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

September 14, 2013 3:44 pm | By More

Introduction

Okay. We will kick off lecture today. We’re going to move onto dental tissues and their replacements. It’s actually probably a good timing following up on Professor Ritchie’s lecture where he talked about bone fracture and moved that into the role of fracture in teeth. Actually if you read the College of Engineering LabVIEW notes this month, the article featured actually is on Professor Ritchie, I think you will find that interesting, he talks a lot about how it’s moved from fracture of ceramics and engineering materials into fracture mechanics of bone. So it’s a nice short article that I think you’ll find quite relevant.

Dental tissues and replacements overview

Okay. So we’ve got a few examples with us today that we’ll go through as we get to the end of class but we’re just going to start with just a basic overview of dental tissues and their replacements and so the first slide just really is an overview, talk a little bit about the structure of a tooth, we’re going to look at this in cross-section in terms of structure. So again this builds on what Professor Richie talked about in terms of the constituents of occlusals and how they’re oriented and how that plays a role in the basic mechanical properties. Then we will look at what it takes to actually replace a tooth. So obviously there’s a cross-section, you can see the screw threads here. This is not a real tooth, this is a man-made replacement. And then we’ll look at just titanium and osseointegration and then we will finish up with TMJ implants and look at some of the issues there.

And one of the things that you will hopefully see as themes that a lot of the dental issues and dental materials very much so are like orthopedics. In fact, the two fields go hand-in-hand. We borrowed some things from them. Bone cement is namely the primary material we borrowed from their community. So the dental adhesive that many of you have probably been exposed to is really the same basic chemistry that gave us bone cement for the connection between the metal implant and the bone.

So dental issues, when we think about orthopedics it’s easy to think about total hip replacements, total knee replacement, shoulder replacements or something afflicting the elderly with osteoarthritis. We can get a little more close to home with athletes when we talk about ligament tearing or tendon rupture, or even talking about meniscus tears of the knee. And so then the athletes start to have some relevance. But when you talk about dental, it’s right down to childhood. So you can start to talk about dental decay and loss of teeth right down to a small child who actually has an appropriate tooth protection. And a lot of this has been channeled by the use of fluorine in our water to actually change the solubility of the enamel and also to improve some of the mechanical properties. So lot of people today in this culture don’t experience some of the dental decay that had been experienced in previous decades.

Periodontal disease

Periodontal disease, so again this is disease where you look at the loss of the bone in the gum line, which then becomes supporting structure for your teeth and so as we lose bone, whether it’s due to by a mechanical loading or whether it’s due to disease or biochemical factors, that becomes a support structure for the actual tooth itself. How many of you have worn braces of some sort? Okay. So here we go, there’s a relevance. Orthodontics, so we won’t really spend a lot of time in here on that but understand that that’s an enormous concept and of mechanical loading of moving teeth. So it’s moving teeth, but it’s also remodeling the bone and actually movement of the supporting structure around the tooth structure itself. So a lot of remodeling has to occur. So again there is a lot of linkage to orthopedics when we talk about orthodontics, and we’ve got two guest lecturers coming up, one from Nitinol Device company and he may not only talk about nitinol as a material for cardiovascular issues and stents but he may actually bring up the use of nitinol as a good material for braces or orthodontics because you could have low force control material because of super elasticity.

And then restorative treatments, and restore treatments can be anything from actually putting on a ceramic crown or just actually doing a reblend of the tooth structure. Something we’re going to see in dental materials that we don’t really see anywhere else are thermal expansion issues and hopefully that makes sense, right, I am sipping here on a hot cup of tea and right before I had my cup of tea I was drinking a cold cup of water. So immediately you start thinking about your temperature differentials that you put in your mouth, right? So every time you’ve had a nice ice cream cone and followed up with hot chocolate you’ve run your tooth through a large delta teeth. So. Delta teeth is really something we don’t tend to think about in the body right, we tend to think a 37 C operational temperature for physiological conditions and plus or minus a degree or so depending on what the situation is but that’s about it. You start talking about dental applications and you could easily have a swing of 50 degree C. So thermal expansion issues are there, and they are there cyclically every day.

So remember we talked about fatigue issues, we said well, fatigue was due to cyclic loading of material, where you can cyclically load due to mechanical issues but you can also cyclically load because of total expansion issues. So every time you put a filling in, you need to be thinking about what the thermal expansion of that material is relative to the tooth surrounding it. So you load up internally with stresses just with the things like amalgams and resins just because of temperature fluctuations.

