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Anatomy of the spine

The vertebral column (or backbone) is composed of 32 vertebrae separated by fibro-cartilaginous discs. 
It consists of five areas: cervical, thoracic, lumbar, sacral and coccygeal. 
The cervical vertebrae are formed by small bodies, the sacral and lumbar are much larger, this is because the weight of the body becomes heavier in the lower vertebrae. 
In the sagittal plane, the spine presents 4 curvatures: 2 kyphosis and 2 lordosis.

The lordosis is at the cervical and lumbar level, with a curve with the concavity facing backwards. 
Cervical lordosis is less pronounced in comparison to the lumbar spine. 
Kryososis is the opposite of lordosis, it is located in the thoracic and sacral spine, including the coccyx where it is more pronounced than in the thorax. 
The rachis has a shape that resembles two “s” in the column.


Vertebra top view

If the curvature is accentuated on the frontal plane it is a scoliosis, ie a permanent lateral and rotational deviation of the vertebral column. 
The scoliosis can affect any part of the spine: cervical, thoracic , or lumbar spine.

A nonhomogeneous load causes greater pressure toward the part where the vertebra is tilted to the side. The consequence is a higher consumption of joint cartilage typical of arthrosis . 
The vertebrae of the spine increase from top to bottom because the lower part should support more weight while the coccygeal vertebrae that should not support the weight are small. 
The vertebrae are formed in the front by a body and behind the vertebral arch. 
There are seven processes that originate from the arch: 
• Two above and two below the joint between the vertebrae. 
• Two transverse and a spiny one that constitute the origin or the insertion of the dorsal muscles. 
There are seven processes that originate from the arc:

Many muscles and ligaments give stability to the spine.

To hold together the bodies there is the anterior and posterior longitudinal ligament, whereas for the vertebral arch and posterior processes there is the yellow, interspinal, supraspinal, intertransversal ligament and the interaphysial ligaments.

Back vertebrae

Biomechanics of the spine

The spine has three main functions. 
The first is to protect the spinal cord that is located within the vertebral foramen and surrounded by very strong ligaments. 
The second function is to support the majority of body weight and keep the head erect. 
The third function is to attenuate internal and external trauma and micro-trauma that the spine transfers to the sacroiliac joint or to the upper limbs, head and trunk. This serves to defend the visceral structures it protects. 
The spine has 4 physiological curves in the sagittal plane: 
• Cervical and lumbar lordosis (forward convexity). 
• Dorsal kyphosis (backward convexity) and Sacro-coccygeal.

A column with curves can withstand a greater pressure, precisely if it multiplies the weight that can support a column straight to the square of the number of curves, plus one. 
In the case of the rachis we obtain: 3 * 3 + 1 = 10, so it can support a weight ten times bigger than a straight column. 
The bone is composed of a cortical (compact) part and a spongy part, especially present in the bones such as the heel and the vertebrae that are composed by vertical and horizontal trábeculas. 
Depending on the orientation of this structure, there is a greater resistance in some senses than in others along lines of force, for example, the vertically organized trabeculae that support the weight of the standing body.
In the vertebrae this orientation implies a lower solidity in the front with a risk of fracture much greater than other parts. 
Functionally, we consider the vertebral column as a tripod with a previous support surface at the body level and two posterior support surfaces at the level of the lower processes. 
The vertebral body, in front of the rachis, almost completely supports body weight, while later processes regulate the movements.


Joint mobility

The vertebral joints can be divided into 2 types: 
• Between the bodies there is the symphysis that has limited mobility in all directions. 
• Between the lower and upper processes of the successive vertebrae are called amphiarthroses and are responsible for the movement of the spine. 
The possible movements are flexion-extension, lateral tilt and rotation.

