Mandible

The mandible, or lower jaw bone, is the largest of the facial bones (Figure 13-2). This highly mobile bone is suspended from the cranium by muscles, ligaments, and the capsule of the TMJ. Many of the important bony landmarks are described as follows.

The body is the horizontal portion of the mandible with sockets for the lower 16 adult teeth (see Figure 13-2). The ramus projects vertically from the body of the mandible. The angle of each mandible provides the attachment for the masseter and medial pterygoid muscles. The mandibular condyle is the convex portion of bone arising from the ramus. Each condyle articulates with the concave mandibular fossa of the temporal bone, forming the TMJ (see Figure 13-1). The coronoid process is the thin triangular projection of bone arising from the anterior aspect of the ramus. The mandibular notch extends between the coronoid process and the mandibular condyle (see Figure 13-2).

Temporal Bone

The mandibular fossa of the temporal bone articulates with the mandibular condyle, forming the TMJ. The anterior aspect of the mandibular fossa is marked by the articular eminence (Figure 13-3). Just posterior to the mandibular fossa is the external auditory meatus: The external opening for the ear. The zygomatic process of the temporal bone projects anteriorly, forming the posterior half of the zygomatic arch (see Figure 13-3). The zygomatic arch is formed by the union of the zygomatic process of the temporal bone and the temporal process of the zygomatic bone (cheek bone) (see Figure 13-1). The zygomatic arch serves as the proximal attachment for the masseter muscle. The temporal fossa is a slightly depressed area on the side of the skull formed by the union of five different cranial bones (see Figure 13-1).

Maxillae

The right and left maxillae unite to form a single maxilla, or upper jaw. The maxilla is firmly fused to adjacent bones of the face, including sphenoid, nasal, and zygomatic bones. The inferior aspects of the maxilla contain sockets for the upper teeth (Figure 13-4).

Zygomatic Bone

The zygomatic bone forms the cheek region and lateral orbit of the eye (Figure 13-5). As has been mentioned, the temporal process of the zygomatic bone forms the anterior half of the zygomatic arch (see Figure 13-1).

Sphenoid Bone

The sphenoid bone (Figure 13-6) is a single, deep bone that runs transversely across the cranium. The greater wings are located on either side of the cranium, just anterior to the temporal bone. Projecting inferiorly are the medial and lateral pterygoid plates (see Figure 13-6); the lateral plate provides proximal attachments for the medial and lateral pterygoid muscles.

Hyoid Bone

The hyoid bone is located at the base of the throat, just anterior to the third cervical vertebra. The very mobile hyoid serves as an attachment for several muscles involved with moving the tongue, swallowing, and opening the jaw (see Figure 13-11.

Supporting Structures

• Articular disc: The articular disc of the TMJ rests between the mandibular condyle and the mandibular fossa of the temporal bone (Figure 13-7, A). This prominent structure consists of dense fibrous connective tissue. The disc provides joint stability, reduces joint contact pressure, and helps safely guide the condyle across the rough articular eminence of the temporal bone.

• Capsule: The TMJ is surrounded by a fibrous capsule. Laterally, the capsule thickens and is called the lateral ligament of the TMJ. The capsule and the lateral ligament provide stability to the TMJ during chewing motions (Figure 13-7, B).

Protrusion and Retrusion

Protrusion, also referred to as protraction, describes the anterior translation of the mandible (Figure 13-8, A). As will be described, protrusion is an important component of opening the mouth.

Retrusion, also called retraction, is the opposite of protrusion. This motion occurs as the mandible translates posteriorly—an important motion for closing the mouth (Figure 13-8, B).

Lateral Excursion

Lateral excursion occurs as the mandible translates from side to side (Figure 13-9). This motion is used to grind food between the teeth.

Depression and Elevation

Depression of the mandible opens the mouth, whereas elevation closes the mouth (Figure 13-10). Both motions play a fundamental role in eating, yawning, and talking. The adult mouth can be opened an average of slightly more than 2 inches. This degree of opening is often gauged by the ability to fit three knuckles (proximal interphalangeal joints) into the mouth.

As is illustrated in Figure 13-10, A, fully depressing the mandible—opening the mouth—requires extreme anterior translation (protrusion) of each mandibular condyle relative to its mandibular fossa. Normally, the articular disc translates anteriorly along with each mandibular condyle, helping to properly guide the motion (Figure 13-10, B). The disc is “reseated” into the joint as the mandible is elevated and retrudes (retracts) during closure of the mouth. In both opening and closing the mouth, the articular disc is essential in minimizing the contact stress between the mandibular condyle and the articular eminence of the temporal bone.