Kinesiology of Mastication and Ventilation

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Chapter 11

Kinesiology of Mastication and Ventilation

CHAPTER AT A GLANCE

PART 1: MASTICATION

Mastication is the process of chewing, tearing, and grinding food with the teeth. This process involves an interaction among the central nervous system and muscles of mastication, the teeth, the tongue, and the pair of temporomandibular joints (TMJs). The joints form the pivot point between the lower jaw (mandible) and the base of the cranium. The TMJ is one of the most continuously used joints in the body, not only during mastication, but also during swallowing and speaking. The first part of this chapter focuses on the kinesiology of the TMJs during mastication.

OSTEOLOGY AND TEETH

Regional Surface Anatomy

Figure 11-1 highlights some of the surface anatomy associated with the TMJ. The mandibular condyle fits within the mandibular fossa of the temporal bone. The condyle can be palpated just anterior to the external auditory meatus (i.e., the opening into the ear). The cranial attachment of the temporalis muscle fills a broad, slightly concave region of the skull known as the temporal fossa. The temporal, parietal, frontal, sphenoid, and zygomatic bones all contribute to the temporal fossa.

Additional surface anatomy associated with the TMJ includes the mastoid process of the temporal bone, the angle of the mandible, and the zygomatic arch. The zygomatic arch is formed by the union of the zygomatic process of the temporal bone and the temporal process of the zygomatic bone.

Individual Bones

The mandible, maxillae, temporal, zygomatic, sphenoid, and hyoid bones are all related to the structure or function of the TMJ.

MANDIBLE: The mandible is the largest of the facial bones (see Figure 11-1). It is a very mobile bone, suspended from the cranium by the muscles, ligaments, and capsule of the TMJ. Muscles of mastication attach either directly or indirectly to the mandible. Muscle contraction positions the teeth embedded within the mandible firmly against the teeth embedded within the fixed maxillae.

The two main parts of the mandible are the body and the two rami (Figure 11-2). The body, the horizontal portion of the bone, accepts the lower 16 adult teeth (Figure 11-3). The rami of the mandible project vertically from the posterior aspect of the body (see Figure 11-2). Each ramus has an external and internal surface and four borders. The posterior and inferior borders of the ramus join at the readily palpable angle of the mandible. The masseter and medial pterygoid muscles—two powerful muscles of mastication—share similar attachments in the region of the angle of the mandible.

At the superior end of the ramus are the coronoid process, mandibular condyle, and mandibular notch. The coronoid process is a triangular projection of thin bone that extends upward from the anterior border of the ramus. This process is the primary inferior attachment of the temporalis muscle. The mandibular condyle extends upward from the posterior border of the ramus. The condyle forms the convex bony component of the TMJ. Extending between the coronoid process and mandibular condyle is the mandibular notch. The mandibular neck is a slightly constricted region located immediately below the condyle. The lateral pterygoid muscle attaches to the anterior-medial surface of the mandibular neck, within a depression called the pterygoid fossa (Figures 11-2 and 11-4).

MAXILLA: The right and left maxillae fuse to form a single maxilla, or upper jaw. The maxilla is fixed within the skull through rigid articulations to adjacent bones (see Figure 11-1). The maxillae extend superiorly, forming the floor of the nasal cavity and the orbit of the eyes. The lower horizontal portions of the maxillae accept the upper teeth.

TEMPORAL BONE: Two temporal bones exist—one on each side of the cranium. The mandibular fossa forms the bony concavity of the TMJ, highlighted in a side view in the lower part of Figure 11-5. The highest point of the fossa is the dome, often very thin and membranous (see main illustration in Figure 11-5). The fossa is bound anteriorly by the articular eminence and posteriorly by the postglenoid tubercle and the tympanic part of the temporal bone. On full opening of the mouth, the condyles of the mandible slide anteriorly and inferiorly across the pair of sloped articular eminences.

