opisthion

Opisthion is a cranial base landmark used in anatomy and craniometry. It denotes the midpoint of the posterior margin of the foramen magnum, the large opening at the base of the skull through which the brainstem continues as the spinal cord. The opisthion lies on the midline of the skull and is paired with the basion, the corresponding midpoint of the anterior margin of the foramen magnum.

These two points define the basion–opisthion line, a reference in skull-base measurements. The distance between basion and opisthion, often called the length of the foramen magnum, is used in anthropometric studies and radiologic assessment of cranial base relationships. In cephalometric analysis, several cranial base angles use basion and opisthion as references.

Variation in skull morphology can affect the precise location of the opisthion on radiographs, and interpretation can depend on projection and image quality. The term originates from Greek and is employed in anatomy, anthropology, dentistry, and radiology as a standard cranial landmark for describing the posterior boundary of the foramen magnum.