Vincent J Devlin MD and Darren L Bergey MD

OSTEOLOGY

1. Describe a typical thoracic vertebra.

T1 and T10 to T12 possess unique anatomic features due to their transitional location between the cervicothoracic and thoracolumbar spinal regions, respectively. Thoracic vertebra two through nine are termed typical thoracic vertebra because they share common structural features (Fig. 2-1):

• Vertebral body: Heart-shaped in cross-section. Posterior vertebral height exceeds anterior vertebral height, resulting in a wedged shape of the vertebral body when viewed in the lateral plane. This wedge shape is responsible for the kyphotic alignment in the thoracic region.

• Costovertebral articulations: The lateral surface of the vertebral body has both superior and inferior facets for articulation with adjacent ribs.

• Costotransverse articulation: Rib articulation with the transverse process of vertebra.

• Vertebral arch: formed by lamina and two pedicles, which support seven processes: Spinous process (1)

Transverse processes (2) Superior facets (2) Inferior facets (2)

Facet for costal tubercle

Sup. articular process

Pedicle rSup. costal facet

Sup. articular process

Pedicle rSup. costal facet

Inf. costal facet LInf. articular process

Spinous process

LATERAL VIEW

Inf. costal facet LInf. articular process

Spinous process

LATERAL VIEW

Transverse process

Pedicle

Costal facet

Spinous process Lamina

Sup. articular process

Vertebral foramen

Costal facet

Transverse process

Spinous process Lamina

Pedicle

Costal facet

Costal facet

SUPERIOR VIEW

SUPERIOR VIEW

Body

Body

Sup. articular process

Lamina

Spinous process

POSTERIOR VIEW

Sup. articular process

Lamina

Spinous process

POSTERIOR VIEW

Figure 2-1. Typical thoracic vertebra. (From Pansky B: Review of Gross Anatomy. 4th ed. New York, Macmillan, 1979, with permission.)

2. What are the unique anatomic features of the first thoracic vertebra?

T1 vertebral body dimensions resemble a cervical vertebra more closely than a typical thoracic vertebra. The T1 vertebral body possesses a well-developed superior vertebral notch. The T1 spinous process is very prominent and may be larger than the C7 spinous process. The first rib articulates with T1 vertebral body via a costal facet.

What are the unique anatomic features of T10, T11, and T12?

• Lack of costotransverse articulations (T11 and T12)

• Ribs articulate with vertebral bodies and do not overlie the disc space

• Vertebral body dimensions increase and approximate lumbar vertebral dimensions

• Facet morphology transitions from thoracic to lumbar in function and appearance

• T12 transverse process consists of three separate projections

4. What anatomic relationships are useful in determining the level of a thoracic lesion on a thoracic spine radiograph?

The first rib attaches to the T1 vertebral body. The second rib attaches to the T2 vertebral body. The third rib articulates with both the second and third vertebral bodies and overlies the T2-T3 disc space. This latter pattern continues until the tenth vertebral body. The tenth, eleventh, and twelfth ribs articulate only with the vertebral body of the same number and do not overlie a disc space.

5. Describe the anatomy of the thoracic pedicles.

The paired pedicles arise from the posterior-superior aspect of the vertebral bodies. The superior-inferior pedicle diameter is consistently larger than the medial-lateral pedicle diameter. Pedicle widths are narrowest at the T4 to T6 levels, with medial-lateral pedicle diameter increasing both above (T1-T3) and below this region. The medial pedicle wall is two to three times thicker than the lateral pedicle wall across all levels of the thoracic spine. The medial angulation of the pedicle axis decreases from T1 to T12. The site for entry into the thoracic pedicle from a posterior spinal approach is in the region where the facet joint and transverse process intersect and varies slightly, depending on the specific thoracic level.

ARTICULATIONS, LIGAMENTS, AND DISCS

6. What anatomic structures provide articulations between the thoracic vertebral bodies? Between the vertebral arches?

The structures that provide articulations between the thoracic vertebral bodies are:

1. The anterior longitudinal ligament

2. Posterior longitudinal ligament

3. Intervertebral disc

Five anatomic elements provide articulations between the adjacent vertebral arches:

1. Articular capsules: Thin capsules attach to the margins of the articular processes of adjacent vertebra.

2. Ligamentum flavum: Yellow elastic tissue that connects laminae of adjacent vertebrae and attaches to the ventral surface of lamina above and to the dorsal surface and superior margin of the lamina below.

