See how they’re made: Dental crowns

We’ve just gone through the sequence of making a set of dentures. One of the other things that technicians do is to make crowns and bridges. And these are either single crowns, or they can be bridges where we’ve got two or three crowns replacing a missing tooth, as in that case there. And bridges can be quite destructive or minimal. This is a minimal bridge where we’ve just taken a very small scrape off the teeth, either side of the missing gap, and we’ve put a little metal collar in there, which is bonded onto that tooth. But mainly, the tooth structure stays as it is.

Whereas the one I showed you a minute ago, we have to prepare both the teeth either side of the gap, which is a little bit more destructive because, to be fair, they were fairly healthy teeth, just to replace that missing one. So we make a variety of different crowns and bridges. And as you can see on this model, we use a variety of different metals. This one is an old gold crown, which most people are familiar with. It’s got a gold colour to it. It’s made out of high gold content alloy. But the downside to that is that it doesn’t look very aesthetically pleasing and, obviously, looks like a metal crown.

So we can also produce crowns that look like your tooth. So they’ve got a metal backing, which, again, would be a palladium alloy. And onto the front of it is veneered some porcelain, which looks like your tooth. We can also make crowns in all ceramic materials, like this one, where there’s no metal at all in there. And that is, really, the best aesthetics we can achieve. The downside to that is that they tend to be a little bit weaker than the metal-supported ones. And so if you’ve got a very tight bite where your lower teeth are biting onto your upper teeth quite closely, we would normally provide you with a metal backing which strengthens the crown.

So how do we produce these things? Well, 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. So when we make any metal restoration, whether it be an old gold crown or a crown which has got metal on one part of it and ceramic veneered on the other, we always make the metal part in wax first. So if I just quickly run through what we do, we would get our die.

We put a little bit of separator on it to stop the wax from sticking to the model, perhaps a little bit on the adjacent teeth and perhaps on the opposing teeth just so that when we’re checking the occlusion, the wax doesn’t stick to that. And then light the Bunsen burner, and then 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, we can work individually on the die. We can get around all sides of it, and we start to build up the wax onto the tooth.

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. So 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 mould, we’re going to fire molten metal. And it can be, as we’ve just described, either gold or a palladium-based, platinum-based metal or even a nickel-chrome metal, a nonprecious metal, depending on 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 and attach this sprue to the thickest part of the crown, which, in this case, is the medial buckle cusp. And then, very carefully, so we don’t destroy our carving, smooth that round so it joins the wax pattern like that. I’ll 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, it’s a cone shaped little projection which fits on the bottom of the ring, so that when we fill it with a 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. Pop the crown onto there. So at the end of the cone is our crown suspended on that wax tube.

So the ring goes over the top, and 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, and 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 can 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. We cut off the sprue, and we’re left with a 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 powders, the porcelain powders, and fire them onto the metal. So he sinters it on to the metal substructure to form that nice ceramic coating. So that, very quickly, is how we make crowns and bridges.