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).

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.

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

ACCESSORY LIGAMENTS: The stylomandibular and sphenomandibular ligaments are the accessory ligaments of the TMJ. Both are located medial to the joint capsule (Figure 11-12). The ligaments help suspend the mandible from the cranium and likely have only a limited dynamic role in mastication.

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 inch) 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).

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.

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.