Episode Transcript
[00:00:06 - 00:00:29] Robert: Welcome back to the Evolution of Dental Podcast, brought to you by Evolution Dental Science, where we explore the stories of the people and the technology shaping the world of dentistry. I'm your host, Rob Norton. Today we're joined by Doctor Nate Lawson of UAB, who is a fascinating doctor who has done both clinical work as well as material science research. Doctor Lawson, welcome to the podcast!
[00:00:30 - 00:00:31] Nate: Thanks Rob, thanks for having me.
[00:00:32 - 00:00:40] Robert: Thanks for being here. So can you tell us a little bit about how you got into this? Like, did you wake up one day and go, I'm going to research materials or dentistry or how did that come together?
[00:00:41 - 00:02:58] Nate: Yeah. So my story was that I had always wanted to go to dental school or become a dentist since I was a kid, because I have a lot of aunts and uncles. Like my mom, on her side of the family, everybody is a dentist, or married to a dentist. And so I always wanted to go to dental school. I went to college in New Orleans at Tulane, and I did engineering because I was so bad at foreign languages. I thought it'd help my GPA if I did engineering and didn't have to take a foreign language. And I like engineering. And then when I was looking at getting into dental school, I went by the LSU dental school to look for a research project to do. And I ran into this guy named John Burgess who was teaching there. Some people may know him. He’s kind of a legend in the field of dental materials testing. So I did research with him. And when I was applying for dental school, it was right around the time of Hurricane Katrina. So that was my senior year of college. And so I had wanted to go to the northeast just for fun, to go to that area, to go to dental school. And then Burgess had transferred to here to UAB. To continue his materials research because LSU was kind of underwater at that period of time. And so he said, Nate, would you want to come up here and go to Dental school in Alabama? So we continued to work together for another six years when I was in graduate school. I went through a quarter life crisis, worked a couple of clinical jobs. And then Burgess at that period of time, thought he wanted to retire, and he called me and said, Nate, would you want to come back to Alabama and teach at the dental school and kind of take over my empire of dental materials teaching? And, you know, the pay is so bad and academics is like, well, maybe I'll do this for you in a little while while I figure my life out. And I came down here and started working with him. And first of all, I just loved it. I mean, I love my job, I love coming in and doing what I do every day. Also within nine months, I met Rachel, who became my wife, who's from here. So now I'm stuck here. I'm never getting out. So that's also what's kept me in this job. But yeah, I came down to, to kind of take over this program as it's a two year master's program. We teach dentists and people that have a lot of internationally trained dentists and some US dentists how to evaluate dental materials out of a two year master's program. And along the way, we evaluate tons of new dental materials. And then I love speaking. So I get to go out, speak about it and do CE events. And we do a lot of publishing. But yeah, I love my job, so I love it. I like this job.
[00:02:59 - 00:03:02] Robert: So Hurricane Katrina, that was like what, 2005 or something like that?
[00:03:03 - 00:03:04] Nate: 2005, Wow that’s crazy. Great memory.
[00:03:05 - 00:03:23] Robert: You said that it was literally underwater over there. So there's been a lot of changes in the world of dental materials since 2005 to, I guess, 20 years later now, right? So one of the big questions is always: zirconia or e.max? What do you think?
[00:03:24 - 00:05:04] Nate: Oh, yeah. It's great. So, yeah, I mean, if you want to just back up even a step, yeah you brought up a great point. Like I graduated dental school in 2011 here at the UAB School of Dentistry. And I mean, my first crown was a full gold crown. And then I think that was the only one I did. And after that, everything's PFM. I never got to touch e.max. And then zirconia like BruxZir®, was just coming out of Glidewell when I was finishing up school, so none of us did zirconia crowns. Some people did like anterior e.max veneers or crowns. I didn't. I was a bad dental assistant, they never let me touch complicated stuff. So I then went to go practice in a DSO setting and we were doing these cheap $40 like probably base metal alloy PFM crowns. And it wasn’t until I came back and started teaching in 2014 that I came back to the school and realized that they had that just in that short period of time, they made this transition to being a completely-zirconia school. And, you know, I was thinking about how we went through this ceramic revolution. Probably I'm thinking because of zirconia being this cool material where you could do these minimally invasive preparations of, like, manufacturers would say, 0.6mm of material thickness. Now, we're telling our students it’s closer to 0.8mm material thickness. So these really conservative preparations have, you know, tooth color rather than gold. Also with the high strength of zirconia, you can cement it rather than having to adhesively bond it. And then also just the cost. I mean, when I did a PFM crown, I don't know what the metal cost was going to be on that thing. So my lab bill is kind of like, you know, at least in the lab that we're using at the dental school here, it's kind of a variable cost. Whereas the zirconia, they've gone up a little bit in price, but they're pretty affordable.
[00:05:05 - 00:05:53] Robert: Yeah, metal fluctuates literally every day. I mean, obviously depending on whether you're using a chrome cobalt, you know, or you're in some kind of non-precious or some gold alloy. I mean, Lord have mercy, gold's up past $5,000 an ounce now, which if you told anybody in 2005 that, well, they’d certainly buy more gold. But yeah, and, rising gold costs, at least for the lab side for sure, is a huge contributing factor to the well, maybe we do need to change what we're doing here. And look at this scary technology that surrounds zirconia, because, you know, the cost of zirconia even now and then it's $5-10 a unit just in base materials versus gold, which can be hundreds of dollars or chrome cobalt, any to those, you know. So yeah, you witnessed the revolution in the ceramic world.
