The Thoracic Region

Vertebral Bodies

The size of the thoracic vertebral bodies is approximately the same in males and females of all races and of all adult ages (Masharawi et al., 2008; Limthongkul et al., 2010). The vertebral bodies of the typical thoracic vertebrae (T2 to T8) are larger than those of the cervical region (Fig. 6-1). The volume of the vertebral bodies increases from T1 to T12 (average 15.0 cm³; range 5.2 to 39.5 cm³) (Limthongkul et al., 2010). Thoracic vertebral bodies appear to be heart-shaped when viewed from above, primarily because of a marked concavity of the posterior aspect of the vertebral body in the region adjacent to the vertebral foramen. The posterior edge of the superior surface of upper thoracic vertebral bodies exhibits small remnants of the cervical uncinate processes (Dupuis et al., 1985).

The left-right (lateral) width of the thoracic vertebral bodies is greater than the anterior-posterior (length) and superior-inferior (height) dimensions. The lateral width decreases from T1 through T3 and then gradually increases throughout the thoracic region (continuing into the lumbar region until L4-L5). The decreasing width of the upper thoracic vertebral bodies may allow for increased lateral flexion (and possibly rotation) of the cervical region (Masharawi et al., 2008). In addition, the lateral width of a thoracic vertebra is smaller at the superior versus inferior vertebral margin of the vertebra and the lateral width of the inferior aspect of each vertebra is always larger than the superior width of the subjacent vertebra. Consequently, in a coronal (frontal) section the vertebral bodies have a trapezoid shape (i.e., narrower superiorly) and the intervertebral discs have an inverted trapezoid shape (i.e., narrower inferiorly) (Masharawi et al., 2008). Typical thoracic vertebrae also are more flattened on their left than right surfaces because of pressure from the thoracic aorta.

The thoracic vertebral bodies are wedged-shaped, the superior-inferior height being smaller anteriorly than posteriorly. The wedging increases from T1 to T7 and then begins to decrease incrementally until L2 (Masharawi et al., 2008). The posterior aspect of a typical thoracic vertebra is approximately five times the height of the posterior aspect of the intervertebral disc immediately above the vertebra (Harrison et al., 2003).

The superior-inferior height of most thoracic vertebral bodies is asymmetric with the right side usually greater than the left side. This “left lateral wedging” is typically found in one or several (usually 3) contiguous vertebrae, “interrupted” by a single vertebra of symmetric left and right heights, followed by additional vertebrae with left lateral wedging. This pattern is repeated throughout the thoracic region. Further investigation is needed to determine if the thoracic intervertebral discs normally compensate for the left lateral wedging of the vertebral bodies. The mild lateral wedging may help to provide stability to the thoracic region during the normal spinal loading (i.e., ground reaction force) that occurs throughout the day. The left lateral wedging is more pronounced in females (92% of females) than in males (86% of males). The female-predominant left lateral wedging may explain the clinical findings that the large thoracic curves of most adolescent idiopathic scolioses are most often found in girls and are usually convex to the right (Masharawi et al., 2008).

The superior-inferior height of the T1 and T2 vertebral bodies is greater than the anterior-posterior length. This ratio is reversed throughout the rest of the thoracic region. Consequently, the T1 and T2 vertebral bodies are more rounded when viewed from the left or right sides. This configuration may help to accommodate the large amount of flexion and extension that occurs in the cervical region (Masharawi et al., 2008). The T2 vertebral body is somewhat cervical in appearance, being slightly larger in transverse than anteroposterior diameter. The body of the T3 vertebra is the smallest of the thoracic region; the vertebral bodies gradually increase in size below this level. The vertebral bodies of T5 through T8 become more and more heart-shaped. This means that the concavity of the posterior aspect of the vertebral bodies becomes more prominent. The heart-shaped appearance also is accentuated because the anteroposterior dimension of the vertebral bodies increases more than the transverse (lateral) dimension at these levels.

The T9 through T12 vertebral bodies begin to acquire lumbar characteristics (see the following discussion) and enlarge more in their transverse than anteroposterior dimension. The T12 vertebral body is similar in shape to that of a lumbar vertebra.

The thoracic vertebral bodies become stronger from upper to lower thoracic vertebrae. This results from an increase in bone density that is probably a response to the increase in compressive forces placed on the successively lower vertebral bodies (Humzah & Soames, 1988).

The neurocentral synchondrosis (see Chapter 12), found between the neural arches and centra of developing vertebrae, fuses first in the lumbar and cervical regions and last in the thoracic region. This fusion may be incomplete in the thoracic region, even in adults (Edelson & Nathan, 1988).

Osteophytes are more prominent at T9-10 than at other thoracic levels. Generally, the aorta decreases the formation of osteophytes (bone spurs) on the thoracic vertebral bodies. Recall that the thoracic aorta courses along the left side of the thoracic vertebral bodies. It then begins to move to the anterior surface of the lower thoracic vertebrae, before passing through the aortic hiatus of the diaphragm. Consequently, osteophytes are found more on the right than left sides of the typical thoracic vertebral bodies (Nathan, 1962; Edelson & Nathan, 1988).

Typical thoracic vertebral bodies have four small facets, two on each side, for articulation with the heads of two adjacent ribs. These facets are known as costal demifacets (literally, “half-facets”) because the head of each rib articulates with both the superior demifacet of the vertebra with the same number and the inferior demifacet of the vertebra above (see Fig. 6-9). For example, the head of the sixth rib articulates with the superior demifacet of T6 and the inferior demifacet of T5. A ridge on the head of each rib, known as the crest of the head, is located between the two articular surfaces of the rib head. The crest of the head of each rib has a ligamentous attachment (intraarticular ligament) to the intervertebral disc (IVD) between adjacent thoracic vertebrae. A fibrous capsule surrounds each vertebral demifacet and continues to the rib surrounding the articular surface on the corresponding half of the rib head. The capsule is lined by synovium, making the costovertebral joint (costocorporeal joint) a synovial joint (diarthrosis). The radiate ligament extends from the head of each rib to the adjoining vertebral bodies and the surface of the intervening IVD (see Costocorporeal Articulations).

