Nothing’s Perfect – DEAL WITH IT.

In lab tests, all endodontic sealers leak to some degree. All of them. The Journal of Endodontic Practice recently published an extensive research review strongly suggesting that creating a perfect apical or coronal seal is impossible.² But that doesn’t mean that the materials you choose or your technique aren’t important. Perfection doesn’t seem to be necessary for clinical success. When both obturation and the coronal restoration are judged “good” (notice I didn’t say “perfect”), the clinical success of RCT runs more than 90%. When both are judged “poor,” clinical success drops to less than 20%.³

Figure 1 . Roots were filled using a single point plus sealer. In this magnified view of the MetaSEAL sample from the occlusal you can see where the sealer penetrated the gutta percha point to bond to it. You can also see where the sealer penetrated into the tubules.

A good seal under a wide range of conditions

The team of polymer chemists who developed MetaSEAL™ focused on creating a resin that not only provided a good seal, but provided it under a wide range of clinical conditions. They wanted a simple technique that didn’t involve priming. And they wanted it to work with both gutta percha and resin points plus all the common irrigants.

At the latest symposium of the International Association for Dental Research, a couple of interesting papers suggested that they achieved their goal.

For example, researchers in Turkey compared the 1-week and 3-month apical leakage of RealSeal-R (Pentron), AH Plus (Dentsply) and MetaSEAL (Parkell). After 4 months there wasn’t much difference between them. But at 1 week MetaSEAL was the clear leader. The most effective early seal was provided by MetaSEAL with a gutta percha point. The second most effective was MetaSEAL with a Resilon point. Then came RealSeal with Resilon and finally AH Plus with gutta percha.⁴

In another study, endodontically-treated roots were cut into horizontal slabs, and the interface between sealer and canal examined microscopically. MetaSEAL with gutta percha showed significantly more gap-free interfaces (about 40% fewer) than either Epiphany with Resilon points or AH Plus with gutta percha points. ⁵

Figure 2: Traditional self- and dual-cure systems (top) distribute free radicals more-or-less evenly throughout the resin. Since polymerization occurs everywhere at the same time, the material shrinks toward the center of its mass ... and away from the canal walls.
MetaSEAL’s unique water-loving catalyst (below) concentrates free radicals along (even within!) the damp canal dentin, so that’s where setting initiates. As a result the resin shrinks toward the canal walls.

Why does MetaSEAL seal so effectively?

Two reasons:

1) When it’s exposed to moisture, the unusual self-cure catalyst in the powder generates free-radicals to initiate cure.

2) Its long curing time assures a good plastic flow during set.

Everything else being equal, self-cure resin tends to shrink toward the spot where it first starts to cure. Take a peek at Figure 2. The “Conventional Resin” on top is a typical self-cure resin sealer with free radicals distributed throughout the resin. As it cures, the sealer shrinks toward the center of its mass and therefore tends to pull away from the canal wall. The result is a gap.

In contrast, MetaSEAL’s hydrophilic catalyst tends to seek out the moisture within the canal wall (Figure 1, bottom). That’s where the free radicals congregate, and that’s where polymerization initiates. As a result, shrinkage pulls the resin toward the wall.

Because MetaSEAL sets slowly, when the curing resin along the canal wall tries to pull resin toward it, the rest of the material is still fluid enough to supply this additional material. The result is a strong, densely impregnated hybrid layer.

So that’s why the researchers found fewer gaps between the sealer and the canal wall. (Or at least that’s what we strongly suspect.)

Taking pictures along the canal

To demonstrate this point, researchers performed RCT on several extracted teeth, irrigating with 3% EDTA and 10% sodium hypochlorite. Then a single gutta percha point was used as a carrier to fill the canal with one of three sealers - MetaSEAL™ – Sealapex™ - or Epiphany™.

Figure 1 (page 1) shows a microscopic view of the MetaSEAL sample from the coronal. You can see the gutta percha point. The dark band around the point is the gp-hybrid layer where the MetaSEAL’s 4-Meta resin has penetrated the point and bonded to it. The white doughnut is the MetaSEAL, and the grey material that forms the rest of the picture is the dentin.

The teeth were sliced vertically and treated with mild acid and bleach to make the features stand out. Finally, the dentin/sealer interface of each sample was examined using a scanning electron microscope at one-millimeter steps from the apex (Figure 3).

At all the examined sites in the MetaSEALed canal, the SEMs revealed formation of a dentin hybrid. The hybrid layer ranged from 1 to 5 microns, and the resin tags ranged from 1 to 40 microns long. There were no contraction gaps.

In the Epiphany sample, no hybrid layer was observed at any site. Though there were some tags, they were relatively rare. Areas of the sealer/dentin interface showed gaps and poor polymerization at the interface. This was presumably due to setting-inhibition caused by the NaOCl.

The Sealapex sample showed no resin tags or hybrid layer, and there was extensive gapping between sealer and dentin wall. Furthermore, the resin appeared to have collapsed along part of the interface. This again may have been the influence of the sodium hypochlorite irrigant.

As I mentioned, the primary characteristic you care about when choosing a sealer is its ability to prevent penetration of the canal. The performance of resin sealers can be limited by the irrigants used during canal preparation. Therefore, to measure the effect of irrigant on the effectiveness of cure, bond strength to dentin that had been prepared using different irrigants was examined. In this case we’re using bond strength as a corollary for the degree-of-cure. We don’t care much what the adhesion is to the canal wall, but we definitely care that the material is efficiently polymerized.

As you can see in TABLE A (below) MetaSEAL’s adhesion to the canal was surprisingly unaffected by the different irrigants.