WEBVTT 1 00:00:00.120 --> 00:00:02.720 You know, when most of us picture a high stakes 2 00:00:02.799 --> 00:00:07.280 chemistry lab, we usually think of like sterile white rooms, right, 3 00:00:07.839 --> 00:00:11.640 fume hoods scientists and hazmat suits measuring volatile liquids and 4 00:00:11.679 --> 00:00:14.960 these perfectly controlled, temperature regulated conditions. 5 00:00:15.000 --> 00:00:19.320 Oh. Absolutely a space where literally every single variable is 6 00:00:19.359 --> 00:00:22.600 locked down. I mean, humidity is constant, pressure is monitored, 7 00:00:22.839 --> 00:00:25.039 and contamination is just non existent. Right. 8 00:00:25.079 --> 00:00:27.280 But the reality is, and I find this so fascinating, 9 00:00:27.399 --> 00:00:31.760 one of the most intense, unpredictable and just genuinely hostile 10 00:00:31.800 --> 00:00:35.119 chemical environments on Earth is actually sitting right behind your lips, 11 00:00:35.560 --> 00:00:36.320 the human mouth. 12 00:00:36.439 --> 00:00:39.079 Yeah, it really changes how you view a routine filling, 13 00:00:39.079 --> 00:00:39.960 doesn't it totally? 14 00:00:40.479 --> 00:00:43.920 And the people running these volatile chemistry experiments every single 15 00:00:44.000 --> 00:00:46.679 day are your dental team, which is exactly our mission 16 00:00:46.759 --> 00:00:48.240 for you today in this deep dive. 17 00:00:48.520 --> 00:00:51.679 I love that framing because placing dental materials isn't merely 18 00:00:51.719 --> 00:00:55.119 about plugging a hole in a damaged tooth. It's really 19 00:00:55.159 --> 00:01:00.640 intricate masterclass in material science, biochemistry, and structural engineering exactly. 20 00:01:00.759 --> 00:01:03.520 So, whether you are a dental student prepping for your boards, 21 00:01:03.679 --> 00:01:08.560 or maybe a newly qualified dental nurse perfecting your armamentarium, setups, 22 00:01:09.120 --> 00:01:12.040 or even just a fiercely curious learner who wants to 23 00:01:12.079 --> 00:01:14.560 know the exact science of what goes into your tooth. 24 00:01:15.000 --> 00:01:17.719 We are getting into the microscopic chemistry today. 25 00:01:17.560 --> 00:01:20.680 Yes, based on the highly practical principles from Carmen Scheller 26 00:01:20.719 --> 00:01:24.079 Sheridan's Basic Guide to Dental Materials. And to figure out 27 00:01:24.079 --> 00:01:27.280 what materials work, we first have to understand the specific 28 00:01:27.840 --> 00:01:30.280 brutal properties of the environment they're going into. 29 00:01:30.599 --> 00:01:33.799 Right, the mouth is like a hurricane zone with acid rain. 30 00:01:33.920 --> 00:01:36.840 That is the perfect way to describe it. The baseline 31 00:01:36.879 --> 00:01:40.000 conditions of the oral cavity are just extreme. Like, let's 32 00:01:40.000 --> 00:01:40.760 start with the chemistry. 33 00:01:40.799 --> 00:01:41.680 Okay, lay it on us. 34 00:01:41.879 --> 00:01:44.319 So the normal resting pH of your mouth is around 35 00:01:44.319 --> 00:01:47.840 seven point zero, with seven point five being completely neutral. 36 00:01:48.120 --> 00:01:50.239 But every single time you eat a piece of fruit, 37 00:01:50.359 --> 00:01:53.719 or drink a soda or consume carbohydrates that the local 38 00:01:53.719 --> 00:01:57.040 bacteria break down, that environment becomes highly. 39 00:01:56.799 --> 00:01:59.959 Acidic, and saliva acts as a natural buffer. 40 00:02:00.319 --> 00:02:04.480 Right it does. Yeah, it contains bicarbonates that neutralize those acids. 41 00:02:05.280 --> 00:02:08.759 But the restorative materials placed in your mouth still have 42 00:02:08.879 --> 00:02:13.960 to continuously withstand these radical chemical fluctuations without dissolving or degrading. 43 00:02:14.240 --> 00:02:17.919 Wow, So the material has to be chemically indestructible basically, 44 00:02:17.960 --> 00:02:22.000 but it also faces massive physical trauma. I mean, the 45 00:02:22.039 --> 00:02:24.479 biting force of human jaws is immense. 46 00:02:24.560 --> 00:02:26.960 Oh, it's huge, hundreds of pounds of pressure. So if 47 00:02:26.960 --> 00:02:28.759 you put a material in there, it needs a really 48 00:02:28.840 --> 00:02:31.639 high yield strength so it doesn't undergo what we call creep. 