Several structures attach to the thoracic vertebral bodies. Table 6-1 summarizes these attachments.

Pedicles

The pedicles of the thoracic spine are long and stout (Fig. 6-1). In fact, the pedicles in the lower thoracic region are larger than the pedicles of the upper lumbar vertebrae (Ofiram et al., 2007). The size of the thoracic pedicles varies considerably among both individuals and vertebrae, but the left and right pedicles of the same vertebra usually are similar (McLain, Ferrara, & Kabins, 2002). The T4 pedicles are the narrowest (left-to-right dimension) and then the pedicles of T5 to T12 become increasingly wider (Kretzer et al., 2011). The pedicles of males are wider than those of females (Pai et al., 2010; Kretzer et al., 2011), and there is ethnic variation in the size of the pedicles; for example, the dimensions of thoracic pedicles are smaller among East Indians and white populations (Acharya, Dorje, & Srivastava, 2010).

Unlike the pedicles of the cervical vertebrae, cancellous bone, rather than cortical bone, predominates in the thoracic pedicles. However, like the cervical pedicles, the cortical bone of the lateral wall of a typical thoracic pedicle is thinner than that of the medial wall (Kothe et al., 1996).

The thoracic pedicles become larger along their inferior surface from T1 to T12. Also, unlike the cervical pedicles, which attach at a significant lateral angle with the cervical vertebral bodies, the thoracic pedicles form only a slight lateral angle in the transverse plane with the thoracic vertebral bodies (and T12 forms no lateral angle with the vertebral body in this plane, i.e., a 90º angle to the vertebral body). The thoracic pedicles incline slightly superiorly in the sagittal plane (Marchesi et al., 1988). They also attach very high on their respective vertebral bodies; as a result, no superior vertebral notch is associated with typical thoracic vertebrae. T1, which is atypical, does have a superior vertebral notch (see First Thoracic Vertebra later in this chapter). On the other hand, the inferior vertebral notches of the typical thoracic vertebrae are very prominent.

Transverse Processes

The transverse processes (TPs) of typical thoracic vertebrae project obliquely posteriorly (see Chapter 2) (Fig. 6-1). They also lie in a more posterior plane than those of the cervical or lumbar regions, being located behind the pedicles, intervertebral foramina, and articular processes of the thoracic vertebrae (Standring et al., 2008). The TPs also become progressively shorter from T1 to T12; therefore the distance between the tips of the left and right TPs is the greatest at T1 and then decreases incrementally until T12, where the TPs are very small. This distance increases in the lumbar region (see Chapter 7). As with other components of thoracic vertebrae, the left and right transverse processes are asymmetric in length, with the left transverse process being longer than the right. In addition, the transverse process of males are larger than those of females (Masharawi & Salame, 2011).

Each thoracic TP possesses a facet for articulation with the articular tubercle of the corresponding rib (e.g., the TP of T6 articulates with the sixth rib). This facet is appropriately named the transverse costal facet, or costal facet of the transverse process, and is located on the anterior surface of the TP.

The first six transverse costal facets are concave and face not only anteriorly but also slightly laterally. The transverse costal facets inferior to T6 are more planar (flatter) in shape and face anteriorly, laterally, and superiorly. The forces applied to the ribs during movements, load carrying, or muscular contraction are transmitted through the TPs to the laminae of the thoracic vertebrae (Pal et al., 1988).

The TPs serve as attachment sites for many muscles and ligaments. Table 6-2 lists the most important attachments to the TPs of thoracic vertebrae.

Articular Processes

The superior articular processes of the thoracic spine are small superior projections of bone oriented in a plane that lies approximately 60 to 75 degrees to the horizontal plane (25 to 40 degrees to the coronal [frontal] plane) (White & Panjabi, 1990; Masharawi et al., 2004). This makes them much more vertically oriented than the cervical superior articular processes. The thoracic superior articular processes and their facets face posteriorly, slightly superiorly, and laterally (see Fig. 6-1). The left and right superior articular facets are usually asymmetric with the right facets being slightly more vertically oriented and the left ones facing slightly more laterally and also being slightly longer from superior to inferior (Masharawi et al., 2004, 2005). The inferior articular processes and their facets match those of the superior ones facing in the opposite direction—that is, anteriorly, slightly inferiorly, and medially, with matching asymmetry as well. The facet orientation and asymmetry are the same for males and females of all ages and racial origins (Masharawi et al., 2004).

The orientation of the thoracic articular processes and their articulating facets allows a significant amount of rotation to occur in this region (see Ranges of Motion in the Thoracic Spine). Flexion and extension are limited primarily by the orientation of the thoracic facets (Oda et al., 1996), and lateral flexion is limited partly by the orientation of the facets. However, the firm attachments of the thoracic vertebrae to the relatively immobile thoracic cage, by means of the costocorporeal and costotransverse articulations, are the primary constraints to axial rotation and lateral flexion of the thoracic spine.

Spondylolysis is a fracture of the region between the superior and inferior articular processes (pars interarticularis). It is usually found in the lumbar region (see Chapter 7 for a discussion of this condition), has been rarely found in the cervical region, and is considered extremely rare in the thoracic region. The first case reported in the English literature was that of Shimada and colleagues (2006), who reported a case of bilateral spondylolysis at T11 with forward slippage of the T11 vertebral body (spondylolisthesis).

