INTRODUCTION
The thoracic region is the largest segment of the spine consisting of 12 vertebrae. It has load bearing function, contributes to erect posture, protects the spinal cord and also the thoracic viscera such as heart, lungs, and great vessels. Certain anatomical and functional properties of the thoracic spine make it susceptible to various pathological conditions. Due to its articulations with rib cage, the thoracic spine is the least mobile spine region. This property makes it less vulnerable to the degenerative conditions such as disc herniation, canal narrowing and listhesis, compared with the lumbar and cervical regions. On the other hand, probably due to high amount of bone tissue, most of the spinal metastases are seen in the thoracic region. Starting from intrauterine life, the sagittal alignment of the thoracic spine is kyphotic, ranging between 20 to 40 degrees in adults. Sagittal and coronal plane alignment problems are not rare in the thoracic area, and may be observed in the form of some specific diseases such as adolescent idiopathic scoliosis.
Anatomical knowledge is the foundation of surgical techniques, as well as all medical skills. This chapter aims to give a detailed description of the thoracic spine as well as its relationship with the neighboring structures. Special emphasis is placed onto the clinical relevancy, whenever applicable. Information contained here is given in a topographical, layer-by-layer fashion starting from outside to inside. Thus, we discuss those aspects and structures of the spine in a topographical order—surface topography and landmarks, muscles, the vertebra, the intervertebral disc, the ligaments, facet joints, rib cage, spinal cord, the viscera.
SURFACE TOPOGRAPHY AND LANDMARKS
Skin surface landmarks give valuable information about location of underlying spinal structures; thus, they may beuseful when evaluating spinal deformities and also planning surgery.
Spinous process of C7, vertebra prominens, lies in the midline at the border of cervical and thoracic regions. Of the note that, the first prominent spinous process at the base of the neck frequently belongs to the C6 vertebra, not the C7.
In the erect posture and neutral shoulder position, the scapula corresponds to the upper half of the thoracic spine, and its medial border is approximately 5–6 cm far from the midline (Fig. 1). The superior angle of the scapula is level with the first thoracic vertebra and the first rib. The spine of the scapula corresponds to the level of the T3, and the inferior angle is level with the T7-T8.
Thoracic spine is kyphotic, ranging between 20 to 40 degrees in adults. Apex of the physiologic thoracic kyphosis generally corresponds to the T6-T7 levels.
MUSCLES
The spine is surrounded and supported by various muscles. Those muscles do not only supports spine stability and motion; but also related with breathing and movements of the upper and lower extremity.
Back muscles are complex, and can be classified into extrinsic and intrinsic groups, or anterior and posterior groups. The intrinsic back muscles attach directly to the vertebral column and responsible for its posture and movements. On the other hand, the extrinsic back muscles do not attach to the spinal column directly and mainly are related with limb and respiratory movements.2
Figure 1: In the neutral shoulder position, the superior angle of the scapula is level with the first thoracic vertebra and the first rib; the spine of the scapula corresponds to the level of the T3; and the inferior angle is level with the T7-T8
Figure 2: Posteriorly, there are three groups of back muscles: superficial, intermediate, and deep muscles. Superficial and intermediate groups are classified as extrinsic, and deep muscle group as classified as intrinsic muscles
Posterior Muscles
Posteriorly, there are three groups of back muscles—superficial, intermediate, and deep muscles. Superficial and intermediate groups are classified as extrinsic, and deep muscle group as classified as intrinsic muscles (Fig. 2).1
The superficial extrinsic back muscles include m. trapezius, latissimus dorsi, levator scapulae, and rhomboids. This muscle group connects the upper limbs to the trunk and controls limb movements, and receives their nerve supply from the anterior rami of cervical nerves.
Musculus trapezius arises from the external occipital protruberance, nuchal ligament, and the C7 to T12 spinousprocesses, and it inserts on the spine of the scapula, acromion, and the lateral third of the clavicle. It receives its motor fibers from the spinal accessory (11th cranial) nerve).
The latissimus dorsi muscle arises from the posterior iliac crest, the lumbar aponeurosis, and the lower four ribs. It attaches to the intertubercular groove of the humerus. It adducts, extends, and internally rotates the arm. The latissimus dorsi is innervated by the thoracodorsal nerve.
The intermediate extrinsic back muscles include serratus posterior superior and inferior. Those are thin, superficial respiratory muscles of the thoracic wall, and innervated by intercostal nerves. The serratus posterior superior lies deep to the rhomboids, and the serratus posterior inferior lies deep to the latissimus dorsi.