49 00:02:31.840 --> 00:02:34.560 Creep. That sounds ominous. What exactly is that. 50 00:02:34.479 --> 00:02:38.319 Creep is a really fascinating mechanical failure. It's the gradual, 51 00:02:38.960 --> 00:02:42.840 permanent distortion of a crystalline material under constant static load. 52 00:02:43.360 --> 00:02:46.919 So imagine an old metal amalgam filling Okay, got it. 53 00:02:47.000 --> 00:02:49.960 Over years of heavy chewing, the atomic planes within that 54 00:02:50.000 --> 00:02:52.960 metal can actually slip past one another. The filling slowly 55 00:02:53.039 --> 00:02:55.159 microscopically flattens and spreads outwards. 56 00:02:55.199 --> 00:02:58.360 Wait, so it literally acts like a slow moving wedge 57 00:02:58.400 --> 00:02:59.439 splitting wood. 58 00:02:59.319 --> 00:03:02.680 Exactly, And that slow spreading can exert so much lateral 59 00:03:02.719 --> 00:03:05.479 pressure that it fractures the natural cusp of the tooth 60 00:03:05.520 --> 00:03:05.960 holding it. 61 00:03:06.159 --> 00:03:09.719 Man. That explains the physical pressure, But honestly, perhaps the 62 00:03:09.719 --> 00:03:12.599 biggest enemy in this environment is fluid. Like if you 63 00:03:12.719 --> 00:03:17.400 have an acidic, wet, constantly moving environment keeping saliva from 64 00:03:17.479 --> 00:03:20.520 just seeping underneath these materials seems impossible. 65 00:03:20.599 --> 00:03:23.919 It is the ultimate battle and restorative dentistry. We're constantly 66 00:03:23.960 --> 00:03:29.439 fighting micro leakage, which is the ingress of saliva, bacteria 67 00:03:29.479 --> 00:03:32.919 and other oral fluids into the microscopic interface between the 68 00:03:33.000 --> 00:03:35.439 natural tooth tissue and the restorative material. 69 00:03:35.599 --> 00:03:38.240 Because no matter how seamless that filling looks to your 70 00:03:38.280 --> 00:03:41.680 naked eye in the battery mirror, microscopically there's almost always 71 00:03:41.680 --> 00:03:44.680 a gap, always, and when a cavity is drilled, it 72 00:03:44.759 --> 00:03:48.280 exposes the dentene layer beneath the heart enamel. I always 73 00:03:48.319 --> 00:03:52.240 picture dentene not as solid bone, but as like thousands 74 00:03:52.240 --> 00:03:56.080 of microscopic straws. Dental tubules just bundle together, That's a 75 00:03:56.080 --> 00:03:58.120 great analogy, and they lead straight down to the live 76 00:03:58.199 --> 00:04:00.240 nerve of the tooth. So if fluid gets to that 77 00:04:00.319 --> 00:04:02.520 micro gap at the surface, it just gets sucked right 78 00:04:02.560 --> 00:04:03.360 down those straws. 79 00:04:03.599 --> 00:04:07.319 Yeah, and that fluid movement inside the tubules is exactly 80 00:04:07.360 --> 00:04:10.319 what triggers the pain receptors in the pulp. It causes 81 00:04:10.360 --> 00:04:14.199 that sharp saving of post operative sensitivity when you drink 82 00:04:14.240 --> 00:04:15.360 cold water. Ouch. 83 00:04:15.639 --> 00:04:17.000 Yeah, we've all felt that. 84 00:04:17.279 --> 00:04:21.920 Right and worse bacteria travel down those same tubules, causing 85 00:04:21.959 --> 00:04:25.959 recurrent decay beneath the filling or even irreversible inflammation of 86 00:04:26.000 --> 00:04:26.959 the pulp, and the. 87 00:04:26.920 --> 00:04:30.399 Pulp faces other threats too, right, like thermal shock because 88 00:04:30.399 --> 00:04:31.959 metals conduct heat rapidly. 89 00:04:32.120 --> 00:04:34.319 Oh definitely. If you have a deep metal filling and 90 00:04:34.399 --> 00:04:37.959 drink hot coffee, that thermal energy travels instantly to the nerve. 91 00:04:38.079 --> 00:04:41.279 And there's also galvanic corrosion, which glows my mind, like 92 00:04:41.319 --> 00:04:43.439 if you have a gold crown on a top tooth 93 00:04:43.439 --> 00:04:46.040 and a silver amalgam filling on the bottom tooth and 94 00:04:46.079 --> 00:04:48.959 they touch while bathed in saliva, which is an electrolyte 95 00:04:49.279 --> 00:04:51.519 by the way, you create a literal battery in your mouth. 96 00:04:51.639 --> 00:04:55.279 Yes, ions literally transferred between the dissimilar metals. It causes 97 00:04:55.319 --> 00:04:58.279 a small electric shock and slowly corrodes the restoration. 98 00:04:58.560 --> 00:05:01.800 It's just wild to some. The design constraints here. A 99 00:05:01.920 --> 00:05:06.639 dental material must survive continuous acid baths, resist atomic level 100 00:05:06.639 --> 00:05:11.560 deformation under intense pressure, seal a microscopic interface to block bacteria, 101 00:05:11.959 --> 00:05:15.399 insulate the nerve from extreme temperature, avoid acting as a 102 00:05:15.439 --> 00:05:20.000 literal electrical conductor, and ideally look exactly like natural enamel. 