Foramen magnum

The foramen magnum is the largest foramen of the skull and is part of the occipital bone ¹. It is oval in shape with a large antero-posterior diameter ².

Gross Anatomy

The foramen magnum is found in the most inferior part of the posterior cranial fossa ³. It is traversed by vital structures including the medulla oblongata ¹. In the midline, the anterior margin is the basion and the posterior margingmargin is the opisthion.

Relations

• anteriorly: basilar part of occipital bone
• anterolaterally: occipital condyles, hypoglossal canal, jugular foramen
• posteriorly: squamous part of occipital bone with the internal occipital crest

Contents

• medulla oblongata
• meninges
• spinal root of the accessory nerve
• vertebral arteries
• anterior and posterior spinal arteries
• tectorial membrane and alar ligaments ²

Radiographic features

CT

• at the level of the foramen magnum CSF is seen to surround the medulla which is reduced or absent in tonsillar herniation

Related Pathology

• foramen magnum meningioma ²
• cerebral herniation
• chiari malformations