 In conclusion, our study showed that: (1) the natural CSAs of masticatory muscles changed markedly along their entire length; and that (2) during jaw-opening, the CSAs did not always change uniformly from origin to insertion. As such, these results are valuable for various clinical issues. The method enables us to determine the CSA accurately in a standardized way, taking into account individual differences in muscle orientation, and thus facilitates the comparison of CSAs in children, normal individuals, and patients with jaw and muscle deformities. The results of the present study are also highly relevant to biomechanical studies of the craniofacial region.
We are grateful to Ms. Satoko Nishida for her assistance in the data analysis and to Prof. Theo van Eijden for critically reading the manuscript. We also thank Prof. Brian Quinn, who is expert in the English language, for reviewing the manuscript. This study was supported by a Grant in Aid of Scientific Research from the Ministry of Education, Japan (Nos. 10771033, 12771123, 14571792). A preliminary report of this study was presented at the 81st General Session & Exhibition of the International Association for Dental Research, 2003, held in Göteborg, Sweden. Received for publication July 15, 2004. Revision received March 30, 2005. Accepted for publication March 31, 2005.
Journal of Dental Research, Vol. 84, No. 6,
570-575 (2005) This article has been cited by other articles:
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