Once the foundation (core) for the crown has been done, you are ready to have the tooth prepared for the crown. The stages are as follows:
- Anaesthetic. First the area is made numb using anaesthetic.This is necessary most of the time but not always. If for example, the tooth has been root filled and the margins of the crown preparation are to stay above the gum line (supra- gingival), no anaesthetic may be needed. However if the dentist will be working near or under the gum, it is better to have some in- it also helps to take your mind off the area allowing you to relax.
- Impressions. Sometimes impressions may have been taken at the consultation appointment for study models. These are poured up in stone to give the dentist a replica of your teeth in order to assess them, your bite and the tooth for which the crown is planned. The model of the tooth can be used to help make the temporary crown that is worn whilst the final crown is being made. If these impressions have not been taken at the consultation- I normally do them whilst you are going numb to save time; they are generally an alginate impression or a putty mould of the tooth to be prepared.
- Crown preparation. Here the dentist will use the high-speed hand piece (called the 'air rotor') to file the tooth down slightly in all dimensions to make space for the crown. How much they file the tooth down depends on the type of crown being made (and hence the material it is being made of). A gold crown is very strong in thin sections and so minimal filing is needed. All porcelain crowns however, generally require more tooth/core to be taken to allow sufficient thickness of material for strength. Some of the new Zirconia porcelains, whilst not the best looking (still tooth coloured) allow for a more minimal preparation due to their incredible strength. Some dentists may scrape instruments called ‘margin trimmers’ around the edges of the tooth, (which doesn’t make for the greatest sound and feeling), in order to smooth the margins to make sure they get a good impression and ultimately a great fitting crown.
Some slightly more technical 'dentist type' details relating to the crown preparation are:
(i) The taper of the walls-
It is important that the walls of the preparation are kept as tall as possible and that they are not too tapered. The more vertical, the better the crown will stay on. But not too vertical so as the crown will not go on! In terms of taper 5-10% is much better than if they are very tapered e.g. 20%. Sometimes the remaining tooth will dictate how much this is possible.
(ii) The walls should be parallel-
If you look down from above on the preparation and close one eye, you should be able to see all the margins of the tooth – this means there are no undercuts, that the impression will record the core accurately and the crown will be able to be made correctly and seated down fully.
(iii) There are no sharp edges-
Everything should be rounded and smooth- ideally following the pattern of the top of the tooth. Sharp corners can cause stresses to concentrate in those areas and potentially lead to fracture of the crown.
(iv) The margins should be at an appropriate level-
If the surface of the crown will be visible in the mouth then the margin should be at or just below the gum line. But where the crown margins cannot be seen- where possible (and with your approval)- it's best that they are slightly above the gum margin to ensure the most accurate impression and to aid cleaning of the final crown, thus preventing associated gum problems. I say, 'where possible', because there are times that it will need to be prepared lower down e.g. a very short tooth for extra retention or in order to create a good ferrule to resist chewing forces. The type of margin (finishing line) will also depend on the type of crown and can be a chamfer or a shoulder.
Once the dentist is happy with the preparation of the tooth, it's necessary to take an impression on which the lab will make the crown. This step is very crucial- the impression has to be very accurate to ensure a good fitting crown.
If the crown preparation is at or below the gum, then in order to make sure that the margins are accurately recorded then dentist may wish to push your gum back a little to expose them more clearly.
We have two ways of doing this and dentists will each have their preference. The most common way is to use something called 'retraction cord
'. This is a thin cord that’s pushed into the gum crevice surrounding your tooth which helps move the gum away from the tooth, so that the impression material can flow down next to it more easily. This can taste quite revolting, if it is soaked in a haemostatic agent to stop the bleeding and can sometimes be a little sore on the inside of the tooth if the nerve in that area is not numb.
One or two cords may be placed and they maybe left in or taken out prior to the impression- it just depends on the circumstances and the preference of the dentist. If you have cord placed, this can be a little sore after the anaesthetic has worn off but should be back to normal within a day or so. Another option, is using a material such as Expasyl which is squirted into the crevice around the tooth. This stops any bleeding and gently expands to push the gum out of the way. It must be thoroughly rinsed off before taking the impression.
The material used for this impression is different to that taken for the opposing model and temporary. Generally a newer silicone based material is used. These materials are very accurate and therefore take longer to set. An alginate impression may set in about a minute (depending if it's a hot or cold day), but a silicone material could take up to 5 minutes- it depends on the silicone, some set more quickly than others.
Some dentists will use a single material whilst others (me included) will use two different materials- a thicker material in the tray (‘heavy bodied’) and a thinner more fluid material that they squirt in and around the tooth in question (‘light bodied’). The heavier material forces the lighter material all around the tooth and the two are bonded together during the setting process. There are various differences and techniques that dentists can use to do this stage- ultimately a very accurate impression of the crown preparation and the teeth either side is needed.
It is very important the tooth is nice and dry and free from saliva and blood so the dentist may pad you out with cotton wool, dry tips and spend a second or two drying the tooth. If the tooth gets wet, the impression is likely to be less accurate and may become distorted.
