Human Anatomy - Kotsan I. Y. 2009

Skeleton of the Head
Joints of the Skull Bones

The BONES OF THE Skull are connected via almost all Types of bone joints: continuous joints, or synarthroses (fibrous, cartilaginous, and bony), and discontinuous joints, or diarthroses.

Among the fibrous JOINTS OF THE skull, we distinguish sutures (suturae), gomphoses (gomphosis), and interosseous membranes, known as fontanelles (fonticuli).

Sutures constitute the vast majority of fibrous joints in the skull. As a rule, the names of sutures derive from the bones they connect—for example, the frontonasal, frontozygomatic, and temporozygomatic sutures. However, certain sutures have their own unique names. For instance, the suture between the Frontal bone and the two parietal bones is called the coronal suture (sutura coronalis); the one between the Occipital bone and the two parietal bones is the lambdoid suture (sutura lambdoidea); and the suture between the paired parietal bones is the sagittal suture (sutura sagittalis).

Depending on their shape, sutures are classified into serrate (sutura serrata), squamous (sutura squamosa), and plane or harmonic (sutura plana) sutures. Serrate sutures connect almost all bones of the skull vault (coronal, lambdoid, and sagittal). The squamous suture connects the temporal and parietal bones, whereas plane (harmonic) sutures join the facial bones.

Gomphosis represents a specific type of fibrous joint. This term refers to the articulation between a tooth and the Bone tissue of the dental alveolus, where a thin layer of Connective Tissue lies between the tooth and the bone.

Interosseous membranes (fontanelles) are present in the skulls of newborns, reflecting The Development of the cranial vault bones from embryonic connective tissue. The anterior (frontal), posterior (occipital), sphenoid, and mastoid fontanelles are distinguished.

Cartilaginous joints of the skull, known as synchondroses, are subdivided into temporary and permanent. Temporary synchondroses exist in children between individual bone parts (for example, among the four PARTS OF THE occipital bone). Permanent synchondroses are located at the skull base between the petrous part of the Temporal bone and the basilar part of the occipital bone (synchondrosis petrooccipitalis), as well as the synchondrosis situated in the sphenopetrosal fissure between the petrous part of the temporal bone and the Sphenoid bone (synchondrosis sphenopetrosa).

In some individuals, apart from permanent sutures and synchondroses, additional inconstant variants may occur (such as a persistent metopic or frontal suture when both halves of the frontal squama fail to fuse, sagittal and transverse sutures in the occipital bone, or a synchondrosis between the squamous and lateral parts of the occipital bone). Inconstant skull bones—namely, sutural (Wormian) bones and fontanelle bones—may also appear within the sutures.

Between the ages of 20 and 30, Ossification of the sutures begins; Fibrous connective tissue is replaced by bone tissue, and such a bone-mediated joint is then termed a synostosis. Synostoses are categorized into primary synostoses, which develop at the site of fibrous connective tissue (fontanelles, sutures), and secondary synostoses, which arise in place of Cartilage (temporary synchondroses).

Discontinuous joints (diarthroses) in the skull occur exclusively at the articulation between the Mandible and the temporal bones, which are connected via the temporomandibular joint.

The temporomandibular joint (articulatio temporomandibularis) is formed by the HEAD of the mandible and the mandibular fossa of the temporal bone. It is a simple, complex, bicompartmental joint (containing an intra-articular disc) and is combined in function (since the right and left joints function simultaneously). By shape, it is ellipsoidal, and by function, it is triaxial (owing to the presence of the disc). The movements possible in this joint include depression and elevation of the mandible, protrusion, retrotrusion, and lateral movements (side-to-side). The joint is reinforced by the lateral, sphenomandibular, and stylomandibular ligaments. The lateral ligament (lig. laterale) extends from the Base of the zygomatic process of the temporal bone to the posterolateral surface of the neck of the mandible. The sphenomandibular ligament (lig. sphenomandibulare) originates from the spine of the sphenoid bone and attaches to the lingula of the mandible. The stylomandibular ligament (lig. stylomandibulare) spans from the styloid process of the temporal bone to the Medial surface of the posterior border of the mandibular ramus (near the mandibular angle).