THORACIC VERTEBRAE

The thoracic vertebrae are 12 (T1 – T12) in number, forms middle part of the spine known as thoracic area and are sandwiched between cervical vertebrae and lumbar vertebrae. They help in forming the rib cage as it articulates with the ribs. The thoracic vertebrae are separated by intervertebral discs. For each of the 12 thoracic vertebrae, there is a corresponding pair of ribs attached to the facets on the sides of the body of vertebrae. Thoracic vertebrae increase in size as they descend towards the lumbar vertebrae; this is because the lower vertebrae must be able to support more of the body’s weight when a person is standing due to the effects of gravity.

The function of the thoracic vertebrae is protecting the spinal cord, in the rib cage it helps to protect all the internal organs, branching spinal nerves and it enables flexibility and mobility of the body.

The thoracic vertebrae are also divided into the typical and atypical vertebrae as per its characteristic features. The typical thoracic vertebrae are from T2 to T9. The atypical thoracic vertebrae are T1, T10, T11 & T12.

TYPICAL THORACIC VERTEBRAE

The typical vertebra has the vertebral body, vertebral arch (laminae, pedicles & vertebral foramen) and 7 processes (spinous, transverse - 2, articulating – 4).

🔹 Vertebral Body:

The thoracic vertebrae have a medium sized and heart shaped vertebral body also known as corpus. The body is larger than the cervical vertebrae but smaller than the lumbar vertebrae. The bodies are broad anteroposteriorly as in the transverse direction. The bodies bear the majority of the force and body weight applied on the vertebrae. They increase in size from the above to below which helps in the greater range of flexion – extension seen at the cervical and lumbar ends of the thoracic vertebral column. The vertebral body on either side has 2 partial facets; superior & inferior costal demi facets which are small, smooth and somewhat concave; which provides the articulation with the head of its corresponding rib and the rib below. The vertebral body consists of a trabecular bone, which contains the red marrow, surrounded by a thin external layer of compact bone.

 

🔹 Vertebral Foramen:

The vertebral foramen is small and circular hence the pedicles do not diverge as they do in the cervical vertebrae. The spinal in thoracic vertebral column becomes smaller and more circular than the cervical. This foramen has a large opening located posterior to the vertebral body. The intervertebral foramen are two small and circular openings on either side of the vertebra in each intervertebral level.

 

🔹  Laminae:

The laminae are short, thick and broad. It overlaps from above downwards of the sub-adjacent vertebrae like tiles on the roof & connects with the pedicles to protect the spinal cord.

 

🔹  Pedicles:

The pedicles are directed backwards and slightly upwards. They are cylindrical bony protrusions whose superior and inferior surfaces are marked with several notches which combines and forms the intervertebral foramina. On the posterior side, pedicles articulate with the laminae on both the sides and forms the neural arch. The neural arch combines with the posterior surface of the vertebral body and forms the vertebral foramen.

 

🔹  Spinous Process:

The spinous process is long and slant inferiorly. It projects posteriorly and inferiorly from the vertebral arch and overlaps the below vertebrae to various degrees; moreover, it overlaps T5 – T8 above and below ones have less oblique overlapping. It helps in protecting spinal cord from any injury and entering any object.

 

🔹  Transverse Processes:

The transverse processes are long, thick, strong & large winglike structure which are directed laterally and backwards from the junction of the pedicles and laminae. It has a clubbed end and a small concave surface known as the transverse costal facet for the articulation with the tubercle of the rib. In the upper six vertebrae, the costal facets on the transverse processes are concave, and face forwards and laterally. In lower four, the facets are flat and face upwards, laterally and slightly forwards. In the last two vertebrae, the articular facets are absent.

 

🔹  Articulating Processes:

There are 4 articulating processes; 2 superior articulating processes & 2 inferior articulating processes. These processes are located on the both the sides of the vertebrae and each of these processes have its articulating facets i.e., superior and inferior articulating facets.

The superior articulating processes are 2 thin & flat bony plates that projects upwards from the junction of the laminae and pedicles. The inferior articulating processes are 2 projections that goes downwards from the vertebral bodies from the junction of the laminae and pedicles. The superior and inferior facets are named as per its position at the vertebral body which helps in the articulation with the head of ribs. The superior and inferior costal facets are located on the sides of each vertebral body. They consist of cartilage lined depressions, which articulate with the heads of the ribs.  The superior facet articulates with the head of the adjacent rib, and the inferior facet articulates with the head of the rib below. In the majority of the vertebrae (T2-T9) these facets are demi-facets.

The point at which superior and articular facets meet is known as a facet, or zygapophyseal, joint. These maintain vertebral alignment, control the range of motion, and are weight-bearing in certain positions.

ATYPICAL VERTEBRAE

The atypical vertebrae have some additional atypical features.

