title: Outer shape changes of human masseter with contraction by ultrasound morphometry.
abstract: OBJECTIVE
  The multipennate masseter has a complex internal architecture, which suggests the contraction is not uniform within the muscle. The heterogeneous contraction may cause inequable changes of the muscle's outer shape. This study aimed to elucidate the outer shape changes of the whole masseter with clenching by multipoint measurement of serial ultrasound images.
  
  
  METHODS
  Serial coronal images (perpendicular to the FH plane, 3 mm interval) of the right masseter of five healthy men were obtained with a real-time ultrasonograph equipped with a 13 MHz linear-array transducer. To define the relationship between the transducer and cranium, we used a 3D mechanical stage with a face-bow. Registrations were made during muscle relaxation and maximal clenching. The distance between the lateral and medial outlines of the muscle was measured at intervals of 1mm from the origin to insertion in each image as the thickness at the corresponding measured point.
  
  
  RESULTS
  The thickness of the relaxed and contracted muscle (R and C) and its difference (Delta) varied among the measured points. Muscle thickness at most measured sites increased with clenching, whereas it decreased at several sites, mainly near the origin and insertion. There were positive correlations in every subject both between R and C, and between C and Delta. On the other hand, the correlation between R and Delta were negative or weak.
  
  
  CONCLUSIONS
  Changes of thickness with clenching showed great disparity within the masseter, which may result from the complexity of the contraction properties due to the multipennate structure and functional heterogeneity.
authors.affiliation: Division of Advanced Prosthetic Dentistry, Tohoku University, Graduate School of Dentistry, 4-1 Seiryo-machi, Aoba-ku, Sendai 9808575, Japan. kei-thk@umin.ac.jp
authors.forename: Kei
authors.initials: K
authors.lastname: Kubo
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Kawata
authors.forename: Toru
authors.initials: T
authors.lastname: Ogawa
authors.forename: Makoto
authors.initials: M
authors.lastname: Watanabe
authors.forename: Keiichi
authors.initials: K
authors.lastname: Sasaki
mesh_terms: D000328:Adult; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D008406:Masseter Muscle / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000502:physiology*; D009119:Muscle Contraction; D009126:Muscle Relaxation / Q000502:physiology; D014463:Ultrasonography