[00:05:54 - 00:09:52] Nate: Yeah. Yeah, and it happened quickly. It was like right in that period of time, like the 2000’s. I think there was a study that was done, I think Christianson did 2008 versus 2013 of like Glidewell’s, production of all ceramic versus PFM. And it went from like 28% to like 72% of their crowns were ceramic. But I ignored your initial question which was like e.max or zirconia. I just wanted to talk about the other thing first. But yeah, as far as zirconia vs e.max, I mean, I think of it from like different applications. I mean, like when I think of e.max I'm thinking of anything bonded. I do a lot of like partial coverage. I've gotten into this whole world of like, adhesive dentistry and doing these, really, a lot of these partial coverage restorations where I can be more tooth-saving with my preparations. I could bond zirconia. I mean, we've done a lot of research that you can bond zirconia, but I think it's just more traditional to bond lithium disilicate. There's a longer track record of doing it. So with a lot of my partial coverage restorations I'm picking lithium disilicate. Anterior restorations I feel like there’s kind of like some considerations. I mean, I guess if there's like a, you have to block something out, like if you, you could use a PFZ or porcelain-fuzed zirconia. Lithium disilicate would be a great option if you got a good color substrate. But then even though you don't have to bond lithium silicate, I still like bonding it. So if I got like, deep margins and I think it's going to be hard to bond in the anterior case, that might be a reason to think about using zirconia, maybe using like, one of the translucent shades of zirconia. And then I would say for posterior crowns, my workhorse is still zirconia. And for bridges it's the only thing I would ever consider for a bridge would be zirconia. So. Yeah, I mean, I think that I can't imagine being limited to just one of those. Like, if somebody said you had to do dentistry and you had to buy a machine that was going to make all of your restorations for you, and you had to buy a zirconia machine and a lithium disilicate machine I'd be like… that'd be really hard. That would make me really sad. And if still you could only use one composite. I'd be like, I could probably do that. If you only can only use e.max or zirconia, I don't think I could do it.
[00:08:18 - 00:09:52] Robert: Yeah, it'd be sidelining yourself into a particular workflow. And I'd say that that's probably a lot to do with strength, wouldn't you say?
[00:08:25 - 00:10:13] Nate: Yeah, and bondability. I mean, like I said, I guess you can bond zirconia, but it's just more… Even as someone that bonds zirconia and believes in zirconia bonding and talks about zirconia bonding. If I'm doing a bonded restoration, there's something about the track record with that material’s bonding that makes me feel a little more comfortable. And so, yeah. And then I guess, yeah, zirconia having the advantage of strength and not just strength, but also like, pre-recording we were having this conversation a little bit about some of these advanced topics and ceramics. And so it's not just the strength. It's also the toughness of the material, like the ability to heal cracks. And so I was telling you, we were talking about Weibull analysis of, like, ceramic materials, meaning that you might have something that has a really high strength, but it's not as reliable of a material because it's susceptible to crack damage. So if you make ten specimens out, you might have like, the specimens that don't have any cracks do really well. But if you have a couple specimens that have these cracks, they cause ultimate failure of the material, then it's like, it's not a very reliable material. Whereas if you had a material that was like, zirconia, what's so special about it is it's got this ability to heal its own cracks that like, you know, really makes it kind of a special, especially to the high strength. I don't know if you want to talk about different kinds of zirconia-
[00:09:52 - 00:10:20] Robert: No, I absolutely want to talk about different types of zirconia. In fact, I think that the healing aspect of it is probably widely unknown. I think more people are familiar with micro fractures than probably the ability for the material to heal itself and the unique nature of both the grain structure and the chemical nature of zirconia versus, say, lithium disilicate derivatives like e.max or others. So yeah, I'd love to share more about that, please.
[00:10:21 - 00:11:59] Nate: Yeah. No, I mean, so that process of transformation, toughening of zirconia is I think, you know, I still remember some of the initial commercials of zirconia crowns where they would take them and just hammer them with a hammer and drive them into pieces of wood and like. Toughness of zirconia comes from this transformation toughening, which means like you were mentioning earlier, zirconia is just a collection of crystals all packed up, one next to each other. What causes any brittle material, like any ceramic, to fail, is a crack propagating through the material and cracks can propagate through ceramics. And zirconia has this neat thing where the initial formulations of zirconia have all of those crystals that we call grains that are all next to each other. And if a crack starts to form, the atomic, the crystalline structure of those crystals or grains will rearrange itself into a different atomic structure to make those crystals slightly bigger. And what that means is that when the crack is trying to go through there, it gets through a patch of crystals. Those crystals rearrange themselves, make themselves slightly bigger, and it kind of compresses that crack and stops it from growing further into the material. And like, that's kind of a unique function of zirconia, like you're saying. It doesn't happen in lithium disilicate. There's some materials that are like “doped” or reinforced with zirconia. If, you know, if the materials, you know, just partially zirconia is not , that's not going to be as helpful. But yeah. So to give materials like a pure crystalline material like zirconia and it's the original formulations of zirconia, they have all these so-called tetragonal crystals and they can heal their own cracks.
[00:12:01 - 00:12:11] Robert: That's probably a term that some of us have heard, but many of them are unfamiliar. So everyone's heard of cubic zirconia. What's the difference between, like, a cubic versus a tetragonal zirconia grain?
[00:12:12 - 00:14:31] Nate: Yeah! So that's a great, great point. So yeah. Like when I think of zirconia, I mean, it's a ceramic, and ceramics are composed of metallic and nonmetallic elements held together by covalent or ionic bonding in these lattice formations. The lattice formations are just like the arrangements of that zirconium and oxygen molecules. And zirconia, I think back to like, I don't know, when I was in high school chemistry, I remember them doing these experiments with tennis balls, and they'd take a little plastic bin and they'd put tennis balls in where they put, one layer of tennis balls, and then the next layer, they put the tennis balls in the spaces in between each tennis ball, and they show you how you could pack them into that arrangement. And they said, you can also pack them in a way where you put the tennis balls on top of each other, and that's a different type of atomic packing or a type of crystal structure. So there's different kinds of crystalline structure arrangements of those atoms. And so, cubic is a more symmetrical arrangement. And the advantage of the cubic arrangement is that since it's more symmetrical, the spaces between atoms are more, there's more directions that light can pass through the spacing between atoms, which allows tetragonal, I mean, cubic to be a more translucent crystal because again, it's more symmetrical. There's more directions from which light can pass through atoms to get through to make it more translucent. That's the advantage of cubic. Tetragonal on the other hand, it doesn't have as many directions that light can pass through. But the advantage is that the tetragonal crystal, when it feels a stress next to it like a crack, those atoms can rearrange into a third arrangement, which is called monoclinic, and in monoclinic, atoms are just spaced 3-4% further apart. So the volume of the tetragonal crystal is 3-4% smaller than monoclinic. Or a better way to say it's monoclinic is 3-4% bigger than its tetragonal. And so again, what happens is when you have a crack pass through a bunch of tetragonal crystals, the atoms rearrange themselves to become the monoclinic crystal. The crystal grows and as the crystal grows it compresses that crack and stops it from driving its way through that zirconia crown or bridge or whatever hybrid or whatever you just made out of zirconia. So that's like the magic beauty of the zirconia, it’s that they have a lot of to tetragonal in them.