3. Supraspinous ligaments: Strong fibrous cord that connects the tips of the spinous processes from C7 to sacrum.

4. Interspinous ligaments: Interconnect adjoining spinous processes. Attachment extends from base of each spinous process to the tip of the adjacent spinous process.

5. Intertransverse ligaments: Interconnect the transverse processes. The pattern described above continues in the lumbar region as well.

7. What are the two types of articulations between the ribs and the thoracic vertebra?

The two types of articulations between thoracic vertebra and ribs are costovertebral and costotransverse. The costovertebral articulation is the articulation between the head of the rib (costa) and the vertebral body. The articular capsule, radiate ligaments, and intraarticular ligaments stabilize this articulation.

The costotransverse articulation occurs between the neck and tubercle of the rib (costa) and the transverse process. The ligaments that stabilize this articulation include the superior and lateral costotransverse ligaments (Fig. 2-2). The T11 and T12 transverse processes do not articulate with their corresponding ribs.

Sup. and inf. articular processes V/ Rib 7

Demi-facets for head of rib Spinous process

-Trans. process 1 costotransverse lig.

Trans. proc.^^raCo T7nJ

Sup. costotrans. lig.\

Intertrans. lie

articulation /jlMC^ of the of rib _ !—. ^rnMmW neck Synovial mgrnmf neck

X cavities

\ ^^f^SP" ^^Cross-section \ x Radiate ligr vert. body

Neural foramen Intervertebral disc

/ertebral body 6 .

Figure 2-2. Extrinsic ligaments of the thoracic spine. (From Johnson RM, Murphy MJ, Southwick WD: Surgical approaches to the spine. In Herkowitz HN, Garfin SR, Balderston RA, et al., editors. RothmanSimeone The Spine. 4th ed. Philadelphia: Saunders; 1999, with permission.)

Figure 2-2. Extrinsic ligaments of the thoracic spine. (From Johnson RM, Murphy MJ, Southwick WD: Surgical approaches to the spine. In Herkowitz HN, Garfin SR, Balderston RA, et al., editors. RothmanSimeone The Spine. 4th ed. Philadelphia: Saunders; 1999, with permission.)

8. Describe the anatomy of the facet joints in the thoracic region.

The facet joints are located at the junction of the vertebral arch and the pedicle. The paired superior articular processes face posterolaterally, and the paired inferior articular processes face anteromedially. The thoracic facets are oriented 60° in the sagittal plane and approximate the coronal plane with a slight medial inclination (20°). Flexion-extension is minimal at T1-T2 and maximal at T12—L1, where facet joint orientation transitions to a lumbar pattern. Axial rotation is maximal at T1—T2 and minimal at the thoracolumbar junction. Lateral bending is more equally distributed across the thoracic region. Motion of the thoracic vertebrae is limited by anatomic constraints, including the rib cage and its attachment to the sternum, ligamentous attachments at the costovertebral and costotransverse joints, narrow intervertebral discs, and overlap of the adjacent lamina and spinous processes.

NEURAL ANATOMY

9. Describe the contents of the spinal canal in relation to the vertebral segments in the thoracic and thoracolumbar spinal regions.

During childhood, the distal end of the spinal cord migrates proximally due to more rapid longitudinal growth of the osseous spinal elements and generally reaches the lower border of L1 by 8 years of age. In the adult, the spinal cord occupies the upper four-fifths of the vertebral canal. It extends from the foramen magnum and ends distally at the level of the L1-L2 disc space (Fig. 2-3). The inferior region of the spinal cord, named the conus medullaris, is characterized by

