Throughout the years, the lateral cephalogram has been used to accurately evaluate craniofacial structures and their growth since it was introduced in 1931 by Broadbent. Necessary reference points and segments are translated anatomically in the skull into radiographic landmarks in the cephalogram. In 1872, a standard horizontal reference line was idealized by Von Ihering and was later on chosen to be a universal reference plane during the Frankfurt Congress of Anthropology, an international congress of anatomists and physical anthropologists in 1884.(5) It is named as the Frankfurt Horizontal Plane. It is defined posteriorly by the superior part of the external auditory meatus (anatomic porion point) and anteriorly by the inferior rim of the orbital cavity (Orbitale).
The anatomic porion point poses difficulty in being located radiographically due to the density and overlapping right and left sides of the temporal bone and the kilovoltage of the machine. The machine (metallic) porion, in turn, was used to replace the anatomic porion point. It is located at 4.5mm above the center of the cephalostatic radiopaque image of the metal ear rods on the auricular olives. The Frankfort Horizontal Plane (FHP) is established by either of these points. However, in 2015, Romano, et al. identified a degree of variability of FHP locations due to the two possible porion points (anatomic or machine porion) leading to alteration of some cephalometric measures, entailing different treatment measures for the patient.(5)
Certain cephalometric analyses such as McNamara(1), Ricketts(6), Tweeds(7), however, advocated the use of anatomic porion over the machine porion to determine the Frankfort Horizontal plane. McNamara noted that the machine porion can be approximately 1 centimeter away from the clinical position of the anatomic porion. The internal auditory meatus, smaller in size, may be located posteriorly and superiorly from the external auditory meatus. The other analyses such as Steiner’s(4), Jarabak(2), Coben(1), uses an alternative horizontal reference line, the Sella turcica (S) – Nasion (N) plane which on the average is oriented 6 to 7 degrees to the Frankfort Plane. This however decreases accuracy even though it increases reproducibility among the practitioners.(2)
Figure 1. Green point – Anatomical porion, the midpoint of the upper contour of the external auditory canal; Yellow point – Machine porion, the midpoint of the upper contour of the metal ear rod of the cephalometer; Red point – Orbitale, lower border of the upper orbital rim.
References:
- Coben, E., (June 1955)The integration of facial skeletal variants: A serial cephalometric roentgenographic analysis of craniofacial form and growth. Am J of Orthodontics Vol 41 Issue 6, p407-434
- Jarabak JR, Fizzel, J.A. Technique and treatment with lightwire appliances. St Louis, MO: CV Mosby; 1972.
- McNamara J. A., Jr (1984). A method of cephalometric evaluation. American journal of orthodontics, 86(6), 449–469. https://doi.org/10.1016/s0002-9416(84)90352-x
- Proffit, WR., Fields HW, et al. (2007) Contemporary Orthodontics 4th ed. Mosby p175
- Romano, Fábio & Ramalli, Edvaldo & Tavares, Stenyo & Neto, João & Magnani, Maria & Nouer, Darcy. (2015). Comparison between cephalometric measure using anatomic and metallic porion point. Brazilian Journal of Oral Sciences (ISSN: 1677-3217) Vol 4 Num 13. 4. 10.20396/bjos.v4i13.8641821.
- Ricketts RM: Perspectives in the clinical application of cephalometrics. Angle Orthod 51: 115105, 1981.
- Orthodontics – E-book: Current Principles and Techniques by Lee Graber, R. Vanarsdall et al. https://books.google.com.ph/books?id=N0SwDAAAQBAJ&pg=PA509&dq=tweeds+analysis&hl=en&sa=X&ved=2ahUKEwifmJqBxb_0AhWHvpQKHbJEAwMQ6AF6BAgLEAI#v=onepage&q=tweeds%20analysis&f=false