103 00:05:20.120 --> 00:05:23.120 It's an impossible wish list, and because no single permanent 104 00:05:23.199 --> 00:05:25.839 material can do all of that flawlessly, we actually have 105 00:05:25.879 --> 00:05:30.279 to construct restorations in layers. We build from the pulp outwards. 106 00:05:30.120 --> 00:05:32.439 Right, so we prepare the tooth and lay down protectors 107 00:05:32.480 --> 00:05:35.800 before we even think about placing the final heavy duty filling. 108 00:05:35.639 --> 00:05:39.839 Exactly, which introduces our foundational layer of materials, liners and bases. 109 00:05:40.360 --> 00:05:42.680 And one of the most historically significant and widely used 110 00:05:42.680 --> 00:05:47.519 materials in this category is zinc oxide eugenol or zoeze. 111 00:05:47.560 --> 00:05:50.319 It's basically the multi tool of the bentle tray, right. 112 00:05:50.480 --> 00:05:52.519 I mean it works as a thick insulating base, a 113 00:05:52.600 --> 00:05:56.959 temporary filling, elluding cement to secure temporary grounds, or even 114 00:05:56.959 --> 00:05:58.439 a surgical dressing for gums. 115 00:05:58.800 --> 00:06:02.480 It is incredibly You generally see it in two main variations. 116 00:06:02.920 --> 00:06:06.040 Type I is unreinforced, meaning it has low strength and 117 00:06:06.120 --> 00:06:10.000 high solubility strictly for temporary use, and then type two 118 00:06:10.120 --> 00:06:13.240 is reinforced with resins or alumina, which gives it moderate 119 00:06:13.240 --> 00:06:14.879 strengths so it can last a bit longer. 120 00:06:15.120 --> 00:06:18.759 And the clinical chemistry of zoe is brilliant. The powder 121 00:06:18.839 --> 00:06:22.759 is primarily zinc oxide and the liquid is ugenol, which 122 00:06:22.800 --> 00:06:24.800 is an essential oil derived from cloves. 123 00:06:25.000 --> 00:06:27.839 Yeah, the clove smell is unmistakable in a dental office. 124 00:06:28.160 --> 00:06:32.879 When mixed, they form an amorphous zinc eugenolate matrix. But 125 00:06:32.959 --> 00:06:35.439 the true clinical value lies in the eugenol. 126 00:06:35.000 --> 00:06:39.000 Itself because it has those profound antimicrobial properties. 127 00:06:38.800 --> 00:06:42.279 Yes, and unique Abtundon effect, meaning it physically soothes and 128 00:06:42.279 --> 00:06:44.079 sedates an inflamed dental pulp. 129 00:06:44.279 --> 00:06:47.120 It is essentially a therapeutic blanket for an angry nerve, 130 00:06:47.480 --> 00:06:49.959 which is awesome. But it makes me wonder. If coe 131 00:06:50.000 --> 00:06:52.639 is so suiting and provides such a great temporary seal, 132 00:06:53.000 --> 00:06:55.360 why not just use it underneath every single filling as 133 00:06:55.399 --> 00:06:57.399 a universal base layer. Ah. 134 00:06:57.600 --> 00:07:02.000 That brings up a critical contraindication. Literally, every dental professional 135 00:07:02.120 --> 00:07:06.360 must memorize. You can never ever use zinc oxide usanol 136 00:07:06.439 --> 00:07:11.959 underneath a modern resin composite filling. The chemistry is fiercely incompatible. 137 00:07:12.040 --> 00:07:14.800 Okay, let's break down why that happens. Because this is fascinating. 138 00:07:15.279 --> 00:07:19.000 Resin composits harden or cure through a chemical process called 139 00:07:19.120 --> 00:07:20.680 free radical polymerization. 140 00:07:20.959 --> 00:07:23.439 Right, the blue curing light exactly. 141 00:07:23.680 --> 00:07:26.759 The curing light activates a molecule that generates free radicals, 142 00:07:26.759 --> 00:07:30.040 which then aggressively link all the resin monomers together into 143 00:07:30.079 --> 00:07:35.199 a solid plastic. But eugenol is a highly effective antioxidant. 144 00:07:35.319 --> 00:07:37.759 It is literally a free radical scavenger. 145 00:07:38.000 --> 00:07:40.319 So when the ugenol seeps out of the zoe base 146 00:07:40.399 --> 00:07:43.680 and touches the resin, it acts like a sponge, sweeping 147 00:07:43.759 --> 00:07:46.160 up all those free radicals before they can even initiate 148 00:07:46.160 --> 00:07:46.839 the chain reaction. 149 00:07:47.040 --> 00:07:49.639 Wow, So the composite sitting directly on top of the 150 00:07:49.720 --> 00:07:52.759 zoe will just remain a soft, uncured paste forever. 