Next, the four chewing muscles proper (temporal, masseter, medial pterygoid, and lateral pterygoid) and the suprahyoid and perioral musculature will be described in preparation for the clinical examination that will be addressed later.
77 Muscles of mastication
Drawing of the muscles of mastication. In the narrowest sense these include only the temporal, masseter, medial pterygoid, and lateral pterygoid muscles. The suprahyoidal musculature is also shown here because it is of interest in the diagnosis and treatment of functional disturbances. The sternocleidomastoid muscle is not included here because it belongs to the musculature of the neck.
78 Function and structural adaptation of the muscles of mastication
Antagonistic muscle activity serves not only to execute mandibular movements, but also helps stabilize the joints. Functional demands can bring about changes in tonus, response to stimuli, and muscle length. Adaptation depends upon the combination of fibers present. Chronic overloading may lead to inflammation, ruptures, or ossification.
Temporal Muscle
The temporal muscle is a compartmentalized muscle that arises from the superior and inferior temporal lines of the temporal bone. It inserts on the coronoid process and on the anterior edge of the ascending ramus of the mandible. Three functional parts can be distinguished (Zwijnenburg et al. 1996). The anterior part has muscle fibers that pull upward and serve as elevators (Moller 1966). The middle part effects closure of the jaws and, with a posterior vector, retrusion (Blanksma and van Eijden 1990). According to DuBrul (1980) the posterior part is involved primarily with jaw closure and only to a minimal extent with retrusion. Nevertheless, experimental studies have revealed a distinct retrusion when the posterior part is activated (Zwijnenburg et al. 1996). During normal opening and closing movements, as well as during tooth clenching, the activity in all three parts is at a nearly equal high level. During chewing, however, there are great differences between the anterior and posterior parts. The activity is greater on the working side than on balancing side (Blanksma and van Eijden 1995). During lateral jaw movement, EMC activity is markedly lower where there is canine guidance than where there is group function (Manns et al. 1987).
79 Macroscopic anatomical preparation
The pars anterior and pars media of the temporal muscle consist of approximately 47% fatigue-resistant type-I muscle fibers with a low threshold of stimulation (Eriksson and Thornell 1983). The content of thinner type-IIB fibers is about 45%, leading to a higher concentration of fibers in the muscle (Stalberg et al. 1986). Type-NA muscle fibers are not present at all and those of type IIC and/or IM account for only about 4% (Ringqvist 1974). From the collection of B. Tillmann
80 Schematic drawing of the right temporal muscle
The muscle comprises a pars anterior (1), pars media (2), and pars posterior (3). Although the sarcomere lengths are the same in the various parts, there are significant differences in the lengths of the muscle-fiber bundles (21.7-28.9 mm) which indicates different functional demands (van Eijden et al. 1996).
81 Insertion of the temporal muscle on the disk-capsule complex
Left: Medial view. Some of the horizontal fibers (arrows) insert onto the middle and lateral third of the middle and lateral third of the disk (Merida Velasco et al. 1993, Bade et al. 1994).
Right: Insertion of the temporal muscle viewed from above. Easily identified is the tendon (*) of the pars posterior, which inserts on the lateral portion of the disk-condyle complex.
Masseter Muscle
The masseter muscle consists of a superficial and a deep part. The origin of the superficial part is on the zygomatic arch and its insertion is on the lateral masseteric tuberosity at the angle of the mandible. The deep part also arises on the zygomatic arch but inserts on the lateral surface of the ascending ramus. Portions of the deep part also insert on the joint capsule and the disk (Frommer and Monroe 1966, Meyenberg et al. 1986, Dauber 1987). In this way the masseter can influence the capsule receptors by changing the capsule tension. The lowest EMG activity and the greatest chewing force in this muscle can be demonstrated at a jaw opening of 15-20 mm (Manns et al. 1979, Lindauer et al. 1993, Morimoto et al. 1996). Seventy-four percent of the masseter’s muscle spindles are to be found in the deep part (Eriksson and Thornell 1987). These muscle spindles have large diameters and a four-fold higher concentration of intrafusal fibers. From this it can be deduced that there are special functions for different areas of the muscle. The masseter muscle shows no significant difference in EMG activity between subjects with canine guidance and those with group function (Borromeo et al. 1995).
82 Pars superficialis of the masseter muscle
Left: Schematic drawing of the pars superficial. The masseter has a higher concentration of capillaries relative to the diameter of the fibers than all the other skeletal muscles (Stal et al. 1996).
Right: Macroscopic anatomical preparation of the masseter muscle. The resultant force of the pars superficialis is in an anterosuperior direction. The posterior part of the pars superficialis is composed of up to 45% type-IIB fibers (Eriksson and Thornell 1983), which have a high threshold and are fatigue-resistant.
83 Pars profunda
Left: Schematic drawing.
Right: The pars profunda revealed in an anatomical dissection. The muscle has a relatively broad insertion on the zygomatic arch from which it pulls on the lateral surface of the ascending ramus of the mandible. The posterior part of the pars profunda also inserts into the lateral third of the disk-capsule complex (Merida Velasco et al. 1993, Bade et al. 1994). Only approximately 25% of the muscle fibers of the pars profunda are of type IIB.
84 Origin and insertion of the masseter muscle
Schematic drawing showing the areas of origin and insertion of the masseter muscle. The origin of the pars superficialis is on the inferior surface of the zygomatic arch anterior to that of the pars profunda. The insertion of the pars superficialis lies on the lateral surface of the angle of the mandible. At the inferior border of the mandible it is continuous with the medial pterygoid muscle on the inner surface of the mandible. The pars profunda inserts above the masseteric tuberosity on the ascending ramus of the mandible.
Medial Pterygoid Muscle
The medial pterygoid muscle, together with the temporal and masseter muscles, are the jaw-closing muscles. This muscle has its origin in the pterygoid fossa of the pterygoid process of the sphenoid bone. From there it extends interiorly, posteriorly, and laterally to the inner side of the angle of the mandible, where it connects with the masseter to form a muscle sling. The course of the medial pterygoid muscle closely parallels that of the pars superficialis of the masseter. The medial pterygoid muscle functions primarily during jaw closure, but also takes part in protrusive movements. Unilateral contraction results in mediotrusion. Because of its oblique course in the frontal plane, this muscle