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Human Masticatory Muscle Volume and Zygomatico-mandibular Form in Adults with Mandibular Prognathism

¹ Department of Orthodontics and Dentofacial Orthopedics, Graduate School of Dentistry, Osaka University, 1-8 Yamadaoka, Suita, Osaka, 565-0871, Japan;
² Department of Oral and Maxillofacial Radiology, Graduate School of Dentistry, Osaka University;
³ Department of Pediatric Dentistry and Clinical Genetics and 3D-Laboratory, School of Dentistry, Faculty of Health Sciences, University of Copenhagen, Denmark; and
⁴ The Copenhagen Craniofacial Unit (CCFU), Department of Clinical Genetics, Copenhagen University Hospital;

* corresponding author, ktakada{at}dent.osaka-u.ac.jp

	ABSTRACT

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Although several investigators have reported associations between masticatory muscles and skeletal craniofacial form, there is no agreement on the association. We tested the hypothesis that masticatory muscle volume correlates with the size and form of the adjacent local skeletal sites. For this purpose, we investigated the morphological association of the cross-sectional area and volume of temporal and masseter muscles with zygomatico-mandibular skeletal structures using computerized tomography (CT) in 25 male adults with mandibular prognathism. Muscle variables significantly correlated with widths of the bizygomatic arch and temporal fossa but not with the cranium width. Masseter volume significantly correlated with cross-sectional areas of the zygomatic arch and mandibular ramus. Masseter orientation was almost perpendicular to the zygomatic arch and mandibular antegonial region. The zygomatic arch angle significantly correlated with the antegonial angle. The results of the study suggest that the masticatory muscles exert influence on the adjacent local skeletal sites.

KEY WORDS: CT • mandibular prognathism • masticatory muscle • zygomatic arch

	INTRODUCTION

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Since the wide dissemination of the work of D’Arcy Thompson (1942), there has been a general consensus that biologic forms are influenced by function as well as by genetics. In craniofacial biology, this has involved the understanding that craniofacial forms are determined, least in part, by biomechanics (Enlow, 1968). Sassouni (1969), in his hypothetical two-dimensional model of the face and cranium based on dry skulls and lateral cephalograms, extrapolated that a flat mandible was associated with extensive development and vertical orientation of jaw-closing muscles, whereas a steep mandible correlated with underdevelopment and posterior positioning of jaw-closing muscles.

Several investigators have reported significant associations between masticatory muscle size and skeletal craniofacial form (Weijs and Hillen, 1984, 1986; Gionhaku and Lowe, 1989; Benington et al., 1999), while others have found very few correlations between the size of jaw-closing muscles and craniofacial morphology (Hannam and Wood, 1989; van Spronsen et al., 1991). Strictly speaking, even when the differences in measuring methods are taken into account, there has been no agreement on the association between craniofacial form and the size of related muscular structures.

To understand biomechanical relationships between hard- and soft-tissue structures, we believe that it is indispensable to examine the structural association between muscles and the adjacent local skeletal sites on which muscle forces are exerted, rather than examining correlations between the overall craniofacial skeletal structure and the size of masticatory muscles within a traditional cephalometric paradigm.

Mandibular prognathism, a gross skeletal deformity of the craniofacial area seen frequently among the Japanese population (Takada et al., 1993), often requires orthognathic surgery. Because the surgical approach necessitates invasion into the mandibular ramus and gonial region, an understanding of the association between the morphology of the skeletal and muscular components of craniomandibular structures is indispensable in achieving successful treatment and post-treatment stability. In addition, such an understanding will provide meaningful insight into the principles that account for relationships between form and function of the craniomandibular apparatus.

The purposes of the present study were to: (1) examine computerized tomography (CT) images of the craniofacial structure in adult patients with mandibular prognathism; (2) based on CT’s three-dimensionality, investigate whether the temporal and masseter muscle volumes correlate with transverse head dimensions; (3) investigate whether the masseter muscle volume correlates with the size of the zygomatico-mandibular skeletal sites, the form of the antegonial region, and orientation of the masseter muscle; and (4) investigate whether the masseter muscle orientation has influence on the inclination of the zygomatico-mandibular skeletal sites.