The styloid process is a long slender extension of bone that protrudes from the inferior aspect of the temporal bone (see Figure 11-1). The pointed process serves as an attachment for the stylomandibular ligament (to be discussed further) and three small muscles (styloglossus, stylohyoid, and stylopharyngeus). The zygomatic process of the temporal bone forms the posterior half of the zygomatic arch (see main illustration in Figure 11-5).

ZYGOMATIC BONE: The right and left zygomatic bones constitute the major part of the cheeks and the lateral orbits of the eyes (see Figure 11-1). The temporal process of a zygomatic bone contributes the anterior half of the zygomatic arch (see Figure 11-5). A large part of the masseter muscle attaches to the zygomatic bone and the adjacent zygomatic arch.

SPHENOID BONE: Although the sphenoid bone does not contribute to the structure of the TMJ, it does provide proximal attachments for the medial and lateral pterygoid muscles. When articulated within the cranium, the sphenoid bone lies transversely across the base of the skull. The relevant osteologic features of the sphenoid bone are its greater wing, medial pterygoid plate, and lateral pterygoid plate (Figure 11-6). When a section of the zygomatic arch is removed, the lateral surfaces of the greater wing and lateral pterygoid plate are revealed (Figure 11-7).

HYOID BONE: The hyoid is a U-shaped bone that can be palpated at the base of the throat, just anterior to the body of the third cervical vertebra (Figure 11-8). The body of the hyoid is convex anteriorly. The bilateral greater horns form its slightly curved sides. The hyoid is suspended primarily by a bilateral pair of stylohyoid ligaments. Several muscles involved with moving of the tongue, swallowing, and speaking attach to the hyoid bone (see Figure 11-21).

Teeth

The maxillae and mandible each contain 16 permanent teeth (see Figure 11-3 for names of lower teeth). The structure of each tooth reflects its function in mastication (Table 11-1).

TABLE 11-1.

Permanent Teeth

image

Each tooth has two basic parts: crown and root (Figure 11-9). Normally the crown is covered with enamel and is located above the gingiva (gum). The root of each tooth is embedded in alveolar bone. The periodontal ligaments help attach the roots of the teeth within their sockets.

Cusps are conical elevations that arise on the surface of a tooth. Maximal intercuspation describes the position of the mandible when the cusps of the opposing teeth are in maximal contact. The term is frequently used interchangeably with centric relation, especially in describing the relative position of the articular surfaces within the TMJ. The relaxed postural position of the mandible allows a slight “freeway space” (interocclusal clearance) between the upper and lower teeth. Normally the teeth make contact (occlude) only during chewing and swallowing.

ARTHROLOGY OF THE TEMPOROMANDIBULAR JOINT

The temporomandibular joint (TMJ) is a loosely fitting articulation formed between the mandibular condyle and the mandibular fossa of the temporal bone (see Figures 11-1 and 11-5). It is a synovial joint that permits a wide range of rotation as well as translation. An articular disc cushions the potentially large and repetitive forces inherent to mastication. The disc separates the joint into two synovial joint cavities (Figure 11-10). The inferior joint cavity is between the inferior aspect of the disc and the mandibular condyle. The larger superior joint cavity is between the superior surface of the disc and the segment of bone formed by the mandibular fossa and the articular eminence.

Although the right and left TMJs function together, each retains its ability to function relatively independently. Mastication is typically performed asymmetrically, with one side of the mandible exerting a greater biting force than the other. The dominant side is often referred to as the “working” side, whereas the nondominant side is referred to as the “balancing” side.24 Different demands are placed on the muscles and joints of the working and balancing sides.

Osseous Structure

MANDIBULAR CONDYLE: The mandibular condyle is flattened from front to back, with its medial-lateral length twice as long as its anterior-posterior length (see Figure 11-3). The condyle is generally convex, possessing short projections known as medial and lateral poles. The medial pole is more prominent than the lateral. While the mouth is opening and closing, the outside edge of the lateral pole can be palpated as a point under the skin just anterior to the external auditory meatus.

