Lecture on Dental Materials P2


So as we bring that process away we lose some of our protective barrier. Underneath this we have dentin and so we’ve got a dentin structure that provides for us, again you’ve got a number of these occlusals that are oriented in different directions, so they take an orthogonal profile but they take all different orientations as we move through the dentin. And that provides for us a highly tough material, but an anisotropic material because these all take profiles and so they are – in this configuration underneath the enamel and as we rotate around the pulp it actually starts to spiral around, so it becomes orthogonal by the time we get to the root. And so these perform different functions, we tie into the periodontal membrane and the bone below and so again very much like cartilage where we actually tie-in we become orthogonal in this direction here and then as we’re up here these periodontal structures actually take the perpendicular to the enamel. So they actually scale themselves as needed relative to load and structural support.


The pulp from a bio standpoint, again very important is our blood supply, it’s the nourishment. So we have to remember that we’ve got a lot of cellular turnover just like we have in bone. So we actually rely on structural remodeling of this material. We have a periodontal membrane, so again we’ve got a structure between the bone itself, so we’ve got our bone, we’ve got cementum layer, professor Ritchie talked little bit about this, we’ve got a periodontal membrane and then we’ve got our vascular and nerve supply, which is why if any of you’ve actually gone through a large temperature range, you probably hit strains in your tooth that in some instances gives you sensitivity, that’s the nerve endings. So anything that does that — anything that we can do that actually brings about nerve response you’re going to feel it, right?


So on my cartilage this is very similar to bone in terms of nerve supply and blood supply. So you’ve got an interesting anisotropic structure that provides for you some unique properties. This is taken out of the paper that’s posted, so it’s the Journal Of Dentistry, again it’s a structural paper. [The Marshall group], UCSF School of Dentistry, they’ve teamed up with professor Ritchie, they’ve done a lot of fracture mechanics works. They’ve also done a lot of nanoindentation work. So they’ve done a lot of nice work where they’ve taken these structures in cross-section and only looked at them micro-structurally which is part of that paper, but they’ve also proved them with a nanoindentation technique. So you can take this in cross-section and then you can actually probe about what the harness is as you move from the enamel to the dentin and through the junctions and this ties in nicely with looking at fracture mechanics issues which are micromechanics based. So you can look at the actual orientation of your occlusals, you can look at the relative mechanical properties and you can break it down to a nano-scale.


So this is a big challenge not just for teeth but all biological materials, when you have a higher RQ like this, whether it’s bone or whether it’s a dental tissue, how do you actually get the mechanical properties of something complex like this? So just going back to what we know about mechanical testing back to our E 45 days, you’re not going to machine this into a little tensile dogbone, right and just go pull on it, get a modulus. You could but what does it tell you? It’d give you some globally averaged tensile modulus. So you could machine this into a little plug and you can load it up in compression and again you could get a globally averaged parameter but I wouldn’t really tell you about what the different constituents contribute and the same issue with fractures, it’s really complicated to try to break apart the fracture process. But if you could have a technique that can come in and actually probe out mechanical properties at these microstructural levels you can get a better understanding of what each of these contributes, which is why nano and microscale mechanical testing is really important for us today.


Questions? (inaudible)


Lisa Pruitt: Well you can do it. Okay. So you can do it in few planes, right? One would be that we’ve taken in this plane here, so we could have dentin and a lot of times you want that, right? Because you want to move through the enamel dental junction. So you’ve got dentin, you’ve got this structure here. So if I can have it in cross-section and then pot it top down, I can come in and let this be my nanoindenter tip, I can come in, in cross-section, so now if I look at this inside profile I’ve got this mounted in cross-section, I can come down with a small tip, and I can actually march across and measure low displacement behavior as I move through different junctions. Question?


(Question Inaudible)


Yeah, that’s right. So okay, okay – so then you bring up – so the question is. That’s okay when we’re talking about enamel, but actually, even with enamel, because — the assumption with enamel is because of its crystalline structure and the scale of the crystalline structure that is a more isotropic structure altogether. So we just assume that you got isotropic behavior, which means we can approach it from any angle and the properties okay. So one way to handle that would be you could do different cross-sections, which is what we do a lot of our tissues. So you could take different cross-sections almost – you had your histology lecture? Okay. So in histology we take a very thin section of tissue, you could take different sections in different orientations and so this could be cross-section in this plane, we could then do cross-sections this way, right? So we could do something like this and then we could do top-down indents, so we could march across this way and get that direction.


