Chapter 3: Skeleton Trunci: Thoracic Cage & Vertebral Column

The skeleton trunci (axial skeleton of the trunk) has three parts:

• the sternum;
• 12 pairs of costae (ribs);
• the columna vertebralis (vertebral column).

The first two, together with the 12 thoracic vertebrae, make up the skeleton thoracis, the bony cage of the chest. It encloses the cavitas thoracis, which opens above at the apertura thoracis superior (bounded by vertebra T1, the first ribs and the manubrium) and below at the apertura thoracis inferior (closed by the diaphragm).

Sternum

The sternum is a flat bone in the anterior midline, in three parts.

Manubrium sterni

• Incisura jugularis (suprasternal notch): the notch on the upper border, easily felt at the base of the neck.
• Incisura clavicularis: on each side of the jugular notch, where the sternal end of the clavicle articulates (sternoclavicular joint).
• Incisura costalis I: the facet for the first costal cartilage.
• Incisura costalis II: only half a facet on the manubrium. The other half is on the body, because the second costal cartilage meets the sternum exactly at the manubriosternal joint.

Corpus sterni

• Facies anterior (planum sternale): shows faint transverse ridges where its four original segments (sternebrae) fused.
• Facies posterior: smoother and slightly concave.
• Costal notches along each side receive costal cartilages II–VII.

Processus xiphoideus

The small lowest part. It is cartilaginous in young people and ossifies with age. It lies at the fossa epigastrica (the "pit of the stomach"). It joins the body at the symphysis xiphosternalis, and the costal margins diverging from it form the angulus infrasternalis (subcostal angle).

The sternal angle

The manubrium and body meet at a slight forward angle, the angulus sterni (sternal angle, angle of Louis). It is the most useful landmark on the chest:

• it can be felt as a transverse ridge;
• the second costal cartilage joins here, so counting ribs and intercostal spaces starts at this point;
• it lies at the level of the T4/T5 intervertebral disc, the boundary between the superior and inferior mediastinum.

Costae

Each rib is an os costale (the bony part) joined anteriorly to a cartilago costalis.

Parts of a typical rib

• Caput costae (head): articulates with the vertebral bodies. Its articular surface is divided by a ridge, the crista capitis costae, into two facets: one for its own vertebra and one for the vertebra above.
• Collum costae (neck): the short segment between head and tubercle.
• Tuberculum costae (tubercle): articulates with the transverse process of its own vertebra.
• Corpus costae (shaft): thin and curved. It bends sharply forward at the angulus costae (angle).
• Sulcus costae (costal groove): along the inferior border of the inner surface. It carries the intercostal vein, artery and nerve.

Typical and atypical ribs

Ribs III–IX are typical: head with two facets and a crista, neck, tubercle, angle and costal groove. Ribs I, II, X, XI and XII are atypical:

• Rib I is the shortest, broadest, flattest and most sharply curved.
  • Its head has a single facet and no crista.
  • Its upper surface carries the tuberculum musculi scaleni anterioris, where scalenus anterior inserts.
  • In front of that tubercle is the sulcus venae subclaviae (for the subclavian vein); behind it is the sulcus arteriae subclaviae (for the subclavian artery and the lower trunk of the brachial plexus).
• Rib II is about twice as long as rib I and has a typical head with a crista. Its outer surface has the tuberositas musculi serrati anterioris, where the first digitations of serratus anterior attach.
• Rib X has only one facet on its head.
• Ribs XI and XII also have a single head facet, and have no neck or tubercle and an indistinct angle. They end in the abdominal wall musculature.

Costal cartilages: true, false and floating ribs

• Costae verae (true, vertebrosternal ribs) I–VII attach directly to the sternum by their own cartilage.
• Costae spuriae (false, vertebrochondral ribs) VIII–X attach to the cartilage of the rib above. Their cartilages form the arcus costalis (costal margin).
• Costae fluctuantes (floating ribs) XI–XII end freely in the muscles of the abdominal wall.

Many textbooks count all of ribs VIII–XII as false ribs, with XI–XII as the floating subset. Both schemes give the same attachments.

Spatium intercostale

An intercostal space (spatium intercostale, abbreviated SIC in Indonesian clinical notes) is the space between two adjacent ribs. It takes the number of the rib above it: the first intercostal space lies between ribs I and II.

Intercostal spaces are how organs are located on the chest. The apex beat (ictus cordis), for example, is normally felt in the fifth intercostal space in the left midclavicular line.

Joints of the thoracic cage

Muscles of the thoracic wall

The thoracic wall muscles fill the intercostal spaces and move the ribs in breathing. They lie in three layers:

All are supplied by the intercostal nerves (ventral rami of T1–T11) and the subcostal nerve (T12).

Blood supply, veins and lymph of the thoracic wall

Arteries

Each intercostal space has one posterior and (in the upper spaces) two small anterior intercostal arteries, which join in the space.

