In anatomy, the axis (from Latin ''axis'', "axle") or epistropheus is the second cervical vertebra (C2) of the

spine Spine or spinal may refer to: Science Biology * Vertebral column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoolog ...

, 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 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 mid ...

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. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the

transverse atlantal ligament In anatomy, the transverse ligament of the atlas is a ligament which arches across the ring of the Atlas (anatomy), atlas (the topmost Cervical vertebrae, cervical vertebra, which directly supports the skull), and keeps the odontoid process in cont ...

which retains the process in position. The apex is pointed and gives attachment to the

apical odontoid ligament The ligament of apex dentis (or apical odontoid ligament) is a ligament that spans between the axis (anatomy), second cervical vertebra in the neck and the skull. It lies as a fibrous cord in the triangular interval between the alar ligaments, whic ...

. Below the apex, the process is somewhat enlarged and presents on either side a rough impression for the attachment of the alar ligament; these ligaments connect the process to the

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobe ...

. 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 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 foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblon ...

. 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 In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...

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 is large, but smaller than the atlas. The

transverse processes 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 i ...

are very small, and each ends in a single tubercle. Each process is perforated by the

transverse foramen 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 ...

, which is directed obliquely upward and laterally. The superior articular surfaces 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 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 ...

. The superior vertebral notches are very shallow, and lie behind the

articular processes The articular processes or zygapophyses (Greek ζυγον = "yoke" (because it links two vertebrae) + απο = "away" + φυσις = "process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vertebr ...

. The inferior vertebral notches lie in front of the articular processes, as in the other cervical vertebrae. The spinous process is large, very strong, deeply channelled on its under surface, and presents a bifurcated extremity.

Variation

Contact sports are contraindicated for individuals with anomalous dens, as any violent impact may result in a catastrophic injury. This is because a malformed odontoid process may lead to instability between the atlas and axis (the C1 and C2

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 vertebrae, viz., one centre for the body, and two for the vertebral arch. 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 The epiphysis () is the rounded end of a long bone, at its joint with adjacent bone(s). Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, including the epiphyseal plate (growth plate). At the joi ...

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 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