Zygapophysial (Z) Joints

The zygapophysial (Z) joints are important structures clinically. These joints are thought to be the source of pain in 48% of the cases of chronic thoracic pain (Schulte et al., 2010). The capsules of the thoracic Z joints are similar to those of the cervical and lumbar regions. However, there are fewer mechanoreceptors in the Z joint capsules of the thoracic region than in the cervical or lumbar regions (McLain & Pickar, 1998).

Z Joint Synovial Folds (Meniscoids)

Z joint synovial folds (meniscoids) have been found to protrude into approximately 62% of thoracic Z joints, which is a lower incidence than the cervical and lumbar regions. Some thoracic Z joints have more than one synovial fold (Ley, 1974; Schulte et al., 2010). The folds are usually located in the inferior (caudal) aspect of the joints, although they have been identified in all regions of the joint. The folds are smaller in the thoracic region than in the other regions of the spine, but as in the cervical and lumbar regions, the thoracic synovial folds are believed capable of becoming entrapped (i.e., trapped between the articular facets) or extrapped (i.e., trapped between the articular process and Z joint capsule), both situations being a theoretical source of back pain (Kos, Hert, & Sevcik, 2002). In addition, Z joint synovial folds have been identified as a source of intraarticular bleeding associated with destruction of facetal cartilage (Schulte et al., 2010).

Thoracic Z joint synovial folds usually originate from a base of well-vascularized connective tissue and extend as flat, solid structures between the articular facets. The synovial lining is found at the base of these folds and usually does not extend between the facetal surfaces, which is different from the lumbar region where the synovial lining has been found to extend along the entire surfaces of the folds. The thoracic folds are longest in the lower thoracic Z joints, are of intermediate length in the upper thoracic region, and are shortest in the mid-thoracic region. These synovial fold lengths correspond to the mobility of the thoracic Z joints (most mobile to least mobile = lower, upper, and middle thoracic Z joints, respectively) (Schulte et al., 2010).

Approximately 6% of thoracic Z joint synovial folds are fatty in nature, also well vascularized, but are found exclusively in the joint recesses and do not extend between the articular facets. Occasionally, the adipose tissue found in the base of the solid (preceding paragraph) and fatty types of synovial folds passes through the ligamentum flavum and becomes continuous with the epidural adipose tissue of the vertebral canal (Schulte et al., 2010).

Another 4% of thoracic Z joint synovial folds are simply well-defined thickenings of the joint capsule (Schulte et al., 2010).

Laminae

The laminae in the thoracic region are short from medial to lateral, broad from superior to inferior, and thick from anterior to posterior. As with other components of thoracic vertebrae, the left and right laminae are asymmetric in superior-to-inferior length, with the right laminae being longer than the left. In addition, the laminae of males are larger (superior-to-inferior) than those of females (Masharawi & Salame, 2011). They completely protect the vertebral canal from behind. Therefore no space exists between the laminae of adjacent vertebrae in a dried preparation. This is unique to thoracic vertebrae. The rotatores longus and brevis muscles partially insert on the laminae of the thoracic vertebrae.

Vertebral Canal

The vertebral canal in the thoracic region is more rounded in shape than in any other region (Fig. 6-1, A, C, and D). It is also smaller in the thoracic region than in either the cervical or the lumbar region (see Fig. 2-16). Compared to the other regions of the spinal cord, the thoracic spinal cord also is smaller.

Spinous Processes

The spinous processes of thoracic vertebrae generally are large (Fig. 6-1, B and E). The upper four thoracic spinous processes project almost directly posteriorly. The next four (T5 through T8) project dramatically inferiorly. The spinous process of T8 is the longest of this group. The last four thoracic spinous processes begin to acquire the characteristics of lumbar spinous processes by projecting more directly posteriorly and being larger in their superior-to-inferior dimension (see Unique Thoracic Vertebrae). The spinous processes of thoracic vertebrae serve as attachment sites for many muscles and ligaments. Because of the length of the thoracic region, attachments vary somewhat from the upper to lower thoracic vertebrae. Table 6-3 lists the attachments to the upper and lower thoracic spinous processes.

Intervertebral Foramina

The intervertebral foramina (IVFs) are covered in detail in Chapter 2. The IVFs in the thoracic region differ from those of the cervical region by facing directly laterally rather than obliquely anterolaterally. The lateral orientation of the thoracic IVFs is similar to that found in the lumbar region.

Unique to the thoracic region is that the T1 through T10 IVFs are associated with the ribs. The eleventh and twelfth ribs are not directly associated with IVFs. More precisely, the following structures are associated with the T1 through T10 IVFs: the head of the closest rib (e.g., T5-6 IVF associated with head of sixth rib), the articulation between the rib head and the demifacets of the vertebral bodies, including the associated ligamentous and capsular attachments with the vertebral bodies and the interposed IVD (see Costocorporeal Articulations). All these structures help to form the anterior and inferior boundaries of the first 10 thoracic IVFs. Pathologic conditions of these articulations may compromise the contents of the thoracic IVFs (Standring et al., 2008). For example, osteoarthritis (development of osteophytes) of the costocorporeal articulations may cause stenosis of the thoracic intervertebral foramina (Bailey & Casamajor, 1911).

Approximately one twelfth of the IVF contains a spinal nerve in the thoracic region, whereas up to 50% of the cross-sectional area of the IVF contains a spinal nerve (and dural root sleeve) in the cervical region (Sunderland, 1974), and approximately one third of the IVF is filled with a spinal nerve in the lumbar region. This may be one reason why radiculopathy as a result of IVD protrusion is much less common in the thoracic region than the lumbar or cervical areas. Thoracic disc protrusion is also less common than cervical or lumbar disc protrusion. One reason may be that the thoracic spine is rendered less movable than the cervical and lumbar regions. This is because the thoracic region is strongly supported by the ribs and sternum. The reduced motion may result in a reduction of stress on the thoracic IVDs.