[00:14:32 - 00:14:52] Robert: That's a beautiful explanation. And, so the chemical differences between, say, more high strength zirconia versus higher translucency. What variation is there? Is there a different chemistry that goes in with that? And how does that influence how a doctor should choose which material to use, or lab for that matter?
[00:14:53 - 00:15:00] Nate: Well, yeah. That's such a great question, Rob. Yeah. And I know that you know the answer and you're setting me up for these, all of these good answers. But like I said-
[00:15:00 - 00:15:15] Robert: Haha! This is something I find fascinating and I've done a lot of research on over the years. I had my own zirconia lab for, you know, ten years. So this is something I poured myself deep into. And, it's just fascinating to have a conversation with somebody who's been more deeply involved in the research aspect of it. So-
[00:15:15- 00:15:16] Nate: I know, yeah, you know I love talking about it!
[00:15:16- 00:15:22] Rob: I know enough to ask the question, you know more about the answer than I do! And I think it would be very helpful for the audience, for sure!
[00:15:19 - 00:15:22] Nate: No, no. I can tell by the way you asked the question, you know plenty!
[00:17:10 - 00:18:04] Nate: Oh my gosh, So yeah, the answer is, I mean, so yeah. So to explain to our audience, you know, the chemical difference between, like, let's say zirconia that's got a lot of cubic in it versus one that doesn't have any cubic in it, is this dopant they add in it called yttria? So yttria is added at a very small percentage, so you know, with the zirconium and oxygen that, at a 3mol% of yttria, you will stabilize, about, you know, I would say predominantly tetragonal crystals. I mean, there are some papers that I have looked at and they said, oh, it's actually only 95% or 92% tetragonal. But in my mind, I think of it as if you've got 3 mol percent yttria, then you've stabilized all to tetragonal crystals. And as you increase the amount of yttria, you go up to like 4mol%, then you get about 25% cubic, 75% tetragonal. And when you get it to 5Y, I think of it as 50-50, 50% to tetragonal, 50% cubic. And so that means that when you've got more yttria in there, you're going to make a more translucent zirconia because it will have more of the cubic. But, you know, the disadvantage, of course, is that now you're not going to be able to do as much of that transformation toughening. You won't be able to heal cracks as well. So that's why when you get up to zirconias that have 5mol% yttria, they're prettier and they're more translucent, but you're losing a lot of the strength and toughness like ability to heal cracks. And again as I know you know Rob, they'll talk about this yttria percentage is 3mol% yttria, the way they abbreviate this is 3Y as 3mol%, 4Y is 4%molrather than 5Y is 5mol% yttria. So 3mol% yttria is strong but opaque, 5Y, 5mol% yttria is translucent but weaker. And so of course yeah, that would get to like how you would use these materials clinically. And yeah, the clinical use for them- everybody's got their own opinions on I mean, I have my own opinions on them. I like using 3Y as much as possible, 4Y I might use it in the case of an anterior bridge. And then maybe 5Y I would use those if I'm doing an anterior restoration where I don't put porcelain on it like a monolithic. And maybe I would normally use lithium disilicate, but maybe if I don't want to bond it and I just want to use conventional cement, I could use a 5Y zirconia and cement that like if my margins were deep or something like that. So that's kind of my, I guess, “take” on the different generations. Reasonably there's so much more things we could go into, like multilayered zirconias.
[00:18:05 - 00:18:42] Robert: That was actually my next question was actually, I know you feel that it influences multilayer. Do you like multilayer or pre-shaded materials? And, what are some of the advantages of using pre-shaded or multilayer versus, say, traditional staining? Because I'm not sure every doctor knows that. I know a lot of labs are probably familiar with that, especially if they've seen it growing over the years. But, there's more than just efficiency trade-offs. I feel with the pre-stained versus the custom hand-dipped stains or sometimes painted, you know, with the cases like, the zirconia stains. Yeah. Like what has your experience been with that? But from a material research perspective as well as a practicing doctor.
[00:18:43 - 00:22:15] Nate: Yeah, that's a good point. So, yeah, when you talk about, like, multi-layered zirconias I think initially used to be like, you know, I think of like the KATANA, like the KATANA HTML and STML’s and that kind of thing. The multi-layer was just a varied gradation in colors, like, you know, you could have a yellower cervical area and a less yellow incisal area of your zirconia disc. And so the multi-layer only implied chroma or to color differences. And now there's, more recently, there's been those multi-layer discs where the powders at the bottom of the disc are like a 3Y zirconia. And then they might have a gradation of powders of, you know, some 3Y-4Y mixed, and then the top layer being a 4Y. And you've got some that, you know, you could vary the powders in any kind of way. And manufacturers will tell us to an extent how the powders are varied in these multi-layered blocks. The first thing you were talking about was the multi-layered as far as color, the alternative to multi layering the color would be like you said, you could take a pre centered zirconia like in the white chalky state. And like you said, you could dip in stains that go internal with the zirconia. And then I guess the advantage of that from a laboratory standpoint I'm imagining is the fact that you could just carry one, you know, a fewer number of shades of your pucks and then use these stains to stain your puck accordingly. As far as, like material disadvantages of that, I don't know of anything like adding the stains necessarily making the zirconia weaker or less, tough or anything like that. I'm not aware of it, I don’t know, I'm not saying that I know, but I'm just not aware of anything that being a disadvantage. I mean, and then the alternative to staining would be to like, you know, mill it all out of one solid color and then do external stains, like, I think of like the Mios system or something like that where you’re painting on external stains, which I think, one of my good buddies does a lot of research in North Carolina, Taiseer Sulaiman. Well he compared zirconia stains versus lithium disilicate external stains. I have to go back to the literature to see which one he said stains on better. But in our experience, when we've tried to toothbrush off like zirconia stains, or put them in lactic acid and just leave them in a bucket for like a week to try to get it off, we can't even get that stuff off of there. So I think that those external stains can stay on pretty well, but I guess there is always the possibility if a patient maybe has a lot of acid and brushes their teeth vigorously so that the external stains could come off. And so now you'd have a ugly, you know, white crown underneath the external staining. And then now. So, I guess my thought with the multilayer as far as color is that, you know, I think the disadvantage with any shaded block is that of course your inventory is going to be higher. But the advantage being that you can get internal staining within the block that, like, isn't reliant on external staining that can potentially get acid eroded off. We did a review article and I kind of remember looking through that literature and, you know, there wasn't something that stood out that made me think, oh, dip stains are bad. So, the next, yeah, then the next topic is like the multi-layer blocks where you've got something like different yttria, percentages in the different layers. And that's something we have looked into a little bit more because it's like when you evaluate that, it's like, how do you evaluate the way we evaluate the strength of the materials?
