Right: During grinding of the anterior teeth, the lower head holds the mandible forward and the activity of the elevators decreases because of the reflex inhibition from the periodontium (Widmalm et al. 1987).
Fibers of the lateral pterygoid muscle were found to insert into the periphery of the disk in 22% of the joints studied. In these cases 88% of the muscle fibers were from the upper head and 12% from the lower head (Abe et al. 1993). The smaller the area of attachment of the lateral pterygoid muscle to the condyle, the greater the tendency for disk displacement (Dreger 1994). The upper and lower heads have an antagonistic action. Numerous EMG studies (Molin 1973; Cibbs et al. 1984; Juniper 1983, 1984; Widmalm et al. 1987; Yoshida 1995) have revealed that the lower head is always active during excursive mandibular movements (jaw opening, protrusion, mediotrusion), whereas the upper head is active during incursive movements (jaw closing, retrusion, laterotrusion), serving to hold the disk-condyle complex continuously against the slope of the eminence and to restrain it during incursive movements (Wood et al. 1986). The high proportion (80%) of type-l muscle fibers (low stimulation threshold, fatigue-resistant) is also indicative of a continuous holding action with a low level of force (Eriksson et al. 1981, Mao et al. 1992). Because the upper head becomes longer during its holding action, this can be referred to as eccentric muscle activity (Wilkinson 1988).
94 MRI of a left lateral pterygoid muscle in the sagittal plane
According to Abe et al. (1993), in 12% of individuals the lateral pterygoid muscle divides into three parts as seen here. According to Ögüt-cen-Toller and Juniper (1994) the musde becomes segmented into three parts in week 12 of embryonic development to form an upper (1), middle (2), and lower (3) head.
95 Attachment areas of the lateral pterygoid muscle on the condyle
From Moritz and Ewers 1987. In agreement with numerous other studies, the upper head always inserts on the condyle (1), and in 60% of joints it also inserts into the anteromedial portion of the disk-capsule complex (gray). The lower head always inserts into the pterygoid fovea (2). An overly strong upper head cannot by itself displace the disk anteriorly.
96 Lateral pterygoid muscles in the horizontal plane
Left: Diagram showing the angulation of the lateral pterygoid muscle to the midsagtttal plane (modified from Christiansen et al. 1988). With the jaws closed, the angle averages 39Q and ranges from 22Q to 52Q. In protrusion the origin and insertion come closer together and the angle becomes greater (Okeson 1998).
Right: MRI showing the lateral pterygoid muscles (1) in the horizontal plane.
Gross Anatomical and Histological Studies of the Masticatory Muscle Insertions
The tendons of the muscles of mastication attach to bone by means of special insertion structures (Goss 1940, Long 1947, Symons 1954. Cooper and Misol 1970, Chong and Evans 1982). A fundamental distinction must be made between periosteal-diaphyseal insertions (Biermann 1957) and chondral-apophyseal insertions (Knese and Biermann 1958). A periosteal-diaphyseal insertion may be flat or circumscribed. The structural makeup of an insertion should be such that it can equalize the different moduli of elasticity of the tissues. Pathological changes in these areas bring about the clinical picture of insertion tenopathy (Becker and Krahl 1978, Tillmann and Thomas 1982). In the extremities there is a direct relationship between the mode of osteogenesis in the region and the histological makeup of the tendon attachment structures (Evans et al. 1991). This type of relationship has not been demonstrated in the masticatory structures. Insertion tenopathies occur primarily in chondral-apophyseal insertion structures. It has been suggested that the cause lies in a disturbed collagen synthesis combined with a low content of glycosaminoglycans in the fibrocartilaginous tissue (Hems 1995).
97 Muscle origins and insertions on the side of the skull
left: Schematic drawing showing the origins and insertions of the masseter and temporal muscles. Both muscles have periosteal and cartilaginous areas of insertion.
Right: Histological preparation of the insertion of the tendon of the temporal muscle. The tendon inserts on the coronoid process by means of cartilaginous structural elements (arrows).
98 Muscle origins and insertions on the medial surface of the mandible
Left: Schematic drawing of the origins and insertions of lateral and medial pterygoid muscles and the mylohyoid muscle. Except for the medial pterygoid, the insertions of all these muscles are entirely periosteal.
Right: Histological preparation of the insertion of the lateral pterygoid muscle in the pterygoid fovea. The insertion is entirely periosteal (arrows).
99 Muscle origins and insertions on the posterior mental protuberance
Schematic presentation of the origins and insertions of the geniohyoid, genioglossus, and digastric muscles. In this region there are both cartilaginous and periosteal areas of insertion.
Right: Histological preparation of the insertion of the digastric muscle. The insertion of this muscle is primarily periosteal, but it does have some regions of cartilaginous insertion. Histology by B. Tillmann (Figs. 97-99, right)
Force Vectors of the Muscles of Mastication
No chewing muscle contracts in isolation. Every muscle contraction contributes to a resultant force vector that acts upon the mandible, the teeth, and the temporomandibular joint (Hannam 1994). From this it follows that only limited parts of a muscle are active during certain functions. In addition, the mandible is an elastic structure and this property can cause the loading vectors to be completely different depending on the muscle activation and the function at the moment (Korioth et al. 1992). The maximum force that a muscle can develop can be calculated from the dimensionless product of its cross-sectional area with a value of approximately 35 (Weijs and Hillen 1984, Korioth et al. 1992). As a general rule, decompensated muscles exhibit a loading vector in the direction of their contraction. Causes for this can be a muscle hypertonus related to central stimuli or the occlusion, or an inflammation of the bilaminar zone. The pattern for complex activation of the muscles of mastication depends much less upon the available muscle force than upon the direction of the resultant force. A complex pattern of activation, therefore, allows for unconscious improvement of the muscle forces for optimum chewing efficiency (Mao and Osborn 1994).
100 Muscle vectors in the sagittal plane
Directions of individual force vectors (arrows). The resultant force on the temporomandibular joint is directed anterosuperiorly. The masseter muscle pulls at an angle of approximately 70° to the occlusal plane and the medial pterygoid muscle at an angle of approximately 80” (Weijs and van Spronsen 1992). Individual variations in these orientations amount to only about 10° (Hagiwara et al. 1994). The physiological cross-sectional area of the temporal muscle ranges from 1.8-2.9 cm2 (van Eijden et al. 1996).