Lecture on Dental Materials P5

 

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.

 

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.

 

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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