Components of the Thoracic Cage

The components of the thoracic cage (Fig. 6-2) include the following:

Superior Thoracic Aperture (Thoracic Inlet)

The thoracic cage is bounded superiorly by the superior thoracic aperture and inferiorly by the inferior thoracic aperture. The superior thoracic aperture is bounded by the following: T1, first ribs (left and right), and superior aspect of the sternum. The superior thoracic aperture allows connection of the anatomic structures of the thorax and the neck.

The term thoracic inlet has a slightly different meaning. It refers to the superior thoracic aperture, the region just above the first rib, and the opening between the clavicle and the first rib. Ironically the term thoracic outlet syndrome is frequently used to describe symptoms and signs arising from compromise of the neural or vascular structures as they pass through the region of the thoracic inlet. The symptoms associated with this syndrome typically are felt in the distal aspect of the upper extremity rather than the area of neurovascular compromise (Bland, 1987). The occurrence of thoracic outlet syndrome remains a matter of clinical debate, with some authorities stating that true compression of these structures is extremely rare. Others are convinced that such compression is relatively common. This section discusses the areas and structures typically associated with the thoracic outlet syndrome.

The right and left subclavian arteries and veins pass through the superior thoracic aperture. These vessels may be compromised by pathologic conditions of the lower cervical or upper thoracic viscera. Examples include lymphosarcoma affecting the lymphatics of the thoracic inlet (Moore, 1992) and tumors of the apex of the lung (Pancoast tumor), esophagus, and thyroid gland.

As the subclavian arteries and veins exit the superior thoracic aperture, they are met by the inferior structures of the brachial plexus. These neural structures include the anterior primary divisions of C8 and T1 and their union as the inferior trunk of the brachial plexus. All these vascular and neural structures pass over the first rib. The subclavian vein passes over the first rib in front of the anterior scalene muscle. The subclavian artery and lower (inferior) trunk of the brachial plexus course directly across the first rib between the insertions of the anterior and middle scalene muscles and are thought to be vulnerable in this region. Anomalous insertion of the scalenes or an anomalous inferior trunk of the brachial plexus that pierces either the anterior or the middle scalene muscles may provide the means by which these structures can become entrapped. Extension of the neck and rotation to the same side closes the interval between the anterior and middle scalene muscles, providing another possible mechanism of compromise. An elongated TP of C7 or a cervical rib (see Chapter 5) can dramatically crowd this region, and many believe that either one is a significant contributor to “thoracic outlet syndrome” (Bland, 1987; Foreman & Crofts, 1988). Cervical ribs range considerably in size, and even the smallest cervical rib can be associated with fibrous bands that course from the cervical rib to the first rib or sternum (van Es, Bollen, & van Heesewijk, 2010). Any or all of these structures could restrict the subclavian vessels and inferior trunk of the brachial plexus.

The subclavian artery becomes the axillary artery at the lateral border of the first rib. Surrounding the transition region of the subclavian artery to the axillary artery are the divisions of the brachial plexus, which soon combine into the cords of the plexus. The divisions and cords accompany the axillary artery beneath the clavicle and can be compressed between the clavicle and the first rib.

The axillary artery is surrounded by the cords of the brachial plexus as the artery passes beneath the coracoid process of the scapula. The axillary vein accompanies the artery in this region. The pectoralis minor muscle passes anterior to these structures as it inserts onto the coracoid process. The axillary artery, axillary vein, and the cords of the brachial plexus may be compressed against the coracoid process and the tendon of the pectoralis minor muscle during abduction and lateral rotation of the arm.

Terminology for Thoracic Outlet Syndrome

Thoracic outlet syndrome is characterized generally by numbness, paresthesias, pain, or a combination of these symptoms along the posterior aspect of the shoulder, axilla, and arm regions with or without vascular compression. However, there is much confusion related to the terminology surrounding thoracic outlet syndrome. Because of this confusion, Ranney (1996) has suggested a completely different nomenclature for the condition. Figure 6-3 illustrates the anatomic structures involved and the regions used in Ranney’s (1996) nomenclature.

Ranney (1996) first suggests that the term thoracic outlet syndrome be replaced with the term cervicoaxillary syndrome. Subdivisions of this syndrome would be named based on the specific anatomic region associated with the pathologic compression of neural or vascular tissues. Second, he suggests that the terms superior thoracic aperture and inferior thoracic aperture should be used instead of the clinically confusing terms thoracic inlet (for the superior thoracic aperture) and thoracic outlet (for the inferior thoracic aperture). Next, he proposes that the appropriate term scalene triangle be used for the space bordered by the anterior scalene muscle anteriorly, the middle scalene muscle posteriorly, and the first rib inferiorly. He then suggests that the superior aspect of this triangle, housing the C5, C6, and usually the C7 cervical nerve roots, be called the cervical outlet. (Compression of these structures in this region sometimes is called scalenus syndrome or scalenus anterior syndrome.) The term thoracic outlet would be reserved for the inferior aspect of the scalene triangle for the region that normally houses the C8 and T1 nerve roots and the subclavian artery. The subclavian artery or vein or the divisions or cords of the brachial plexus can be compressed in the more distal costoclavicular space. This space is bounded by the clavicle superiorly and first rib inferiorly. Finally, the pectoralis minor space is the region bounded superiorly by the pectoralis minor muscle and coracoid process to which it inserts, as well as the thoracic cage inferiorly. The term hyperabduction syndrome has been used to identify compression of the axillary artery and the cords of the brachial plexus against the pectoralis minor tendon during prolonged abduction of the upper extremity. With these terms and definitions in mind, Ranney (1996) suggests that the term cervicoaxillary syndrome (CAS) be used for compression of neural or vascular structures anywhere along the thoracic outlet, costoclavicular space, or pectoralis minor triangle, and that the more specific subtypes of CAS to include thoracic outlet, costoclavicular, and pectoralis minor syndromes be used once the precise location of compression is identified. The term cervical outlet syndrome would be used to describe compression of the C5, C6, or C7 nerve roots in the upper portion of the scalene triangle and the terms scalene syndrome and scalenus anticus syndrome would be discarded.