151 00:07:52.959 --> 00:07:55.920 Yep, the restoration will completely fail. Yeah, it's a perfect 152 00:07:55.920 --> 00:07:59.000 example of why the dental nurse or assistant must intimately 153 00:07:59.079 --> 00:08:02.360 know the chemical constant of their armamentarium absolutely. 154 00:08:02.720 --> 00:08:05.800 And speaking of the dental assistant, mixing zoe is highly 155 00:08:05.879 --> 00:08:08.920 technique sensitive too. It's a three to one powdered to 156 00:08:08.959 --> 00:08:12.920 liquid ratio. But before dispensing the powder, the protocol requires 157 00:08:12.920 --> 00:08:14.160 you to fluff the bottle. 158 00:08:14.560 --> 00:08:18.439 Yes, fluffing is key. Shaking the closed powder bottle incorporates 159 00:08:18.480 --> 00:08:22.879 air because zinc oxide powder settles and packs really densely 160 00:08:22.920 --> 00:08:26.120 over time. If you dip a measuring scoop into densely 161 00:08:26.160 --> 00:08:30.079 packed powder, you'll extract significantly more mass than if the 162 00:08:30.079 --> 00:08:31.000 powder is aerated. 163 00:08:31.360 --> 00:08:33.960 Oh I see, so fluffing just ensures a consistent, predictable 164 00:08:34.039 --> 00:08:36.080 volume for the chemical reaction exactly. 165 00:08:36.279 --> 00:08:39.159 You dispense this onto a wax paper pad, divide the 166 00:08:39.159 --> 00:08:42.399 powder into quarters, and use a broad bladed spatula to 167 00:08:42.440 --> 00:08:45.039 force the powder into the liquid. You keep folding it 168 00:08:45.080 --> 00:08:47.799 in until the mixture loses its stickiness and reaches a firm, 169 00:08:48.080 --> 00:08:50.399 putty like consistency. You should be able to roll it 170 00:08:50.440 --> 00:08:52.240 into a neat little ropeer bawl okay. 171 00:08:52.320 --> 00:08:55.440 So Zoe is excellent for sedation, but if the cavity 172 00:08:55.480 --> 00:08:57.919 is so deep that the pulp is actually exposed or 173 00:08:57.919 --> 00:09:01.320 microscopically close to being exposed, we pivot to a different liner. 174 00:09:01.440 --> 00:09:01.600 Right. 175 00:09:01.799 --> 00:09:02.879 Calcium hydroxide. 176 00:09:03.120 --> 00:09:07.519 Yes, calcium hydroxide is fascinating because it doesn't just passively 177 00:09:07.559 --> 00:09:10.879 sit there. It actively forces the tooth to heal itself. 178 00:09:11.120 --> 00:09:13.080 Wait, really, how does it do that? 179 00:09:13.240 --> 00:09:17.000 It does this through its extreme alkalinity. Calcium Hydroxide has 180 00:09:17.000 --> 00:09:19.320 a pH fro round eleven to twelve. When you place 181 00:09:19.360 --> 00:09:23.200 this highly alkaline material directly over and exposed nerve, it 182 00:09:23.279 --> 00:09:27.960 causes a very mild localized chemical irritation on micronecrosis of 183 00:09:28.000 --> 00:09:28.919 the immediate tissue. 184 00:09:28.960 --> 00:09:32.679 Oh wow, so the tooth's pulp perceives this alkaline burn 185 00:09:32.759 --> 00:09:33.360 as a threat. 186 00:09:33.639 --> 00:09:37.120 Exactly in defense, the specialized cells in the pulp called 187 00:09:37.240 --> 00:09:41.320 odonta blasts kick into overdrive and rapidly secrete a brand 188 00:09:41.360 --> 00:09:42.519 new layer of hard tissue. 189 00:09:42.559 --> 00:09:45.600 They build a secondary dentine bridge like They literally build 190 00:09:45.639 --> 00:09:47.799 a bone like wall to seal themselves off from the 191 00:09:47.879 --> 00:09:51.120 chemical threat. It's forced biological regeneration, it really is. 192 00:09:51.399 --> 00:09:53.559 And after applying these liners, if we need a thicker 193 00:09:53.600 --> 00:09:57.720 structural base, we might use zinc phosphate cement. However, the 194 00:09:57.759 --> 00:10:01.120 mixing of zinc phosphate powder and phosphoric acid liquid creates 195 00:10:01.120 --> 00:10:04.240 a violently exothermic reaction. I mean, it gives off a lot. 196 00:10:04.120 --> 00:10:05.799 Of heat, which is why you always mix it on 197 00:10:05.840 --> 00:10:08.639 a cool, thick glass slab instead of the paper pad. 198 00:10:08.840 --> 00:10:11.200 The glass acts as a heat sink right right. 199 00:10:11.519 --> 00:10:14.159 It pulls the thermal energy away, so the material doesn't 200 00:10:14.159 --> 00:10:18.279 set instantly, giving the dentist more working time. But because 201 00:10:18.279 --> 00:10:21.440 that liquid is pure phosphoric acid, the initial placement is 202 00:10:21.519 --> 00:10:23.759 highly acidic and can shock the nerve. 