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Anatomy Atlases: Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal System: Occipital Bone

Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Cranium

Occipital Bone

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD

Peer Review Status: Internally Peer Reviewed


The articular surface of the occipital condyles may possess a transverse ridge of cartilage that corresponds to a groove on the articular surface of the atlas, or notches at the margin may partly divide the condyle in two parts.

A shallow fossa is occasionally found on the ventral surface of the basilar portion anterior to the pharyngeal tubercle, which has been interpreted as a vestige of the canal of the notochord.

Precondylar tubercles, unilateral or bilateral, just anterior to the anterior margin of the foramen magnum, were found in 10% of 1527 skulls (Broman).

The occipital condyles vary considerably in their position relative to the sides of the foramen magnum. The condyles articulate with the atlas; occasionally, a facet located on the anterior margin of the foramen magnum, the so-called third occipital condyle, articulates with the dens. The frequency of accessory condyles was 0.64% in a study of 157 skulls.

Various structures similar to parts of the atlas have been seen around the foramen magnum and have been described as occipital vertebra (termed, manifestation of occipital vertebra). The atlas may be fused., in part or completely, with the occipital bone (termed, assimilation of the atlas). About 0.5 to 1% of skeletons show such variation, which has been interpreted by some authors as a cranial shift in the regional grouping of vertebrae of the vertebtal column. Signs believed associated with assimilated or occipital vertebrae around the foramen magnum include the following: (a) a massive paramastoid process; (b) an enlarged jugular process; (c) the anterior margin of the foramen magnum thickened and raised to form a bar of bone between the condyles; (d) the hypoglossal canal divided by a bony bridge; and (e) a tertiary condyle and a facet or other marking for the apex of the dens on the anterior margin of the foramen magnum.

If the atlas is "assimilated," the occipital condyles are replaced by the inferior facets of the atlas (atlantoaxial type). If there is duplication of the atlas, the condyles are of the occipitoatlantal type. In a series of 1,246 skeletons, 13 (1.04%) exhibited two or more characteristics of manifestations of an occipital vertebra; 2 (0.16%) showed assimilation of the atlas and were among 10 (0.80%) which showed definite cranial shifting of intersegmental boundries of the vertebral column (Lanier, 1939 a,b).

Elevation of an area between the supreme and superior nuchal lines is termed torus occipitalis. In these cases, the inion may be greatly enlarged. Other names are "occipital spur or torus occipitalis", it is the insertion site of the ligamentum nuchae.

Sutural bones are usually small, irregularly shaped ossicles, often found in the sutures of the cranium, especially in the parietal bones. When the lateral portions of the transverse occipital sutures persist, the anomaly is termed sutura mendosa. Sutura mendosa, which starts from both lambdoidal sutures, represents the remainder of a transverse occipital suture. This suture forms an interparietal bone (Inca bone or intercalary bone or sutural bone). As many as 172 sutural or wormian bones have been found in one skull. They are rarely found in the sutures of the face.

The hypoglossal (canal) foramen may be doubled.

The jugular foramen may be divided into two parts by intrajugular processes.

The condyloid canal is absent in 25% of skulls. In some cases it communicates with the hypoglossal canal. Accessory condyloid canals have been reported.

The grooving on the inner surface of the occipital bone is variable. In about 17% of cases the sagittal sulcus turns to join the left transverse sulcus. The sagittal sulcus may bifurcate, with the larger groove turning to join the right transverse sulcus and the smaller one joining the left transverse sulcus (about 15% of cases). In rare cases, the larger groove joins the left and the smaller the right. In very rare cases, the right and left groove appear equal in size.

The crista occipitalis in some subjects is a sulcus (occipital sulcus) rather than a ridge. In a case reported by LeDouble, the sagittal and occipital sulci were doubled or paired; both sulci joined the transverse sulci on their respective sides. The trocular fossa (housing the torcular herophili or confluens of the sinuses) is usually on the right side but may be in the median plane or on the left side. A shallow fossa of varying size may occasionally be found on the dorsal aspect of the foramen magnum, and has been named the vermian fossa or the middle cerebellar fossa of Verga. It is bounded by the limbs of the internal occipital crest or ridge, which diverge around the foramen magnum giving the fossa a somewhat triangular shape. It may be divided into upper and lower parts by a ridge of bone. The fossa houses part of the inferior vermis of the cerebellum.

The foramen magnum varies in shape in children and adults. Lang (1991) has classified the shapes in 5 groups as follows: appears as two semicircles (adults in 41.2 % and children in 18.4%); appears as an elongated circle (adults in 22.4% and children in 20.4%)", egg-shaped (adults in 17.6% and children in 25.5%); rhomboidal (adults in 11.8% and in children 31.6%); and rounded (adults in 7% and in children in 4%).

The ligamentum muchae may contain sesamoid bones.

Occipital Condyles

Anomalies of Occipital Bone at Foramen Magnum (Occipital Condyles).

Sutural Bones. Sutures. Torus Occipitalis. Parietal Foramina.

Vermian Fossa

Torus Occipitalis

Absence of Coronal and Sagittal Sutures

Skull, Internal Views

Wormian Occipital and Interparietal Bones

Pterygoid Canal


References

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