The Great Divide:
As Related to the Growth of the Jaws
and the Treatment of Malocclusion

For many years I have felt that we need a landmark which will divide the structures which are displaced forward by growth from those structures which are displaced posteriorly. A line from sella extended downward perpendicular to Frankfort we have termed the "Great Divide," much like the imaginary line S-N. It can be thought of as dividing anterior from posterior cranio-facial growth, much like the east-west water divide of the Rocky Mountains which was discovered by the great American explorers Lewis and Clark in 1805. It is to horizontal growth what Sella Horizontal is to vertical growth (see article 24). All anterior and posterior growth is buttressed against this imaginary line. see Fig.1.

Fig. 1. Showing the Great Divide.

It is well known that the cranium grows concentrically around sella tercica. This being true then it follows that if we choose two points on the circumference say 4 mm apart, the two points will move away from one another as the individual grows.

Further, if we take any radius of the circle and choose two points say 2 mm on each side on the radius on the circumference, these points will move away from one another as the individual grows.

Also, if we draw a line from sella downward perpendicular to Frankfort, a radius of a circle, then structures on each side of the vertical line will grow away from each other. This simple explanation is probably unprovable scientifically, but this does not mean that it is not true. It verifies the truth of the "great divide" concept. In order to not mislead the reader it must be said that obviously sella is not in the exact center of the cranium. But since the cranium grows concentrically around it, it can be thought of as being the center of a circle for purposes of illustration.

I am sure that many investigators have drawn a line down from sella for various purposes, but not for the purpose of dividing posterior growth from anterior growth. Of course the real verification is the actual measurement of the structures. This we have done and will be discussed later.

Fig.1.5 Illustrating the principle of the "Great Divide." Asthe cranium grows concentrically, any diameter divides the growthperpendicular to this diameter. Therefore the great divide (adiameter of the circle representing the cranium), dividesanterior growth from posterior growth. This has been verified byactual measurements. A section of the circumference of thecranium at age 3 months becomes greatly enlarged at age 16years. In this illustration A 7 mm section becomes 14 mmsection.

Nearly all anatomical structures in front of this line are displaced forward by growth while all structures posterior to this line are displaced posteriorly by growth. The entire maxilla and upper teeth are buttressed against this divide. The external auditory meatus, the fossa, articulare and basion move posteriorly and are buttressed against the great divide.

In a random sample growth study of ninety-five subjects we related the pterygomaxillary fissure to the great divide, and to our surprise it moved posteriorly more often than forward. In the majority of individuals the movement was straight downward, but in ten percent the movement was posteriorly. In less than ten percent the movement was forward. Bjork¹ related the posterior nasal spine to a perpendicular to S-N and found that it moved posteriorly in ten of twenty-one cases shown. Thus, the pterygomaxillary fissure and the posterior nasal spine are exceptions to our rule of the great divide.

The growth of the mandibular condyles is buttressed against the temporal bone; the temporal bone moves posteriorly with relation to the great divide and the chin moves forward. The total purpose of vertical condylar growth is to move the chin forward and its direction depends upon the vertical growth of the molar teeth.

Fig. 2. A low angle case Male age 8 to 17

During a nine year growth study of patient number one (Fig. 2), articulare moved six mm posteriorly with relation to the great divide and the condyle grew forward five mm with relation to the mandible. The gonion angle changed from 104° to 95° and the S-N to mandibular plane changed from 26° to 6°. All of these complex anatomical changes occurred and yet the gonion angle moved downward, parallel to the great divide. This is a remarkable statistic. In this case 5 mm of forward growth of the condyles and 30 mm of vertical condylar growth is accompanied by 10° of forward rotation of the mandibular plane, resulting in an S-N to mandibular plane of 6°. When the posterior migration of the fossa approximately equals the forward growth of the condyle, as in this case, it aids time one and time two rami to remain parallel to one another. This type of growth almost always causes a marked forward rotation of the mandibular plane.

