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Chapter (I) Anatomy of cervical spine Chapter (I) Anatomy of cervical spine The cervical spine consists of seven vertebrae, with the two upper vertebrae; atlas (C1) and axis (C2) being atypical vertebrae and the remaining five vertebrae (C3-C7) being classified as typical vertebrae.The vertebral bodies of atlas and axis have unique anatomical appearances and consequently unique biomechanical properties. In the lower cervical spine (C3-C7) osseous elements extends posteriorly from the vertebral bodies, consisting of pedicles , articular columns, transverse processes, laminae and spinous processes. The primary functions of the cervical spine are to supply , support and mobility to the head and provide protection to the spinal cord. (Williams & Newell, 2005). CERVICAL VERTEBRAE: (Williams & Newell, 2005). Cervical vertebrae are the smallest of the true vertebrae, and can be readily distinguished from those of the thoracic or lumbar regions by the presence of a foramen in each transverse process. The first, second, and seventh present exceptional features; the following characteristics are common to the remaining four (Figs. 1, 2). The body is small, and broader from side to side than from before backward. The anterior and posterior surfaces are flattened and of equal depth; the former is placed on a lower level than the latter, and its inferior border is prolonged downward, so as to overlap the upper and forepart of the 4 Chapter (I) Anatomy of cervical spine vertebra below. The upper surface is concave transversely, and presents a projecting lip on either side; the lower surface is concave from before backward, convex from side to side, and presents laterally shallow concavities which receive the corresponding projecting lips of the subjacent vertebra. The pedicles are directed lateral ward and backward, and are attached to the body midway between its upper and lower borders, so that the superior vertebral notch is as deep as the inferior, but it is, at the same time, narrower. The laminae are narrow, and thinner above than below; the vertebral foramen is large, and of a triangular form. The spinous process is short and bifid, the two divisions being often of unequal size. The superior and inferior articular processes on either side are fused to form an articular pillar, which projects lateral ward from the junction of the pedicle and lamina. The articular facets are flat and of an oval form: the superior look backward, upward, and slightly medial ward: the inferior forward, downward, and slightly lateral ward. The transverse processes are each pierced by the foramen transversarium, which, in the upper six vertebrae, gives passage to the vertebral artery and vein and a plexus of sympathetic nerves. Each process consists of an anterior and a posterior part. The anterior portion is the homologue of the rib in the thoracic region, and is therefore named the costal process or costal element: it arises from the side of the body, is 5 Chapter (I) Anatomy of cervical spine directed lateral ward in front of the foramen, and ends in a tubercle, the anterior tubercle. The posterior part, the true transverse process, springs from the vertebral arch behind the foramen, and is directed forward and lateral ward; it ends in a flattened vertical tubercle, the posterior tubercle. These two parts are joined, outside the foramen, by a bar of bone which exhibits a deep sulcus on its upper surface for the passage of the corresponding spinal nerve (Williams & Newell, 2005). Fig. (1): A typical cervical vertebra (Quoted from Gray's Anatomy, 2005) Fig. (2): Side view of a typical cervical vertebra (Quoted from Gray's Anatomy, 2005) 6 Chapter (I) Anatomy of cervical spine Atypical cervical vertebrae :( Gray's Anatomy,2005 ) 1. Atlas:- Anterior arch: forms about one-fifth of the ring: its anterior surface is convex, and presents at its center the anterior tubercle for the attachment of the Longus colli muscles and the anterior longitudinal ligament; posteriorly it is concave, and marked by a smooth, oval or circular facet (fovea dentis), for articulation with the odontoid process(dens) of the axis. Posterior arch : The posterior arch forms about two-fifths of the circumference of the ring: it ends behind in the posterior tubercle, which is the rudiment of a spinous process and gives origin to the Recti capitis posterior and the ligamentum nuchae. The diminutive size of this process prevents any interference with the movements between the atlas and the skull. Lateral masses: The lateral masses are the most bulky and solid parts of the atlas, in order to support the weight of the head. Each carries two articular facets, a superior and an inferior arch. Vertebral foramen The anterior or smaller receiving the odontoid process of the axis the posterior transmitting the spinal cord (medulla spinalis) and its membranes 7 Chapter (I) Anatomy of cervical spine Transverse processes The transverse processes are large; they project laterally and downward from the lateral masses, and serve for the attachment of muscles which assist in rotating the head. They are long, and their anterior and posterior tubercles are fused into one mass; the foramen transversarium is directed from below, upward and backward. (Gray's Anatomy,2005) 2. Axis (c2) : (Gray's Anatomy,2005) The most distinctive characteristic of this bone is the strong odontoid process known as the dens which rises perpendicularly from the upper surface of the body The dens, also odontoid process or peg, is the most pronounced feature, and exhibits a slight constriction or neck where it joins the main body of the vertebra. The dens is a protuberance (process or projection) of the axis(second cervical vertebra). The condition, where the dens is separated from the body of the axis, is called osodontoideum, and may cause nerve and circulation compression syndrome. On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position. The apex is pointed, and gives attachment to the apical odontoid ligament; below the apex the process is somewhat enlarged, and presents on either side a rough impression for the attachment 8 Chapter (I) Anatomy of cervical spine of the alar ligament; these ligaments connect the process to the occipital bone. The pedicles are broad and strong, especially in front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces. The laminae are thick and strong, and the vertebral foramen large, but smaller than that of the atlas. The transverse processes are very small, and each ends in a single tubercle; each is perforated by the transverse foramen, which is directed obliquely upward and laterally. The superior articular surfaces are round, slightly convex, directed upward and laterally, and are supported on the body, pedicles, and transverse processes. The inferior articular surfaces have the same direction as those of the other cervical vertebrae. The superior vertebral notches are very shallow, and lie behind the articular processes; the inferior lie in front of the articular processes, as in the other cervical vertebrae The spinous process is large, very strong, deeply channelled on its under surface, and presents a bifurcated extremity. (Gray's Anatomy,2005) 9 Chapter (I) Anatomy of cervical spine The Seventh Cervical Vertebrum : (Williams & Newell, 2005). (Fig. 3) The most distinctive characteristic of this vertebra is the existence of a long and prominent spinous process, hence the name vertebra prominens. This process is thick, nearly horizontal in direction, not bifurcated, but terminating in a tubercle to which the lower end of the ligamentum nuchae is attached. Fig. (3): Seventh cervical vertebrum (Quoted from Gray's Anatomy, 2005) 10 Chapter (I) Anatomy of cervical spine The transverse processes are of considerable size, their posterior roots are large and prominent, while the anterior are small and faintly marked; the upper surface of each has usually a shallow sulcus for the eighth spinal nerve, and its extremity seldom presents more than a trace of bifurcation. The foramen transversarium may be as large as that in the other cervical vertebrae, but is generally smaller on one or both sides; occasionally it is double, sometimes it is absent. On the left side it occasionally gives passage to the vertebral artery; more frequently the vertebral vein traverses it on both sides; but the usual arrangement is for both artery and vein to pass in front of the transverse process and not through the foramen. Sometimes the anterior root of the transverse process attains a large size and exists as a separate bone, which is known as a cervical rib (Williams & Newell, 2005). Intervertebral Disc: (Furman, 2010) The intervertebral disc consists of an outer annulus fibrosus and an inner gel-like nucleus pulposus. The intervertebral disc is thicker anteriorly, contributing to the normal cervical lordosis. The C6-7 disc is the thickest disc of the cervical spine. The nucleus pulposus is the central portion of the disc, a remenant of the notochord. It is composed primarily of glycosaminoglycans and type 2 collagen, which have the capacity to bind to large amount of water. The nucleus pulposus is avascular and receive nutrition through diffusion and imbibition of fluids (Bland & Boushey, 1990). 11 Chapter (I) Anatomy of cervical spine The annulus fibrosus is formed mainly of type 1 collagen, it is a multi-laminated ligament that encompasses the periphery of the disc space. Attaches to the end plate cartilage via collagen fibers which run obliquely to the vertebral body and in opposite direction to the annular fibers of the adjacent layer. The annulus is also attached to the apophyseal ring and blends centrally with the nucleus pulposus. Its outer one third has been found to be innervated by the sinuvertebral and the vertebral nerves. The end plates of the vertebral bodies are lined with hyaline cartilage and bind the disc to the vertebral body. The adult cervical disc has a crescentic shape anteriorly, with the apex of the crescent at the uncovertebral joints on each side. The posterior annulus has multiple vertical fissures. In addition, the nucleus of the cervical disc tends to be poorly centralized when compared with the lumbar disc. In the lumbar disc, the nucleus tends to be well localized in the center of the disc, and the posterior annulus tends to remain relatively intact when compared with the cervical disc (Furman, 2010). 