Craniofacial Muscles

94 M. Lewis et al.
Fig. 6.2 MRI scans indicating the greater thickness of masseter muscle in a short face (SF) and a
thinner muscle in a long face (LF) compared to an individual with normal (Nor) vertical facial
morphology (reproduced with permission of P.R. van Spronsen)
Furthermore, the orientation of the masseter muscle fi bres in prognathic patients
compared to controls was found to be in a more forward direction, forming an
obtuse angle with the Frankfort horizontal plane (Ariji et al. 2000 ; Kitai et al. 2002 ) .
It has been suggested that the more upright the direction of the masseter muscle
fi bres (as in short face patients) in relation to the Frankfort horizontal or functional
occlusal planes, the greater the occlusal forces (Kitai et al. 2002 ) .
Other studies have assessed the relationship between the volume of the masseter
muscle and speci fi c craniofacial skeletal parameters. The results have indicated a
positive correlation between masseter muscle volume and the ramus height (Kubota
et al. 1998 ) , posterior face height (Benington et al. 1999 ) , and the cross-sectional
area of the zygomatic arch (Kitai et al. 2002 ) , whilst a negative correlation was
observed in relation to mandibular inclination and gonial angle (Kubota et al. 1998 ;
Benington et al. 1999 ) . No relationship was found between masseter muscle volume
and cranial width (Kitai et al. 2002 ) . Furthermore, general anterior and posterior
craniofacial vertical dimensions were more related to masseter muscle volume than
cross-sectional area (Boom et al. 2008 ) .
6.3 Fibre Typing
The actin–myosin relationship is key to skeletal muscle contraction (Barany 1967 ;
Barany et al. 1967 ) . The sarcomeric myosin fi lament is one of the most important
elements of the contractile mechanism, consisting of two myosin heavy chains