Craniofacial Muscles

6 Masticatory Muscle Structure and Function
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elevate the mandible, whilst simultaneous contraction of both the medial and lateral
pterygoids of both sides of the jaws protrudes the lower jaw. However, when the
medial and lateral pterygoid muscles contract on one side only the mandible rotates
and protrudes to the opposite side as occurs in chewing movements.
The lateral pterygoid muscle also has two heads: the superior head that arises
from the infratemporal surface of the greater wing of the sphenoid bone, and an
inferior head from the lateral surface of the pterygoid plate of the maxilla. The fi bres
from both heads converge as they pass posteriorly and laterally to be inserted into a
small depression on the anterior surface of the neck of the mandibular condyle and
the anterior aspect of the articular disc of the temporomandibular joint. Contraction
of the lateral pterygoid muscles draws the mandibular condyle downwards and forwards
onto the articular eminence as occurs during opening of the mouth. As noted
above, working together, the medial and lateral pterygoid muscles are involved in
many of the complex movements of the mandible as occur during suckling and
mastication.
The masseter muscle is a quadrilateral-shaped muscle composed of three superimposed
layers, which blend together anteriorly. The super fi cial layer is the largest
and arises as a thick aponeurosis from the zygomatic process of the maxilla and the
anterior two-thirds of the lower border of the zygomatic arch. The fi bres pass downwards
and backwards to be inserted into the angle region and the lower half of the
lateral surface of the ramus of the mandible. The middle fi bres arise from the deep
surface of the anterior two-thirds of the zygomatic arch and the lower border of the
posterior third and are inserted into the middle of the ramus of the mandible. The
deep fi bres arise from the deep surface of the zygomatic arch and are inserted into
the upper part of the ramus and coronoid process of the mandible.
The principal action of the masseter muscle is to elevate the mandible with a
small effect in lateral and protrusive movements and minimal activity in the rest
position. The relative involvement of the three different fi bre layers during functional
movements has been fully elicited through electromyographic studies (Vitti and
Basmajian 1977 ) .
The size, volume, thickness, cross-sectional area and the direction and orientation
of the masseter muscle have been measured using different types of magnetic
resonance imaging (MRI) (van Spronsen et al. 1992 ) , computerised tomography
(CT scan) (Kitai et al. 2002 ) , and bilateral ultrasonography (US) (Kiliaridis and
Kalebo 1991 ; Trawitzki et al. 2006 ). The masseter muscle has been reported as
short, thin, of low volume, and having a small cross-sectional area in orthodontic
Class III, prognathic (Ariji et al. 2000 ; Kitai et al. 2002 ; Trawitzki et al. 2006 ) and
long face patients (Kiliaridis and Kalebo 1991 ; van Spronsen et al. 1992 ) when
compared to patients with ideal antero-posterior and vertical facial balance and proportions.
Multi-disciplinary treatment of prognathic patients can increase the thickness
of the masseter; however, this is never to the levels seen in patients with
‘normal’ or ideal facial form (Trawitzki et al. 2006 ) . When compared to other facial
patterns, long face individuals showed the thinnest, whilst short face patients exhibited
the thickest masseter muscle volume compared to average face subjects
(Satiroglu et al. 2005 ; van Spronsen 2010 ) (Fig. 6.2 ).