The maximum range of motion of the trunk is: 
• At 75/80 ° bending • At 25/30 ° 
extension • At 30/35 ° 
lateral tilt 
• At 40/45 °

At cervical level mobility is greater than the other parts, in fact here the movements can achieve: 
• 50 ° flexion 
• 45 ° extension • 45 ° 
lateral inclination 
• 65 ° rotation

The flexion-extension is a movement performed on the sagittal axis and the transverse axis. During extension the space of the intervertebral foramen decreases, vice versa in flexion. 
This movement is performed almost entirely by the cervical and lumbar part, while the thoracic is limited mainly because of the joints with the back and also because the disc has a thickness smaller than the vertebral bodies. 
In the lumbar spine the range of motion is 60 ° in flexion and 45 ° in extension. 
Lateral tilt . The movement is in the frontal plane and in the sagittal axis of the body, it is limited in case of scoliosis. 
Rotation . Rotation occurs on the transverse and longitudinal axis.


Anatomy of the cervical vertebrae

In the cervical vertebral, thoracic and lumbar spine, the vertebrae are formed by a body and 7 processes. 
In each of these groups, the shape and size of the bones are different from one another. 
A typical cervical vertebra is small and has a large hole in the center. 
The transverse processes are in the lateral part, each of them has a hole called transverse foramen, where it passes the artery and the vertebral vein. 
The 7th cervical vertebra is called prominent, does not have a foramen to the cervical artery and has a very long spinous process and not bifid, this vertebra goes out a lot.

The cervical vertebrae have peculiar characteristics, the first one is called Atlas and the second Axis.

The Atlas does not have a body, but has two arcs, one normal and the other where the body is, they are called the anterior and posterior arch. 
In this vertebra the thorny process lacks. 
In the Atlas, there is a large vertebral foramen partially filled by the vertebral joint with the second vertebra.

The second vertebra is called axis, above the body it has a prominence called the Tooth (Odontoid Process). 
The apex shows the beginning of a spiny bifid process. 
The tooth of the apex (odontoid process) is very long, above the Atlas and reaches the level of the great occipital foramen. In violent movements, if fractured, it can affect the spinal cord and can cause death. 
Death by hanging is caused by the rupture of the odontoid process. 
It is important not to move the neck in case of large fractures because the odontoid emerges into the large occipital foramen and may compress the brain stem and the elongated spinal cord where the respiratory centers meet. 
The destruction of this part of the brain causes death, because here there are vital centers.
The spine connects to the base of the skull through the Atlas, at the level of the articular facets, on both sides of the great foramen of the occipital bone. 
The joint is called Atlo-occipital and allows only flexion-extension movements. 
The rotational movement of the head is at the level of the articulation between the Atlas and the Axis.


The thoracic vertebrae

The thoracic vertebrae have a larger body than the cervical vertebrae, but these have a larger transverse diameter. 
Descending towards the lumbar vertebrae, the spiny process is more inclined and is not bifid. 
The transverse processes present a joint facet. 
The first thoracic vertebra presents a single costal facet in the body. From the second to the tenth there are two hemifacts because the ribs are articulated with the corresponding vertebra and with the preceding vertebra. 
The vertebral foramen is smaller than the cervical vertebrae and has the shape of a pentagon. 
The transverse process of the thoracic vertebrae allows the ribs to rise with the contraction of the intercostal muscles.
The rib head articulates with two vertebrae except the first, eleventh, and twelfth vertebrae. The outas articulate with the rib of the corresponding number.


The lumbar vertebrae

The lumbar vertebrae are five, have large body and vertebral foramen very small compared to the vertebral body, have a large spiny process in quadrilateral form. 
The vertebral foramen is larger than the thoracic vertebra but smaller than that of the cervical with a shape similar to a triangle.

The upper articular facets have a concave surface, while the lower ones have a convex surface. 
The upper articular processes look forward and the lower backward processes in the thoracic is the opposite.

The fifth lumbar vertebra has some differences compared to the other four, in the sagittal plane has a shape similar to a rectangular trapezoid with the minor side towards the thorny process. 
The transverse processes and the lower articular facets are also in a more lateral position than the other vertebrae.


The sacred bone

Situated between the lumbar spine and the coccyx, it is shaped like an inverted pyramid. 
It is formed by five vertebral bodies united without interposition of intervertebral discs. 
Laterally, the transverse processes are united forming protrusions called sacral wings. 
The intervertebral foramen is called the sacral foramen, there are four posterior foramina per side, of which the spinal nerves come out. 

