anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

, the axis (from Latin ''axis'', "axle") or epistropheus is the second

cervical vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sa ...

(C2) of the spine, immediately inferior to the

atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...

, upon which the head rests. The axis' defining feature is its strong odontoid process (bony protrusion) known as the dens, which rises dorsally from the rest 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 muscle The longus colli muscle (Latin for ''long muscle of the neck'') is a muscle of the human body. The longus colli is situated on the anterior surface of the vertebral column, between the atlas and the third thoracic vertebra. It is broad in the m ...

Odontoid Process of Axis (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. On its anterior surface is an oval or nearly circular facet for articulation with that on the

anterior arch of the atlas In anatomy, the atlas (C1) is the most superior (first) cervical vertebra of the spine and is located in the neck. It is named for Atlas of Greek mythology because, just as Atlas supported the globe, it supports the entire head. The atlas is t ...

. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position. The apex is pointed and gives attachment to the apical odontoid ligament. Below the apex, the process is somewhat enlarged and presents on either side a rough impression for the attachment of the

alar ligament In anatomy, the alar ligaments are ligaments which connect the Axis (anatomy), dens (a Process (anatomy), bony protrusion on the second cervical vertebrae, cervical vertebra) to Tubercle (anatomy), tubercles on the Anatomical terms of location#Med ...

; these ligaments connect the process to the occipital bone. 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-

weight bearing In orthopedics, weight-bearing is the amount of weight a patient puts on an injured body part. Generally, it refers to a leg, ankle or foot that has been fractured or upon which surgery has been performed, but the term can also be used to refer to ...

joint. The alar ligaments, together with the apical ligaments, are attached from the sloping upper edge of the odontoid peg to the margins of the foramen magnum. The inner ligaments limit rotation of the head and are very strong. The weak apical ligament lies in front of the upper longitudinal bone of the cruciform ligament and joins the apex of the deltoid peg to the anterior margin of the foramen magnum. It is the fibrous remnant of the notochord.

Other features

The pedicles are broad and strong, especially in the front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces. The laminae are thick and strong. They play a large role in the stability of the cervical spine alongside the laminae of C7. The

vertebral foramen In a typical vertebra, the vertebral foramen is the foramen (opening) formed by the anterior segment (the body), and the posterior part, the vertebral arch. The vertebral foramen begins at cervical vertebra #1 (C1 or atlas) and continues inferio ...

is large, but smaller than the

. The transverse processes are very small, and each ends in a single tubercle. Each process is perforated by the transverse foramen, which is directed obliquely upward and laterally. The superior

articular surface A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

s are round, slightly convex, directed upward and laterally, and are supported on the body, pedicles, and transverse processes. The inferior articular surfaces have the same direction as those of the other cervical vertebrae. The superior

vertebral notches The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

are very shallow, and lie behind the articular processes. The inferior vertebral notches lie in front of the articular processes, as in the other cervical vertebrae. The

spinous process The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

is large, very strong, deeply channelled on its under surface, and presents a bifurcated extremity.

Variation

Contact sport Contact sports are sports that emphasize or require physical contact between players. Some sports, such as mixed martial arts, are scored on impacting an opponent, while others, including rugby football, gridiron football and Australian rules fo ...

s are

contraindicated In medicine, a contraindication is a condition that serves as a reason not to take a certain medical treatment due to the harm that it would cause the patient. Contraindication is the opposite of indication, which is a reason to use a certain tre ...

for individuals with anomalous dens, as any violent impact may result in a

catastrophic injury Catastrophe or catastrophic comes from the Greek κατά (''kata'') = down; στροφή (''strophē'') = turning ( el, καταστροφή). It may refer to: A general or specific event * Disaster, a devastating event * The Asia Minor Catastro ...

. This is because a malformed odontoid process may lead to instability between the

and axis (the C1 and C2 cervical vertebrae).

Development

The axis is

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

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

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

, viz., one centre for the body, and two for the

vertebral arch The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

. 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

odontoid process In anatomy, the axis (from Latin ''axis'', "axle") or epistropheus 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 (bon ...

, consist originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed. 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

epiphyseal plate The epiphyseal plate (or epiphysial plate, physis, or growth plate) is a hyaline cartilage plate in the metaphysis at each end of a long bone. It is the part of a long bone where new bone growth takes place; that is, the whole bone is alive, wit ...

on the undersurface of the body of the bone.

Clinical significance

Fracture of dens

Fractures of the dens, not to be confused with

Hangman's fracture Hangman's fracture is the colloquial name given to a fracture of both pedicles, or '' partes interarticulares'', of the ''axis vertebra'' ( C2). Causes The injury mainly occurs from falls, usually in elderly adults, and motor accidents mainly d ...

s, 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 surger

File:PdensfracX.png, A fracture of the base of the dens as seen on plain X-ray File:PdensfracCT.png, A fracture of the base of the dens as seen on CT File:OdontoidType3Mark.png, Type 3 odontoid fracture File:Type2DensFracMark.png, Type 2 dens fracture

Additional images

File:C2 from top animation small.gif, Its shape and position (shown in red) from above. The skull is shown in semi-transparent. File:Cervical vertebra 2 close-up top animation.gif, Axis seen from above File:Human axis bone.stl, 3D image File:Gray305.png, Posterior atlantooccipital membrane and atlantoaxial ligament. (Axis visible at center.) File:Gray308.png, Median sagittal section through the occipital bone and first three cervical vertebræ File:Gray994.png, Sagittal section of nose mouth, pharynx, and larynx File:Dens axis.jpg, Axis on X-ray File:Atlasbogenspalt.jpg, Unfused arch of atlas at CT

References

External links

*Netter, Frank
''Atlas of Human Anatomy''
, "High Cervical Spine: C1-C2" {{Authority control Bones of the thorax