HUMAN ANATOMY ATLAS - G.L. Bilich - 2014

System of Articulations

JOINTS OF THE SKULL BONES

The BONES OF THE Skull are interconnected primarily by continuous joints—sutures and synchondroses. Only the Mandible forms a paired temporomandibular joint with the Temporal bone.

The temporomandibular joint (articulatio temporomandibularis) is formed by the mandibular fossa of the temporal bone and the HEAD of the condylar process of the mandible (Figs. 122, 123; Tab. 23). The articular tubercle is located anterior to the fossa. An oval-shaped biconcave articular disc (discus articularis), composed of fibrocartilage, lies between the articular surfaces, dividing the joint cavity into two compartments: superior and inferior. In the superior compartment, the articular surface of the temporal bone articulates with the upper surface of the articular disc. The synovial membrane of this compartment (membrana synovialis superior) lines the inner surface of the capsule and attaches along the margins of the articular Cartilage. In the inferior compartment, the head of the mandible articulates with the lower surface of the articular disc. The synovial membrane of the inferior compartment (membrana synovialis inferior) lines not only the capsule but also the posterior surface of the neck of the condylar process, which is enclosed within the capsule.

The loose articular capsule attaches anteriorly to the articular tubercle on the temporal bone and posteriorly at the level of the petrotympanic fissure. On the condylar process, the articular capsule attaches anteriorly along the margin of the head and posteriorly 0.5 cm below the head of the mandible. The articular capsule is fused circumferentially with the articular disc. The capsule is thin anteriorly, whereas posteriorly it thickens and is reinforced by several ligaments (Fig. 124). The fan-shaped lateral ligament (ligamentum laterale), which reinforces the Joint Capsule laterally, originates from the Base of the zygomatic process of the temporal bone. The fibers of this ligament run posteriorly and inferiorly, attaching to the posterolateral surface of the neck of the condylar process. The sphenomandibular ligament (ligamentum sphenomandibulare), located on the medial side of the joint, originates from the spine of the Sphenoid bone and attaches to the lingula of the mandible. The stylomandibular ligament (ligamentum stylomandibulare) originates from the styloid process of the temporal bone and attaches to the inner surface of the posterior border of the ramus of the mandible near its angle. This ligament is situated medially and posteriorly to the temporomandibular joint. Both of these extra-capsular ligaments are separated from the joint capsule by adipose tissue.

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Fig. 122. Right temporomandibular joint. Lateral view, zygomatic arch removed, joint capsule opened:

1 — Stylomandibular ligament; 2 — Hcad of mandible; 3 — Aiticular tuberclc; 4 — Joint capsule; Articular cap- sule; 5 — Articular disc; 6 — Temporal bone, squamous part; 7 - Lateral pterygoid, upper hcad; superior liead; 8 — Lateral pterygoid, lowerhead; inferior head; 9 — Medial pterygoid; 10— Buccinator

Fig. 123. Left temporomandibular joint, capsule opened, all surrounding Tissues removed, lateral view:

1 — Articular tubercle; 2 — Joint capsule; Articular capsulc; 3 — Articular disc; 4— Head of mandible; 5— Stylomandibular ligament

The temporomandibular joint is an ellipsoid, complex, biaxial, combined joint. The right and left joints function synchronously, executing movements around the vertical and frontal axes. Around the frontal axis, the mandible elevates and depresses; around the vertical axis, it performs lateral side-to-side movements to the right and left. Due to the extensive articular surface on the temporal bone, the articular processes, along with the entire mandible, can glide forward and backward. During anterior mandibular movement, the condylar processes shift onto the articular tubercles, returning to their initial position in the mandibular fossae upon posterior movement.

When the mandible is depressed, the mental protuberance moves downward and slightly backward, tracing an arc concave posteriorly and superiorly. This movement comprises three phases. In The first phase (slight depression of the mandible), movement around the frontal axis occurs exclusively within the inferior compartment of the joint, while the articular disc remains in the mandibular fossa. In the second phase (pronounced depression), while hinge movement of the condylar heads continues in the lower compartment, the cartilaginous disc and the condylar head glide forward onto the articular tubercle. The condylar processes of the mandible displace anteriorly by approximately 12 mm. In the third phase (maximum depression), movement takes place solely in the inferior compartment around the frontal axis, with the articular disc positioned on the articular tubercle.