203 00:10:23.600 --> 00:10:27.159 Which reinforces why we need that calcium hydroxide liner underneath 204 00:10:27.159 --> 00:10:28.200 it in the first place. 205 00:10:27.960 --> 00:10:31.879 Precisely so once the pulp is sedated, protected, and insulated, 206 00:10:32.279 --> 00:10:35.360 we finally move to the final layer. The heavy lifters 207 00:10:35.720 --> 00:10:40.039 the direct permanent restorations, and for decades the dominant choice 208 00:10:40.039 --> 00:10:45.240 for posterior heavy chewing teeth was dental amalgam amalgam. 209 00:10:45.559 --> 00:10:48.480 It's a fascinating metallurgical process. I mean. It begins as 210 00:10:48.480 --> 00:10:51.519 a powdered alloy of silver, tin, and copper, but to 211 00:10:51.559 --> 00:10:54.919 bind those metal particles together into a solid mass, we 212 00:10:55.039 --> 00:10:57.000 use elemental liquid mercury YEP. 213 00:10:57.120 --> 00:11:00.320 The process is called trituration. Inside a sealed case absol, 214 00:11:00.399 --> 00:11:03.279 a machine rapidly vibrates the powder and liquid together. 215 00:11:03.080 --> 00:11:05.879 And the liquid mercury just dissolves the outer layer of 216 00:11:05.919 --> 00:11:09.159 the silver and tin particles, forming a new crystalline matrix 217 00:11:09.159 --> 00:11:11.600 that binds the remaining unreacted cores together. 218 00:11:11.960 --> 00:11:16.279 Exactly, it creates appliable silvery paste that can be forcefully 219 00:11:16.320 --> 00:11:20.279 packed or condensed into the cavity preparation before it crystallizes 220 00:11:20.399 --> 00:11:23.360 into this brutally hard, durable material. 221 00:11:23.600 --> 00:11:27.799 But of course, handling unmixed elemental mercury requires strict protocols. 222 00:11:28.039 --> 00:11:32.279 Dental professionals must avoid inhaling vapors or allowing any skin contact, 223 00:11:32.399 --> 00:11:32.679 and for. 224 00:11:32.679 --> 00:11:35.440 The patient placing a rubber dam, which is a latex 225 00:11:35.519 --> 00:11:39.639 or nitrial sheet isolating the tooth is completely non negotiable 226 00:11:39.840 --> 00:11:43.799 to prevent any accidental swallowing or inhalation of metal fragments 227 00:11:44.039 --> 00:11:45.480 during placement or removal. 228 00:11:45.759 --> 00:11:49.559 Right now, amalgam offers unmatched longevity and compressive strength, but 229 00:11:49.639 --> 00:11:53.159 its metallic silver appearance is obviously a huge esthetic drawback for. 230 00:11:53.200 --> 00:11:55.639 A lot of people it is, and it also doesn't 231 00:11:55.679 --> 00:11:58.960 chemically bond to the tooth. It relies entirely on mechanical retention. 232 00:11:59.399 --> 00:12:02.000 The dentist literally has to cut undercuts into the tooth 233 00:12:02.039 --> 00:12:05.200 structure so the metal physically locks into place once it hardens. 234 00:12:05.480 --> 00:12:08.600 So when patients demand a restoration that perfectly mimics natural 235 00:12:08.600 --> 00:12:12.039 tooth structure, we turn to resin composits. These are a 236 00:12:12.080 --> 00:12:16.200 matrix of synthetic polymers packed with microscopic glass or silica 237 00:12:16.200 --> 00:12:19.519 of filler particles to give them strength and optical translucency. 238 00:12:19.799 --> 00:12:23.120 They are the esthetic champions for sure, used for everything 239 00:12:23.159 --> 00:12:27.799 from direct veneers on front teeth to large fillings on molars. However, 240 00:12:28.200 --> 00:12:32.759 Achieving that perfect invisibility requires a very specific clinical sequence. 241 00:12:32.879 --> 00:12:34.840 Yeah, let's talk about that, because this is a crucial 242 00:12:34.879 --> 00:12:37.480 optical trap for the clinical team. The shade of the 243 00:12:37.519 --> 00:12:40.600 composite must be selected using a shade guide under natural 244 00:12:40.679 --> 00:12:43.919 lighting conditions before the tooth is isolated and dried out. 245 00:12:44.080 --> 00:12:48.200 Emphasize before, absolutely When a tooth is isolated under bright 246 00:12:48.240 --> 00:12:52.200 surgical lights and high volume suction, it quickly loses noisture. 247 00:12:52.879 --> 00:12:56.919 A desiccated dry tooth scatters light completely differently than a 248 00:12:57.000 --> 00:12:57.720 hydrated one. 249 00:12:57.799 --> 00:13:01.240 It appears significantly wider, chalky, and more opaque. 