In an eight year growth study of patient number two (Fig. 3), articulare moved posteriorly 5 mm with relation to the great divide, the S-N to mandibular plane changed from 131° to 130°, the gonion angle changed from 124° to 119°, and the condyle grew 10 mm posteriorly with relation to the mandible. The gonion angle seems to grow roughly down the great divide throughout growth; even though the gonion varies a great deal in size, varies with different types of growth, and varies with different types of morphology. In this case 10 mm of posterior horizontal condylar growth and 25 mm of vertical growth resulted in 1° of forward rotation of the mandible and 4° of gonion reduction. This type of growth will rarely cause more than minor rotation and this may be either forward or backward. It is important to measure the growth of the condyles perpendicular to Frankfort instead of S-N. There is from 2 to 3 mm difference in the distance from the fossa to S-N when measured perpendicular to S-N as compared to measuring perpendicular to Frankfort, depending upon the size of the S-N to Frankfort angle.

Fig. 3. A high angle case Male age 8 to 16

Fig. 4. The two individuals shown in Figs. 2 and 3 superimposed on S-N

The tracings of patients one and two (Figures 2 and 3) are superimposed on S-N and shown in Figure 4. It will be observed that the two basions are almost identical, the maxillae are in quite similar positions, and the chins have the same anteroposterior positions. Here is where the similarity ends.

The growth of the mandibles caused all of the non-linear chaotic movement. The vertical and horizontal condylar growth (vertical 30 mm and forward horizontal 5 mm) in patient number one, the posterior horizontal condylar growth in patient number two (10 mm), and the S-N to Mn angle change of 20° in patient No. 1, caused most of the dramatic differences in morphology. Please see Figures 2 and 3 for anatomical details.

Solow² has taught us that the ramus grows approximately parallel to its original inclination. We have related structures posterior to the great divide, to the divide itself to try to determine how this phenomenon happens. In a growth study of 95 subjects we related the following landmarks to the great divide: (1) articulare, (2) basion, (3) gonion angle, (4) pterygomaxillary fissure, and (5) posterior nasal spine.

The investigations of Fred Schudy (1966)³ and Bjork (1972)¹ have taught us that the growth of the corpus causes a reduction in the gonion angle. Then in patient number one (Figure 2) the growth of the corpus would have tended to move the gonion angle posteriorly with relation to the great divide were it not for the fact that the dorsal movement of articulare was approximately equal to the forward growth of the condyle with relation to the mandible. Then in effect the growth of the corpus helped to initiate the compensations which allowed the gonion angle to move down parallel to the great divide.In growth there is a negative correlation of above the one percent level of confidence between the angle NSGo and the posterior growth of the condyle with relation to the mandible (1966 ³ - p. 195, Fig. 6). This could have some significance. It could mean that the condyle grows posteriorly to prevent the gonion angle from moving posteriorly.

Condylar Growth

An interesting observation is that while vertical condylar growth's one function is to move the chin forward, in a random sample of 95 subjects ages 6 to 17, its correlation with SNB is .1294 with a probability of .211 (no significants). This means that there is almost no relationship between the vertical condylar growth and SNB, while horizontal condylar growth has a high correlation with SNB (.3814) and a high probability (0.000) in the 95 subject matrix. How could this possibly be true? The explanation is that vertical condylar growth is conditioned by the vertical growth of molar teeth while horizontal condylar growth is not.

In these studies we found a great deal of objective evidence which we will not discuss here. This objective evidence kept cropping up so often that we feel justified in offering the opinion that the vertical growth of the lower molars partially causes the condyle to grow posteriorly.

The Effective Growth of the MandibleThe effective growth of the mandible is represented by the distance condylion to gnathion. It is generally considered that the growth of the condyle is responsible for the increase in condylion-gnathion distance; however the change in the gonion angle, the growth of the ramus and the growth of the corpus contribute to the effective growth of the mandible, see Figure 5 and Figure 6.

