Friday 29 April 2016

Warning: Very Boring Article About Dentistry

Listen: this is probably going to be a boring post for the vast majority of you. It’s about dentistry, after all.

Since it’s boring, you don’t really need to read further; I won’t mind.

Well, for those of you who have chosen to read on, here’s what:

Recently – very recently, as of this morning – I finished constructing a complete denture for a specimen with no teeth in her lower jaw. Not only did she have no teeth in her lower jaw, she had almost no gum left either. This is something that happens in people who lose their teeth at an early age (this specimen was 57 and had lost her teeth years ago), and especially if they lose their teeth to gum disease. Gum disease – gingivitis and periodontitis – causes the bone and gum to shrink drastically around the roots of teeth, causing them to loosen and fall out. The remaining bone that used to be around the roots of the teeth is then useless since there is no tooth for it to contain, so it’s quite quickly absorbed by the body. In the end there’s only the basal jawbone left, and even that slowly disappears over the years. I have seen people with such attenuated jaws that they have literally broken their mouths simply from the effort of chewing.

I am not making this up.

Now, if we are going to give removable false teeth (dentures) to people, those dentures have to attach to something to stay in place. If there are other teeth, the dentures can be attached to them in a variety of ways, for example clasps and retainers, or by being adapted around them. However, if there are no remaining teeth, then the dentures are entirely dependent on their contact with the gum to stay in place, by the phenomenon (which you may vaguely remember hearing of in school physics class) called surface tension. The greater the extent of the remaining gum, the greater the surface tension, and the better the denture stays in place.

However, when the jawbone is badly shrunken, we have a whole mess of problems. In the first place, the lack of bone means that there’s hardly any gum at all, and the little that’s left is usually a roll of soft tissue that slides around under pressure instead of staying in one place. Secondly, if there’s some gum, the margins of the denture can be kept far enough away from the cheeks, lips and tongue that they don’t push it away when the person talks or eats. However, when there’s no gum, even the slightest movement of the tongue or cheeks tends to push the denture off the tissue.

Obviously, then, it’s not easy to do such a denture. Often, the best option would be implants – titanium screws fitted surgically in the jawbone on top of which the denture can be attached – but they are expensive, and in many cases they are simply not possible. So we have to do the best we can.

Before I go further, I need to take a few minutes to explain, as simply as possible, how we go about making a denture in people with no teeth:

First, we take an impression of the jaw using a stainless steel tray of the appropriate size using a relatively stiff material. This is because the tray doesn’t approximate very closely to the jaw shape and we need a better adaptation to get a good impression. What this relatively stiff material is varies; in the past I used to use (as they still use in dental colleges) a heat-softened wax called composition material, but these days I use a heavy-bodied silicone rubber. Then, you can do two things; you can either make a model of the jaw by pouring plaster of Paris into the impression, and make a second tray on that; or you can simply scrape the top surface of the impression away. In either case, a soft, flowing material (I used to use zinc oxide paste, an extremely messy substance; now I use a light bodied silicone rubber) is placed on the new tray or the scraped impression to take a second, more detailed impression. Clear so far?

Once this second impression hardens, we pour plaster of Paris into it to make a model (cast) of the jaw. On this cast we adapt a baseplate of hard wax or acrylic resin (generally hard wax when there’s sufficient gum available, and the much stronger acrylic resin when it isn’t). This acrylic resin is “cold cured” – doesn’t need heating to harden. On top of the baseplate we attack a curved block of softer modelling wax in which we then put the acrylic denture teeth and try them in the mouth. 

Once the position of the teeth, their appearance, etc are finalised, we make a mould around the wax, teeth and cast of more plaster of Paris. Then we boil off the wax and remove the baseplate, fill the mould with denture base acrylic, which is “heat cured” – needs heating to harden – and then heat it until it bonds with the teeth. We can then remove it and polish it, and that is the denture. 

