Cranial joints

Classification of skull bone joints

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

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

Fibrous joints: types of sutures

The most common type of joint in the adult skull is sutures (suturae). These are specialized fibrous joints classified according to the spatial configuration and edge shape of the articulating bones. Several key types of sutures are distinguished:

1. Serrate suture (sutura serrata). The bone edges have pronounced teeth that tightly interlock, providing maximum mechanical strength. This type includes the sagittal and coronal sutures of the cranial vault.
2. Plane suture (sutura plana). Characterized by even, smooth edges of the contacting bones. This type of joint is most typical for the facial bones of the skull, where significant deforming loads are absent.
3. Squamous suture (sutura squamosa). Formed by the overlap 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 shape of the teeth. A prominent representative of this type is the lambdoid suture.
5. Schindylesis (schindylesis). A unique type of fibrous joint where the sharp edge of one bone wedges into a cleft or groove 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 profound changes during ontogenesis. In newborns, the bones of the cranial vault lack tight serrate sutures. Instead, broad areas of unossified membranous (fibrous) tissue are present — fontanelles (fonticuli cranii).

Several fontanelles are distinguished:

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

The presence of fontanelles is critical for the birth process, as they allow the skull bones to shift relative to each other (moulding of the head). Additionally, they ensure unhindered brain growth in the first months of life. As the child develops, the fibrous tissue of the fontanelles ossifies and they close, gradually transforming into typical sutures.

In the cranial 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 the longitudinal growth of the cranial base.

Formation of synostoses

Over time, as the organism ages, skull plasticity decreases. Almost all skull bone joints show a tendency to transform into synostoses — continuous bony fusions.

This process affects both cartilaginous and fibrous joints. 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 become overgrown with bone tissue. The transition into synostoses makes the skull of an adult and elderly person a monolithic framework, maximizing its strength and protective properties regarding the central nervous system.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Sagittal and coronal sutures: topography of serrate joints
• Anatomical structure of plane sutures of the facial cranium
• The process of ossification of the spheno-occipital synchondrosis
• Timing of closure of the anterior, posterior, and lateral fontanelles

Other topics in the "Arthrology" section

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