TRANSCRITO DO AMERICAN JOURNAL OF ORTHODONTICS - JUNE, 1976 - Volume 69, Number 6
 

Orientation-Sella-nasion or Frankfort horizontal

Robert M. Ricketts, D.D.S., M.S.,
Robert J. Schullof, B.A., M.A.,
Math. Stat., and Lindy Bahga, B.A. Math.
Pacific Palisades, Calif.
 
In the selection of the most appropriate line for cephalometric orientation for descriptive skeletal morphology, the focus has centered on sella-nasion (SN), the Frankfort horizontal plane (FH), and the basic cranial plane, basion-nasion  (BN).  In this regard, the intent of this article is to discuss and evaluate the first two means of orientation mentioned in five areas of consideration: (1) clinical significance, (2) anatomic significance, (3) measurement accuracy, (4) application in description, and (5) application in growth forecasting.

Clinical significance

Perhaps this aspect of a plane for orientation is the most important to the practicing clinician. To answer the question “What is the clinical significance of sella nasion and the Frankfort horizontal plane?” the following comparison is submitted.  By definition, the  sella-nasion plane is sellected from point sella, located by inspection in the center of sella turcica, and point nasion, located at the suture junction  of the  frontal  bone  with the nasal bone ( Fig. 1 ).  In contrast  to this selection of points in the interior of the skull,  the Frankfort horizontal plane is  selected from  the points  porion,  located at the superior border of the external  auditory  meatus,  and orbitale, located at the inferior border of the orbital rim.  Because both porion  and orbitale  are located in the external portion of the skull, it follows that the  soft-tissue  orbit and ear tragus can be visualized clinically, and a horizontal from this plane appears to be useful for clinical  visualization  (Fig. 2,  A and B).  Sella-nasion, however, cannot be visualized clinically and therefore cannot be utilized as a tool for actual direct clinical communication (Fig. 3).
Another factor to be considered in this comparison for clinical significance is facial  orientation. From  Frankfort  horizontal  plane,  the clinician is able to demonstrate the manner in which the face, the chin, and the palate are oriented around it. Again,  the inability to visualize the sella-nasion plane negates this aspect of the comparison.
 

  Fig. 1. Cross-section photograph indicating that  
points sella and nasion are selected in the interior  
of the skull. 

NOTA:   Esta imagem não conseguimos copiar com 
clareza  do original. Assim usamos uma outra da 
Anatomia da Cabeça e Pescoço de:
Roberto M.H. MacMinn - 
Roberto T. Hutchings - 
Bari M, LOgan 
 
  Fig. 2. A, Porion and orbitale are selected in the external  
portion of the skull. B, Photograph of patient showing  
that since both porion and orbitale are located in the  
external portion of the skull the soft-tissue orbit and ear  
tragus can be visualized clinically and a horizontal from  
this plane appears to be useful for clinical visualization. 
A final consideration in support of the selection of the Frankfort horizontal plane for communication is that professional journals require it for illustrations. This, of course, is in agreement with Down’s and Tweed’s original objectives, and the variation of Frankfort plane to the true horizontal has been shown to vary around zero degrees. In other words, the average Frankfort plane in the population does represent a horizontal to the earth’s surface.

Anatomic significance

The discussion of the anatomic significance of the planes of orientation is quite basic and straightforward. Although nasion bears anatomic significance to the face, sella, in contrast, bears none. As the housing for the hypophysis of the brain and the pituitary gland, sella is totally unrelated and not relevant to the face or jaws.
 

Fig. 3.  Photograph of patient illustrating that sella-nasion  
cannot possibly be visualized clinically since there is no  
correspondence with soft-tissue structures. This implies  
that sella-nosion is useless as o tool for direct clinical  
communication. 
 
 
However, both of the Frankfort-horizontal plane, (porion and orbitale) have distinct biologic and anatomic significance. They are related to basic sense organs or neural functions as keys to the vital centers. The orbital rim may grow slightly downward but still represents effective competion of size at an early date. Porion, representing a point in the temporal bone, is in juxtaposition to the semicircular canals. The temporal bone contains the organ of hearing and provides protection to the vital vegetative center, such as the carotid canal, the seventh and eighth nerves, and the jugular foramen. Therefore, with this direct relationship with the basic sense organs of sight and hearing, the Frankfort horizontal has a relationship to the face that is not apparent in sella-nasion.
With respect to sella-nasion, it does serve as a basic framework of the anterior cranial base and fossa which is the superstructure scaffolding of the nasal capsule or upper face. However, it bears little significance to the mandible and its morphology.

Measurement accuracy

Probably the greatest objection to the Frankfort horizontal plane use is the difficult of selection of porion. A study was performed in order to evaluate this objection. Four technicians were asked to test this problem. In order to include a wide range of quality, seven head films were taken at random from different samples. Each film was marked on the corners for reference, so that comparisons could be made by two fixed points.
 

  Fig. 4. A 5-degree change in the reference line results in a  
simultaneous change in the factors being measured, thus  
increasing their linear correlation. 
The accuracy or agreement in the selection of sella, nasion, porion, orbitale, and basion was compared, together with Frankfort horizontal plane, sella-nasion, and basion-nasion. The tracings of one technician were used as a base line 0 for comparison of all others. The standard deviation (S.D.) of difference between the technicians was calculated as a measure of the variability of one plane over the other as far as selection alone is concerned. The results showed the following standard deviations: S-N, ± 0.66 degrees; FH, ± 0.70; and BaN, ± 0.72. Thus, no statiscally significant differences were seen in the difficulty of selection of any of the three planes.
It should be realized that trained technicians performed this study and that the machine ear rod was not employed in these studies; nor is it used in present day clinical analysis. To repeat, the ear rod is untrustworthy and can vary as much as 1 cm. from true porion, and its use may lead to gross error. Therefore, the true porion or ear canal outline must be sought and employed if the Frankfort plane is to be used and trusted.