The articular surface of the mandibular condyle is lined with a thin but dense layer of fibrocartilage. This tissue absorbs forces associated with mastication better than hyaline cartilage, and it has a superior reparative process.65 Both of these functions are important, given the extraordinary demands placed on the TMJ.

MANDIBULAR FOSSA: The mandibular fossa of the temporal bone is divided into two surfaces: articular and nonarticular. The articular surface of the fossa is formed by the articular eminence, occupying the sloped anterior wall of the fossa (see Figures 11-5 and 11-10). This thick and smooth load-bearing surface is lined with a thick layer of fibrocartilage. Full opening of the mouth requires that each condyle slide forward across the articular eminence. The slope of the articular eminence is, on average, 55 degrees from the horizontal plane.29 The magnitude of the slope partially determines the kinematic path of the condyle during opening and closing of the mouth.

The nonarticular surface of the mandibular fossa consists of a very thin layer of bone and fibrocartilage that occupies much of the superior (dome) and posterior walls of the fossa (see Figure 11-5). This thin region is not an adequate load-bearing surface. A large upward force applied to the chin can fracture this region of the fossa, possibly even sending bone fragments into the cranium.

Articular Disc

The articular disc within the TMJ consists primarily of dense fibrocartilage that, with the exception of its periphery, lacks a blood supply and sensory innervation. The histology of this tissue is generally similar to that of other load-bearing intra-articular discs of the body, such as the disc within the distal radio-ulnar joint and the meniscus of the knee. The fibrocartilage in the TMJ is flexible but firm owing to its high collagen content. The entire periphery of the disc attaches to the surrounding capsule of the joint.

The disc is divided into three regions: posterior, intermediate, and anterior (see Figure 11-10). The shape of each region allows the disc to accommodate to the varying contours of the condyle and the fossa. The posterior region of the disc is convex superiorly and concave inferiorly. The concavity accepts most of the condyle, much like a ball-and-socket joint. The extreme posterior region attaches to the loosely organized retrodiscal laminae, containing collagen and elastin fibers. Connections made by the laminae anchor the disc posteriorly to bone. A meshwork of fat, blood vessels, and sensory nerves fills the space between the superior and inferior laminae.

The intermediate region of the disc is concave inferiorly and generally flat superiorly. The anterior region is nearly flat inferiorly and slightly concave superiorly to accommodate the convexity of the articular eminence. The anterior region of the disc attaches to several tissues.

The thickness of the disc varies between its anterior and posterior regions. The thinnest intermediate region is only 1 mm thick.36 The anterior and posterior regions, however, are about two to three times thicker. The disc is constricted at its intermediate region.55 The constriction, flanked by the adjacent thicker anterior and posterior regions, forms a dimple on the disc’s inferior surface. In maximal intercuspation, the dimpled intermediate region of the disc should fit between the anterior-superior edge of the condyle and the articular eminence of the fossa. The disc position protects the condyle as it slides forward across the articular eminence during the later phase of opening the mouth widely.

The articular disc maximizes the congruency within the TMJ to reduce contact pressure. The disc also adds stability to the joint and helps guide the condyle of the mandible during movement. In the healthy TMJ, the disc slides with the translating condyle. Movement is governed by intra-articular pressure, by muscle forces, and by collateral ligaments that attach the periphery of the disc to the condyle.

Capsular and Ligamentous Structures

FIBROUS CAPSULE: The TMJ and disc are surrounded by a loose fibrous capsule. The internal surfaces of the capsule are lined with a synovial membrane. Superiorly the capsule attaches to the rim of the mandibular fossa, as far anterior as the articular eminence. Inferiorly the capsule attaches to the periphery of the articular disc and to the superior neck of the mandible. Anteriorly the capsule and part of the anterior edge of disc attach to the tendon of the superior head of the lateral pterygoid muscle (see Figure 11-10).

