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 quite happy and she’s looking for answers). Her basic question is up on my website under my article, On Centric Relation. And 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?”
So, let’s get to it. Figure 1 shows a healthy temporomandibular joint. The condyle is well-centered in the glenoid fossa. 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. The image was produced with an iCat. I consider this a reasonable example of radiographic centric relation. The reason I use the word, “reasonable,” is that technically this condyle is slightly displaced to the posterior. For those of you familiar with Harold Gelb’s 4/7 position, 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 quite a few others.
Unhealthy, painful, clicking, troublesome TM joints generally have condyles posterior– and often superior– to centric, as you see in Figure 2. This is a joint that is certainly not in centric relation (CR) by anybody’s definition in 2012.
The imaging technique is important and may even be critical. The gold standard is the MRI and I plan to devote a future post to magnetic resonance imaging. The advent of cone beam computed tomography (CBCT) has greatly increased our ability to evaluate the joint and can provide even more overall information than the MRI.
Dr. Pete Dawson in his text, Functional Occlusion from TMJ to Smile Design, states that the most commonly used technique for imaging the TM joint is the transcranial x-ray. Done properly, the transcranial is an inexpensive alternative the CBCT or MRI. The transcranial does have limitations to be sure, but when carefully done, shows relative condylar position in the glenoid fossa. Would I trust a transcranial film to be accurate to the nearest millimeter? No. But again, if well done, it will show relative position and can also show boney changes such as the flattening of condyles, beaking, cratering, etc, that are present in osteoarthritis. Transcranials just don’t show those changes as well as CBCT. See Figures 3 and 4.
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 of 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 cortical plate or flattening of the superior surface of the condyle on a panoramic x-ray. The reason for this stems from the angulation of the x-ray beam through the condyle itself. Because the x-ray beam is directed from a position under the joint, the lateral pole of the condyle is projected upward and therefore details are lost because the image is superimposed over other boney structures. A panoramic may provide lots of information about other things, but don’t depend on a panoramic to tell you much of anything about the joint. See Figures 5 and 6 for details.
When I answered the email about actually finding centric relation, I explained that it might well require a two day seminar for dentists to cover the techniques I employ. For starters, I do joint images, Joint Vibration Analysis and T-Scan before attempting to find CR. Next, I already have an idea of what will be required to achieve well-centered condyles. In other words, if my diagnostic image shows both condyles posterior to centric by about 2mm, I know what change is necessary to correct the position.
Here is the bottom line: whatever technique I decide to use, I always get an image of my treating position.
Sometimes, although this may sound sacrilegious to the bimanual manipulation crowd or the TENS (Myomonitor™) group, I sometimes simply do a bite registration at a slightly open vertical and then adjust the articulator to achieve that 2mm protrusion. The bite registration and resulting treatment orthotic are done on articulator mounted models. I always x-ray the result to make sure I have achieved the joint space I desire.
Using the Leaf Gauge as has been described elsewhere. A good example can be seen in the video below. I have used this technique a handful of times and it appears to work well. Again, I always image the result.
I use bimanual manipulation occasionally but not often.
I tried the Myomonitor technique a total of four times after initially learning it at LVI and at a Bioresearch meeting. All four bite registrations failed to produce CR on imaging (four tries gives me an obviously small sample, but all four registrations produced joint images that showed condyles posterior to centric. The images were done on an iCat).
If the patient has a canted frontal occlusal plane (see TMJ Pain, Whose Fault Is It? for a photo of canted occlusal plane if you’re unfamiliar with the concept), I almost always use an Aqualizer ™ as part of the bite registration. This is because the person with a canted frontal plane will have unequal occlusal force as measured right to left on the T-Scan. The Aqualizer ™ will exactly equalize those forces during the bite registration. I keep the Aqualizer ™ in place anywhere from a few minutes to a few hours—sometimes even overnight—before doing the bite registration. It amazes me how well the Aqualizer ™ serves to get interfering occlusal contacts out of the way and let me easily guide the patient to CR most of the time.
Again, the result is verified radiographically.
So, is this the answer dentists have been looking for? Does this always guarantee a comfortable position? No. Nothing in my experience is 100%. However, in doing a chart audit of several hundred patients treated over the past couple of years, I will say that I find a comfortable and functional joint position and vertical dimension over 95% of the time. Maybe someday, someone will find a perfect solution for every single person who suffers from headaches, jaw pain and so on. I don’t believe it exists just 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 as long as there is adequate space behind, above and in front of the condyle. When beginning TMJ treatment, that’s what I aim to produce. And I want to see it on a film.