Generality
The axis, or axis, is the second cervical vertebra of the spine.
Together with the atlas (first cervical vertebra) through the atlo-epistropheic articulation, the axis is a very particular vertebra; in fact, it has a vertebral body of small dimensions, from which originates, on the upper side, a prominence called tooth, which is absent in all the other vertebrae of the spine.
Thanks to the tooth, the axis acts as a rotation pivot for the atlas: this mechanism allows the human being to turn his neck and head with greater ease.
Together with the atlas, the axis constitutes the segment of the cervical column most subject to fracture; the fracture of the axis is a much feared accident, because it can lead to permanent paralysis or, in the most unfortunate cases, to the death of those who are its victims.
What is the Epistrofeo?
The axis is the second cervical vertebra of the spine.
The epistrophe is a singular vertebra, with completely distinctive characteristics, which owes its anatomical importance to its particular relationship with the atlas, that is, the first cervical vertebra (with respect to the episcope, therefore, the atlas is in a superior position ).
The axis is also known by the names of: axis, dentate vertebrae and vertebra C2 (where the letter C stands for "cervical" and the number 2 indicates the positioning along the cervical tract of the spine).
To understand: revision of the Vertebral Column and Vertebrae
- The spine, or spine, is the bone structure that:
- It runs vertically along the center of the back;
- It is the backbone of the human body;
- It houses and protects the spinal cord (which, along with the brain, makes up the central nervous system ).
- Starting from the apex, the vertebral column can be divided into 5 segments (or sections): the cervical segment, the s. thoracic, the s. lumbar, the s. sacral and the s. coccygeal;
- The vertebral column is composed of 33-34 overlapping irregular bones, called vertebrae, which are separated from each other by a thin fibrocartilage structure called the intervertebral disk;
- Of the 33-34 vertebrae forming the spine, 7 belong to the cervical tract, 12 to the thoracic tract, 5 to the lumbar tract, 5 to the sacral tract and 4/5 to the coccygeal tract;
- Although their specific anatomy varies in relation to the tract of spine considered, all the vertebrae present:
- A cuboid-shaped element in a ventral position, called a vertebral body ;
- An arched formation in dorsal position, called vertebral arch ;
- A hole between the arch and the body, whose name is a vertebral hole ;
- A prominence in the center of the outer edge of the arch, called the spinous process ;
- A prominence for each external side of the vertebral arch, called the transverse process .
Anatomy
Jointed with the atlas through the so - called atlo-epistropheic articulation, the axis is a very particular vertebra. In fact, it is characterized by a body of small dimensions and with an anomalous morphology, from which it originates, on the upper side, a prominence called tooth or odontoid process, which is absent in all the other vertebrae of the spine.
Further details on the peculiarities of the epistemology and details related to localization, components and connected ligaments and muscles will be discussed in the next sections of this chapter.
Location of the Epistrophy
The axis resides between the atlas (first cervical vertebra), superiorly, and the third cervical vertebra, inferiorly.
The axis is located in the upper back of the neck, just above the lower border of the mandible .
Characteristics of the components of the Epistrophy
VERTEBRAL BODY
The vertebral body of the axis develops downwards, towards the third cervical vertebra. As anticipated, it is not a large structure, as is the case for thoracic vertebrae or lumbar vertebrae.
On the front (or ventral) surface, in a central position, the body of the axis is a longitudinal crest, which is interposed between two depressed areas, on which the two so-called long muscles of the neck are connected .
At the level of the lower surface, the axis presents a concavity in a central position and a convexity on each side.
DENTAL OR DENTISTRY PROCESS
The tooth is the most characteristic anatomical element of the axis.
It develops upwards starting from the upper surface of the vertebral body, assuming the appearance of a rounded prominence up to the apex, which is instead pointed.
Anteriorly, the tooth is provided with an oval area - called in a generic way facet - which serves to articulate the axis to the vertebral arch of the atlas; on the back, instead, it has a shallow groove, which serves to hook the so-called transverse ligament of the atlas .
The transverse ligament of the atlas is a thick and strong band of fibrous connective tissue, which has the task of fortifying the union between the axis of the atlas and the vertebral arch.
In the apical position, on the tip, the tooth has an area that serves to hook the so-called apical odontoid ligament ; a little further down, instead, it gives space to two distinct areas, which have the task of anchoring the two so-called wing ligaments .
The apical odontoid ligament (or ligament of the dental apex) and the two alar ligaments are the bands of fibrous connective tissue that join, respectively, the apex of the tooth of the axis and the area just below the aforementioned apex to the occipital bone (for the precision, to the so-called condyles of the occipital bone).
As the reader will have the opportunity to deepen later, the tooth contributes in a decisive way to the functions of the axis and to the relationship between the latter and the atlas.
VERTEBRAL ARCH
As a rule, the vertebral arch of a generic cervical vertebra consists of:
- The two peduncles, which correspond to the portions of the arch immediately connected to the vertebral body;
- The two intervertebral holes, which are the channels used for the passage of the spinal nerves exiting the spinal cord;
- The lamina, which is the curved bone segment that runs from a peduncle to a peduncle and from which the transverse processes and the spinous process originate;
- The two upper articular processes, which are two growths emerging from the upper side of the vertebral arch, with the important task of joining the vertebra of belonging to the upper vertebra (through the lower articular processes of this);
- The two lower joint processes, which are two growths emerging from the lower side of the vertebral arch, with the fundamental function of connecting the vertebra of belonging to the inferior vertebra (through the superior articular processes of this).
