Lecture on Dental Materials P3

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.

 

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.

 

Comments on the mechanical property aspects, I don’t mean to be negative, it’s just — I want a great sense of awareness I think from the class, so you’re going to your case studies, I think the case studies we’ve chosen for you that come from the literature are from good scientific groups. You always want to be looking at these papers with a critic’s eye. You always want to be thinking about what were the conditions for which the data was collected, what are the conditions for which the analysis is done, so when you’re looking at failures what’s the pool, are you looking at pools of athletes for these implants, are you looking at pools of people who chew eight packs of gum a day versus one pack of gum a day, there’s all sorts of conditions that you want to think about.

 

So for our dental biomaterials we’re going to see a lot of similarities of what we’ve seen in orthopedics but we’re going to see some subtleties. Again we’re going to touch on the one subtlety, which is temperature. Amalgams which was much more common in older days, but we still refer to that technology today, or what we loosely call fillings. So if any of you ever had a cavity and again cavities are not nearly as prevalent as they were before, we have fluoride treatments.

 

Implants, again you could have loss of tooth for a number of reasons, right? You could have loss of tooth because of loss of structural support. So you could actually have loss of support of the underlying bone. You could have poor mechanical loading of the teeth itself. You could have a brawl in the bar. You could play hockey. There’s number of reasons that one can lose a tooth. And with that there’s a lot of technology involved in what do you do to restore a tooth. The worst thing you can do is not put the tooth back in, because when you don’t put the tooth back in, then all the other teeth get loaded in a flexural mode because the bending orientation’s changed. All the stresses on the underlying bone structure of the jaw also change and so again you just start a process essentially like osteoarthritis where you get some of those effects, or osteolysis where you change the bone structure and then you actually start to have bone loss.

 

So when we look at fillings, again we’re going to look at just a few scenarios. Amalgams, acrylic resins, so this would be polymer resins or polymethyl methacrylate type resins. Titanium dominates when we look at dental materials, because when we look at either tying into the jawbone or for support you’ll notice – in fact, you will notice a very similar technology to what we see in orthopedics, right? You will see a polyethylene liner, you see titanium backing but you get really osseointegration, you get good mechanical loading, when you talk about anything that gets embedded into the jawbone you’ve got a 99% chance that it’s titanium based.

 

Teeth, again when we talk about the tooth itself, you’re talking about the crown, you’re talking about – if someone actually needs the dental implant, you don’t give them a titanium tooth, we give them a titanium abutment substructure and then you attach to that porcelain a resin or ceramic, right?

 

Braces, pretty much dominated by two materials: stainless steel, which are continually loaded through plastic deformation or tightening of the wire or Nitinol, which is a constant low-force mechanism and then again, your basic acrylic resins, so this is really where we borrowed in orthopedics this whole technology of having a very good adhesive that could bond between bone and a metal. So we learned a lot from adhesive technology from the dental community. So the whole acrylic-based polymer what also builds us bone cement came from dentistry.

 

So again, motivation to replace a tooth, there’s is root support and chewing efficiency, there’s prevention of bone resorption, but most of all it’s cosmetic, right? Most people don’t want to walk around without a tooth present. So there’s the cosmetic component of all that. But there is a real mechanical issue here, so there is root support and actual prevention of bone resorption. So it’s very similar to the stress shielding issue, you need to have bone loading, which would have come from the tooth and you need that stress transported back to the underlying bone, you take the tooth away, you take the stress away, you take the bone away and when the bone resorbs, then the adjacent teeth go. And so what starts as a slightly unpleasant cosmetic appearance becomes a very unpleasant cosmetic appearance very quickly.

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