The deep (intrinsic) back muscles attach directly to the vertebral column and innervated by the posterior rami of spinal nerves. They maintain posture and control movements of the spine. Those muscles are covered by a fascia, which attaches medially to the nuchal ligament, the tips of the spinous processes of the vertebrae, the supraspinous ligament, and the median crest of the sacrum. It attaches laterally to the transverse processes and to the angles of the ribs. The thoracic and lumbar parts of this deep fascia form the thoracolumbar fascia.
The deep back muscles can again be grouped into the superficial, intermediate, deep, and minor deep layers.
- Superficial layer includes splenius capitis and cervicis muscles. These muscles cover and hold deeper intrinsic muscles in position. They arise from the midline and extend superolaterally to the cervical vertebrae (splenius cervicis) and cranium (splenius capitis).
- Intermediate layer of intrinsic back muscles include iliocostalis, longissimus and spinalis muscles. This group is also known as erector spinae and acts as the main extensor of the spine. They occupy the area between the spinous processes and the angles of the ribs. From medial to lateral, their order is: the spinalis (the medial column), the longissimus (intermediate column), and the iliocostalis (the lateral column). Each column is further divided according to their region: the spinalisand longissimus are subdivided into the capitis, cervicis, and thoracic portions. Iliocostalis is subdivided into the cervicis, thoracis and lumborum.
- Deep layer of intrinsic back muscles also termed as transversospinal muscle group, and include m. semispinalis, multifidus, and rotatores. They occupy the groove between the transverse and the spinous processes. They are difficult to accurately identify from each other, since they are all short and muscles, situated deep to the erector spinae, and run obliquely.3 The semispinalis is the superficial member of the group. It is divided into three parts according to the superior attachments—semispinalis capitis, semispinalis thoracis, and semispinalis cervicis. The multifidus is the middle layer of the group and consists of short, triangular muscular bundles that are thickest in the lumbar region. The rotatores are the deepest of the three layers of transversospinal muscles and are best developed in the thoracic region.
- Thelast group of deep intrinsic muscles are minor deep layer. These are the interspinal, inter transverse, and ribs elevators. Those are thin and poorly developed muscles in the thoracic region.
Anterior Muscles
The muscles anterior to the spine is found in the cervical and lumbar regions, not in the thoracic region. Those are longus colli, longus capitis, quadratus lumborum and psoas muscles. They function as spine stabilizers.
Some anterior muscles do not attach to the spine directly, but play important roles in spinal stability and motion (abdominal muscles) or pelvic stabilization, forming a stable base for the vertebral column (glutei and hamstrings).
THE VERTEBRA
The thoracic vertebrae are intermediate in size between the cervical and lumbar vertebrae. There are regional variations from T1 to T12. T1 shows some similarities to the cervical vertebrae with uncinate processes protruding from its superolateral edges. T2 to T8 are quite uniform, and are considered as typical thoracic vertebrae. T9 to T12 are the transition vertebrae, and shows some similarities with lumbar vertebrae. Thoracic region can also be arbitrarily subdivided as upper (T1-T4), middle (T5-T8), and lower (T9-T12) thoracic areas.
Vertebral Body
Thoracic vertebral bodies are heart-shaped, with a deeper anterior posterior dimension than mediolateral width. The dimensions of the bodies gradually increase from superior to inferior. The left side of the vertebral body may be slightly flattened due to pulsations of the aorta. As a unique feature, posterolateral corners of the vertebral bodies has articular surfaces (costal facets). These facets articulate with head of the ribs (Fig. 3).
Pedicle
The pedicles are cylinder-like structures, and connect the vertebral bodies to the posterior elements. They have a cortical shell and variable amount of trabecular core, and their medial cortex is thicker than the lateral cortex. They are oval in cross section, having larger heights and smaller widths. However, their elliptical shapes are highly variable.2 The medial wall of the pedicle is bordered by the exiting nerve root and the thecal sac. Laterally the pedicle is bordered by the costovertebral ligaments, joints, and ribs. Superiorly and inferiorly, the pedicles create the upper and lower walls of the neural foramen.
Pedicle screw is the strongest spinal anchor and have revolutionized spinal instrumentation. For a safe insertion, a complete understanding of the quantitative pedicle anatomy is a must. To achieve this, sizes (height, width, length) and direction (angles on axial and sagittal planes) of the pedicles should be acknowledged. The pedicle diameter is usually largest in the low thoracic region, smallest in the mid-thoracic, and medium in size in the upper thoracic.3 The smallest sizes of pedicles are found between T3 and T6 and may be 10 mm in height and 4 mm in width. The largest pedicle diameter is usually in the low thoracic region, being 14 mm in height and 8 mm in height at T12 level.
Pedicle axes show convergence at the axial plane. These transverse angles decrease from superior to inferior. The largest transverse angulations are observed at T1 (30 degrees). A steady reduction in transverse angulations is observed as one moves caudally, and it decreases to 0 degree at T12 level.
