The thoracic spine is also called the dorsal spine and is constituted by the next 12 vertebrae after the cervical spine. These vertebrae are named T1 to T12 (or D1 to D12) from above downwards.
If you palpate back of the neck from above downwards in the midline, you would come across a sudden projection or bump in the lower part of the neck. This projection is the posterior spinous process of C7 vertebra and projects out due to its large size. C7 connects on top of T1.
Similarly, the lowest vertebra of the thoracic spine, T12, connects with L1 or first vertebra in the lumbar spine. Between these two are ten other thoracic vertebra connected to each other.
Unique Features of Thoracic Spine
Following features are unique to the thoracic spine when compared to the cervical spine
Each thoracic segment is accompanied by a pair of ribs. The ribs articulate with the spinal column posteriorly, and the sternum anteriorly.
The first, eleventh, and twelfth pair of ribs articulate with their named vertebra only.
The second through tenth ribs articulate both with their named vertebral body and with the intervertebral disk and the vertebra above.
Each pair of ribs also articulates with the anterior surface of the transverse process of its named vertebrae.
The thoracic spine and rib cage provide both stability and protect vital organs in thorax.
Discs are cushions in between two vertebrae that act as shock absorbers. In the thoracic spine, the intervertebral discs are thinner than at the cervical or lower spine.
Identification of Thoracic Vertebra
The thoracic vertebrae are identified by the presence of costal facets on the sides of the vertebral bodies. Costal facets of each vertebra articulate with the corresponding rib. The costal facets may be two or only one on each side. There are twelve thoracic vertebrae [both typical and atypical thoracic vertebrae], out of which the 2nd to 8th are typical and the remaining 5 i.e.1st, 9th, 10th, 11th and 12th are atypical.
Typical Thoracic Vertebra
The body of the typical thoracic vertebra is heart-shaped with roughly the same measurements from side to side and anteroposteriorly. On each side, it bears 2 costal facets (demifacets). The superior costal facets are larger and placed on the upper border of the body near the pedicle. They articulate with the head of the numerically corresponding rib. The inferior costal facets are smaller and placed on the lower border in front of the inferior vertebral notch. They articulate with the next lower rib.
The vertebral foramen is comparatively small and circular.
The vertebral arch has its pedicles directed straight backwards. The superior vertebral notch is shallow, while the inferior vertebral notch is deep and conspicuous.
The laminae overlap each other from above.
The superior articular processes project upwards from the junction of the pedicles and laminae. The articular facets are flat and are directed backwards and a little laterally and upwards. This direction permits rotatory movements of the spine.
The inferior articular processes are fused to the laminae. Their articular facets are directed forwards and slightly downwards and medially.
The transverse processes are large and are directed laterally and backwards from the junction of the pedicles and laminae. The anterior surface of each process bears a facet near its tip, for articulation with the tubercle of the corresponding rib. In the upper 6 vertebrae, the costal facets on the transverse processes are concave and face forwards and laterally. In others the facets are flat and face upwards, laterally and slightly forwards.
The spinous process is long, and is directed downwards and backwards. The 5th to 9th spinous process are the longest, more vertical and overlap each other. The spinous process of the vertebra near cervical and lumbar junctions are less oblique in direction.
- The upper and lower borders of the body – the anterior and posterior longitudinal ligaments.
- Laminae [The upper border and lower part of the anterior surface]- Ligamentum flavum or yellow ligament.
- The transverse process
- Lateral costotransverse ligament (at the tip)
- Superior costotransverse ligament (lower border)
- Inferior costotransverse ligament (anterior surface)
- Intertransverse muscles (to upper and lower borders)
- Levator costae muscle (posterior surface).
- The spinous process
- Supraspinous and interspinous ligaments
- Several muscles including the trapezius, the rhomboids, the latissimus dorsi, the serrate posterior superior and inferior
Atypical Thoracic Vertebrae
First Thoracic Vertebra
The body of T1 vertebra resembles that of a cervical vertebra. It is broad and not heart-shaped. Its upper surface is lipped laterally and beveled anteriorly. The superior costal facet on the body is complete. It articulates with the head of the first rib. The inferior costal facet is a “demifacet” for the second rib.
The spine is thick, long and nearly horizontal.
The superior vertebral notches are well marked, as in cervical vertebrae.
Ninth Thoracic Vertebra
It resembles a typical thoracic vertebra except that the body has only the superior costal facets (demifacets). The inferior costal facets are absent.
Tenth Thoracic Vertebra
It resembles a typical thoracic vertebra except that the body has a single complete superior costal facet on each side, extending on to the root of the pedicle.
Eleventh Thoracic Vertebrae
The body has a single large costal facet on each side, extending on to the upper part of the pedicle.
The transverse process is small, and has no articular facet. Sometimes it is difficult to differentiate between vertebrae T10 and T11.
Twelfth Thoracic Vertebra
The shape of the body, pedicles, transverse processes and spine are similar to those of a lumbar vertebra. However the body bears a single costal facet on each side, which lies more on the lower part of the pedicle than on the body.
