Skip to 0 minutes and 10 secondsOne of the other things that technicians do is to make crowns and bridges. And if we look at this demonstration model, it shows us quite a few different types of crown and the different metals that we use. So for instance, this one is a gold crown. It's quite a good restoration in that it lasts a long time. It's long-wearing, and it fits very, very well. Its downside is that aesthetically it's not very pleasing. It doesn't look like a natural tooth, and we usually use it at the back of the mouth, as you can see here, on molar teeth that don't show too much. As we come towards the front of the mouth, we can use a variety of different restorations.
Skip to 0 minutes and 55 secondsOne of the commonest is what we call the metal ceramic crown. Where we've got a metal substructure, a metal base, and veneered onto that is a layer of porcelain, which looks like the patient's tooth. See one at the front of mouth there. So we've got a metal backing to provide strength, and on the front of it has been veneered some porcelain. We can also make all-porcelain restorations, which is this one here. Which is aesthetically very, very good, but strength-wise isn't perhaps as strong as the metal ceramic crown next door.
Skip to 1 minute and 32 secondsSo if we've got a very tight bite, patient's got a bruxis habit where they grind their teeth very heavily, we would probably go and choose a metal ceramic crown over an all-ceramic one. But if pressure and force isn't a consideration then the all-ceramic crown is a good one to pick. We can also make bridges. And these come in a variety of different styles. This is one where we've got what we call a 3-unit bridge. We've got one missing tooth, and we've used the two teeth either side to provide support to allow us to replace that one individual tooth.
Skip to 2 minutes and 12 secondsThe downside of this type of bridge, is that we've had to prepare the two teeth either side to accept the metal caps. And it's fairly destructive, in the sense that we've had to hack away at two healthy teeth to replace that one missing tooth. And this again is a metal ceramic bridge, so we can see that we've got ceramic on the side that you would see. And we've got metal at the back to provide strength. We can provide what we call minimum preparation bridges. So we've lost a tooth there. We've taken a very small amount of tooth material off the palatal aspects of those two teeth.
Skip to 2 minutes and 54 secondsAnd we're using little wings, which the dentist will bond on to those two teeth to hold that missing tooth in place. So that's a minimum preparation bridge, and it's much less destructive than the one that I've just shown you. Other bridges, typically cantilever bridges, where we only use one tooth-- in this case the canine-- and we've got a little gold wing which will be cemented onto the back of that, which holds that lateral tooth in place. But we've not had to use the central on this side. And again, there are various considerations we have to look at before we can decide to use that kind of restoration. So how do we produce these things?
Skip to 3 minutes and 36 secondsWell, your dentist will prepare the tooth. So he'll grind away the bad bits of the tooth, and produce a nice little preparation. He then takes a very accurate impression, sends it to his technician, who casts a model and makes an individual, what we call die, of each of the teeth that we're going to be treating. And in this case we're going to make a gold crown on that one. And so we've got the nicely prepared tooth. We've got the rest of the model. And we've got the opposing dentition, so that the technician can use that to check the occlusion and to wax up the occlusal surface so it fits together with the opposing cusps on the opposing teeth.
Skip to 4 minutes and 21 secondsAnd we're going to use some blue, what's typically called inlay wax. Which is a hard blue wax, which is very accurate. And we usually use this coloured wax when we're building up our crowns. So, to start with, you could work individually on the die where you can get around all sides of it. And we start to build up the wax onto the tooth.
Skip to 4 minutes and 53 secondsWe gradually add the wax until we've recontoured the tooth back to the contour it was before the dentist started to cut away the defective part of the enamel and dentine. And we've got enough wax on. The first thing to do is to check the occlusion, that we're not too high. So we put the teeth together, press them together. And the lower cusps will bite into that wax, and start to create an indentation in the wax, which then the technician can use to form the cusps in the upper so that they don't interfere with the occlusion. And the patient doesn't feel as if they're biting a little bit too high on their new crown.
Skip to 5 minutes and 40 secondsSo once we're happy with it, we've got to convert this wax pattern into metal. And it's called the lost wax casting process. We've got wax, we're going to make a mould of it. We're then going to heat the wax away, and into the mold we're going to fire molten metal. And it could be, as we've just described, either gold or a palladium-based, platinum-based metal, or even a nickel chrome metal, a non-precious metal, depending what kind of restoration we're making. So we would melt the metal and fire it in. So to do that, to make the mould, we need a little sprue.
Skip to 6 minutes and 21 secondsAnd attach this sprue to the thickest part of the crown, which in this case is the mesial buccal cusp. And very carefully, so we don't destroy our carving, smooth that down so it joins the wax pattern.
Skip to 6 minutes and 42 secondsLike that. Just let that set a bit. And then, we're going to embed it inside this metal ring. So this is called a casting ring. This is a casting cone. And if you can see inside there, there's a cone-shaped little projection, which fits on the bottom of the ring, so that when we fill it with our refractory material it will form a cone shape at one end. At the other end will be our wax pattern suspended on the wax tube. Put the crown onto there, so that the end of the cone is our crown suspended on that wax tube.
Skip to 7 minutes and 33 secondsSo the ring goes over the top, our crown's inside. And then we pour in a refractory material called investment, which makes a mould of the crown and the sprue, the wax sprue. Once that's set, we can then take off the cone and we're left with the wax embedded inside the investment. That goes into a furnace, and it's heated up to about 700 degrees centigrade to 900 degrees centigrade, depending on what metal we're going to fire inside it. During that heating process, a couple of things happen. One, the wax melts out of the investment mould, so we're now left with a void inside the investment exactly the same diameter and size as our wax pattern.
Skip to 8 minutes and 24 secondsAnd the investment expands slightly to compensate for the shrinkage of the metal as it cools. And then once we've got to that stage, we put the little casting ring inside a furnace and we fire centrifugally. So we melt the metal, and then we spin it into the mould. So it spins around very quickly. The metal goes down the cone part of the casting ring into the mould, and fills the mould with molten metal. So it can either be the gold that we're going to use for that crown, or it could be the nickel chrome or platinum alloy that we're going to use for that one. Once the metal's cooled, we can then take away the investment material.
Skip to 9 minutes and 9 secondsWe cut off the sprue, and we're left with the crown exactly the same size as the wax pattern. We then grind off any excess sprue, polish it with rubber wheels so it's nice and bright and shiny, as you can see there . And if it's a gold crown, well that's it. It then goes back to the dentist, who then cements it into the patient's mouth. If it's for a metal with porcelain fused on it, the technician will then mix the porcelain powders and fire them onto the metal. So he sinters it onto the metal substructure to form that nice ceramic coating. So that very quickly is how we make crowns and bridges.
See how they're made: Dental crowns
In this video we again see Dr Tony Johnson, Dental Technician, in the dental laboratory. This time he shows how dental porcelain and metal are used to create dental crowns.
If you would like to see this process in even more detail, we have added some additional YouTube resources showing how gold and ceramic crowns are made.
© University of Sheffield