Vertebrae Cervicales Superioressay

The cervical spine is made up of the first 7 vertebrae, referred to as C1-7 (see the images below). It functions to provide mobility and stability to the head while connecting it to the relatively immobile thoracic spine. The cervical spine may be divided into 2 parts: upper and lower. 

Upper Cervical Spine

The upper cervical spine consists of the atlas (C1) and the axis (C2). [1, 2, 3, 4] These first 2 vertebrae are quite different from the rest of the cervical spine (see the image below). The atlas articulates superiorly with the occiput (the atlanto-occipital joint) and inferiorly with the axis (the atlantoaxial joint). The atlantoaxial joint is responsible for 50% of all cervical rotation; the atlanto-occipital joint is responsible for 50% of flexion and extension. The unique features of C2 anatomy and its articulations complicate assessment of its pathology.

Atlas (C1)

The atlas is ring-shaped and does not have a body, unlike the rest of the vertebrae. Fused remnants of the atlas body have become part of C2, where they are called the odontoid process, or dens. The odontoid process is held in tight proximity to the posterior aspect of the anterior arch of the atlas by the transverse ligament, which stabilizes the atlantoaxial joint. The apical, alar, and transverse ligaments, by allowing spinal column rotation, provide further stabilization and prevent posterior displacement of the dens in relation to the atlas.

The atlas is made up of a thick anterior arch, a thin posterior arch, 2 prominent lateral masses, and 2 transverse processes. The transverse foramen, through which the vertebral artery passes, is enclosed by the transverse process.

On each lateral mass is a superior and inferior facet (zygapophyseal) joint. The superior articular facets are kidney-shaped, concave, and face upward and inward. These superior facets articulate with the occipital condyles, which face downward and outward. The relatively flat inferior articular facets face downward and inward to articulate with the superior facets of the axis.

According to Steele's rule of thirds, at the level of the atlas, the odontoid process, the subarachnoid space, and spinal cord each occupy one third of the area of the spinal canal.

Axis (C2)

The axis has a large vertebral body, which contains the odontoid process (dens). The odontoid process articulates with the anterior arch of the atlas via its anterior articular facet and is held in place by the transverse ligament. The axis is composed of a vertebral body, heavy pedicles, laminae, and transverse processes, which serve as attachment points for muscles. The axis articulates with the atlas via its superior articular facets, which are convex and face upward and outward.

Embryology

C2 has a complex embryologic development. It is derived from 4 ossification centers: 1 for the body, 1 for the odontoid process, and 2 for the neural arches. The odontoid process fuses by the seventh gestational month.

At birth, a vestigial cartilaginous disc space called the neurocentral synchondrosis separates the odontoid process from the body of C2. The synchondrosis is seen in virtually all children aged 3 years and is absent in those aged 6 years. The apical portion of the dens ossifies by age 3-5 years and fuses with the rest of the structure around age 12 years. This synchondrosis should not be confused with a fracture.

Parts of the occiput, atlas, and axis are derived from the proatlas. The hypocentrum of the fourth sclerotome forms the anterior tubercle of the clivus. The centrum of the proatlas sclerotome becomes the apical cap of the dens and the apical ligaments.

The neural arch components of the proatlas are divided into rostral and ventral components. The rostral component forms the anterior portion of the foramen magnum and the occipital condyles; the caudal component forms the superior part of the posterior arch of the atlas and the lateral atlantal masses. The alar and cruciate ligaments are formed from the lateral portions of the proatlas.

Vasculature

There is an extensive arterial anastomotic network around the dens, fed by the paired anterior and posterior ascending arteries arising from the vertebral arteries around the C3 level and the carotid arterial arcade from the base of the skull. The anterior and posterior ascending arteries reach the base of the dens via the accessory ligaments and run cephalad at the periphery to reach the tip of the process. The anastomotic arcade also receives tributaries from the ascending pharyngeal arteries that join the arcade after passing through the occipital condyle.

