Axis (anatomy)
Axis | |
---|---|
Details | |
Identifiers | |
Latin | axis, vertebra cervicalis II |
MeSH | D001368 |
TA98 | A02.2.02.201 |
TA2 | 1050 |
FMA | 12520 |
Anatomical terms of bone |
In anatomy, the axis (from Latin axis, "axle") is the second cervical vertebra (C2) of the spine, immediately inferior to the atlas, upon which the head rests.
The axis' defining feature is its strong odontoid process (bony protrusion) known as the dens, which rises from the superior aspect of the bone.
Structure
The body is deeper in front or in the back and is prolonged downward anteriorly to overlap the upper and front part of the third vertebra.
It presents a median longitudinal ridge in front, separating two lateral depressions for the attachment of the longus colli muscles.
Dens
The dens, also called the odontoid process, or the peg, is the most pronounced projecting feature of the axis. The dens exhibits a slight constriction where it joins the main body of the vertebra. The condition where the dens is separated from the body of the axis is called os odontoideum and may cause nerve and circulation compression syndrome.
The internal structure of the odontoid process is more compact than that of the body. The odontoid peg is the ascension of the atlas fused to the ascension of the axis. The peg has an articular facet at its front and forms part of a joint with the anterior arch of the atlas. It is a non-
Other features
The
The
The vertebral foramen is large, but smaller than the atlas.
The
The superior
The inferior articular surfaces have the same direction as those of the other cervical vertebrae.
The superior
The
Variation
Contact sports are contraindicated for individuals with anomalous dens, as any violent impact may result in a catastrophic injury.[3] This is because a malformed odontoid process may lead to instability between the atlas and axis (the C1 and C2 cervical vertebrae).
Development
The axis is ossified from five primary and two secondary centres.
The body and vertebral arch are ossified in the same manner as the corresponding parts in the other
The centres for the arch appear about the seventh or eighth week of fetal life, while the centres for the body appear in about the fourth or fifth month.
The dens, or
During about the sixth month of fetal life, two centres make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above; the interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage.
The base of the process is separated from the body by a cartilaginous disk, which gradually becomes ossified at its circumference, but remains cartilaginous in its center until advanced age.
In this cartilage, rudiments of the lower epiphyseal lamella of the atlas and the upper epiphyseal lamella of the axis may sometimes be found.
The apex of the odontoid process has a separate centre that appears in the second and joins about the twelfth year; this is the upper epiphyseal lamella of the atlas.
In addition to these, there is a secondary centre for a thin
Clinical significance
Fracture of dens
Fractures of the dens, not to be confused with Hangman's fractures, are classified into three categories according to the Anderson Alonso system:
- Type I fracture - Extends through the tip of the dens. This type is usually stable.
- Type II fracture - Extends through the base of the dens. It is the most commonly encountered fracture for this region of the axis. This type is unstable and has a high rate of non-union.
- Type III fracture - Extends through the vertebral body of the axis. This type can be stable or unstable and may require surgery.[1]
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A fracture of the base of the dens as seen on plain X-ray
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A fracture of the base of the dens as seen on CT
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Type 3 odontoid fracture
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Type 2 dens fracture
Additional images
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Its shape and position (shown in red) from above. The skull is shown in semi-transparent.
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Axis seen from above
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3D image
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Posterior atlantooccipital membrane and atlantoaxial ligament; the axis is visible at center.
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Median sagittal section through the occipital bone and first three cervical vertebra
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Sagittal section of nose mouth, pharynx, and larynx
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Axis on X-ray
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Unfused arch of atlas at CT
See also
References
This article incorporates text in the public domain from page 99 of the 20th edition of Gray's Anatomy (1918)
- PMID 29018648.
- PMID 8621347.
- ISBN 9780892031726.
External links
- Netter, Frank. Atlas of Human Anatomy Archived 2017-11-20 at the Wayback Machine, "High Cervical Spine: C1-C2"