Floating Ribs

Floating ribs are the 11th and 12th pairs of ribs in the human rib cage. Unlike other ribs, they do not have any anterior attachment to the sternum or to other ribs. Instead, their anterior ends are free within the musculature of the posterior abdominal wall. Despite being small and short, floating ribs play a role in protecting the kidneys and supporting muscle attachment.

Location

Floating ribs are located at the inferior end of the rib cage. Each arises from a thoracic vertebra (T11 or T12) and extends laterally and slightly anteriorly into the posterior abdominal wall. They are situated below the false ribs (ribs 8–10) and form part of the posterior lower thoracic region.

Definition

Ribs are classified as "floating" if they do not attach to the sternum or to the costal cartilage of another rib. Only ribs 11 and 12 fall into this category.

Anatomical Features

Floating ribs are short, narrow, and lack many of the features present in typical ribs:

• Head: Articulates with the body of its corresponding thoracic vertebra (T11 or T12)
• No tubercle: These ribs do not articulate with the transverse processes of the vertebrae
• No neck: The neck region is absent or minimal
• No anterior articulation: The anterior end is free and embedded in abdominal wall musculature
• Smooth and rounded tip: Often embedded in muscle tissue (e.g., quadratus lumborum)

Articulations

• Posterior: Each floating rib articulates with its corresponding thoracic vertebra (T11 or T12)
• Anterior: No connection to the sternum or costal cartilage

Function

• Provide limited protection to upper abdominal organs, particularly the kidneys
• Serve as attachment sites for posterior abdominal wall muscles
• Contribute to thoracic cage flexibility and lower ribcage mobility

Muscle Attachments

• Quadratus lumborum: Inserts on the inferior border of rib 12
• Latissimus dorsi: May have fascial attachments in this region
• External and internal obliques: Attach near the tips of floating ribs

Blood Supply

• Arterial: Posterior intercostal arteries (branches of the thoracic aorta)
• Venous: Posterior intercostal veins draining into the azygos and hemiazygos veins

Innervation

• Intercostal nerves T11 and T12 (T12 is also called the subcostal nerve)

Clinical Relevance

• Fractures: Floating ribs are more mobile but less protected, making them susceptible to trauma
• Kidney protection: Rib 12 provides partial coverage for the superior pole of the kidneys
• Pain syndromes: Floating rib syndrome (also known as slipping rib syndrome) may cause pain due to hypermobility and irritation of intercostal nerves
• Surgical access: Rib 12 is often used as a landmark or access point in posterior approaches to the kidney (e.g., nephrectomy)