Doctors, do you leave your crowns just out of occlusion? Have another look.

Dr. Halligan
Dr. Halligan

To the dentist: are you placing crowns that are just out of occlusion? I know it is fairly common practice and is even advocated in some circles (see my article Last Tooth Standing). When I was younger and just starting out in dental practice, I would often tell the lab to leave crowns just shy of occlusion. It seemed a good alternative to grinding them in.

Omer Reed always told his students, “Cement by primary intent. If your laboratory technician knows what he’s doing, you shouldn’t need to do a try in and adjust.”

By asking the lab to leave them just out of occlusion, there was minimal adjustment, if any.

But when I was seeing TMD patients referred by their dentists for consultation, I noticed a significant downside, particularly when multiple crowns were placed in a quadrant and all of them out of occlusion.

I recall one young man who was referred by a local oral surgeon who was puzzled by the patient’s TMJ symptoms when CT scans of the joints appeared to be normal. When I interviewed this patient, he said, “My best time, my most pain-free time, is when I first wake up. Then there is no pain at all, and no tension. But as the day goes on the pain comes and gets worse. By dinner time, it is fairly severe. The pain is right here.” He placed an index finger against his right masseter to show me. “Then, I go to bed and sleep with no pain.”

I guess he must not be a clencher or grinder.

Now, the story becomes more interesting. “My dentist has done a bunch of crowns over the last three or four years, all of them on the right side. I’ve noticed that when I get a new crown, it doesn’t quite touch when I bite down. I guess that’s so the new crown won’t bother me when I start eating on it. The new crowns were always comfortable right away. Pretty smart, right?”

We move to an exam room and I take a look. Dentition: normal class I occlusion, normal overbite and overjet, four PFM crowns upper right (Nos. 2,3,4,5) and one PFM lower right (#30). Range of motion normal. Areas painful to palpation? Only the right masseter muscle. Joint noises recorded with JVA, by Bioresearch, are very slight and within normal limits. Joint images from the oral surgeon show normal condyles centered in the glenoid fossae.

Then I did a computer analysis of the bite with T-scan. Here is the result with the patient closed in his habitual—normal—occlusion.

(T-scan image)

Obviously, there is very heavy occlusion on the left side and very light contact on the right. Note that the molars on the right do not contact at all.

What are we seeing here? Five teeth on the right side were restored with crowns—one at a time over a period of a few years. All apparently left just out of occlusion at cementation. Well, won’t supra-eruption occur and bring those restored teeth into occlusion within a short time? The answer appears to be, do not count on it!

I showed the patient his T-scan results. “You have close to eighty percent of your biting force on the left side and twenty percent on the right. Every time you close, your brain, the motor cortex, tells your jaw muscles to contract until the teeth touch. Oops! The right-side teeth never come together. The right-side cheek muscle, the one that’s sore, just keeps on trying all day long. The good news is that you don’t have a joint problem. The bad news is that your dentist is going to have to re-do some crowns. Get them into contact when you close, and that muscle pain will go away.

“My dentist said maybe I need a night guard.”

“You said you don’t have nighttime pain and that your most pain-free time is first thing in the morning, right?”

“That’s right.”

“So, what’s a night guard going to do for you?”

“Oh, right. Not much, huh?”

Correct. A night guard will do nothing for this person except possibly make him worse.

I understand that dentists don’t want to do a lot of adjustment on restorations when they come back from the lab. Accurate models and bite registrations—including digitized ones these days—and an excellent lab are part of that puzzle. But if you do several restorations on one side and they are all out of occlusion, you’ll eventually take away that one holding cusp, and create an imbalance.

In my dental school days, we were instructed to take excellent full-arch impressions of both arches, even if just restoring a single tooth. A few of the instructors approved a technique of doing a quadrant bite tray for single crowns. One day, the department chair saw me using a quadrant tray. “I know that technique is officially approved, but I don’t like it. How can you observe canine guidance? How can you take note of the opposite side? You can’t. Just make full arch models. Do what you can to get the bite exactly right.”

It took years for me to see the wisdom of his words (meaning I disregarded them!) Then over years of seeing patients sent for TMJ exams and consultations, I’ve seen plenty with occlusal problems—many with that last tooth standing (i.e. in contact) restored just out of occlusion.

Last tooth standing isn’t just a myth. Gradually taking away the natural balance of forces from right to left is creating one of those mysteries of TMJ pain. Often it isn’t the TMJ at all, just the muscles’ reaction to getting the bite way out of balance. Certainly, you can get away with leaving a crown or two out of occlusion. But eventually, one last crown done that way will take away an important holding contact and create a painful occlusal problem. Just food for thought.