[00:22:16 - 00:22:29] Robert: Yeah. Is it actually an advantage, because it's built out as being like, well, you have the strength of the high strength at the base layers, but then you have the translucency and the esthetics on the incisal layers of having the, translucent zirconia. So does that really pan out in your research?
[00:22:29 - 00:24:09] Nate: So the little bit that we've done with it, we took a look at the 4Y-5Y material. And we milled a crown out of it where the cervical aspect of the crown was in the 4Y, and then the incisal occlusal portion was in the 5Y, and then fractured the crown. And we got similar strength as the 4Y, you know, because we were like, if we fracture the crowns, is it going to be a, you know, the same number load, the same amount of load it takes to fracture the crown as a 5Y crown, or a 4Y crown? And it actually did fracture closer to the strength of the 4Y crown. So in that way I do think it like gave, you know, some of the strength advantages of the stronger layer. I mean of course not every, I mean every zirconia puck is not the same. So the way one might be layered might be better than another. I know there's many different brands out now that have layered zirconia pucks. So like how well those different layers are adhered to each other. And it's not just about the way that they fabricate the puck. It's also the fact that, like different types of zirconia, 3Y, 4Y and 5Y have different sintering parameters. So like making sure that when you're sintering them all at the same time, that things are fusing well, is like you know, there's technology that goes into that. So, I think that just because it can work well for some materials doesn't mean there's, you know, you can pick up. You can go to LabDay in Chicago and find, you know, somebody’s multi-layer zirconia they made in their basement and buy it and center it and expect that it's going to do well. I think that you'd have to go with a company that's actually done some research into it. I mean, like, if I was going to buy someone’s basement zirconia, I'd be more comfortable buying their solid 3Y than buying their 3Y-4Y-5Y.
[00:24:10 - 00:24:47] Robert: Absolutely. I mean, there's so many different manufacturing techniques that go into zirconia. And the sources of the powder because, I mean, there's really only, I mean, correct me if I'm wrong, but as far as I'm aware, there's only two sources for zirconium powder for which, like the Tosoh powder that goes into manufacturing zirconia. And have you seen any? What kind of trends do you see from the different mined sources of the original powder and also the different manufacturers, like, have you seen some that are better than the others? And some of them have different grain sizes from one to the next. How much of a difference do you think that makes?
[00:24:06 - 00:25:06] Nate: When I think about different technological differences in the fabrication of the zirconia puck, I know about it more from the stage of like, like what you do with the powder. I know when they make the disc, there's different ways that you can press it. They can press the powders into that disc. So like because it's uniaxial only just pressing from the top and the bottom-
[00:25:05 - 00:25:08] Robert: Right, versus like hydrostatic which presents from all directions.
[00:25:09 - 00:25:56] Nate: Exactly. Yeah. And I think that just gets to consistency with, I’m imagining consistency with like shrinkage when you're trying to center these things and making sure you're having even shrinkage of restorations. So like, I could I mean, again, I haven't tested this. My assumption when something is hydrostatically pressed from all directions that if you're trying to do like a six unit fixed bridge or something like that, the fit would be better in something that's been hydrostatically pressed, because the particles are more evenly condensed into that puck. So when they all get smaller as they center, it would more likely shrink from all directions better in a hydrostatically pressed puck versus one that's only been isostatically pressed.
[00:25:57 - 00:27:01] Robert: Yeah. That has been my experience, especially when you get into larger, more full arch restorations and especially when you're that or like you're saying like large span bridges where you don't actually have as much material connecting one unit to the next, like if you have like 6 or 8 units versus say an all-on-X, that’s all implants supported, but it's one giant chunk of zirconia, which is its own challenge. Have you like, for example, with that, have you seen a lot of discrepancies in, say, fluorescence and opalescence in different manufacturers or different ways of processing your zirconia? And, one thing that I kind of bring to mind with that as well is what's your opinion on like fast sintering? Because I know some people, one of the big disadvantages for zirconia over the years has been, say, an 8 to 10 hour, sintering time in the, in the oven versus now some of these manufacturers are coming out with other materials or ovens or both that supposedly center in, say, 2 or 3 hours. What are your thoughts on that? I know I threw a huge question at you. There's a lot there.
[00:27:02 - 00:27:23] Nate: So the first one, it's funny that you mentioned to me about fluorescence, I mean... My understanding of opalescence is like depending if the angle which you view something you can get different colors, and like that's almost like a weird thing. But sometimes I look at zirconia crowns. I'm like, this thing looks like a pearl. Like I almost don’t want it to be so opalescent like that. It’s almost unnatural.
[00:27:23 - 00:27:29] Robert: Especially if you polish it. It looks very shiny, but it's not natural looking necessarily.
