Frankfurt’s plane

 

The Frankfurt plane or Frankfort plane (both spellings are widely used, and the term is entered as Frankfort horizontal plane in several medical dictionaries, although Frankfurt is the modern standard spelling of the city it is named for) was established at the World Congress on Anthropology in Frankfurt am Main, Germany in 1884, and decreed as the anatomical position of the human skull. 

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, and also close to the position the head is normally carried in the living subject.

 

The plane is also called the auriculo-orbital plane.

Note that in the normal subject, 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.

For purposes of comparison of human skulls with those of some other species, notably hominids and primates, the skulls may be studied in the Frankfurt plane; nonetheless, the Frankfurt plane is not considered to be the anatomical position for most non-primate species.

The Frankfurt plane may also be used as a reference point in related fields. For example, in prosthodontics, the Frankfurt-Mandibular plane Angle (FMA) is the angle formed at the intersection of the Frankfurt plane with the mandibular plane.

It is almost identical with Reid’s base line, except that the posterior point passes through the upper margin of the external acoustic meatus. This plane is accepted by the physical anthropologists as the standard orientation of the human skulls for the study of characteristics of different races.