"My name is Bond. Dentin Bond."
Ahhh, might-have-beens. But I suppose "James" was a pretty good choice for 007's given name too...
The best dental restorations today are adhesive restorations; they're bonded onto or into the teeth that they help repair and maintain. Like all technologies, dental adhesion is in a constant state of change and improvements are being made all the time. Let's look at this vital aspect of modern dentistry and share a little insight into how the best restorations are currently achieved.
There are two fundamental parts of a tooth that we have to consider when we set out to bond to them: the enamel, and the dentin.
When we bond to enamel, we apply and then rinse away a 37% phosphoric acid solution. No worries, your stomach is more acidic than this. The solution etches the surface of the enamel, which is a substance made up of crystals. Highly magnified, it looks like this afterwards:
The surface becomes rough on a very small scale. We then apply a resin (the first part of these composite resin fillings of ours) that becomes micromechanically interlocked into the roughened surface. The strength of this bond is incredible. We can bond composite resin onto teeth in very functional areas and it often stays on for years without breaking or staining. We measure bond strengths in MegaPascals or MPa, and 30 is a very good number to achieve. (Don't worry about exactly what a MegaPascal is; all we need to do for the purposes of this discussion is to look at relative numbers.) We need to hit at least 17 MPa to keep restorations sealed and on the tooth.
The first person to achieve this breakthrough was Michael Buonocore D.D.S., in 1955.
Bonding to dentin, though- that was a lot more difficult. After removing decay with a drill there is a smear layer (yes, that's really its scientific name) of microscopic debris left all over the inside of the cavity prep. This is removed with the etchant. There are also tubules which are normal channels that run through the dentin:
According to the brilliant David Pashley:
"The success of bonding resins to acid-etched enamel is because of the fact that enamel contains little protein, and it can be dried without causing any collapse of the roughened surface. When Buonocore et al tried those same procedures on dentin, they were disappointed to find that the resin-dentin bond strengths to acid-etched dentin were very low (5 to 10 MPa) and were about the same as resin-dentin bonds made to smear layer-covered dentin. They did not know that acid-etching dentin transformed the surface from a hard, mineralized surface to a very soft, mineral-free, collagen-rich surface that collapses when air-dried. Although it is now known that acid-etching removes the smear layer and smear plugs, thereby making the tubules available for resin tag formation, drying the surface collapses the spaghetti-like collagen fibrils, making resin infiltration difficult."
Spaghetti. Spaghetti is the key to understanding all of this. Because once we've etched and rinsed the dentin, it looks like this:
The problem is, we can't leave the dentin soaked, or worse yet, underwater. We have to dry it. (And dentists, being as a class a somewhat compulsive lot, really tend to dry the teeth we work on, with a blast of air that the wind tunnel at NASA's Langley Research Center would be jealous of.) Un-etched dentin has a compressive strength of 20,000 MegaPascals. Etched dentin has a compressive strength of one MegaPascal. Just one. So here we create this wonderful substance to bond to- the dentin has this spaghetti-like mass of collagen which is highly chemically reactive and has a vast surface area- and what do we do to it? We blast it down into a flat lifeless mass, like pizza dough that's been flattened and pulverized and all dried out. The bond fails every time. The stress in the filling material as it cures (usually with a light activation procedure) is higher than the bond to the dentin, and so gaps form. Bond strength is terribly low. Teeth are sensitive. Painful to chew on because they're not bonded, the fillings actually move about when we bite. Cavities recur more easily. The left in this picture is collapsed, the right is properly expanded; in reality it can be far worse than the image indicates:
It's a disaster, and all because the pasta wasn't respected.
What, then, can we do? Well, in 1985 or so, two independent researchers solved this technical problem of dentin bonding with an innovation called self-etching primers. These researchers were Takeo Fusayama and John Gwinnett:
These researchers developed the class of dental bonding agents called self-etching primers. These primers etch and bond all in the first application of liquid, and they take a novel approach- they enlist the smear layer into the bond itself. They churn it up and make a hybrid layer out of it, a layer that is composed of the resin from the filling and also the debris from the smear layer, all activated and mixed into the spaghetti-like mass of etched dentin. This technique can now get into the 32MPa range on dentin, and also treats the enamel properly to get an enamel bond as well. It looks like this:
This technique is so excellent that, since adopting it some 12 or 14 years ago, I can say that I have never had a restoration fail from leakage and lack of bonding in its own right- they may fail from fracture, or decay around the restoration, but I do not see recurrent decay under the restoration primarily; it has to start externally, due to the patient overdoing the sugar and underdoing the flossing.
We still use etch-and-bond separately for front teeth and situations where the enamel bond is the most important factor, like chipped front teeth. This method still gives higher enamel bonds. We bias the relative bond strength between enamel and dentin to the clinical situation. There are also one-bottle self-etch primers now which I suppose are being made to "simplify" the procedure; however for various technical reasons, in late 2010 these products do not get good results at all. In fact there are only two existing self-etch products (both two-bottle) that I'd even consider using to restore a back tooth with a bonded composite resin restoration- Kuraray's SE Bond and Danville Engineering's Prelude SE. They are so superior to any others that they are the only choice in 2010, and there's a lot of science to back it up.
The longevity of these restorations is quite good as well. Silver amalgam restorations seal against the tooth by corroding, a process that reaches its peak at the two year point. After that it's all downhill. Composite resins bonded with self-etch primers seem to hold their bond strength to both enamel and dentin for at least 8-12 years, after a small initial decrease. Their longevity may be quite longer.
For the technically minded in the audience- ok, for geeks like me- here are two of my source articles; they are excellent reviews of the literature and the progress in this vital aspect of dentistry:
The reason for this rather technical review? If you have a composite resin restoration placed and it's very sensitive, especially to cold and to chewing on it, there are three main possible causes:
-The bite is "high" and needs to be adjusted;
-The decay was deep and a root canal issue is brewing; or
-Total etch was employed and the bond is not good. I'm sticking my neck out when I say this, but the science tells us that total etch is too technique sensitive to rely upon, and thus self-etch is the only rational way to restore a back tooth with composite resin at the present time.