Ligaments

The craniocervical junction and the atlantoaxial joints are secured by the external and internal ligaments. The external ligaments consist of the atlanto-occipital, anterior atlanto-occipital, and anterior longitudinal ligaments. The internal ligaments have 5 components, as follows:

  • The transverse ligament holds the odontoid process in place against the posterior atlas, which prevents anterior subluxation of C1 on C2

  • The accessory ligaments arise posterior to and in conjunction with the transverse ligament and insert into the lateral aspect of the atlantoaxial joint; the apical ligament lies anterior to the lip of the foramen magnum and inserts into the apex of the odontoid process

  • The paired alar ligaments secure the apex of the odontoid to the anterior foramen magnum

  • The tectorial membrane is a continuation of the posterior longitudinal ligament to the anterior margin of the foramen magnum

  • The 3 cm × 5 mm accessory atlantoaxial ligament not only connects the atlas to the axis but also continues cephalad to the occipital bone; functionally, it becomes maximally taut with 5-8° of head rotation, lax with cervical extension, and maximally taut with 5-10° of cervical flexion; it seems to participate in craniocervical stability; future improvements in magnetic resonance imaging (MRI) may lead to better appreciation of its structure and integrity of this ligament [5]

Lower Cervical Spine

The 5 cervical vertebrae that make up the lower cervical spine, C3-C7, are similar to each other but very different from C1 and C2. Each has a vertebral body that is concave on its superior surface and convex on its inferior surface (see the image below). On the superior surfaces of the bodies are raised processes or hooks called uncinate processes, each of which articulates with a depressed area on the inferior lateral aspect of the superior vertebral body, called the echancrure or anvil.

These uncovertebral joints are most noticeable near the pedicles and are usually referred to as the joints of Luschka. [6] They are believed to be the result of degenerative changes in the annulus, which lead to fissuring in the annulus and the creation of the joint. [7] These joints can develop osteophytic spurs, which can narrow the intervertebral foramina.

The spinous processes of C3-C6 are usually bifid, whereas the spinous process of C7 is usually nonbifid and somewhat bulbous at its end.

Anterior and posterior columns

The subaxial cervical spine can conveniently be divided into anterior and posterior columns. The anterior column consists of the typical cervical vertebral body sandwiched between supporting disks. The anterior surface is reinforced by the anterior longitudinal ligament and the posterior body by the posterior longitudinal ligament, both of which run from the axis to the sacrum.

Articulations include disk-vertebral body articulations, uncovertebral joints, and zygapophyseal (facet) joints. The disk is thicker anteriorly, contributing to normal cervical lordosis, and the uncovertebral joints in the posterior aspect of the body define the lateral extent of most surgical exposures. The facet joints are oriented at a 45º angle to the axial plane, allowing a sliding motion; the joint capsule is weakest posteriorly. Supporting ligamentum flavum, posterior, and interspinous ligaments also strengthen the posterior column. [8]

Nerve supply

In the neuroanatomy of the cervical spine (see the image below), the cord is enlarged, with lateral extension of the gray matter consisting of the anterior horn cells. The lateral dimension spans 13-14 mm, and the anterior-posterior extent measures 7 mm. An additional 1 mm is necessary for cerebrospinal fluid (CSF) anteriorly and posteriorly, as well as 1 mm for the dura. A total of 11 mm is needed for the cervical spinal cord. Exiting at each vertebral level is the spinal nerve, which is the result of the union of the anterior and posterior nerve roots.

The foramina are largest at C2-C3 and progressively decrease in size down to C6-C7. The spinal nerve and spinal ganglion occupy 25-33% of the foraminal space. The neural foramen is bordered anteromedially by the uncovertebral joints, posterolaterally by facet joints, superiorly by the pedicle of the vertebra above, and inferiorly by the pedicle of the lower vertebra. Medially, the foramina are formed by the edge of the end plates and the intervertebral discs.

Interconnections are present between the sympathetic nervous system and the spinal nerve proper. The spinal nerves exit above their correspondingly numbered vertebral body from C2-C7. Because the numbering of the cervical spinal nerves commences above the atlas, 8 cervical spinal nerves exist, with the first exiting between the occiput and the atlas (C1) and the eighth exiting between C7 and T1.

Vasculature

The vascular anatomy consists of a larger anterior spinal artery located in the central sulcus of the cord and paired posterior spinal arteries located on the dorsum of the cord. It is generally accepted that the anterior two thirds of the cord is supplied by the anterior spinal artery and that the posterior one third is supplied by the posterior arteries.