[00:27:30 - 00:28:50] Nate: Yeah. And that's not something we’ve ever tested, because I don't know how you measure opalescence in a laboratory. I'm sure there's some kind of way of, like, reflecting from different angles and measuring the wavelengths of light coming back. But we don't have a machine that does that. And then fluorescence, I guess, first, I guess fluorescence is like when you have UV light that hits a specimen and then how much visible light gets reflected back off of it, I guess. First I thought the relevance of that was only like if the patient's dancing and like a club, you know, like a disco tech, and they're like smiling and like, how much, how does one tooth look relevant to the other. And I was like, that's a very specific thing, especially in the US where almost nobody goes clubbing. At least not that I know, but like, I guess there's also some aspect of it, there is some UV light coming from my window of natural light that could possibly reflect something back differently. So we're actually undergoing it. We're doing a study right now. We’re sending in specimens to The University of Michigan, and we’re testing fluorescence of- Actually we got this research grant from the American Academy of Esthetic Dentistry to measure the fluorescence of 3D printed zirconia. I have to say, I didn't propose the project. Someone else, a different faculty member, proposed it. For me. I was like, that's a very specific thing to worry about. But we got like a $5,000 grant to measure that. So we sent out the specimens last week, so we’ll know more about it. And we also were comparing 3Y, 4Y and 5Y, in printed zirconia as far as their fluorescence. We’ll know more about that later.
[00:28:51 - 00:30:17] Nate: The other question you asked me that was a great question was about the fast sintering of zirconia. So we definitely did a project in that one where we got some. KATANA had an 18 minute sintering zirconia that we evaluated and we compared it against a KATANA that was fired for eight hours. And then we also took some Tosoh zirconia that wasn't in a sample, and wasn't intended to be fast sintered. And we sintered that for 18 minutes and 8 hours. We found out that the KATANA zirconia was intended to be fired in 18 minutes. We could fire it, and we got the same strength and the same optical properties we looked at under SEM, and there's no porosity in it. We had our sample specimen that we made of the Tosoh powders. And when we fast-fired that, we got full porosity, it is really opaque, low strength. And in eight hours, it was fine. So I think that there's again, there's technology that goes into that to allow them to do that fast sinter. Do I know what the technology is? No. I have no idea. But you know, as a classical engineer, a lot of times we just study, to see what happens to things, to try understanding what happens. We’re like the chemists, the physicians, we're just engineers. We're just looking to get the strength. So it can work, we can get the strength in We’re just looking to get the strength. It can work, we can get the strength and translucency rather, and absence of porosity in an 18 minute sinter with a material that's intended for that function.
[00:30:17 - 00:31:06] Robert: I think it's fascinating to see that zirconia is coming up in this new direction of addressing the short turnaround times. One of the big disadvantages, of course, was up against lithium disilicate’s e.max options is how rapidly you could process them versus zirconia and taking just by its nature up until apparently now like a full data process, and how that that compares with the upcoming 3D printed options, both zirconia and some of the nano ceramic hybrids, which I really want to get into with you. But before we do that, I really want to know more about how you test these things. Like what kind of processes and machines, what kind of material science goes into testing this? Both the differences, the toughness, the flexural strength, fracture strength. What goes into that in your lab and your research? What kind of tools do you use?
[00:31:07 - 00:35:34] Nate: This is really like, this is like a great line of questions. Sometimes I listen to like an interview or a podcast. And I'm like, thinking to myself like, how is anybody getting any relevant information? We're just bs-ing around about useless stuff. But there are great questions like, I love these. So when we measure strength in the laboratory, there's different tests we can do, as you mentioned. I mean, there's flexural strength tests. Almost nobody does a tension strength test, meaning they don't just pull the piece of zirconia apart and nobody does a straight compression test because, I mean, I would break my machine before I could break a piece of zirconia in compression. I can never, I would never be able to smash it. So we do flexural strength tests, meaning, like we have a bar of something, and we, you know, we support it, and then we break it from the middle. And so there's two big kinds of flexural strength tests. So there's a three-point bend flexural strength test, and then there's biaxial flexural strength test. And that sounds like a nerdy thing to differentiate about but it actually makes a big difference. I’ll share a story from several years ago where they made a big deal out of this. So the typical has been a three-point bend flexural strength test where we have a bar, we support it on two points and we break it from a third. That's why it's a three point bend flexural strength test. And then we get a value, you know, like a three-way zirconia is around 1200 mega pascals. The strength of 5Y might be something around 5-600 mega pascals of strength. There's another kind of test you can do, which is a biaxial flexural strength test, which is instead of having a bar, you have a disc and you support it on three balls on the bottom, and you have a fourth ball from the center that comes and breaks it. And that one will also give you a value of, you know, 1200 Mega pascals, values like that. The difference being that with a three-point bend flexural strength with ceramics, you're always going to get lower values than with a biaxial flexural strength. Because with the ceramic, when you make a three-point bend flexural strength test, you've got you know, you got students or you or someone’s a technician, someone’s making the specimens. They've got to like make that bar that has the sharp corners in it and the sharp corners when you put them under stress, when you test it, they can have little flaws in there that can cause a fracture. When you have a biaxial flexural strength disc, the highest amount of load is right underneath the indenter, which is in the middle of that circle. So there's no cracks or anything there. So ceramics do much better under biaxial flexural strength than a three-point bend flexural strength. Why that’s relevant is like you might go out, pick up a brochure and then say, this one’s got a functional strength of 1500 mega pascals. And look at this junk over here. It's got a fluxual strength of 600 mega pascals. You got to say, are they both biaxial flexural strength? And remember, I love Ivoclar, a client and I actually love e.max as a product. But I remember when they came out the e.max 500 I was like what? What is this, a new e.max? Now we had just started measuring biaxial flexural strength rather than three-point bend flexural strength, so now it's 500. It used to be 348 or whatever. So it was like, the way you test flexural strength, the values you get can be highly dependent if it’s biaxial or three-point bend flexural strength testing. And honestly, for ceramics, it's probably more conventional to measure biaxial like a disc specimen and get those higher values. But you just have to make sure you don't have someone comparing a biaxial value to a three-point bend value if you want to try to pick which zirconia you're going to buy. And then you mentioned some other tests, like there's a test that's really important for ceramics called fracture toughness where you have some type of a bar, you start a crack in it and you see what load does it take to drive that crack to failure? Because ceramics all fail by crack propagation. And honestly, it's funny because I think there's like one guy in the US that does the best job at fracture toughness testing. Tom Hill. at Ivoclar. They make the specimens. It's super hard to make them. We don't even try to make the specimens anymore. It's so hard to make the specimen and get that crack in there and get the crack the right length and measure it. Make sure it's right. We just can't even do it anymore. There's people that do it well in Europe and they report values. But like it's a super hard test to do. It's really easy to make that crack with 3D printed materials because they're softer and you can use a little saw blade to make the crack. But like for ceramics, it's really hard to do that. So if somebody reports fracture toughness to you and they're a dependable source, that value of the data is actually really cool. It’s really great to get that information. Those are probably two of the, you know, when I think of strength of material, I think primarily of the, you know, three-point device of flexural strength, and also what's its fracture toughness. Those would be numbers that I would be looking out for.
