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

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.

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.

Biology and structure

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.

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

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.

Mechanical property aspects

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.

Dental biomaterials

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.

Amalgam 

So anyone know what amalgam is? It isn’t that way to get that right. One of the components is and you can see why we don’t have so much of this technology any more, mercury. But there was a time when this was the dominant one and probably if you’ve got grandparents or great grandparents, you may even have a scenario where they had silver or gold as their filling, right? So not very cost effective, but it made for a good filling, mostly because it was malleable and inert. So it’s liquid at room temperature, reacts with silver and tin and essentially forms a plastic mass [at times]. So in essence it was the precursor to the bone cement concept, right? You could have something that was workable, shapeable like a dough and then you could plug it in and it would set. So in a matter of moments, you actually had a hard material that was capable of supporting load.

Nickel titanium, which also we call nitinol. So that’s a 50:50 alloy. Gold, again not so common anymore. Probably the acrylic resins at this point dominate. So those are based on polymethyl methacrylate type resin chemistry, very much like bone cement, sets up very, very quickly. Dental works a little bit different from the orthopedics and not — Dr. Reese made a comment about bone cement, I think I made a comment about the bone cements, Dr. Andy Combs made a comment about bone cement, it’s a two-part system, you’ve got pre-polymerized powder and then bring it in except that it’s a chemical hazard to get the monomer in here. You’ve got a monomer that starts that process, you’ve got a liquid vial and then you’ve got a little packet of pre-polymerized powder, put the two together in a bowl, mix it up and you essentially start to dough a mass. So it starts as something that’s almost fully liquid like pancake batter and then ends up something like plateau. And so in a matter of 3 to 5 minutes you move from a liquid to something that feels like plateau. And as it goes through its polymerization process you’ve got a very high temperature increase, so up to 100 degrees or 150 or so degrees Fahrenheit. So it gets so hot you can’t even hold it in your hand any more. So that part of the experiment is great to show the class, it’s the monomer part. So I used to bring it and do in class and I think truly it’s an environmental hazard. So we won’t do that.

Just take my word on it but that same concept really was important but have any of you had filling work in the last few years of any sort? Did you experience that technology? Probably not. So more likely what they did is that they took the same resin chemistry, but they used a UV curable polymer. So more likely they just set something up in your mouth and then a lot of times, they just do UV exposure and this thing sets up in a matter of moments. But they also have quick setting, but yeah, most of us, right, we have our jaws probe tube and probably aren’t seeing anything. But the chemistry is very, very similar to what we’ve seen in orthopedics and the scent is very similar. So if you recall that odor, you can imagine working in the OR, where you’re going to have 12 to 15 packets of bone cement go by. So environmental issues are an issue which probably explains some of the spacesuit technology. You were in surgery recently? Yeah. So they were in full mask, not just for blood contamination, but there’s a lot of — anytime there’s bone cement you have to look at the outgassing of that monomer. So it’s a real environmental concern. It’s really bad. Yeah, you probably weren’t suited up with that protective gear, yeah.

So if you go and watch a surgery, just be prepared for the other side of that. Actually if any of the nurses are expected, I believe they have to leave the room, it’s that bad. Yeah, so it’s a – there are some downsides of medical technology.

Thermal expansion coefficients

Okay. Thermal expansion coefficients, so like I said, it’s the only place in the class where there is an opportunity to talk about thermal stresses because we really don’t have to deal with that anywhere else. Just a simple analysis, here’s our thermal expansion coefficient alpha, which is the length change delta L, normalized by the initial length for a given temperature. So what that means it is we’re looking at a structure for simplicity sake, which is L0, we subject it to a delta T and then moving through that delta T, we get a coefficient of thermal expansion. So there are only a few scenarios where we have materials that give us a negative thermal expansion. So for the most part we apply delta T, that temperature increase, we move from L to a deformed or expanded length L and so it’s alpha is our change now. So this essentially is this differential here, so it’s the total length change delta L normalized by the initial length L0, multiply that by delta T. And the strain that we get as a result of that – and again we’re assuming isotropy. So isotropy is assumed, the way to get around that would be to do this directionally, right? So just take different orientations and then you could get thermal expansion efficient in a longitudinal axial or circumferential direction.

