THE RACHIS

It is an articulated structure, made up of mobile elements, the vertebrae, which extends from the upper cervical part to the lower lumbar part. These elements are integral with the lower part formed by the sacrum itself located above the coccyx.

This bone rod is median, posterior to the abdomen, and it is at the same time flexible, mobile and resistant.

The 3 functions of the spine:

1. static organ that supports the weight of the body. The mechanical stresses increase as the column is lowered. The whole is transmitted via the sacrum to the iliac wings, then to the 2 lower limbs by the coxo-femoral joints;

2. dynamic organ: the vertebrae are articulated relative to each other, which allows a certain number of movements which can be of great amplitude at the cervical level, like the mobility of the head, but less at the level of the thoracic cage. All this vertebral mobilization is made possible by the existence of muscles which will on the one hand ensure the static and the maintenance of the spine and on the other hand the mobility;

3. protective organ: the vertebrae are hollowed out by a canal, this superposition of the vertebrae allows the formation of the spinal canal, which contains the spinal cord from the first cervical vertebra to the L1 / L2 level. Below it is the ponytail, ie the end of the spinal cord.

The spinal canal is doubled inside by the meninges.

1.1. Dimensions

The height of the spine is approximately 70 cm, this height changing with age. Between the adult size of 20 years and the sixties, it loses approximately 5 cm. Each vertebra is separated from the other by an intervertebral disc, it is a cartilage rich in water and resistant. It contributes to the stability of the spine. The difference in height comes from the fact that it loses part of its volume of water.

1.2. Constitution

There are 3 parts:

• high: cervical region – supports the head -. It consists of 7 vertebrae. It is arranged above the thoracic region;

• medium: thoracic region. There are 12 vertebrae, the size of which gradually increases from top to bottom. It is located above the lumbar region;

• low: lumbar region. It consists of 5 vertebrae, the size of which gradually increases from top to bottom. Their size is greater than the thoracic vertebrae.

These 3 regions represent the mobile elements that explain the mobility of the spine. The lumbar region is articulated with the sacrum at L5, consisting of 5 bony pieces welded together, the first 2 of which occupy half the height. The sacrum is located above the Coccyx which extends the sacred crest, also made up of 4 elements welded together.

Each of the vertebrae is articulated with its above and underlying counterpart on 3 planes:

• 1 anterior plane: at the level of the intervertebral disc. This allows the stability of one vertebra with respect to the other, but also its mobility, as well as the absorption of shock waves. The goal is to best protect the brain. These discs play the role of the suspension.

• 2 posterior planes: The articulars guarantee stability and allow rotational movements of one vertebra with respect to the other. The shape of the joints varies from one level to another. They adapt perfectly to constraints. Indeed, there are mechanisms specific to each level: at the cervical level, flexion, extension of the head, significant mobility of the vertebrae, unlike the lumbar region. The lumbar region supports the most constraints: the most frequent pathology is the herniated disc often accompanied by neurological disorders.

The spine is not rigid: there are curvatures that will determine an increase in resistance.

• cervical curvature: convex in front, vertex in C3

• back curvature: concave forward

• lumbar curvature: concave backwards.

The lateral exaggeration gives kyphosis, the anteroposterior exaggeration gives lordosis. Kyphosis is often accompanied by scoliosis.

The spine is well protected but there are skin elements that allow us to understand the level examined: the spinous processes, similarly, if we draw a line joining the 2 iliac crests, we are at level L4.

The spinous processes and the sacred holes are perceptible.

2. Cervical spine

There are 7 cervical vertebrae. This part has 3 roles:

1. it supports the head and allows significant movements of mobility.

2. protection of the spinal cord by, in front, the vertebral body and the neural arch behind; which limits the spinal canal which contains the spinal cord, surrounded by meninges, and, between the meninge close to the bone and the spinal cord, there are the lepto-meningeal spaces which contain cerebrospinal fluid (CSF for short) which bathes the spinal cord over its entire length. On each side of the spinal canal, there is a vertebral foramen which contains the vertebral artery

3. laterally, on each side, there is a channel which allows the exit of the spinal nerves: the channel of conjugation.

2.1. Vertebral body

It is also called spondyle. It has the shape of a rectangular parallelepiped. The lateral part of the upper edge is raised, constituting on each side the unciform processes called uncus.

2.2. Neural Arc

It consists of the pedicles which attach it to the vertebral body, and which extend back towards the midline by the blades. It is triangular in shape with a posterior apex and it delimits behind the spinal canal which contains the spinal cord and the CSF with the meninges.

2.3. Vertebral processes around this complex:

• These are the transverse processes which are implanted laterally to the vertebral body, on the right and on the left, they are perforated with an orifice: the transverse foramen. Across the cervical region, the transverse foramines juxtapose to form the transverse canal. The end of this process has 2 tubers.

• At the back, the 2 blades extend through the median spinous process, oblique at the bottom and which is at its very posterior part, for the cervical, bituberculate. That of the seventh cervical vertebra is the most salient.

• Between the transverse and spinous processes, there are the articular processes which are located at the junction of the pedicles at the front and the blades at the rear. They have 2 facets, an upper and a lower. The principle is simple: the upper facet is articulated with the lower facet of the upper vertebra.

With aging, the uncus, the posterior edges and the vertebral processes can thicken, which implies an amputation of part of the spinal canal, i.e. compression of the spinal cord and therefore signs of suffering. Ditto for the spinal nerve.

3. Morphological specificities – Atlas – Axis

3.1. Atlas

Vertebra C1: it is called the Atlas.

It is characterized by its shape: a ring, and also by the fact that there is no vertebral body. There are 2 large, bulky lateral masses, which articulate with the occipital condyles. They are joined by 2 arches: anterior and posterior.

The anterior arch is concave behind. It has a prominent tuber on the midline. Its posterior surface has an articular surface with the tooth of the Axis, the odontoid process of C2.

The lateral masses are characterized by the existence of a large concave articular surface in its major axis. They form the imprint of a foot and are related to the occipital condyles.

On the lateral side, on each side, there is a transverse process hollowed out with an orifice, specific to all the cervical vertebrae, for the passage of the vertebral artery. The posterior arch is strongly concave forward and also has a small tuber on the midline.

The vertebral foramen is therefore broad and is limited by the lateral masses and the anterior and posterior arches. It is wide because there is no vertebral body.

3.2. Axis

It is the C2 vertebra. The vertebral body, spondyle, is extended upwards by the odontoid process, the tooth of the Axis. This tooth fits immediately behind the anterior arch of C1. It has a narrowed portion at its base: the neck of the tooth.

On each side of the axis tooth there are 2 articular processes on the right and on the left themselves related to those of C1. The tooth is blocked, limited behind by a very resistant ligament: the transverse Atlas ligament. It blocks the tooth against the anterior arch.

The posterior part, the posterior neural arch, is characterized by the fact that the pedicles are long and fragile

3.3. Thoracic Spinal Canal

In front of the vertebral body, there is the anterior vertebral common ligament Likewise, the posterior vertebral common ligament lines the posterior part of the vertebral bodies, plus the anterolateral part of the spinal canal.

The yellow ligament lines the posterior surface of the spinal canal.

The inter osseous ligament connects the spinous processes.

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