HOW TO PREVENT FRACTURES WHEN YOU
TEMPORIZE A POST-CORE BUILD UP.

A small modification in the way I temporize post/cores has significantly reduced premature fractures ... even when the provisional period is a lot longer than I intended

Martin B Goldstein, DMD
Wolcott, CT

When you bond a prefab endodontic post, there are a number of situations where you may decide to defer prepping the core and taking the impression. Maybe you’re running behind schedule. Or perhaps the patient can’t afford the definitive crown at this point. There may be insurance issues. Or you may want to see how the gingiva heals before you begin C&B.

So you build a direct composite crown build up over the post, and explain to the patient that they must call to schedule the rest of the treatment.

But you don’t hear from them again. Until months later, you get ...

The call

The overly agitated patient tells you that your crown build-up has broken, leaving a toothless gap. To make matters worse, Mrs. Jones has a wedding this weekend and there’s no way she can attend the affair in this state.

Granted, the tooth wasn’t crowned as instructed, but all Mrs. Jones knows is that your handiwork failed very quickly and at the worst possible time. To remain in her good graces, you better find some way to fit her into your already crammed schedule. Looks like lunch is optional today.

So you regroup, send out for something to eat and tell her to come right in, hoping that what’s left of Mrs. Jones’ #10 is salvageable.

After several schedule-wrecking, lunch-destroying fractures like Mrs. Jones’s, I began over-engineering my post-retained temporary crowns. And I’ve encountered almost no fractures since.

Consider a typical “at or below the gingivae” coronal fracture that requires a post.

To keep it simple, let’s assume an endodontically treated bicuspid has fractured at the gum-line. (It’s one I happen to have clear, illustrative photos of.) You’re faced with a partially submerged root surface with a pink eye of gutta percha peeking out somewhere center-root. Gingivae may have begun to creep into the fractured root recesses. Root caries may be present as well (fig 1). You know at a glance that the bicuspid needs a post, core and crown. While my points are illustrated with a bicuspid, the same principals hold true for the typical anterior fracture.

Your first inclination might be to localize the tissue, blow away the gingiva over the root with a highspeed instrument and remove whatever caries you find. Following placement of the post, you might build the composite crown using a clear former. That way the crown will be retained adhesively by bonding to the root interface as well as mechanically retained by the head of the post (fig 2).

A better alternative

Think back to having read something, somewhere about a “ferrule”. A what?!! The ferrule (and I’ve no idea of the origin of the term) refers to 2mm or so of sound root structure apical to the core that the margins of the crown should engage (fig 3). A ferrule makes post-retained full-coverage restorations significantly more retentive and dramatically strengthens the tooth to resist fracture. It surrounds the circumference of the tooth, holding it together like the metal bands around the head of a wooden mallet.

We encounter the ferrule in other areas of dentistry, such as those small but necessary hex locks that join our implant components. Apparently, a circumferential, sleeve-like engagement of as little as two millimeters will resist dislodgment and breakage of many things dental.

Though the importance of the ferrule is widely acknowledged in literature, it refers primarily to the definitive fixed crown. I’ve found that adding a ferrule to my post-retained temporary crown build-ups has made a dramatic difference in their success.

The walls of the ferrule prep should be as parallel as possible to maximize strengthening. If significant coronal tooth structure remains, that’s not a problem. In some cases, like this bicuspid, we’ll have to accept a certain degree of taper.

In an ideal situation, we would also like to have a 2mm zone of biologic width apical to the ferrule. Fact is, in a core buildup, we often don’t have enough room. So we’re left with a choice between recommending a crown-lengthening procedure or making do with what’s left. Unless space is a serious problem, I concentrate first on establishing a fracture-resistant buildup and later worry about the biologic zone.

Here’s how you apply the ferrule rule to temporizing that fractured tooth.

You’ve encountered minimal root decay, cleaned it up and had the wisdom to pick up a perio probe. You’ve found almost 3 mm of sulcus depth surrounding most of the remaining root.

First, place your post according to the manufacturer’s instructions. It’s not unusual to encounter slight bleeding during trough creation, so you may as well bond the post in place now, while the environment is as blood-free as possible. Pictured here is a fiber post that has been fitted (fig 4). After bonding the post in place, I stabilize it with a college pliers or hemostat and trim it to proper height.

Now the core.

You recall the numerous times your patients have not returned for crowns as advised and you remember the ferrule effect. You’re going to play it smart. You’ve decided to expose two mms of submerged root before placing your build up.