Further forceful Mouth opening may cause the mandibular head to slip anteriorly past the articular tubercle into the infratemporal fossa, resulting in dislocation of the temporomandibular joint. The Mechanism of mandibular elevation reverses the stages of depression. When the mandible is protruded, movement occurs exclusively within the superior compartment of the joint. The articular processes, together with the articular discs, glide forward onto the tubercles in both the right and left temporomandibular joints.

During lateral Displacement of the mandible, movements in the right and left temporomandibular joints differ. For instance, when the mandible moves to the right, the condylar head and disc in the left temporomandibular joint glide forward onto the articular tubercle, meaning gliding occurs in the superior compartment. Simultaneously, in the right joint, the condylar head rotates around the vertical axis passing through the neck of the condylar process. When the mandible moves to the left, anterior gliding of the head and articular disc occurs in the right joint, while rotation around the vertical axis takes place in the left joint.

Fig. 124. Extra-capsular ligaments of the right temporomandibular joint, medial view:

1 — Pterygoid process, laleral piate; 2 —- Pterygospinotis ligament; 3 — Mandibular notch; 4 — Stylomandibular ligament; 5 — Sphenomandibular ligament; 6 — Pterygoid process, medial piate

Table 23. Temporomandibular joint

Depression of the mandible is effected by the contraction of the paired digastric, mylohyoid, and geniohyoid muscles. Elevation of the jaw is similarly performed by the paired temporalis, masseter, and medial pterygoid muscles. Protraction is driven by the lateral pterygoid muscles and anterior fascicles of the masseters, whereas retraction (return to the initial position) is handled by the lower (posterior) fascicles of the temporalis muscles. Lateral excursions of the mandible to the right and left are executed by the unilateral contraction of the contralateral lateral pterygoid muscle.

The capsule of the temporomandibular joint is vascularized by Branches of the maxillary artery; venous Blood drains into the periarticular venous plexus and subsequently into the retromandibular vein. Lymphatic drainage proceeds to the deep parotid and ultimately to the deep cervical Lymph Nodes. Innervation is supplied by the auriculotemporal nerve (a branch of the mandibular nerve).

Continuous joints of the skull bones are represented by fibrous joints—sutures in adults and interosseous membranes (syndesmoses) in newborns. Synchondroses are present in the region of the skull base.

The bones of the cranial vault are interconnected via serrate and squamous sutures. The medial borders of the parietal bones are joined by the serrate sagittal suture (sutura sagittalis). The anterior borders of the parietal bones articulate with the posterior border of the Frontal bone via the serrate coronal suture (sutura coronalis). The posterior borders of the parietal bones form the serrate lambdoid suture (sutura lambdoidea) with the anterior border of the Occipital bone. The squamous part of the temporal bone articulates with the Parietal bone and the greater wing of the sphenoid bone, forming the squamous suture (sutura squamosa). In the facial Skeleton, sutures are smooth, flat, or plane (Tab. 24).

The cranium also features non-constant (temporary) sutures that form As a result of delayed fusion or non-fusion of individual ossification centers. For instance, the upper part of the occipital squama is sometimes separated wholly or partially from the rest of the occipital bone by a transverse suture. Occasionally, the two halves of the frontal bone fail to fuse. In such cases, the sagittal suture (known as the metopic suture) extends from the glabella or slightly above it. When an intermaxillary or incisive bone is present, an incisor suture is formed. Doubling of the parietal bone results in an interparietal suture. Additionally, accessory sutures may occur, such as the squamomastoid and sphenomandibular sutures (between the pterygoid process of the sphenoid bone and the body of the Maxilla). In the second half of human life, most sutures undergo complete or partial obliteration, during which the Connective Tissue membrane between the cranial bones is replaced by Bone tissue.

Cartilaginous joints, or synchondroses, of the skull (Table 25) are located in the region of the cranial base and are formed by fibrocartilage. Typically, human cartilage tissue is replaced by bone with age. By the age of 20, the spheno-occipital synchondrosis is replaced by a synostosis.

Table 24. Plane Sutures of the Skull

Table 25. Synchondroses of the Skull