Facet Joints

The facet joints in the cervical spine are diarthrodial synovial joints with fibrous capsules. The joint capsules are more lax in the lower cervical spine than in other areas of the spine to allow gliding movements of the facets. The joints are inclined at an angle of 45° from the horizontal plane and 85° from the sagittal plane. This alignment helps prevent excessive anterior translation and is important in weight-bearing. [9]

Nerve supply

The fibrous capsules are innervated by mechanoreceptors (types I, II, and III), and free nerve endings have been found in the subsynovial loose areolar and dense capsular tissues. [10] In fact, there are more mechanoreceptors in the cervical spine than in the lumbar spine. [1] This neural input from the facets may be important for proprioception and pain sensation and may modulate protective muscular reflexes that are important for preventing joint instability and degeneration.

The facet joints in the cervical spine are innervated by both the anterior and posterior rami. The atlanto-occipital and atlantoaxial joints are innervated by the anterior rami of the first and second cervical spinal nerves. The C2-C3 facet joint is innervated by 2 branches of the posterior ramus of the third cervical spinal nerve innervate, a communicating branch and a medial branch known as the third occipital nerve.

The remaining cervical facets, C3-C4 to C7-T1, are supplied by the posterior rami medial branches that arise 1 level cephalad and caudad to the joint. [11, 12] Therefore, each joint from C3-C4 to C7-T1 is innervated by the medial branches above and below. These medial branches send off articular branches to the facet joints as they wrap around the waists of the articular pillars.

Intervertebral Discs

Intervertebral discs are located between the vertebral bodies of C2-C7. Intervertebral disks are located between each vertebral body caudad to the axis. These disks are composed of 4 parts: the nucleus pulposus in the middle, the annulus fibrosis surrounding the nucleus, and 2 end plates that are attached to the adjacent vertebral bodies. They serve as force dissipators, transmitting compressive loads throughout a range of motion. The disks are thicker anteriorly and therefore contribute to normal cervical lordosis.

The intervertebral disks are involved in cervical spine motion, stability, and weight-bearing. The annular fibers are composed of collagenous sheets (lamellae) that are oriented at a 65-70° angle from the vertical and alternate in direction with each successive sheet. As a result, they are vulnerable to injury by rotation forces because only one half of the lamellae are oriented to withstand force applied in this direction. [1]

The middle and outer one third of the annulus is innervated by nociceptors. Phospholipase A2 has been found in the disc and may be an inflammatory mediator. [13, 14, 15]

Ligaments

Although the cervical spine consists of 7 cervical vertebrae interspaced by intervertebral disks, the complex ligamentous network keep the individual bony elements behaving as if they were a single unit.

As noted, the cervical spine can be viewed as being made up of anterior and posterior columns. It can also be useful to think in terms of a third (middle) column, as follows:

  • The anterior column consists of the anterior longitudinal ligament and the anterior two thirds of the vertebral bodies, the annulus fibrosus, and the intervertebral disks

  • The middle column is composed of the posterior longitudinal ligament and the posterior one third of the vertebral bodies, the annulus fibrosus, and the intervertebral disks

  • The posterior column is made up of the posterior arches, including the pedicles, transverse processes, articulating facets, laminae, and spinous processes

The longitudinal ligaments are vital for maintaining the integrity of the spinal column. Whereas the anterior and posterior longitudinal ligaments maintain the structural integrity of the anterior and middle columns, the posterior column alignment is stabilized by a complex of ligaments, including the nuchal and capsular ligaments, and the ligamentum flavum.

If 1 of the 3 columns is disrupted as a result of trauma, stability is provided by the other 2, and cord injury is usually prevented. With disruption of 2 columns, spinal cord injury is more likely because the spine may then move as w separate units.

