title: Relationships between the orientation and moment arms of the human jaw muscles and normal craniofacial morphology.
abstract: It has been suggested that subjects with increased vertical craniofacial dimensions have relatively oblique orientated jaw muscles with a reduced possibility to restrain the vertical component of craniofacial growth. To test this hypothesis, relationships were investigated between the spatial orientation of the jaw muscles and the craniofacial morphology. Computer reconstructions of the external shape of the jaw muscles of 30 adult males with a normal skull were made with the use of serial magnetic resonance imaging (MRI) scans. The orientation of the jaw muscles was defined by a regression line through the centroids of the serial cross-sections. Sagittal and frontal projections of the moment arms of the muscles were measured with respect to the centre of the ipsilateral condyle. Craniofacial morphology was analysed three-dimensionally using lateral head films and coronal MRI scans. The cephalometric data were analysed statistically using regression and factor analyses. Six cephalometric factors with Eigen values higher than 1 were correlated with jaw muscle orientation and moment arm data, using a multiple regression analysis. The anterior face height factor was significantly correlated with the orientation of the jaw opening muscles in the sagittal plane but was not significantly correlated with the orientation of the mandibular elevators. The sagittal moment arms of the mandibular elevators showed significant correlations with the factors describing the gonial angle and the posterior face height. It was concluded that the variation of spatial orientation of the human jaw closing muscles is predominantly associated with the variation of mandibular morphology (expressed by the gonial angle) and the posterior face height. The orientation of the jaw opening muscles shows significant relationships with anterior vertical craniofacial dimensions. The hypothesis that persons with an increased anterior face height have relatively oblique orientated jaw elevators was rejected.
authors.affiliation: Department of Orthodontics, Academic Center for Dentistry Amsterdam (ACTA), The Netherlands.
authors.forename: P H
authors.initials: PH
authors.lastname: van Spronsen
authors.forename: J H
authors.initials: JH
authors.lastname: Koolstra
authors.forename: F C
authors.initials: FC
authors.lastname: van Ginkel
authors.forename: W A
authors.initials: WA
authors.lastname: Weijs
authors.forename: J
authors.initials: J
authors.lastname: Valk
authors.forename: B
authors.initials: B
authors.lastname: Prahl-Andersen
mesh_terms: D000328:Adult; D019412:Anatomy, Cross-Sectional; D002508:Cephalometry; D005147:Facial Bones / Q000033:anatomy & histology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D008334:Mandible / Q000033:anatomy & histology; D008335:Mandibular Condyle / Q000033:anatomy & histology; D008406:Masseter Muscle / Q000033:anatomy & histology; D008410:Masticatory Muscles / Q000033:anatomy & histology* / Q000502:physiology; D008445:Maxillofacial Development; D011626:Pterygoid Muscles / Q000033:anatomy & histology; D012044:Regression Analysis; D012886:Skull / Q000033:anatomy & histology*; D013703:Temporal Muscle / Q000033:anatomy & histology; D014716:Vertical Dimension