Human Anatomy - Kotsan I. Ya. 2009

Bones and Their Joints
Continuous joints of bones - synarthroses

In its development, the human Skeleton passes through three stages: Connective Tissue, cartilaginous, and bony. Since the transition from one stage to another is also associated with Changes in the tissue located in the gaps between the bones, the articulation of bones goes through the exact same three phases in its development, resulting in Three types of synarthroses:

I. If connective tissue remains in the space between the bones after birth, the bones will be joined by means of connective tissue. This type of connection is called a syndesmosis.

II. If the connective tissue in the space between the bones transforms into Cartilage, which persists after birth, the bones will be joined by means of cartilaginous tissue. This type of connection is termed a synchondrosis.

III. If the connective tissue between the bones transforms into Bone tissue—either directly or via an intermediate cartilaginous stage—the bones will be joined by means of bone tissue. This type of connection is called a synostosis.

The Nature of bone connections is not static throughout an individual's lifetime. Corresponding to the three stages of ossification, syndesmoses can transition into synchondroses and synostoses. The latter represent the final phase of skeletal development.

Syndesmosis (syndesmosis), or fibrous connection, is a continuous articulation of bones by means of connective tissue. These include connective tissue membranes (membranae), ligaments (ligamenta), gomphosis, and sutures (suturae).

1. If connective tissue occupies a large space between the bones, such a connection takes the form of interosseous membranes, for example, between the BONES OF THE forearm or leg.

2. If the intermediate connective tissue resembles fibrous bundles in its Structure, fibrous ligaments are formed (such as the ligaments of THE Vertebral Column).

3. If one bone is essentially driven into the substance of another with a thin layer of connective tissue securing them in between, this type of connection is called a gomphosis (dentoalveolar syndesmosis).

4. If the intermediate connective tissue takes the form of a very thin layer between the adjoining bones, sutures are formed. Examples include the sutures between the Skull bones. According to the shape of the articulating bone edges, the following types of sutures are distinguished:

a) serrate suture (sutura serrata) — when the Teeth on the edge of one bone fit into the gaps between the teeth of another (found between most bones of the cranial vault: coronal, sagittal, and lambdoid sutures);

b) squamous suture (sutura squamosa) — when the bevelled edge of one bone overlaps the matching edge of another like fish scales (between the edges of the temporal and parietal bones);

c) plane suture (sutura plana) — the harmonious apposition of non-serrated bone edges (between most bones of the facial skeleton).

Synchondrosis (synchondrosis), or cartilaginous connection, is a continuous articulation by means of cartilaginous tissue. Movements at a synchondrosis are limited and spring-like in character. They depend on the thickness of the cartilage layer: the thicker the layer, the greater the mobility.

Depending on the type of cartilage (hyaline or fibrocartilage), the following are distinguished:

a) hyaline synchondrosis (for example, between the first rib and the Sternum);

b) fibrocartilaginous synchondrosis. The latter occurs where there is high resistance to mechanical stress (for example, between the vertebral bodies. Here, fibrocartilaginous synchondroses act as buffers due to their elasticity, absorbing shocks and jolts.

Depending on their duration of existence, synchondroses are classified as:

1. Temporary — they exist only up to a certain age, after which they are replaced by synostoses. For example, the synchondroses between the three bones that form the hip bone.

2. Permanent — they persist throughout life. For example, the intervertebral discs in the cervical, thoracic, and lumbar Regions of the spine.

Synostosis (synostosis), or bony connection, is a continuous articulation of bones by means of bone tissue. If the actual connective tissue (skull sutures) or cartilaginous tissue (connections between the sacral vertebrae) between the articulating bones transforms into bone tissue, the bones become united by bone tissue.