The capsule of the TMJ provides significant support to the articulation. Medially and laterally the capsule is relatively firm, providing stability to the joint during lateral movements such as those produced during chewing. Anteriorly and posteriorly, however, the capsule is relatively lax, allowing the condyle and disc to translate forward when the mouth is opened.

LATERAL LIGAMENT: The primary ligament reinforcing the TMJ is the lateral (temporomandibular) ligament (Figure 11-11, A). The lateral ligament has been described as a combination of horizontal and oblique fibers (see Figure 11-11, B).71 The more superficial oblique fibers course in an anterior-superior direction, from the posterior neck of the mandible to the lateral margins of the articular eminence and zygomatic arch. The deeper horizontal fibers share similar temporal attachments. They course horizontally and posteriorly to attach into the lateral pole of the mandibular condyle.

The primary function of the lateral ligament is to stabilize the lateral side of the capsule. Tears or excessive elongation of the lateral ligament may cause the disc to migrate medially by an unopposed pull of the superior head of the lateral pterygoid muscle. As described in the discussion of arthrokinematics, the oblique fibers of the lateral ligament have a special function in guiding the movement of the condyle during opening of the mouth.55

Osteokinematics

The osteokinematics of the mandible are most often described as protrusion and retrusion, lateral excursion, and depression and elevation (Figures 11-13 to 11-15). All of these movements occur to varying degrees during mastication. For a more detailed analysis of mandibular movements, the reader is encouraged to consult the classic work by Posselt,61 thoroughly summarized by Okeson.55

PROTRUSION AND RETRUSION: Protrusion of the mandible occurs as it translates anteriorly without significant rotation (see Figure 11-13, A). Protrusion is an important component of the mouth’s opening maximally. Retrusion of the mandible occurs in the reverse direction (see Figure 11-13, B). Retrusion provides an important component of closing the widely opened and protruded mouth.

LATERAL EXCURSION: Lateral excursion of the mandible occurs primarily as a side-to-side translation (see Figure 11-14, A). The direction (right or left) of active lateral excursion can be described as either contralateral or ipsilateral to the side of the primary muscle action. In the adult, an average of 11 mm (about imageinch) of maximal unilateral excursion is considered normal.74 Lateral excursion of the mandible is usually combined with other relatively slight translations and rotations. Normally the specific path of movement is guided by the shape of the mandibular fossa and position of the articular disc.

DEPRESSION AND ELEVATION: Depression of the mandible causes the mouth to open, a fundamental component of chewing (see Figure 11-15, A). Maximal opening of the mouth typically occurs during actions such as yawning and singing. In the adult the mouth can be opened an average of 50 mm as measured between the incisal edges of the upper and lower front teeth.2,35,74 The interincisal opening is typically large enough to fit three adult “knuckles” (proximal interphalangeal joints). Typical mastication, however, requires an average maximal opening of 18 mm—about 36% of maximum (sufficient to accept one adult knuckle). Being unable to fit two knuckles between the edges of the upper and lower incisors is usually considered abnormal in the average sized adult. Elevation of the mandible closes the mouth—an action used to grind food during mastication (see Figure 11-15, B).

Arthrokinematics

Movement of the mandible typically involves bilateral action of the TMJs. Abnormal function in one joint naturally interferes with the function of the other. Depending on the osteokinematics, the arthrokinematics of the TMJ normally involve both rotation and translation. In general, during rotational movement the mandibular condyle rolls relative to the inferior surface of the disc, and during translational movement the mandibular condyle and disc slide essentially together. The disc usually moves in the direction of the translating condyle.

PROTRUSION AND RETRUSION: During protrusion and retrusion the mandibular condyle and disc translate anteriorly and posteriorly, respectively, relative to the fossa (see Figure 11-13). The condyle and disc follow the downward slope of the articular eminence. The mandible slides slightly downward during protrusion and upward during retrusion. The path of movement varies depending on the degree of opening of the mouth.