But what these techniques allow for is for you to dissect the problem. So it allows you to look at different orientation effects, it allows you to come into the tissue in this orientation and then it allows you to take a different cross-section and come out of it from these sides. So you look at different effects of orientation. And the comment being if I just tried it to load the whole structure instead of taking cross-sections or are looking at very tiny parameters, a globally averaged, and so I can’t look at the role of orientation of a tubule, I can’t look at what happens to dentin by itself is maybe you changed a drug, something that didn’t come up with Professor Ritchie’s lecture, he talked a little bit about pharmaceutical treatment. There is a lot of work in bone right now, where if you look at pharmaceutical treatment, question is what happens to the quality of the bone? And again if you try to macroscopically characterize the mechanical properties you might miss out what goes on at the microstructural levels. So that’s where combinations of imaging with things like micro CT coupled with nanoindentation and modeling become very powerful tools.


So some of the classic mechanical testing protocols that we learned in E45 that worked really well for steels, well for engineering materials probably miss we have these highly complicated hierarchical structures. But that’s a challenge, how do we learn from biology and also it teaches us because it lets us ask the question of how can we better design engineering materials to give us this type of wear resistance and this type of fracture resistance? Most of us would love to have tooling materials that would have the same wear resistance as enamel, right? We put diamond like coatings on carbide or carbide treated steel to get better machining properties and reality is we don’t even come close to what we get out of biology. So there’s a lot — it works both ways, we can learn from tissues to understand disease but we can also learn from healthy tissues to better engineer materials.


That’s a bizarre looking plot, huh? So the concept of what I was trying to teach here, we should probably have a lecture on nanoindentation one day but the idea is that we would come in with an indentured tip into this structure. It would make an indent into the material and we can get low displacement behaviors. So we can monitor load and displacement and from that we can back out of unloading stiffness, and we can get a representation of the elastic modulus. And so the nice thing about doing to set nanoindentation scales is we can change out that tip geometry to be anything from a diamond pyramid tip on nanometer length scale all the way to a spherical tip that span several microns. So you can start to probe out with this technique nanometer length scales all the way to micron or hundreds of micron length scales, that becomes important because then you can start to see well, what are the cellular contributions, what are the trabecular orientation contributions, what are all the sub-structural contributions? So there’s not one biological tissue that isn’t built on hierarchy of these constituents. And so if you really want to get clear about the mechanics you have to start asking the deeper question because if we just go to this idea of well, we will make a little dogbone and we will load it up and get stress strain behavior, well that’s fine but it globally averages everything. And so we miss on all the constituent elements.


And same thing if we were to do a little compression test, we could still get a compressive modulus, we could still get compressive yield but we’re globally averaging everything that goes on. So we have no way to deal with size scales in that context.


So again this is — if you’re interested in that type of work in dental, this is a good group to watch for. The Marshall group has done a lot of work on looking at nanoindentation and how it plays a role on the basic mechanical properties and then recently they’ve coupled as I said with Professor Ritchie’s group looking at how the microstructure plays a role in fracture mechanics. So it’s a good combination, always when you can get your biological groups together with your mechanics groups. And hopefully you’re trying to learn out as we go through the course, right? There are some benefits to both, there’s benefits of understanding biology and structure and there’s benefits to really understanding mechanics and if you can bring the two together you’ve got a lot of power.


Okay. So again just different constituents, you’ve got enamel, so again very unique material and offering wear resistance. So it’s our hardest substance in the body, it’s calcium phosphate salt type crystals, they are large hexagonal type structures. Again nanometer length scales, so again that’s that whole nanostructured material, dentin composed largely of type 1 collagen fibrils, so again you’ve got a lot of fibrous tissue and you’ve got it blended with nanocrystalline apatite mineral. And then as Rob said very similar to bone in its microstructure. So that’s another thing you want to think about, every time we’re studying these materials we want to ask ourselves where can we learn from, so the literature from orthopedics offers us a lot of insight to dental and vice versa.


Dentin, the dentinal tubules radiate from the pulp. So we saw that in that one image. So we get radiation of these tubules and again they’re just marching around in a radial orientation. So they’re taking different orientations depending on where we are whether they’re tying into the jaw, whether they’re supporting below the enamel. The pulp is again almost like our bone marrow, it’s what provides for us a lot of the elements of vascularization and blood supply, it’s innervation, it’s a very important structure and so we talk about root canals, you get a lot of work into what happens at the pulp level.


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.

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