·       T1: It has a whole superior costal facet, an inferior costal demi facet & a transverse costal facet. It is the only vertebra to articulate with the 1^st rib. It has a long, thick & almost horizontal spinous process like that of C7 vertebra.

·       T10: It has a transverse costal facet & superior costal demi facet on each side & lacks an inferior costal demi facet.

·       T11: The body has a single large facet on each side of the vertebra which extends till the upper part of the pedicle. The transverse process is small & has no articular facet. T10 may resemble the atypical nature of the 11 and 12 vertebrae. When that is the case, T9 lacks an inferior demifacet, as it would not be needed to articulate with the 10th rib.

·       T12: The shapes of the body, pedicles, transverse processes and spine are similar to that of the lumbar vertebra. But body has a single costal facet which usually lies on the lower part of the pedicle than the body. The transverse process is small has no facet but has superior, inferior and lateral tubercles. The superior is largest, projects upwards and corresponds to a lumbar mammillary process, though it does not lie as close to the superior articular process; the lateral tubercle is the homologue of a transverse process; and the inferior is the homologue of a lumbar accessory process. The superior and inferior processes are surprisingly long in some specimens.

INDIVIDUAL THORACIC VERTEBRAE

·       T1 (First Thoracic Vertebra): T1 has a unique structure featuring an entire articular facet for the first rib's head and a demi-facet for the upper half of the second rib's head. Its spinous process is thick, long, and horizontal, and its transverse processes are long, with deep upper vertebral notches. T1 resembles cervical vertebrae in its broad and concave body. The thoracic spinal nerve 1 exits beneath T1.

·       T2 (Second Thoracic Vertebra): Slightly larger than T1, T2 continues the thoracic spine's structural pattern. The thoracic spinal nerve 2 exits below it.

·       T3 (Third Thoracic Vertebra): T3's primary feature is its role in the pathway for the thoracic spinal nerve 3, which exits beneath it.

·       T4 (Fourth Thoracic Vertebra): T4 is at the same level as the sternal angle, an important anatomical landmark. The thoracic spinal nerve 4 exits underneath T4.

·       T5 (Fifth Thoracic Vertebra): Similar in location to T4, T5 aligns with the sternal angle. The trachea bifurcates into the primary bronchi at this level, although variations occur. The thoracic spinal nerve 5 exits beneath T5.

·       T6 (Sixth Thoracic Vertebra): This vertebra accommodates the thoracic spinal nerve 6, which exits below it.

·       T7 (Seventh Thoracic Vertebra): The thoracic spinal nerve 7 passes beneath T7. This vertebra marks the midpoint of the thoracic spine.

·       T8 (Eighth Thoracic Vertebra): Found at the level of the xiphoid process of the sternum, T8 is another important anatomical marker. The thoracic spinal nerve 8 exits beneath it.

·       T9 (Ninth Thoracic Vertebra): T9 may or may not have demi-facets for rib articulation. It aligns with the xiphoid process in the axial plane. The thoracic spinal nerve 9 exits below T9.

·       T10 (Tenth Thoracic Vertebra): T10 features a single entire articular facet on each side, located on the pedicles. It does not have facets below it, as subsequent ribs only articulate with one vertebra. The thoracic spinal nerve 10 exits beneath it.

·       T11 (Eleventh Thoracic Vertebra): Transitioning toward lumbar vertebrae, T11 has thicker pedicles and larger rib articulation facets. The transverse processes are short and lack articular facets. The thoracic spinal nerve 11 exits beneath T11.

·       T12 (Twelfth Thoracic Vertebra): T12 most closely resembles a lumbar vertebra, with convex inferior articular surfaces and subdivided transverse processes featuring superior, inferior, and lateral tubercles. These features align with the lumbar region's mammillary and accessory processes. The thoracic spinal nerve 12 exits beneath T12.

MUSCLE AND LIGAMENT ATTACHMENTS

Muscles Attached to Thoracic Vertebrae

The thoracic vertebrae anchor many muscles that play essential roles in posture, spinal movement, and respiration:

