Lecture on Dental Materials

 

Lisa Pruitt: 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.

 

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, 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.

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