The pedicles are also show angling at the sagittal plane. This angling is at the posterior superior to anterior inferior direction, and is around 20 degrees. However, large pedicle heights allow for different trajectories for pedicle screw insertion. Utilization of polyaxial screws enables the surgeon to connect those screws (with different sagittal plane angulation) with a rod easily.4
Probably the most important point during pedicle screw insertion technique is the selection of the entrance point. The center of the pedicle projection lies at the intersection of a line drawn parallel to the edge of the transverse process in its superior one-third, and a line drawn 1 to 2 mm medial to the lateral edge of the lamina. Thoracic pedicle screws may also be placed in a lateral extrapedicular fashion to engage the lateral aspect of the pedicle and the medial aspect of the rib. This may be a necessary if the pedicle is very thin, or as an salvage technique. In this case, entrance point is placed more lateral, on the transverse process.
Transverse Process
The transverse processes join the pedicles and the laminae at their bases. They extend lateral and posterior, to make room for the ribs to pass anterior to them. They articulate with the ribs on their anterolateral surface. T1 transverse processes is the largest and then gradually decrease in size toward T12. The angulation in the transverse plane changes from fairly flat at T1 to more posteriorly protruding at T12. At the lumbar spine, this posteriorly directed and small transverse processes transform to tiny accessory processes. On the other hand, ribs transform to transverse processes of the lumbar spine.
Lamina and Spinous Process
The thoracic vertebrae typically have long, slender spinous processes that point downward and overlap vertebral arches of the vertebra below. Thus, thoracic laminectomy requires removal of the inferior portion of the spinous process of the cranial vertebra. They serve as attachment points for the segmental spinal muscles and the thoracolumbar fascia.
Articular Process
The thoracic facets oriented in the coronal plane. Thus, thoracic facets allow primarily lateral bending and axial rotation. The superior articular facet of the caudal vertebrae forms the roof of the neural foramen.
Intervertebral Disc
Thoracic discs are thinner and narrower than those in the cervical and lumbar regions. Mean disc height in the thoracic spine is around 5 mm. However, disc size gradually increases from superior to inferior, likewise vertebral bodies. Figure 4 presents radiographical and direct (cadaveric) measurements of the thoracic disc heights level by level.4
The size of the nucleus pulposus is quite small in the thoracic region.
Figure 4: Direct and radiographic measurements of thoracic disc heights (n = 129).2 Horizontal black lines represents median values
Thus, a nuclear protrusion is very rare in the thoracic spine, and small in size if it exists. Most of the protrusions are usually of the annular type and may be calcified.
LIGAMENTS
There are seven spinal ligaments in the thoracic spine (Fig. 5).
Anterior Longitudinal Ligament
The anterior longitudinal ligament (ALL) runs down the anterior surface of the all over the spine, from the occiput to the sacrum. Its main function is to restrict the extension. The ALL is thicker and narrower in the thoracic region. The ligament is thick and slightly narrower over the vertebral bodies and firmly attached to the edges of the vertebral bodies. Conversely, the ALL thinner and slightly wider over the intervertebral discs and loosely adheres to the annulus fibrosus.
The ligament has three layers—superficial, intermediate and deep. The superficial layer traverses three or four vertebrae, the intermediate layer covers two or three and the deep layer is only between individual vertebrae. The deep layers of the ligament, as it crosses the vertebral body, blend with the periosteum.
Posterior Longitudinal Ligament
The posterior longitudinal ligament (PLL) is situated within the vertebral canal. It arises from the posterior aspect of the basiocciput, is continuous with the membrane tectoria, and runs over the posterior surfaces of the bodies of the vertebrae, down to the coccyx. It is wider over the intervertebral disc than over the vertebral body. The PLL is composed of smooth, shining, longitudinal fibers, denser and more compact than those of the ALL, and consists of superficial layers occupying the interval between three or four vertebrae, and deeper layers which extend between adjacent vertebrae.
Intertransverse Ligament
The intertransverse ligaments are situated between the transverse processes. In the thoracic region, they are rounded cords, and closely connected with the deep muscles of the back.
Capsular Ligaments
A capsular ligament is a part of the articular capsule that surrounds a synovial joint. In the spine, 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.
Ligamentum Flavum
The ligamentum flavum connects the antero-inferior edge of the lamina to the postero-superior edge of the lamina below. It extends from C2 to S1. This ligament is composed mainly of elastic fibers, and assists the vertebral column to resume after flexion. Its hypertrophy is one of the major reasons of spinal stenosis. The ligamentum flavum is thicker in the thoracic region.
Interspinous Ligament
The interspinous ligaments connect adjacent spinous processes, and runs obliquely from the antero-inferior aspect of the spinous process above to the postero-superior aspect of the spinous process below. The ligament is thin and membranous, narrow and elongated in the thoracic region.
