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.