[00:35:34 - 00:35:41] Robert: Sounds to me like the biaxial strength test is a lot more applicable to dentistry anyway, considering the kind of environment that a crown is subjected to.
[00:35:42 - 00:37:34] Nate: Yeah, yeah, to an extent. I was just teaching class an hour ago and we were talking about where the crowns fail. I mean crowns, there's a theory that crowns fail at like the, at the interproximal margin, where you kind of have this moon. The interproximal margin isn't as flat as, like the buccal and lingual margin, you kind of have this kind of curvature to it. And when the patient bites down, it has kind of this opening force at that, interproximal margin, which can cause cracks to grow through the material. And so that's one place where cracks are supposed to originate. And then additionally, if there's any kind of defects on the intaglio surface of the crown, they can originate. so that intaglio kind of fractures. Those ones, yeah, definitely would be more applicable to a biaxial flexural strength type setup… There is a third way which we do test the crowns, and sometimes we literally just make a crown and then we break it. When you do that, all you can do is compare a different type. You can compare like, you know, one brand to another. Like the value I give you is meaningless. Like if I told you I fractured the crown at 700 Newtons. Well, it makes a difference if the crown is a millimeter thick, was it 1.2mm thick? Was it a molar or a premolar? So the value is useless. All I can say is, well, our new zirconia broke at 700 Newtons and our old one broke at 400 newtons. And so the new one's better. But you can't really just report that number. So we do a lot of that kind of testing too, in the lab where we will make a prosthesis like, an implant supported bridge or a distal cantilever bridge or, you know, a crown or, you know, some type of or anomaly or something. And then we either fatigue it or we just fracture it and look at the load it takes for it to fail.
[00:37:35 - 00:38:29] Robert: That makes a lot of sense. I mean, because that's a little bit more applicable to just daily dentistry anyway. And you mentioned e.max 500 earlier, which I think is an interesting little outlier on its own. So you do hear a lot of words like, you know, 500 MPa, 1000-1500 MPa or Mega Pascal, thrown around as comparison numbers for these materials, but also the nature of zirconia versus lithium disilicate materials like e.max is a little different. I mean, the chemistry is different, so the behavior of the material is different. Do you think that the same number from one to the next is directly comparable? Like for example, if you have like a 500 MPa zirconia versus a 500 MPa e.max situation. And one other follow up to that, just kind of baking that in, no pun intended, is do you see a difference between, say, pressed e.max versus milled e.max in comparison to, say, zirconia?
[00:38:30 - 00:42:30] Nate: Ok yeah. So I guess, getting into the first question first is like, if you had e.max at 500 Mega pascals, and let's say you had a 5Y zirconia at 500 Mega pascals. Are both of those, like, or is that the same thing? I mean, there's two ways I could look at it. I mean, first, I mean, there's, you know, with e.max, it could also depend on, is it bonded to the tooth structure? Or is it conventionally cemented with RMGI? Because if you can bond something to the tooth structure then that's going to make it stronger. Whereas if you conventionally cemented, it's not going to reinforce the material nearly as much. So there's that kind of differentiation. And then I guess there's something else that I should mention. And it's funny because, again, I was just teaching this in class an hour and a half ago is that, you know, when you talk about these values like 500 mega pascals, that's static strength, because sometimes people say to me, well if zirconia has got 1200 mega pascals of strength, and then that means in your machine, you know, when you work it all out that you probably fracture it around like maybe a thousand newtons of force, you chew a piece of bread with 40 newtons of force, you chew a carrot with 100 newtons of force, and maybe your maximum clench is like 500 newtons. And then the zirconia goes up to 1000 newtons before it breaks. Why does it even need to be any stronger? Well, the idea is that you just don't. You know, your teeth aren't meant to chew one piece of food one time. I mean, your teeth are going to be chewing food cyclically for millions of cycles. And so, whatever value something fails within a static load like a one time test, it's going to be a much, much lower value than if it fails under fatigue. So that's another kind of test I forgot to mention that we also do is we'll make specimen discs or bars, or crowns. And then we got another machine which just fatigues the hell out of them. Just slam them for like a million cycles at a specified load. And then we see, you know, what load they can go to. Like, sometimes we set a run off at a million cycles. So like, you know, just because it has the ultimate strength of being able to go to 1000 Newtons, when at a one time maybe it only can go to 250 Newtons at a million cycles. And so, if you have an e.max 500 in the other zirconia 500, what would probably be even more interesting or more relevant is like, what load can it go to at a million cycles? And then for the second part of the question, pressed versus milled, as far as strength, we've never tested. I've looked into the literature and that, if there is a difference I would think of it being relatively minuscule as far as strength goes? I mean, there's also this question of like, how accurate are the margins of milled versus press? And again, I remember writing this review article and looking at the data on it in the research articles that have been published has shown no difference in the fit and the margins of milled versus pressed. If you talk to most dentists and labs though, I feel like they would disagree with that statement. I think anecdotally, most people expect that a pressed, lithium disilicate crown is going to fit better than milled. I mean, personally, I think our lab is pressing them all for us. So I don't have as much experience with milled lithium disilicate. But yeah, as far as you know, it's interesting that I'm going to have this. I have this continuing education event I'm going to be doing in a couple months and it's going to myself. It's going to be a practicing dentist, and there's going to be somebody coming from a lab. And we're all going to be talking actually about this similar topic of zirconia and lithium disilicate. I think there's like so many different perspectives of what you can come at it. In my personal biases, I'm so focused on research that I'm like, if the research says this, this is what I believe. But somebody’s like, well, I've done 10,000 crowns or I have a lab and I make a million crowns. And I can tell you, you know, that, we get sent back, you know, milled ones and the pressed ones all seem to fit. So there's so many different ways to look at it that I think this is yeah. For my particular bias as far as studies like, I haven't seen a huge difference in pressed versus milled.