SKULL

SKULL

Cervical -<

Thoracic^

Vertebral pedicles

Lumbar

Sacral <

Coccyx

SKULL

Cervical -<

Thoracic^

Vertebral pedicles

Lumbar

Sacral <

Coccyx

3-month fetus

C6 C7 C8 T1

3-month fetus

C6 C7 C8 T1

SKULL

Thoracic

Lumbar m

Adult m

Co s Cervical m

Thoracic

Lumbar

Sacral s m y

Co Coccyx

Adult

Figure 2-3. Relationships between vertebral levels, spinal nerves, and spinal cord segments in the three month fetus and adult (dorsal view). In the fetus, the spinal cord extends the full length of the vertebral column, the spinal cord segments and vertebral levels correspond, and the spinal nerves course horizontally to exit from their intervertebral foramina. However, in adults the spinal cord ends at the L1-L2 vertebral level, only upper cervical cord segments correspond to their vertebral levels with lower cord segments at progressively higher vertebral levels, and lower spinal nerves pursue increasingly more vertical courses. (From Shenk C: Functional and clinical anatomy of the spine. Phys Med Rehabil State Art Rev 1995;9(3):577.)

the presence of both spinal cord and spinal nerve elements within the dural sac. Distal to the termination of the spinal cord (conus), the lumbar, sacral, and coccygeal roots continue as a leash of nerves termed the cauda equina. The filum terminale is a fibrous band extending from the distal tip of the spinal cord and attaching to the first coccygeal segment. Enlargements of the spinal cord between C3 and T2 (cervical enlargement) and between T9 and T12 (lumbar enlargement) correlate with the origin of nerves supplying the upper and lower extremities. The spinal cord possesses a trilayered covering termed meninges and consisting of dura mater, arachnoid mater, and pia mater. The dura mater is the only meningeal layer that extends the entire length of the vertebral column from the foramen magnum to S2. Between the arachnoid and pia mater is the subarachnoid space, a large interval filled with cerebrospinal fluid.

10. Describe the anatomy of thoracic spinal nerves.

Dorsal (sensory) and ventral (motor) roots originate from the spinal cord to form a spinal nerve in the region of the intervertebral foramen. The spinal nerve divides in the region of the foramen into a posterior (dorsal) primary ramus (innervates the posterior aspect of the associated dermatome and myotome) and an anterior (ventral) primary ramus, which continues as the intercostal nerve. The thoracic spinal nerves are numbered according to the pedicle of the vertebral body that the nerve contacts. For example, the T6 nerve root passes beneath the pedicle of the T6 vertebra.

11. Describe the contents of a thoracic neurovascular bundle.

Each neurovascular bundle is composed of a posterior intercostal vein, posterior intercostal artery, and anterior primary ramus of a spinal nerve (mnemonic: VAN superior to inferior). The neurovascular bundle lies immediately below the inferior edge of each rib in the neurovascular groove.

12. Where is the thoracic portion of the sympathetic trunk located?

The thoracic portion of the sympathetic trunk is located along the anterior surface of the rib head. The sympathetic chain or trunk consists of a series of ganglia that extend from the skull to the coccyx. There are two sympathetic chains, located on each of the anterolateral surfaces of the vertebral column. Each consists of approximately 22 ganglia. Each ganglia gives off a gray ramus communicans that joins the adjacent spinal nerve just distal to the junction of the anterior and posterior roots.

13. What is the innervation of the diaphragm?

Innervation of the diaphragm is provided by the phrenic nerve, which originates from the C2 to C4 segments. Because the diaphragm receives its innervation and blood supply centrally, it can be incised and retracted from its insertion along the thoracic wall to permit surgical exposure of the thoracolumbar vertebral bodies without compromising its neurovascular supply.