As I said, in cases where the gum is badly shrunken, what happens is that the remaining bone has only a tiny roll of soft tissue over it, and that’s all the gum there is in the mouth. The heavy impression material, instead of recording this gum, presses it flat, and the light flowing material can’t take a proper impression of something that has been flattened. So you end up with a cast with no gum whatever. Of course, the final denture that comes out of the mould then has no gum either, is totally flat at the bottom, and floats around in the specimen’s mouth without being seated on the gum at all.

Now, long ago, when I was a student in dental college, I had an idea. I suggested it to the teachers, who shot it down immediately. But this specimen of mine brought this idea back to my mind.

My idea was, when you make the cast and adapt the acrylic resin baseplate, you should then take that baseplate, grind the bottom hollow, fill it with more semi liquid acrylic resin, which as I said is cold-cured and so doesn’t need heat to harden, and put it into the specimen’s mouth. Allow it to harden in the specimen’s mouth, take it out, grind and shape it until it fits well, and then pour a second cast from that baseplate. Use the baseplate on the second cast to make the denture as described.

Since I am no longer under the jurisdiction of the teachers, I tried it in the specimen and the first and second casts show the difference clearly.

First, here’s the specimen’s mouth, showing almost no gum at all.

This is the cast poured from the first impression. Note that you can see no gum at all, especially in the front of the jaw (to the left of the photo).

This is the cast poured from the baseplate I adapted in the specimen’s mouth. Look at the left of the cast, and you’ll see a roll of gum tissue which was absolutely not visible in the first cast. Also, since I adapted the baseplate in the mouth, I trimmed the edges so that the lip and tongue movements didn’t push it away.

This is the wax block adapted on the baseplate.

This is the baseplate, seen from the lower surface.

These are the teeth placed in the wax, after positioning in the specimen’s mouth. Note that on the right side of her mouth the teeth are much higher and towards the cheek than on her left side, where they are depressed and irregular. The reason for this is that on the right side her upper teeth are placed further towards the cheek, while on the left side her upper teeth have migrated downwards until they’re almost touching the lower gum. Therefore I had to arrange the teeth irregularly in order for her to be able to chew evenly. Otherwise, if I’d placed the two sides at the same level, I’d have had to leave a gap between the upper teeth and the denture teeth on the right side, and when she chewed on the left side the denture – having no teeth opposite it on the upper right – would have tilted off the gum.

And yet, my erstwhile teachers would have intensely strongly frowned on my arrangement of teeth here, and would have demanded that I mount the teeth in the wax as though the human specimen was a practice set-up in the laboratory, where everything is supposed to conform to an ideal. No wonder the usual specimen in the clinics back in the college usually went away far from happy.

Here’s the denture after being separated from the mould. It’s not yet been polished.

And here is the cast, still in the lower half of the mould.

The cast doesn’t usually survive the denture preparation, but this one did. I gave it to the specimen, in the same state as you can see in this picture. The idea is that if the denture breaks at any time, she can bring the pieces and the cast back to me, and I can place the pieces on the cast and repair the denture with fresh material along the fracture line. If the cast isn’t there, the broken pieces have to be approximated as best one can, stuck together with hard wax, a new cast poured, and the denture then repaired. There’s always some distortion and the denture then has to be readjusted in the specimen’s mouth, a troublesome and far from always successful endeavour.

So here is the denture, after polishing:

And here it is in the specimen’s mouth:

Was she happy? Yes, she was happy. She was happy enough to pay me extra and insist I take the money.

It feels good when somebody actually appreciates the effort you go to for them!


  1. Well done! I still have all my teeth (except wisdom, obviously)and was just at the dentist a few days ago for a regular checkup. I am trying really hard to make sure they last until I'm dead.

  2. Fine, well done!

  3. I found this utterly fascinating. She is lucky to have you as a dentist. The whole process is innovative. Your teachers were fools.

  4. That's amazing. If you were my dentist I might actually go more than once a decade.


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