	SUBJECTS & METHODS

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Subjects
Twenty-five Japanese males (mean age, 23 yrs 4 mos; range, 16 yrs 4 mos-32 yrs 11 mos), seeking orthodontic treatment because of mandibular prognathism and malocclusion, participated in this study. Each was diagnosed cephalometrically as having a skeletal Class III jaw base relationship based on the ANB angle (mean = -3.1°; range = -10.5° to +0.0°). ANB angles were smaller than the Japanese normative mean minus one standard deviation (SD) for healthy adult males. Means and ranges of several cephalometric variables were as follows: SN (mean = 72.8 mm; range = 68.5 mm to 80.5 mm), SNA (mean = 81.2°; range = 76.0° to 88.0°), and Mp to FH (mean = 29.5°; range = 18.7° to 43.0°). The patients had good general and dental health, complete or nearly complete dentition, and no history of temporomandibular joint disorders. The subjects gave consent to participate in the study after receiving a full explanation of its aim and design. The current experiment was approved by the Ethical Committee of the Osaka University Graduate School of Dentistry.

Recording Method
CT images were recorded for each subject with the use of a helical-type CT scanner (GE, Milwaukee, WI, USA). Scanning planes were parallel to the occlusal plane, and the scanning ranged from vertex to menton. Slice thickness was 2.0 mm, with a slice gap of 0.5 mm. Field of view was 25 cm, and the number of matrices was 512, providing one pixel size of 0.49 mm.

Data Analyses
CT image data were transferred to a workstation (Advantage Workstation 3.1^TM, GE, Milwaukee, WI, USA) and a graphics computer (Silicon Graphics, Inc., Mountainview, CA, USA). From the CT dataset, craniofacial skeletal structures were segmented on the basis of a threshold CT value, which was determined as 160 Hounsfield Unit (HU). Settings for evaluation of the temporal and masseter muscles included a window width of 350 HU and a window level of 35 HU. 3-D reconstruction of skeletal structures was carried out, and several anatomic landmarks were determined visually on the surface of the 3D object with the software package, Analyze^TM (Biomedical Imaging Resource, Mayo Clinic and Foundation, Inc., Rochester, MN, USA) (Fig. 1A). Landmark positions were identified on the axial, coronal, and sagittal slices. Frankfurt horizontal plane (FHP) was defined as a plane through the po_r, the po_l, and the or_l. Mid-sagittal plane (MSP) was defined as a plane perpendicular to the FHP through the bs and the midpoint between the or_l and the or_r. Frontal plane (FP) was defined as a plane perpendicular to the FHP and the MSP through the bs.

View larger version (63K): [in this window] [in a new window]   Figure 1. 3-D reference points of the craniofacial structures and zygoma, cranium, and temporal fossa widths (A) Orbit (left, right) (orl, orr), the most inferior point of the orbit bone ridge; porion (left, right) (pol, por), the most latero-superior point of the external auditory meatus; basion (bs), the coronal midpoint on the anterior margin of the foramen magnum; zygoma anterior (zga), the most anterior point of the origin of the masseter muscle; zygoma posterior (zgp), the buccolingual midpoint of the deepest point on the antero-inferior region of the zygomatic arch; zygoma exterior (zge), the most exterior point of the upper margin of the zygomatic arch; cranium exterior (cre), the deepest point of the cranium on the temporal fossa; apex of the condylar head (cd), the most superior point of the condylar head; mandible anterior (mna), the buccolingual midpoint of the most anterior point of the insertion of the masseter muscle on the mandible; mandible posterior (mnp), the buccolingual midpoint of the most posterior point of the insertion of the masseter muscle on the mandible; gonion (gn), the buccolingual midpoint of the most prominent point of the mandibular angle; mandibular ramus notch (mnn), the deepest point of the mandibular notch; and mandibular ramus center (mnc), on axial slices the buccolingual midpoint of the given line through the open site of the mandibular foramen, which is perpendicular to the anteroposterior axis of the ramus. (B) Zygoma width (ZgW), the distance between bilateral zge points projected to the intersection between the FHP and the FP; cranium width (CrW), the distance between bilateral cre points projected to the intersection between the FHP and the FP; and temporal fossa width (TFW), the distance between the zge and the cre points projected to the intersection between the FHP and the FP.