[00:42:31 - 00:43:29] Robert: Well, that's what makes this conversation so fascinating, at least to me anyway. Is that lining up the anecdotal experiences against actual research numbers? So the anecdotal versus the research is always interesting to put next to each other, especially with having the numbers to line up with that. And speaking of one thing that I hear a lot of anecdotal information about that I don't really see the research as much following up yet is the advent of the new 3D printed permanent crown options, as well as the temporaries both zirconia and the nano ceramic hybrid options, which, there seems to be a varying kind of narrative there, whereas some manufacturers seem to be saying that it's a nano ceramic hybrid based on the material science. And this is how it works, versus we're just trying to make sure that it qualifies for the insurance so that it's 51% zirconia, and that way it can be billed as a ceramic restoration as opposed to something else. What are your opinions on that? What's been your experience with that so far?
[00:43:30 - 00:46:40] Nate: Yeah, I mean, it's interesting if you read like the ADA definition of ceramic that they've I guess the big modification, honestly, was that it's always been something that was predominantly, well not always, but I think since like the advent of milled composite, they have the definition that if something's predominantly refractory compounds that’s predominantly ceramic, then it [is] defined as a ceramic. And so it's funny. I ChatGPT’d the word predominantly and in ChatGPT “predominantly” means over 70%, but somebody has reinterpreted it to mean that predominantly means 51% filler, that it can qualify as a ceramic. And the big change that came recently was that they took away the words like “press milled” or, they left it ambiguous as to the fabrication technology. So that meant if it was printed and there was more than 51%, now we've decided that the ADA could classify this as ceramic so you could bill it out as a ceramic. I first I thought that that was the motivation to which they were trying to introduce filler particles beyond 50% or beyond 48%, because we did notice initially that when you start looking at strength, you don't have to get to that 50% mark to start seeing the highest strength, like we saw some of these, like 30 some percent filled materials. We had a publication out that was like, I think we were comparing like ceramic crowns from SprintRay to like OnX. I think it was maybe the original OnX or something. The original OnX had like similar strength as the ceramic crown, even though the OnX was like less filled. And so it's not like you have to get this certain filler percentage to achieve strength. But there are two other properties that you do need to get to. In order to achieve that, you do need to increase filler to achieve. One of those is modulus. The stiffness of the material is highly correlated with strength, I mean highly correlated with filler. Filler and strength are not as correlated, but filler and modulus are pretty well correlated. So like all these composites are like if you look at like Filtek Supreme direct composite, modulus is around 12. Still less than Dentin, but around 12. If you look at 3D printed it's like 8-7. And then it isn't until you start looking at some of the 3D printed that are in like the 60-70 percentile where you start seeing this modulus is even somewhat similar to direct composites. Another thing is hardness. In order to get something harder, you add more filler particles. Hardness though the intent of making something hard, because you don't care how hard something is. Like I don’t need to have hard, hard teeth. They just want wear-resistant teeth. And so hardness is kind of an implication of where resistance is. Interestingly enough, I'm like sharing with you so much stuff that, I was at some point kind of holding back a little bit because we haven't really published any of this yet, but hardness of 3D printed specimens hasn't necessarily correlated with wear. We're still messing around with that in the lab. And then our assumption was harder specimens wear less. That hasn't totally held true, but that's that. I'm not just not sharing this to hold back. It's just I haven't figured that out. We haven’t tested-
[00:46:41 - 00:46:45] Robert: It’s ongoing research, but that's interesting that so far the hardness is not necessarily correlated with wear.
[00:46:46 - 00:47:03] Nate: Hardness is not necessarily correlated with wear. Hardness is very easy to measure. There's not a lot to it, it's a very easy property to measure. Wear is trickier. There's so many variables that go into how you replicate wear in a laboratory that I don't know that our wear methodology is going to totally represent clinical situations.
[00:47:04 - 00:47:11] Robert: Right, there's only so much you can test. But I mean, you have to have some kind of numbers to go on. So it's better to test than not to test, right?
[00:47:11-00:47:12] Nate: Yeah.
[00:47:13 - 00:47:49] Rob: With the ceramics coming from, say, resin based 3D printing versus there is, not even on the horizon anymore. There's a few companies that are 3D printing sort-of traditional-ish zirconia. Where do you see the future here? Do you really see that 3D printed zirconia will catch up? I mean, I know it's hard to speculate, but if you could, if you could sort of dive into your crystal ball, with your experience and your insights, do you see that catching up? Do you see the future taking off with the resins, or do you think, the other materials, the more traditional materials like e.max and traditionally milled zirconia will remain dominant into the future?
