P.H. van Spronsen

Comparison of Jaw-muscle Bite-force Cross-sections Obtained by Means of Magnetic Resonance Imaging and High-resolution CT Scanning

Cross-sectional areas of the jaw muscles were determined by means of magnetic resonance imaging (MRI) in 12 healthy adult male subjects. These findings were compared with the cross-sectional areas of the jaw muscles of the same subjects, obtained by means of computer tomography (CT) in a previous study (Weijs and Hillen, 1985). Significant correlations (r>0.7) were found between the CT and MRI cross-sections of the masseter, medial pterygoid, and temporalis muscles. The low correlation between the CT and MRI cross-sections of the lateral pterygoid muscle could be explained by the different imaging techniques (slice thickness) of MRI and CT scanning. CT and MRI cross-sectional areas of the masseter and medial pterygoid muscle (but not the temporalis muscle) showed highly positive and significant correlations with the maximal voluntary bite force. In living subjects, the cross-sections of the masseter and medial pterygoid muscles can be visualized with CT and MRI. Compared with CT, MRI has some advantages, such as the absence of adverse effects (no radiation) and the excellent soft-tissue imaging. Furthermore, a series of frontal, horizontal, sagittal, and angulated MRI scans can be made without modification of the patients position, facilitating reconstruction of the jaw muscles.

A Comparison of Jaw Muscle Cross-sections of Long-face and Normal Adults

Long-face subjects have smaller maximum molar bite forces than do normal individuals. This has been attributed both to differences in moment arms and size of the jaw muscles. In this study, a comparison was made between the mid-belly cross-sectional areas of the jaw muscles of 13 long-face and 35 normal adults by means of serial MRI scans. The subjects were selected on the basis of anterior lower face height as a percentage of anterior total face height. These and other cephalometric variables were measured from lateral radiographs. In the long-face group, the cross-sectional areas of the masseter, medial pterygoid, and anterior temporal muscles were, respectively, 30%, 22%, and 15% smaller than in the control group. By a discriminant analysis and a multivariate analysis of variance, these differences were found to be significant (p < 0.001). The findings of this study hint that differences in the sizes of the jaw muscles of long-face and normal subjects might explain, in part, the observed differences in maximum molar bite force.

A COMPARISON OF HUMAN MASSETER MUSCLE THICKNESS MEASURED BY ULTRASONOGRAPHY AND MAGNETIC RESONANCE IMAGING

Non-invasive imaging techniques such as computerized tomography, magnetic resonance imaging (MRI), and ultrasonography enable measurements of the cross-section and thickness of human jaw muscles in vivo, providing an indication of the maximal force a muscle can exert. In 15 adult Caucasian men the thickness of the masseter muscle was registered bilaterally on three different levels by ultrasonography. Scans were made on the contracted and relaxed muscle. A comparison was then made with measurements from serial MRI scans, using univariate analysis of variance for repeated measurements and Pearsons correlation coefficients. Variances of the repeated measurements were calculated for the different scanning levels and the different muscle conditions and tested for homogeneity. For both the ultrasound and MRI measurements there was no difference in thickness between the left and right muscle. The registration level with highest reproducibility was halfway between the origin and insertion. Measurements from the contracted muscle were more reproducible than those from the relaxed muscle. The relaxed muscle thickness measured by ultrasonography was smaller than that measured by MRI. The correlation between ultrasound and MRI was significant for the upper and middle level of scanning (p < 0.001). The highest correlation was found between MRI (relaxed) and ultrasound (contracted) at the middle level (R = 0.83, p < 10(-6)). The conclusion is that ultrasonography is an accurate and reproducible method for measuring the thickness of the masseter in vivo. It allows for large-scale longitudinal study of changes in jaw-muscle thickness during growth in relation to change in biomechanical properties of masticatory muscles.

