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UPDATE (11/14/2007) : Added videos of the Gold Casting Process.

Our final project in Restorative II is to make a gold crown from start to finish. Looking at all the steps required to make a gold crown can give you an ulcer. In total it took about 2-3 weeks of work. When I say 2-3 weeks this doesn’t mean 2-3 weeks of solid work. We had a couple of practicals to do in this time along with daily work and physiology every morning. There are some things about making a crown that SEEM like they are a pain, but I chalk this up to being inexperienced. I have written about the learning curve in dental school before and this project is no different. At first it can be uncomfortable. With experiance it become second nature and then it becomes fun. One important thing before I discuss the process: You have to be anal over the quality of your work, because if that gold crown comes out and doesn’t fit your tooth…you’re STARTING OVER!

So to ease the burden of some of the new dental students or to those of you who have not done this yet here are the things we have been doing. Those of you who have done this before, then please share your tips!
We made a crown preparation of tooth #3 (maxillary first molar). After the preparation is done (MAKE SURE IT IS EXCELLENT) you make a temporary for it and you also take an impression using a custom tray. The custom tray is made out of a material called Triad. It is made on a stone model of your typodont. Here is what it looks like:

To see how we made the custom tray see my article on “Making a Custom Tray“.

After the custom tray has been made you take an impression of the typodont with the prepared crown on it. The impression material is called polyvinyl siloxane or as it is commonly known, reprosil. There is reprosil putty which was recently nominated as my best friend (I don’t have very many as you can see by all the time I have for this website.) There is also light body, medium body, and heavy body reprosil. It comes in tubes and is applied using a gun. Here is the impression and the gun:

Here it is in the typodont head:

After the impression has set and been removed check it for bubbles, especially around your prep and the finish line of the crown. Any bubbles in this area and you should take a new impression. If the prep is good then you are ready to pour up the jade stone using the vacuum mixer. The vacuum mixes the stone under a vacuum (duh) and gets rid of bubbles. It also mixes the stone to a nice creamy mixture. You basically follow the directions. It isn’t hard. You pour the stone into your impression that has been blocked out with wax to provide a good base.

It helps if you have someone show you how to do it as seen here:

Let the stone set and remove it slowly from the Reprocil impression. THIS can be DIFFICULT! I saw about 6 people break their anterior teeth while trying to separate the two. I used a buffalo knife to chip away at excess stone around the edge of the custom tray until I could wiggle the stone back and forth enough to life it a little bit out of the impression. I then used my buffalo knife to pry the stone out of the impression by using small evenly dispersed pressure around the whole rim of the custom tray. It should eventually just pop out.

Here is the jade stone, cut down and looking beautiful. This should look EXACTLY like your mouth.

When the stone has set and has been cut to the proper dimensions (there are specifications of how thick things should be, for example: The distance from the finish line to the base should be 10-15mm, the buccal-lingual distance should be 20mm, etc.) then it is time to drill holes into your stone. Don’t worry, this won’t destroy all of your work. There are various methods to make a gold crown. Temple utilizes the pindex system. The pindex system gets its name from the pindex drill. The pindex drill puts small holes into the bottom of the base of similar size and length. Here is how it works.

A small light shines down from above onto the jade stone. This light indicates where the drill will be drilling on the opposite side. The drill comes in from below and drills a hole into the bottom of the stone. Here are some visually stimulating pictures:

The above picture is where you would place the stone.

If you look closely you can see where the light shines down onto the tooth. This indicates where the drill is drilling from the bottom. The number of drill holes is determined by the number of segments the jade stone will be cut into. This gold crown project requires the stone to be cut into 4 pieces. The importance of this will be illustrated below. Typically two holes are required per segment, but it varies. Rarely is there a blanket statement in dentistry. Here are all the holes (I may have added some to show a classmate how to do it…)

The holes are for pins that will be glued in using a special glue called cyanoacrylate (super glue). Yes, it is that technical The pins will be used to keep the segments of the stone in the correct placement. This will also be explained below. Here are the pins:

The wax around the base is for yet another step. A box of wax is added to the stone along with the wax seen above to block out the “tongue”. My camera battery died during these steps and I couldn’t take a couple of pictures but will explain and the next pictures will illustrate the idea. Plastic sleeves are placed over the pins and then wax is placed on the tips of the pins. Then wax is placed all around the jade stone and stone is poured in all around the pins. The idea is to create a thick base for the jade stone. When it has set the jade stone can be cut into pieces and the pins allow the jade stone to be taken in and out of the new base. Here is the base setting up:

Here is a shot of my workspace. There are many other small tasks going on during this process that can be done while waiting for stone to set. Getting out materials for the next step, reading about the next step, etc.