So the strain is just that thermal expansion coefficient times the delta T. And we look at the biometric thermal expansion coefficient we’ve got the biometric strain, which is three times alpha. So again just alpha coming back to give us deltal L over L0 times delta T. So just a very simple expression.

And I gave you on this worksheet that you downloaded, what happens as a result of thermal expansion coefficient? So it’s a really simple problem. It’s not to show dissimilar the schematic that we have here. Not so dissimilar from what we had when we talked about transferring stresses or looking at composite type behavior. So we’ve got an internal structure, which is in gray. That’s our filling and we’ve essentially reamed out a hole in the tooth structure. So we’re going to assume that we reamed out a nice cylindrical hole. So again you’d have to account for what the geometry is of the hole itself and so we considered here a 2 mm diameter hole, which is 4 mm in length in a molar tooth. So we’ve got a hole that we’ve created, that’s got 2 mm diameter and it’s sitting in a tooth structure. So here is our tooth but we’re just going to schematically say well, it’s got a boundary and we’re going to be [interested]. So we’ve got a diameter of 2 mm and we’ve got a length of that hole which is 4 mm.

And then we look at what’s going to happen to the coefficient of thermal expansion? So in other words, what’s going to happen if we think about this rigid boundary of the tooth acting on this material? So we’ve got a void space and then we’re going to fill that void space with a material. And then we’re going to subject it to a temperature fluctuation, delta T. And then the question is what’s going to happen to that structure? Well, there’s going to be a thermal expansion of that material as it’s heated and then we have to look at thermal expansion mismatch between the enamel itself and the amalgam or resin. So we’re going to just treat this as enamel, and this is going to be our amalgam or resin. And we just look at the difference between the two.

So this basic problem looks at what happens with the delta T at 53 degree C? So we’ve got a delta T of 50 degree C and then we’ve got different thermal expansion coefficients. So we’ve got the thermal expansion coefficient of the amalgam, so there is our mercury alloy of 25 times 10 to the minus 6, again this is millimeters per millimeter. So it’s a length change per original length per degree C. So it’s unit less per degree C or strain units per degree C or strain units per degree C, alpha of enamel. So again a very small thermal expansion coefficient, which probably makes a lot of sense if we think about what thermal expansion coefficient means. So enamel will be a highly ceramic structure. So we’ve got very little exchange of strain as a function of temperature, so only 8.3. And then you look at the polymer, so again we think back to what polymers were, they were these open structured chain materials that are isotropic for the most part, but they’re also randomly organized in space. So there is lot of room for expansion.

And so even though these are somewhat rigid polymers, you’ve got a coefficient of thermal expansion here for a typical acrylic resin on the order of 81 times 10 to the minus 6. So 81 versus 25 versus 8.3. And then you look at – okay, well what’s the elastic modulus of these materials? So the elastic modulus of that alloy, so again steel versus a polymer, the amalgam is 20 gigapascals for elastic modulus. The resin is two and a half gigapascals, so again a much smaller elastic modulus, it’s a polymer. And then we go back to our expression for what’s the change in volume. So again the volumetric strain took the form of three alpha times delta T, right?

So the volumetric strain took a form of three, so we look at delta V, so that was our volumetric strain, when we look at delta V we’ve got whatever the initial volume is and then we’re multiplying that times – three times alpha times delta T. So we’ve got change in volume is the initial volume times three times thermal expansion coefficient and it’s assuming isotropy times delta T. And then you plug this in, and say, well, what’s going to be the change in volume if we use the mercury-based amalgam? So again just geometry, what’s the cross-sectional areas? We’ve got pi times the radius squared. So pi times one millimeter squared times the length, so we’ve got 4 mm of length, then we’ve got three times that difference now in thermal expansion coefficient. So we’ve got 25, which came from the amalgam, and then we subtract away 8.3, so we subtract away the thermal expansion from the enamel, that was to — 10 to the minus 6 power times the temperature flux 50 degree C. And so you get a volume change of 0.03 mm cube.

If you do the same thing for the resin, so again the same geometry would be pi times one millimeter squared, piR squared times the length 4 mm times three, and then the difference would be instead of having 25 minus 8.3, I’d have 81 minus 8.3, same temperature. And so now the volume change is 0.14 mm cube, so relatively large volume change. If I look at just a one-dimensional force span, you’ve got the forces of the elastic modulus times the strain times the area. So you’ve got the elastic modulus times delta T so that 50 degree C times the change that we have on the amalgam resin minus the enamel. So the difference between thermal expansion coefficient, whether it’s the amalgam or whether it’s the resin and subtract away from that the enamel and then you’ve got the perimeter of your pi DH, there’s your diameter, the height. And so you roll that out and you look at the forces and the amalgam force is 420 Newtons, the force in the resin is 228 Newtons. So the forces are relatively high.

