Ribs and Sternum Joints: Types, Classification, and Functional Significance

What is the Type of Joint Between Ribs and Sternum?

The joints between the ribs and the sternum are primarily a combination of cartilaginous (synchondrosis) and synovial (plane/gliding) joints, with the specific classification depending on the particular rib and its attachment point.

Understanding Thoracic Articulations

The human rib cage is a complex structure designed to protect vital organs and facilitate respiration. Its intricate framework involves several types of joints that allow for both stability and necessary flexibility. When discussing the connection between the ribs and the sternum (breastbone), it's crucial to differentiate between two primary sets of articulations: the costochondral joints and the sternocostal (or chondrosternal) joints.

Joint Classifications: A Quick Review

To fully appreciate the rib-sternum connections, a brief understanding of joint classifications is helpful:

• Fibrous Joints (Synarthroses): Immobile joints where bones are joined by fibrous connective tissue (e.g., sutures of the skull).
• Cartilaginous Joints (Amphiarthroses): Joints where bones are united by cartilage.
  • Synchondrosis (Primary Cartilaginous Joint): Bones joined by hyaline cartilage, typically temporary and immobile (e.g., epiphyseal plates, first sternocostal joint).
  • Symphysis (Secondary Cartilaginous Joint): Bones joined by fibrocartilage, allowing limited movement (e.g., pubic symphysis, intervertebral discs).
• Synovial Joints (Diarthroses): Freely movable joints characterized by a joint capsule, synovial fluid, and articular cartilage (e.g., knee, shoulder, most sternocostal joints).

The Costochondral Joints

These joints are found where the bony portion of each rib meets its respective costal cartilage. The costal cartilages are bars of hyaline cartilage that extend from the anterior ends of the ribs.

• Type of Joint: All costochondral joints are classified as synchondroses, a type of primary cartilaginous joint.
• Structure: Here, the bone of the rib directly fuses with the hyaline cartilage. There is no joint capsule, synovial fluid, or significant movement.
• Function: While essentially immobile, the presence of cartilage here is critical for the overall elasticity and flexibility of the rib cage, allowing it to expand and contract during breathing without the bones directly rubbing against each other.

The Sternocostal Joints (Chondrosternal Joints)

These are the joints formed between the costal cartilages and the sternum. Their classification varies depending on the specific rib.

• First Sternocostal Joint:

  • Type of Joint: This is a synchondrosis (primary cartilaginous joint). The first costal cartilage directly articulates with the manubrium (the upper part of the sternum) via hyaline cartilage.
  • Movement: This joint is largely immobile, providing a stable anchor for the upper rib cage.

• Second to Seventh Sternocostal Joints:

  • Type of Joint: These are predominantly synovial plane (gliding) joints.
  • Structure: Each joint possesses a fibrous capsule, a synovial membrane, synovial fluid, and articular cartilage covering the articulating surfaces of the costal cartilage and the sternum.
  • Ligaments: They are reinforced by radiate sternocostal ligaments anteriorly and posteriorly, which fan out from the costal cartilage to the sternum. The second sternocostal joint is unique in having an additional intra-articular sternocostal ligament that divides the joint cavity into two, connecting the second costal cartilage to the sternum at the junction of the manubrium and body.
  • Movement: These synovial joints allow for slight gliding and rotational movements, which are crucial for the expansion and contraction of the thoracic cavity during respiration.

• Ribs 8, 9, and 10: These ribs do not directly articulate with the sternum. Their costal cartilages connect to the costal cartilage immediately above them, forming interchondral joints, which are also synovial plane joints.

Functional Significance in Respiration

The combination of fixed cartilaginous joints and slightly mobile synovial joints in the thoracic cage is perfectly engineered for breathing:

• Elasticity: The costal cartilages provide elasticity, allowing the rib cage to spring back after muscle contraction during exhalation.
• Movement for Ventilation: The synovial sternocostal joints, along with the joints between the ribs and vertebrae (costovertebral and costotransverse joints), facilitate the "pump handle" and "bucket handle" movements of the rib cage.
  • Pump Handle Movement: Primarily involves the upper ribs (2-6) and leads to an increase in the anterior-posterior diameter of the thorax.
  • Bucket Handle Movement: Primarily involves the lower ribs (7-10) and leads to an increase in the transverse (side-to-side) diameter of the thorax. These movements collectively increase the volume of the thoracic cavity, reducing intrathoracic pressure and drawing air into the lungs during inspiration.

Clinical Relevance

Understanding these joints is vital for diagnosing various conditions:

• Costochondritis: Inflammation of the costal cartilages, particularly at the sternocostal junctions, causing localized chest pain.
• Tietze's Syndrome: A rare inflammatory condition involving swelling of one or more costal cartilages, most commonly affecting the second or third sternocostal joint.
• Rib Fractures: While the costal cartilages are flexible, severe trauma can lead to fractures of the ribs or separations at the costochondral or sternocostal junctions.

Conclusion

The joints between the ribs and the sternum represent an elegant interplay of anatomical design and physiological function. From the stable, yet elastic, synchondroses of the costochondral and first sternocostal joints to the subtly mobile synovial plane joints of the lower sternocostal articulations, each plays a critical role in providing protection, structural integrity, and the dynamic flexibility essential for the mechanics of breathing. This intricate system underscores the sophisticated engineering of the human body.