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


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Introduction

The foramen magnum is of interest for various scientific disciplines: forensic and also physical anthropology; compare anatomy; biology; surgical procedure of the foramen structures and also for the craniovertebral junction. There are morphological differences in between men and 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 part of the occipital bone. Its contents include the medulla oblongata, meninges, spinal root of cranial nerve XI, vertebral arteries, anterior and also posterior spinal arteries, the tectorial membrane, and also alar ligaments.

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


Structure and Function

The foramen magnum attributes as a i 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 kind joints v the an initial cervical vertebra.

The place of the foramen is necessary for attitude in orthostatism, as it enables a exactly relationship between the skull and the cervical spine.In fossil hominins, the position of the foramen is indicative of the bipedalism of our ancestors.


Embryology

All the skeleton of the skull build from paraxial mesoderm and also neural crest cells. Throughout gastrulation in the 3rd week, mesenchymal cells move through the primitive streak to kind the mesoderm. In ~ the end of the 3rd week, the paraxial mesoderm is segmented right into somites that distinguish to become sclerotomes.<1> During the 4th week, the occipital sclerotomes concentrate around the notochord under the affect of signals from the rhombencephalon.

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

Fetuses at 9 main gestation have an ossification center roughly the hypoglossal canal in every exoccipital part and a single median ossification center in the basioccipital cartilage.<3> At 12 weeks gestation, a pair the ossification centers in the supraoccipital cartilage fuse together to form the supraoccipital bone. Rostral to the supraoccipital bone, the 2nd pair the ossification centers in the membranous part fuse to form the interparietal bone.<4> The intraparietal portion ossifies intramembranous while the rest of the occipital bone ossifies endochondrally using cartilage as a precursor.<1> The supraoccipital and also interparietal bones then fuse midline, however at this allude in advancement are quiet separated laterally by the mendosal suture. In ~ 14 weeks, ossification the the basioccipital wake up and breakthroughs laterally into the ventral section of the condylars, while concurrently the ventral portions development into the dorsal portions. Also, throughout the 14th week, the fusion of the supraoccipital and interparietal skeletal progress practically to completion. The complete union of these segments go not take place until between 2 and also 4 years of age.<3> By the 16th week of fetal development, every intramembranous ossification centers are typically fused developing a lattice the trabeculae overlaying the external surface of the occipital squama.<4> The exoccipitals remain separated native both the basioccipital and also the supraoccipital segment by synchondroses and also will not fuse until in between 2 and 4 year of age.<3>


Blood Supply and Lymphatics

The vertebral arteries and also the anterior and posterior spinal arteries traverse the foramen magnum. The vertebral artery branches turn off of the subclavian artery and has 4 segments. The an initial preforaminal segment (V1) constitutes the section from the subclavian to the transverse foramen of C6. The 2nd foraminal segment (V2) travels v the transverse foramen of C6 with C2. Once the artery emerges from the foramen the C2, that is considered V3. This third segment, the extradural segment, continues through the transverse foramen that C1, the suboccipital triangle, and the foramen magnum. The suggest at which the artery penetrates the dura and also arachnoid mater indicates the start of the fourth and final segment of the vertebral artery, the intradural segment. The right and also left fourth segments the the vertebral artery incorporate to type the basilar artery at the level that the pons.<5>

In addition to the basilar artery, the 4th segment that the vertebral arteries provides rise to the anterior spinal artery, the posterior spinal artery, perforating branches come 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 conus medullaris. The lateral medulla, cerebellar tonsils, worse vermis, and choroid plexus rely on the posterior inferior cerebellar arteries, and also the penetrating branches supply sections of the medulla, the olives, and the inferior cerebellar peduncle.


Nerves

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 motor function.