After it is set, the dentist will take the impression out- sometimes it can have a very good grip (remember it's very accurate) and therefore can take a bit of effort. The material is slightly flexible, so it won’t get stuck and don’t worry- it will not pull out any of your teeth!
The impression must be inspected under a bright light to check that it has accurately recorded the whole of the crown preparation and teeth either side. Sometimes you can get drags, distortions, air bubbles or find that material hasn’t flowed properly around the margins. If this happens, unfortunately the impression must be retaken. Since it is this impression that will determine how well the final crown fits, (it is the only thing the lab has to work on)- it's crucial that it is right - I have had to take 3 impressions on occasions to get an appropriately accurate record.
The dentist needs to record how you bring your teeth together; we call this your 'occlusion
' or bite. This allows the technician, once he has poured up the impressions in stone to create replicas of your top and bottom teeth, to put them on an articulator to mimic how your teeth interlock. They can then make a crown that fits with the way you currently bite.
Generally a silicone material e.g. blue bite will be squirted into your mouth- you will then be asked to close together normally and stay together whilst it sets- which is pretty quick- say 30 seconds. Wax may also be softened and used. Sometimes, the way you bite (your occlusion) is obvious, especially if you have no spaces and in this circumstance the dentist may not need to take a bite impression at all.
Different dentists will make you a temporary crown at different times during the appointment but the process is pretty much the same. The mould or matrix that was taken before your tooth was prepared is filled with a cold cure acrylic material, just in the area of the tooth for the crown. This material fills the space between the filed down tooth and the original impression taking just over a minute to set to the point where it needs to be taken out. If it is taken out too soon, it's still gooey and if it sets to hard, it can be difficult to remove.
Once the dentist has polished this up, the result is a tooth coloured temporary crown that will sit over your tooth while the lab is making the real crown. This is important in a number of ways. The most important reason is that if a tooth is vital (alive) i.e. it has not had a root canal, preparing the tooth exposes the dentine tubules that are directly connected to the nerve.
If bacteria in saliva and plaque are allowed to get into these tubules in between appointments- because the tooth has not been sealed by a good temporary- then the nerve is much more likely to die off over time.
This is the main reason why some teeth prepared for crowns die off and eventually need root canal. Other reasons for a good fitting temporary are to prevent sensitivity, to ensure that the teeth either side and those opposing the prepared tooth, stay where they are, i.e. it maintains the space to ensure the final crown fits.
Even slight movement of the teeth next door will mean the dentist has to adjust the crown to make it fit. The temporary crown is cemented in with a temporary cement (weak cement) to ensure it can be easily removed at the next visit and the bite adjusted so it's comfortable.
Here’s some advice I give my patients with a temporary crown:
(i) Take it easy on that side- appreciate you have a temporary and it's much weaker than a normal tooth so treat it with care. Often just being aware and conscious that you have a temporary on that side will do the trick and prevent you chewing too hard.
(ii) Where possible consider chewing on the other side.
(iii) Pull the floss through when you are flossing, rather than pulling it back up which may dislodge the crown.
The last thing the dentist needs to do before you leave is take the shade. There are a couple of different ways to do this. The most common method is to simply match the shade of the teeth either side using a shade guide. Vitapan and Are, are the two principle guides used in Australia. Some laboratories and probably some practices have digital shade taking devices for crucial areas.
These machines take digital photographs of the teeth on either side and the remaining tooth before the temporary is placed and with the specialist technology, the appropriate shade is determined.
Often this will be 2-3 different shades for the different parts of your teeth. For example, the cervical portion of the tooth, (that nearest the gum) tends to be slightly darker. The body of the tooth is the main colour while the incisal tip, (the top part) is often very white or translucent. Have a look at your own teeth and see how the colour changes from top to bottom and also between different teeth.
The technicians are able to get quite arty and add little touches of colour and specks to make teeth look as symmetrical as possible- we call this characterization. It is the job of the dentist to communicate this information with the laboratory for a great result. In cases where all the teeth are to be crowned or veneered then it is much easier to get them all to match, because all the crowns are made from one mix of porcelain and as all of the surfaces are porcelain, they will reflect light in the same way.
Porcelain mimics teeth very well and is improving all the time but it still reflects light in an ever so slightly different way.
The untrained eye may not notice it and when the teeth are wet with saliva, it can be difficult to distinguish. Still, I am trying to help set realistic expectations. One of the most difficult circumstances to match is a single central incisor i.e. where one tooth is natural and one needs a crown. This is because the eye will notice asymmetry more easily here. As a result some dentists may advise that a veneer or crown be placed on the other central incisor so that the mouth is symmetrical ,thus providing optimum aesthetics.
The shade should be taken in natural light, not artificial light - near a window is the best place and it is always a good idea to get the opinion of the dental nurse, besides having a look yourself in the mirror too, to check that you are happy.
The dentist will communicate the time frame, type of crown, shade and any special instructions, such as a rest seat for a partial denture
on a lab sheet which they send off to the laboratory with your impressions and bite. The lab will pour up your teeth, place them on an articulator to mimic the way you bite together and make the crown which will fit your tooth.