Would it be wise to establish a stable landmark by drawing a line through sella parallel to Frankfort? The Frankfort line from the first film of a series could be transferred to all subsequent films.The measurement from gonion to SN perpendicular to Frankfort helped us establish the "mechanism of jaw growth" concept, which taught us the relationship between vertical and horizontal growth and taught us how and why the mandible rotates forward and backward.
When measuring from the gonion angle to SN, there is a certain amount of inaccuracy due to the backward slope of SN as related to Frankfort. When the gonion angle and other anatomical parts grow posteriorly the measurement would be from a point farther posteriorly on SN, which would produce a measurement slightly less than the ramus and other parts really grew. Please see Fig. 1.
While measuring the ramus growth from gonion to SN has served us well, still if we can make the measurement more dependably accurate, we have a professional obligation to do so. By using the "sella horizontal" line as a reference then all vertical measurements would be accurate regardless of how much the anatomical part moved anteriorly or posteriorly.
This would eliminate the difficulty of accurately vertically measuring the upper first molar as it moves forward in growth along an ever widening portion of the angle between SN and Frankfort. To be able to be completely accurate in the vertical measurement of the upper first molar is an important item. It is one of the anatomical parts which has been difficult to measure vertically, partly because it moves forward in growth, but mostly because we have not established a stable horizontal line from which it could be measured.
At the degree of enlargement shown in Fig. 1, at the first film the upper first molar is approximately 12 mm anterior to the great divide, and at the 2nd film the molar is approximately 16 mm anterior to the great divide having moved forward approximately 4 mm. When this amount of forward movement is measured vertically from SN, there is approximately 2.5 mm more vertical movement than when measured from sella horizontal.
When we measure structures which move posteriorly from the great divide, there will be the same difference with the same amount of movement but it will be diminished instead of exaggerated. In other words anatomical points which move forward from the great divide are exaggerated when measured from SN, as compared to measuring from sella horizontal; while structures which move posteriorly are diminished when measured from SN.
This is why we must measure vertically from line, sella horizontal. To accurately measure horizontally we must use a line perpendicular to the earth's surface - "the great divide".
In recent articles authors have used a horizontal line six degrees or seven degrees down from sella/nasion as a basis for vertical measurement.
Six degrees down from SN would be accurate only when the SN/Fr angle is twelve degrees. Then six degrees down from, SN is only an approximation, and still depends on SN. The SN/Fr angle can vary from 1.5 degrees to 15 degrees. The saddle angle, Na/Sella/Ba, can vary from 119.5 degrees to 140 degrees. If the SN/FR angle were 1.5 degrees, then 7 degrees down from SN would be 5.5 degrees in error.
When we establish the line "Sella Horizontal" parallel to the earliest Frankfort, we eliminate the guess work and have a completely stable land mark. All anatomical points in the craniofacial complex can be accurately vertically measured from this line.
When we establish the "Great Divide," a line from sella downward perpendicular to Frankfort, we have a stable land mark from which all anatomical points can be accurately horizontally measured.
Through the years there has been considerable discussion which condemns the use of Frankfort as being unreliable as a basis for measuring because it changes with growth. Much of this criticism seems pure folly; because even an arbitrary line approximately parallel to the Frankfort of the first film would serve perfectly satisfactorily.After all, Frankfort is the most ideal reference for vertical measurement of the craniofacial complex. Then to establish a line, through sella parallel to the Frankfort of the first film, is the most logical basis for vertical measurement; and the great divide, a line perpendicular to Frankfort in the first film, is the most logical basis for horizontal measurement.Nasion has been used as a landmark for vertical and horizontal measurement as long as we have had cephalometrics, and if one prefers it can still be used for that purpose. Nasion will always serve as a dependable landmark for vertical measurements on and near the profile of the hard and soft tissues.To be able to accurately measure the craniofacial complex both vertically from the sella horizontal line parallel to Frankfort and horizontally from the great divide which extends downward from sella, it will solve the problem of how to measure cephalometric radiograms. If investigators will begin using these stable landmarks they will soon become standardized. Using a one millimeter grid greatly facilitates measuring vertically and horizontally. Please see Fig. 1 and Article 1.
This system of measuring can be likened to longitude and latitude. Any point on the globe can be pinpointed by expressing its location in degrees of longitude and latitude. By the same token, any point on the lateral cephalometric radiogram can be pinpointed by expressing its location in millimeters and fractions of mm of vertical and horizontal position.
Showing two stable landmarks for accurately measuring vertical and horizontal growth. The "Sella Horizontal," a line through sella parallel to Frankfort is a stable landmark from which all points in the craniofacial complex can be measured vertically. The "Great Divide," a stable landmark extending downward from sella perpendicular to Frankfort, is a line from which all points in the craniofacial complex can be measured horizontally. This line divides anatomical structures which move forward in growth from structures which move posteriorly in growth. Please see Article No. 1.
Fig. 1.The great divide would be identified by the number "zero," all points anterior to this line would be identified by a "plus" and all points posterior to this vertical line would be identified by a "minus." This north-south meridian divides structures which grow anteriorly from those which grow posteriorly.
The causative increments emanate from the vertical and horizontal stable meridia while the resultant increments are on the perimeter of the complex. The zone of demarkation between the causative increments and the resultant increments is not and cannot be clearly defined at this time.
While the growth of the posterior part of the face may not necessarily cause the peripheral areas to grow, it may be considered to be the cause since peripheral growth is "stacked on top" of what we consider to be causative growth.
While the temporomandibular joint grows posteriorly from the great divide, it acts as a buttress for the forward growth of the chin. The causative growth at the posterior part of the face must be measured from the vertical and horizontal meridia, but the resultant vertical growth at or near the profile may be measured from any landmark which the investigator chooses.
As is well known, the growth of the craniofacial anatomy is so complex that it defies complete understanding at this time. There will always be controversy regarding the analysis of this portion of the human anatomy.
Now that we have stable landmarks both vertically and horizontally, we at long last can standardize the measuring process. For the first time in the history of orthodontics we can scientifically measure the craniofacial complex both vertically and horizontally.
Again it is hoped that colleagues will comment!! It would be considered an act of friendship and good will if a member of the academic world would comment-either for or against.
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