The Thoracic Cage

Core Concepts

The sternum: manubrium, body, and xiphoid

The sternum is a flat, dagger-shaped bone in three parts:

• Manubrium — the wide upper piece. Its jugular (suprasternal) notch is the dip you can feel at the base of the throat; its clavicular notches articulate with the clavicles. Rib 1 attaches here, and the costal cartilage of rib 2 attaches at the manubriosternal junction.
• Body (gladiolus) — the long middle piece; costal cartilages of ribs 2–7 attach along its edges.
• Xiphoid process — the small, inferior tip. It attaches no ribs, starts as cartilage, and commonly ossifies later in life (the timing varies). It lies near the level of vertebra T9–T10 (commonly taught) and can be injured by forceful compression — a key reason CPR hand placement avoids it.

The sternal angle (angle of Louis) is the palpable ridge where manubrium meets body; it marks the second rib and, in the mediastinum, the level where the trachea divides (commonly taught as T4–T5). Count ribs from this landmark, not from the clavicle, because rib 1 is largely hidden behind the clavicle.

The ribs: true, false, and floating

All 12 pairs of ribs articulate posteriorly with the thoracic vertebrae (costovertebral joints). They are classified by their anterior attachments:

• True (vertebrosternal) ribs, 1–7: each attaches directly to the sternum by its own costal cartilage.
• False (vertebrochondral) ribs, 8–10: their cartilages do not reach the sternum; each joins the cartilage of the rib above, ultimately connecting through rib 7's cartilage.
• Floating (vertebral) ribs, 11–12: no anterior attachment at all — they end in the body wall muscles.

A typical rib has a head (two facets that articulate with the bodies of two adjacent thoracic vertebrae), a neck, a tubercle (articulates with the transverse process of the same-numbered vertebra — ribs 11–12 usually lack this joint), and a curved shaft. Along the inferior inner edge of the shaft runs the costal groove, which shelters the intercostal nerve, artery, and vein (the neurovascular bundle) — one reason procedures are performed at the upper border of a rib.

Joints and cartilage: how the cage stays flexible

The ribs meet the vertebrae at costovertebral joints (head ↔ vertebral bodies) and costotransverse joints (tubercle ↔ transverse process). In front, costochondral joints join bone to costal cartilage, and sternocostal joints join cartilage to the sternum; the false ribs connect to each other at interchondral joints. The costal cartilages are hyaline cartilage that keeps the anterior chest springy — they also commonly calcify with age, which is why older adult chests feel less flexible (a normal age-related change).

The cage in breathing: pump handles and bucket handles

Breathing is a volume game: air enters when the thoracic cavity expands. The cage is built to expand in two directions (commonly taught model). The upper ribs (2–6) act like pump handles — their anterior ends rise, pushing the sternum forward and increasing the front-to-back (anteroposterior) diameter. The lower ribs (7–10) act like bucket handles — they swing upward and outward, increasing the side-to-side (transverse) diameter. The intercostal muscles run between adjacent ribs and help lift them, while the diaphragm (the primary muscle of inspiration) descends, enlarging the vertical dimension. Every inspiration, the cage widens in all three directions; every expiration, it recoils.

Landmarks and clinical relevance

The sternal angle → rib 2 → count down: this is how chest X-rays and physical exams locate rib levels, and it is why the angle is the most tested sternal landmark. The xiphoid process is avoided during chest compressions because it is thin and easily fractured. Rib fractures are the most common significant chest injury (commonly taught); multiple adjacent ribs fractured in two or more places can produce a flail segment that moves paradoxically with breathing (flail chest) — a serious condition requiring emergency care. Structural variations such as pectus excavatum (sunken sternum) and pectus carinatum (pigeon chest) are developmental differences that range from cosmetic to functionally significant and need professional assessment.

How It Works / Step-by-Step Process

Counting ribs on a chest X-ray (or finding an intercostal space in a physical exam) is a standard routine:

1. Find the sternal angle — the ridge where the manubrium meets the body (visible on the lateral view, palpable in a thin person).
2. Identify rib 2 at that level, then count down along the anterior chest.
3. Skip rib 1 if you started at the clavicle — it hides behind the clavicle and is easy to miss.
4. Label the rib level and side (e.g., "left 5th intercostal space, midaxillary") — the language clinicians use to describe chest findings and procedure sites.

Common Confusions