Fig. 5. Showing the effect of a small gonion angle on the effectiveness of the growth of the condyles. (45% effective)

Fig. 6. Also showing the effect of a large gonion angle on the effectiveness of the growth of condyles. (240% effective)

It may be noted that in patient number one in Figure 5, the gonion angle closed 17° (113° to 96°); while in patient number two, (Figure 6), the gonion angle was reduced by only 4° - 126° to 122°. This disparity of gonion reduction is what caused the dramatic difference in the effectiveness of ramus growth (45% in Figure 5 as opposed to 240% in Figure 6).

In 1974 in his post doctoral thesis, George Schudy⁴ discussed in detail the difference in the mechanism of forward movement of pogonion in high and low angle patients. In this discussion he pointed out the role of the forward bending of the gonion angle. This can be found on page 47 in the original article and page 355 in the book by Schudy and Schudy⁵.

The Behavior of the Lower 3rd Molar Tooth Bud

In many cases the lower 3rd molar tooth bud does most of its downward decending before the beginning of calcification of the 3rd molar tooth. In other cases the bud decends as much as 3 or 4 mm after the beginning of calcification. This study indicates that the loser 3rd molar tooth bud is not a dependable landmark for superimposition.

Post Treatment Growth

Post treatment growth is one of the most important subjects in the treatment of malocclusion and in the field of clinical orthodontics. This is because it has such a dramatic effect on the treated case. As treatment progresses the orthodontist must carefully consider how his or her treatment will be affected by post treatment growth -- whether favorably or unfavorably.

When all aspects of treatment have been attended to, post treatment growth will be our faithful ally. When the following aspects of treatment have been properly accomplished:

1. When overbite has been reduced to one mm
2. When the upper incisors are about 24° to line NA and have been appropriately intruded if needed
3. When the lower arch has been completely leveled without intrusion of incisors and cuspids
4. When good cuspid interdigitation has been achieved and tested for permanency (we should not hesitate to remove enamel from the appropriate teeth to accomplish this), and
5. When the case has been carefully retained with a fixed retainer from cuspid to cuspid

then post treatment growth will usually improve the superior result.

When all of the above has been accomplished, then post treatment growth will usually translate the upper molars and bicuspids forward, or tip them forward, tip the incisors and cuspids forward, reduce the overjet, improve or maintain the overbite and improve the interdigitation. See Figure 7.

Fig. 7. This shows the effect of post treatment growth on tooth and jaw position.

The lower arch will not be affected in any way because it is protected by a fixed retainer. Since overbite has been reduced to one mm the lower incisors will contact the upper incisors at the incisal edge and tip them forward.

The "poor treatment scenario" goes like this: three or four mm of overbite still remains, upper incisors are inadequately torqued, cuspid and molar relation is poor, lower arch is not completely leveled, lower incisors and cuspids have been intruded, upper molars are not rotated properly, and of course aesthetics is not good because the upper incisors have been moved downward when they should have been intruded. There is evidence that too much vertical has been induced, probably by cervical headgear or long class two elastics.

Now when post treatment growth ensues, the upper incisor will not tip forward because the lower incisors contact them about the middle of the crown. The mechanical disadvantage caused by poor overbite correction and poor torque now tends to force the lower incisors lingually and encourages bite closure and crowding of lower incisors.

In other words post treatment growth produces a complete disaster. Many orthodontists do not understand these principles or they would work harder to try to avoid such adverse effects. It is important for every clinical orthodontist to thoroughly understand what post treatment growth does to the treated case.

In Figure 8 please note that there was very little if any vertical growth of the molars and no increase in anterior dental height. Since the case was well treated and retained with a cuspid to cuspid fixed retainer, all teeth remained in good alignment. The mandibular teeth maintained a constant relation to their base. The maxillary molars and bicuspids were translated forward and the incisors were tipped forward by the growth of the mandible. Of special interest is the fact that the upper molars moved forward about the same as the chin moved forward. This case proves the fact that if a case is well treated and retained, it will withstand the onslaught of very unfavorable growth.