Application in description

To evaluation the relative usefulness of Frankfort horizontal plane and sella-nasion as descriptive horizontal reference lines, the following procedure was used.
A key use of a horizontal reference line is to determine the relative horizontal positions of the maxilla and the mandible in the face. In the Frankfort horizontal plane system, angles made by the lines nasion to A point (N-A) and nasion to pogonion (N-PO) to FH are used as indicators of horizontal position. In the sella-nasion system, angles made by the lines sella-nasion-A point (SNA) and sella-nasion-B point (SNB) were used. If the reference line is to be considered reliable for description, the correlation between the measurements of the maxilla and the mandible to the reference line should be minimal. The higher the correlation, the more the reference line itself will affect the measuresurements.
 

  Fig. 5.  A study was performed to compare two reference systems for determining not only the mean  
direction of growth with respect to  
these reference lines but also the standard deviation of change. The  
two reference frames compared  
were (1) FH and pterygoid vertical (PTV) and (2) SN with a perpen- 
dicular through sella as a Y axis. 
A study performed on a group of a forty untreated cases indicated dear results.4 SNA and SNB*  were correlated 0.75 in an initial series and 0.59 in the final, while N-Po to FH and N-A to FH showed correlations of only 0.22 and 0.17, respectively. Therefore, it was concluded that FH was the more reliable reference frame (Fig. 4).
In addition, the reference points for FH and BaN orientation are more distant from each other than S-N,  which provides a wider margin in selection without significantly altering the interpretation.

Application in growth forecasting

 A factor underlying the whole controversy is the lack of direct relation of the glenoid fossa to the S-N line, together with an absence of statistical correlation with the mandible. Many clinical orthodontists and students of morphology believe that prediction of chin location dominates concern. Attempts to relate any part of the mandible to the anterior cranial base alone have  been  unsuccessful. Particularly when correlations and predictions are attempted.2 Also, if the  clinician does not need to use the anterior cranial base for description as the previous argument indicates, it follows that he need not use it for prediction if other reference lines offer a means for a more accurate forecast.
 In order to avaluate the application of these lines of orientation to growth forecasting, a study was performed. The study was to include a comparison of two reference systems for determining not only the mean direction of growth with respect to these reference lines but also the standard deviation of change. Points Xi, Po, A point, end of the nose, and B6 were studied. The two reference frames evalueted in the study included ( 1 ) FH and pterygoid vertical ( PTV ) and ( 2 ) SN with a perpendicular through sella as a Y axis ( Fig. 5 ). For this study, a sample of fifty untreated cases from the University of Michigan growth studies was used. There was 9.42 years of average growth involved. Calculations to determine the average change in the X and Y directions were made, as well as the root-mean-squared errors in the X and Y directions. It was evident that in every instance the reference frame which utilized the Frankfort horizontal was ap-preciably better than that which utilized sella-nasion ( Table I ).

             Table I. Constants calculations

Calculations were made to determine the average change in the X and Y directions as well as the root-mean-squared errors. It is obvious that in every incidence the reference frame wich utilized the Frankfort horizontal was apreciably better than that wich utilized sella-nasion.
 
 Summary

 The controversy regarding the most apropriate line for cephalometric orientation has been resolved and can be summarized in the following five areas of consideration:
1. Clinical significance. The ability of the clinician to visualize the Frankfort horizontal plane affords him the opportunity for effective clinical communication, which is lacking with sella-nasion. In addition, he is able to demonstrate the orientation of the face, chin, and palate to Frankfort horizontal plane, which is also lacking in the sella-nasion system.
2. Anatomical significance. The direct relationship of the Frankfort horizontal plane with the basic sense organs of sight and hering displays a relationship to the face. Sella, in contrast, relates to the brain and not the face.
3. Measurement accuracy. Studies performed to test the accuracy of selection of the planes under consideration showed no significant differences when true porion and not the car rod was used and when experienced technicians performed the tracings.
4. Applications in description. If the reference line is to be considered reliable for description, the correlation between the measurements of the maxilla and the mandible to the reference line should be minimal. In a study performed, SNA and SNB displayed a significantly higher correlation than N-Po to FH and N-A to FH.
5. Application in growth forecasting. A study was performed in order to evaluate the application of these lines of orientation to growth forecasting. In every instance the reference frame which utilized the  Frankfort horizontal plane was appreciably better than that which utilized sella-nasion.


*The use of SN-Po rather than SNB gave the same result, so that use of B point was not a factor.
 
 

REFERENCES

1. Downs, W. B.: Variations in facial relationships: Their significance in treatment and prognois, AM. J. ORTHOD. 34: 812-840, 1948.
2. Johnston, L. E.: A statistical evaluation of cephalometric prediction, Angle Orthod. 38: 284-304, 1968.
3. Ricketts, R. M.: Foundation for chephalometric communication, AM. J. ORTHOD. 46: 330, 1960.
4. Ricketts, R. M., Bench, R. W. And Schullof, R. J.: Master study (unpublished).
5. Schullof, R. J., and Bagha, L.: A estatistical evaluation of the Ricketts and Johnston growth forecasting methods, AM. J. ORTHOD. 67: 258-276, 1975.
6. Steiner, C. C.: Cephalometrics for you and me, AM. J. ORTHOD. 39: 729-755, 1953.