★ How Dental PFM Crowns are Made | Step-by-Step Photos ★
The Hidden World of Crown Making
Before I learnt how to make dentures, crowns and orthodontic appliances myself, I had not even considered that these things were custom made individually by actual people - I just never thought past when the dentist takes the impression of the teeth. But it is a job, and it's not one that many people will have heard of, never mind know much about it - I mean I only found out about it accidentally! But now I have first-hand experience of the intricate, time-consuming process that is making a crown, and I want to share how difficult, unrecognised and undervalued a craft it really is.
If you ever have to have a crown, just think that someone, somewhere tailor made it from scratch, especially for you :-)
I hope you find this insight interesting.
Video Showing The Making Of A Gold Crown
Step 1 - Cleaning Impressions
These are the impressions of the upper and lower teeth which are delivered to the lab. These can range from clean to gross! First they MUST be disinfected.
The impression on the right has a section of differently coloured impression material where the crown is destined for. This gold material is runnier than the blue material and so is better at capturing detail.This is why it is being used for the area where the crown is to be fitted - it really needs to be accurate.
A PFM crown is a porcelain-fused-to-metal crown, consisting of a metal substructure in a cap shape (made to fit perfectly over the patient's tooth) with a porcelain coating.
A patient requiring a PFM crown needs to have their damaged tooth ground down by a dentist to remove the damaged tissue and to create enough room to fit the metal and porcelain crown into.
The impressions above belong to a patient who did not have enough undamaged tooth tissue left to support a crown, so the whole tooth was removed and a tapered hole was drilled into their alveolar (gum) ridge. A metal post will be made to fit into this hole and this will support the PFM crown as the tooth would have done.
You might be able to see the gold spike in the impression shown above, where the gold impression material has been pushed into the drilled hole of the patient's gum. This spike shape will be exactly replicated in metal to form a post.
Step 2 - Positioning the Impression
Plasticine is rolled out into a long strip and cut so that it just comes up to the slot in the back of the metal tray. This is so that a plastic plate can be slotted correctly onto the metal tray in the next step.
The 'important' impression i.e. the one containing the impression of the crucial tooth which requires the crown, is raised to make it level and centred, plus to keep it approx. 1-2cm below the plasticine strip.
Note that the impression has been trimmed with a scalpel to remove excess impression material and to make the impression flatter and more level on top.
Step 3 - Preparing the Mould
The plasticine is wrapped around the impression and some is used to fill the hole in the centre.
The top of the plasticine should be flat and level, and the top should lie in line with the bottom of the slot in the back of the metal tray.
The plasticine must have no holes or gaps.
Step 4 - Baseplate Preparation
Next, a white plastic baseplate is inserted into a drilling machine. The pointed metal rod at the top lines up with a drill that is underneath the baseplate.
Step 5 - Drilling the Baseplate
The silver metal tray is secured onto the black metal block containing the white baseplate. This will ensure that the impression lines up with the white baseplate below. The black surface can slide around.
The metal rod above the impression is in line with a drill beneath the white baseplate, and is used to direct the drill into the baseplate to create holes in the correct places. Later on in the process, the cast made from this impression will be split into sections - in this case, it will be 3; the crown section will need to be removeable so the cast will need to be split either side of the place where the crown will be fitted.
Therefore, the places to put the holes must be decided in this step. There needs to be at least 2 holes drilled into each section, depending on it's size. Metal pegs will be put into these holes in the next step to help with relocation later.
Step 6 - Add the Metal Pegs
Metal pegs are inserted into the holes. Note that the crown will be positioned where there are 2 pegs close together.
Step 7 - Pour the Dental Stone
Dental stone powder is mixed with water in a vacuum mixer. The mixture is slowly poured into the impression whilst holding the tray on a vibrating table to prevent air bubbles being trapped. Air bubbles can ruin the resulting stone cast.
Some of the stone mixture is poured onto the white baseplate, to try to prevent air bubbles being trapped around the pegs.
Step 8 - Make The Opposing Cast In Plaster
The white baseplate is pressed into the slot in the metal tray, making sure it is pressed down onto the plasticine.
The other impression (of the lower teeth) is filled with plaster on a vibrating table, and a pile of plaster is put on the worktop. When the pile has set slightly so it's no longer runny, the impression is turned over onto the pile, and the plaster is shaped around the impression tray shape. The impression must be level and centred.
The stone only needs to be used for the 'important' upper impression (rather than plaster), because this cast is the one that will be worked on and so needs to be more hard and durable.
Step 9 - Remove The Plasticine & Impression Tray
When the plaster and stone have set, the plasticine and impression trays are carefully removed to leave you with a plaster model of the lower teeth, and a stone model of the upper teeth attached to a white baseplate with metal pegs.
Step 10 - Top & Bottom Views of the Upper Model
This is the upper model from the top, and from the bottom when the white baseplate is removed.
The metal pegs allow the cast to be removed from the baseplate and then connected again in exactly the same position.
Step 11 - Sanding The Upper Model
Excess stone material is removed from inside and outside the tooth arch. The model is then sanded and smoothed on the sides to basically make it presentable and easier to handle.
Step 12 - Separating the Crown Area from the Rest of the Model
The model is put back onto the baseplate and it is sawn into 3 pieces with a cut either side of where the crown is needed. The gingival margin, which is the top edge of the gingiva (gum) surrounding the tooth crown - and is where the crown will need to meet the gum, is marked out in red.
The upper half of this stone section is sanded into a smooth tapered shape up to the red line.
Step 13 - Make the Wax Post
Surface sealer and lubricant is used to coat the crown section of the model so that wax will not stick to it.
