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You are watching: Largest foramen in the human skeleton

StatPearls . Sweetheart Island (FL): StatPearls Publishing; 2021 Jan-.



The foramen magnum is of attention for assorted scientific disciplines: forensic and physical anthropology; compare anatomy; biology; surgery of the foramen structures and for the craniovertebral junction. There are morphological differences between men and also women. The sagittal diameter is higher in the male, together is the transverse diameter.

The foramen magnum is the biggest foramen of the skull. The is located in the many inferior section of the cranial fossa together a component of the occipital bone. Its materials include the medulla oblongata, meninges, spinal root of cranial nerve XI, vertebral arteries, anterior and also posterior spinal arteries, the tectorial membrane, and alar ligaments.

On the foramen magnum, there space two craniometric points: the basion, the median allude of the front leaf of the hole, and the opisthion, posterior correspondence.

Structure and also Function

The foramen magnum attributes as a passage of the main nervous device through the skull connecting the mind with the spinal cord. Top top either next of the foramen magnum is one occipital condyle. This condyles type joints v the an initial cervical vertebra.

The place of the foramen is essential for posture in orthostatism, as it allows a correct relationship in between the skull and the cervical spine.In fossil hominins, the place of the foramen is indicative of the bipedalism of our ancestors.


All the skeletal of the skull construct from paraxial mesoderm and neural crest cells. Throughout gastrulation in the third week, mesenchymal cells migrate through the primitive streak to kind the mesoderm. In ~ the finish of the 3rd week, the paraxial mesoderm is segmented right into somites that distinguish to end up being sclerotomes.<1> During the fourth week, the occipital sclerotomes concentrate roughly the notochord under the affect of signals from the rhombencephalon.

These sclerotomes derive membrane and cartilage, the developmental origin of the occipital bone. 4 primary cartilaginous centers, the anterior basioccipital (basilar), the lateral exoccipitals (condylars), and the posterior supraoccipital (squamous), in addition to a 5th membranous element, referred to as the interparietal combine roughly the foramen magnum to form the occipital bone. The mendosal suture runs horizontally between the emerging inferior supraoccipital and also superior interparietal bones.<2>

Fetuses in ~ 9 mainly gestation have an ossification center about the hypoglossal canal in every exoccipital component and a solitary median ossification center in the basioccipital cartilage.<3> At 12 main gestation, a pair of ossification centers in the supraoccipital cartilage fuse together to kind the supraoccipital bone. Rostral to the supraoccipital bone, the second pair the ossification centers in the membranous part fuse to form the interparietal bone.<4> The intraparietal portion ossifies intramembranous if the remainder of the occipital bone ossifies endochondrally using cartilage together a precursor.<1> The supraoccipital and interparietal bones then fuse midline, however at this allude in advance are tho separated laterally by the mendosal suture. At 14 weeks, ossification that the basioccipital wake up and developments laterally into the ventral part of the condylars, while at the same time the ventral portions breakthrough into the dorsal portions. Also, throughout the 14th week, the blend of the supraoccipital and also interparietal skeleton progress nearly to completion. The complete union of these segments walk not occur until between 2 and also 4 years of age.<3> By the 16th week that fetal development, every intramembranous ossification centers are usually fused developing a lattice the trabeculae overlaying the outside surface of the occipital squama.<4> The exoccipitals remain separated indigenous both the basioccipital and the supraoccipital segment by synchondroses and will no fuse until in between 2 and 4 year of age.<3>

Blood Supply and also Lymphatics

The vertebral arteries and also the anterior and also posterior spinal arteries traverse the foramen magnum. The vertebral artery branches turn off of the subclavian artery and also has four segments. The first preforaminal segment (V1) constitutes the portion from the subclavian to the transverse foramen the C6. The 2nd foraminal segment (V2) travels v the transverse foramen of C6 v C2. Once the artery emerges from the foramen that C2, it is thought about V3. This 3rd segment, the extradural segment, proceeds through the transverse foramen that C1, the suboccipital triangle, and the foramen magnum. The allude at i beg your pardon the artery penetrates the dura and also arachnoid mater shows the beginning of the fourth and final segment the the vertebral artery, the intradural segment. The right and also left fourth segments that the vertebral artery incorporate to kind the basilar artery at the level that the pons.<5>

In enhancement to the basilar artery, the 4th segment of the vertebral arteries offers rise to the anterior spinal artery, the posterior spinal artery, perforating branches to the medulla, and also the posterior worse cerebellar artery. The anterior spinal artery feeds the top cervical spinal cord and also inferior medulla if the posterior spinal artery feeds the dorsal spinal cord and also conus medullaris. The lateral medulla, cerebellar tonsils, worse vermis, and choroid plexus count on the posterior worse cerebellar arteries, and the penetrating branches supply sections of the medulla, the olives, and the inferior cerebellar peduncle.


Cranial nerve XI, or the accessory nerve, originates from the upper spinal cord and also medulla and also enters the skull v the foramen magnum. This nerve innervates the sternocleidomastoid and trapezius muscles and provides engine function.


