The occipital condyles are undersurface protuberances of 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 ...

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, which function in articulation with the superior facets 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 ...

vertebra. The condyles are oval or reniform (kidney-shaped) in shape, and their anterior extremities, directed forward and medialward, are closer together than their posterior, and encroach on the basilar portion of the bone; the posterior extremities extend back to the level of the middle of the foramen magnum. The articular surfaces of the condyles are convex from before backward and from side to side, and look downward and lateralward. To their margins are attached the capsules of the atlanto-occipital joints, and on the medial side of each is a rough impression or tubercle for the alar ligament. At the base of either condyle the bone is tunnelled by a short canal, the hypoglossal canal.

Clinical significance

Fracture of an occipital condyle may occur in isolation, or as part of a more extended

basilar skull fracture A basilar skull fracture is a bone fracture, break of a bone in the base of skull, base of the skull. Symptoms may include Battle sign, bruising behind the ears, periorbital ecchymosis, bruising around the eyes, or hemotympanum, blood behind the ...

. Isolated condyle fracture is a type of craniocervical injury. The classification of Anderson and Montesano distinguishes three types of occipital condyle fracture: * Type I: Isolated impaction fracture of the occipital condyle, due to compression by the atlas or dens. This injury is usually stable; significant displacement of fragments is rare. * Type II: Occipital basilar skull fracture extending into the condyle, resulting from direct trauma. The craniocervical junction usually stays stable, but neurologic injury may occur from the blow to the head. * Type III: Isolated avulsion of the condyle with displacement towards the alar ligament, due to forced rotation / lateral bending. This injury tends to be unstable and may co-occur with atlanto-occipital subluxation or dislocation. Neurological injury may occur and range from minor to instantly fatal. Minimally displaced fractures are treated conservatively. Surgery may become necessary if there is significant compression of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

, the lower

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

s or cervical arteries. Bilateral condyle fractures (e.g. as part of an atlanto-occipital dislocation or the "occipital ring fracture") are rare, but often fatal. Symptoms of an isolated occipital condyle fracture resemble those of other craniocervical injuries, including high cervical pain, reduced range of motion, unusual head / neck posture, prevertebral swelling, and possibly lower cranial nerve (IX, X, XI, XII) deficits, tetra

paresis In medicine, paresis (), compound word from Greek , (πᾰρᾰ- “beside” + ἵημι “let go, release”), is a condition typified by a weakness of voluntary movement, or by partial loss of voluntary movement or by impaired movement. Whe ...

or abnormal breathing. Among these, cranial nerve deficits are the most characteristic due to the proximity of the injury to the

jugular foramen A jugular foramen is one of the two (left and right) large foramina (openings) in the base of the skull, located behind the carotid canal. It is formed by the temporal bone and the occipital bone. It allows many structures to pass, including the ...

and hypoglossal canal. Onset of neurologic symptoms may be immediate or delayed. The prevalence of occipital condyle fractures is not definitely known. Once thought to be a rare injury, it is nowadays believed to constitute 1-3% of all blunt craniocervical traumas. It is most commonly seen in high-energy trauma, often associated with other skull and/or cervical spine injuries.

In reptiles and birds

The presence of a single occipital condyle in dinosaurs (including birds) and

crocodilian Crocodilia () is an Order (biology), order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorp ...

s contrasts with the condition in

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s and

synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...

s (including

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s such as ''

Homo sapiens Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

''), where two occipital condyles are present. Here, the occipital condyle is a single rounded projection that is present on the rear of the skull and articulates with the first cervical (neck) vertebra. Functionally, it allows the head to move from side to side, up and down, as well as to rotate. A combination of a number of smaller bones (such as the basioccipital and exoccipitals) participate in the formation of this structure. In most dinosaurs, the occipital condyle is situated at the rear part of the skull, below the ''

foramen magnum The foramen magnum () 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 oblongata, passes thro ...

'', and points toward the posterior of the animal. Some exceptions to this exist, such as in the iguanodontian '' Anabisetia saldiviai'' where the condyle points downward. In some dinosaurs, where the skull has not been preserved, the presence of a small occipital condyle suggests to some paleontologists that the skull of the dinosaur was relatively small.

References

External links

lesson9
at The Anatomy Lesson by Wesley Norman (Georgetown University) * *
occipital condyles on Triceratops
{{Authority control Bones of the head and neck

Clinical significance

In reptiles and birds

See also

References

Further reading

External links