I do a lot of crown buildups …and I blame Dr. 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 GO A WHOLE LOT SIMPLER
|Table. Automix Core Composites: Properties and Costs|
|*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. Craig RG, Peyton FA. The microhardness of enamel and dentin. J Dent Res. 1958;37:661-668.
2. Craig RG, Peyton FA. Elastic and mechanical properties of human dentin. J Dent Res. 1958;37:710-718.
3. Parkell Today. 2002; April:22.
4. Miller MB. Core Materials. In: REALITY. Houston, Tex: Reality Publishing. 2002;16:129-147.
5. Latta MA, Kelsey MN, Naughton. Physical properties of two chemically-cured core materials. J Dent Res. 2002;3(Abstract No. 2666): 335.
Over the past 2 years a new category of composite resin has been developed specifically for cores. The two I’m most familiar with are Jeneric 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 (see Table).
Don’t confuse these materials with conventional restorative composites. The objective of a good restorative composite is to simulate the smooth, translucent, nonstaining, 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 I should add economy and make it four words. Build-It is just a fraction of the cost of other 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 buildup 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 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 buildup. 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 (4 minutes in the case of Absolute Dentin). 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 for just a moment while prepping and you’ll have a tough time telling tooth from core material.
Buildups have never been this fast—no crown formers, no etching. While in some instances, a crown 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 eliminating 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 2 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) (Parkell) 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.
|Figure 1. The bicuspids appeared ready for the “biologic waste” bag.||Figure 2. 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. Before attaching the mixing tip, my assistant expressed a small amount of material to ensure 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.|
|Figures 6a, 6b, and 6c. After curing the bonding agent on both teeth, the core material is expressed directly onto the teeth. Both cores are built up, one tooth right after the other.|
|Figure 7. Notice that the material held its shape nicely, like soft ice cream. Retraction cord is placed.||Figure 8. Because the core resin was dual-cure, I could zap the exterior with my light and know that the interior would self-cure. Electrosurgery further exposed the crown margins.|
|Figure 9. 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 preps took just 20 to 30 minutes.||Figure 10. The definitive crowns bonded (cemented) in place.|
In Figure 1, 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 often have calcified, nonreactive pulps like this. Both teeth were asymptomatic, so their only crime was utter dilapidation and lack of function.
The dearth of chewing capacity was made clear to the patient using a dramatic digital image. 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. There was no pulp in sight (Figure 3). The dentin was remarkably sound—discolored, but rock-hard. The bonding agent was applied (Figure 4). Using a “self-etch” system is the other half of the secret to rapid-fire cores. Of the several I’ve used, I like Parkell Touch&Bond’s ease of use. The total bonding procedure for both teeth took less than 2 minutes, and required a single drop of bonding agent. To summarize, the steps are:
(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 seconds.
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 to ensure that both components are flowing evenly (Figure 5). 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 6a and 6b), I “served up” the core material (in this case, Absolute Dentin) 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 6c).
Immediately after light curing the resin (40 seconds total), I rough-prepped the buildups and packed retraction cord (Figure 7) while the material within the core self-cured. Using my electrosurgery unit, I further exposed the crown margins before beveling the prepared shoulders (Figure 8). Remember: core-retained crowns are best served by providing a 1.5- to 2-mm ferrule so that the margins are placed on sound dentin. Modern materials haven’t changed this fundamental rule.
The properly formed crown preps were then ready for a bloodless impression (Figure 9). 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 20 to 30 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 the patient’s bicuspids. (Figure 10).
No, this procedure won’t change the course of dentistry as we know it, but if you haven’t tried one of the new generation of core buildup 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 buildup/prep procedures.
Like the man said, it just keeps getting better and better.
1. CRA Newsletter, June 2000.
2. REALITY. Houston, Tex: Reality Publishing. 2001; 15: 1-183.
3. Latta MA, Kelsey MN, Naughton. Physical properties of two chemically-cured core materials. J Dent Res. 2002;3(Abstract No. 2666): 335.