[00:47:50 - 00:50:55] Nate: Yeah, that's a great point. I mean, I would have thought, my assumption would have been that, like 3D printing and ceramics would be where we would have to go in order to like, get properties that are going to match this ceramic series, because I just don't see how we're going to be able to go from a 1200 Mega Pascal strength zirconia that we’re used to like using an abusing, making three-unit posterior bridges and short second molar crowns and stuff like that, to all of a sudden go into a material that's got a flexural strength of like 100 Mega pascals, you know, to use the workhorse material, I just don't see it. But like, I could see us going to like, an 700 or 800 Mega Pascal 3D printed, zirconia. So we've just done the very initial work with, I mean, we're just kind of getting into testing of 3D printed zirconia. Like I said, I've seen like Cento and 3DCeram and some of these companies, those are the ones I'm aware of, that do 3D printed zirconia. What I’ve seen with that has been interesting in that it's a very long processing time apparently, to debind. I mean, when they print it, obviously they're printing ceramic particles bound in resin. And so they have to get rid of all that resin and the process of getting rid of all that resin is a technique-sensitive thing. And like we did have this. We have a resident, he's a ceramic engineer and he's one of our students. And he found a company on the internet that they were like, oh, we 3D print zirconia and we just burn the resin in the furnace and then you can sinter it and they sent us a specimen. And the stuff was like, so garbage. And we look at it on a microscope, it’s just full of holes like the density of the stuff is like 80 some percent, whereas most stuff is like 100% dense and like it has 10 billion problems with it. It's ugly. We haven't tested strength yet, but I'm sure the strength is gonna be terrible. So like that whole debinding process is like, very critical, I think to get rid of prostheses. You have good esthetic properties- We haven't started testing the strength yet, but it's probably also going to affect strength. So I have seen some papers from very respectable research groups like Bart Van Meerbeek and stuff, where they've shown that 3D printed zirconia with similar strength is milled, zirconia. But I think the processing right now would be so cumbersome for the labs, like even to get our research specimens. I mean, the cost, the price that we're paying for a specimen is, like, pretty high. So it's definitely not there yet. Commercially- And even the ones that we're getting, definitely the opacity is not good. So, yeah, it definitely needs some improvements. I think that if they learn how to, you know, debind and sinter it better to make something that looks nicer and that’s also more efficient. That would be great. I don't know if. I mean, it'd be I mean, it'd be awesome if you can have that chairside. You know, something that you could chairside 3D print and then debind and sinter. But we're a long way from that as far as I've seen right now. So I think that but if I was going to invest money into something like, you know, put your money in now and then you get to go in a time machine and go for ten years, I'd probably put it in the in the 3D printed zirconia.
[00:50:56 - 00:51:37] Robert: Really, in spite of the fact that? I mean, that's interesting because it sounds like, with the debinding process here instead of going down in time, like from the 8 to 9 hour sintering, where the mills are kind of down, like you said, with the new curing stuff like the 18 minutes, which is very impressive. Going kind of backwards, adding in like an additional day's worth of processing. But also, I mean, we can just remember where CEREC and Lava Crowns came from where it looked like a, you know, a chunk of soap 15 years ago, 20 years ago. And now these are some of the highest esthetic restorations we can provide. So, you know, it's interesting perspective on that for certain.
[00:51:38 - 00:51:56] Nate: Yeah. I would only do that if I get to put my money down and then hop in the time machine. like painfully waiting for the technology to progress. You know, I don't think it's, I don't think it's going to happen like tomorrow. I don't know, maybe chat, or maybe AI robots will expedite the process of that, we'll see it, figure it out in two years. Yeah.
[00:51:57 - 00:51:58] Robert: AI material science research?
[00:51:59 - 00:52:07] Nate: Yeah. ChatGPT, I mean... How do we make this faster? Oh, that was how! Heh.
[00:52:07 - 00:52:36] Robert: Yeah, you know, let it do 10,000 simulations for you and then come back with the best guess. And, speaking of. So, you're saying that you would, for example, if you were looking at maybe investing in either a 3D printer or milling machine, which do you think you'd put your money into? I know that they're not 1 to 1 comparative options. There, but especially if you're like a clinic and you're looking at getting one or the other or both. Where do you see your investment going?
[00:52:37 - 00:54:00] Nate: Yeah, hmm. I guess probably… I mean, I've got both. And the one I've used more is the 3D printer because I use a lab to make all my rounds. And then I've got a mill that I could be milling all of my- I do a lot of e.max onlays, and I could, be learning how to like, design my own e.max onlays and then mill them myself, but I don't, I use the lab to do it. And with the 3D printer. I use that for making splints and models, splints and models, and I don't use any of them for crowns. So if I only had to get one of them, I mean, a 3D printer is like a $10,000 caboodle. You know, mill is like, you know, you got to have design software. I mean, assuming you already have a scanner, you have to get design software and then you have to get a mill, which is expensive, then replace the burrs and buy the blocks and everything like that. It’s a big investment. So I mean, I guess in that way, but personally I would get the 3D printer, but I wouldn't personally be using it for crowns though. If I went to a practice and they said, you can only have one of these two things to make crowns out of. Like, you can't use a lab. And now we live on the moon and so, you know, there's no labs. You get to take, like, one thing on your spaceship with you. I'd probably take the mill. You know, I feel like you have to do all the dentistry for all your family members, you know, if they’re going to live on the moon. I would say, okay, I'll take that. I'll take the mill.
[00:54:01 - 00:54:33] Robert: That makes sense. Like the 3D printer is a little bit more of a Swiss Army knife when it comes to different dental applications. But as far as making long term restorations, the mill is probably the answer. Milling e.max for sure is going to be a lower, lower barrier of entry. Because I mean with zirconia, you also have to have the zirconia sintering furnace, which is its own animal there. So yeah, I can see where they come from with that for sure. And man, this has been a fascinating conversation. I really appreciate your time with all this.
[00:54:34 - 00:54:52] Nate: Yeah, thanks Rob! Yeah, I remember when- people don't know this because they didn’t get to see the pre-interview when I started this interview in a terrible mood because I'm having a terrible clinic day. But you put me in a good mood, so I really enjoyed the conversation. Now I have to go back and deal with my terrible clinical problems. But yeah, it was fun.
[00:54:53 - 00:54:58] Robert: I'm glad to hear that. Thank you for being with us. Where can people find you online if they want to know more about your research or your practice or your work?
[00:54:59 - 00:55:36] Nate: Yeah, sure. We have an Instagram account. It's called @DentinalTube. It's dentinal like dentinal tubules, and tube like YouTube. And then I have my website, my speaking website, which is my name, Doctor Nate Lawson… in case you don’t know, my joy in life is going out to different clubs and nerding out about dental materials. You know, we talked about direct and indirect. So all my speaking information is on that website, drnatelawson.com. Yeah, that's where they go to find me digitally. So yeah. Thanks for letting me get a shout out to those. And. Yeah, thanks for having me.
[00:55:37 - 00:55:39] Rob: Absolutely. Thank you again for being a part of this. We really appreciate you.
[00:55:40 - 00:55:41] Nate: Awesome! Thank you.
[00:55:41 - 00:55:55] Robert: And thank you everybody for watching. This has been the Evolution of Dental Podcast. Please look for us on all your major platforms. Don't forget to like, subscribe, share this video with your friends and please never stop evolving.
