Porion
The porion is the point on the human skull located at the
upper margin of each ear canal (external
auditory meatus, external acoustic meatus). It lies on the superior margin of
the tragus. It is a cephalometric
landmark with significance in biological
anthropology and in clinical applications such
as oral and maxillofacial surgery.
The porion is one of the three anatomical points used to
determine the Frankfurt plane. The Frankfurt plane (also called the
auriculo-orbital plane) was established at the World Congress on Anthropology
in Frankfurt, Germany in 1884, and decreed as the anatomical position of the
human skull for comparative craniometric measurements. It was decided that a
plane passing through the inferior margin of the left orbit (the
point called the left orbitale) and the upper margin of each ear
canal or external auditory meatus, a point called the porion, was most nearly
parallel to the surface of the earth at the position the head is
normally carried in the living subject.
In normal subjects, both orbitales and both porions
lie in a single plane. However, due to pathology, this is not always the case.
The formal definition specifies only the three points listed above, sufficient
to describe a plane in three-dimensional space.
The Mastoid Index (a craniometric measurement) is the distance
from the porion to the asterion.
The determination of the Frankfort plane differs between
skeletal and soft tissues, soft tissue using the tragus as the
landmark in place of the porion