250 00:13:01.320 --> 00:13:03.960 Exactly so, if you match your composite to a dry tooth, 251 00:13:04.360 --> 00:13:06.600 the filling will look far too bright and visible an 252 00:13:06.639 --> 00:13:10.039 hour later when the patient's enamel rehydrates with their saliva. 253 00:13:10.240 --> 00:13:13.720 That is such a smart practical tip. But beyond esthetics, 254 00:13:13.799 --> 00:13:17.879 resin composits do have significant limitations. We already established they 255 00:13:17.879 --> 00:13:21.120 fail if contaminated by eugenol, but they also suffer from 256 00:13:21.120 --> 00:13:24.039 a physical phenomenon called polymerization shrinkage. 257 00:13:24.240 --> 00:13:28.639 Right, let's visualize the chemistry of that shrinkage before curing. 258 00:13:28.960 --> 00:13:32.360 The composite is appliable paste full of individual free floating 259 00:13:32.399 --> 00:13:34.320 monomer molecules, and when you hit. 260 00:13:34.240 --> 00:13:37.879 Them with the blue curing light, those individual molecules rapidly 261 00:13:37.919 --> 00:13:42.159 snap together, forming tight covalent bonds to create long polymer 262 00:13:42.240 --> 00:13:43.240 chains exactly. 263 00:13:43.720 --> 00:13:47.200 But because covalent bonds pull atoms physically closer together than 264 00:13:47.200 --> 00:13:49.960 when they were just floating freely in the paste, the 265 00:13:50.039 --> 00:13:54.120 overall volume of the material literally shrinks, usually by about 266 00:13:54.159 --> 00:13:55.399 two to three percent. 267 00:13:55.120 --> 00:13:57.279 Which doesn't sound like a lot. But if you place 268 00:13:57.320 --> 00:14:00.240 a massive volume of composite into a cavity all at 269 00:14:00.279 --> 00:14:04.840 once and cure it, that shrinking mass generates immense stress, 270 00:14:05.039 --> 00:14:05.840 huge stress. 271 00:14:06.039 --> 00:14:08.279 It physically pulls away from the walls of the tooth. 272 00:14:08.720 --> 00:14:12.519 That shrinkage stress tears the adhesive bond, immediately creating the 273 00:14:12.600 --> 00:14:15.480 exact microleakage gap we are trying so hard. 274 00:14:15.320 --> 00:14:17.679 To prevent, which is why dentists have to place and 275 00:14:17.720 --> 00:14:21.360 cure a composite in tiny incremental layers, carefully managing the 276 00:14:21.399 --> 00:14:24.759 shrinkage vectors. It is highly highly technique sensitive. 277 00:14:25.000 --> 00:14:29.759 It is furthermore, composites demand an absolute bone dry environment. 278 00:14:30.360 --> 00:14:32.639 If you're trying to fill a cavity that extends deep 279 00:14:32.639 --> 00:14:35.919 below the gum lines, subgingibly where blood and genual fluid 280 00:14:35.919 --> 00:14:40.320 are constantly weeping. Achieving isolation is nearly impossible. 281 00:14:39.879 --> 00:14:42.879 And a single microscopic drop of moisture will just completely 282 00:14:42.960 --> 00:14:44.320 destroy the resin bond. 283 00:14:44.399 --> 00:14:48.960 Completely, which creates a real clinical dilemma. Amalgam is incredibly strong, 284 00:14:49.320 --> 00:14:53.799 but ugly and requires removing healthy tooth structure for mechanical locking. 285 00:14:54.639 --> 00:14:58.120 Resin composite is beautiful, but shrinks, pulls away from the margins, 286 00:14:58.200 --> 00:14:59.679 and demands zero moisture. 287 00:15:00.120 --> 00:15:02.720 So is there a hybrid material. When faced with a 288 00:15:02.759 --> 00:15:06.159 situation that requires a chemical bond without extreme moisture sensitivity, 289 00:15:06.240 --> 00:15:08.879 what do we do? We utilize glass ionomers. 290 00:15:08.960 --> 00:15:13.519 Yes, glass ionomers, cements or gis operate on completely different chemistry. 291 00:15:13.840 --> 00:15:17.240 They consist of a fluoro iluminosilicate glass powder mixed with 292 00:15:17.320 --> 00:15:18.720 a polyocrylic acid. 293 00:15:18.519 --> 00:15:22.000 Liquid, and when combined, an acid base reaction occurs. The 294 00:15:22.039 --> 00:15:25.200 acid actually extracts calcium and phosphate ions from the natural 295 00:15:25.240 --> 00:15:28.320 tooth structure, integrating them into its own matrix, so it 296 00:15:28.360 --> 00:15:31.840 creates a direct chemical bond to both enamel and dentine. 297 00:15:31.960 --> 00:15:36.000 It essentially fuses with the tooth. But the true superpower 298 00:15:36.039 --> 00:15:39.440 of glass ionomers is their role as a therapeutic reservoir. 299 00:15:40.600 --> 00:15:44.519 Because they contain that fluoroluminosilicate glass. They have the highest 300 00:15:44.559 --> 00:15:47.320 release of fluoride of any restorative material. 