Rapid Fire Crown Build-Ups

By Martin B. Goldstein, DMD
Wolcott, MA

I do a lot of crown build-ups … and I blame Gordon Christensen for most of them. A number of years ago I attended one of his CE courses, and came home persuaded that old restorations don’t belong in new crown preparations. The possibility of leakage or secondary caries is just too high to risk using them as the base for expensive C&B work.

When I began to routinely remove old amalgams and composites, I was shocked at the gunk I discovered under apparently sound restorations. In most instances, these conditions never showed up on film. To be sure, the “sub-amalgam mush” that I was encountering didn’t belong under my new crowns.

Of course, this philosophy substantially increased the time I spent on a typical crown preparation. After removing the old restorations and cleaning up any surprises, I’d apply a bonding agent to the remaining tooth structure, build it up with composite (any shade I happened to have too much of), and then cut a conventional crown prep. This could add as much as 15 minutes to the procedure.

Things just got a whole lot simpler

Over the past two years a new category of composite resin has been developed specifically for cores. The two I’m most familiar with are Pentron’s Build-It^® and Parkell’s Absolute Dentin™. In my hands, they both perform admirably, but there are other similar materials, so shop around until you find one with the properties you like.

Don’t confuse these materials with conventional restorative composites.

The objective of a good restorative composite is to simulate the smooth, translucent, non-staining, wear-resistance of Mother Nature’s enamel. A good core composite, on the other hand, simulates the hardness, strength and supportive function of dentin … not enamel. In a core, cosmetics and wear-resistance are irrelevant. And the hardness should be substantially less than that of a traditional hybrid composite. If the core is too hard (or too soft, for that matter), your bur will skip or jack-rabbit as it transits from core to dentin.

Unlike restorative composites, core composites are self-cure or dual-cure. This allows fast bulk-placement without the hassle of incremental buildup.

So What’s the Big Deal?

In three words: “convenience, “ “convenience” and “convenience.” (I guess you better make that “four” words, because the “economy” is also excellent. Build-It is just a fraction of the cost of all restorative composites and most hand-spatulated core resins … and Absolute Dentin costs even less than Build-it.)

I hate using words like “remarkable,” but in this instance, it’s a good fit. I’d suggest that you ditch your clumpy, can’t-get-into-small-spaces, difficult-to-bond paste-paste material for one of these new automix build up resins. Here’s why:

The cartridges slip into a standard impression gun. The automix tip ensures a perfect mix every time without the air-inclusions created when you hand spatulate. And with a mini-tip snapped into the mixing tip, you can precisely express small amounts into, YES!, even deep post preparations!

This easy placement is accompanied by intimate contact with the bonded surface. Voids are rare. It’s as if the goop melds with the tooth structure underneath. There’s no need to condense. Of course you need to apply a good bonding agent first (more about that shortly), but you’ll discover that these cores don’t pop out during impressions, tooth preparation or removal of temporaries.

Paradoxically, even though these resins flow enough to conform to the tooth surface, they have enough body to stand at attention. Upon placement, they hold their shape, much like soft ice cream as it is dispensed into a cone.

Because these materials are dual cure, you extrude the entire core in one shot; no laborious incremental build-up. You can speed polymerization by using your curing light, and still be certain that the deep material far beyond the reach of your light will self-cure (four minutes in the case of AD). Shallower buildups can be prepped almost immediately following light cure.

What’s more, what flowed like soft ice cream a few minutes before now cuts like dentin. Close your eyes while prepping (not for too long, please), and you’ll have a tough time telling tooth from core material.

As Buffalo Springfield once observed on a somewhat different subject, “There’s something happenin’ here”, except in this case, “it IS perfectly clear.” Core production just became a LOT easier.

Build-ups have never been this fast. (No Crown Formers, No etching.)

While in some instances, a former or matrix may make your life easier, in most cases, you’ll find that the non-slumping nature of these materials and their intimate contact with the surface eliminates the need for a former.

The marriage of “no-etch” bonding agents with these automix core materials allows a high-speed production-line approach. Suppose I’m restoring a number of broken-down teeth and want to create optimal crown preps. After removing any old restorations and caries, I apply a self-etch bonding agent to all the teeth. I light-cure the surfaces, and express the core material using a single mixing tip. I zap them with my light to initiate the dual-cure set. There’s no mixing; no separate etching; no spatulating or loading a syringe.

Word of caution: Over the past two years, research has identified bonding problems between certain bonding agents and specific core materials.^1,2 Though some lecturers have speculated that only self-cure bonding agents adequately bond to self-cure core resins, the compatibility problem appears to be more complex than that. For example, I’ve had good results using Touch&Bond (light-cure) with both Build-It and Absolute Dentin (dual-cures). Other studies have found bonding problems between self-cure core materials and certain self-curing bonding agents.³ Bottom line: Even though they may bond well to your restorative composites, certain bonding agents adhere very poorly to certain core resins. Whether your bonding agent is self-cure, dual-cure or light-cure, it is a good idea to confirm its compatibility with your core material.

Case in Point:

In figure 1 you’ll note two sad looking bicuspids that appear ready for the scrap heap. The radiograph, however, (figure 2) reveals sound bone and pulps that are far from the action. Seniors (and Charles has been a card-carrying member of AARP for a good many years) often have calcified, non-reactive pulps like this. Both teeth were asymptomatic, so their only crime was utter dilapidation and lack of function.

After I explained what the lack of chewing function meant to his health and happiness and showed him the broken down state of his teeth on a digital image, he agreed it was time for action and requested that a salvage operation be undertaken. Fortunately, cleanup did not result in pulpal exposure, so it was decided that we would shoot for two garden-variety crown buildups and full crown restorations. If symptoms arose, root canal therapy would be an option.

