The Behavior of the Mandibular
Molars
in Relation to Their Base
There exists a wide difference of opinion in regard to how the mandibular molars move in relation to their base, during the growth of the jaws. Björk has shown by metallic implants that, according to his interpretation, the mandibular molars move forward and upward a great deal. Others have questioned this interpretation.
In this narrative we will try to present some facts which hopefully will shed some light on this difference of opinion. First, we must state that all movements of teeth and jaws shown by the implant studies are accurately recorded in the SN superimposition. Also, we must state that all superimposition should be recorded in relation to the Frankfort plane, not S-N.
Please remember that Björk's findings are an unrealistic portrayal of the actual changes that occurred. Here the condyle is free to move, whereas in actual life it is fixed in the fossa, and the gonion angle does the moving. Please see Figure 1.
Fig. 1: This illustration shows that while the lower first molar did grow upward remarkably, it was not due to superimposition on the implants. Please see explanation in the text. (From the marvelous research work of Dr. Björk.)
The World Conference of Scientists held in Frankfort, Germany in 1929 decided that the eye/ear plane is most nearly parallel to the earth's surface. Since this is true we should use this plane, or another one parallel to it, to superimpose cephalometric radiograms-making our analysis "square with the world."
To make any sense out of the complexity of the craniofacial anatomy, we must adopt a plane which is parallel to the earth's surface. In the individual shown in Figure 1, the rotation of the mandible as a result of implant superimposition plus the growth reduction of the gonion angle, resulted in a 13° reduction of the angle of the ramus. Only in recent years have we known the importance of the growth reduction of the gonion angle. When we factor in all these known abstruse phenomena, the complexity becomes overwhelming.
Sixty-five years should be sufficient time to establish and standardize an accurate method of superimposition, which is based on landmarks that are square with the world and completely stable.
Figure 1 shows a reproduction from the work of Björk which depicts the correct behavior of the mandibular first molar, according to the implants, with relation to the corpus of the mandible. Please notice that when we superimpose on the lower implants the mandible rotates 8° in this particular individual as it grows. This rotation has to be contained within the S-N superimposition.
The center of rotation is at the intersection of the two mandibular planes. If we have two tracings, and place a pin through both tracings at the intersection of the two planes, and then rotate the two tracings from the implant superimposition to the mandibular plane superimposition, we can see how the molars move-see whether the first molar moves vertically as Björk reported. When we execute these procedures we find that the first molar does not move vertically but moves 3 mm forward. Here is where a big puzzle becomes evident.
In a random sample of the growth of 95 subjects ages 6 to 17, Fred Schudy found an average of .7 mm of posterior movement of the first molar on its base. Then Schudy measured the anteroposterior width of the symphysis and found it to increase an average of .4 mm. The .7 mm of posterior movement minus the .4 mm of addition at pogonion yields a total of .3 mm of posterior movement of the first molar on the mandible. Please see Figure 2.
Fig. 2: This is the same individual shown in Figure 1. When we superimpose on pogonion the first molars move posteriorly, as well as the incisors. (The reader must be informed that the author has a high regard for Dr. Björk's integrity and his professional contribution.)
There we have two opinions. One shows evidence that the lower implants are associated with a great deal of upward and forward movement of the first molar on the corpus of the mandible. The other shows objective evidence that the implant rotation does not (maybe slightly) cause the first molar to move upward, and reports evidence to show that first molars do not move forward on the mandible.
This puzzle will remain forever unless two research projects are performed. (1) Unless Björk's research is redone, superimposing on "sella horizontal" and the "great divide," and (2) unless a large sample is measured to confirm Schudy's findings. Let us be optimistic and expect it to be done in the future.
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