Dental Materials: Structural Aspects of Biomaterials Lecture – Professor Lisa Pruitt (Transcript)

Cementum, so again this is -coarsely fibrillated bonelike substance, again Rob made a comment about this in terms of the cementum line at that juncture and providing good mechanical properties. You may recall that it was actually that juncture that provided to your fracture toughness of the material. And so when you enter the dental enamel line to the cementum, see if we’ve got picture of this again, you’ve got your enamel, you’ve got your dentin, but you also have the cementum structure. And so the cementum structure actually marks you as the transition zone between them and so this place is where you stop cracks and this becomes a source of how you actually create (inaudible) lot of toughness. So mechanistically very similar to what we see in our bone materials.

And then the periodontal membrane is very much like what we see at the baseline of cartilage into bone. So it anchors the root into the alveolar bone. And so a lot of times when we talk about loss of bone it becomes loss of connection to the substrate of the bone structure. So you’ve got a bone line, or jaw bone, that runs underneath the teeth, the teeth are embedded deep into that bone structure.

So again just a little bit of the biology of the tissues, from the enamel you’ve got 96% mineral. So you’ve got 1% protein and lipid, remainder balance — small balance is water, they’re long crystals hexagonal in shape. So you’ve got little single crystals at the nanometer length scale. So again in terms of materials research, a lot to be learned here. They are 48 nanometers in their hexagonal diameter. But they are thousand nanometers in length.

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.

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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.

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Elastic modulus

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.

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