When the stone has set you can take the wax off and remove the wax that was over the pins to block out the stone. The result is as follows:

After peeling out the wax from the area and trimming everything up to look nice and neat you are ready to cut the jade stone. The idea is to cut out your single tooth prep so it can be worked on outside of the mouth and away from interferences. Wax can be placed on it easier and when you are done it can be placed back into the articulator as it was before. The pins help keep everything in the right orientation. After the jade stone has been cut it is ready to be put into the articulator. To spare me a long and boring explanation, I will let you experiance the fun of mounting the stone yourself. This is what it will look like:

As you can see the pins guide the teeth back into the proper spot and everything is held in place to keep it organized. The articulator represents the mouth in your patient and allows you to do the various movements in someones mouth like lateral or protrusive movements.

CONGRATULATIONS!!! You are half way to making your gold crown. Give yourself a thumbs up!

Those of you who know me on SDN will recognize this kid as the Cheerio King. A thumbs up from him means you are really awesome. He rarely gives them out. I get about 3 a day which proves to you how….

Ahem, I was getting carried away. What is the next step? Well, now comes the waxing up. Due to the stressful week of two physiology tests, a physiology quiz and getting ready for the final…well, I wasn’t in the mindset to stop and take a picture. I got so in the groove of what I was doing that I spaced it. You’ve seen other wax ups on the site. I will quickly tell you what you do from wax up to final product.

1. Wax up crown on the removable die. The die is the crown prep that has been cut out of the green jade stone (seen in pictures below).
2. Attach a sprue to the wax crown and put it in a metal ring.
3. Mix some high strength stone in the vacuum mixer and pour it in the ring around the crown.
4. Place the metal ring with the wax that is covered in beauty stone in a water bath for 30 minutes.
5. Then place the metal ring inside an oven so the wax melts out of the stone for an hour.
6. After the wax melts out you now have a metal ring with stone inside of it that is molded like the wax that has melted out. The gold will be spun into here to replicate the wax that has melted out.

When your metal ring has been in the oven long enough it is time to melt your gold. I have made some videos of the process. Watch them below ( I am having trouble uploading them…I will do it later, stay tuned)

UPDATE (11/10/07): The videos can be viewed HERE.
GOLD CASTING MOVIE PART ONE

GOLD CASTING PART TWO

Here are some shots of the oven (900 degrees Fahrenheit) and the gold caster:

When the gold crown comes out it is very hot. Let it cool on the bench for a few minutes and then immerse it in water. If done right the crown will come out smooth and dull. No air bubbles or porosities should be present. If there are some defects then there is probably something you overlooked. Many things can go wrong when making a gold crown. For example the wax sprue could be too thick, too thin, too long, or not attached to the wax crown correctly. These are easily avoided by paying attention to detail.

Here are some pictures of the gold crown as it goes from a rough product to a smooth and shiny final product worthy of the typodont mouth.

So there you have it. Nice and shiny and it got three checks. Each check is worth 3 points, so I got a 91% on this thing. Go to the Flickr page and click on the original sized photo. You can see a small pit on the margin and some of my anatomy was over carved leaving an over-reduced height of contour. Overall it was a decent crown that would have been clinically exceptable in the clinic. I also had fun on the project. The learning curve was definitely there, but over time and with some practice and quality control I think I would be able to do all these steps in a good afternoon of solid work. Remember that you want to refine your technique in the pre-clinic because in two years you will be working with a real live human patient who breathes and talk and has problems with their mouth that you are supposed to fix.