LATERAL EXCURSION: Lateral excursion involves primarily a side-to-side translation of the condyle and disc within the fossa. Slight multiplanar rotations are typically combined with lateral excursion.55 Figure 11-14, B, shows an example of lateral excursion combined with slight horizontal plane rotation. The left condyle forms a pivot point within the fossa as the right condyle rotates slightly anteriorly and medially.

DEPRESSION AND ELEVATION: Opening and closing of the mouth occur by depression and elevation of the mandible, respectively. During these movements, each TMJ experiences a combination of rotation and translation among the mandibular condyle, articular disc, and fossa. No other joint in the body experiences such a large proportion of translation and rotation. Because rotation and translation occur simultaneously, the axis of rotation is constantly moving. In the ideal case the movements within both TMJs result in a maximal range of mouth opening with a minimal stress placed on the articular surfaces.

The arthrokinematics of opening the mouth are depicted for an early and a late phase in Figure 11-16. The early phase, constituting the first 35% to 50% of the range of motion, involves primarily rotation of the mandible relative to the cranium.66,88 As depicted in Figure 11-16, A, the condyle rolls posteriorly within the concave inferior surface of the disc. (The direction of the roll is described relative to the rotation of a point on the ramus of the mandible.) The rolling motion swings the body of the mandible inferiorly and posteriorly. The axis of rotation is not fixed but migrates within the vicinity of the condyles.24,60

The rolling motion of the condyle stretches the oblique portion of the lateral ligament. The increased tension in the ligament helps to initiate the late phase of the mouth’s opening.57,71

The late phase of opening the mouth consists of the final 50% to 65% of the total range of motion. This phase is marked by a gradual transition from primary rotation to primary translation. The transition can be readily appreciated by palpating the condyle of the mandible during the full opening of the mouth. During the translation the condyle and disc slide together in a forward and inferior direction against the slope of the articular eminence (see Figure 11-16, B). At the end of opening, the axis of rotation shifts inferiorly. The exact point of the axis is difficult to define because it depends on the person’s unique rotation-to-translation ratio. At the later phase of opening, the axis is usually below the neck of the mandible.24

Full opening of the mouth maximally stretches and pulls the disc anteriorly. The extent of the forward translation (protrusion) is limited, in part, by tension in the stretched, elastic superior retrodiscal lamina. The intermediate region of the disc translates forward while remaining between the superior aspect of the condyle and the articular eminence. This placement of the disc maximizes joint congruency and reduces intra-articular stress.

The arthrokinematics of closing the mouth occur in the reverse order of that described for opening. When the mouth is fully opened and prepared to close, tension in the superior retrodiscal lamina starts to retract the disc, initiating the early translational phase of closing. The later phase is dominated by rotation of the condyle within the concavity of the disc, terminated when contact is made between the upper and lower teeth.

MUSCLE AND JOINT INTERACTION

Innervation of the Muscles and Joints

The muscles of mastication and their innervation are listed in Table 11-2. Based primarily on size, the muscles of mastication are divided into two groups: primary and secondary. The primary muscles are the masseter, temporalis, medial pterygoid, and lateral pterygoid. The secondary muscles are much smaller. The primary muscles of mastication are innervated by the mandibular nerve, a division of the trigeminal nerve (cranial nerve V). This nerve exits the skull via the foramen ovale, which is just medial and slightly anterior to the mandibular fossa (see Figure 11-5).

