Dental proprioception is a very finely tuned sense in most people. The anterior teeth can feel the difference between the thickness of an eyelash and a hair from the eyebrow—a difference of less than 1 100,000th of an inch.
And yet, as dentists we’ve all seen patients whose proprioception is so faulty that they give dentists clearly wrong information. For example, a patient may say, “My right molars are hitting hard.” Yet when checking with articulation paper, or better yet T-scan, the dentist finds that it’s the left molars that are hitting first and hardest.
It’s wise to be cautious at this point, because the patient’s sense of what’s happening is usually correct. If he says he’s hitting hard on a right side tooth, he’s usually right.
But there are exceptions. Even after researching the literature, the reason remains unclear. How can such a finely tuned sense, related to the sense of touch, go so completely wrong? So far, we just don’t know. As some of you can attest, it can indeed go totally out of kilter!
When checking and adjusting an orthotic or splint it is not unusual for the patient to tell me he feels it hitting hard on one side, yet when I analyze with both T-scan and articulation paper I find hard contact on the opposite side. I’ve long given up arguing with the patient; I just go into the lab and adjust what I see and give the appliance back to the patient who then says, “Yeah, that’s better.”
Last week I saw a woman who demonstrated this to the extreme. A local dentist had completed full mouth restoration and the patient felt that the bite was wrong, that her posterior teeth did not contact at all. Her chiropractor told her he was sure she had a TMJ problem.
On manual exam, Susan had middle temporalis pain bilaterally and mild right temporomandibular joint capsule pain. That’s it. No masseter pain, no tenderness in the posterior neck and shoulder muscles. She was able to open 45 mm without pain and there was no joint noise.
Joint imaging showed normal joint architecture (condyles well centered in the glenoid fossae), and computerized joint noise testing (JVA by Bioreseach) detected no vibrations.
“But, you know,” Susan said, “Only my front teeth touch. I can’t close on my back teeth.”
So I placed a T-scan sensor over the lower arch and asked her to close. My computer screen showed very even contact right to left and excellent contact on the posterior teeth. In fact if anything the posterior teeth hit harder than the anteriors.
Could the T-scan result be inaccuarate?
So I picked up a sheet of shim stock (Occlusal registrations strips by Artus—thickness 12.7 microns or .0005 inch) I showed it to Susan. “This is a very very thin piece of plastic, about a thousandth of an inch thick. Let’s see what happens when you close on it.”
I placed it over her lower right first molar and asked her to close. She bit down and I tried to pull the shim stock out from between her teeth. I couldn’t; she had a good solid grip on it. Her eyes went wide with astonishment. I tried the left side. Same result.
“You mean, my back teeth come together?”
“Quite firmly in fact. You have very solid contact on your back teeth. The only puzzle is why it doesn’t feel that way. You could feel your grip on this little sheet of plastic, couldn’t you?”
So, I did not recommend any treatment for Susan. She had normal temporomandibular joints (TMJ), minor tenderness perhaps due to tension or stress. Her occlusion was remarkably good with only one small problem: on left lateral excursion she had an interference on the left second molars. In my report, I pointed out the interference to her dentist so that he could do a minor adjustment in that area.
Two questions come to mind. Could this one interference cause her proprioception to send the wrong information up the afferent nerve pathways to the brain, or is there a deeper problem? So far, there is no explanation. The literature points to MS as one possible cause for faulty proprioception but it is generally considered a mystery.
The next question is how did the dentist achieve the nearly perfect result that he did when placing dental restorations on a patient who had no clue what she was feeling? Pretty remarkable.