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The occipital condyles are undersurface protuberances of the
occipital bone The occipital bone () is a 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 lobes of the cer ...
in
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
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 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 geogra ...
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 break of a bone in the base of the skull. Symptoms may include bruising behind the ears, bruising around the eyes, or blood behind the ear drum. A cerebrospinal fluid (CSF) leak occurs in about 20% of cases and ...
. Isolated condyle fracture is a type of craniocervical injury. The classification of Anderson and Montesano distinguishes three types of occipital condyle fracture: * Type I: Isotated 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 ...
,
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, 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 ...
s or cervical arteries. Bilateral condyle fractures (e.g. as part of an
atlanto-occipital dislocation Atlanto-occipital dislocation, orthopedic decapitation, or internal decapitation describes ligamentous separation of the spinal column from the skull base. It is possible for a human to survive such an injury; however, 70% of cases result in immedia ...
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 () is a condition typified by a weakness of voluntary movement, or by partial loss of voluntary movement or by impaired movement. When used without qualifiers, it usually refers to the limbs, but it can also be used to desc ...
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 (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest livin ...
s contrasts with the condition in
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s and
synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
s (including
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s such as ''
Homo sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture ...
''), 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 ( 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 ...
'', 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.


See also

*
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 geogra ...
* Condyloid fossa


References


Further reading

* * * * * * * * * *


External links

* * *
occipital condyles on Triceratops
{{Authority control Bones of the head and neck