The central part of the disc within the TMJ lacks sensory innervation. The periphery of the disc, capsule, lateral ligament, and retrodiscal tissues, however, possess pain fibers and mechanoreceptors.75,87 In addition, mechanoreceptors and sensory nerves from oral mucosa, periodontal ligaments, and muscles provide the nervous system with a rich source of proprioception. This sensory information helps protect the soft oral tissues, such as the tongue and cheeks, from trauma caused by the teeth during chewing or speaking. Furthermore, the sensation helps coordinate the neuromuscular reflexes that synchronize the functional interaction among the muscles of the TMJ and in the craniocervical region. The sensory innervation from the TMJ is carried through two branches of the mandibular nerve: auriculotemporal and masseteric.75

Muscular Anatomy and Function

PRIMARY MUSCLES OF MASTICATION: The primary muscles of mastication are the masseter, temporalis, medial pterygoid, and lateral pterygoid. Refer to Appendix III, Part C for a summary of muscle attachments.

Masseter: The masseter is a thick, strong muscle, easily palpable just above the angle of the mandible (Figure 11-17, A). The muscle, as a whole, originates from the zygomatic arch and zygomatic bone (see Figures 11-1 and 11-5) and inserts inferiorly on the external surface of the ramus of the mandible (see Figure 11-2).

The masseter has superficial and deep heads (see Figure 11-17, A). The fibers of the larger, more superficial head travel inferiorly and posteriorly, attaching inferiorly near the angle of the mandible. The fibers from the smaller deep head attach inferiorly to the upper region of the ramus of the mandible, close to the base of the coronoid process.

The actions of both heads of the masseter are essentially the same. Bilateral contraction elevates the mandible to bring the teeth into contact during mastication.55,75 The line of force of the muscle is nearly perpendicular to the biting surface of the molars. The primary function of the masseter, therefore, is to develop large forces between the molars for effective grinding and crushing of food. Bilateral action of the masseters also protrudes the mandible slightly. Unilateral contraction of the masseter, however, causes slight ipsilateral excursion of the mandible. Such an action may occur during a lateral grinding motion while chewing (Figure 11-18). The multiple actions of the masseter are necessary for effective mastication.

Temporalis: The temporalis is a flat, fan-shaped muscle that fills much of the concavity of the temporal fossa of the skull (see Figure 11-17, B). From its cranial attachment, the muscle forms a broad tendon that narrows distally as it passes through a space formed between the zygomatic arch and the lateral side of the skull (see Figure 11-5). The muscle attaches distally to the coronoid process and to the anterior edge and medial surface of the ramus of the mandible (see Figure 11-2). Bilateral contractions of the temporalis muscles elevate the mandible. The more oblique posterior fibers elevate and retrude the mandible.55

Similar to the masseter, the temporalis courses slightly medially as its approaches its distal attachment. Unilateral contraction of the temporalis, therefore, as when chewing in a side-to-side manner, causes slight ipsilateral excursion of the mandible (see Figure 11-18).

Medial Pterygoid: The medial pterygoid muscle arises from two heads (Figure 11-19, A). The much larger deep head attaches on the medial surface of the lateral pterygoid plate of the sphenoid bone (see Figures 11-5 and 11-6). The smaller superficial head attaches to a region of the posterior side of the maxilla, just above the third molar (see Figure 11-7).75 Both heads course nearly parallel with the masseter muscle and attach on the internal surface of the ramus, near the angle of the mandible (see Figures 11-2 and 11-4).

The actions of the two heads of the medial pterygoid are essentially identical. Acting bilaterally, the medial pterygoid elevates and, to a limited extent, protrudes the mandible. Because of the oblique line of force of the muscle relative to the frontal plane, a unilateral contraction of the medial pterygoid produces a very effective contralateral excursion of the mandible (see Figure 11-18).

SPECIAL FOCUS 11-1   imageFunctional Interactions between the Masseter and Medial Pterygoid Muscles

The medial pterygoid and masseter muscles form a functional sling around the angle of the mandible (Figure 11-20). Simultaneous contractions of these muscles can exert a powerful biting force that is directed through the jaw and ultimately between the upper and lower molars. The maximal bite force in this region averages about 422 N (95 lb) in the adult, twice that generated between the incisors.42

image

FIGURE 11-20.

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