Anatomy: Posterior Fossa, Hindbrain & Craniocervical Junction
Basion
The basion is a bony landmark at the front edge of the foramen magnum used to measure tonsillar descent and craniocervical alignment in Chiari malformation.
In short: Basion
- It is the midpoint of the front edge of the foramen magnum.
- It is a reference point for craniometric measurements.
- The line from basion to opisthion is used to measure tonsillar descent.
- It is used in assessing craniocervical alignment.
What it is
The basion is a bony landmark at the midpoint of the anterior (front) edge of the foramen magnum. It serves as a reference point for measurements used in evaluating Chiari malformation and craniocervical alignment.
Understanding Basion
Radiologists use the basion as a fixed reference for several measurements. The line drawn from the basion (front) to the opisthion (back) across the foramen magnum — the basion-opisthion line, or McRae's line — is used to measure how far the cerebellar tonsils descend below the foramen magnum, the key measurement in Chiari I. The basion is also used in measurements assessing craniocervical alignment and stability (such as the basion-dens interval and basion-axial interval), which help evaluate for conditions like basilar invagination and instability. Understanding the basion clarifies one of the landmarks that makes the precise measurements in Chiari evaluation possible.
Why it matters
Understanding the basion helps families see how a fixed bony landmark enables the measurement of tonsillar descent and craniocervical alignment that underpins Chiari evaluation.
Common questions
What is the basion?
The midpoint of the front edge of the foramen magnum.
What is it used for?
As a reference point for craniometric measurements.
How does it relate to tonsillar descent?
The basion-opisthion line is used to measure descent.
Bottom line
The basion is a bony landmark at the front of the foramen magnum used to measure tonsillar descent and craniocervical alignment in Chiari.
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