In the vertebral arch of the axis, the two peduncles are very broad, especially on the anterior portion, where they connect to the vertebral body and al dente; the intervertebral holes are normal and have the task of guaranteeing the passage to the second pair of spinal nerves (NB: the intervertebral holes originate from the superposition of two vertebrae); the lamina is broad and massive, largely tracing the appearance of the peduncles; finally, the two upper articular processes and the two lower articular processes are in fact, respectively, a slight upper extension and a slight lower extension of the two peduncles.
Joining the lower articular processes of the atlas, the articular processes of the axis contribute in a decisive way to the atlo-epistropheic articulation, which in fact joins atlas and axis.
SPINY PROCESS
Originating on the outer side of the vertebral arch, halfway between the two peduncles, the spinous process of the axis is a large bone projection, which at one point of its lower portion forks giving rise to a groove.
The spinous process of the axis serves, on a par with what happens in the other vertebrae of the spine, to anchor the muscles and ligaments of the back.
In all vertebrae, including the epistrophy, the spinous process represents the most ventral bone portion.
TRANSVERSE PROCESSES
Connected to important neck muscles, the transverse processes of the axis are two small lateral projections of the vertebral arch, which originate more or less at the peduncles and, with oblique orientation, develop slightly downwards.
As in all cervical vertebrae, even in the axis, the transverse processes are provided with a hole, called a transverse hole (or transversal hole ), through which the vertebral artery and the vertebral vein pass.
Did you know that ...
The transverse hole is a peculiarity of the cervical vertebrae; this means that only the transverse processes of the cervical vertebrae are provided.
VERTEBRAL HOLE
In line with all the other cervical vertebrae, the axis possesses a vertebral hole of considerable size, in which one of the earliest parts of the spinal cord passes.
Muscles and Ligaments connected to the Epistrophy
If we want to summarize the structure of the muscles and ligaments connected to the axis, it appears that:
- On the tooth 4 important ligaments are anchored: the transverse ligament of the atlas, the apical odontoid ligament and the two alar ligaments. These ligaments are part of the aforementioned atlo-epistropheic articulation, which links the atlas to the axis;
- On the transversal processes, part of the levator scapulae, middle scalene and neck splenium are inserted, which play a fundamental role in the rotation of the head;
- On the spinous process, part of the nuchal ligament and part of the semispinal muscles of the neck are inserted, the posterior rectum greater than the head, inferior oblique, spinal of the neck, interspinous and multifidus (NB: these muscles contribute to the musculature of the upper section of the back);
- On the lamina are found some of the yellow ligaments, whose function is to consolidate the relationship between the axis and the contiguous vertebrae.
Ossification
The axis is a bone whose final formation is contributed by 5 primary ossification centers and 2 secondary ossification centers :
- Of the 5 primary ossification centers, one resides where the vertebral body takes life; two reside where the vertebral arch will appear; two others are located where the base of the tooth will form;
- Of the 2 secondary ossification centers, one takes place where the apex of the tooth takes shape, while the other where the lower apex of the body appears;
- The first of primary ossification centers to be activated are the two present on the vertebral arch (between the VII and the VIII week of fetal life); then follow, in order, the primary ossification center present on the vertebral body (between the IV and V month of fetal life) and the two primary ossification centers present at the base of the tooth (VI month of fetal life);
- From the primary ossification centers of the vertebral arch depends the formation of the vertebral arch and of the various related processes (spino process, transverse processes, etc.); the formation of the body, excluding the lower apex, depends on the primary ossification center of the vertebral body; finally, the generation of the tooth, excluding the upper apex, depends on the primary ossification centers present at the base of the tooth;
- The formation of the upper apex of the tooth and the lower apex of the body depend on the two secondary ossification centers.
Did you know that ...
Before ossification, the future tooth of the axis is actually the body of the atlas.
Function
The axis covers two functions:
- Thanks to the tooth, it acts as a rotation pivot for the overlying atlas (or first cervical vertebra).
The rotation of the atlas on the tooth of the axis is an extremely important mechanism for the human being, because it allows the latter to turn his head and neck with greater ease;
- It acts as a coupling point for important neck muscles (which also contribute to the rotation of the head and the neck itself) and for important back muscles;
- Protects a short segment of spinal cord.
diseases
The axis constitutes with the atlas the segment of the cervical column more subject to fractures .
Fracture of the Epistrophy: what it is, causes and types
The fracture of the axis is a much feared injury, as it can result in permanent paralysis or, in the worst cases, in the death of those who are its victims.
The most common causes of fracture of the axis are trauma due to violent collision at the neck and trauma due to violent movements of the head; these two situations (violent shock injuries to the neck and traumas caused by violent head movements) are, in most cases, the result of road accidents.
According to the so-called Anderson-D'Alonzo classification, there are three types of fractures in the axis :
- Type I, which groups all the fractures of the apex of the tooth of the axis.
Features: it is a usually stable fracture;
- Type II, which includes all the fractures of the basis of the tooth of the axis.
Characteristics: it is a usually unstable fracture;
- The fracture of the type III epistrophe, which brings together all the fractures of the vertebral body of the axis.
Features: can be stable or unstable.