The Relation of Other Structures to the Great Divide

In 30 percent of the 95 subjects studied the gonion angle and articulare moved down parallel to the great divide or moved the same direction the same amount, keeping the ramus axis unchanged. In 54 percent the gonion moved posteriorly more than articulare, causing the angle ramus to great divide to radiate downward with the apex directed upward. In 11 percent of the sample this angle had its apex downward, indicating that articulare moved posteriorly more than gonion.

The Growth of the Mandible as Related to Dr. Bjork's Implant Studies

The thing that makes Dr. Bjork's implant studies so confusing is that he took the mandible out of the fossa, analyzed it completely out of anatomical context, and never returned to the fossa. Any conclusions which he may have reached could not possibly be valid.

It is difficult to understand how the difference between time one and time two measurement of condyle movement in the lower implant superimposition (20.4 mm), can be termed growth, when the distance from condyle to gnathion increased only 13 mm. The 20.4 mm measurement is only displacement, not growth.

According to Bjork's measurements in S-N superimposition in case No. 14, the distance from S-N to gonion angle increased 11 mm (condyle grew 8 mm and the fossa moved down 3 mm making a total of 11 mm). According to Bjork's implant superimposition the condyle grew 20.4 mm. Whatever growth changes that are registered by the implant superimposition have to fit between the time two fossa and the time two gonion angle of the S-N superimposition. The moot question is, how do you fit a 20.4 mm segment into a 11 mm slot?

Had he only superimposed on the implants to show how, not how much the mandible grew, he would not have been guilty of duplication; but when he applied millimeters and angles, then in a very real way he has counted the rotation of the mandible twice. This produced an incorrect interpretation.

Conclusion

We had hoped to find some high correlations of some anatomical entities with changes in the angulation of the ramus, but found none. This was a disappointment. How the ramus angulation remains relatively constant is a mystery to me.

All growth of the craniofacial complex should be measured parallel or perpendicular to the great divide. When growth is measured perpendicular to S-N there is as much as 2 to 3 mm difference in the distance from condyle to S-N measured perpendicular to Frankfort. The distance from condyle to gnathion does not represent the relevant growth activity of the mandible. The effective length of the mandible varies as does its component parts (corpus, ramus, and gonion angle). The mechanism of jaw growth controls the vertical and horizontal movement of the symphysis and the rotation of the mandible. The lower 3rd molar tooth bud can migrate downward several mm. This disqualifies it as a stable landmark.

The pterygo-maxillary fissure moves posteriorly, with relation to the great divide, more often than it moves forward. Post treatment growth is one of the most important subjects in the treatment of malocclusion because it has such a dramatic effect on the treated case.

We need top level discussions of the most scientific method of superimposition with a view to reaching a consensus. Sixty-five years of cephalometrics should be sufficient to reach a consensus on the most scientific method of measuring growth.

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Bibliography

¹Bjork A., Skieller V., Facial development and tooth eruption: an implant study at the age of puberty. Am J Orthod 1972;62:339-83.

²Solow B., Sierback-Nielsen S., Cervical and craniocervical posture as predictors of craniofacial growth. Am J Orthod Dentofac Orthop 1992;101:499-58.

³Schudy, Fred F., The Association of Anatomical Entities As Applied to Clinical Orthodontics, Angle Orthodontist Vol. XXXVI, No. 3, July 1966.

⁴Schudy G.F., A longitudinal cephalometric study of post-treatment craniofacial growth: its implications in orthodontic treatment. Am J Orthod 1974;65:39.

⁵Schudy, Fred F., D.D.S., and Schudy, George F., D.D.S., M.S., The Vertical Dimension, D. Armstrong, Houston, Texas 1972.

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