Cephalometrics has been with us for the past sixty-five years, but we still do not have a standardized method of superimposing tracings.If we could agree upon the best method it would have far reaching effects. Most published articles must describe the cephalometric system used to evaluate the data. It would be more efficient and save journal space if the author did not need to define the method of cephalometric assessment. It would be understood. If we had a standardized system of superimposition and assessment, our literature would be easier to read and understand.
In addition to regional superimposition there are at least four methods of overall superimposition which have been used.
- Superimposition on S-N and registering on the center of sella tercica.
- Superimposition on nasion basion line.
- Superimposition on basion horizontal parallel to Frankfort.
- Superimposition on Frankfort horizontal.
It is important to include in any analysis, midline landmarks. This would immediately eliminate Frankfort horizontal. Since I have never used the basion-nasion line I do not feel capable of intelligently discussing its merits.
It is not the purpose of this narrative to list all the favorable and unfavorable characteristics of methods mentioned. I am sure there are other methods of analysis and superimposition of which I am not aware, which have been proposed through the years. This brings us quickly to the fact that we now have only two methods to compare-namely S-N and basion horizontal.
First we will discuss superimposition on S-N. To make a thorough study we secured head films from Western Reserve University. Here we would like to pay tribute to this institution and to Dr. Holly Broadbent Jr., who has charge of preserving the wonderful records. Dr. Holly Broadbent Sr. is responsible for these records and every orthodontist in the world owes him a debt of gratitude.
The radiograms were of just one patient, No. 3092. There were eight films from age 3 months to 16 years. Please see Fig. 1. It will be observed that the cranium grows concentrically around sella turcica. There is a greater increase in the posterior one-half of the cranium than in the anterior one-half. This growth is remarkably orderly-almost always increasing the same amount at any two time points at any point on the circumference.
This is the age 3 months tracing superimposed on the 16 year tracing. The S-N lines are superimposed and registered on the center of sella. It can be seen that the diameter of foramen magnum diminished 5 mm. Basion moved 10 mm posteriorly with relation to sella and downward 14 mm. Fig. 1.
Nasion is not distinguishable in the early months and years. During this time we must rely on the landmarks of the anterior cranial base. It is obvious that S-N maintains about the same relationship to the facial complex from 3 months to 16 years in this particular patient. This anatomic feature highly recommends S-N as a method of superimposition.
We will now discuss basion horizontal as a method of superimposition. Superimposition on S-N causes the facial complex to grow downward and forward in an orderly manner. Superimposition on basion horizontal causes the facial complex to grow upward and forward, completely opposite to reality.
Any accepted method of superimposition must be capable of using the "mechanism of growth" concept. The mechanism of growth has been shown to be valid. No one in the last thirty-seven years has objected to any part of it. It has been endorsed by a number of authors. Since the basion horizontal method divides vertical growth into two parts, (1) the upward growth of line S-N and (2) the downward growth of the mandibular plane, this makes it difficult to accurately measure the vertical growth of the face.
Even though the basion horizontal system has been well documented and successfully used for a long period of time, it has basic and anatomic shortcomings. It is predicated on the assumption that foramen magnum does not move posteriorly with relation to the cranium. This is a hazardous assumption.
Dr. Eugene Coben developed the basion horizontal system when he was in graduate training. He has spent his life further refining and teaching the system. He has done a magnificent job of documentation. I appreciate his dedication and his contribution to orthodontics. I am sure he has most of the answers, and he may have answers to some of my objections. The 1 mm grid, which is used with the system, is an excellent method of measuring vertical and horizontal growth.
It may be noted in Fig. 1 that the cranium grows concentrically around sella, in an orderly manner; and that the facial complex grows downward and forward in an orderly manner. However in Fig. 2 it may be noted that when we superimpose on basion there is complete chaos. (In the spirit of fair play we must inform the reader that in the basion horizontal superimposition basion is not used in this way). Registering on basion does show us that basion is not basically related to craniofacial anatomy.
This is the age 3 months tracing superimposed on the 16 year tracing. The orientation is to Frankfort and the registration is on basion. Please note that this superimposition causes complete chaos. (The reader must be informed that in the basion horizontal system of superimposition, basion is not used in this manner.) The diameter of foramen magnum diminished 5 mm. Fig. 2.
The axis of foramen magnum changed 7°-meaning that basion moved downward 7° with relation to the posterior aspect of the foramen, and also in relation to the cranium.
Basion moved 10 mm backward and 14 mm downward in relation to sella. It doesn't seem logical to use a landmark that moves so much as a point of registration. Please see Fig. 1.
The diameter of foramen magnum decreases 5 mm from 3 months to 16 years. This is an astounding statistic. Let us assume that I am wrong-that foramen magnum did not decrease in size. The posterior aspect of the foramen is clearly visible in all films. In the 16 year film the image of the posterior aspect is 32 mm from basion. In the 3 month old film, if we extend basion down to a point 32 mm from the posterior aspect, it will be completely through the osseous tissue and extend 2 mm into the spinal cord. This logical deduction, by extrapolation, should convince all doubters. See Fig. 1 and Fig. 2.
Discussions of cranial base growth seldom if ever mention millimeters. We do not know whether they are talking about 1/10 of a mm or multiple millimeters. The authors Fumio Ohtsuki, Debabrata Mukherjee, Arthur B. Lewis and Alex Roche, stated, "Craniofacial region will still remain a topic of great concern for physical anthropologists and orthodontists."
Clinical orthodontists need not be particularly concerned since the minute details of the cranial base growth are included in overall measurements.
We took the illustration from Coben's book (1986), made tracings, superimposed them on S-N, and registered on the center of sella turcica. Please see Fig. 3 and Fig. 4. It will be seen that the chin moved forward on a trajectory of 36° to horizontal; however when we superimposed on S-N the chin really moved forward on a trajectory of 50° to horizontal-a difference of 14°. The discrepancy was because, in the basion horizontal method, vertical growth is divided into two parts-one part moving upward and one part moving downward. These facts do not recommend the basion horizontal as a preferred method of superimposition.
This was taken from the work of Coben. The tracings were registered on basion. The chin moved forward on a trajectory of 36° to horizontal. Please see text.
Fig. 3.Here we traced the work of Coben and superimposed on S-N. The chin grew forward on a trajectory of 50° to horizontal. Please see text.
Fig. 4.The basion horizontal system is incompatible with the "mechanism of growth" concept. Developed in 1960, this concept measures growth increments and compares vertical to horizontal growth increments, to explain the function of the jaws. It is a principle of jaw function, and principles do not change -- like the law of gravity or the sun coming up in the morning. A principle is right one hundred percent of the time. A system of superimposition must be capable of measuring and using growth increments, the building blocks, in explaining the nature of growth of the jaws and their function. We have found no advantage of the basion horizontal system over the S-N system but we have pointed out several disadvantages.
Thus, we have pointed out considerable objective evidence which indicates that S-N superimposition is far superior to all other methods of superimposition.
It is hoped that our profession will readily accept the truth of this report and will soon begin discussions with a view to reaching a consensus.
1Scott, James H.: The Analysis of Facial Growth, A. J. Ortho. Vol 44, page 507-512. 1958. Bibliography
2Neilsen, Ib Leth: Cephalometric Evaluation of Growth and Treatment Changes, University of California, San Francisco.
3Fumio Ohtsuki, Debabrata Mukherjee, Arthur B. Lewis and Alex F. Roche: Growth of the Cranial Base and Vault Dimensions in Children, The Journal of the Anthropological Society of Nippon, vol. 90, No. 3, July 1982.
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