On Centric Relation

Greetings! The following is the first in a series of articles on various aspects of dentistry derived from my years of experience in a TMD head, neck and facial pain practice. With apologies to a well-known talk radio personality, please think of it as the fusion of entertainment and enlightenment. Enjoy.

“Nobody’s right if everybody’s wrong.” Buffalo Springfield, For What it’s Worth

I’m going to begin by tipping my hat to a couple of guys who both deserve some recognition. Pete Dawson has done as much or more to elevate the level of dentistry around the world as anyone. Whole new generations of excellent teachers in dentistry, people such as John Kois, owe a great debt to the work that Dr. Dawson did before them.

Bill Dickerson, at LVI, has raised the level of esthetic dentistry done in this country and around the world as no other one person has done. Color mapping, the idea that cosmetic dentistry doesn’t just mean making the anteriors white, proper proportions, attention to the potential dark corridor that may remain after your cosmetic restorative dentistry, are all concepts that many dentists have first learned from Bill.

But if you like to explore occlusal concepts on the internet, as I do, you’ve doubtless run into the ongoing blog wars between Drs. Dawson and Dickerson. Who’s right? Who’s wrong? I’m not sure anyone is 100% correct, but I will offer a few opinions based on research and my own observations over the past few years.

The best way to measure the reliability of an occlusal concept is in the way that concept works to produce complex dentistry: the full mouth reconstruction.

During the past year, twelve patients who had all undergone full mouth restorations were referred to me for evaluation of their TMJ pain. In fact, on average I have seen about one completed full mouth case per month over a span of several years. Some of these patients have been restored with neuromuscular concepts, i.e. restored in myocentric, some restored in Centric Relation using a bimanual manipulation technique, and a few patients have been restored in terminal hinge–a position found by pushing the chin straight back with one thumb (Is anybody still doing that? Yep. Sure enough).

To find the so-called “myo-centric” position, a dentist uses a myomonitor or ultra low frequency TENS with electrodes placed over the coronoid notches to stimulate the masseters and temporalis muscles. Theoretically, pulsing the muscles this way relaxes them and guides the mandible into a correct position.

In Pete Dawson’s opinion, this simply doesn’t work. In his words, the results are “inconsistent and unreliable.” Based on my observations, I have to agree. I saw seven full mouth reconstructions done in myocentric within the psst few months, all resulted in severe TMJ pain, and joint images showed results all over the map: some of these patients had posterior displacement of condyles and anterior disc displacements, some had condyles so far anterior that they were half way down the slope of the eminence, and–astoundingly!–many had both at the same time: one condyle jammed back of center and one well forward. The dentists should have recognized something was wrong even without joint images. The mandibular midlines for all these patients were off–sometimes so far off it was obvious just from looking at the patients’ chins.

Now to be fair, keep in mind that I’m seeing the failures. These could very well be a minority. And also bear in mind that I’m not singling out any particular teacher or group–some dentists claiming to use neuromuscular concepts may not be fully or well trained.

I’ve asked a few neuromuscular type dentists how the results I’ve seen occurred. No one seems to know. And yet it happens. A lot.

I have an opinion on how these various results–especially the shift in midline–occur, but it is an opinion.

If you’re unfamiliar with the use of the myomonitor to record bite position, just Google “TENS to record bite position.” You’ll get a number of possible sites to explore. Clayton Chan does as good a job as anyone to explain it.

The ultra low frequency TENS delivers pulses to the 5th and 7th cranial nerves via electrodes placed over the coronoid notch and this causes contraction of the elevator (closing) muscles. What if those electrodes are just slightly misplaced so that the lateral pterygoid–an opening muscle that depresses the mandible moving it down and forward, and a muscle that along with the medial pterygoid can move the mandible side to side–is stimulated?

What if it is stimulated only on one side? My guess is that is what’s happening in many of these cases.

So, I’m with Dr. Dawson on this one. In my observation, the results of using myomonitor are inconsistent just as Dawson says.

Now, does that mean I think neuromuscular concepts are best left to die? Perhaps. But maybe there is some hope. Dr. Chris Stevens, a neuromuscular dentist and lecturer in Wisconsin, takes a bite registration using TENS, and then takes a corrected tomogram of the result. At that point he asks himself a question: does this position look good to me? If not, if for example one condyle is posterior and one is anterior I’m sure he says: Well, I guess not. So he makes an adjustment or retakes the bite. I did the level one LVI class years ago, and the use of joint images was not taught at that time. I know that 3 D cone beam image interpretation is now being taught at LVI. In a recent email, Bill Dickerson informed me that a lot has changed in the LVI curriculum. I may just have to go back!

Whether you’re reading an advertisement for a center for dental education or listening to a politician, I think it’s wise to maintain a certain level of sceptism.

How about bi-manual manipulation to find CR? The Dawson Center advertises that his students reliably find CR in their first half day of classes. Maybe. But probably not.

Let’s first be clear on definitions as the term CR is thrown around rather loosely these days. CR, as defined by the American Academy of Prosthodontics is the center of the condyle centered in the glenoid fossa with the articular disc centered over the condyle. The actual terminology used is the most convex portion of the condyle centered in the most concave portion of the glenoid fossa with the disc in place. The Dawson Center adds: with the condylar-disc assembly braced against the posterior slope of the eminence. I’m okay with that definition, but the trend more recently is to place the condyles slightly forward of center.

Now, tell me please, can you take your two hands–your thumbs against the chin and the rest of the fingers under the border of the mandible–and CENTER the condyle in an invisible glenoid fossa and know that’s what you’ve done? And do you know the disc is centered over the condyle? One of the practitioners who’s full mouth reconstruction patient I was seeing said, “I know the position is correct because it’s repeatable.”

And here I agree with Bill Dickerson: “Just because it’s repeatable doesn’t mean it’s comfortable.”

After all, terminal hinge is repeatable.

Within the past year, three patients restored in CR were referred to me because of TMJ pain. Based on tomograms of the joints, all three cases of full mouth reconstruction done in CR found with the bimanual manipulation method had condyles posterior to centric with either partial or complete anterior disc displacement.

I know that Pete Dawson says you know an articular disc is in place if you can load the joint by placing upward force on the angle of the mandible and evoke no pain. That may be true much of the time. Because, if the disc is out of place such joint loading puts pressure on the highly enervated and tender retro-discal tissue. But there are two problems with this simple test: 1) it doesn’t test for the partial disc displacements and 2) the retro-discal tissue when exposed to force over time–think of a person whose disc has been out of place for years–will toughen or callus. Load that joint all you want and it won’t necessarily hurt. But there’s still joint dysfunction and muscle imbalance causing pain.

The terminal hinge patients? Obviously, that concept died a thankful death years ago. Only a handful of dentists still use that technique, even though I still see it advocated in some circles.

Bottom line? I think the day is dawning that will see a radiographic centric as at least part of the answer. Think a TENS device will automatically find the proper vertical and anterior-posterior position for you? I don’t believe so. Do you think you can find a position that puts the center of the condyle in the center of the glenoid fossa with the articular disc centered on top of the condyle with your hands? You may come close. But without other data including joint imaging, I think you’re dreaming.

Please feel free to call or email us with your comments about this article as well as suggestions for future articles.

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