Then a thin plastic post is used to help molten wax fill the hole in the model completely, add strength and keep the wax post together.
Step 14 - Shape the Top of the Post in Wax
A rough tooth shape is built up in wax on top of the post. Enough space is left on every side of the wax to fit the crown into. Note that the wax is not flat on top because a v shape means that the crown will have more support from the post and more surface area to attach the crown to. This will help the crown stay in place despite the strong biting/eating forces which will act in many directions on the crown.
If the patient has enough tooth material to support a crown, these steps are obviously missed out as a post will not be needed. Instead, the ground down tooth will look similar to the wax shape shown above, and a crown will be built onto that (see later steps for the crown manufacture).
Step 15 - Sprue the Wax Post
The wax post is removed from the model and attached to a yellow cone shape via a sprue (a wax rod).
Everything in wax is going to be surrounded with a special investment material and the wax will be removed, leaving a cavity where the wax was. A sprue is a wax channel which, when removed, is designed to leave a tunnel from the outside of the investment material to the inside post shape.
The cone shape and the sprue are made to be smooth and rounded, to transport the molten metal used when casting to the post shape as smoothly and as quickly as possible - any obstructions may create turbulence and bubbles, and will slow the metal down. Slowing the movement of the metal down is not recommended because the metal cools quickly and it must not set at all before it reaches the shape to be cast.
The position of the sprue on the wax post is important. It must touch the wax post at the largest/thickest section and then direct the metal down the path of least resistance.
Step 16 - Pour the Investment Material
A flexible tube mould is pressed down onto the yellow cone so that the wax post and sprue are enclosed near the centre of the tube. A small amount of debubbleiser (great name!) is sprayed onto the wax post, which is to make the investment flow easier over the wax to try and prevent bubbles.
Investment material is mixed in a vacuum mixer, and this is poured slowly into the tube.
Step 17 - Burnout & Casting
When the investment is set, it is removed from the tube. The cone is taken out and the cylinder is placed in a furnace for the wax to be burnt out to leave just a cavity in the investment where the wax previously was.
The investment cylinder is then put into a casting machine so that it lines up with a crucible (in which the metal is placed). The crucible and cylinder are then enclosed in a vacuum and the metal is melted (which you can see in the bottom right of the photo above). When the metal is at the right temperature (i.e. completely molten), the metal is poured into the cavity of the cylinder. The cylinder is removed from the machine with tongs and left to cool.
Step 18 - Removing the Metal Casting
A hammer is used to break the metal casting out of the investment material. Any excess investment is removed from the metal.
Step 19 - Sandblasting the Metal Casting
A sandblasting machine is used to fire a stream of fine granules at the surface of the metal casting to clean and polish the surface.
The excess metal is removed from the casting and a fine grade bur* such as a diamond bur is used all over the metal post to remove any bobbles/irregularities and to smooth the surface. The bur is not used to remove any metal because this may cause the post to be ill-fitting for the patient.
The post is checked to see that it fits snugly in the stone model.
*A bur is an abrasive attachment for an electric handpiece, which rotates fast like a drill. There are many available with different designs, different uses, and different grades.
Now we move onto the manufacture of the crown.
First, the casting wax is built up into what the shape of the metal substructure will be. The wax must be melted before it is added to the crown, as it needs to be closely adapted. The margin around the bottom of the crown must go up to the red line which was drawn on previously. If the wax doesn't go up to the red line, the resulting crown will not meet the patient's gum, and the gap will allow bacteria and food to seep under the crown leading to infection.
The blue collar on the inside of the crown is a ledge for the porcelain layer to rest on.
The wax crown is then cast in metal just as the post was in previous steps.
So now there is a post (on the left of the photo) and the metal subsructure of a jacket crown (on the right of the photo). The crown fits perfectly over the post, with a tiny, uniform gap between the two for the cement that will hold them together in the patients mouth.
It's not shown in previous steps, but a spacer is used on top of the post before the wax is added so that there is enough of a gap between the crown and the post for this very thin layer of cement.
The crown is 'degassed' in the furnace and sandblasted before the coat of opaque porcelain, which stops the metal colour from showing through, is added.
The lip at the back of the crown is not covered in porcelain. This metal lip is to support the porcelain, add strength and maintain a clear, clean and definite margin.
Next, layers of porcelain are built up and fired in the furnace.
The porcelain mix is created by mixing porcelain powder with a liquid (often water), and this is added to the crown using a paintbrush. Tissue paper is used to absorb excess moisture.
The pink coloured porcelain turns yellow when fired in the furnace, and is used to represent the darker tooth material dentine, which is found underneath the enamel in natural teeth.The white porcelain mix turns into a light coloured and quite transparent porcelain when fired, which replicates how enamel looks in natural teeth.
The dental technician aims to make the porcelain coating the correct shape, but a larger size than the finished crown needs to be. This is because the porcelain shrinks slightly during firing.
The technician will use a photo to use the right shades of porcelain, and build up the layers in the correct way to give the same appearance as the patient's adjacent teeth. You need a good eye for colour here!
After firing, a bur is used to grind the porcelain into exactly the correct shape so that the sides are in line with the adjacent teeth, the curve of the tooth is smooth and matches the other teeth, and it is the correct size. The bur is also used to grind detail into the porcelain to match the same tooth (premolar) on the other side of the dental arch.
The lower teeth, which were cast in plaster earlier, are used to check that the crown is in the correct position in relation to these opposing teeth, and that they fit together correctly. There must be no premature contacts when biting/eating.
The last furnace firing programme is used to glaze the crown and produce a shiny finish. The metal collar at the back of the crown is polished and it's finished!
Stain could also be used on the crown to add more specific detail.