Let’s create space for the ferrule

Do you have an electrosurge unit handy? If there was ever a case that begged for electrosurgery, this is it! Precision, nearly bloodless tissue removal performed in just a few minutes. (See below.)

The electrode cauterizes as it cuts, so it minimizes the possibility of blood seeping into the buildup. But if there’s adequate sulcus, I’ll also pack a hemostatic cord prior to tissue removal. Now it’s simply a matter of creating a nice little trough around the root that’s about 1 mm wide and 2 mm deep (Fig 5).

In essence, I do what it takes to permit introduction of a flame-shaped diamond into the sulcus without having to wade through tissue. I create a beveled root surface that will mate with the composite. Shoot for a 2 mm occluso/gingival prep if possible. Try to keep the bevel as close to parallel as is practical but still slightly beveled.

My favorite technique for creating crown build ups involves a clear strip-off crown form that’s been cut and festooned to closely fit around the newly exposed beveled root. I drill a porthole in the crown former that’s large enough to fit a composite compule tip. Since I plan to inject the material, my composite must have sufficient “flowability”. And since I want it to survive even if the patient doesn’t return as directed, it must have reasonable strength. I used Caulk’s TPH for this application, but if you prefer to use your favorite hybrid you can make it more flowable by heating a compule in warm water. The engagement of the ferrule prep, however, makes the strength of the core material somewhat less critical since this zone of attachment will account for much of the build-up’s strength and resistance.

I re-etch the exposed root and apply another coat of my favorite bonding agent. I then seat the crown former over the beveled root and maneuver it until there appears to be close adaptation to the tapered, exposed root. The taper allows a sleeve of composite to encase the root while still being confined to the boundaries of the crown form. Ideally, all of the bevel should be engaged. I then begin to pump composite through the port hole until the crown form is completely filled and excess begins to force its way out at the gingival aspect (Fig 6).

I digitally stabilize the crown form during injection of the composite paste. I remove as much flash as possible prior to curing. In some instances I will have the patient close to full occlusion so long as the crown form won’t be significantly deformed in order to minimize occlusal adjustments. If closure is not an option, I may compress the crown form with fingers placed on the buccal and lingual to achieve better interproximal contacts. Once I’m satisfied that the crown form is in proper position, the build up is zapped with the curing light from four directions, twenty seconds each way.

If you’re concerned about the interproximal contacts, you can create mesial/distal contact ports in the strip crown. But this will make it more difficult to remove the form. Frankly, if the contacts are seriously inadequate I simply add more composite after I’ve finished.

I remove the crown form by slicing it labially top-to-bottom with a #15 blade. This enables me to slip an explorer under the form and pry/peel off the shell (fig 7).

Occlusion is adjusted, flash trimmed and if really, really necessary, tighter contacts are added. The thickness of the crown form will prevent you from creating normal contacts. Since this is supposed to be an interim restoration, the threat of open contacts is far less critical than the chance of fracture. If you are certain that this build up will remain uncovered for an extended period, feel free to establish better contacts via class two or class three preps accompanied by conventional matrix techniques for contact creation.

Let’s finish up

This patient did in fact return in a timely fashion for crown preparation.

In figure 8 you can see how the ferrule area has been reprepped to receive the crown margins formerly occupied by the interim crown build up. Again, I point out the tapered parallel walls that extend well past the post and core/root interface. The impression (fig 9) assures me that the likelihood of crown failure is minimal given the generous amount of root surface engaged.

As you can see in figure 10, when my patient returned for final cementation, the tissue had fully recovered from the electrosurge procedure. The gingival contour made possible by the beveled root enabled placement of a physiologic composite crown with proper emergence profile, followed by a similarly contoured temporary crown. Both were well-received by the surrounding tissue as will be the finished restoration. The gingiva was happy! (OK, maybe just content.) and the resulting crown is well supported.

So what’s the big deal? Well, there are four things I hope you’ll take away from this article.

1. For many reasons, your patient may not have the definitive crown placed as soon as you recommend. This poses particular dangers for teeth with endodontic posts. Therefore, it makes sense to design a provisional crown buildup that will withstand prolonged use. Doing so will reduce the number of lunches you end up taking in.

2. Teeth with posts will hold up much better if the provisional crown engages a long beveled root surface. Remember the ferrule effect!!!

3. An electrosurge offers a fast, almost blood-free way to create working room for the ferrule prep. The surge should be located just as conveniently as your favorite handpiece. If it’s at hand, you’ll use it every day.

4. If you place the crown form first and then inject into it, you’ll get a more reliable crown orientation. This also assures that the composite will fully engage the root taper to create a ferrule.