The sternocleidomastoid muscle acts to flex and also extend the neck and turn the head come the contralateral side. The trapezius muscle shrugs the shoulders.

Physiologic Variants

There room a few significant anatomical measurements concerning the foramen magnum: the transverse diameter, the anteroposterior diameter, and also the foramen magnum table of contents calculated by separating the anteroposterior diameter through the transverse diameter. These dimensions exhibit physiologic variance in between skulls.<6> For example, a foramen magnum index greater than 1.2 is thought about an ovoid variant.<7> Other called shapes of the foramen magnum include rhomboid, circle, heart, pear, and hexagon. Details names space inconsistent in between studies.<6> Additionally, the foramen might be asymmetrical. The protrusion of the occipital condyles right into the foramen magnum demonstrates another source of anatomical variance. The hypocondylar arch is a function of embryologic skulls that usually regresses in ~ birth however rarely is maintained.<7>

Surgical Considerations

The anatomical variance that the foramen magnum may have some impact on details surgical procedures such together vertebral artery and posterior inferior cerebellar artery aneurysm repairs, foramen magnum meningioma resections, and also foramen magnum decompression among others. In an ovoid kind foramen magnum, it is an overwhelming for a operated doctor to adequately expose the anterior section of the foramen. Additionally, the occipital condyle and also jugular tubercle space the key bony importance obstructing the anterolateral portion of the foramen magnum. Extension of the occipital condyles right into the foramen magnum may show the need for much more extensive bony removed in specific procedures.<7>

Clinical Significance

There space a number of pathological claims directly associated with the foramen magnum. Elevated intracranial pressure (greater than 20 mm Hg) frequently results from resources of edema such as stroke, trauma, fixed effect, and infection and represents a major prognosis. As the full volume in the skull is fixed and also consists that blood, cerebrospinal fluid (CSF), and brain tissue, the enhanced pressure may lead to compensation via cerebellar tonsil herniation v the foramen magnum. The clinical picture of any herniation has hypertension, bradycardia, and respiratory depression, yet cerebellar herniation specifically causes compression the the respiratory centers of the medulla and may it is in fatal. The anatomy the the foramen magnum may influence the displacement that the herniated tissues.<6>

In an additional subset that clinical associations with this anatomic structure, patients with Chiari malformations have statistically bigger foramen magnums contrasted to the common population. Chiari ns is the herniation that the cerebellar tonsils (a solitary tonsil 5 mm or both tonsils 3 mm) with the foramen magnum. If some patients room asymptomatic, there is a wide range of severity the clinical symptoms. The mildest being infrequent exertional headaches. Severe instances present with far-reaching myelopathy and brainstem compromise. Usual complications include syringomyelia and hydrocephalus. In babies, over there are linked sleep apnea and also feeding difficulties.<8>

Chiari II is the inferior displacement of the vermis, cerebellar tonsils, medulla, and fourth ventricle through the foramen magnum. Clinical presentation is an ext severe in Chiari II 보다 Chiari I and is strongly associated with lumbar myelomeningocele and supratentorial anomalies together as corpus callosal dysgenesis, heterotopias, and sulcation abnormalities. Around 80% to 90% of children with Chiari II malformations current with hydrocephalus an additional to fourth ventricle obstruction request shunt placement. Patients with Chiari III malformations present with encephalocele in addition to the abnormalities that Chiari II.<8>

A mass to represent another form of compression surrounding to the foramen magnum. Meningiomas space a bright slow-growing main nervous device tumors that the arachnoid cells of the dura mater many often emerging in the basal region of the cerebrum. Hardly ever they take place in the foramen magnum presenting v posterior headache, paresthesias, and also motor deficits. The as whole symptomology might be varied as result of the anatomic proximity come the cerebellar tonsils, caudal medulla, reduced cranial nerves, rostral spinal cord, and also upper cervical nerves.<9>

A complication of chronic rheumatoid arthritis demonstrates another example of pathology associated with the foramen magnum. Rheumatoid arthritis-associated C1-C2 side erosion and laxity that ligamentous restrains results in upright atlantoaxial subluxation. This defect may further lead come a protrusion of the odontoid with the foramen magnum and also compression the the midbrain. Various other etiologies the atlantoaxial subluxation incorporate trauma and also congenital conditions such as down syndrome. Clinical presentation varies widely as part patients experience couple of symptoms, but others experience significant instability and neurologic compromise.

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The occipital condyles consist of the osseous lateral limits of the foramen magnum. In patients presenting through high-energy trauma instrument of injury or high influence axial loads during various sporting occasions (i.e. American football, hill climbing, excessive sports events), early out diligence to dominion out injury to the occiput-cervical junction, as well as the remainder of the cervical spine is warranted via accurate history taking, physical examination, and appropriate imaging. At advanced (i.e. Level I) and local trauma centers, computed tomogram imaging is performed as component of the progressed imaging protocol. The is necessary to recognize that also displaced occipital condyle fractures have the right to be to let go on initial radiographs. In the proper clinical scenario, factor to consider should be offered to transferring the patience complaining that persistent neck pain even in the lack of neurologic symptoms upon presentation.<11>