Fatigue and fracture, so again large mastication forces, so we chew and most of us chew more than a couple times a day, right. So we’ve got large cyclic loads, you can have forces as high as 900 Newtons acting on a tooth. So you get pretty high stresses and you get large levels of fatigue loads. So fatigue issues are important. And fracture, it’s a fracture of the tooth, not many of us have probably fractured a tooth but certainly fracture of a tooth is an issue. It’s more – I think as we talk about aging and disease with dental is more that we have loss of the tooth structure, so that the loss of the jaw bone – so we will talk a little bit about that today, but you could actually need to rebuild the joint structure for movement, but you can actually have loss of the bone that actually supports the structure around the tooth itself. So this is an indicator here just the tooth loss because of poor bone support. And once that process starts, it’s like everything else in orthopedics. You can change the biomechanical loading, once you lose one tooth you’re setting yourself up to lose multiple tooth, so it’s not a singular event. So it’s one thing to lose your tooth in a bar brawl, it’s another thing to lose it because of biomechanical loading.

So you probably have seen this in your dentist’s office, like how to brush your teeth. This actually – I think a pretty nice schematic, there’s two in the posted slides, one that comes out Dr. Sally Marshall’s paper, which I think is a nice read, it gives you a little bit of structure on dentins that’s the PDF that was posted. Again a lot of that work becomes collaborative with Professor Ritchie’s work so that they have teamed up to look at the role of all these constituents on the fracture behavior of these materials. So again you’ve got different structures acting here. So enamel probably just from Trivia you realize is the hardest material in the body. It’s a material that provides great resistance to damage to the underlying structure. Its role is really important because enamel is the bearing surface, it’s the one that comes in contact with other teeth, it’s the one that when you’re chewing gum, it’s being subjected to the continuum mastication and the chewing forces, as you’re eating food, it’s always in contacts. So high abrasive forces, so all the same things we had to think about for the hip and the knee suddenly come into play for the loading of the teeth, right?

So if we just think about this for a moment, and we just back off for the image of this tooth we can expect high compressive forces, we can expect shear forces, most of us, it’s right after lunch right, we kind of think about how we chew we also have it out of plane motion, right? So we probably also get a little bit of a torquing motion on that jaw. So you get high compressive axial loading, you get shear just due to the motion of the teeth and then you can also go little bit of out of plane because our jaws actually have out of plane motion as well. So you’ve got high, high levels of stresses and these stresses also have contact. So we don’t just think about forcing on the tooth itself, so we’ve got this enamel structure. We think about its counter bearing, so there’s another tooth that comes in and meets the structure. So you’ve also got contact tissues, so you’ve got dental contact and suddenly it’s not looking so different from orthopedics, is it? So suddenly it just looks just like it could be a knee joint, we still have high compressive loads. We still have actions of shear. We can still have out of plane motion. So we can essentially have the rolling sliding type combinations that we had in the knee and we remember when we talked about orthopedics that every time we had contact, then we also had stress distributions that build up due to contact, right? We also remember that we’ve got stress profiles that build up under these contact zones.

So we’ve got compressive forces, we’ve got shear, we’ve got torsion, we’ve got contact. We have cyclic loads. So immediately we need to be thinking about where, we need be thinking about fatigue, and we need to be thinking about fracture. And so these types of stresses as overloads can give us a fracture scenario. So that when we talk about materials that go in and try to replace the dental tissues, we have to remember what they are being mechanically subjected to. This is just the mechanical aspects, and then we have to remember that inside this situation unlike the joint we’re also going to have a delta T component. So we’re going to have a strain that’s going to develop as a function of our thermal expansion coefficient and a temperature range. So we’re going to see strain buildups that can occur just because of thermal expansion and thermal expansion mismatch when we talk about fillings.

So that the stress states when we talk about dental our — again very similar to what we see in orthopedics added with that the thermal effects. Okay, so let’s just look at the tooth again. So in terms of enamel, the role of enamel is really to provide wear resistance, it’s to prevent fracture and fatigue in the sense that — again we’re going to go back to what we learn in orthopedics, anything that we can do to prevent the initiation of a flaw is going to be good for fatigue, propagation and it’s going to be good for initiation and propagation and it’s going to be good for fracture toughness. So anytime we have good mechanical integrity of enamel we’re setting ourselves up for the good protective shield. Most of us know or have experienced at some level, what can happen with loss of enamel. So loss of enamel can come about through resorption abilities, so relative changes in saliva, pH or fluoride treatments can make the enamel more porous, more susceptible to damage. You can have mechanical fractures of the enamel itself and you can have just loss of the enamel over time just because you literally wear it away.

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.

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