VASCULAR STRUCTURES

14. Describe the vascular supply of the thoracic spinal cord.

As in the cervical region, single anterior and paired posterior spinal arteries supply the spinal cord. Radicular (segmental) arteries enter the vertebral canal through the intervertebral foramina and divide into anterior and posterior radicular arteries, which supply the anterior and posterior spinal arteries, respectively. The majority of the vascular supply of the spinal cord is supplied by the anterior spinal artery. In the thoracic spine, the radicular arteries originate from intercostal arteries. The intercostal arteries arise segmentally from the aorta and course along the undersurface of each rib. Segmental arteries supplying the spine branch off from the intercostal arteries at the level of the costotransverse joint and enter the spinal canal via the intervertebral foramen. The number of radicular arteries is variable throughout the thoracic spine. The radicular artery of Adamkiewicz is the largest of these segmental arteries and is a major blood supply to the lower spinal cord. It originates from the left side in 80% of people and usually accompanies the ventral root of thoracic nerves 9, 10, or 11. However, it may originate anywhere from T5 to L5. Careful dissection near the intervertebral foramen and costotransverse joints is necessary to prevent injury to this vascular supply.

15. Explain the watershed region and critical supply zone of the thoracic spinal cord.

The blood supply of the spinal cord is not entirely longitudinal. It is partly transverse and dependent on a series of radicular arteries that feed into the anterior and posterior spinal arteries at various levels. The limited number of radicular arteries supplying the thoracic spinal cord results in a less abundant blood supply in this region compared with the cervical and lumbar regions. Branches of the anterior median spinal artery supply the ventral two-thirds of the spinal cord, whereas branches of the posterior spinal arteries supply the dorsal third of the cord. The region where these two zones meet is relatively poorly vascularized and is termed the watershed region. The zone located between the fourth and ninth thoracic vertebrae has the least profuse blood supply and is termed the critical vascular zone of the spinal cord. This region corresponds to the narrowest region of the spinal canal. Interference with circulation in this zone during surgery is most likely to result in paraplegia. Surgical dissection in this region of the spine requires added care. Segmental vertebral arteries should be divided as far anteriorly as possible. Dissection in the region of the intervertebral foramen and costotransverse joint should be limited, and electrocautery should not be used in this area.

FASCIA, MUSCULATURE, AND RELATED STRUCTURES

16. Describe the anatomy of the posterior muscles of the thoracic and lumbar spinal regions.

The anatomy of the posterior muscles of the back is confusing because of the multiple overlapping muscle layers and the fact that distinct muscle layers are not seen during posterior surgical dissection. It is helpful to divide the back muscles into three main layers:

• Superficial layer: Consists of muscles that attach the upper extremity to the spine. The trapezius (innervated by spinal accessory nerve), latissimus dorsi (thoracodorsal nerve), and levator scapulae muscles (dorsal scapular nerve) overlie the deeper rhomboid major and minor muscles (dorsal scapular nerve) (Fig. 2-4).

• Intermediate layer: Consists of the serratus posterior superior and inferior. These muscles of accessory respiration are innervated by the anterior primary rami of segmental nerves (Fig. 2-5).

• Deep layer: Consists of the intrinsic back muscles, which function in movement of the spinal column. These muscles are innervated by the posterior rami of segmental thoracic and lumbar spinal nerves (Fig. 2-6).

The muscles comprising this deep layer can be subdivided into three layers:

1. Splenius capitis and splenius cervicis

2. Sacrospinalis (erector spinae), subdivided into spinalis, longissimus, and iliocostalis portions in the thoracic region

3. Semispinalis, multifidi, rotatores, intertransversari, and interspinales

17. Why should a spine specialist understand the anatomy of the thoracic cavity?

There are two important reasons why a spine specialist must possess a working knowledge of anatomy and pathology relating to the thoracic cavity. First, extraspinal pathologic processes within the thoracic cavity (e.g. aneurysm, malignancy) may mimic the symptoms of thoracic spinal disorders. Second, surgical treatment of many types of spinal problems involves exposure of the anterior aspect of the thoracic spine.

The thoracic cavity contains the pleural cavities and the mediastinum. The pleural cavities contain the lungs. The mediastinum is the intrapleural region that separates the pleural cavities and is subdivided into four regions that contain the following structures:

1. Superior mediastinum (thymus gland, aortic arch and great vessels, trachea, bronchi, esophagus)

2. Anterior mediastinum (thymus gland, sternopericardial ligaments)

3. Middle mediastinum (pericardial cavity and related structures)

4. Posterior mediastinum (esophagus, thoracic aorta, inferior vena cava, azygos system, sympathetic chain)

Sternocleidomastoid m.