The transverse process is small and has no facet, but has superior, inferior and lateral tubercles.
The inferior articular facets are lumbar in type, and are directed laterally (everted), but the superior articular facets are thoracic in type.
Joints of Thoracic Spine
Symphyses and facet joints are present in other regions of spine as well. Costovertebral joints are unique to thoracic spine.
Adjacent vertebral bodies are joined by intervertebral discs, made of fibrocartilage.
These are synovial joints formed by the articulation of superior and inferior articular processes from adjacent vertebrae.
Following joints are unique to thoracic spine.
Each costovertebral joint consists of the head of the rib articulating with:
- Superior costal facet of the corresponding vertebra
- Inferior costal facet of the superior vertebra
- Intervertebral disc separating the two vertebrae
The joint is stabilized by the intra-articular ligament of the head of the rib which attaches the rib head to the intervertebral disc. Only slight gliding movements can occur at these joints.
The costotransverse joints are formed by the articulation of transverse processes of a thoracic vertebra and the tubercle of the adjacent rib. They are present in all vertebrae except T11 and T12.
Ligaments of Thoracic Spine
The thoracic spine is strengthened by the presence of numerous ligaments.
Anterior Longitudinal Ligament
The anterior longitudinal ligament runs down the anterior surface of the vertebral bodies from the occiput to the sacrum. The anterior longitudinal ligament is thicker and narrower in the thoracic region. The ligament firmly attaches to the edges of the vertebral bodies.
This is thick and slightly more narrow over the vertebral bodies and thinner but slightly wider over the intervertebral discs. It also loosely attaches to annulus of disc.
It is a three-layered ligament – superficial, intermediate and deep.
Posterior Longitudinal Ligament
The posterior longitudinal ligament arises from the posterior aspect of the basiocciput, is continuous with the membrana tectoria, and runs over the posterior surfaces of the bodies of the vertebrae, down to the coccyx. It is within the vertebral canal.
These ligaments are situated between the transverse processes. In the thoracic region, they are closely connected with the deep muscles of the back.
The capsular ligaments are attached to the articular margins of the articular processes. The fibers are oriented perpendicular to the facet joint and are stronger in the thoracic and lumbar region than in the cervical region.
Also called Yellow Ligament, the ligamentum flavum connects the anteroinferior edge of the lamina to the posterosuperior edge of the lamina below. The ligamentum flavum is thicker in the thoracic region. This ligament is composed mainly of elastic fibers, and that elasticity serves to preserve the upright posture, and to assist the vertebral column to resume after flexion.
The interspinous ligaments connect adjacent spinous processes, and runs obliquely from the anterior inferior aspect of the spinous process above to the posterior superior aspect of the spinous process
This strong ligament is a strong fibrous cord, which connects together the tips of the spinous processes from the C7 to the sacrum.
In the cervical spine, the interspinous and supraspinous ligaments thicken and combine to form the nuchal ligament
Ligaments Unique to Thoracic Spine
Radiate ligament of head of rib
This ligament fans outwards from the head of the rib to the bodies of the two vertebrae and intervertebral disc.
This connects the neck of the rib and the transverse process.
Lateral costotransverse ligament
It extends from the transverse process to the tubercle of the rib.
Superior costotransverse ligament
It passes from the upper border of the neck of the rib to the transverse process of the vertebra superior to it.
Muscles of Thoracic Spine
There are two types of muscles in the back
- Extrinsic which are responsible for limb and respiratory movements
- Intrinsic which maintain posture and carry out spinal movements.
Extrinsic Back Muscles
The extrinsic back muscles include superficial and intermediate muscles.
- Trapezius, latissimus dorsi, levator scapulae, and rhomboids
- Connect the upper limbs to the trunk act to move limbs
- serratus posterior superior and inferior
- Thin, superficial respiratory muscles
Intrinsic Back Muscles
The intrinsic or deep back muscles maintain posture and control movements of the vertebral column.
The deep back muscles can be grouped into superficial, intermediate, and deep layers
- Superficial layer
- Splenius capitis
- Splenius cervicis muscles.
- Intermediate layer [Erector spinae muscles]
- Spinalis muscles.
- Chief extensor of the vertebral column.
- Deep Layer [Transversospinal muscle]
Apart from these, the following muscles are present.
- Interspinal which connect spinous processes
- Intertransverse which connects transverse processes
The anterior musculature is found in the cervical and lumbar regions, not in the thoracic region.
Blood Supply of Thoracic Spine
The branches of the posterior intercostal arteries are main blood suppliers of the thoracic spine.
The first two posterior intercostal arteries are branches of the subclavian artery. the remaining intercostal arteries are branches of the thoracic aorta.
Intercostal arteries ranch out into the periosteal and equatorial arteries. these further branch into anterior and posterior canal branches.
A venous plexus is present inside the vertebral canal and another one outside. BIth of this plexus lack valves. THis allows for blood movements according to pressure gradient.
The blood finally drains into the segmental veins of the trunk.