8.3. The Temporomandibular Joint

The temporomandibular joint (TMJ) allows for opening (mandibular depression) and closing (mandibular elevation) of the mouth, as well as side-to-side (mandibular excursion) and protraction/retraction motions of the lower jaw. This joint involves the articulation between the mandibular fossa and articular tubercle of the temporal bone, with the condyle (head) of the mandible. The main components are the joint capsule, articular disc, mandibular condyles, articular surface of the temporal bone, temporomandibular ligament, stylomandibular ligament, sphenomandibular ligament, and lateral pterygoid muscle.

The capsule is a dense fibrous membrane that surrounds the joint and incorporates the articular eminence. It attaches to the articular eminence, the articular disc and the neck of the mandibular condyle.

Ligaments

There are three ligaments associated with the temporomandibular joints: one major and two minor ligaments. These ligaments are important in that they define the border movements, or in other words, the farthest extents of movements, of the mandible. Movements of the mandible made past the extents functionally allowed by the muscular attachments will result in painful stimuli, and thus, movements past these more limited borders are rarely achieved in normal function.

The major ligament, the temporomandibular ligament, is actually the thickened lateral portion of the capsule, and it has two parts: an outer oblique portion (OOP) and an inner horizontal portion (IHP). The base of this triangular ligament is attached to the zygomatic process of the temporal bone and the articular tubercle; its apex is fixed to the lateral side of the neck of the mandible. This ligament prevents the excessive retraction or moving backward of the mandible, a situation that might lead to problems with the joint. More details.

The two minor ligaments, the stylomandibular and sphenomandibular ligaments are accessory and are not directly attached to any part of the joint.

The stylomandibular ligament separates the infratemporal region (anterior) from the parotid region (posterior), and runs from the styloid process to the angle of the mandible; it separates the parotid and submandibular salivary glands. It also becomes taut when the mandible is protruded.

The sphenomandibular ligament runs from the spine of the sphenoid bone to the lingula of mandible. The inferior alveolar nerve descends between the sphenomandibular ligament and the ramus of the mandible to gain access to the mandibular foramen. The sphenomandibular ligament, because of its attachment to the lingula, overlaps the opening of the foramen. It is a vestige of the embryonic lower jaw, Meckel cartilage. The ligament becomes accentuated and taut when the mandible is protruded.

Synovial cavity

The unique feature of the temporomandibular joint is the articular disc. The disc is composed of dense fibrous connective tissue that is positioned between the two bones that form the joint. The temporomandibular joints are one of the few synovial joints in the human body with an articular disc, another being the sternoclavicular joint. The disc divides each joint into two. These two compartments are the upper and lower synovial cavities. The synovial membrane lining the joint capsule produces the synovial fluid that fills these cavities.

The posterior region (retrodiscal tissue) has blood vessels and nerves. The anterior portion, in contrast, is avascular and lacks innervation. Few cells are present, but fibroblasts and white blood cells are among them. This area is thinner but denser than the peripheral region.

Movements of the mandible

Protraction and Retraction

Protraction and retraction are anterior-posterior movements of the scapula or mandible. Protraction of the scapula occurs when the shoulder is moved forward, as when pushing against something or throwing a ball. Retraction is the opposite motion, with the scapula being pulled posteriorly and medially, toward the vertebral column. For the mandible, protraction occurs when the lower jaw is pushed forward, to stick out the chin, while retraction pulls the lower jaw backward.

Depression and Elevation

Depression and elevation are downward and upward movements of the scapula or mandible. The upward movement of the scapula and shoulder is elevation, while a downward movement is depression. These movements are used to shrug your shoulders. Similarly, elevation of the mandible is the upward movement of the lower jaw used to close the mouth or bite on something, and depression is the downward movement that produces opening of the mouth.

Excursion

Excursion is the side to side movement of the mandible. Lateral excursion moves the mandible away from the midline, toward either the right or left side. Medial excursion returns the mandible to its resting position at the midline.

Summary

The temporomandibular joint (TMJ) is the synovial articulation between the mandibule and the temporal bone. The mandibular condyle articulates with the mandibular fossa but the two bones are separated by an articular disc. The TMJ is a highly versatile joint, allowing for protraction, retraction, elevation, depression and excursion of the mandible.

Key terms

TMJ, mandibular fossa, mandibular condyle, joint capsule, articular disc, temporomandibular ligament, stylomandibular ligament, sphenomandibular ligament, lateral pterygoid muscle, masseter muscle, temporalis muscle, articular eminence, articular tubercle, mandibular protraction, mandibular retraction, mandibular elevation, mandibular depression, mandibular excursion, mandibular rotation, mandibular translation

Figure credits

Figure 1 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 309, Public Domain, https://commons.wikimedia.org/w/index.php?curid=85394

Figure 2 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 310, Public Domain, https://commons.wikimedia.org/w/index.php?curid=85396

Figure 3 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 311, Public Domain, https://commons.wikimedia.org/w/index.php?curid=85397

Figure 4 by OpenStax College – Anatomy and Physiology. Chapter 9.5 Fig. 2. https://commons.wikimedia.org/wiki/File:Body_Movements_II.jpg