Inion

The inion is the most posterior point of the external occipital protuberance, a protrusion on the occipital bone at the back of the skull. It serves as a midline cranial landmark used in anatomy, anthropology, and radiology.

Anatomically, the inion lies on the midline of the occipital bone at the apex of the external occipital protuberance. It is typically the most prominent point at the back of the skull and can be palpated in many individuals, though soft tissue, hair, and individual variation may affect palpation.

In practice, the inion is used as a reference point in various measurements and planes. In craniometry and cephalometrics, the distance from nasion to inion helps define cranial height, and other measurements may involve inion with additional landmarks such as opisthion or bregma. In medical imaging and neurosurgery, the inion contributes to orientation and alignment for CT or MRI scans and for planning surgical approaches that reference the posterior skull.

Related posterior skull landmarks include the opisthion, the midpoint at the posterior foramen magnum, and the external occipital protuberance as a broader feature. The term inion derives from its Latin-root meaning related to the back of the head and is used primarily in anatomical, anthropometric, and clinical contexts.