Thoracic Spine Anatomy
The thoracic region consists of 12 vertebrae, and is the largest segment of the spine. Due to its articulations with rib cage, the thoracic spine is more rigid than the cervical and lumbar regions. It protects thoracic viscera such as heart, lungs, and great vessels. Starting from early fetal period, the sagittal alignment of the thoracic spine is kyphotic, ranging between 20 to 40 degrees in adults.
The thoracic vertebrae are intermediate in size between the cervical and lumbar vertebrae. There are regional variations from Tl to T12. Tl 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 subdivided as upper (T1-T4), middle (T5-T8), and lower (T9-T12) thoracic areas.
Vertebral bodies: Thoracic vertebral bodies are heart-shaped, with a deeper anterior posterior dimension than medio-lateral width. The left side of the vertebral body may be slightly flattened or depressed due to pulsations of the aorta. The dimensions of the bodies increase as one moves down the spinal column (see picture).
Pedicles: The pedicles connect the vertebral bodies to the posterior elements. They are cylinder-like structures 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. The medial wall of the pedicle is bounded by the exiting nerve roots and the thecal sac. Laterally the pedicle is bounded by the costovertebral ligaments, joints, and ribs. Superiorly and inferiorly, the pedicles are bounded by the adjacent neural foramen. Pedicle sizes are around 6 to 8 mm from T1 to T12. The smallest sizes of pedicles are found between T3 and T6 and may only 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. In the proximal thoracic spine, pedicle sizes are slightly larger than those of middle thoracic levels. Pedicle axes show convergence at the axial (transverse) plane. These transverse angles are also show regional differences. The largest transverse angulations are observed at T1 (30 degrees). A steady reduction in transverse angulations is observed as one moves caudally, with the transverse inclination of the T12 pedicles being approximately 0 degrees. The pedicles are also show downward inclination at the sagittal plane. This inclination is at the posterior superior to anterior inferior direction, and is around 20 degrees.Determining the posterior projection of the pedicle on the posterior elements is of crucial importance during pedicle screw placement. 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.
Facet Joints: 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.
Transverse Processes: 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.
Lamina and Spinous Processes: 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 rostral spinous process. Both the laminae and the spinous processes slope in a superior anterior to posterior inferior direction. They serve as attachment points for the segmental spinal muscles and the thoracolumbar fascia.
Each thoracic segment is accompanied by a pair of ribs. The ribs articulate with the spinal column posteriorly, and the sternum anteriorly. Ribs 1 to 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 (see picture).
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 disk 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 (see picture).
The functions of the ligaments are to allow and limit physiologic motion, to contribute to the spinal stability, and to protect the spinal cord in trauma. There are seven spinal ligaments (see picture).
Anterior Longitudinal Ligament: The anterior longitudinal ligament (ALL) runs down the ventral surface of the spine from the occiput to the sacrum. The ALL is thicker and narrower in the thoracic region. The ligament is thick and slightly more narrow over the vertebral bodies and firmly attached to the edges of the vertebral bodies. On contrary, the ALL thinner but slightly wider over the intervertebral discs and it is loosely attached to the disk annulus. 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 membrana tectoria, and runs over the posterior surfaces of the bodies of the vertebrae, down to the coccyx. It is wider over the intervertebral disk than over the vertebral body. This is important in understanding the typical location for a disc herniation. 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 vertebae, and deeper layers which extend between adjacent vertebrae.
Intertransverse Ligaments: 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: In general, 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 anteroinferior edge of the lamina to the posterosuperior edge of the lamina below. They are best seen from the interior of the vertebral canal. It extends from C2 to S1. The ligament consists of two lateral portions which start out on either side of the roots of the articular processes, and extend backward to the midline; the posterior margins of the two portions are in contact and to a certain extent united, there are small gaps for the passage of small vessels. 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. Hypertrophy of this ligament is one of the major reasons of spinal stenosis.
Interspinous Ligaments: 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 below. The ligament is thin and membranous, and narrow and elongated in the thoracic region. They meet the ligamentum flavum in front and the supraspinal ligament behind.
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.
Because most body weight lies anterior to the vertebral column, the many strong muscles attached to the spinous and transverse processes are necessary to support and move the vertebral column. Those muscles and the complex neuromuscular controls is essential to provide spine stability, and to produce balanced spinal motion during physiologic activities. The spinal musculature does not only play an important role in spine stability, motion, and balance; but also interrelate with the upper and lower extremity function.