View larger version (93K): [in this window] [in a new window]   Figure 2. Cross-sectional areas of skeletal sites and muscles, volumes of muscles and angular variables on the MSP. (A) Zygomatic arch cross-sectional area (ZgCA), cross-sectional area of the zygomatic arch intersected by the plane through the zgp point, which is parallel to the FP; mandibular ramus cross-sectional area (MnCA), cross-sectional area of the mandibular ramus intersected by the plane through the mnc point, which is perpendicular to the line through the mnn and the mnc; (B) Temporal muscle cross-sectional area (TCA), cross-sectional area of the temporal muscle intersected by the plane which is parallel to the FHP and 1 cm superior to the zge (Weijs and Hillen, 1984); masseter muscle cross-sectional area (MsCA), cross-sectional area of the masseter muscle intersected by the plane through the midpoint between the zga and the mna, which is perpendicular to the line through the zga and the mna; temporal muscle volume (TV), volume of the temporal muscle inferior to the plane which is parallel to the FHP and 1 cm superior to the zge; and masseter muscle volume (MsV), volume of the masseter muscle. (C) Gonial angle (GnA), angle between mna-gn and gn-mnp; masseter anterior orientation angle (MsA), angle between mna-zga and the FHP; masseter zygomatic angle (MsZgA), angle between zgp-zga and zga-mna; masseter antegonial angle (MsAgA), angle between zga-mna and mna-gn; (D) Zygomatic angle (ZgA), angle between pol-zga and the FHP; and antegonial angle (AgA), angle between cd-mna and the FHP.

Statistical Analyses
Pearson correlation coefficients were calculated among the temporal muscle cross-sectional area, masseter muscle cross-sectional area, temporal muscle volume, and masseter muscle volume; among the temporal muscle volume, masseter muscle volume, and the widths of the bizygomatic arch, the cranium, and the temporal fossa; among the zygomatic cross-sectional area, the mandibular ramus cross-sectional area, and masseter muscle volume; between masseter muscle volume and the gonial angle; between masseter muscle volume and masseter anterior orientation; and between the zygomatic angle and the antegonial angle. The p < 0.01 level of significance was chosen for all tests. Analyses were performed with the use of statistical software (Stat View 5.0, Abacus Concepts, Inc., Berkeley, CA, USA).

	RESULTS

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Coefficients of variations (CV) for the two repeated measures for each variable in 10 patients ranged from 0.4% to 0.6% for the linear measurements, from 0.3% to 3.6% for the area measurements, 1.4% for the volumetric measurements, and from 1.1% to 8.9% for the angular measurements.

Means, standard deviations, and ranges for all variables are given in Table 1. Significant positive correlations were found among temporal muscle cross-sectional area, masseter muscle cross-sectional area, temporal muscle volume, and masseter muscle volume (Table 2-1). Only the volume data on the temporal and masseter muscle were used for subsequent analyses, because the volume data correlated highly with the cross-sectional area data.

There were significant positive correlations among the zygomatic arch cross-sectional area, the mandibular ramus cross-sectional area, and masseter muscle volume (Table 2-3).

Masseter muscle volume did not significantly correlate with the gonial angle and the masseter anterior orientation. The masseter zygomatic angle was 95.1 + 6.2 degrees, and the masseter antegonial angle was 87.8 + 8.3 degrees (Table 1).

A significant positive correlation was found between the zygomatic arch angle and the antegonial angle (r = 0.724, P < 0.0001).

	DISCUSSION

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This report documents the association of the temporal and masseter muscle volumes with the size and morphology of their adjacent local skeletal structures in adults with mandibular prognathism. Although previous reports have examined correlations between overall craniofacial skeletal structure and the orientation and size of the masticatory muscles (Weijs and Hillen, 1984, 1986; Gionhaku and Lowe, 1989; Hannam and Wood, 1989; van Spronsen et al., 1991), no agreement has been found on the association between craniofacial form and muscle structures.

This examination of the structural association between the masticatory muscles shows significant positive correlations among the temporal muscle cross-sectional area, masseter muscle cross-sectional area, temporal muscle volume, and masseter muscle volume. These results agree with previous reports documenting significant positive correlations between cross-sectional areas of the anterior temporal muscle and other jaw-closing muscles (van Spronsen et al., 1991) and strong positive correlations between volume and maximum cross-sectional area for the masseter muscle (Gionhaku and Lowe, 1989).

