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Questions & Answers on Dental Occlusion

dental occlusion questionsDrs. Dawson and Wilderman continue a previous discussion on the Whitepaper, "Why Front Teeth Don't Hit First in CR." To see this previous discussion click here.

Dr. Wilderman: 

If the teeth are part of the mandible , and the mandible is one bone, don't the teeth and bone have to move in the same direction? If the condyle moves downward and backward, wouldn't the teeth have to separate during this movement as they would also be moving downward and backward? If the teeth are biting into the leaf gauge with their own closing muscle power, and the power is applied from behind the teeth, these two things seem to be mutually impossible to occur at the same time. Otherwise you are saying that the mandible can pivot with the teeth as the fulcrum.

Dr. Dawson:

Bruce- I don't blame you for being skeptical. We repeated our centri-chek recordings on many different patients before we figured out how the condyles could be moved down and back when the elevator muscles were pulling up. Remember this only happens if the anterior guidance is very steep, as in a deep, tight overbite. When the leaf gauge is placed between the anterior teeth, the vector of force is horizontally directed distally. THE POSTERIOR TEETH ARE SEPARATED BY THE LEAF GAUGE, and the only resistance to the steep anterior vector is a much flatter vector of the posterior incline of the fossa. And that incline only engages a small portion of the medial third of the condyle. Remember also that with separation of the posterior teeth, the lateral pterygoid muscle releases contraction. The result is that the steeper anterior vector wins the battle, allowing the mandible to be moved distally. Several studies have shown that even very light chinpoint guidance causes distalization of the condyles. So what do we learn from this?

  1. Design anterior deprogramming devices with a flat surface.
  2. Always load test for centric relation before accepting any CR recording.
  3. Don't use a leaf gauge if you have a steep AG.
  4. The distalizing effect of a steep AG results from a slide… not a pivot.

Thanks Bruce for your question. I hope this answers it. You can verify this effect yourself with a centri-chek. Your question is important because it illustrates the need for careful techniques when recording the uppermost (CR) condylar position… the key to successful occlusal treatment.

Dr. Wilderman:

I am not questioning your observation of the phenomena of distalization of the condyle.  What I still am questioning is your explanation how and why it occurs during biting into a leaf gauge.

It has been a long time since college where I was a dual major in Chemistry and Physics.  I am by no means an expert on either topic. What I do know is that the life sciences such as anatomy and physiology, are made up of theories on how systems operate.  The hard sciences, like physics are governed by laws, such as Newton's 1st, 2nd and 3rd laws of motion. I like to say that we can argue about the anatomy and physiology of how we think a system in the body operates. But however we believe that system operates, it must obey the laws of physics. I do not think your explanation obeys those laws.

In the example you gave of " Several studies have shown that even very light chinpoint guidance causes distalization of the condyles" I would agree 100% with that statement.  However in that example an external force is being applied by another person on the masticatory system.  That vector of force could direct the mandible in any direction.

My question is directed at the forces being applied by our OWN system, not an external force on the system.  Forces and vectors of forces in the human system are solely applied across joints by muscles.  ONLY muscles can apply forces, hence only muscles can cause movement. An object such as a leaf gauge cannot apply a force nor can it change the direction of force. (I should more correctly say it can apply an equal, but opposite force).  The classic example of this is if you are driving down the highway and a bug hits your windshield, which had more force applied to it, the bug or the windshield?  It's a trick question. Of course the correct answer is neither.  The forces are equal and opposite.

Newton's first law basically states that F=MA.  in other words, without force, acceleration (or movement) cannot occur. The only forces applied by the human system on the mandible comes from the elevator and closing muscles.  Someone could argue about where those muscles insert or originate.  Someone could argue that a particular individual has some weird anatomic anomaly. But these are arguments based on anatomy and physiology.

Physics dictates that the only forces applied to the mandible by the elevator muscles (regardless of what is in between the teeth) are in the posterior, superior direction.  You, yourself have professed this point many times.  What I am saying here is that while we agree on the observation that at times the mandible can move distally, we differ in our explanation of how that occurs.  The only other possible explanation that does not violate the laws of physics is that there is another force that is being applied to the mandible in the direction of motion.  The logical conclusion to me is that it is the muscle in-coordination that you have professed all these years that displaces the condylar disc and PUSHES the mandible downward and backward.  That force could certainly explain the discs movement and hence the mandibles movement in that direction.  Maybe we are talking semantics here, but I think it is an important point.

The different "camps" that you have often battled with over how the masticatory system operates could be neutralized by an argument that is based on Physics, and not Anatomy and Physiology.  I believe I have developed a rudimentary teaching aid that uses simple junior high math and high school physics that helps people understand the very principles that you have been preaching all these years.

If you agree that my explantion on how distalization of the mandible occurs, and it makes sense or would like to hear more about it, I would love to discuss it with you in greater detail.

Thank you again for your time in reading this as well and my two other emails.  I have learned so much from you over the years and my practice has flourished from the knowledge I have gained from your courses.


Dr. Dawson:

I like it that you are continuing to question my rationale for why the condyles go down and back when the anterior teeth are compressed against a nearly vertical vector of ditalizing force versus a much flatter more minimal resistance force at the posterior incline of the fossa. I will stick by that observation until I can be convinced it is wrong and your explanation doesn't do that.

I definitely agree that when a disk is partially displaced the condyle can be forced distally past the posterior band by a steep anterior vector. However if this occurs, the condyle will go back and up… Not back and down.

Regardless of either your explanation or mine, we know from numerous Centri- check recordings, that using the leaf gauge with compression against a very steep anterior guidance forces the condyle back and down. That is why ( if a leaf gauge is used) we advocate using bilateral manipulation in combination with the leaf gauge to be sure the condyles are seated to the most superior position. It is also why we advocate a flat surface on any anterior deprogrammer device.

So let's just concentrate on the clinical implications of achieving a perfected centric relation...and that means complete seating of both condyles UP. Newton's Law won't change that.

Peter E. Dawson, D.D.S. is considered to be one of the most influential clinicians and teachers in the history of dentistry. He authored the best selling dental text, Evaluation, Diagnosis and Treatment of Occlusal Problems, which is published in 13 languages. His latest book is entitled Functional Occlusion: From TMJ to Smile Design. He is the founder of the “Concept of Complete Dentistry Seminar Series (SM)” as well as The Dawson Academy. In addition to numerous awards and special recognitions, Dr. Dawson is the past president and life member of the American Equilibration Society, a past president of the American Academy of Restorative Dentistry and the American Academy of Esthetic Dentistry.