Joints of the Skull Bones

Classification of Skull Joints

Globally, all joints of the head can be divided into several basic categories depending on the type of tissue located between the connecting bone surfaces. According to anatomical nomenclature, the following main types of articulations are distinguished:

• Fibrous joints: Composed of dense fibrous connective tissue. In the skull, these are represented primarily by sutures.
• Cartilaginous joints: Formed by strong cartilage tissue (synchondroses), which is often temporary.
• Synovial joints: Classic interrupted joints (e.g., temporomandibular joint) possessing a joint cavity.
• Bony joints: The result of complete replacement of connective or cartilage tissue by bone (synostoses).

Fibrous Joints: Types of Sutures

The most common type of joint in the adult skull is the suture (sutura). These are specialized fibrous joints classified based on the spatial configuration and shape of the articulating bone margins. Several key varieties of sutures are distinguished:

1. Serrate suture (sutura serrata): Bone margins have prominent interlocking teeth that fit tightly together, providing maximum mechanical strength. Examples include the sagittal and coronal sutures of the cranial vault.
2. Plane suture (sutura plana): Characterized by even, smooth edges of contacting bones. This type of articulation is most typical for the bones of the facial skeleton, where significant deforming loads are absent.
3. Squamous suture (sutura squamosa): Formed by the overlapping of a pointed, bevelled edge of one bone over the edge of another, resembling fish scales. A classic example is the junction between the parietal and temporal bones.
4. Denticulate suture (sutura denticulata): A structural variant of the serrate suture, but with a more complex tooth shape. A prominent representative of this type is the lambdoid suture.
5. Schindylesis: A unique variant of a fibrous joint in which the sharp edge of one bone wedges into a cleft or sulcus of another. In the human skull, this type occurs strictly between the vomer and the sphenoid bone.

Age-Related Features: Fontanelles and Cartilage

The anatomy of skull bone joints undergoes massive changes during ontogenesis. In newborns, the bones of the cranial vault lack tight serrate sutures. Instead, wide areas of unossified membranous (fibrous) tissue are present—the fontanelles (fonticuli cranii).

Fontanelles include:

• Unpaired anterior fontanelle
• Unpaired posterior fontanelle
• Paired lateral fontanelles

Preserving fontanelles is critical for labor, allowing cranial bones to shift relative to one another (molding of the head). Additionally, they ensure unrestricted brain growth during the first months of life. As the child develops, the fibrous tissue of the fontanelles ossifies and closes, gradually transforming into typical sutures.

In the skull base of children and adolescents, specific cartilaginous joints are also present. The most important of these is the spheno-occipital synchondrosis—a layer of cartilage tissue ensuring longitudinal growth of the cranial base.

Formation of Synostoses

Over time, as the body ages, skull plasticity decreases. Almost all skull bone joints demonstrate a tendency to transition into synostoses—continuous bony fusions.

This process affects both cartilaginous and fibrous articulations. For example, the temporary spheno-occipital synchondrosis eventually ossifies completely, firmly fusing the sphenoid and occipital bones into a single basilar structure. Similarly, the sutures of the cranial vault gradually overgrow with bone tissue. The transition into synostoses makes the adult and elderly skull a monolithic framework, maximizing its strength and protective properties for the central nervous system.