301 00:15:47.639 --> 00:15:51.639 They constantly leach fluoride ions into the surrounding tooth structure, 302 00:15:52.120 --> 00:15:56.360 remineralizing the enamel and halting secondary decay. I read they 303 00:15:56.360 --> 00:15:59.200 can even recharge their fluoride levels when the patient uses 304 00:15:59.200 --> 00:15:59.960 fluoride toothpage. 305 00:16:00.240 --> 00:16:03.440 They absolutely can. It acts like a biological battery fighting 306 00:16:03.440 --> 00:16:04.480 off acid attacks. 307 00:16:04.639 --> 00:16:06.399 That is so cool. But the trade off for this 308 00:16:06.519 --> 00:16:09.799 chemical bonding and fluoride release is structural weakness. 309 00:16:09.440 --> 00:16:12.559 Isn't it Sadly? Yes, gis are brittle. They have a 310 00:16:12.639 --> 00:16:16.120 high wear rate and low tensile strength. You would never 311 00:16:16.240 --> 00:16:19.080 ever place a glass ionomer on the heavy chewing surface 312 00:16:19.080 --> 00:16:22.919 of a molar A Class two restoration. The massacatory forces 313 00:16:22.960 --> 00:16:24.080 would just crush it. 314 00:16:24.120 --> 00:16:26.440 And they are also highly sensitive to water balance. During 315 00:16:26.480 --> 00:16:29.759 the initial setting phase, overwetting washes out the ions and 316 00:16:29.879 --> 00:16:32.320 overdrying cracks the matrix exactly. 317 00:16:33.000 --> 00:16:36.679 And here is a critical procedural rule for handling gis. 318 00:16:37.720 --> 00:16:41.159 You should never apply a standard resin bonding agent to 319 00:16:41.279 --> 00:16:42.960 the tooth before placing a. 320 00:16:42.919 --> 00:16:47.600 Glass ionomer oh because if the material chemically bonds applying 321 00:16:47.600 --> 00:16:51.519 a resin layer first acts as a physical barrier exactly. 322 00:16:51.159 --> 00:16:54.320 You block the polycrylic acid from reaching the tooth's calcium, 323 00:16:54.679 --> 00:16:57.759 and you completely trap the fluoride inside the material. You 324 00:16:57.759 --> 00:17:00.120 are effectively painting over the medicine. 325 00:17:00.200 --> 00:17:02.600 That makes perfect sense. So to bridge the gap between 326 00:17:02.639 --> 00:17:06.319 the weakness of glass ionomers and the moisture sensitivity of resins, 327 00:17:06.759 --> 00:17:08.920 manufacturers created compomers right. 328 00:17:09.039 --> 00:17:12.480 These are primarily resin composites modified with some of the 329 00:17:12.640 --> 00:17:16.400 hydrophilic monomers and glass particles found in GIS and for. 330 00:17:16.359 --> 00:17:19.519 The dental nurse, compomers are a dream to handle. They 331 00:17:19.519 --> 00:17:22.079 don't require measuring powder and liquid. They just come pre 332 00:17:22.200 --> 00:17:24.799 mixed in light blocking syringes or compulars. 333 00:17:24.880 --> 00:17:27.759 Yeah, very user friendly. They offer better wear resistance than 334 00:17:27.799 --> 00:17:30.839 gid's and release a moderate amount of fluoride. But because 335 00:17:30.880 --> 00:17:33.559 the resin component is dominant, they lack the ability to 336 00:17:33.640 --> 00:17:35.559 chemically fuse to the tooth on their own. 337 00:17:35.640 --> 00:17:38.079 So they require an adhesive bonding agent and they must 338 00:17:38.119 --> 00:17:41.359 be like cure typically forty seconds for every two millimeters 339 00:17:41.400 --> 00:17:42.799 of depth, which brings us. 340 00:17:42.720 --> 00:17:47.240 To the core mechanism of modern dentistry adhesion. If resins 341 00:17:47.319 --> 00:17:50.359 and compomers don't chemically fuse to the tooth like gis do, 342 00:17:50.960 --> 00:17:54.119 how do we anchor them permanently to a wet, smooth surface. 343 00:17:54.480 --> 00:17:57.960 We rely on micromechanical retention, and it starts with a 344 00:17:58.079 --> 00:18:01.920 highly corrosive chemical thirty seven seven point five percent phosphoric 345 00:18:02.039 --> 00:18:04.920 acid gell, commonly known as acid etchant. 346 00:18:05.079 --> 00:18:08.960 Yes. The dnist applies this brightly colored blue gell directly 347 00:18:08.960 --> 00:18:12.319 to the enamel and dentine. The acid selectively dissolves the 348 00:18:12.319 --> 00:18:16.319 inorganic mineral content the hydroxy appetite crystals. 349 00:18:15.880 --> 00:18:19.160 And on the enamel it leaves behind this microscopic jagged 350 00:18:19.240 --> 00:18:22.880 landscip of exposed enamel prisms, while on the dentan it 351 00:18:22.960 --> 00:18:26.160 uncovers a spongy, porous network of collagen fibers. 