Foramen magnum

The foramen magnum is a bony opening in the region of the posterior cranial fossa, through which the medulla oblongata passes and thereby connects with the spinal cord.

Anatomy

The foramen magnum is located in the occipital bone. Alongside the medulla oblongata, the following structures pass through the foramen magnum:

• Spinal root of the accessory nerve

• The left and right vertebral artery

• anterior spinal artery

• The right and left posterior spinal artery

• Spinal vein

• tectorial membrane

• alar ligaments

The foramen magnum marks the transition area of the cranial dura mater into the spinal dura mater.

Osteometric Measurement

The foramen magnum has the following sagittal and transverse osteometric dimensions[^1]:

Dimensions of the Foramen Magnum	Average	Minimum - Maximum
Sagittal Diameter	36.6 mm	30.1 - 42.6 mm
Transverse Diameter	31.1 mm	25.0 - 38.9 mm

Imaging

Clinical Relevance

Herniation

In cases of excessively high intracranial pressure, parts of the brain can be displaced downwards into the foramen magnum, leading to a herniation.

Foramen Magnum Stenosis

In patients with a skeletal dysplasia, such as achondroplasia, stenosis of the foramen magnum can occur[^2].