Right now, I am preparing a presentation for a continuing education course. I have been granted a couple of hours to present my ideas to my colleagues at the premises of the campus of the University of Jyväskylä. I am going to present a novel hypothesis that the anterior teeth provide different kind of sensory information as compared to the back teeth. This idea provides a theoretical basis for the so called canine guidance phenomenon. The dissemination of this idea will cause a revolution in occlusion -related clinical guidelines.
The muscles attached to the mandible are pointing to many different directions. The direction of the motor units of the many muscles are often antagonistic. It should be self-evident that in the different phases of chewing sequence a concerted guidance of activity of different muscle groups are required. I should have to make my audience convinced that the oppositely directed forces of different muscles of the mandible must be monitored and controlled by automatic sensory reflexes.
The town of Jyväskylä has historically been regarded as the cradle of education and learning in Finland. Jyväskylä has been cherished with a moniker “The Athens of Finland”. As the freemen of Athens, so shall I be encouraged by the spirit of pursuit of knowledge, that is so valued in Jyväskylä.
To make a slide show for my presentation is a challenge. My presentation is about the dynamics of mastication; it is about how the jaw bone moves about by the action of muscles. Such an automatic, common thing that normal people would little care to discuss about. Yet I should have to speak about the subject for the whole night! It would be difficult, and probably boring, to try to set out the spatially three-dimensional phenomenon of mastication cycle into its fourth dimension, i.e. the time, just by means of narrative of words. I shall need some pictures to make my point.
Obviously, I turned over to Google Pics, and offered her a variety of my favorite search terms. Yeah, she provided me with some feasible portraits of skull bones and jaw muscles, but not quite what I had wished for. Believe me, a facial view illustration of the lower jaw positioned in slightly lateral extrusion, with the teeth and mandibular bone informatively glowing through the skin is a rarity even in the vast and infinite expanses of the worldwide web. Besides, should I use the search result pictures, there would be the elaborate copyright-permit-issues to deal with.
Ultimately I could not find any other way but to go to the stationery across the street of my practice and buy some paper and ink. What a Saturday! Several hours of toiling, ink stains on my face and hands, but the process of drawing ascertained myself of the importance of my work. The illustrations make the importance of canine guidance to become self-evident. The idea of anterior teeth guiding the movement of the mandible in mastication is so simple and makes so much common sense, and yet it is so very difficult to verbally explain this idea for the uninitiated.
Here’s the illustrations that I created. I hope they make my hypothesis clear. Here’s how the important events take place in the jaw-closing process.
In the figure 1. there is a lateral view of the skull crushing a piece of jelly-bear-candy between the molar teeth. Only the molar teeth are receiving mechanical strain from the piece of candy. Note that the anterior teeth are not in contact. This two-dimensional picture can’t show it, but you can assume that the mandible is slightly in excursion to the lateral, to the working side. The sensory information coming from the molar teeth under pressure load provides a signal for the masseter muscle. The masseter must “GO!”
The next figure 2. is a facial view of a pale-faced gentleman in the action of squeezing a jelly-candy between his left molar teeth. The blue arrow pointing upward shows the direction of forces that are moving the jawbone. The principal muscles that are active in this process are shown in orange color. On the left side of this gentleman the temporal and masseter muscles are active. Contralaterally, in the non-working side, on his right side, the pterygoids, the lateral and medial provide the thrust to keep the mandible to the working side. The orange circle depicts the inevitable about-to-happen point of contact between the left upper and lower canines
Figure 3. shows the moment as the left canine teeth of the working side meet each other as the molar teeth approach each other on their way through the jelly-candy mass. Now, the upper and lower left canines make a sharp contact and there is sensory information coming from the canines. The blue arrow indicates that the information coming from the canines signals the masseter muscle. The masseter must “STOP!” it’s activity.
Figure 4. is to illustrate that after the signal provided by the colliding left canines, a complete revolution has taken place what it comes to the activity of different sets of muscles. The signal from the canines has quieted down the activity of working side temporalis and masseter muscles. The activity of contralateral side pterygoids is no longer needed. The blue arrow below the chin shows the direction of mandible movement from the lateral excursion into the maximum intercuspidation. The only muscle that needs to be active is the medial pterygoid of the working side.
The point of my presentation is, that the mandible is not just a senseless inert nut-cracker-machine, but a sensitive organ providing two different kinds of information, depending from which tooth area the sensory information is coming from. The anterior teeth are crucially important to put off the potentially destructive forces of the masseter and temporalis muscles in the final stage of the chewing cycle. Previously held ideas of chewing and occlusion need to be revised. My forthcoming presentation in Jyväskylä 18th May, and my blog articles present some clinical implications of this paradigm. Eventually, there will be fundamental changes in many dental clinical treatment modalities.