A synchondrosis is a type of joint in the human body. It is a cartilaginous joint, meaning bones are connected by cartilage, not fibrous tissue or a fluid-filled cavity. This arrangement allows for very limited or no movement, playing a role in growth and stability.
Defining Synchondrosis
It is a primary cartilaginous joint where bones are united by hyaline cartilage. This type of cartilage is smooth and glassy, providing a flexible yet firm connection between bones. Unlike highly mobile synovial joints, synchondroses lack a joint cavity and are primarily designed for growth and support rather than extensive movement.
Its main function is to facilitate bone growth, particularly in length, during development. This occurs through a process where the hyaline cartilage within the joint gradually expands and is then replaced by bone tissue. This mechanism is essential for the elongation and development of the skeletal system, especially in children and adolescents.
Common Synchondrosis Examples
One prominent example of a synchondrosis is the epiphyseal plate, commonly known as the growth plate, found in the long bones of the body. These plates are located between the shaft (diaphysis) and the ends (epiphysis) of growing bones. The hyaline cartilage within these plates continuously produces new bone tissue, which allows long bones to lengthen during childhood and adolescence.
Another example is the joint between the first rib and the sternum, known as the first sternocostal joint. Here, the first rib’s costal cartilage directly connects to the manubrium (the upper part of the sternum) via hyaline cartilage. This particular synchondrosis is considered a permanent type, as it typically does not ossify and maintains its cartilaginous connection throughout life, providing stability to the thoracic cage.
The spheno-occipital synchondrosis is located at the base of the skull, connecting the sphenoid bone and the occipital bone. This synchondrosis plays a significant role in the longitudinal growth of the skull base during childhood and adolescence. Its continued growth contributes to the overall elongation of the craniofacial skeleton, influencing the development of facial structures and the neurocranium.
The Temporary Nature of Synchondroses
Many synchondroses are temporary structures that exist primarily during periods of growth and development. As an individual matures, the hyaline cartilage within these joints undergoes a process called ossification, where it is gradually replaced by bone tissue. This transformation effectively fuses the bones that were once separated by cartilage.
Once ossification is complete, the synchondrosis becomes a synostosis, which is a completely fused, bony joint with no movement. For instance, the epiphyseal plates in long bones typically close and ossify by late adolescence or early adulthood, signaling the end of longitudinal bone growth. The spheno-occipital synchondrosis also typically undergoes fusion, often completing its ossification process between the ages of 16 and 25, though this can vary.