We were exhibiting at the International Academy of Gnathology meeting last week so I had the opportunity to sit in on some of the very interesting presentations they had on their program. One of the presentations in particular has set me thinking so I thought I’d share its challenge on our blog. It was given by Dr. Tom Basta, one of the directors of the Foundation for Advanced Continuing Education and titled “Occlusion – Myths & Realities”.
Dr. Basta presented on a number of topics but one stood out to me as I’ve heard a lot of discussion on the topic and was convinced I understood the answer. The topic was: Where does the condyle (or condylar axis) go when a patient encounters a posterior interference in closing from centric relation (CR)?
As someone who was trained as a mathematician (I’m not a dentist, though I’ve now been involved in animating the concepts around occlusion for several years) it has always frustrated me that there is a debate in this area as we are talking about a relatively simple application of high school geometry. We are taking an inflexible object (the jaw) and translating and rotating it by measurable amounts. What happens to the condylar axis is well-defined – i.e. there’s only one answer assuming we agree on two facts: where the jaw starts (CR) and where it ends up in its position of maximum intercuspation (MIP). As far as I can tell everyone agrees on where those two positions are. Some may take longer before they are confident they have found the true CR position and have not found a “CR du jour” position (Dr. Basta’s expression) which is close but not the fully seated position that can be established if the lateral pterygoid muscles are given longer to relax. For the debate I’m considering in this article it actually doesn’ t matter whether you start with a confidently established CR or a quickly established “CR du jour”.
In case you are not aware, I’d summarize the debate I am talking about as a discussion between the following two positions:
- After encountering a posterior interference the jaw slides down and forwards from CR to MIP.
OR - After encountering a posterior interference the jaw pivots about that point of contact so that the condylar axis moves down and backwards from CR to MIP.
Using slightly crude diagrams:
| Jaw in centric relation | Is maximum intercuspation: Down and forwards or ... |
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I’ve heard dentists from both “camps” swear that the majority of their patients exhibit the behavior which they believe happens. How can that be? Surely they have not both attracted patients whose anatomies match their beliefs? That is not credible. Those who argue for “opinion” #1 say that they see the teeth slide forwards and there are no muscles that support the rotation believed to happen for “opinion” #2. Those who argue for “opinion” #2 say that they see the condyle pivoting down and back so that’s what happens.
I’ll give the mathematical answer to this debate in a future article and provide you with the mathematics to prove what happens – along with ways you can prove to yourself what’s happening with each patient’s condyle. However, for now I just want to share one thing that Dr. Basta communicated in his presentation and ask you one question.
What I liked most about Dr. Basta’s presentation is that he, and his FACE partner Dr. Jeff Brucia, take measurements so that they can establish precisely where the condylar axis goes. They do this using physical equipment which is fixed to the patient’s skull and jaw. Consequently they have hundreds of measurements of what happens to the condylar axis. This seemed like a pretty good way of verifying the answer to me!
Therefore my question to you is:
- Do you measure where the condylar axis goes when the patient’s jaw moves from CR to MIP?
If you don’t make that measurement, what makes you confident about your opinion of what happens to the condylar axis?
In my next blog I’ll explain why it is unwise to take a firm opinion on this subject without taking the appropriate measurements.
PS – Add your answers to the comment area below. And you might amplify your answers by telling us what you measure and how you measure it. Would also be good to know if you keep a record of these measurements so you have a database of results (even if it’s contained in a filing cabinet).