But an interesting thing that I put here in gold is that although the resin expands, so if we look at just the delta V, you’ve got a fourfold increase in volumetric expansion. But the reduced stiffness actually results in a lower force. So again it goes back to — you can’t just look at – just when you look at back of the pockets or back of the envelope calculations, if all you had done (inaudible) to volumetric change, you would’ve said okay, just because of that thermal expansion coefficient of the polymer, the polymer does not look like the way to go because you’ve got a very high thermal expansion coefficient, if I run that into biometric changes you’ve got three times alpha, so there’s your roll right here, we’d have a fourfold increase in that volumetric expansion.

But if I convert that back to a force on the actual system because the modulus is so much stiffer for the polymer versus the metal, you end up equalized in terms of the actual forces. So your gut might have been to say, oh, four times the strain, I am going to expect to see a much greater contribution on stress or force. So just little plays on how some of these relationships work. So polymers because they have a lot of modulus make them very forgiving materials in lot of these applications. Question?

Question: Can you explain why you subtracted the alpha of enamel?

Lisa Pruitt: Because you’re looking at the differential of thermal expansion. So you’re looking at what’s the overall change. So you’re assuming at the boundary, the thermal expansion difference between how much – in other words, the thermal expansion is going to occur in the resin but it’s going to also have a temperature effect that’s going to be balanced by what’s going on in the enamel itself. So if you add delta T, you don’t just have the resin expanding, you also have contribution of what your dental tissue is doing as well. So you subtract that away. The same question, okay. Yeah, so we subtract out the counterpart, so we subtract that away because it’s also experiencing a thermal expansion effect.

So again that was very simplistic approach, just taking a simplistic strain, looking at how we can convert that to a simple force but it gives you perspective of just something we would design for differently in dental applications that we would never see in any of the other materials. So delta T issues are an issue.

So if we look at just some of the environmental effects, chewing forces, I think when you first think about dental applications, you don’t tend to think of the forces in the mouth being very high. And the forces in the jaw are extraordinarily high and if you look at [the bright] enamels, you can get extraordinarily high because it relies on their mechanism for prey and predatory effects. But just for a human a chewing force can be up to 900 Newtons. And so you’ve got a high cyclic loading capability. You can have large temperature differences. So we talked about 37 C being the sub-point and here in the mouth you’re looking at potentially a 50 degree C range. So you can run that through and not just singularly in one day but multiple times a day. So you just think about that effect of having something very, very cold, or something very, very hot and probably every one of you has done that, right, at some point, you’ve had something very cold and very hot or vice versa and you probably get a little tinge of nerve response when you did so. So there’s truly a thermal expansion that occurs and you can actually feel that right down to the innervated part of the tissue.

Large pH differences, so again enormous bodies of literature on the role of pH and the role of different types of a composition of saliva in various foods and how that plays a role of pH in the mouth. And it sounds silly, but it makes an enormous difference, large variety of chemical compositions from food, so I am sipping on my — we all have — some of us have coffee, some of us are chewing gum, we’re all loading our teeth in one way or another. So lots of issues.

Going back to what I started with, you’ve got a number of parameters to think about, you’ve got cyclic loads. So we have to think about fatigue resistance, when designing for these TMJ designs there’s a number of issues. There is overall fracture. So you’ve got a post-scenario, so you wouldn’t want to have fracture of the device. You’ve got again a bearing combination. So you’ve got metal on polymer, so we need to be thinking about wear assistance. You’ve got metal that’s now going to live in the presence of saliva, low pH, so you’ve got moisture, temperature and pH issues. So, you’ve got a big-time corrosion problems. And if we thought we had a corrosion design issue when we got to the Morse taper, you stick something in the mouth and talk about having crevice corrosion issues, you’ve got some design standards to worry about.

So the TMJ design probably a little more forgiving in that regard, but you still have to deal with it. You still have to deal with what your local environments are and what the contributions are. We get into an actual dental implant at the tooth level and be subject to what goes on in the mouth itself and corrosion is a big scenario, which is why we won’t stick. It’s typically titanium into a ceramic matted piece of the abutment, so we will see that in a moment.