Supraspinous Ligament
The supraspinous ligament is a strong fibrous cord, which connects together the tips of the spinous processes from the C7 to the sacrum. In the cervical area, it extends as the ligamentum nuchae. The supraspinous ligament consists largely of tendinous fibers derived from the back muscles and is better developed in the upper lumbar region and is often absent in the lower lumbar region. The ligament closely blended with the neighboring fascia. The most superficial fibers of this ligament extend over three or four vertebrae; those more deeply seated pass between two or three vertebrae while the deepest connect the spinous processes of neighboring vertebrae. Between the spinous processes, it is continuous with the interspinous ligament.
Facet Joints
The thoracic facet joints are quite uniform throughout the region. They consist of two adjacent flattened surfaces oriented in the coronal plane. This orientation allows primarily lateral bending and axial rotation. The superior articular facet of the caudal vertebrae situated ventral and forms the roof of the neural foramen. Medially it borders the lateral aspect of the spinal cord.
Rib Cage
Each thoracic segment is accompanied by a pair of ribs. The ribs articulate with the spinal column posteriorly, and the sternum anteriorly. Ribs 1–7 are termed as true ribs. They articulate with the sternum directly. Ribs 8-10 are termed as false ribs, and they articulate with the costocartilage of the rib above. Ribs 11 and 12 are termed as floating ribs, because they do not articulate to either the sternum or the costal cartilage at their distal ends.
Costovertebral Joints (Costal Facets)
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 disc and the vertebra above. Also, each pair of ribs articulates with the anterior surface of the transverse process of its named vertebrae. The ribs are connected to the vertebral column by the costovertebral ligaments. The space between the transverse process, the lateral edge of the pedicle, and the medial edge of the rib constitutes a triangular osteoligamentous zone. This zone can be used for extrapedicular screw placement.
The Spinal Cord
The spinal cord ends approximatelyat the L1 vertebral body level. It is at risk during placement of instrumentation, deformity correction, and from ischemic episodes especially when large kyphotic deformities are corrected. There are certain anatomical properties of the thoracic spine causing high risk of ischemic events. Unlike the cervical and lumbar regions, the thoracic spinal canal is small and circular. Mid-thoracic zone (T4–T9) has the narrowest spinal canal diameter. The spinal cord occupies the space of the spinal canal maximally at the thoracic level. The cord depends for its blood supply on the inner and outer circles.5 The inner circle of three longitudinal arteries descend from the medulla oblongata to the conus medullaris. Their perforating arteries to the spinal cord are larger and more numerous at the cervical and lumbar level than at the thoracic level. Moreover, this arterial circle is characterized in the thoracic spine by a lack of anastomoses. This poor vascularization together with small canal diameter renders the thoracic spine vulnerable to ischemic damage.
The Viscera
The thoracic spine protects not only the spinal cord, but also the thoracic viscera. Understanding the relationship of the spine with neighboring viscera especially important for anterior surgery. Thoracic vertebral levels and their relationship with the thoracic cavity are as follows: T1–T4 corresponds to the superior mediastinum, T5–T9 corresponds to the inferior mediastinum, and T9–T12 corresponds to the basis of lungs and diaphragm attachments (Fig. 6).
The aorta lies at the left side of the vertebral body. Thus, left side is the preferred side of the thoracotomy to reach the spine because it is easier to deal with an artery than that of a vein during surgery. The segmental arteries may be needed to ligate in order to complete an anterior decompression and/or stabilization procedure (Fig. 7).
The diaphragm is attached to both sides of T12, L1 vertebrae and to the right side of L2 (Fig. 8). Musculoskeletal and visceral anatomy dictates the selection of the approach to the spine, either anterior or posterior. Table 1 and Figure 9 outlines the approaches to the cervicothoracic spine and their target levels.
Figure 8: The diaphragm and its relationship with the spine. The diaphragm is attached to both sides of T12, L1 vertebrae and to the right side of L2
REFERENCES
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- Kothe R, O'Holleran J, Liu W. Internal architecture of the thoracic pedicle. Spine. 1996;21:264–70.
- Panjabi MM, O'Holleran JD, Crisco JJ 3rd, Kothe R. Complexity of the thoracic spine pedicle anatomy. Eur Spine J. 1997;6:19–24.
- Kutoglu T, Kilincer C, Hamamcıoglu MK, Tuncbilek N, Okten O, Mesut R, et al. Agreement Between Radiographic and Surgical Measurements of Intervertebral Disc Height: A Cadaveric Study. TrakyaUniv Tip FakDerg. 2010;27(4):385–90.
- Dommisse G. The blood supply of the spinal cord. J Bone Joint Surg. 1974;(2):223–35.