Subclavius Posticus Muscle

An anomalous muscle known as the subclavius posticus muscle (also known as the chondroscapular muscle or the scapulocostalis minor muscle of Gruber) can be associated with CAS. When present, the muscle usually originates along the superior border of the scapula and the transverse scapular ligament and passes anteriorly, inferiorly, and medially to insert onto the costal cartilage of the first rib. Because of its innervation from branches of the brachial plexus (e.g., the suprascapular nerve), the muscle is thought to be related to the subclavius muscle. Anatomic dissections have found the upper and middle trunks of the brachial plexus (see Fig. 5-33), as well as the subclavian artery (Forcada et al., 2001) and subclavian vein (Akita et al., 2000), to show signs of compression by this muscle, suggesting that the muscle could cause some cases of a variant of the costoclavicular syndrome of CAS (using Ranney’s terminology) and also Paget-von Schrötter syndrome (Akita et al., 2000; Forcada et al., 2001). The latter syndrome is thrombosis of the axillary vein, or its proximal continuation as the subclavian vein, that either can develop spontaneously or can be related to strenuous activity involving the upper extremity. The anomalous subclavius posticus muscle is common, being found unilaterally or bilaterally in 8.9% of cadavers (Akita et al., 2000). This anomaly should be visible on magnetic resonance imaging (MRI) (Collins et al., 1995; Akita et al., 2000).

Inferior Thoracic Aperture

The inferior thoracic aperture (thoracic outlet) is bounded by the following: T12, twelfth ribs, anterior costal margins, and xiphisternal joint. The inferior thoracic aperture contains the diaphragm, which serves as the boundary between the thorax and abdomen.

General Characteristics of the Thoracic Cage

The thoracic cage functions to protect underlying structures, support underlying structures (e.g., pericardium via sternopericardial ligaments), serve as attachment sites for overlying muscles, support the skin and breasts, and assist in respiration.

The adult thorax is wider from side to side than front to back. The anteroposterior diameter increases during inspiration. This is quite different than a child’s thorax, which is circularly-shaped and therefore does not allow change to occur during inspiration. In contrast to adults, children rely almost completely on the excursions of the diaphragm for respiration.

Typical Ribs

The typical ribs are ribs three through nine. Each consists of a head, neck, tubercle, and shaft (Fig. 6-4).

The head of a typical rib articulates with two adjacent vertebral bodies (see Vertebral Bodies). Inferior and superior articular facets of the rib head articulate with the superior costal demifacet of the same-number vertebra as the rib and with the inferior costal demifacet of the vertebra above, respectively. The crest of the head is a ridge that runs between the two articular surfaces of the rib head. The crest is joined by the intraarticular ligament to the adjacent IVD. This creates the two separate components of the costocorporeal joints—one superior to the crest of the head of the rib and one inferior to the crest (see Costocorporeal Articulations and Fig. 6-9).

The neck of a typical rib is located between its head and tubercle. The neck serves as the attachment site for the costotransverse ligament and superior costotransverse ligament.

The tubercle of a rib is a process that forms the lateral boundary of the neck and the beginning of the shaft. It possesses an articular facet (articular portion) for articulation with the transverse costal facet on the TP of a typical thoracic vertebra. The tubercle of a rib articulates with the same-number vertebra as the rib (e.g., fourth rib articulates with TP of T4). The tubercle also contains a nonarticular part lateral to the articular portion. The nonarticular region serves as an attachment site for the lateral costotransverse ligament.

The shaft of a rib begins at the articular tubercle and extends distally to the end of the rib at its articulation with the costal cartilage. The typical ribs curve inferiorly and anteriorly. Much of this anterior curve is achieved at the angle of the rib. The angle of the rib is located a few centimeters distal to the articular tubercle and is where the shaft makes the sharpest anterior bend.

A costal depression or groove, located along the inferior aspect of each rib, shelters (from superior to inferior) the intercostal vein, artery, and nerve.

Anteriorly each typical rib attaches to a costal cartilage. The costal cartilage, in turn, joins each of the first through seventh ribs with the sternum. The eighth through tenth costal cartilages articulate with the costal cartilage immediately above. The xiphoid process, seventh costal cartilage, and union of the eighth through tenth costal cartilages together form the substernal angle.

Atypical Ribs

The first, second, tenth, eleventh, and twelfth ribs all have special features (Standring et al., 2008). The first rib is short, flat, and strong. It lies almost completely in the horizontal plane and does not angle inferiorly as do typical ribs. Its superior surface is marked by a scalene tubercle (for insertion of the anterior scalene muscle). The subclavian vein passes anterior to the scalene tubercle (and the anterior scalene muscle), and the subclavian artery and inferior trunk of the brachial plexus course posterior to this tubercle. The first rib usually articulates with only one vertebra (T1). Occasionally the head also articulates with the body of C7.

The second rib is much more typical than the first and is almost twice its size. The major distinguishing characteristic of the second rib is a tuberosity on its superior surface, which serves as the partial origin of the serratus anterior muscle.

The tenth rib has only a single facet, and no crest, on its head. The head articulates with the large, single costal facet on the lateral aspect of the body (close to the pedicle) of T10. Sometimes the head of the tenth rib also articulates with the IVD between T9 and T10.

The eleventh and twelfth ribs are short, and neither possesses a neck or tubercle. They are considered free, or floating, ribs because they do not attach to costal cartilage anteriorly. As with the first and tenth ribs, both the eleventh and twelfth ribs articulate with only one vertebra (T11 and T12, respectively).

