Synchondroses

1. Overview

Synchondroses are a type of cartilaginous joint where two bones are joined by hyaline cartilage. These joints are typically immovable (synarthroses) and are primarily found during growth and development stages. Some synchondroses are temporary and fuse over time, while others persist throughout life, serving structural and functional roles in the axial skeleton.

2. Location

Synchondroses occur in various regions, particularly in growing bones and parts of the thoracic and cranial base. Common examples include:

• Epiphyseal plates (growth plates): Between the epiphysis and diaphysis of long bones during development.

• First sternocostal joint: Between the first rib and the manubrium of the sternum (permanent synchondrosis).

• Spheno-occipital synchondrosis: Between the sphenoid and occipital bones in the cranial base (fuses in adolescence).

• Intra-pelvic synchondroses (in infants): Found temporarily between parts of developing hip bones.

3. Structure

Synchondroses are structurally simple yet biomechanically significant:

• Connecting tissue: Hyaline cartilage unites the two bones.

• Bone surfaces: Covered by smooth cartilage without a synovial cavity.

• No joint capsule: Unlike synovial joints, synchondroses lack a surrounding fibrous capsule.

• Vascularity: Limited blood supply in the cartilage; most nourishment comes via diffusion.

In growing bones, synchondroses are often part of the growth mechanism and later undergo ossification (endochondral fusion).

4. Function

Synchondroses provide both temporary and permanent mechanical functions:

• Allow bone growth: Epiphyseal plates enable longitudinal growth in long bones during childhood and adolescence.

• Provide stability: In permanent synchondroses like the first sternocostal joint, they ensure rigid yet slightly flexible connections.

• Transmit forces: Act as force-transmitting interfaces between adjacent skeletal elements (e.g., ribs and sternum).

5. Physiological role(s)

Synchondroses support critical physiological functions:

• Facilitate development: Essential in endochondral ossification during skeletal growth.

• Maintain thoracic shape: Provide slight flexibility to the upper rib cage for breathing while maintaining stability.

• Cranial base formation: Contribute to the alignment and shape of the skull during early development.

6. Clinical Significance

Though less commonly injured, synchondroses are involved in several clinical scenarios:

• Growth plate fractures:

  • In children and adolescents, trauma can damage epiphyseal synchondroses (Salter-Harris fractures), potentially affecting bone growth.

• Premature closure:

  • Early fusion of epiphyseal plates can lead to limb length discrepancies or skeletal deformities.

• Spheno-occipital fusion issues:

  • Abnormal fusion may contribute to craniofacial anomalies or interfere with cranial base development.

• Costochondritis (rarely involving first rib synchondrosis):

  • Inflammation near the first sternocostal synchondrosis may cause localized chest pain, though most costochondritis affects synovial joints of other ribs.

• Ossification and aging:

  • Many synchondroses ossify with age, reducing thoracic flexibility and contributing to skeletal rigidity in elderly individuals.