12 Chapter (I) Anatomy of cervical spine Fig. (4): Interverteberal disc (Quoted from Furman, 2010) cervical spine facet joint: (Furman, 2010) The lower cervical spine (C3-C7) facet joints are arranged in pairs bilaterally and each joint consist of opposing articular facets, i.e. the inferior articular process (IAP) of the vertebra above and the superior articular process (SAP) of the vertebra below (Figure 5). The facets are oriented slightly oblique to the sagittal plane, nearly 45 degrees with the superior articular facets facing laterally and upwards approximately to the horizontal plane, with gradual changes in orientation in the cervico-thoracic transitional zone. The orientation of these facets allows flexion, extension, lateral bending and rotation of the lower cervical spine. Immediately anterior to each facet joint the 8 cervical spine nerves root exits the spinal canal through the intervertebral foramen formed by the pedicles, vertebral bodies and uncinate processes, intervertebral disc and the articular column. 13 Chapter (I) Anatomy of cervical spine Fig. (5): Facet joint Orientation (Quoted from Furman, 2010) 14 Chapter (I) Anatomy of cervical spine Intervertebral Neural Foramina: The lateral portion of the spinal canal is covered posteriorly by the lateral aspects of a superior and inferior lamina. Ventral to the lamina, the ligamentum flavum is attached the two-thirds of the undersurface of the superior lamina, but inferiorly it is attached only to the superior edge of the lower lamina. Laterally, the ligamentum flavum ends 1 to 2 mm before the medial limit of the neural foramen. The anterior boundary of the cervical neural foramen, from rostral to caudal, is the posterolateral cortical margin of the superior vertebral body, the intervertebral disc covered by the posterior longitudinal ligament (PLL), and a small portion of the posterolateral cortical margin of the inferior vertebral body and its uncinate process. Posteriorly, from rostral to caudal, the neural foramen is bounded by 1 to 2 mm of the superior (descending) facet, followed by the entire ventral surface of the inferior (ascending) facet. The superior and inferior boundaries of the neural foramen are formed by the superior and inferior vertebral pedicles, respectively (Russell and Benjamin, 2004) (Fig.6). Fig. (6): A, axial CT scan through the neural foramen. B, a sagittal reformatted CT scan through the cervical neural foramen (Russell & Benjamin, 2004) 15 Chapter (I) Anatomy of cervical spine Ligaments and Muscles of Posterior Cervical Spine: (Macgregor & Mior, 1989). The functions of a ligament are: (1) to provide stability to the joint, (2) to absorb energy during trauma, and (3) to act as a joint position transducer during physiologic motions. Ligaments, along with the paracervical muscles in the cervical spine, prevent motion between vertebrae that might injure the spinal cord or nerve roots. The cervical spine ligaments have numerous and complex interrelationships (Fig. 7). Running vertically along the anterior and posterior aspects of the vertebral bodies, the anterior and posterior longitudinal ligaments attach to the discs as well. The tightly attached posterior longitudinal ligament is thick in its central portion, which helps to prevent a disc herniation from pressing directly on the cord posteriorly, It also limits flexion and distraction. The interspinous and supraspinous ligaments are also located posteriorly but the interspinous ligaments are not as well developed in the cervical region. The supraspinous and inter-spinous ligaments limit flexion and anterior horizontal displacement. The ligamentum flavum, a yellowish elastic membrane, overlies the space between the laminae of adjacent vertebrae and the neural arches. The 16 Chapter (I) Anatomy of cervical spine posterior location of the ligamentum flavum helps to restrain hyperflexion. The ligamentum flavum becomes shortened and thicker in hyperextension and elongated and thinner in hyperflexion. During hyperextension, it may protrude into the cervical canal as much as 3.5 mm. Impingement on the spinal cord during extension is normally prevented by the elastic properties of the ligament; however, hypertrophy of the ligamentum flavum or loss of elasticity through degeneration may lead to canal narrowing or cord impingement. The capsular ligaments, oriented approximately orthogonal to the articular facets, provide maximal mechanical efficiency in resisting distraction of the facets but relatively poor resistance to shear (Macgregor & Mior, 1989). Fig. (7): Ligaments of cervical spine (Quoted from Furman, 2010) 17 Chapter (I) Anatomy of cervical spine Muscles of the Posterior Cervical and Upper Thoracic Spine The trapezius muscle forms the most superficial layer of the muscles of the back of the neck, which largely originates from the thoracic spinous processes and the ligamentum nuchae. The splenius cervicis and splenius capitis muscles are deep to the trapezius, forming the second layer, they are attached to the cervical transverse processes and the mastoid process respectively. The third layer is formed by the following deep muscles: Semispinalis Capitus (head rotation/pulls backward). Iliocostalis Cervicis (extends cervical vertebrae). Longissimus Cervicus (extends cervical vertebrae). Longissimus Capitus (head rotation/pulls backward). Longissimus Thoracis (extension/lateral flexion vertebral column, rib rotation). Iliocostalis Thoracis (extension/lateral flexion vertebral column, rib rotation). Semispinalis Thoracis (extends/rotates vertebral column). 