Sternocleidomastoid m.

Spinal Cavity Involves
Figure 2-4. Superficial layer of the muscles of the back. (From An HS. Principles and Techniques of Spine Surgery. Baltimore: Williams & Wilkins; 1998, with permission.)

Longissimus capitis m.^

Levator scapulae m. Splenius capitis m

Serratus post. superior m. Splenius cervicis m.

Iliocostal m. Longissimus m Spinalis m. Serratus post. inferior m.

Internal abdominal -oblique m.

Thoracolumbar fascia

Iliocostal m. Longissimus m Spinalis m. Serratus post. inferior m.

Internal abdominal -oblique m.

Thoracolumbar fascia

Transversus abdominis m.

Figure 2-5. Intermediate layer of the muscles of the back. (From An HS: Principles and Techniques of Spine Surgery. Baltimore: Williams & Wilkins; 1998, with permission.)

Superior nuchal line of skull

^ Rectus capitis post. major m. Sup. obliquus capitis m.

Rectus capitis post. major m. Inferior obliquus capitis m. Longissimus capitis m. Semispinalis capitis m.

Spinalis cervicis m.

Longissimus cervicis m. Iliocostalis cervicis m. — Iliocostalis thoracis m. Spinalis thoracis m. Longissimus thoracis m. Iliocostalis lumborum m. T12

Transversus abdominis m.

Figure 2-5. Intermediate layer of the muscles of the back. (From An HS: Principles and Techniques of Spine Surgery. Baltimore: Williams & Wilkins; 1998, with permission.)

Rectus capitis posterior minor m.

Posterior tubercle C1

Spinous process C2

Semispinalis capitis m Spinous process C7

External intercostal m

Semispinalis thoracis m.

Multifidus m

Ant. layer thoracolumbar fascia

Ant. layer thoracolumbar fascia

Iliac crest

Erector spinae m.

Figure 2-6. Deep layer of the muscles of the back. (From An HS: Principles and Techniques of Spine Surgery. Baltimore: Williams & Wilkins; 1998, with permission.)

Iliac crest

Erector spinae m.

' Sup. obliquus capitis m. Rectus capitis post. major m. Transverse process C1 Inferior obliquus capitis m.

Rotatores cervicis longus, brevis m.

Interspinalis cervicis m. Levator costae m.

Rotatores thoracis longus, brevis m.

Levator costarum brevis, longus m.

Interspinalis lumborum m. Lateral intertransversi m. Quadratus lumborum m. Multifidus m.

Figure 2-6. Deep layer of the muscles of the back. (From An HS: Principles and Techniques of Spine Surgery. Baltimore: Williams & Wilkins; 1998, with permission.)

Key Points

1. Thoracic spinal motion is limited by multiple anatomic constraints.

2. The blood supply to the thoracic spinal cord is less abundant than in the cervical or lumbar region.

3. The third through ninth ribs overlap the posterolateral aspect of the adjacent disc space.

4. The intercostal artery and vein are located along the inferior surface of the rib.

Websites

1. See spine anatomy index section, thoracic spine: http://www.spineuniverse.com/displayarticle.php/article1397.html

2. See thoracic spine anatomy: http://www.orthogate.org/patient-education/thoracic-spine/thoracic-spine-anatomy.html

BiBLiOGRAPHY

1. An HS: Principles and Techniques of Spine Surgery. Baltimore: Williams & Wilkins; 1998.

2. Herkowitz HN, Garfin SR, Eismont FJ, et al. Rothman-Simeone The Spine. 5th ed. Philadelphia: Saunders; 2006.

3. Hoppenfeld S, deBoer P: Surgical Exposure of the Spine and Extremities. 3rd ed. Philadelphia, Lippincott; 2003.

4. Schneck C: Functional and clinical anatomy of the spine. Phys Med Rehabil State Art Rev 1995;9(3).

5. Vaccaro AR: Spine Anatomy. In Garfin SR, Vaccaro aR, editors. Orthopaedic Knowledge Update—Spine, Vol. 1. Rosemont, IL: American Academy of Orthopaedic Surgeons; 1997, pp 3-18.

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