porion

Porion is an anatomical landmark used in skull measurement and craniofacial analysis. It is defined as the uppermost point on the margin of the external auditory meatus (the opening of the ear canal). In anatomical terms this point is referred to as the anatomic porion. In radiographic cephalometry, porion is identified on imaging and serves as a standard reference point for measurements and plane construction.

A central application of porion is its role in defining the Frankfort horizontal plane, typically by pairing porion with orbitale to form the Po-Or line. This plane is used to orient cephalometric radiographs and to compare craniofacial relationships across individuals and over time. Porion-based measurements are common in orthodontics, orthognathic surgery planning, and anthropometric research.

Variants exist in practice. Some sources distinguish between the anatomical porion and the radiographic porion, noting that the radiographic point may be influenced by projection and image quality. In cases where the bony porion is difficult to locate, a soft tissue landmark near the ear (soft tissue porion) may be used as an approximation.

Limitations include observer variability in identifying the exact point, differences in head position during imaging, and natural anatomical variation. Despite these factors, porion remains a foundational reference point in skull and facial analysis, contributing to diagnostic assessments, treatment planning, and comparative studies in anatomy and anthropology.