Bones and joints of thorax in detail

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Bones and Joints of the Thorax

The thoracic wall is built from four skeletal elements: thoracic vertebrae (12), intervertebral discs, ribs (12 pairs), and the sternum. Together they form the thoracic cage, which protects the thoracic viscera and changes volume during breathing.

1. Thoracic Vertebrae

There are 12 thoracic vertebrae (T1-T12), each uniquely characterized by articulations with ribs.

Typical Thoracic Vertebra (T2-T9)

Body: Heart-shaped, roughly equal in transverse and anteroposterior dimensions
Vertebral foramen: Circular (smaller than cervical)
Spinous process: Long, pointing steeply downward
Laminae: Broad and overlapping with those of the vertebra below
Superior articular processes: Flat, articular surfaces facing almost directly posteriorly
Inferior articular processes: Project from laminae, articular facets facing anteriorly
Transverse processes: Club-shaped, projecting posterolaterally

Costal Facets on a Typical Vertebra

Each typical thoracic vertebra has three articulation sites per side for ribs:

Superior costal demifacet (on body) - for part of the head of its own rib
Inferior costal demifacet (on body) - for part of the head of the rib below
Transverse costal facet (on transverse process) - for the tubercle of its own rib

Atypical Thoracic Vertebrae

Vertebra	Special Feature
T1	Superior costal facet is complete (full facet, not a demifacet) for the entire head of rib I; also has inferior demifacet for rib II
T10 (often T9)	Only single complete costal facet per side; no inferior demifacets
T11 & T12	Only one complete costal facet per side; no transverse costal facets

Gray's Anatomy for Students, p. 179-180

2. Ribs

There are 12 pairs of ribs, all articulating posteriorly with the vertebral column and each ending anteriorly in a costal cartilage.

Thoracic cage showing true ribs, false ribs, floating ribs, costal cartilages, and intercostal spaces

Classification

Class	Ribs	Anterior attachment
True ribs	I-VII	Costal cartilages attach directly to sternum
False ribs	VIII-XII	Do NOT attach directly to sternum
- Vertebrochondral	VIII-X	Costal cartilages join the cartilage of the rib above
- Floating ribs	XI-XII	No anterior connection to ribs or sternum

Structure of a Typical Rib (Ribs III-IX)

Each rib has a posterior end (articulating with vertebrae) and an anterior end (continuing as costal cartilage). The bony segments from posterior to anterior are:

1. Head

Somewhat expanded, typically presenting two articular facets separated by a crest
Smaller superior facet: articulates with the inferior costal demifacet of the vertebra above
Larger inferior facet: articulates with the superior costal demifacet of its own vertebra
The crest connects via an intra-articular ligament to the adjacent intervertebral disc

2. Neck

Short, flat region of bone connecting the head to the tubercle
Bears a sharp superior ridge (crest of the neck) on ribs 2-12
Directed posterosuperiorly

3. Tubercle

At the junction of neck and shaft
Has an articular facet (for the transverse process of its own vertebra) medially
Has a non-articular roughened area (for ligament attachment) laterally

4. Shaft (Body)

Curved, with the sharpest bend at the costal angle (just lateral to the tubercle)
On its inferior inner surface: the costal groove - protects the intercostal vessels and nerves (absent on ribs 1, 11, 12)
Shafts of ribs 2-12 are twisted about their long axis: external surfaces face slightly downward at the vertebral end and slightly upward at the anterior end

Rib lengths: Rib 1 and 12 are the shortest; rib 7 is the longest. Costal cartilage increases in length from rib 1 to 7, then shortens again.

Atypical Ribs

Rib I

Broadest and most curved; lies in an almost horizontal plane
Flat (superior/inferior surfaces rather than lateral/medial)
Has a scalene tubercle on its superior surface (for scalenus anterior)
Grooves for the subclavian artery and vein on either side of the scalene tubercle
Head has a single facet (articulates only with T1)
No costal groove

Rib II

Flat like rib I, but twice as long
Articulates with the vertebral column in a typical fashion (with T1 and T2)

Rib X

Head has a single facet for articulation with only its own vertebra (T10)

Ribs XI and XII (Floating Ribs)

Articulate only with the bodies of their own vertebrae
No tubercles, no necks (short and pointed anteriorly)
No articulation with transverse processes
Gray's Anatomy for Students, p. 180-182; THIEME Atlas of Anatomy, p. 156

3. Sternum

The sternum is a flat, elongated bone lying in the midline of the anterior chest wall. It is slightly convex anteriorly and has multiple costal notches along its lateral borders.

Sternum - lateral view showing manubrium, sternal angle, body, xiphoid process, and articular facets for ribs

The adult sternum has three parts:

A. Manubrium

The broadest, most superior part
Superior border: wide, with a palpable jugular notch (suprasternal notch) in the midline
On each side of the jugular notch: a large oval clavicular notch for articulation with the clavicle
Immediately inferior to the clavicular notch on each side: a first costal notch (synchondrosis with rib I)
Lower lateral border: a demifacet for the upper half of the second costal cartilage
Clinical: The subcutaneous position of the sternum allows sternal bone marrow aspiration for diagnosis of blood diseases like leukemia

B. Body of the Sternum

Flat; formed by the fusion of four embryonic sternebrae (visible as transverse ridges on the anterior surface)
Lateral borders bear costal notches for ribs III to VII
At its superior end: a demifacet completing the articulation for the second costal cartilage (paired with the manubrium's demifacet)
At its inferior end: a demifacet for the upper part of the seventh costal cartilage