Muscles

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


Physiologic Variants

There are a few far-reaching anatomical measurements concerning the foramen magnum: the transverse diameter, the anteroposterior diameter, and the foramen magnum index calculated by splitting the anteroposterior diameter by the transverse diameter. These dimensions exhibit physiologic variance in between skulls.<6> For example, a foramen magnum index better than 1.2 is taken into consideration an ovoid variant.<7> Other named shapes of the foramen magnum incorporate rhomboid, circle, heart, pear, and hexagon. Particular names room inconsistent in between studies.<6> Additionally, the foramen might be asymmetrical. The protrusion the the occipital condyles into the foramen magnum demonstrates another resource of anatomical variance. The hypocondylar arch is a function of embryologic skulls that generally regresses in ~ birth yet rarely is maintained.<7>


Surgical Considerations

The anatomical variance that the foramen magnum may have some impact on particular surgical actions such together vertebral artery and also posterior worse cerebellar artery aneurysm repairs, foramen magnum meningioma resections, and foramen magnum decompression among others. In one ovoid kind foramen magnum, the is difficult for a operated doctor to adequately disclose the anterior part of the foramen. Additionally, the occipital condyle and jugular tubercle space the key bony prestige obstructing the anterolateral portion of the foramen magnum. Expansion of the occipital condyles into the foramen magnum may indicate the require for an ext extensive bony removal in particular procedures.<7>


Clinical Significance

There are a variety of pathological claims directly connected with the foramen magnum. Elevated intracranial pressure (greater than 20 mm Hg) often results from sources of edema such as stroke, trauma, massive effect, and also infection and represents a severe prognosis. As the complete volume in the skull is fixed and consists that blood, cerebrospinal fluid (CSF), and brain tissue, the enhanced pressure may bring about compensation via cerebellar tonsil herniation v the foramen magnum. The clinical photo of any herniation contains hypertension, bradycardia, and respiratory depression, yet cerebellar herniation specifically causes compression that the respiratory tract centers that the medulla and also may be fatal. The anatomy that the foramen magnum may influence the displacement of the herniated tissues.<6>

In an additional subset of clinical associations through this anatomic structure, patients v Chiari malformations have actually statistically larger foramen magnums contrasted to the regular population. Chiari i is the herniation of the cerebellar tonsils (a single tonsil 5 mm or both tonsils 3 mm) with the foramen magnum. While some patients room asymptomatic, over there is a wide selection of severity of clinical symptoms. The mildest being infrequent exertional headaches. Severe cases present with far-reaching myelopathy and brainstem compromise. Usual complications include syringomyelia and also hydrocephalus. In babies, over there are connected sleep apnea and also feeding difficulties.<8>

Chiari II is the inferior displacement that the vermis, cerebellar tonsils, medulla, and also fourth ventricle with the foramen magnum. Clinical presentation is much more severe in Chiari II 보다 Chiari I and is strongly linked with lumbar myelomeningocele and supratentorial anomalies together as body callosal dysgenesis, heterotopias, and sulcation abnormalities. Roughly 80% to 90% of youngsters with Chiari II malformations current with hydrocephalus secondary to 4th ventricle obstruction request shunt placement. Patients with Chiari III malformations existing with encephalocele in addition to the abnormalities the Chiari II.<8>

A mass represents another kind of compression nearby to the foramen magnum. Meningiomas are a bright slow-growing central nervous device tumors of the arachnoid cell of the dura mater most often occurring in the basal region of the cerebrum. Seldom they occur in the foramen magnum presenting through posterior headache, paresthesias, and motor deficits. The in its entirety symptomology might be varied due to the anatomic proximity come the cerebellar tonsils, caudal medulla, reduced cranial nerves, rostral spinal cord, and upper cervical nerves.<9>

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

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<10>

The occipital condyles comprise the osseous lateral boundaries of the foramen magnum. In patient presenting v high-energy trauma instrument of injury or high impact axial loads throughout various sporting events (i.e. American football, hill climbing, extreme sports events), early diligence to rule out injury come the occiput-cervical junction, and the rest of the cervical spine is warranted via accurate background taking, physical examination, and also appropriate imaging. At progressed (i.e. Level I) and local trauma centers, computed tomogram imaging is performed as part of the progressed imaging protocol. That is necessary to recognize that also displaced occipital condyle fractures can be missed on early stage radiographs. In the appropriate clinical scenario, consideration should be offered to moving the patience complaining of persistent neck pain even in the absence of neurologic symptoms upon presentation.<11>