There are two major groups of muscles in the back. The extrinsic back muscles produce and control limb and respiratory movements. The intrinsic (deep) back muscles act on the vertebral column, maintaining posture and producing its movements (see picture).
Extrinsic Back Muscles
The extrinsic back muscles include superficial and intermediate muscles. The superficial extrinsic back muscles include trapezius, latissimus dorsi, levator scapulae, and rhomboids. The function of this muscle group is to connect the upper limbs to the trunk and produce and control limb movements. Although located in the back, these muscles receive their nerve supply from the anterior rami of cervical nerves and act on the upper limb. The trapezius receives its motor fibers from cranial nerve XI, the spinal accessory nerve. The intermediate extrinsic back muscles include serratus posterior superior and inferior. Those are thin, superficial respiratory muscles. They are mostly described as 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.
Intrinsic Back Muscles
The intrinsic back muscles (also called as deep back muscles, muscles of back proper) are act to maintain posture and control movements of the vertebral column, and innervated by the posterior rami of spinal nerves. The deep muscles of the back are separated from the overlying muscles by fascia. That fascia 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 cervical and lumbar transverse processes and to the angles of the ribs. The thoracic and lumbar parts of the deep fascia constitute the thoracolumbar fascia. In the thoracic region, the fascia over the muscles is thin, attached in the midline to the spinous processes, and laterally, at the edge of the muscles to the ribs and fascia, covering the intercostal muscles.
These muscle groups can be differentiated by the direction of their fibers. A distinction is also made on the length of the muscle fibers, whether they attach to the ribs and also by the area of the muscle—capitis, cervicis, thoracis, and lumborum.
The deep back muscles can be grouped into superficial, intermediate, and deep layers according to their proximity to the surface. There is also another group, which can be classified as minor deep layer muscles. Superficial layer of intrinsic back muscles include splenius capitis and cervicis muscles. The splenii arise from the midline and extend superolaterally to the cervical vertebrae (splenius cervicis) and cranium (splenius capitis). The splenii cover and hold the deep neck muscles in position. The splenius capitis arises from the ligamentum nuchae and the spinous processes from C7 to T3 or T4 and attaches to the superior nuchal line and mastoid process of the skull. The splenius capitis arises from the spinous processes below the splenius capitis, usually T3 to T6.
Intermediate layer of intrinsic back muscles also known as erector spinae muscles and include iliocostalis, longissimus and spinalis muscles. This group is the chief extensor of the vertebral column. These three muscles occupy the area between the spinous processes centrally and the angles of the ribs laterally. They are ordered lateral to medial: the iliocostalis forms the lateral column, the longissimus forms the intermediate column, and the spinalis forms the medial column. Each column is divided regionally into three parts according to the superior attachments: iliocostalis is subdivided into the cervicis, thoracis and lumborum; the longissimus and spinalis are subdivided into the capitis, cervicis, and thoracic portions.
Deep layer of intrinsic back muscles also known as transversospinal muscle group, and include semispinalis, multifidus, and rotatores muscles (Picture 6)
As all transversospinalis mucles have common origins and insertions, the muscle layers are difficult to accurately separate from each other. They are much shorter muscles, situated deep to the erector spinae, and run obliquely. These muscles originate from transverse processes of vertebrae and pass to spinous processes of more superior vertebrae. They occupy the valley between the transverse and the spinous processes.
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.
As it is stated above, the deep back muscles can be grouped into superficial, intermediate, and deep layers. However, there is also another group, which can be classified as minor deep layer muscles. These are: The interspinal (connects spinous processes), intertransverse (connects transverse processes), and elevators of ribs (represent the posterior intertransverse muscles of the neck) are minor deep back muscles that are poorly developed in the thoracic region.
Anterior spinal muscles
Other than all those abovementioned back muscles, there are anterior spinal muscles. The anterior musculature is found in the cervical and lumbar regions, not in the thoracic region. Namely, these are longus colli, capitis, quadratus lumborum and psoas muscles. These are function as spine stabilizers. Some muscles of the body do not attach to the spinal column directly but play a role in spinal stability and motion (abdominal muscles) or in pelvic stabilization forming a stable base for the spine (glutei and hamstrings).