On each side of the sacral bone there is a joint surface called the atrial facet because it is shaped like an ear.
This is articulated with a very thin facet that is in the hip bone forming the sacro-iliac joint. 
The sacrum articulates inferiorly with the coccyx.


The intervertebral disc

Intervertebral disc and nerve root 
© turhanerbas – Fotolia.com

The intervertebral disc is a fibrocartilaginous bearing present between the body of a vertebra and the one below. 
Located between the vertebrae of the cervical, dorsal and lumbar spine, it is not present at the sacral or coccygeal level. 
The shape is almost the same as that of the bodies it separates, it fits well to the contours of these. 
The vertebra has a nearly equal height anteriorly and posteriorly, except the thoracic one that presents a greater height in the back. 
The disks are taller anteriorly in the lumbar and cervical part, vice versa in the thoracic tract, the consequence is the characteristic shape of the spine with the three physiological curves. 
The intervertebral discs gradually increase from the cervical to the lumbar area.
The ratio of proportionality between the height of the disc and that of the bodies is maximal between the cervical vertebrae while it is minimal between the thoracic vertebrae. 
In the treatment where the disc has a thickness greater than the body, the joint amplitude of the rachis is greater. 
The disc is composed of collagen fibers and connective tissue, does not contain blood vessels or nerves. 
It has a rolling function to attenuate the pressure to which the column is subjected during the day, coming from below or from above. 
It is formed by two concentric structures:

  • The inner part is called the nucleus pulposus, of gelatinous consistency, consists predominantly of hydrophilic mucopolysaccharides and water.
  • The outer part is the fibrous annulus, ie a solid, rigid outer capsule, the fibers of which are oblique and separated into concentric bundles with opposite directions. This structure is used to evenly distribute the forces exerted on the spine.

Most of the disc pressure is discharged into the nucleus pulposus. 
The consequence of standing or sitting during the day is that the vertebrae and discs should support the weight of the upper body. 
The consequence is that the water contained in the nucleus pulposus moves towards the vertebral body. Body height is lower at night than about 1-1.5 cm compared to that in the morning. 
The nucleus pulposus is not located exactly in the center of the annulus but is slightly shifted to the back, ie the arch of the vertebra. 
When we perform the flexion-extension or lateral inclination movements of the spine, the disc narrows narrowing in the part where the vertebral column moves away. 
As we age, the discs degenerate, the nucleus pulposus becomesdehydrates  by making more weight on the annulus that, therefore, can be lacerated.

This results in a lower elasticity and a decrease in height. 
Sedentary lifestyle, obesity and poor postural habits favor disc dehydration.

Under these conditions, abrupt movement of the spine may cause an outflow of the nucleus pulposus out of the annulus, ie, a hernia . 
protrusion of the nucleus pulposus that presses on the roots of the spinal cord or spinal nerve is considered as cause of low back pain , whereas the majority of the individuals where a disc hernia has been found does not complain of spine pain or sciatic pain .


Spinal cord

The spinal cord  is a bundle of nerves that forms the central nervous system along with the brain. 
It originates from the brain and more precisely from the bulb and ends up forming the medullary cone at L2 level. 
The spinal cord has a nearly conical shape, is inserted and protected by the vertebral foramen which is the canal between the body and the arch of the vertebra. 
The protection of the bone marrow is given in the first instance by the cerebrospinal fluid and the meninges, that is, a triple layer of membranes: dura mater (the external one), the arachnoid and the pia mater (more internal). 
The neurons are the functional units of the nervous system, they make up the gray matter of the spinal cord that in the transverse plane is shaped like an “H”.
Outside the gray matter is found the white substance composed of ascending and descending nervous bundles, the former carrying sensorial type information, the latter carrying the commands to the muscles. 
The posterior roots are sensitive, while the anterior roots are motile. 

In the lateral part of the spine, in the space between the pedicles of two adjacent vertebrae, there exists the foramen of conjugation, crossed by spinal nerves. 
From the medulla the anterior and posterior spinal nerve roots leave. 
After crossing the foramen of conjugation, they join in a single spinal nerve that innervates the viscera and the locomotor system.

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