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Joints of the Thoracic Cage

Q: What structures form the costal margin?

The costal margin (arcus costalis) is formed by the costal cartilages of the 7th, 8th, 9th, and 10th ribs. This structure is built by the false ribs (costae spuriae — pairs 8–10), whose cartilages do not reach the sternum directly but instead attach sequentially to the cartilage of the rib immediately superior. The convergence of the right and left costal margins at the xiphoid process forms the infrasternal angle (angulus infrasternalis).

Compages thoracis

For medical students2 min readUpdated 2026-10-10

The thoracic cage forms a strong yet mobile framework whose joints play a critical role in the biomechanics of respiration. This complex is primarily composed of joints connecting the ribs posteriorly to the vertebral column and anteriorly to the sternum.

FunctionFacilitating rib rotation during respiratory excursions

Floating ribsThe 11th and 12th ribs lack anterior articulations and end in the abdominal musculature

SynchondrosisThe type of cartilaginous joint attaching the 1st rib cartilage to the sternum

Costovertebral Joints

The posterior thorax exhibits high mobility due to paired combined joints at each vertebral level.

Joint of head of rib (articulatio capitis costae). Formed by the articular surfaces of the rib heads and corresponding costal facets on the vertebral bodies. The joint capsule is tightly fixed strictly along the margins of the articular surfaces. For extra stability, this complex is reinforced by the intra-articular ligament of head of rib (lig. capitis costae intraarticulare) and the radiate ligament (lig. capitis costae radiatum).
Costotransverse joint (articulatio costotransversaria). This articulation is located between the tubercle of the rib and the transverse process of the corresponding vertebra. Its stability is maintained by a robust ligamentous apparatus, including the superior, lateral, and accessory costotransverse ligaments (ligg. costotransversaria superius, laterale et proprium).

Working together, these two articulations form a single axis around which the rib rotates during inspiration and expiration.

Anterior Connections: Ribs and Sternum

Anteriorly, the ribs transition into cartilage and form articulations with completely different biomechanics.

Costochondral joints (articulationes costochondrales). These are the sites where the osseous part of the rib transitions into the cartilaginous part. They are completely immobile.
Sternocostal joints (articulationes sternocostales). Characteristic of the 2nd through 7th ribs. These are true synovial joints securely reinforced externally by radiate sternocostal ligaments (ligg. sternocostalia radiata). An important exception is the 1st rib cartilage, which forms a continuous cartilaginous joint with the sternum — a synchondrosis.
Interchondral joints (articulationes interchondrales). Found between the cartilaginous ends of the false ribs (7th to 10th).

The lowest ribs (11th and 12th) do not reach the sternum at all. They remain free, with their anterior ends embedded directly into the musculature of the abdominal wall.

Intrinsic Ligaments and Age-Related Changes of the Sternum

The sternum itself has a complex structure and fixation system for its segments. In the region where the manubrium transitions to the body, an intra-articular ligament (lig. sterni intraarticulare) is located. Externally, the bone is covered by a dense network of radiate sternal ligaments (ligg. sterni radiata).

With age, the cartilaginous layers between individual fragments of the sternum undergo ossification. This results in the formation of synostoses—solid bony fusions that transform the sternum into a single monolithic bone.

Mnemonic

To remember the ligaments of the costotransverse joint, use the formula "SLS": Superior, Lateral, Superior (proper/proprium) — referring to the lig. costotransversarium superius, laterale et proprium.

Frequently asked questions

What articular surfaces form the joint of head of rib for ribs 2–10?

The joint of head of rib for ribs 2–10 is formed by the articular surface of the head of the rib and the costal demifacets of two adjacent vertebral bodies.

Articular surface of head of rib (facies articularis capitis costae) — divided by the crest of head of rib (crista capitis costae) into superior (smaller) and inferior (larger) facets.
Superior and inferior costal demifacets (fovea costalis superior and fovea costalis inferior) — located on the bodies of adjacent thoracic vertebrae. The superior part of the head articulates with the inferior demifacet of the vertebra above, and the inferior part articulates with the superior demifacet of the vertebra below.

On which vertebrae is the costotransverse joint absent?

The costotransverse joint is absent on the eleventh and twelfth thoracic vertebrae (T₁₁–T₁₂). The transverse processes of these vertebrae lack costal facets (fovea costalis processus transversi), because the corresponding 11th and 12th ribs have a simpler structure and lack a rib tubercle (tuberculum costae). Consequently, no articulation with the transverse processes occurs at these levels.

What is the anatomical name of the joint between the manubrium and the body of the sternum?

Anatomically, the joint between the manubrium and the body of the sternum is called the manubriosternal symphysis (symphysis manubriosternalis). This articulation forms an obtuse sternal angle (angulus sterni) facing posteriorly, which is easily palpable on the anterior chest wall. An intra-articular sternal ligament (lig. sterni intraarticulare) is located at this junction. With age, the cartilaginous layers between sternal segments may ossify, forming synostoses.

What structures form the costal margin?

The costal margin (arcus costalis) is formed by the costal cartilages of the 7th, 8th, 9th, and 10th ribs. This structure is built by the false ribs (costae spuriae — pairs 8–10), whose cartilages do not reach the sternum directly but instead attach sequentially to the cartilage of the rib immediately superior. The convergence of the right and left costal margins at the xiphoid process forms the infrasternal angle (angulus infrasternalis).

Which joints are responsible for thoracic cage excursion during breathing?

Rib rotation during respiration is driven by the combined action of the joint of head of rib and the costotransverse joint.

How does the attachment of the first rib to the sternum differ from the other true ribs?

The first rib articulates with the sternum via a synchondrosis (cartilaginous union), whereas ribs 2 through 7 form mobile synovial sternocostal joints.

Where do the eleventh and twelfth ribs terminate?

They are classified as floating ribs, lacking anterior connections to the sternum or other ribs, and terminate directly within the abdominal muscles.

Go deeper

Structure of the intra-articular ligaments of the head of the rib
Biomechanics of sternocostal joints in true ribs
Formation and anatomy of interchondral joints
Age-related sternal synostoses