Several ligaments of the cervical spine that provide stability and proprioceptive feedback are worth mentioning and are briefly described here. [16, 17]

The transverse ligament, the major portion of the cruciate ligament, arises from tubercles on the atlas and stretches across its anterior ring while holding the odontoid process (dens) against the anterior arch. A synovial cavity is located between the dens and the transverse process. This ligament allows rotation of the atlas on the dens and is responsible for stabilizing the cervical spine during flexion, extension, and lateral bending. The transverse ligament is the most important ligament for preventing abnormal anterior translation. [18]

The alar ligaments run from the lateral aspects of the dens to the ipsilateral medial occipital condyles and to the ipsilateral atlas. They prevent excessive lateral and rotational motion while allowing flexion and extension. If the alar ligaments are damaged, as in whiplash, the joint complex becomes hypermobile, which can lead to kinking of the vertebral arteries and stimulation of nociceptors and mechanoreceptors. This may be associated with the typical complaints of patients with whiplash injuries (eg, headache, neck pain, and dizziness).

The anterior longitudinal ligament (ALL) and the posterior longitudinal ligament (PLL) are the major stabilizers of the intervertebral joints. Both ligaments are found throughout the entire length of the spine; however, the ALL adheres more closely to the disks than the PLL does, and it is not well developed in the cervical spine. The ALL becomes the anterior atlanto-occipital membrane at the level of the atlas, whereas the PLL merges with the tectorial membrane. Both continue onto the occiput. The PLL prevents excessive flexion and distraction. [19]

The supraspinous ligament, the interspinous ligaments, and the ligamentum flavum maintain stability between the vertebral arches. The supraspinous ligament runs along the tips of the spinous processes, the interspinous ligaments run between adjacent spinous processes, and the ligamentum flavum runs from the anterior surface of the cephalad vertebra to the posterior surface of the caudad vertebra.

The interspinous ligament and (especially) the ligamentum flavum control for excessive flexion and anterior translation. [19, 20, 21] The ligamentum flavum also connects to and reinforces the facet joint capsules on the ventral aspect. The ligamentum nuchae is the cephalad continuation of the supraspinous ligament and has a prominent role in stabilizing the cervical spine.

Children have significant anatomical variations in the craniocervical junction compared with adults. Surgical management of craniocervical junction instability in children poses unique challenges. While the indications for cervical fusion are similar to adults with regard to technique, in children, significant anatomical variations in the craniocervical junction complicate the approach and limit the use of internal fixation. Treatment is hindered by the diminutive bone and ligamentous structures, which are often complicated by syndromic craniovertebral abnormalities. Recent advances in imaging have improved outcomes. Menezes reviewed 850 children who underwent craniocervical fusion. The author presents detailed review of the technique of fusion, as well as the indications and means of avoidance of complications, their prevention, and their management. [22]

 

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Understanding the anatomy of the spine is a fundamental basis to knowing your spinal condition.  Dr. Thomas Schuler provides a video overview of spinal anatomy.

>> Click here to Watch Part Two <<

 


Anatomical Terms & Latin meaning

Understanding anatomy means learning a new set of language terms. Anatomy uses a set of Latin based terms that are used to describe all the structures in our body. You can find many of these terms in Our Glossary and we will cover some basic terms here.

Orientation:

  • Anterior – the front portion of the body; located in front of a structure
  • ​Lateral – situated away from the midline of the body
  • Anterolateral – situated or occurring in front of and to the side
  • Inferior – situated below or directed downward
  • Medial – situated closer to the midline of the body
  • Posterior – the back portion of the body; located behind a structure

Planes:

  • Coronal – a plane dividing the body into anterior and posterior portions
  • Sagittal – a plane dividing the body into right and left portions
  • Transverse – a plane dividing the body into superior and inferior portions

Basic Anatomy: 

  • Articular – pertaining to a joint
  • Bone – the hard tissue that provides structural support to the body, primarily composed of hydroxyapatite crystals and collagen
  • Cartilage – a hard, thin layer of white glossy tissue that covers the end of bone at a joint allowing for motion with minimum friction
  • Collagen – a fibrous protein which is a major part of connective tissue, such as skin, tendons, ligaments, cartilage, and bones
  • Joint – the junction between two or more bones that allows varying degrees of motion
  • Ligament – a band of flexible, fibrous connective tissue attaching bones to one another
  • Spinal Cord – the long cord of nerve tissue enclosed in the spinal canal. It serves as a pathway for nervous impulses to and from the brain and is a center for executing and coordinating many reflex actions independent of the brain.
  • Tendon – the fibrous band of tissue that connects muscle to bone, mainly composed of collagen