352 00:18:26.240 --> 00:18:28.880 It essentially takes a microscopic glass window and turns it 353 00:18:28.920 --> 00:18:30.680 into an aggressive molecular velcro. 354 00:18:30.839 --> 00:18:33.839 Molecular velcro I love that you carefully wash the acid away, 355 00:18:34.200 --> 00:18:36.359 dry the tooth slightly, and then paint on a liquid 356 00:18:36.440 --> 00:18:39.599 resin bonding agent. That liquid flows into all those microscopic 357 00:18:39.680 --> 00:18:41.319 jagged peaks and valleys. 358 00:18:41.200 --> 00:18:43.079 And when you hit it with the curing light, the 359 00:18:43.119 --> 00:18:48.559 resin hardens into millions of microscopic solid plastic anchors firmly 360 00:18:48.640 --> 00:18:53.240 locking into the tooth's architecture. The composite filling then bonds 361 00:18:53.319 --> 00:18:55.240 chemically to that layer of anchored resin. 362 00:18:56.039 --> 00:19:00.000 But handling acid engine requires a strict no touch technique. 363 00:19:00.200 --> 00:19:03.799 Right when the assistant wipes away excess, they must use 364 00:19:03.839 --> 00:19:07.240 gauze and extreme care not to contaminate the adjacent tissues. 365 00:19:07.319 --> 00:19:11.559 Oh. Absolutely, And because this entire velcrosystem relies on the 366 00:19:11.599 --> 00:19:15.160 resin penetrating clean, dry pores, it is entirely useless if 367 00:19:15.200 --> 00:19:18.519 a patient cannot tolerate strict moisture control. Yeah, like if 368 00:19:18.519 --> 00:19:21.720 a pediatric patient or an elderly patient with excessive saliva 369 00:19:21.880 --> 00:19:24.680 cannot keep their mouth completely open and dry, the resin 370 00:19:24.720 --> 00:19:26.880 will fail to penetrate and the filling will literally just 371 00:19:26.920 --> 00:19:27.480 fall out. 372 00:19:27.799 --> 00:19:30.119 And in those high moisture cases you pivot back to 373 00:19:30.160 --> 00:19:33.720 the chemical adhesion of glass ionomers. Right now, let's shift 374 00:19:33.720 --> 00:19:36.200 focus a bit. If the tooth damage is too extensive 375 00:19:36.240 --> 00:19:38.880 for a direct filling, the dentist will prepare the tooth 376 00:19:38.880 --> 00:19:42.279 for an indirect restoration, like a laboratory made ceramic crown. 377 00:19:42.920 --> 00:19:45.880 To attach that cran permanently, we require looting agents or 378 00:19:46.000 --> 00:19:46.799 dental cements. 379 00:19:47.079 --> 00:19:51.359 We discuss zinc phosphate earlier, but polycarboxylate cement is a 380 00:19:51.400 --> 00:19:55.319 major player here too. It utilizes a zinc oxide powder 381 00:19:55.599 --> 00:19:58.039 mixed with a polyocrylic acid liquid, and. 382 00:19:57.960 --> 00:20:02.039 The handling of polycarboxylate provides this brilliant visual cue for 383 00:20:02.119 --> 00:20:05.759 the underlying chemistry. The protocol states it must be mixed 384 00:20:05.880 --> 00:20:08.839 rapidly within thirty to sixty seconds on a glass slab, 385 00:20:09.480 --> 00:20:12.759 but the critical indicator is its surface appearance. It must 386 00:20:12.839 --> 00:20:14.680 look glossy when applied to the crown. 387 00:20:14.839 --> 00:20:18.200 Yes, that glossy sheen is vital. It indicates that the 388 00:20:18.240 --> 00:20:22.079 polycrylic acid molecules are still free and available to chemically 389 00:20:22.119 --> 00:20:24.119 react with the calcium in the tooth structure. 390 00:20:24.440 --> 00:20:26.480 So if you are mixing it and the surface turns 391 00:20:26.599 --> 00:20:29.400 dull or mad, it means the polymer chains have already 392 00:20:29.400 --> 00:20:32.640 begun cross linking. The chemical reaction has advanced too far. 393 00:20:32.839 --> 00:20:36.000 Exactly, it physically cannot bond anymore. If it looks mad, 394 00:20:36.119 --> 00:20:37.799 you must throw it in the bin and start over. 395 00:20:37.920 --> 00:20:40.200 That's a great practical tip, but before we even get 396 00:20:40.240 --> 00:20:42.559 to cementing a crown, we have to capture an exact 397 00:20:42.680 --> 00:20:45.200 replica of the prepared tooth to send to the lab. 398 00:20:45.720 --> 00:20:47.880 We need impression materials right. 399 00:20:48.240 --> 00:20:52.400 And for Crown and Bridge work, accuracy is paramount. Polyethro 400 00:20:52.480 --> 00:20:56.799 impression materials are highly favored because their hydrophilic nature allows 401 00:20:56.839 --> 00:21:00.920