I exchanged a couple of emails with a non-dental professional (i.e., she’s a patient in a dental office somewhere; she’s not happy and she’s looking for answers). Her question can be found in my article, On Centric Relation. Although she may not have phrased it this way, the question is, “What the heck is radiographic centric?” And when I answered that question, the follow-up was, “Okay, how do you find it?”
Figure 1 shows a healthy temporomandibular joint. The condyle is well centered in the glenoid fossa—think of that as the socket in a ball and socket joint. That’s an extreme over-simplification, but at least gives you an idea of what the glenoid fossa is, a socket in the cranial base. That semi-circular dark area to the left of the joint in this image is the external auditory meatus, otherwise known as the ear hole. There is disc space above the condyle and there is adequate room posterior to the condyle for the nerves and blood vessels that traverse that space. The patient imaged is comfortable and joint vibrations (sounds) are normal as measured by JVA. This is a reasonable example of radiographic centric relation. The reason I use the word ‘reasonable’ is that technically the condyle is displaced slightly posterior, that is back toward the ear. For those of you familiar with Harold Gelb’s 4/7 position, or with Pete Dawson’s contention that the condyle/disc assembly should be braced against the eminence—that downward sloping portion of the fossa anterior to the condyle—I’m sure you would agree. Still, this is an example of a comfortable joint with good function.
Did I just make up the term “Radiographic Centric?” Yes. But I know the concept is being used by many others, even if the term is not.
Unhealthy, painful, clicking, troublesome temporomandibular joints generally have condyles posterior and often superior to centric as you see in Figure 2. Though hard to imagine in 2021, rearmost, uppermost was once considered a good position for the condyles. But that was the 1960s. This joint position does not meet anyone’s definition of centric relation today. The condyle is back, too close to the ear, and it is up, probably in bone-on-bone contact with the glenoid fossa.
The imaging technique is important and may even be critical. The gold standard is the MRI, and as time goes on, I may devote more space the magnetic resonance imaging. However, cost constraints and difficulty obtaining insurance authorization limit the number of MRIs being done for TMJ issues. In the meantime, the advent of cone beam computed tomography (CBCT) has greatly increased our ability to evaluate the joint.
Dr. Pete Dawson in his text, Functional Occlusion from TMJ to Smile Design, copyright 2007, Mosby, Inc. states that the most used technique for imaging the temporomandibular joint is the transcranial x-ray. Done carefully, the transcranial x-ray is an inexpensive alternative to the CBCT and will show the relative condylar position within the glenoid fossa. But would I trust that image to accurate to the nearest millimeter? No. And the transcranial image will often miss boney changes such as flattening of the condyle, beaking and cratering that are present in an arthritic joint. I will note, however, that the transcranial image is certainly superior to a panoramic film.
Though Dawson’s text is rightly considered one of the best in dentistry, and though 2007 is not that many years ago, the transcranial imaging technique appears to be fading into obsolescence.
One type of image that does NOT work well for joint imaging is the panoramic film. The technology has improved since the early SS White machines from a few decades ago, but even with a modern panoramic, the relative position of the condyle in the glenoid fossa cannot be determined. You cannot see boney changes such as loss of the cortical plate or flattening of the superior surface of the condyle on a panoramic x-ray. This is because the x-ray beam is directed from a position about 30 degrees under the joint and portions of the condyle are projected upward and superimposed over other boney structures in the image. A panoramic may provide lots of information about other things—the position of wisdom teeth, for example, but don’t depend on it to tell you much about the joints. See Figures 5 and 6 for details.
When I answered the question about actually finding centric relation (by today’s definition), I explained that it might well require a two-day seminar for dentists to cover my recommended techniques, so the How-To will need to wait for another day and I don’t believe can be covered in a short article.
Regardless of technique, I believe the result should be verified radiographically. That may be a minority opinion (mine), but that is the ideal as I see it.
So, is this—radiographic centric—the answer dentists have been looking for? Does this always guarantee a comfortable position? No. Nothing in my experience is 100%. However, research involving a chart audit of hundreds of patients treated with these criteria for joint positioning shows that a comfortable joint position is found over 95% of the time. Perhaps someday there will be a perfect solution for every single person who suffers from headaches, jaw pain, and so on, a perfect solution that works every single time. It just doesn’t exist yet.
Dr. Ron Jackson, one of the best teachers in dentistry, made the following statement to me: “There’s a perfect position for every condyle. Unfortunately it’s different for every patient.”
Great quote and probably true. But most joints are happy when there is adequate cushion or space behind and above the condyles. When beginning TMJ treatment, that’s a position the practitioner should have in mind.