Costocorporeal Articulations

The joint between the head of a rib and the adjoining typical thoracic vertebrae consists of articulations with the two adjacent vertebral bodies and interposed IVD (Fig. 6-9). The rib head articulates with the superior demifacet of the same-number vertebra and the inferior demifacet of the vertebra above (e.g., seventh rib articulates with superior demifacet of T7 and inferior demifacet of T6). The crest of the rib head is attached to the adjacent IVD by an intraarticular ligament. This short, flat ligament creates two distinct articular compartments (upper and lower) within the costocorporeal articulation. Both of these compartments are surrounded by a fibrous articular capsule lined with a synovial membrane. These synovial joints can best be classified as having ovoid articular surfaces, and the fibrous capsule extends around the ovoid articular surfaces of both the demifacet and adjacent articular half of the rib head (see Fig. 6-9). The capsule extends to the intraarticular ligament between the upper and lower compartments. The fibers of the fibrous capsule closest to the IVD blend with that structure, and the posterior fibers blend with the costotransverse ligament. In a fashion similar to that found in the Z joints, synovial folds, or menisci, protrude into the costocorporeal articulation, presumably to help lubricate the joint and help with the sliding and rotary motions of the joint (Meyer, 1972). The heads of the first, tenth (occasionally), eleventh, and twelfth ribs form single ovoid synovial articulations with their respective ribs.

The ligaments of this compound joint include the capsular, intraarticular (both described previously), and radiate. Each radiate ligament (see Fig. 6-9) associated with typical vertebrae attaches to the anterior aspect of the head of the articulating rib and the two vertebrae to which the head attaches. In addition, the radiate ligament attaches by horizontal fibers to the IVD between the two vertebrae. The superior fibers attach just above the superior demifacet and ascend to the vertebral body of the superior vertebra. Likewise, the inferior fibers attach just below the inferior demifacet and descend to the inferior vertebral body. The radiate ligament of the first rib has some superior fibers that attach to C7. The radiate ligaments of the tenth through twelfth ribs attach to only the vertebra with which the rib head articulates.

In addition to allowing motion of the ribs, the costocorporeal joints provide stability to the thoracic region during motions in the sagittal (flexion-extension), coronal (lateral flexion), and transverse planes (axial rotation) (Oda et al., 2002).

Confirming the work of Meyer (1972), Erwin and colleagues (2000) found large synovial folds protruding into the costocorporeal joints. In addition, they found that the synovial folds were innervated by free nerve endings and mechanoreceptors immunoreactive to substance P. Substance P is associated with pain transmission and functions in the perception of pain, related reflex muscle responses (flexion reflexes), and reflex responses to pain of the autonomic nervous system and endocrine system. Therefore the costocorporeal joints are similar to the Z joints with respect to being planar synovial joints with nociceptive (pain-sensitive) innervation of both the joint capsule and the synovial folds. The nerve endings are most likely sensitive to tissue strain or tissue damage of mechanical origin. The authors speculated that nociception transmitted from a lesion affecting these nerve endings would probably radiate to the midline thoracic region and anteriorly along the same rib. In addition, pain from the costocorporeal joints also may be associated with atypical chest and arm pain (Erwin, Jackson, & Homonko, 2000). Others (Groen et al., 1987) have identified fibers from the sympathetic chain extending to the costocorporeal joint. The fibers were found to form a dense network that surrounded the joint, and met the criteria of vasomotor fibers that would provide innervation to the vessels extending to the synovial lining of the joint capsule.

Costotransverse Articulation

This joint is composed of the costal (articular) tubercle of a rib articulating with the transverse costal facet of a TP (see Fig. 6-9). Recall that the eleventh and twelfth ribs do not articulate with the TPs of their respective vertebrae. The joint surfaces of the upper five or six costotransverse joints are curved, with the transverse costal facet being concave and the articular tubercle convex. The remaining joints are more planar in configuration (Meyer, 1972; Standring et al., 2008). A thin, fibrous capsule lined by a synovial membrane attaches to the two adjacent articular surfaces. A costotransverse foramen is found between the TP and the rib between the costotransverse and costocorporeal articulations. This foramen is filled by the costotransverse ligament. The costotransverse ligament passes from the posterior aspect of the rib neck to the anterior aspect of the adjacent TP (see Fig. 6-9). For example, the costotransverse ligament of the sixth rib attaches to the posterior aspect of that rib and to the anterior aspect of the TP of T6. Sensory nerve endings also have been found in the costotransverse ligament, indicating that it is pain sensitive (Erwin, Jackson, & Homonko, 2000). Sensory innervation to the lateral and superior costotransverse ligaments (discussed immediately below) has not been investigated adequately.

The ligaments of the costotransverse articulation include the articular capsule, costotransverse ligament (both described previously), superior costotransverse ligament, and lateral costotransverse ligament (see Fig. 6-9). The strong but short lateral costotransverse ligament courses directly laterally from the lateral margin of the TP to the nonarticular region of the costal tubercle of the adjacent rib (see Fig. 6-9). This ligament is found at every thoracic segment. The ligaments of the upper thoracic vertebrae course slightly superiorly, as well as laterally, whereas the lower ones run slightly inferiorly, as well as laterally.

A superior costotransverse ligament courses between the neck of each rib and the TP of the vertebra above. This ligament is divided into two parts (laminae), anterior and posterior. Both laminae course superiorly from a rib neck to the inferior border of the TP immediately above. The anterior lamina angles slightly laterally as it ascends and blends with the posterior intercostal membrane (see Fig. 6-12, B). The posterior lamina angles slightly medially. Because it is more posteriorly placed, this ligament blends laterally with the external intercostal muscle. The intercostal vein, artery, and nerve pass across the anterior surface of these ligaments. A lumbocostal ligament runs from the inferior border of the twelfth rib shaft to the superior surface of the TP of L1.

