The atlanto-occipital joint (''Articulatio atlantooccipitalis'') is an articulation between the atlas bone and 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 lies over the occipital lob ...

. It consists of a pair of condyloid joints. It is a

synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulati ...

Structure

The atlanto-occipital joint is an articulation between the atlas bone and the

. It consists of a pair of condyloid joints. It is a

Ligaments

The ligaments connecting the bones are: * Two articular capsules * Posterior atlanto-occipital membrane * Anterior atlanto-occipital membrane

Capsule

The capsules of the atlantooccipital articulation surround the condyles of the

, and connect them with the articular processes of the

atlas An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...

: they are thin and loose.

Variation

Atlantooccipital fusion, also known as occipitalization of the atlas, is a congenital or acquired anomaly characterized by the partial or complete fusion of the atlas to the base of the occipital bone. It is found in 0.12% to 0.72% of the population. This fusion results in the elimination or reduction of movement and abnormal neck posture, or even narrowing of the foramen magnum compressing the spinal cord.

Function

The movements permitted in this joint are: * (a)

flexion Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, Limb (anatomy), limbs, and specific sections of the body. The terminology used de ...

and extension around the mediolateral axis, which give rise to the ordinary forward and backward nodding of the head. * (b) slight lateral motion, lateroflexion, to one or other side around the anteroposterior axis. Flexion is produced mainly by the action of the longi capitis and recti capitis anteriores; extension by the recti capitis posteriores major and minor, the obliquus capitis superior, the semispinalis capitis,

splenius capitis The splenius capitis () () is a broad, straplike muscle in the back of the neck. It pulls on the base of the skull from the vertebrae cervical vertebra, in the neck and upper Thoracic vertebrae, thorax. It is involved in movements such as shaking ...

sternocleidomastoideus The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the acces ...

, and upper fibers of the

trapezius The trapezius is a large paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the human spine, spine and laterally to the spine of the scapula. It moves the scapula and ...

. The recti laterales are concerned in the lateral movement, assisted by the trapezius, splenius capitis, semispinalis capitis, and the sternocleidomastoideus of the same side, all acting together.

Clinical significance

Dislocation

The atlanto-occipital joint may be dislocated, especially from violent

accident An accident is an unintended, normally unwanted event that was not deliberately caused by humans. The term ''accident'' implies that the event may have been caused by Risk assessment, unrecognized or unaddressed risks. Many researchers, insurers ...

s such as

traffic collision A traffic collision, also known as a motor vehicle collision, or car crash, occurs when a vehicle collides with another vehicle, pedestrian, animal, road debris, or other moving or stationary obstruction, such as a tree, pole or building. Tr ...

s. This may be diagnosed using

CT scan A computed tomography scan (CT scan), formerly called computed axial tomography scan (CAT scan), is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or ...

s or

magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...

of the

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple ani ...

and

neck The neck is the part of the body in many vertebrates that connects the head to the torso. It supports the weight of the head and protects the nerves that transmit sensory and motor information between the brain and the rest of the body. Addition ...

. Surgery may be used to fix the joint and any associated

bone fracture A bone fracture (abbreviated FRX or Fx, Fx, or #) is a medical condition in which there is a partial or complete break in the continuity of any bone in the body. In more severe cases, the bone may be broken into several fragments, known as a ''c ...

s. Neck movement may be reduced long after this injury. Such injuries may also lead to hypermobility, which may be diagnosed with

radiographs Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical ("diagnostic" radiography and "therapeu ...

. This is especially true if traction is used during treatment.

Additional images

File:Gray129.png, Occipital bone. Outer surface.

References

{{Authority control Joints Joints of the head and neck