The Frankfurt plane is a plane formed by drawing a straight horizontal line from the top of the ear canal to the bottom border of the eye along either side of the human skull. The line is called a Frankfurt horizontal line (FHL) and is an important reference point in medical imaging studies. The plane itself is used as a standard reference in anthropometry — the science that studies measurements of human anatomy in order to describe variations in human characteristics.
The plane is also called the auriculo-orbital plane because it passes through the auriculus, or ear, and the orbitales, or eye sockets. It was established in 1884 in Frankfurt, Germany, by the World Congress of Anthropology. The Frankfurt plane became the principal standard of skull measurement because it also identifies the normal plane in which the head is positioned parallel to the ground.
Because the Frankfurt plane serves as a benchmark for making skull measurements, it is also called a craniometric plane. This is simply a point on the skull, or cranium, from which measurements can be made. Craniometric measurements made on the exterior of the skull have endocranial equivalents, which are points within the skull that correspond to the external measurements.
Craniometric measurements are an important factor in administering MRIs and CT scans. In 1962, the World Federation of Radiology defined Reid’s base line — a line derived from the Frankfurt plane — as the zero plane to be used in medical imaging. This line extends the Frankfurt plane from the lower eye sockets through the point of the ear onward towards the back of the head to the center of the occipital bone at the lower part of the skull. The line can be shown graphically with the head leaning slightly backward, and tilting up about 7 degrees from the horizontal Frankfurt plane.
Basically, however, the Frankfurt plane was designed to be a frame of reference for identifying skeletal variations in human anatomy from an anthropological viewpoint. The plane established a standard for measuring the human body and comparing differences in races by matching these variations with known measurements for particular individuals. Since the median path of the Frankfurt plane follows a line 0.827 inches (21 mm) below the center of the external ear canal and varies from 0.20-3.82 inches (5-97 mm) as it progresses to the lower border of the eye, this means that the variations can be due to differences in facial structures. Such variations have been found to characterize different racial types, which begin to differ at the point where the top border of the lateral nasal sinus changes direction.
In this regard, different races have been found to share a relatively uniform degree of change within a particular range of measurements. For example, British measurements have been found to be concentrated within 0.15 inches (3.75 mm) below the ear canal and to be located 0.10 inches (2.66 mm) below the point where the lateral sinus changes direction. Similarly, measurements of the Chinese have been shown to be localized within a range of 0-0.20 inches (0-5 mm) below the ear canal and 0.40 inches (10-0 mm) below the point where the lateral sinus changes direction. The significance of such measurements has not yet been determined; however the evolutionary impact of genetic divergences of the human species may extend far into the future.