Three small muscles have been found between anterior and posterior laminae of the first three superior costotransverse ligaments (Darwish & Ibrahim, 2009). The triangle-shaped muscles attach to the lateral aspects of transverse processes superiorly (forming the broad bases of the triangles) and the necks of the subjacent ribs inferiorly (narrow apices of the triangles). More specifically, the first muscle attaches to the TP of C7 and the neck of the first rib (there is only a single lamina of the superior costotransverse ligament at this level that covers the muscle anteriorly), the second attaches to the TP of T1 and the neck of the second rib, and the third muscle attaches to the TP of T2 and and the neck of the third rib. Because of similar embryologic origin, these muscles may have a function similar to the intertransversarii muscles (Darwish & Ibrahim, 2009).

An accessory ligament is normally found medial to the superior costotransverse ligament. This accessory ligament is separated from the superior costotransverse ligament by a gap that is filled by the posterior primary division as it leaves the mixed spinal nerve to reach the more posterior structures of the spine. More specifically, the accessory ligament originates from the region of the neck of a rib medial to the attachment of the superior costotransverse ligament. It courses superiorly to reach the inferior articular process of the vertebra above, although some fibers reach the TP.

A “first thoracic rib ligament” is found on at least one side of 80% of individuals (Matullo et al., 2010). The 3-cm-long ligament attaches to the inner surface of the neck of the first rib and then distally along the inner surface of the shaft of the first rib. The T1 anterior primary division courses around the inferior surface of this ligament and then passes superiorly between the ligament and the shaft of the first rib to unite on top of the first rib with the anterior primary division of C8 to form the inferior (lower) trunk of the brachial plexus. The first thoracic rib ligament could conceivably be the cause of a type of thoracic outlet syndrome (see Superior Thoracic Aperture, previously) in which the anterior primary division of T1 becomes entrapped as it passes beneath or lateral to the ligament (Matullo et al., 2010).

Sternocostal Joints

A complex type of synarthrosis exists between the first costal cartilage and manubrium. A thin piece of fibrocartilage is interposed between the two surfaces and is tightly adherent to both (Standring et al., 2008). A radiate ligament also unites the two surfaces. The joint between the second costal cartilage and sternum is synovial and is separated into two compartments by an intraarticular ligament. Small synovial joints are located between the costal cartilages of the third through seventh ribs and the sternum. The costal cartilages and sternum are united by the fibrous capsules of the joints and also by the radiate ligaments that pass from the anterior and posterior surfaces of each costal cartilage to the sternum. A small amount of rotation occurs at these joints. This rotation allows the thorax to expand and contract during inspiration and expiration, respectively.

Interchondral Joints

As mentioned, the eighth through tenth costal cartilages articulate with the costal cartilage immediately above. Small synovial joints are formed at the attachment sites for the eighth and ninth ribs. The costal cartilage of the tenth rib usually is continuous with the costal cartilage of the ninth rib, and no true joint unites the two. Occasionally no attachment exists between the tenth rib and the costal cartilage of the ninth rib. Although both the sixth and seventh costal cartilages attach directly to the sternum, their most distal portions also contact one another (see Fig. 6-2). Small synovial joints are also located where these cartilages are in contact with one another.

Intercostal Nerves

The anterior primary divisions of the T1 to T11 nerves are the intercostal nerves (see Figs. 6-11 and 6-12). The ventral ramus of the T1 nerve splits, and the larger branch joins the ventral ramus of C8 to form the lower (inferior) trunk of the brachial plexus. The smaller branch of the ventral ramus of T1 forms the first intercostal nerve. The anterior primary division of the T12 nerve is the subcostal nerve.

Close to its origin, each intercostal nerve (and the subcostal and upper two lumbar nerves) sends a white ramus communicans to the sympathetic ganglion of the same level. These ganglia are located anterior to the intercostal nerves and lie along the lateral aspect of the vertebral bodies.

The intercostal nerves also receive gray rami communicantes from the neighboring sympathetic ganglia (see Fig. 6-12). This is similar to all other anterior primary divisions. The intercostal nerve then continues laterally along the subcostal groove, inferior to the intercostal vein and artery (Fig. 6-13; see also Fig. 6-11). The lateral course of the intercostal nerve, within the posterior intercostal space, is subject to a wide degree of variation. The intercostal nerve frequently runs within the middle of the intercostal space (73% of the time) and occasionally courses along the inferior aspect of the intercostal space just above the subjacent rib (Hardy, 1988).

Each intercostal nerve provides sensory, motor (somatic motor), and sympathetic (visceral motor to blood vessels and sweat glands) innervation to the thoracic or abdominal wall. This is accomplished by means of posterior, lateral, and anterior branches.

Posterior Intercostal Arteries

The third through eleventh intercostal arteries originate from the thoracic aorta and course laterally along the inferior aspect of the corresponding rib (see Figs. 6-11 and 6-13, A). The artery that courses below the twelfth rib is known as the subcostal artery because it lies inferior to the twelfth rib and not between two ribs. The first two intercostal arteries arise from the highest intercostal artery. The highest intercostal artery is a branch of the costocervical trunk, which arises from the subclavian artery.

The intercostal arteries course between the intercostal vein superiorly and the intercostal nerve inferiorly. Because the thoracic aorta lies to the left of the thoracic spine, the right intercostal arteries must cross over the thoracic vertebral bodies to reach the right intercostal spaces. This results in the right intercostal arteries being longer than the left ones.

Each posterior intercostal artery gives rise to dorsal and lateral branches that supply the dorsal (including deep back muscles) and lateral aspects of the intercostal spaces, respectively.