So I said earlier, titanium is really our most successful implant, primarily that’s a biological but also mechanical issue. Excellent fatigue, total life resistance, so it’s got a very high endurance limit, it’s got good stability. But probably the number one reason is you get good bony in growth. So you get a fantastic osseointegration. So it really is our template for our materials so that Ti6Al4V, the aerospace alloy that we see in the thermal stem is the same material that dominates the implants. And so this is just a nice picture to give you – this is an actual biological tooth just to give you perspective of the root structure so that’s just what ties into the bone. This is the part that we see from a cosmetic standpoint. So we all smile, this is the exposed part of our teeth. So this is our enamel, this enamel has been actually worn away.

So, this is what gives you structural anchor into the jaw bone. This is about the same, obviously same scale, so here’s your titanium implant, it’s threaded. So there’s actual contact stresses or fretting stresses to give you a mechanical interlock. One of the biggest challenges is that hopefully became clear when we talked about orthopedics is we don’t put titanium as the bearing material because it’s soft, it’s got very poor wear resistance. So excellent fatigue, excellent corrosion but it’s got poor fretting resistance and poor wear resistance. So it would scratch easily, it would create debris readily. So lot of times what’s done is as you do a surface treatment, you use ion implantation or ion bombardment. You leave the surface in compression and you make it much more resilient to mechanical damage. So you will see it in some orthopedics, but mostly you will see it in dental where you actually have surface implantation techniques and that actually improves the overall wear or the fretting resistance of that material. And again this shows just our implant on X-ray.

So really where we see titanium do well is just how well it fixates. And so the downside of this – and we will see this again when we get to soft tissues is there is always a trade-off. You want good mechanical integrity, you want wear assistance, fatigue resistance, fracture, corrosion, you want this thing to be stable, right? So you want to implant it, you’ve seen this now in Dr. Ritchie’s lectures. There is a good mechanical interlock, right, this thing is hammered in place. So you got good mechanical fixation or you have bone cement, that thing is not going anywhere, unless you get stress shielding or you get osteolysis and you lose the bony support in which case you’re going to retrieve.

The downside on retrieving and you saw this when Mike took out one of the implants there was no bone left. The downside of – if you get good fixation and the device needs to come up for any other reason is that it’s almost impossible to get these devices out without taking the surrounding bone with it. So if you have a recall of a device or you have inflammation or response to something else and you haven’t had enough bony loss, getting this thing out is a real challenge without losing supporting bone. So it’s always a trade-off, but this material really osteintegrates and so the real scenario for using this is you get good bone interfacing, good bony in-growth to the structure and you get a very good biological shield. So that helps prevent the whole corrosion issue. What you don’t want to happen is put this threaded device down below the jawline and actually have a mechanism by which saliva, food, other items work their shelf down and then you literally are setting yourself up for the crevice corrosion issue, right? You’ve got a very small opening, you get oxygen depletion but if you can get osseointegration all the way round you literally get a biological shield. And so you go through a process by which you can build yourself a shield and it’s a biologically stable one. So this is the real reason that this material dominates the market.

I put this in just for completion sake fatigue issues. So about 1 million cycles annually, I think quite honestly that’s probably on the low side. A typical stress is up to 20 megapascals, so again you’re up to the same stress levels we saw in a new design. Critical crack sizes, again that comes back from back calculating up from the stress 20 megapascals and the fracture toughness, you get a very long crack size. So you get something on the order of meters. In other words, this material has got phenomenal fracture toughness, resilience.

Total life approach even after accounting for stress concentrations, so again you get – if we look at a circular fillet you have a stress concentration factor of three. You’ve got a fatigue limit or an endurance limit for titanium on the order of 600 megapascals. So you’ve got a lot of forgiveness in terms of fatigue resistance. So here’s a material, it’s going to do well in terms of fracture. It’s going to do well in terms of fatigue. And this equation here hopefully that’s very familiar. Here’s your DADN meters per cycle is one times 10 to the minus 11, so that’s C. Here’s your delta K, 3.9 so that’s the old DADN is C delta K to the M.