C. Xiphoid Process

Smallest and most inferior part
Highly variable shape: may be wide, thin, pointed, bifid, curved, or perforated
Begins as cartilage, ossifies in adulthood (but may remain cartilaginous in some adults)
Upper lateral margin on each side: a demifacet for the seventh costal cartilage
Has no direct costal cartilage attachments of its own

Sternal Angle (Angle of Louis)

Formed at the junction of manubrium and body of sternum - a slight posterior angulation
A key clinical landmark: marks the level of the 2nd costal cartilage (the second rib is the most superior palpable rib), the bifurcation of the trachea (T4/T5), and the superior border of the pericardium
Gray's Anatomy for Students, p. 182-183; THIEME Atlas of Anatomy, p. 155

4. Joints of the Thorax

A. Costovertebral Joints (Joints of the Rib Head)

Formed between the head of the rib and the costal facets on two adjacent vertebral bodies (plus the intervertebral disc between them).

Type: Synovial (plane/gliding)
The joint is divided into two synovial compartments by an intra-articular ligament that attaches the crest of the head of the rib to the intervertebral disc
Surrounded by a single fibrous joint capsule

Ligaments:

Radiate (stellate) ligament: Fan-shaped; from the head of the rib to the bodies of both adjacent vertebrae and the intervertebral disc between them. This is the primary stabilizing ligament of the joint.
Intra-articular ligament: Connects the crest of the rib head to the intervertebral disc; divides the joint into two compartments

Exceptions:

Rib I, X (and often IX), XI, XII: head articulates with only one vertebra - joint has a single compartment and no intra-articular ligament

Movement: The two compartments allow rotation of the rib neck about its longitudinal axis (upper ribs) or gliding ascent/descent (lower ribs)

B. Costotransverse Joints

Synovial joints between the tubercle of the rib and the transverse process of its own vertebra.

Type: Synovial (plane/gliding)
Thin, weak fibrous capsule
Absent for ribs XI and XII (which have no transverse costal facets on T11/T12)

Ligaments (all extracapsular):

Ligament	Attachment	Position
Costotransverse ligament	Neck of rib → transverse process (same level)	Medial to joint
Lateral costotransverse ligament	Tip of transverse process → non-articular tubercle	Lateral to joint
Superior costotransverse ligament	Superior surface of rib neck → transverse process of vertebra above	Superior

Movement: Slight gliding movements

C. Sternocostal (Sternocostal) Joints

Between the costal cartilages of ribs I-VII and the sternum.

Rib I: Not synovial - fibrocartilaginous synchondrosis (primary cartilaginous joint) between manubrium and first costal cartilage
Ribs II-VII: Synovial joints with thin capsules reinforced by sternocostal ligaments

Special feature of rib II: The joint is divided into two compartments by an intra-articular ligament attaching the second costal cartilage to the manubriosternal junction (sternal angle).

Ligaments:

Radiate sternocostal ligaments: Radiate from the perichondrium of the costal cartilage to the anterior (and posterior) surface of the sternum, blending with the periosteum to form a dense fibrous membrane called the sternal membrane

D. Interchondral Joints

Between the costal cartilages of adjacent ribs - mainly ribs VII to X (may also include V and VI).

Type: Usually synovial, with thin fibrous capsules reinforced by interchondral ligaments
Provide indirect anchorage to the sternum
Contribute to the smooth inferior costal margin (the oblique ridge running from rib VII to rib X along the lower thorax)

E. Joints Within the Sternum

Manubriosternal joint (Sternal angle / Louis' angle)

Type: Symphysis (secondary cartilaginous joint)
Between manubrium and body of sternum
Slight angular movements during respiration
Remains cartilaginous in most adults; may ossify in older age

Xiphisternal joint

Type: Symphysis
Between body and xiphoid process
Often ossifies with age
Gray's Anatomy for Students, p. 184-186; THIEME Atlas of Anatomy, p. 155

5. Movements at the Thoracic Joints and Respiration

The axes of rib movement run parallel to the necks of the ribs:

Upper ribs (I-VI): Axes closer to the coronal plane → rib elevation increases the anteroposterior (sagittal) diameter of the thorax ("pump-handle" movement)
Lower ribs (VII-X): Axes closer to the sagittal plane → rib elevation increases the transverse diameter of the thorax ("bucket-handle" movement)

This coordinated movement at costovertebral and costotransverse joints, combined with diaphragmatic descent, drives the volume changes needed for breathing.

Summary Table: Joints of the Thorax

Joint	Type	Bones	Key Ligaments
Joint of rib head	Synovial (plane)	Rib head + 2 vertebral bodies	Radiate, intra-articular
Costotransverse	Synovial (plane)	Rib tubercle + transverse process	Costotransverse, lateral CT, superior CT
Sternocostal (rib I)	Synchondrosis	Rib I cartilage + manubrium	-
Sternocostal (ribs II-VII)	Synovial	Costal cartilages + sternum	Radiate sternocostal
Interchondral (VII-X)	Synovial	Adjacent costal cartilages	Interchondral
Manubriosternal	Symphysis	Manubrium + body	Fibrocartilage disc
Xiphisternal	Symphysis	Body + xiphoid	Fibrocartilage disc

Sources: Gray's Anatomy for Students (9780323934237); THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System (9781626237186)

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