Anterior Intercostal Arteries

The upper six anterior intercostal arteries arise from the internal thoracic artery (internal mammary artery). The internal thoracic artery arises from the subclavian artery, and then courses inferiorly, behind and lateral to the sternocostal articulations, and divides into the superior epigastric artery and the musculophrenic artery. The musculophrenic artery, in turn, supplies the seventh, eighth, and ninth anterior intercostal arteries. The nine anterior intercostal arteries supply the intercostal muscles anteriorly, as well as the muscles of the anterior thoracic wall and breast.

Intercostal Veins and the Azygos System of Veins

Venous drainage of the thoracic cage is accomplished primarily by the intercostal veins. The intercostal venous blood generally courses in the opposite direction of the arterial supply and drains into the azygos system of veins (see Figs. 6-11 and 6-13, A).

The azygos vein originates from one or more of the following: inferior vena cava, right renal vein, and right ascending lumbar vein. The azygos vein passes through the diaphragm by means of the aortic hiatus. It then courses along the right anterior aspect of the thoracic vertebral bodies. Along its course this vein receives the right lower eight intercostal veins, right superior intercostal vein, and hemiazygos vein. In addition, the accessory hemiazygos vein frequently is a direct tributary of the azygos vein. The right superior intercostal vein drains the upper two to three intercostal spaces and can empty into either the azygos or the right brachiocephalic vein. Other tributaries of the azygos vein include esophageal, bronchial, mediastinal, and pericardial veins. The azygos vein courses superiorly and arches (from posterior to anterior) around the superior aspect of the right primary bronchus. It then terminates by entering the superior vena cava.

The hemiazygos vein originates from the left ascending lumbar vein, left renal vein, or both. The hemiazygos vein enters the thorax through the aortic hiatus. It then continues superiorly along the left anterior aspect of the vertebral bodies. Along its path the hemiazygos vein receives the lower four or five left intercostal veins. It also frequently receives the accessory hemiazygos vein. The hemiazygos vein helps to drain the left mediastinum and left lower esophagus. The hemiazygos vein crosses from left to right at approximately the level of T9 and terminates by emptying into the azygos vein.

The accessory hemiazygos vein connects the middle three or four intercostal veins. It occasionally receives the left superior intercostal vein, which drains the upper two to three intercostal spaces. However, the left superior intercostal vein normally is a tributary of the left brachiocephalic vein. The accessory hemiazygos vein ends either by draining into the hemiazygos vein or by crossing the vertebral column from left to right, just above the hemiazygos vein, to drain into the azygos vein.

Aortic Arch

The aortic arch begins at the heart as the outflow path of the left ventricle. It extends in front of the trachea, swings around the left primary bronchus, and comes to lie to the left of the midthoracic vertebrae (see Fig. 6-11). The arch then continues inferiorly as the descending thoracic aorta. There are three large branches from the aortic arch: the brachiocephalic (innominate) artery, left common carotid artery, and left subclavian artery.

Descending Thoracic Aorta

The descending thoracic aorta is the continuation of the aortic arch (see Figs. 6-11 and 6-13). It begins at approximately the T4-5 disc and continues inferiorly along the left side of the thoracic vertebrae. The thoracic aorta shifts to the midline in the lower thorax, lying on the anterior aspect of the lower thoracic vertebrae. The thoracic aorta gives off bronchial arteries (which supply the lungs) and all the posterior intercostal arteries except the first two on each side (supplied by the highest intercostal artery of the costocervical trunk). The descending thoracic aorta also gives off the left and right subcostal arteries.

Left Vagus Nerve

The left vagus nerve enters the thorax between the left common carotid and left subclavian arteries. It continues inferiorly by crossing the aortic arch, where it gives off the large left recurrent laryngeal nerve. This nerve loops around the aortic arch from anterior to posterior just lateral to the ligamentum arteriosum. It then runs superiorly in a groove between the esophagus and trachea (tracheoesophageal groove) to supply eight of the nine laryngeal muscles on the left side. The main trunk of the vagus nerve continues inferiorly from the arch of the aorta (giving branches to the cardiac plexus) and follows the aorta posteriorly, passing behind the root of the left lung, where it participates in the pulmonary plexus. The vagus nerve then courses medially and comes to lie on the anterior aspect of the esophagus, where it helps to form the anterior esophageal plexus. The anterior esophageal plexus coalesces inferiorly to form the anterior vagal trunk. This trunk exits the thorax by traveling along the anterior aspect of the esophagus through the esophageal hiatus of the diaphragm.

Right Vagus Nerve

The right vagus nerve enters the thorax by crossing the right subclavian artery. The right recurrent laryngeal nerve is given off at this point. This nerve loops around the right subclavian artery and continues superiorly in the right tracheoesophageal groove to supply eight of the nine muscles of the larynx on the right side. The right vagus nerve continues inferiorly in the thorax, contributing to the superficial and deep cardiac plexuses, and runs posterior to the root of the right lung. Here it sends several branches to the posterior pulmonary plexus. The right vagus nerve then travels medially to the posterior aspect of the esophagus, where it forms the posterior esophageal plexus (see Figs. 6-11 and 6-13). The nerves of the posterior esophageal plexus coalesce to form the posterior vagal trunk. The posterior vagal trunk exits the thorax by traveling along the posterior aspect of the esophagus through the esophageal hiatus of the diaphragm.

• Greater splanchnic nerve. This nerve arises from thoracic ganglia 5 through 9 (see Figs. 6-11; 6-12, A; and 6-13, A) and synapses in the celiac ganglion. Some of its fibers do not synapse here but pass directly to the medulla of the adrenal gland, which they innervate.

• Lesser splanchnic nerve. The lesser splanchnic nerve arises from thoracic ganglia 10 and 11 (Standring et al., 2008). It synapses in the aorticorenal ganglion.

• Least splanchnic nerve. This nerve originates from thoracic ganglion 12 and synapses within ganglia of the renal plexus.