The temporal and masseter muscle volumes showed significant positive correlation with bizygomatic arch width. This finding is consistent with the general consensus in previous reports that subjects with strong or thick mandibular elevator muscles have wider transverse head dimensions (Ringqvist, 1973; Weijs and Hillen, 1984; Hannam and Wood, 1989; Kiliaridis and Kalebo, 1991; van Spronsen et al., 1991; Bakke et al., 1992; Kiliaridis, 1995). The temporal and masseter muscle volumes were significantly correlated with the temporal fossa width but not with the cranium width. These results suggest that the greater bizygomatic arch width for those individuals having large temporal and masseter muscles is not due to the wide cranium but rather to the wide temporal fossa, which is filled primarily with the temporal muscle and partly with the masseter muscle.

Significant positive correlation was found between masseter volume and the cross-sectional areas of the zygomatic arch and the mandibular ramus. Force exerted by a muscle is proportional to the product of the physiological cross-sectional area and fiber length and thus to muscle volume (Van Eijden et al., 1997). The present findings may be explained by previous animal experiments demonstrating that bone mass increases as an effect of mechanical load on long bones (Jee and Li, 1990; Jee et al., 1991) and that the increase in bone mass has a linear relationship with the magnitude of strain (Rubin and Lanyon, 1985). Although inheritance clearly has a strong influence on facial features (Proffit, 1993), skeletal morphology is modified by the mechanical stresses placed on it. Mechanical stresses are the principal factor governing the course of skeletal modification (Wolff, 1892; Enlow, 1968).

In the present study, masseter muscle volume did not significantly correlate with the gonial angle and the masseter orientation. These results do not support the hypothetical model proposed by Sassouni (1969). One of the disadvantages of Sassouni’s model would be that the relationship between the masseter muscle and the conventional mandibular inclination was extrapolated. The mandibular inclination depends upon the anterior part of the mandible on which the masseter muscle forces are not exerted. In the present study, the inclinations of the masseter attachment sites were examined and shown to be influenced by masseter muscle orientation.

The perpendicularity between the masseter muscle orientation and the inclinations of the attachment sites may be an expression of a strong force-resisting framework for masticatory stress (Hylander and Johnson, 1997). As Frost (1990) has shown, adaptations to compression or tension cause trabecular bone to align parallel to the line of the mechanical load. The alignment of trabecular bone in stress trajectories crosses at right angles to each other, and the trabecular alignment was perpendicular to the circumference of the bone so as to resist functional forces. The perpendicularity that supports strong bite force may represent an optimal relationship between masseter muscle orientation and the attachment sites. In skeletal Class III patients who undergo orthognathic surgery, the optimal relationship between the muscle and the attachment site may be changed, because the surgical approach necessitates invasion into the gonial region. This may explain why, after orthognathic surgery, bite force does not increase to the same level as is found in normal unoperated subjects (Shiratsuchi et al., 1991; Kikuta et al., 1994). It has been documented that it is necessary to maintain the position of the gonial region, because the backward rotation of this region hampers post-treatment stability in patients with long faces (Proffit and White, 1991).

Findings from the present study show that the rotation of the anterior masseter origin point around the porion correlates with the rotation of the anterior masseter insertion point around the condylar head. This suggests that the position of the anterior portion of the zygomatic arch is influenced by the strain of the masseter muscle caused by the rotation of the anterior masseter insertion point. The great influence of masseter muscle strain on the anterior portion of the zygomatic arch may be explained by an earlier study of adult macaques (Hylander and Johnson, 1997) that documented a steep strain gradient along the zygomatic arch during chewing, with the highest strains along its anterior region.

In conclusion, the results of this study suggest that temporal and masseter muscle volumes exert influence on the size of their adjacent local skeletal sites in which the muscles are accommodated and on which muscle force is exerted. The angles between masseter orientation and the inclinations of the origin and insertion sites are almost 90E, so as to resist functional forces. Although the present results are limited to individuals with mandibular prognathism, the principles described in this study that account for relationships between the local skeletal form and function of the craniomandibular apparatus are biomechanically reasonable and therefore could be generalized to individuals with normal morphology.

	ACKNOWLEDGMENTS

 
This research was supported by The Grant for Development of Highly Advanced Medical Technology. The authors are grateful to Mr. Per Larsen, Department of 3D-Laboratory, School of Dentistry, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark, for his kind help and contribution in data analyses. Preliminary reports were presented at the 79th General Session & Exhibition of the International Association for Dental Research, 2001, Makuhari, Chiba, Japan, and at the 80th General Session & Exhibition of the International Association for Dental Research, 2002, San Diego, CA, USA.

Received September 11, 2001; Last revision August 2, 2002; Accepted August 9, 2002

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