18 Chapter (I) Anatomy of cervical spine (Furman, 2010) Table (1): Function and innervations of cervical spine muscles (Junwei et al., 2003) CERVICAL MUSCLES Sternocleidomastoid FUNCTION Extends & rotates head, flexes vertebral column Scalenus Flexes & rotates neck Spinalis Cervicis Extends & rotates head Spinalis Capitus Extends & rotates head NERVE C2, C3 Lower cervical Middle/lower cervical Middle/lower cervical Extends & rotates vertebral Middle/lower column cervical Semispinalis Capitus Rotates head & pulls backward C1 – C5 Splenius Cervicis Extends vertebral column Longus Colli Cervicis Flexes cervical vertebrae C2 – C7 Longus Capitus Flexes head C1 – C3 Rectus Capitus Anterior Flexes head C2, C3 Rectus Capitus Lateralis Bends head laterally C2, C3 Iliocostalis Cervicis Extends cervical vertebrae Semispinalis Cervicis 19 Middle/lower cervical Middle/lower cervical Chapter (I) Anatomy of cervical spine Longissimus Cervicis Extends cervical vertebrae Longissimus Capitus Rotates head & pulls backward Rectus Capitus Posterior Middle/lower cervical Middle/lower cervical Extends & rotates head Suboccipital Extends head Suboccipital Obliquus Capitus Inferior Rotates atlas Suboccipital Obliquus Capitus Superior Extends & bends head laterally Suboccipital Major Rectus Capitus Posterior Minor The Nuchal Ligament: The nuchal ligament is an intervertebral syndesmosis, a bilaminar fibro-elastic inter-muscular septum interposed between the paired groups of paravertebral muscles of the cerviconuchal region. It extends from the external occipital protuberance to the spinous process of C7, continuing below C7 as the interspinous ligament in the thoracic region. It is a strong fibrous band covered by the multiple layers of the superficial and deep cervical fascias and by the aponeurosis of the trapezius muscle. (Honet & Puri, 1979). Functionally, this ligament is important for maintaining the lordotic alignment of the cervical spine and stabilizing the head during rotation of the 20 Chapter (I) Anatomy of cervical spine cervical spine. It is a reliable landmark and widely advocated as a means to assure a midline orientation. The nuchal ligament has funicular (dorsal) and lamellar (ventral) portions This ligament is best surgically used for bloodless, non disruptive surgical exposure of cervical spine (Honet & Puri, 1979). Nerve Supply: There is an intricate nerve supply to the cervical spine. Nerves from the cervical and brachial plexus divide in branches supplying the cervical spine bones, ligaments, intervertebral discs, muscles and other soft tissues in a segmental organization. Autonomic fibers, relaying in the cervical sympathetic ganglion and coursing with the vasculature as well as arising from the spinal nerves, also supply some of the cervical spine structures. (Versteegen et al., 2000). At each spinal segment the recurrent spinal meningeal nerve (sinuvertebral nerve) of Luschka re-enters the spinal canal via the foramen in order to supply the outer part of the intervertebral disc with sensory and sympathetic autonomic fibers. The nociceptors and mechanoreceptors in the annulus fibrosus mediate pain transmission from structural disruption of the intervertebral disc itself or from the chemically mediated inflammatory effect of phospholipase A2. Pacinian corpuscles and Golgi tendon organs present in the posterolateral region of the outer one third of the annulus transmit proprioceptive information from the intervertebral disc (Versteegen et al., 2000). 21 Chapter (I) Anatomy of cervical spine The medial branches and accessory nerves of the primary dorsal rami supply the articular structures including the capsule and synovial folds, but not the articular cartilage, with nociceptive fibers and mechanoreceptive nerve endings at a segmental organization. Each medial branch curves the ipsilateral articular column and sends ascending and descending branches to the joints above and below. (Bland & Boushey, 1990). The sinuvertebral nerve arises from the ventral ramus (somatic root), whereas the vertebral nerve (autonomic root) is derived primarily from the sympathetic nervous system. However, the vertebral nerve has connections with the cervical ventral rami, which suggests the possibility of the vertebral nerve also conveying somatic afferents from the disc (Bland & Boushey, 1990). Blood Supply: The blood supply to the cervical spine arises primarily from the brachio-cephalic trunk, the common carotid arteries and the vertebral arteries. Spinal and muscular branches supply the vertebral bodies, the posterior elements and the adjacent musculature of the posterior part of the cervical spine (Williams & Newell, 2005). The spinal and accessory branches of the vertebral arteries anastomose longitudinally and course the intervertebral foramen where they continue as anterior and posterior radicular arteries with vascular supply to the facet joints, spinal cord and posterior elements of the spinal column and are surrounded by the accessory vertebral vein. Arteries of the back of the 22 Chapter (I) Anatomy of cervical spine neck include muscular branches of the vertebral artery, occipital and deep cervical arteries as well as radicular and muscular branches of the ascending cervical artery (MacLain, 1994). 23