So for titanium, we said before for alloys it’s typically two to four. So there’s titanium sitting at 3.9 and I said that C was one times 10 to the minus 11, and again that’s meters per cycle. So there’s your funky units. It’s delta K which is megapascal root meters raised to the power 3.9, so you would have that inverse multiply by meters per cycle, that’s the units on C. So C does not have any steady units in case you’ve not figured that out yet. C always depends – yeah, C always depends on what the multiplier is on M, on this megapascal root meter, and that’s a little subtlety that will bug you somewhere probably in your homework, okay, such but remember that C — the easiest way to find C is to always just remember this equation and solve for it, and just remember that M is the scaling parameter.

Okay. This is actually a plot that came out of Professor Ritchie’s group, did a lot of work on fatigue issues on titanium. It’s really that this man not only spent a lot of his time doing fatigue and fracture but also aerospace materials, right? So moving out of aerospace and ceramic materials to dental and bones is actually a pretty good transition. So the only thing you need to really see off this plot is that you’ve got an endurance limit for titanium. So here’s your maximum stress versus cycles to failure. You actually get endurance limit at 600 megapascals. So if you’ve got stresses on the order of 20 megapascals, you’re pretty safe against stress concentration, right?

We said total life tells us about initiation plus propagation, what if something already had a flaw in it. Well, again you’ve got a lot of protection with fracture toughness. So here’s predicted lifetime versus an initial crack length and so — but this is meters, so by the time you get out to the 0.1 meter level, you still get a year of life. So all it takes is 0.01 meters and you’re up towards the eight-year mark and so on and so forth. So if we can only do this well in terms of polyethylene we would be golden, right? So in terms of fracture toughness, years of use, very small crack lengths, all it takes is something on the 0.001 meter crack length and you’ve got decades of use. So titanium is a really good material in this capacity.

So you may think well, do we ever have failures then? And the answer is yes, so we don’t have traditional fatigue failures. We don’t have just a typical monotonic fracture failure, but we can have stress cracking, which is environmentally based. So the coupling of corrosion and stress can set you up for failure of that device. Fretting, so again borrowing back from the Morse taper, the study of the micro motion and continue rubbing of titanium, not having good surface properties, taking away that surface oxide sets you up for wear resistance that is actually quite poor, you create wear debris, you create essentially a third-body wear but you also have a mechanism for loosening or osteolysis. So we do see structural failures, but they typically are combination of stress and environments. So this would be where stress corrosion cracking is an issue.

And again just – these are on your hand-out, this should be an example again of the secondary crack or flaw that’s developed due to stress corrosion mechanisms rather than just cyclic mechanisms. So when you start taking away protective oxides and dropping pHs and in those situations the game changes dramatically.

 Tooth replacement

So just let me walk us through how we actually do a tooth replacement? So this is the actual abutment piece. So we’re going to start with the structural peace, which is going to be underlying implanted device for the titanium. It looks very much like just a [peer-out] mechanical fixation. So internal taper for easy fit, try to avoid stress concentrations, it’s actually got a threaded design, smooth external finish and easy removal of the caps. So again you’re going to switch this out and we’ll walk this through the process as we look at this. It’s going to look a lot like orthopedics. So remember the reamer, so you’re going to drill a hole with the reamer appropriate to the dimensions, not nearly as exciting as watching a hip or knee replacement, okay, very small.

But just take the scale, drop it down, actually in my office I have got very tiny drills if you want to see some dental drills, they are pretty — pretty interesting to look at. They are about the size of needles. Come in, you actually drill a hole with a reamer appropriate to the dimensions of your site. So again very much like what we’ve seen in orthopedics. And then that’s going to be the mechanism for which you place your temporary abutment into your device. So you’ve got your mechanical fixation piece. So here’s your titanium. It’s going to be prepared and actually mechanically fixated into the jawline and then we’re going to have a temporary abutment. So again we have titanium, so we’re going to have this little ball that becomes a temporary tooth and the reason for that is going to become obvious as we go along. We need to have this sealed biologically. We need osseointegration, so we need to have good healing around this. Then we will substitute that out.

