Fluorine, and again we all have seen fluoride in our toothpaste, fluoride in water treatments. It renders the enamel much less soluble. So again it’s your first line of attack for wear assistance, it’s your first line of attack to any substructural damage or cavities if you will in the dentin and it’s really controlled by solubility. And there’s a lot of issues about pH and saliva quality as well. So depending on what dental journal you pick up the focus changes dramatically from a chemical loaded factor versus the mechanical load factor. And just the basic chemical composition of hydroxyapatite. So again just highly crystalline structure predominantly isotropic relative to the role of dentin.
So again this is a more fibrillar structure, so here’s our dentin, you’ve got type 1 collagen fibrils, you still have nanocrystalline apatite, but this time they’re dispersed. You’ve got tubules from that dentin enamel and the cementum enamel junctions to the pulp. So again those tubules are radiating out all the way around and those channels are passed through the odontoblast. So that’s your dentin forming cells. So again a lot of similarity to osteoblasts which build bone during the basic process of remodeling or dentin formation and then you’ve got mineralized collagen fibrils. So again not so dissimilar from bone, you’ve got a lot of collagen in bone but you’ve got a lot of mineralization and these are arranged orthogonal to the tubules. And so again you’ve got a fibrous component that gives you ductility and then you’ve got a rigid component that gives you hardness and strength. And then you’ve got inter-tubular dentin matrix again with nanocrystalline structures. So you’ve got a really unique microstructure built in here. So nanocrystalline and isotropic, highly oriented for very specialized properties.
And then just a relative comparison, there’s lots of places that you can find properties. Again just a comment, this is actually taken out of Biomaterials, the textbook by Park and Lakes, podcast here, (inaudible) correct which is a reasonably good book, because the nice job of reviewing things, it’s just a lot of times he has to rely on what the current literature was at the time and in doing so what you will immediately see is that there is singular values plotted here. So for enamel you see a basic density of 2.2 versus dentin of 1.9. So that makes sense, you’ve got a highly crystalline structure, a lot of repeatability, a lot of ability in spatial form to pack a lot of very tight crystals together. So you’ve got higher density. Dentin, you’ve got radiated tubules, you’ve got more fibrous structures, so you expect the density to be lower.
Elastic modulus, so again this is just a chart that I took from that book. It just gives you a singular tensile modulus. So you might ask yourself, is that the modules that I want? Probably I’d be thinking about compressive modulus, I might be thinking about shear modulus, I might even think about flexural modulus. Those tests are really – how do you — so then, okay that’s easy to be at the critic how do I get those properties, which brings us back to that earlier plot, how do you dissect enamel which has got a length scale that’s very small and how do you get those properties? And so you tend to get a globally averaged value, you isolate it and you get a parameter that gives you a measure and then 48, what they don’t tend to give you in the older literature is 48 plus or minus what? Right, so how many of you are doing biological research? Okay. You want to take a guess of what the plus or minus what would be? At least try. Chang, nanoindentation work, plus or minus what percentage? So variations and that sounds like we don’t know how we are doing in the lab, right?
But the variations between one person’s tooth versus another, so what’s your population that gave you that data? What was the orientation of that? What was the quality? And so just to encourage you to think about these things when you see these lot of textbooks, right? Because everything is nice and easy, there’s little – there’s the chart right there, they put it here for a reason, because they are there, it’s a singular value 48 gigapascals. So what it — the take home message there would be it’s deep. Okay. it’s the hardest material in the body, it’s highly crystalline, so it’s got a high density, you expect it to have high hardness, high modulus. But don’t ever assume that when you see a singular value in biological tissues, that value has meaning, okay. That is a representation for a given set of data and only a given set of data.
Same thing, at least now we know we’re talking about compressive strength, right? So again that would be globally averaged from real compressive tests but again we have to take that from what’s the source, are these 20 to 30-year-olds, are they, as Rob said, are they the people that haven’t had alcohol in their mouth, that makes a difference in the tooth structure. So there’s also parameters with the environment, and again just relative to dentin, so what I tend to — my general rule is this, I tend to look qualitatively at data when I see these things. So I am more interested in comparisons. We expect that the density is higher for enamel versus dentin that’s there. We expect to have a much greater stiffness for the enamel versus the dentin, that’s there. We expect to have a much better compressive strength for the protective enamel coating, again that’s there. It’s not that this isn’t a good starting point, it’s just that you should expect a pretty large standard deviation because of the biological variations between people and the variations in just basic biological structures.
And then we’re going to look at these again in a moment as well, thermal expansion coefficients. So that gets tricky too, when we think about thermal expansion coefficient measurements, I don’t know if any of you have ever done this, it’s really nice when the material is isotropic, right, because we can then run it through a delta T, and we can make displacement measurement, and we can say well, thermal expansion coefficient for steel is X and have some confidence in that number with a really tight standard deviation. When we start thinking about thermal expansion coefficients for dental or other tissues, we really get stuck with what’s the orientation effects because obviously fibrils are going to orient or expand differently in one direction and then will in a different direction. So again, you tend to get globally averaged values and probably if you look in the literature you won’t see thermal expansion properties of any other tissue, but dental tissues for the reason I mentioned before. So for the most part we take the body to be 37C, but we assume that the mouth gets loaded not just mechanically but thermally.