• Posterior intercostal arteries
  • Spaces 1–2: from the a. intercostalis suprema, a branch of the truncus costocervicalis (from the subclavian artery).
  • Spaces 3–11: directly from the thoracic aorta. The subcostal artery runs below rib XII.
• Anterior intercostal arteries
  • Spaces 1–6: from the a. thoracica interna (internal thoracic artery, a branch of the subclavian artery), which runs down about 1 cm lateral to the sternum.
  • Lower spaces: from the a. musculophrenica, one of the two terminal branches of the internal thoracic artery. The deck lists spaces 7–11; most texts give 7–9, with the lowest spaces supplied by posterior intercostal arteries only.
• The other terminal branch of the internal thoracic artery, the a. epigastrica superior, continues into the rectus sheath.

Veins

• Posterior intercostal veins drain to the azygos system: the v. azygos on the right; the v. hemiazygos and v. hemiazygos accessoria on the left. The upper spaces drain through the right and left superior intercostal veins.
• Anterior intercostal veins drain to the v. thoracica interna and the v. musculophrenica.

Lymph

• Anterior chest wall → nodi parasternales (along the internal thoracic vessels).
• Posterior chest wall → nodi intercostales (near the rib heads).
• Diaphragm → nodi diaphragmatici.

From these nodes lymph reaches the ductus thoracicus (on the left) or the right lymphatic duct, which empty at the junction of the subclavian and internal jugular veins.

Clinical: cervical rib and thoracic outlet syndrome

A costa cervicalis (cervical rib) is an extra rib arising from C7, found in a small percentage of people. It can narrow the space through which the subclavian artery and vein and the lower trunk of the brachial plexus leave the thorax. That compression is one cause of thoracic outlet syndrome (TOS):

• pain, numbness and tingling along the medial forearm and hand (from C8–T1);
• weakness of the small hand muscles;
• sometimes a cold, pale hand or arm swelling (vascular compression).

Columna vertebralis

The vertebral column is typically 33 vertebrae in five regions: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral (the os sacrum) and 3–5 (usually 4) fused coccygeal (the os coccygis).

Curvatures

Seen from the side, the adult column has four curves:

• Primary curves, present in the fetus: thoracic and sacral kyphosis (concave forward).
• Secondary curves, which develop after birth: cervical lordosis (as the infant holds up its head) and lumbar lordosis (as the child sits and walks). These are concave backward.

Parts of a typical vertebra

• Corpus vertebrae (body): the weight-bearing cylinder in front.
• Arcus vertebrae (vertebral arch), behind the body:
  • pediculi (pedicles), joining the arch to the body;
  • laminae, meeting in the midline behind.
• Seven processes project from the arch:
  • one processus spinosus (spinous process) posteriorly;
  • two processus transversi (transverse processes) laterally;
  • two superior and two inferior processus articulares (articular processes), which form the facet joints.
• Foramen vertebrale: the hole enclosed by body and arch. Stacked foramina form the canalis vertebralis, which contains the spinal cord.
• Foramen intervertebrale: the gap between the pedicles of adjacent vertebrae, where each spinal nerve exits.

Regional features of the vertebrae

Vertebrae cervicales (C1–C7)

Every cervical vertebra has a foramen transversarium in each transverse process. This is the feature that identifies it as cervical. The vertebral artery and vein pass through the foramina of C1–C6.

Typical cervical vertebrae (C3–C6):

• a small, broad, rectangular body, with raised lateral lips (processus uncinati) on its upper surface;
• a large, triangular vertebral foramen;
• a short, bifid spinous process.

Atypical cervical vertebrae:

• C1, the atlas, carries the skull.
  • It has no body and no spinous process. Instead it is a ring of an anterior and a posterior arch joined by two lateral masses.
  • The fovea dentis on the back of the anterior arch articulates with the dens of C2.
  • The upper surfaces of the lateral masses articulate with the occipital condyles.
• C2, the axis, is the pivot for the atlas.
  • The dens (processus odontoideus) projects upward from its body. Developmentally it is the body of C1. It has an apex and an anterior articular facet for the atlas.
  • Its spinous process is large, strong and bifid.
• C7, the vertebra prominens:
  • has the longest spinous process of the cervical vertebrae, not bifid, easily felt at the base of the neck and used to count spinous processes;
  • has large transverse processes but small foramina transversaria, which usually carry only the vertebral vein. The vertebral artery typically enters at C6.

Vertebrae thoracicae (T1–T12)

• A heart-shaped body.
• A small, round vertebral foramen.
• A long spinous process that slopes steeply downward, overlapping the vertebra below.
• Costal facets, the identifying feature:
  • foveae costales superior and inferior on the body, for the rib heads;
  • a fovea costalis processus transversi on each transverse process (T1–T10), for the rib tubercle.

Because a typical rib head spans two vertebrae, most thoracic bodies carry half-facets (semilunar):

T12 is transitional toward the lumbar form. Its transverse process is reduced to small tubercles (superior = mammillary, inferior = accessory, and lateral).

