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Human masseter muscle studies by magnetic resonance Lam, Ernest W. N.


The human masseter muscle is a structurally complex jaw elevator with the capability of generating high, multidirectional forces. The invasiveness of current anatomical and physiological methods has, however, limited both the number and scope of studies of human masseter muscle structure and function. Therefore the aim of this work was to apply in vivo magnetic resonance (MR) techniques to elucidate the three-dimensional internal architecture of the human masseter muscle and its metabolic response to exercise in order to gain a better understanding of the jaw muscles in health and disease. In the first of these experiments, five adult subjects were selected and examined using cephalometric radiography, magnetic resonance imaging (MRI) and three-dimensional rotational and reconstructive computer graphics to describe the organization of tendon planes within the masseter muscle. Planar quadrilaterals representing putative tendon planes were fitted to the surfaces of the three-dimensional muscle reconstructions, and these were related to the mid-sagittal plane in the coronal and axial views. To confirm whether putative planes disclosed by MRI represented true anatomic entities, a fresh human cadaver head was imaged by MRI and then cryosectioned at millimetre intervals. Planar sections through the reconstructed muscle generated from the cadaver cryosections were correlated with the actual MR images in the same planes. Tendon plane angulation appeared to be related to ramal length and lower face height measured cephalometrically. In the axial view, the tendon planes appeared roughly to follow the angulations of the zygomatic arch and the lateral face of the mandibular ramus. Our results suggest that the angulation of tendon planes, and possibly pennation angles are different depending on the viewing angle, and infer that muscle fibres inserting on either side of a central tendon may need to develop different tensile forces if translation is to occur directly along the tendon axis. In the second, 31P magnetic resonance spectroscopy (MRS) was utilized to examine the masseter muscles of six adult males at rest and performing stereotyped isometric clenching exercises. 31P MR spectra were acquired from three locations within the muscle using a 2cm by 3cm, single-turn, copper receiver coil. The spectra were quantified on the basis of relative peak area and position. The organic phosphate (Pi) to creatine phosphate (PCr) ratio (Pi/PCr), which has been shown to be proportional to free ADP concentration and hence, the metabolic activity, as well as the normalized Pi concentration ([Pi]) and pH, were calculated for each site and exercise. The mean resting Pi/PCr ratio and [Pi] were greater for the deep part of the muscle than for the superficial and intermediate parts. These differences were significant to p<0.01. The mean pH however, was similar in all parts of the muscle at rest. During exercise, a significant increase in mean Pi/PCr was found in the superficial and intermediate parts of the muscle. Both these differences were significant to p<0.05. An accompanying decrease in mean pH was observed in all parts of the muscle during exercise. In the superficial part of the muscle, this decrease was significant to the p<0.05 level, and in the deep part, the decrease was significant to the p<0.001 level. No significant differences were found for these parameters between left and right molar clenching. These results suggest that metabolic activity may be monitored in the masseter muscle using 31P MR spectroscopy and that task-dependent and regional variations in metabolic activity may be demonstrated both at rest and during exercise. They are promising enough to encourage future studies of muscle metabolism in subjects with jaw muscle disorders. These experiments demonstrate the novel application of magnetic resonance techniques for studying craniomandibular morphology and function non-invasively. Collectively, they reveal the anatomical and functional heterogeneity which exist in the human masseter muscle.

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