And I think when you are in surgeries, an example of what one might do in hip replacement as a temporary, there was a case of infection in surgery few weeks ago and Doctor Reese built up a full mask system out of polymethyl methacrylate. So rather than mixing materials and putting in the full implant, you’ve got a temporary system that would come out. So in this case, also you have a temporary system that comes out in about six weeks time or more depending on healing and then you get a full replacement. So if you look at the insertion, so again if you ream out whole hole, here’s your substructures. So here’s the dental sub – substructure that’s threaded, this is going to then be threaded into the jawline. So again you’ve got your bone structure that this is going to be matted too. And so it’s into your prepared socket and then you’re going to have your temporary abutment. So you’ve got mechanical loading or transfer of load to the jawbone. So here is your titanium device. Here’s your temporary abutment. So initially you actually have a little titanium ball that’s actually screwed in and until the whole structure is healed, you get osseointegration and good load bearing capability. So anywhere from 6 to 10 weeks depending on bone growth.

So this little temporary abutment is actually – or this – I will show you about it a bit here. You’re going to have little temporary abutment that’s actually screwed right in. So here is your piece here, you’re actually going to thread that in and so this little piece becomes the temporary, it’s the holding ground until you get full osseointegration of the substructure. So once you’ve got that in place and you’ve got your healed tissue, this would be a top-down view, you’ve actually got a full biological shield. If you try to put the dental replacement on beforehand and you are fully healed, you essentially set yourself up for crevice corrosion, right? Now you’re going to have a material mismatch. If you don’t have good healing around, any type of food or contaminant can actually become a pathway now down towards the crevice. So usually it’s waited until you’ve got absolute full healing, absolute osseointegration, which can be confirmed on X-ray.

So there’s your soft tissue topside looking – before the insertion of the permanent abutment. And then these actually look very much like the human tooth. So these are matched, they can be resin or all ceramic, probably all ceramic is the more common design, but again a lot of resins technology is improving that situation. They can be blended to the exact same color as adjacent teeth. And so now you’ve got an all-ceramic crown and use a dental adhesive. So again it’s that polymethyl methacrylate adhesive and then that’s placed into the abutment below. Now you’ve got your permanent abutment with your integrated crowns, so in cross-section you’ve got your titanium below osseointegration. You’ve got a biological seal and then you’ve got a ceramic crown. And so now you’ve got a tooth that looks like it’s always been there.

And so your net results rather than being without a tooth is you end up with something that looks like it’s always been there. So there’s a lot of work, structural work that’s going on in the field of dentistry and we tend to I think — from an engineering standpoint, we may look at as dentistry is not quite the same caliber as orthopedics in terms of engineering, and it’s absolutely the same caliber if not more. So there’s a lot of mechanical design that goes on into these devices.

So again in the end you have something that’s aesthetically matched with good structural support. And then if you look post-op what this would look like, so here’s your basic radiograph. You would see into the bones your threaded titanium, your abutment attachment and then from the side it looks exactly like an actual tooth. So rest assured if you lose a tooth, it can be recovered.

Okay. So I thought I just saw the same here, because I think in terms of regulatory issues, your first instinct might think class II because it doesn’t seem to be a safety critical application. But because of the osseointegration and because of what’s involved there is actually for this sub part here the underlying structure to the abutment where you’ve got thread titanium you actually have a class II regulation on it. So it requires the PMA or 510(k) and basically you have all the same specifications as our safety critical thermal stems or heart valves. So all these basic specifications, device manufacturing, sterilization, mechanical, bio and clinical studies are in place. So it’s a pretty robust industry in terms of what’s expected mechanically and structurally.

TMJ Concepts

And then just — again just a quick note to finish on the TMJ Concepts. TMJ Concepts are again – actually this is the name of the business TMJ Concepts and it’s right here in Ventura, California and probably the most important thing for us to look at inside view is how similar this device actually is to a hip or knee design. So you’ve got titanium, you’ve got a metal bearing, you’ve got something again that looks a lot like what we see in orthopedics. So the same types of design issues, we have to think about contact stresses, we have to think about fatigue loading. We have to think about wear debris, we have to think about corrosion. So there’s a lot of similarity in terms of what we do with dental work with what we’ve done with orthopedics.

And again here’s just another example that – so nice cast resin of our teeth choppers, and just inside view you can actually see these devices. So I will – I know we are short on times but if anyone wants to come up at the end, you can just take a look. But I think it just gives you a good example of how complex these devices can be and how similar they are in terms of dealing with stresses and design issues that we have in orthopedics. So again just a good appreciation of crossover in our fields. Okay. Let’s stop with that and if anyone wants to come up and take a look, you can check out the devices.

 

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