Greetings again! The following is the second in the series of articles on various aspects of dentistry derived from my years of experience in a TMD head, neck and facial pain practice.
LAST TOOTH STANDING: The myth of the helpful aspect of supra-eruption: Don’t count on it!
When I was in dental school at USC we had one clinical instructor known simply as the Big O. I won’t give his full name, or repeat some of the rumors that circulated about him–and some of those rumors were true, too! Those who were at Southern Cal in those days will certainly remember the guy. I suspect every dental school in the country had and has its own version of the Big O. At USC he was the easy instructor. The one direct antithesis to the nit-picky, super-detailed instructors, who otherwise filled the hallowed clinical sections.
All the students loved him.
Students waited in line to sign up for a cubical in his section, especially if the plan for the day was crown cementation. While most instructors would sit chair side and take a sharp explorer to crown margins and then look for perfect occlusion before granting approval to cement the restoration, the Big O would take a mouth mirror in his left hand, gently pull back the patient’s cheek, take a 1 to 2 second look, grab a pen and put his trademark signature on the chart. You got it: A nice big O!
One of the things the Big O taught was that achieving excellent occlusion at cementation was a waste of time. “Just leave it slightly out of occlusion. The tooth will supra-erupt anyway, and in a week or two it’ll be in perfect occlusion. Why bother with good occlusion? Just don’t make it too high.”
He wasn’t a big believer in nice margins either, and didn’t even check them. If the crown appeared to fit, it got approved. One of his opinions: “Your patient will lose this tooth to perio before he gets recurrent caries anyway, so don’t worry about it.”
Now, I’m sure we all agree that he was wrong about recurrent caries. But you may believe he was right about supra-eruption. In visiting various dental labs, I see those little sheets of foil relief on the opposing models so I know some of you believe the theory. I know I believed it for years. But I have some bad news: supra-eruption only occurs when it works against you; it generally won’t work for you.
What do I mean by that? Tell me how many times this has happened to you. You have a patient scheduled for crown seating, cementation or bonding. Mr. Patient shows up and you or your assistant are ready to remove the temporary or temporaries. Oops. It’s not there. You’re only mildly horrified.
“When did your temporary come off? You should have called me and come in!” You run the question and the sentence together in one sort of breathless phrase.
“Oh, just a few days ago. Maybe a week. It didn’t really hurt or anything, and I knew I was getting the real crown today, so I didn’t worry about it. No big deal, right?”
Of course the tooth supra-erupted, and maybe drifted to the mesial or distal as well. In the best case it only takes a bit more adjustment of occlusion and contacts than you planned, and you only get a little behind schedule and at least you can still cement the crown. In the worst case, you have to reduce the prep because now you have inadequate clearance, new impression, new temp and reschedule. Supra-eruption has just worked against you.
Oh, but sometimes it will work for you, right? J ust like your school’s version of the Big O told you. Sorry, I don’t think so. If you cement a crown that is not in occlusion, most of the time it will stay that way.
Does it matter? Sometimes it matters a lot. What if every one of your patient’s posterior teeth has an old restoration that is bound to fail over the next few years? What if you’ve already done three crowns on the lower right, for example, and all three were placed just out of occlusion and now the fourth tooth, the second molar, fractures? And you do a crown just like all the others, just out of occlusion? Well, that’s okay isn’t it? The other three must have erupted into occlusion by now, haven’t they?
Look, you’re a good dentist. You didn’t leave those crowns with open contacts; when you checked with floss it went through with a nice tight snap. With how much force does a tooth erupt? Can it be measured in Pascals? And how much force can the contacts between the teeth resist? My observations say that weeds might grow up through a tiny crack in a sidewalk, but your crowned tooth is not going to erupt up through a tight contact.
Okay, so what? you may be asking. Here’s a scenario I’ve seen a little too often. A dentist places a crown. Within days the patient has TMJ pain. The dentist checks occlusion. The crown is not high. In fact, it’s not even in occlusion. He refers to me for an exam. I’ve even had dentists tell me on the phone, “I know there can’t be a problem with my crown. It’s not even in occlusion.”
And the dentist is correct. The crown is not in occlusion. But neither are any of the other three crowns in the quadrant, and those were done years ago. One last tooth was holding an occlusal stop, and now that’s gone. Sometimes my exam discloses an actual internal derangement of the TMJ, but often it’s an occlusal problem only. Using T-scan, which, among other wonderful things, can quantify the percentage of occlusal force left to right, I’ve occasionally seen 100% occlusal force on one side and 0% on the other. Even if it’s not that bad, 80% on one side and 20% on the other is something I see fairly often after a dentist does that one last crown in a quadrant. And masseter muscles don’t like that very much. You had an instructor at your school or maybe at a post-grad seminar tell you that you should leave your crowns just out of occlusion because supra-eruption will work in your favor? Sorry, in time you’ll find the truth, a new corollary or variant to Murphy’s law: Supra-eruption will only occur if it serves to screw things up; it will almost never happen if you want it to.
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