In our laboratory class of crown and bridge we have learned how to make different types of crowns. We are now focusing in how to make metal crowns and inlays. A crown restores a whole or part of the coronal portion of the tooth and could be made of ceramic, composite resin or metal. An inlay is a restoration that fits inside of the tooth and can also be made out of the same materials.
Technicians usually receive an impression from the dentist. First off, the procedure starts with the dentist taking the impression of the patient and immediately sends it to the dental lab to be produced and while being processed the dentist will place a temporary crown on the patient until the crown is done and sent back from the dental lab. Once we receive the case, we start things by disinfecting everything that’s sent from the dentist with iodophor and then we can start working on the case. In order for dental technicians to make full metal crowns and inlays we need to make working casts first. Working casts are used to make final dental restorations, which is a positive representation of the patient’s mouth.
Working casts are made by mixing die stone with water. First we add water and then the stone powder. We mix it with a vacuum mixer (or spatula/mixing bowl) and then pour it up using a vibrator, so the stone can flow easily. We always start from the back of the impression all the way around to avoid air bubbles and surface tension. Then, we wait until it sets and then remove it from the impression when it’s ready. We are usually recommended to follow the manufacturer’s instructions to avoid any mistakes. When the upper or lower cast is ready to remove, we trim the rough parts and any excess, which is the base until it reaches 15mm from the gum line. We also trim the opposing cast up to 10mm from the gum line as well. For us to work in each desired area, we need to separate the working areas by using pins and cutting each die.
After the casts are ready we can start using the pindex machine. This machine is used to make holes where the pins are to be inserted. First we mark with a pencil the center of the occlusal surface. Then, we align the light of the machine with the occlusal mark and start drilling. All the holes are to be in a perfect arch and aligned. After drilling we start inserting the double sleve pins in the holes to the cast with preparations and gluing them making sure they are parallel to each other.
When we cut the dies we need them to stay in place to work on them. For this reason we need a base. First we have to apply separator on the preparations so the removal of the base could be easier and does not stick to the pins or break and then we let it dry. The base is made by mixing lab stone or also called buff stone with water, adding water first and then the powder. We mix it with a vacuum mixer (or spatula/mixing bowl) and then we pour it to a cast mold using a vibrator. After pouring we place the pindexed cast into the mold and let it set. We do the same process for the opposing cast. And we let them set completely.
After casts are completely set we start to articulate to see the centric occlusion of the patient. We use plastic disposable articulators to start mounting. We glue the articulator to the back of the casts’ bases. When the glue has dried we can start cutting the dies by drawing lines in the working areas indicated by the dentist. We draw the lines in the mesial and distal sides of the preparations. There are 3 different methods of cutting dies; using a cutting disc, hand saw or a cutting die machine. I personally feel more comfortable using a hand saw.
When we are cutting the dies we have to be very cautious. We have to make sure it is straight and not to cut the pins or touch the marginal areas. After cutting the dies we need to start trimming using a carbide bur. Trimming, means to remove the excess stone from each die for easier insertion of the die into the cast when we are working on it. Furthermore, after trimming the die, we have to start ditching with a round carbide bur. Ditching is removing the excess of stone by the purpose of exposing the margins of the preparations. We have to be very careful to do not even touch or chip the margins. After exposing the margins we mark them with a marking pencil. We have to be aware that we cannot use a lead pencil since this will ruin the wax and the casting process. We could apply die hardener to strengthen the stone so it doesn’t chip easily. We also need to apply 1 coat of die spacer in order to create a space for the inside of the restoration for the dentist’s use to apply whichever bounding agent.
After all of this process, we are now ready to start waxing. We can choose any technique we prefer. I personally like using an electric waxer or using Bussan burner with a spatula #7. There are also many types of waxes used for different types of restorations. For example: casting wax, margin wax, etc. Before waxing we always need to apply a layer of separator in the die in order to be able to remove the wax up without damaging it.
In order to wax a tooth we need to have in mind the form and function of each tooth we are working on and the opposing ones. The reason we marked the margins was to make sure the wax is really close to the margin, there cannot be a space between the die and the wax.
After waxing, there are other processes we need to follow; spruing, investing, casting and divesting. Spruing is when we attach our wax ups to the wax screws. We first cut the tip of the screws and then use boxing(red) wax to connect it to the wax ups. We place the screw at a 45° angle of the wax up. Then we make ball-like shape using the same boxing wax and place it at the center of the casting ring and it should be no more than 1-2mm and then seal it. After sealing it, we use a heated tool to make holes in it so we can place the sprues in it. We will spray debubblizer and then, using a vibrator, pour the investment in the casting ring. Then we place it in a pressure pot in order for it to become hard by removing the air trapped and make it dense. Then we wait 15 minutes for it to set.
Finally, we place the casting ring ready in a burn out oven and remember to always be very cautious when using a burn out machine; we use it to cast the metal later on. We have to wait until it reaches 900° for about 45 minutes and then we let it cool off. Then we use the winding machine to shoot the metal up the crown melting the wax inside and replacing it with the metal. We wait for it to cool off again and start hitting the investment material until we are left with the full metal crown and inlay. Then we use a machine to sand off the small spaces left. As a result we have our complete metal crown and inlay and it is ready to send back to the dentist.