The bicuspids were cleaned up. As you can see there was no pulp in sight (figure 3). The dentin was remarkably sound … discolored, but rock-hard.

The bonding agent was applied (figures 4 & 5). Using a “self-etch” system is the other half of the secret to rapid-fire cores. Of the several I’ve used, I’ve found Parkell’s Touch&Bond’s speed and ease-of-use too much to resist. The total bonding procedure for both teeth took less than two minutes and required a single drop of bonding agent.

1. Apply bonding agent to both teeth and allow it to sit for 20 seconds
2. Air dry to evaporate solvent.
3. Apply a second coat and air dry immediately.
4. Cure with a halogen curing light for 10 secs.

That’s it. Now I was ready to build the cores with Absolute Dentin.

It’s time for Mr. Softy:

Another word of caution: Before attaching a new mixing tip to the core material cartridge, express a little material directly from the cartridge onto a pad in order to assure that both components are flowing evenly (figure 6). Then after affixing the tip, express a little more before going to the mouth. This is very important. Unlike impression materials, core materials are used in very small quantities. If the component in one side of the cartridge starts flowing just slightly before the other, the material will not have the proper 1:1 mix ratio. The resulting core will have poor physical properties or may not even set.

With the mini-tip snapped into the static mixer (figures 7&8), I “served up” the core material directly from the cartridge onto the tooth surface. Note the absence of crown forms or matrix. The material possesses enough body to stack nicely without extra support (figure 9).

Immediately after light curing the resin (40 seconds total), I rough prepped the buildups and packed retraction cord (figure 10) while the material within the core self-cured.

Using my electrosurge, I further exposed the crown margins before beveling the prepared shoulders (figure 11). Remember: core-retained crowns are best served by providing a 1.5-2mm ferrule so that the margins are placed on sound dentin. Modern materials haven’t changed this fundamental rule.

The properly formed crown preps are ready for a bloodless impression (figure 12). Note the intimate relationship of the core material to the underlying tooth. This occurs directly from the cartridge without need for condensation. The entire buildup and prep time was twenty to thirty minutes. Using my old materials, this would have taken at least 45 minutes.

Several weeks later, two all-ceramic crowns were bonded into place, providing a new life for Charles’ bicuspids (Figure 13).

Conclusion:

No, this won’t change the course of dentistry as we know it, but if you haven’t tried one of the new generation of core build up materials, you’re missing out on a truly wonderful time saver. These highly filled dual-cure resins require no hand mixing and can be syringed directly onto the tooth. They will instantly streamline your crown build up/prep procedures.
Like the man said, it just keeps getting better and better.

This article is an expansion of an article that first appeared in Dentistry Today (July, 2002). It is with the kind permission of that publication that it appears here.

*Dentin is a highly variable material, so its physical properties can vary substantially. These properties, which have been drawn from published literature are merely representative.
**The clinical benefit of fluoride in core materials is still debated.

1 CRA Newletter, June 2000
2 REALITY 2001, pg. 1-207
3 Latta MA, et al. Dentin bond strength of resin core pastes using auto-cured adhesives. Jour Dental Res. 81:Spec, Abstr #1153, Mar 02

1 Craig, R. G., and Peyton, F. A. The microhardness of enamel and dentin. J. Dent. Res., 37(4):661-668, 1958
2 Craig, R. G., and Peyton, F. A. Elastic and mechanical properties of human dentin. J. Dent. Res., 37(4):710-718, 1958
3 According to mfg
4 Reality - Vol 16, 2002
5 Latta MA, et al. Physical properties of two chemically-cured core materials. Jour Dent Res. Abstr #2666, Mar 02

Figures 1&2: As you can see in this mirror shot (above), the bicuspids appeared ready for the “biologic waste” bag.
But the radiograph below showed the bone was sound and the pulps were nowhere near the action.

Figure 3: Even after caries elimination, I was far from the pulp. Though discolored, the tooth structure was perfectly sound.

Figure 4: “No-Etch” bonding agents complement the speed and ease of these automixing composites. (But whatever agent you use, be sure to confirm that it is compatible with your core resin.)



Figure 5: Because they eliminate the need to isolate, etch, rinse and dry, no-etch agents really speed the process. A single drop of Touch&Bond was sufficient for both teeth.



Figure 6: Before attaching the mixing tip, my assistant expressed a small amount of material to assure that both sides were flowing evenly. As you can see, in this case one side started slightly before the other. . If she had simply attached the mixing tip, the first material I expressed wouldn’t have been a proper 1/1 mix. But now that they are both flowing she can safely attach the tip.



Figure 7: With the small intraoral tip attached to the mixing tip, it’s easy to express core material directly from the cartridge onto the tooth.



Figure 8: I built up both cores, one tooth right after the other. Notice that the material held its shape nicely, like soft ice-cream.



Figure 9: Because the core resin is dual cure, I could zap the exterior with my light and know that the interior will self-cure.



Figure 10: Core material mimics the hardness of dentin (not enamel), so tooth preparation is a piece of cake. My handpiece doesn’t jump as the bur goes from tooth … to core material … and back to tooth. The entire procedure for both cores combined took just 20-30 minutes.



Figure 11: Prior to cutting the bevel, I refined the gingiva around the crown margins using my electrosurge.

Figure 12: Notice that the margins will sit on 1.5-2mm of solid tooth structure to create a ferrule.



Figure 13: The final all-ceramic crowns.