Jaw Muscle Orientation and Moment Arms of Long-face and Normal Adults

Long-face subjects have strongly reduced bite forces relative to normal subjects. This difference cannot be fully explained by the reduced cross-sectional area of the jaw muscles. In this study, we investigated whether the orientation and moment arms of the jaw muscles of normal and long-face subjects are different, and if so, to what extent these differences contribute to the observed differences in maximum molar bite-force levels. Three MRI scan series with different orientations were made of the jaw muscles of 30 normal and 13 long-face subjects. These served as the basis for computer reconstructions of the external shape of the muscles. The spatial orientation of the jaw muscles was defined by the regression line through the centroids of the muscular cross-sections. The moment arms of the jaw muscles and the bite point of the first mandibular molar were measured with respect to the center of the ipsilateral condyle. The muscular variables-including angles, moment arms, and mechanical advantage-were analyzed with a discriminant analysis and a multivariate analysis of variance (MANOVA). Differences in the spatial orientation of the temporalis muscle and the anterior digastric muscle contributed most to the distinction of the normal and long-face group. With MANOVA, it was shown that the normal and long-face group did not significantly differ with respect to the jaw muscle moment arms and mechanical advantage data. Only small differences were found between the sagittal muscle angles of the masseter and anterior digastric muscles in the two groups. In both the normal and long-face group, the orientation and moment arm data of the right and left muscles differed significantly. It was concluded that the variation of the spatial orientation of the jaw muscles is small and does not significantly contribute to the explanation of the different molar bite-force levels of long-face and normal subjects. Therefore, it is tempting to assume that the jaw muscles of normal and long-face subjects are different with respect to the maximum force they can exert per unit of cross-sectional area.

Computer-assisted estimation of lines of action of human masticatory muscles reconstructed in vivo by means of magnetic resonance imaging of parallel sections

The orientation of these lines of action was estimated in 9 healthy subjects, by reconstructing the muscle shape from a series of parallel sections obtained by MRI. In order to gain insight into sources of error, the lines of action of the masseter and medial pterygoid were estimated from two mutually perpendicular series of sectional images. Average results were compared with anatomical data from the literature. The results indicated that the accuracy of the estimate was principally dependent on the reliability of the reconstructions; the average accuracy of the estimated orientations was about 5 degrees.

Brain and Oral Function: Oral Motor Function and Dysfunction (1995). T. Morimoto T. Matsuya and K. Takada (Eds). Publisher: Elsevier Science, Amsterdam, The Netherlands. Price: Dfl. 435.00 US

This book contains an elaborate selection of papers from the Osaka International Oral Physiology Symposium on Brain and Oral Function held in 1994. All papers presented at this symposium focus on orofacial function and dysfunction from a neuroscience viewpoint and can be divided into seven subsections: (1) histochemistry and contractile properties of the muscle fibres of the masticatory muscles; (2) periodontal sensation and its contribution to the control of the masticatory muscles; (3) central connections of muscle spindle afferents and the reflex control of masticatory force; (4) central rhythm generator of jaw and tongue movements; (5) physiological and pharmacological properties of the synapse between premotor neurons and the trigeminal motorneurons; (6) the cerebral mechanism of jaw and tongue movements; and (7) abnormal rhythmic oral movements and clinical evaluation of craniofacial motor dysfunctions. This book is especially recommended for those performing basic reasearch in the physiological mechanisms of jaw movements and provides insight into the control of the human masticatory muscles and jaw movements. For instance, the role of the human periodontal mechanoreceptors, electromyogram-force relationships, and right-left asymmetries in masticatory muscle activity are discussed. For clinicians relevant papers can be found in the section dealing with temporomandibular joint and craniomandibular disorder that focus on patients with chronic orofacial pain, and the section about posture and electromyography that partly deals with sleep apnoea syndrome. The contributions about the relationships between facial morphology and jaw muscle function and oral sensation and bite force may also be of interest to orthodontists. R H. van Spronsen