1. Erector Spinae: This group includes the iliocostalis, longissimus, and spinalis muscles, which extend the spine and maintain upright posture. The thoracic segments anchor these muscles along their vertebral attachments.
2. Interspinales: Small muscles between the spinous processes that assist with minor extension and stabilization of the spine.
3. Intertransversarii: These muscles connect transverse processes and facilitate lateral flexion and stabilization.
4. Latissimus Dorsi: Originates partially from the lower thoracic vertebrae (T7–T12 spinous processes) and plays a role in arm adduction, extension, and internal rotation.
5. Multifidus: A deep stabilizing muscle attaching to the spinous processes, laminae, and transverse processes. It provides fine control over spinal movements.
6. Rhomboid Major and Minor: Attach to the spinous processes of the upper thoracic vertebrae (T2–T5 for major and C7–T1 for minor) and retract the scapula.
7. Rotatores: Deep muscles running from the transverse process of one vertebra to the spinous process of the next, aiding rotation and proprioception.
8. Semispinalis: Part of the transversospinalis group, this muscle spans from the transverse processes of lower thoracic vertebrae to the spinous processes of higher vertebrae, extending and rotating the spine.
9. Serratus Posterior Superior and Inferior: These muscles assist in respiration by elevating (superior) or depressing (inferior) the ribs. They attach to the spinous processes of thoracic vertebrae (T1–T4 for superior and T11–T12 for inferior).
10. Splenius Capitis and Splenius Cervicis: These muscles span from the spinous processes of the upper thoracic vertebrae (T1–T6) to the occipital bone (capitis) or cervical transverse processes (cervicis), aiding neck extension and rotation.
11. Trapezius: Attaches to the spinous processes of thoracic vertebrae (T1–T12) and moves the scapula, assisting with posture and upper limb movements.
12. Levator Costae: Attaches posteriorly at the apex of the transverse processes, aiding in elevating the ribs during inspiration.
13. Longus Colli: Originates from the upper thoracic vertebral bodies and aids in neck flexion.
14. Psoas Major and Minor: Attach near the lower border of T12 and assist in hip flexion and trunk stabilization.

Ligaments Attached to Thoracic Vertebrae

The thoracic spine is reinforced by multiple ligaments, contributing to spinal stability and integrity:

1. Anterior Longitudinal Ligament: A strong ligament running along the anterior surface of the vertebral bodies, preventing hyperextension of the spine.
2. Posterior Longitudinal Ligament: Found along the posterior surface of the vertebral bodies within the vertebral canal, this ligament limits spinal flexion.
3. Ligamentum Flavum: Connects the laminae of adjacent vertebrae and provides elasticity, aiding in returning the spine to its normal position after flexion.
4. Interspinous Ligament: Connects adjacent spinous processes, contributing to the stabilization of the spine during movement.
5. Supraspinous Ligament: Runs along the tips of the spinous processes, providing additional support against flexion.
6. Costotransverse Ligaments:
• Costotransverse Ligament: Connects the neck of the rib to the transverse process of the vertebra.
• Lateral Costotransverse Ligament: Joins the transverse process to the tubercle of the rib.
• Superior Costotransverse Ligament: Links the upper border of the rib's neck to the transverse process of the superior vertebra.

BLOOD & LYMPHATIC SUPPLY

The arterial blood supply to the thoracic vertebrae is derived primarily from the posterior branches of the segmental arteries. These include the posterior intercostal arteries, which arise from the thoracic aorta. Each posterior intercostal artery divides into dorsal branches near the vertebrae, supplying the vertebral body, transverse processes, spinous processes, and associated muscles and ligaments. Additionally, these branches form connections with smaller vessels, such as the spinal branches, which enter the vertebral canal through the intervertebral foramina. Once inside, they supply the spinal cord, meninges, and surrounding structures.

The venous drainage parallels the arterial supply and is primarily handled by the posterior intercostal veins, which drain into the azygos and hemiazygos systems. These veins receive blood from the vertebral venous plexuses, which consist of internal and external networks. The internal vertebral venous plexus lies within the epidural space and communicates with the external venous plexus on the vertebrae's outer surfaces, providing a pathway for blood drainage and potential spread of infections or metastases.

The lymphatic drainage of the thoracic vertebrae is directed to the paravertebral lymph nodes and posterior intercostal lymph nodes, which are situated along the intercostal spaces. These nodes receive lymph from the bony vertebrae, surrounding ligaments, and associated soft tissues. From here, lymphatic vessels transport the lymph to the thoracic duct, which is a major lymphatic channel responsible for returning lymph to the venous circulation at the junction of the left subclavian and internal jugular veins.

The thoracic duct itself is positioned near the thoracic vertebral column and serves as a critical conduit for lymph drainage from most of the body below the diaphragm and the left thoracic region. The lymphatic drainage system not only removes waste products but also plays a vital role in immune surveillance and response within the vertebral and spinal structures.

OSSIFICATION

All vertebrae begin ossification in the embryonic period of development around 8 weeks of gestation. They ossify from three primary ossification centers: one in the endochondral centrum (which will develop into the vertebral body) and one in each neural process (which will develop into the pedicles). This begins at the thoracolumbar junction and proceeds in the cranial and caudal directions. The neural processes fuse with the centrum in between three and six years of age. During puberty, five secondary ossification centers develop at the tip of the spinous process and both transverse processes and on the superior and inferior surfaces of the vertebral body. The ossification centers on the vertebral body are responsible for the superior-inferior growth of the vertebrae. Ossification completes around the age of 25.