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Spinal Curvatures

The human spine has natural curvatures. When you look at a back from behind, the spine should be straight and centered over the pelvis. However, when you look at the spine from the side, the curves are designed to maintain balance as the spine is behind organs in the chest and abdomen. The spine has two alternating curves to create an “S” like shape. In the neck and low back there is normally an inward curvature or sway back known as lordosis. In the thoracic spine and sacrum there is an outward curvature known has kyphosis or hunchback. These curves normally balance out each other so that when the patient stands they are well balanced with their head straight above their hips when viewed from the side. Standing in this position minimizes the effect of gravity and allows the patient to stand with the best posture and use the least energy when moving or walking.


There are seven cervical (C) vertebrae, twelve thoracic (T) vertebrae, and typically five lumbar (L) vertebrae.

View of the spine from the side, back and front

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Vertebrae

The spine is made of a column of bones. A round bone called a vertebral body forms each bone. The body is located in the front of the spine. A bony ring attaches to the back of the vertebral body, forming a canal for the spinal cord and nerves. This bony ring is formed by two sets of bones. One set called the pedicle bone attaches to the back of each vertebral body to the side, transverse processes. Processes are outgrowths of bone and then subsequently named for where the outgrowth occurs. A lamina (Latin for plate) bone connects to the other end of the pedicle, one on the left and one on the right to connect the transverse process with the spinous process. The spinous process is your 'back bone'. Each vertebral segment creates a bony circle, called the spinal canal or vertebral foramen, that protects the spinal cord and spinal nerves.

The lamina bones form a protective roof over the back of the spinal cord. When the vertebra bones are stacked on top of each other, the canal forms a long tube that surrounds and protects the spinal cord as it passes through the spine.


Joints

A joint is a connection between two bones. Our spine is made up of multiple joints at each level and many of these joints in the spine are synovial joints. Synovial joints are the most common and most movable type of joint in our body. Synovial joints have a joint capsule surrounding the bony surfaces of the joint and lubricating synovial fluid in the joint capsule.

  • The skull is connected to the cervical spine at C1 by synovial joints called the occipital-cervical joint.
  • Each spinal motion segment of the spine has a pair of facet joints that provide the posterior support for the spine.
  • A pivot joint is found at the very top of the spine, called the atlanto-axial joint. This joint is between the very first and second cervical vertebrae which are very specialized to the spine to allow for rotational movement.

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MOTION COMPLEX

Each level of your spine functions as a three-joint complex. There are two facet joints in the back and a large disc in front. This tripod creates great stability, supports all your weight above each level and provides support for you to move in all directions. Vertebrae are stacked one on top of another and are separated by intervertebral discs, which act as an elastic cushions or shock absorbers. The first two cervical vertebrae are an exception and do not have discs. The interbody space is the disc space that is located between the vertebral body bones.

 


 

Cervical Vertebrae

There are seven cervical vertebrae, named C1-C7, designed for flexibility and movement. The cervical spine has a lordotic shape, or a backwards “C” shape.

The first two cervical vertebrae are very specialized to allow us to turn our head from side to side. The first cervical vertebra (C1) is called the atlas and is aptly named after the Greek god Altas, who carried the world on his shoulders. This bone is formed like a ring that sits upon the second cervical vertebra (C2). The second cervical vertebra (C2) is called the axis as it is the line upon which the head and C1 rotate upon. The C1 vertebra connects the skull to the cervical spine. These two vertebrae have different anatomy than the rest of the spine. The C1 vertebra is formed like a ring that sits on top of C2. The C2 vertebra has a bony knob that fits into the front portion of the ring of the C1 vertebra. This bony knob is called the odontoid process or dens. It is held in place by a special ligament that holds it tightly to the front of the ring of the C1 vertebra. While the cervical spine is very flexible, it is also at greatest risk for injury from strong sudden movements, as there is less muscular support.

A cervical vertebrae

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Thoracic Vertebrae

There are twelve thoracic vertebrae, named T1-T12, specialized for stability. The thoracic spine aids in keeping the body upright, protects vital chest organs and articulates with each rib to form the rib cage. Each rib is firmly connected to each level of the thoracic spine. The thoracic spine has a kyphotic shape, a “C” shape, and the discs in this part of the spine are relatively thin.


Lumbar Vertebrae

There are usually five lumbar vertebrae, named L1-L5, designed for weight bearing loads and movement. In some people, they may have developed four or six lumbar vertebrae. In some cases one of the bones of the sacrum, the base of the spine, forms as a vertebra instead of the sacrum. This is called a transitional (or sixth) vertebra and is simply a bony anomaly.

The lumbar spine is shaped like the cervical spine; it is lordotic like a backwards “C”. The two lordotic curves in the neck and low back are balanced by the thoracic kyphotic “C” curve so the spine’s center of gravity is overall balanced in an “S” shape. The vertebrae in the lumbar spine are the largest of the entire spine, designed to hold increasing forces of weight. The lumbar spinal canal is also the largest, allowing for more space for the nerves.


Sacrum and Coccyx

The sacrum is made of five fused vertebrae that form a single bone. The sacrum is shaped like an inverted triangle with the base at the top. It acts as a wedge between the two iliac pelvic bones. On both sides of the pelvis, the sacrum articulates with the ilium through the sacroiliac joints. The coccyx is formed by the fusion of four to five rudimentary vertebrae, commonly referred to as the tailbone.


Sacroiliac Joint

The sacroiliac (SI) joint is a strong weight bearing joint in the pelvis that connects the sacrum and pelvis. There are two joints, one on each side of the sacrum. This joint is reinforced by strong surrounding ligaments as shown in the image. Both joints move together as single unit to transmit upper body forces and provide shock absorption for the spine. A series of ridges and valleys in the joint fit together like a lock and key, much like if you put your knuckles together. There is a small amount of movement in this joint to allow for a walking gait pattern in normal human locomotion. Just like other joints in the body, this joint can become inflamed, unstable and dysfunctional.

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Intervertebral disc

An intervertebral disc is a strong ligament that connects one vertebral bone to the next. The discs are the shock-absorbing cushions between each vertebra of the spine. The disc is made up of three basic structures: the annulus fibrosus, the nucleus pulposus, and the vertebral end-plates. All three disc structures are made of different percent compositions of proteoglycan (proteins that bind water), collagen (the main protein in connective tissue) and water. The varying compositions create different functions. All these properties make the disc unique and highly specialized.

Discs in the spine increase in size from the neck to the low back as there are increasing needs for shock absorption due to weight and gravity. These specific disc ligaments function just like knee ligaments and shoulder ligaments do.  They allow the spine to move so we can bend forward, backward and sideways. Just like other ligaments, the discs can be injured.

The intervertebral disc is essential for providing spinal stability and proper alignment, withstanding compressive shock forces, and allowing for movement between vertebral bones. The intervertebral disc is an avascular tissue, meaning it does not naturally have a blood supply. Capillaries from the surface of the vertebral bodies supply the disc with nutrients. Thus, the disc is dependent on the blood supply to the vertebral bones.

Each disc has a strong outer ring of fibers, called the annulus fibrosus. The annulus is comprised of specialized sheets of connective tissue, called lamellae, which are layered for strength. The annulus has a higher amount of collagen that makes the outer layer extremely stiff. The nucleus pulposus is the soft, jelly-like center. It serves as the main shock absorber and is held in place by the outer annulus. The nucleus consists of a gel like matrix that provides maximum hydration. It functions to distribute pressures in all directions within each disc under compressive forces.


Spinal Cord and Nerves

The spinal cord connects the nervous system from the brain to the rest of the body. The spinal cord leaves the brain through a hole in the base of the skull called the foramen magnum. The spinal cord and nerves travel from the cervical spine down to the lowest point of the spine, the sacrum. Spinal nerves exit the spinal canal between the vertebrae at each level. Two nerves exit each level, one on the left and one on the right. These nerves exit through openings called foramen. The nerves leave the spinal cord and travel to specific destinations in the body. The nerves leaving the neck travel to both arms and those from the low back to each leg. The specific pattern each nerve innervates is called a dermatome pattern.

 

 

 

 

 

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