85 Posterior view of the medial pterygoid muscles in the frontal plane
This preparation clearly reveals the relationship of the medial pterygoid muscles (1) to the lateral pterygoids (2) and the masseters (3) and the muscular slings they form with the latter. The strength of a muscle depends on its cross-sectional area (Sasaki et al. 1989). In patients with dentitions damaged by periodontal disease, the chewing force was found to be reduced by 25% in the masseters and 10% in the medial pterygoids (Gilbert and Newton 1997).
86 Schematic drawing of the medial pterygoid muscle
Like the pars superficialis of the masseter, the composition of the anterior part of the medial pterygoid is 64% type-I muscle fibers (low threshold, fatigue-resistant) and 27.5% type-IIB (high threshold, fatigue-resistant) (Eriksson and Thornell 1983). In the posterior part, on the other hand, 44% of the fibers are of type I and 45% are of type IIB.
87 Medial view of an anatomical dissection of a right medial pterygoid muscle (1)
The muscle runs inferiorly and posteriorly from the fossa of the pterygoid process to the inner side of the angle of the mandible. As in the masseter, the muscle fibers are extensively interlaced with no uniform direction of alignment. They are comparatively short, but have a relatively large cross-section (van Eijdenetal. 1995).
Suprahyoid Musculature
The suprahyoid muscles, together with the lateral pterygoids, make up the jaw-opening muscles. The suprahyoid musculature includes the digastric, mylohyoid, geniohyoid, and stylohyoid muscles. The digastric muscle has two bellies. The posterior belly has its origin medial to the mastoid process and extends anteriorly, inferiorly, and medially to the hyoid bone; here it attaches to the bone and connects with the anterior belly by means of an intermediate tendon. The anterior belly inserts in the digastric fossa on the inner side of the mandible. When the hyoid bone is braced by the infrahyoid musculature, contraction of the digastric muscle tends to open and retract the jaw. The mylohyoid muscle extends from the step-like myohyoid line on the inner aspect of the body of the mandible to the body of the hyoid bone. The geniohyoid muscle arises at the inner side of the mandibular symphysis, and from there its parallel fibers run directly to the body of the hyoid bone. The pair of geniohyoid muscles can function together as jaw openers, but they can also lift the floor of the mouth and pull the hyoid bone forward. The stylohyoid muscle runs from the styloid process of the temporal bone to the greater horn of the hyoid and serves to stabilize the hyoid.
88 Mylohyoid muscle
Anatomical preparation of a right mylohyoid muscle (1). The anterior and middle fibers of the mylohyoid muscle insert in the center at the mylohyoid raphe. The fibers of the posterior portion insert on the hyoid bone under the geniohyoid muscle (2). These muscles are active primarily during jaw opening, swallowing, and protraction of the tongue, but they also show some activity during lateral jaw movements (Vitti and Basmajian 1977).
89 Suprahyoid musculature from a left posteroinferior view
The digastric muscle with its anterior belly (1) and posterior belly (2) can be seen from this perspective. The mylohyoid (3) and stylomandibular (4) muscles are also visible. When the mandible is stabilized by the closing muscles (elevators), contraction of the suprahyoid musculature lifts the hyoid bone. This is an important part of the act of swallowing.
90 Suprahyoid musculature
preparation of the left half of the jaw showing the posterior belly of the digastric (1), stylohyoid (2), styloglossus (3), and stylopharyngeus (4) muscles as well as the stylomandibular ligament (5). Except for the digastric muscle, all these structures have their origin on the stylohyoid process (6). Muscle fibers of type I, type IIA, and type IIB each make up one-third of the digastric muscle (Eriksson et al. 1982). From the collection of B. Tillmann (Figures 85, 87, 88, 90)
Lateral Pterygoid Muscle
The lateral pterygoid muscle is made up of two functionally different parts (McNamara 1973): an upper (superior) head and a lower (inferior) head. In 12% of the specimens studied. Abe et al. (1993) could also identify a third, intermediate muscle belly.
The upper head arises on the greater wing of the sphenoid bone. It always has an insertion in the upper portion of the pterygoid fovea, but it may also insert to varying degrees into the disk-capsule complex (Chissin 1906, Troiano 1967, Porter 1970, Mahan et al. 1983, White 1985, Meyenburg et al. 1986, Dauber 1987, Carpentier et al. 1988, Merida Velasco et al. 1993, Bade et al. 1994). In 30-40% of temporomandibular joints, the upper head inserts exclusively on the condyle (Moritz and Ewers 1987. Wilkinson 1988, Luder and Bobst 1991, Heylings et al. 1995, Naidoo and Juniper 1997).
The lower head arises on the lateral face of the lateral lamina of the pterygoid process and inserts in the pterygoid fovea. It is possible that fibers from the upper and lower head are intermingled, but in any event, fibers of the lower head can radiate into the disk-capsule complex (Dauber 1987).
91 Lateral pterygoid muscle
Left: Lateral view of an anatomical preparation. This muscle is characterized by relatively long fiber bundles with small cross-sections (van Eijden et al. 1995) in its upper head (1) and lower head (2). The upper head always inserts on the condyle, and in 60% of specimens it also inserts on the disk-capsule complex.
From the collection of B. Tillmann
Right: Schematic drawing.
1 Upper head
2 Lower head
92 EMC activity of the muscles of mastication at rest and during jaw opening
After juniper 1984
Left: In the relaxed mandibular position only minimal EMC activity can be detected from the upper head of the lateral pterygoid muscle.
Right: During jaw opening the upper head is active and serves to protrude the condyles. Activity of the suprahyoid musculature was not measured in this study.
93 EMC activity during grinding of the teeth
After juniper 1984.
Left: