Anatomy and Function of the Spine

The vertebral column is composed of vertebrae, discs, ligaments and muscles. The function of the spine is to provide both mobility and stability to the torso. The muscles and ligamentous supporting structures of the spine enable the spine to function in an upright position, and also allow greater degrees of motion to permit the head and trunk to assume a variety of positions for various activities. Some of these motions include rotation, lateral bending, extension and flexion.

The spinal column acts to support the head and trunk and allows humans to stand erect and balance the trunk over the pelvis. The support allows us to ambulate in an upright posture. The spinal column also protects the neural elements consisting of the spinal cord and nerve roots.

Curves of the spine

When viewing the spine from a posterior view, the normal spine is straight and symmetrical. When viewing the spine in a lateral view, the spine has three natural curves. When these three curves are in alignment, the body is in a balanced position. These curves also allow shock absorption against axial load. In the cervical (neck) region, the curve is referred to as lordosis. In a lordotic curve, the convexity of the curve is positioned anteriorly ii. In the thoracic (mid-back) region, the curve is referred to as kyphosis. In a kyphotic curve, the concavity of the curve is positioned anteriorly. And finally, in the lumbar spine, there is also a natural lordotic curve.

The spine is made up of 33 bones (vertebrae) divided in to five main regions. The firs seven vertebrae are known as the cervical vertebrae. These are denoted as C1-C7. (C1 is the most cranial vertebra0> the next 12 vertebrae proceeding inferiorly are the thoracic vertebrae. these vertebrae are denoted as T1-T12. Caudal to the thoracic region is the lumbar region. The lumbar region contains five vertebrae that are denoted as L1-L5. The sacrum is the next most caudal region, which contains five fused vertebrae referred to as Si-S5. The final region of the spine is called the coccygeal region. The coccyx usually contains four bones and is denoted as c1-c4.

Bony anatomy of the spine

In the spine, the vertebral bodies are stacked on each other and are divided by intervertebral discs. (Learn more about them in the section below titled "The disc".)

Each spinal vertebra possesses two main parts: a solid anterior section called the vertebral body and a posterior arch of bone.

The vertebral body is the largest part of a vertebra. In fact, the vertebral body bears 80 percent of the loads applied to the spine. It is flat on the superior and inferior surfaces where the intervertebal discs rest and is composed of spongy cancellous bone with a thin layer of dense cortical bone surrounding it. The cancellous part of the vertebral body is highly vascular and is a storage site for many blood cells and nutrients. The cortical bone is the main structural support for the vertebral body. Although the vertebral body appears square when viewed on an x-ray from a lateral or anterior view, the body is rounded and appears oval when viewed axially.

The posterior arch of bone is made up of five main parts: pedicle, lamina, spinous process, transverse process, and articular process.

The pedicle is a short, thick segment of bone that projects posteriorly from the vertebral body. There are two pedicles (one on each side) on every vertebra. The pedicle has a dense cortical outer wall that surrounds cancellous bone. The pedicle is the bony bridge between the vertebral body and the posterior arch.

The lamina is a broad plate of cortical bone that extends from each pedicle and completes the primary posterior arch by fusing together in the middle. The main function of the lamina is to provide a shield over the spinal cord.

The spinous process projects posteriorly from the junction of the lamina and is the most superficial portion of the spine when viewed from behind. (In fact, if you take your index finger and touch the back of the base of your neck you can feel the spinous process of C7.) The spinous process is an attachment point for muscles and ligaments.

Extending laterally from the junction of the pedicle and lamina are the transverse processes. There are two transverse processes for 3every vertebra (one on each side). This is also a major attachment point for muscles and ligaments.

Lastly, the posterior arch contains four articular processes. Two of the processes project superiorly from the junction between the pedicle and lamina and two project inferiorly from the junction between the pedicle and lamina. The superior articular processes have articulating surfaces (facets) that face posteriorly. Conversely, the inferior articular processes have articulating surfaces that face anteriorly. When the vertebrae are stacked on top of each other, the facets join to form a pair of joints (one on each side) between each pair of vertebrae.

Although each region of the spine has the same general structure, it is important to identify the characteristics that make each spinal region unique.

Cervical spine

There are seven cervical vertebrae. The cervical spine protects the brainstem and spinal cord, supports the skull and is extremely flexible. Its curvature is a normal lordosis or forward bending. The first and second cervical vertebrae have unique shapes, whereas the rest of the bones of the cervical spine (C3-7) have a very similar appearance to each other.

The first cervical vertebra is also known as the atlas. The atlas articulates with the occiput of the skull. It is unique in shape due to its lack of vertebral body. During the development of the atlas, the body meant for C1 becomes fused to the C2 vertebra to become the dens or odontoid process. The atlas also has no spinous process and the posterior arch of the bone is plainly a ring of bone that connects two lateral masses. Since the joints (facets) of the cervical spine are mainly in the transverse plane, the facets sit on top of each other and form what is known as the lateral mass or articular pillar. The lateral masses of the cervical spine, when considered as a unit, are like a column of bone that lends support to the occiput.

The second cervical vertebra is also known as the axis. The axis has its identifying feature, the dens or odontoid process referred to above. The dens is a toothlike projection that projects superiorly and is attached to the anterior arch of C1 by the transverse ligament of the axis. The dens acts as a center of rotation for the atlas. The C2 vertebra therefore has an anterior body (with the dens projection), large lamina and a large spinous process.

The remaining cervical vertebrae (C3-7) are similar in shape. In the vertebrae of the cervical spine, as well as every other vertebra (except the coccyx) there is an opening formed by the posterior arch of bone. The opening is referred to as the spinal or vertebral foramen (opening). The spinal cord passes through the spinal foramen. The cervical vertebrae however have an extra foramen located in the transverse processes of C2 to C6. This is called the foramen transversarium (or transverse foramen) and serves as a passageway for the vertebral artery. The superior surface of each vertebral body in the lower cervical spin is concave and has projecting lips (uncus or uncinate process) that form the uncovertebral joint with the vertebra above. These joints provide stability from a lateral flexion and extension point of view. The spinous processes from C3 to C6 are bifid (split in two). This bifurcation houses the nuchal ligament, which inhibits passive flexion of the head.

Thoracic Spine

The thoracic spine consists of 12 vertebrae (T1 - T12) also referred to as the dorsal vertebrae. The thoracic region is rigid and is the section of the spine that articulates with the 12 pairs of ribs and aids in support of the thorax or trunk of the body.   Because of the presence of the ribs, the thoracic spine is more stable and less mobile than cervical spine above it or lumbar spine below it. The thoracic spine has natural kyphosis and the facets of the thoracic spine are mainly in the coronal or frontal plane. The individual vertebrae of the thoracic spine are similar to each other in shape, although they do get increasing larger toward the more caudal end of the thoracic region. The spinous processes in the thoracic spine are much larger than their cervical counterparts. Another uniqueness of the thoracic spine is the extra facet joints called the costal facets, which are on the vertebral body as well as the transverse processes to allow articulation with the ribs. It should be noted that there are not transverse foramen in t he region of the spine and none of the spinous processes are bifid.

Lumbar Spine

The lumbar spine is composed of five vertebrae, carries a large share of the body's weight and consequently also possesses the largest vertebrae. The curvature of the lumbar spine is lordotic (like that of the cervical spine) and the facets in the lumbar spine are in the sagittal plane. Due to the facet orientation, the lumbar spine is resistant to axial rotation, but allows for significant flexion and extension of the joints. The lumbar spine is fairly mobile compared to the rigid thoracic spine, but is less flexible than the cervical spine. The lumbar vertebrae are fairly flat and broad in shape and have larger posterior elements than any other region.

Sacrum

The sacrum contains five fused vertebrae that transmit the body's weight to the hips by articulating with the pelvic girdle through the sacroiliac joints. The posterior or dorsal surface of the sacrum has crests (median sacral crest) over the fused spinous, articular and transverse processes. The most superior portion of the fist sacral vertebra is called the sacral promontory due to its anterior protruding ridge.   The broad flat region of the anterior surface of S1 where it articulates with the ilium of the pelvis is referred to as the sacral ala.

Coccyx

The final region of the spine is the coccyx. This is the most caudal portion of the spinal column. The coccyx usually contains four or five bones and is functionally insignificant. It represents the vestigial tail of our mammalian antecedents. The bones of the coccygeal region have no pedicles, lamina or spinous processes and thus have no foramina.

Spinal Cord and nerves

The spinal cord begins at the foramen magnum (Latin for big hole), the opening at the base of the skull where it connects with the medulla oblongata (brain stem) and is the distal continuation of the central nervous system. The spinal cord is contained within the spinal foramen and is therefore protected by the bony skeleton of the spine.

The spinal cord tapers near the L1 level. The term for this tapered section is the conus medullaris. At the distal tapered end, the cord forms a bundle of filaments within the spinal canal that, because it resembles a horse's tail is called the cauda equina (Latin for horses' tail). The cord is relatively straight but does possess a cervical enlargement and a lumbar enlargement. These enlargements are due to the number of nerves transmitting to the upper and lower extremities.

There are three membranes that surround the cord. These membranes are referred to as the meninges. The most external membrane is called the dura mater and forms a loose sheath that does not adhere to the bones of the spinal canal. The second membrane is called the arachnoid. This layer is thin and its outer surface is in contact with the inner surface o the dura mater. Space between the layers where they are not connected is called the subdural space. The last, (most internal) membrane is the pia mater. This membrane closely adheres to the cord and forms a sheath for each filament of the spinal nerves. There exists a considerable space between the pia mater and the arachnoid. This space is referred to as the subarachnoid space and contains a serous secretion called the cerebro spinal fluid (CSF). This fluid expands the arachnoid to fill the entire space included within the dura mater.

The individual spinal nerves at each vertebral level originate inside the spinal cord and exit the spine through what is known as the intervertebral foramen. This foramen exists when one vertebra is stacked on another vertebra. These foramen are on each side of the spinal column just superior to the junction of the articular processes.

There are 31 pairs of spinal nerves that correspond with he regions through which they pass:

The spinal nerves are numbered top to bottom depending on their location of exit. Between the occiput and C1 exits the C1 nerve root. Between C1 and C2 exits the C2 nerve root. There fore, in the cervical spine, the nerve root exits above the level for which it is named. That is why in the cervical spine, although there are only seven vertebra, there are eight pairs of nerve roots. At the cervical-thoracic junction a change in numbering takes place and for the remainder of the spine, the nerve root is named for the level above where the nerve root exits. Thus, for example, the T1 nerve root exits between T1 and T2 and so on until the L5 nerve root exits below the L5 pedicle (or between L5 and S1).

Each spinal nerve has two roots, a dorsal or posterior root that provides sensory information and a ventral or anterior root that acts as a motor nerve.

The last main anatomical consideration concerning the spinal cord and nerves is a phenomenon called a plexus. A plexus is a center or network of nerves. A plexus exists when additional nerves are needed to innervate a specific area of the body. The four main divisions are as follows:

Cervical Plexus - formed by the anterior division of the four upper cervical nerves.

Brachial Plexus - formed by the union of the anterior division of the four lower cervical and first thoracic nerve.

Lumbar Plexus - formed by the anterior division of the four upper lumbar nerves and supplemented by the twelfth thoracic nerve.

Sacral Plexus - formed by the lumbo sacral cord, the anterior division of the three upper sacral nerves and part of the fourth sacral nerve.

Spinal Ligaments and Muscles

The ligamentous structures of the spinal column are extremely important in maintaining stability of the spine. When injuries occur in the spine, the bony damage as well as the ligamentous damage is classified to determine the surgical plan. During reduction of some fractures, the ligaments can actually help restore the bony anatomy to its anatomical position.

There are six major ligaments that are of particular importance. They are the anterior longitudinal ligament, posterior longitudinal ligament, the supraspinous ligament, interspinous ligament, ligamentum flava and the facet capsule.

A ligament is connecting tissue that connects bone to bone (unlike a tendon which connects bone to muscle). The anterior longitudinal ligament (ALL) begins at he anterior aspect of the base of the occipital bone and is attached to the atlas and to the anterior surface of all vertebrae including part of the sacrum. The ligament is approximately one inch wide.

The posterior longitudinal ligament (PLL) arises from the posterior aspe3ct of the base of the occipital bone, covers the dens and runs over the posterior surfaces of all vertebral bodies down to the coccyx. The ALL and PLL stabilize the spine against excessive flexion and extension.

The supraspinous ligament attaches the tip of each spinous process. (In the cervical spine, it starts as the nuchal ligament).

The interspinous ligament connects adjacent spinous processes. This ligament, unlike those mentioned above, is not a continuous ligament but a series of short ligaments between each segment.

The ligamentum flava also known as the yellow ligament (due to its color) connects the borders of adjacent lamina from the second cervical vertebra to the first sacral vertebra. This ligament possesses a great amount of elastic fibers and represents the most pure elastic tissue in the human body.

Finally, each joint that is formed by the apposition of the facets on the superior and inferior articular processes (sometimes called the zygapopyseal joint) is surrounded by a capsular membrane. These joints are much like other joints in the body (i.e. knee, elbow, shoulder) and have hyaline cartilage on the facet, synovial fluid and the facet capsule that surrounds the joint. The muscles that surround the spine create a soft tissue envelope that permits movement and provides stability of the spinal column. The paraspinal muscles of the back including the splenius capitus, splenius cervicus, erector spinae, transversospinalis and iliopsoas serve to stabilize the multiple bones of the vertebral column, influence posture and extend, flex and rotate the spinal column. During posterior spinal surgery, the muscles are removed from their vertebral attachment points with a Cobb elevator (a sharp, flat-bladed instrument) or similar instrument. Once the muscles have been treated, the surgeon can gain exposure of the bony anatomy and ligamentous structures.

The disc

The intervertebral discs are fibrocartilagenous structures that constitute about one-quarter of the entire height of the spine. Their function is to absorb pressure and distribute weights.

A disc contains two main structures - a soft, gel-like center called the nucleus pulposus and a tough outer ring called the annulus fibrosus.

The intervertebral discs are subjected primarily to compressive loads as a result of the direct weight of the trunk. The discs are also subject to other types of loads and stresses, when physiologic motions of flexion, extension and lateral bending occur, the discs become subject to tensile stresses. Tensile stresses tend to stretch or elongate the discs. When the torso rotates with the pelvis, torsional loads are produced which results in shear stresses. These shear stresses occur most often in the lumbar region during rotation.

The discs deform to accommodate the stress or external pressure acting on them. When the stress of external pressure is released (i.e. when lying down), the built-up internal pressure of the disc forces the endplates to separate. This separation restores the vertebral column length and physiological curves, while permitting the vertebrae to continue carrying out their normal function.

Injury

The spinal cord is considered part of the upper motor neuron system and is more sensitive to injury than the nerve roots and cauda equina that are considered part of the lower motor neuron system. Often injury to the spinal cord will be permanent whereas injury to the nerve roots and cauda equina usually have a much better prognosis.

When spinal injuries occur, they are classified primarily as complete or incomplete. Complete lesions are characterized by a total loss of motor, sensory and reflex functions below the level of injury. Complete cord injuries are named by the last function nerve root (i.e. CD4 quadriplegia). Incomplete injuries means there is some function, which is highly dependent on the level and neurological progress. To further classify these injuries, a common system is the Frankel classification. The division of the is classification are as follows:

1. Complete (A) - There is a complete loss of motor and sensory power below the lesion.
2. Sensory Only (B) - There is some sensation present below the level of the lesion, but motor paralysis is complete below that level.
3. Motor Useless (C) - There is some motor power present below the lesion, but it is of no practical use to the patient.
4. Motor Useful (D) - There is useful motor power below the level of the lesion and these patients can move their lower limbs and many can walk with or without aids.
5. Recover (E) - The patient is free of neurological symptoms, i.e. no weakness, no sensory or motor loss.

Another important determining factor in spinal cord or nerve injury is the level at which the lesion occurs. The more cranial the injury, the more restricted the patient. Complete injuries in the upper cervical spine will result in quadriplegia (all four limbs affected) where as complete injuries more caudally in the spine may result in paraplegia (lower extremities affected). The following list provides a brief summary of injuries and their associated levels. Note what a large difference in function, one level lower can make.

To help diagnose injury to the spinal nerves, dermatomes are used. Dermatomes (Greek for skin slices) are areas of overlying skin that are innervated by a specific spinal nerve and supply branch.

In addition to dermatomes, there are motor levels that correspond with the spinal nerves that are tested during a physical examination. A variety of reflex tests (especially deep tendon reflexes) are performed to help identify the level of injury. The reflex tests are helpful because they are diagnostic for sensory and motor functions of the nerves, spinal cord and muscle groups innervated by the nerve.            

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