Vertebrae lumbales (L1–L5)

• A large, kidney-shaped body that carries the most weight.
• No foramen transversarium and no costal facets.
• A triangular vertebral foramen.
• Thick, strong pedicles and laminae.
• A short, square, hatchet-shaped spinous process pointing straight back, leaving room to pass a needle between the processes (lumbar puncture).
• L5 is the largest. Its body is taller in front than behind, which shapes the lumbosacral angle, and the iliolumbar ligament attaches to its transverse processes. L5 is sometimes partly fused to the sacrum (sacralisation).

Os sacrum

The sacrum is five fused sacral vertebrae, forming a triangle wedged between the hip bones.

• Facies pelvina (anterior, concave):
  • the basis ossis sacri (base) above, with the promontorium projecting at its front edge;
  • the alae (wings) to each side;
  • lineae transversae, ridges marking where the bodies fused;
  • four pairs of foramina sacralia anteriora, for the ventral rami of the sacral nerves;
  • the apex ossis sacri below.
• Facies dorsalis (posterior, convex, rough):
  • the crista sacralis mediana (fused spinous processes);
  • the crista sacralis medialis/intermedia (fused articular processes);
  • the crista sacralis lateralis (fused transverse processes);
  • four pairs of foramina sacralia posteriora, for the dorsal rami;
  • the hiatus sacralis at the lower end, where the laminae of S5 (and often S4) fail to meet, flanked by the cornua sacralia. It is the entry point for a caudal epidural injection.
• Laterally, the ear-shaped facies auricularis forms the sacroiliac joint with the ilium. Behind it the tuberositas ossis sacri anchors the strong interosseous sacroiliac ligaments.
• The canalis sacralis is the continuation of the vertebral canal through the sacrum.

Os coccygis

The coccyx is usually four (three to five) small fused vertebrae. The first has cornua coccygea, which articulate with the sacral cornua.

The intervertebral disc

Between adjacent vertebral bodies (from C2–C3 down to L5–S1) lies a discus intervertebralis. Discs make up about a quarter of the length of the column and act as shock absorbers.

• Anulus fibrosus: concentric lamellae of fibrocartilage forming the outer ring. The fibres in neighbouring lamellae cross obliquely.
• Nucleus pulposus: the gel-like core, a remnant of the notochord (chapter 2). It is compressed under load and shifts away from the side of bending.

Clinical: disc herniation (hernia nucleus pulposus, HNP). Trauma or repeated heavy loading can tear the anulus, letting the nucleus pulposus bulge out. It usually goes posterolaterally, where the posterior longitudinal ligament is thin, and compresses the nerve root. The commonest levels are L4–L5 and L5–S1, causing low back pain with pain radiating down the leg (sciatica).

Joints of the vertebral column

• Intervertebral joints: symphyses between the bodies, through the discs.
• Artt. zygapophysiales (facet joints): synovial plane joints between the inferior articular process of one vertebra and the superior articular process of the one below. The orientation of their surfaces decides which movements each region allows:
  • cervical: oblique (sloping from front to back), allowing flexion, extension, lateral flexion and rotation;
  • thoracic: near-vertical, in the coronal plane, allowing rotation; flexion and extension are limited by the ribs;
  • lumbar: curved and interlocking, in the sagittal plane, allowing flexion and extension but almost no rotation.
• Artt. uncovertebrales (joints of Luschka): small joint-like contacts between the uncinate processes of C3–C6 and the body above. Bony spurs here with age can narrow the intervertebral foramen.
• Art. atlantooccipitalis: the occipital condyles on the lateral masses of the atlas. It moves in flexion and extension (nodding "yes").
• Artt. atlantoaxiales: a median pivot joint (dens against the anterior arch of the atlas and the transverse ligament) and two lateral plane joints. Together they allow rotation (shaking the head "no").

Kinesiology

The vertebral column as a whole performs flexion, extension, lateral flexion and rotation. It is most mobile in the cervical and lumbar regions and least mobile in the thoracic region, which is braced by the ribs.

Ligaments

Muscles of the back

The back muscles fall into three groups:

The erector spinae columns can be remembered as "I Love Spine", from lateral to medial.

Blood supply of the vertebral column

Clinical correlations

• Spina bifida: the vertebral arches of the lower vertebrae fail to fuse during development, leaving the vertebral canal open behind (see chapter 2, neural tube defects).
• Disc herniation (HNP): see above.
• Abnormal curvatures:
  • Scoliosis: a lateral curvature, usually with rotation of the vertebrae.
  • Hyperkyphosis: an exaggerated thoracic curve ("hunchback"), for example from osteoporotic wedge fractures.
  • Hyperlordosis: an exaggerated lumbar curve, for example in late pregnancy or with abdominal obesity.

Key teaching point

Identify a vertebra by elimination. A foramen transversarium means cervical; costal facets mean thoracic; neither, with a large body, means lumbar. For ribs, count from the sternal angle (rib II), and remember that the intercostal neurovascular bundle runs under each rib, so needles go in over the top of the rib below.