Academy of Dental Knowledge: June 2022

Dental Crowns
Crowns are a far more complex procedure than most people realise. To make a good quality crown, a dentist has to work in within a fraction of a millimetre. There is almost no margin for error. Getting the appearance correctly and communicating accurately with the dental lab can be an issue. Added to these, discussions about the balance between appearance and maintaining tooth tissue can be tricky. It can take a lot of painstaking adjustment at every step. Add in time pressure and any other added difficulties and it's a recipe for a stressed dentist, particularly if the dentist has recently graduated with limited experience.
A calm and supportive dental nurse can work slickly with the dentist and makes the entire procedure easier. The roles of a good dental nurse and efficient lab technician are vital to the success of crowns.
Diagram showing transvers view of a tooth showing shape and thickness of a dental crown in pink colour.

Planning for Making a Crown

The first stage is the treatment planning, and deciding whether or not to place a crown. A decision has to be made about whether a filling is sufficient, whether a crown is most appropriate, or whether the tooth has become unrestorable and an extraction is the only choice. Crowns can be quite destructive due to the amount of tooth material to be removed. On the contrary, the advantage of the crowns is that they can be protective to the tooth and can help to hold the tooth together just like a helmet on the head, preventing the tooth splitting outwards, specifically after a root canal treatment. No restorative material can replace original tooth tissue so unnecessary tooth destruction must be avoided. so although they certainly have their place, crowns should not be placed without good reason.
The next big question comes is whether the patient's priority is the appearance or saving tooth tissue. Sometimes choosing to aim for the optimum appearance can be done without endangering the pulp of the tooth but at other times a compromise has to be made in order to keep the tooth alive and healthy. Some patients will be willing to accept a metal-coloured tooth quite happily to keep as much of their own tooth as possible because the metal crowns can be made much thinner, but for others that will not be an acceptable option.
At most occasions, compromises are done and agreed upon, like a metal band around the gingival margin of the tooth, or just having the visible part of the crown tooth coloured and the rest metal coloured. Discussing the patients' priorities and making sure that they know what exactly they will get end of treatment is a very important part of the process, to ensure their happiness and satisfaction.
Another important part of the planing is checking whether the tooth is vital. Often crowns are placed following root canal treatment. Therefore, it is important to check that any infection under the tooth has improved. If the tooth is not root canal treated, then vitality tests and radiographic examination is a must for long term prognosis.

Crown Preparation
The first stage of the crown preparation appointment is normally making an impression for making a temporary crown while the permanent crown is being fabricated. The impression can be made using alginate or putty. Alginate needs to be mixed smoothly and the impression needs to be accurate with no big air bubble in the relevant area specially the margins. Putty needs to be used if the impression may be needed at a later date, as alginate impressions soon lose their shape.

Then comes the actual ‘crown preparation’ stage. Local anaesthetic is normally given to anaesthetise the tissues around the tooth being prepared. Cutting the tooth for the crown is the time where the dentist's real skill is needed. The amount of tooth tissue removed and the space needed for the crown depends on the material to be used for each part of the crown. A thicker porcelain is needed for porcelain whereas metal need to be kept thinner. For a patient, the difference between little tooth tissue removal and too much removal is fairly small. The dentist will try to create a clear margin, ideally finishing on natural tooth rather than filling material, for the dental lab to work on. On one hand, a written laboratory authorisation form will communicate a lot of the information about the crown to be made, the work done on the actual tooth can give a lot of information about what the dentist is intending for the tooth and how they want the lab to make the crown. The margin needs to be clear in order for it to be picked up clearly in the impression so that the lab has something useful to work with and so that the end result is good.

Shade Selection
After the crown prep is completed, the shade(s) for the crown needs to be decided on, in the case of porcelain. A shade guide is used and the dentist will look at different shades compared with the surrounding teeth. If the crown is full porcelain, rather than part porcelain with metal underneath, the dentist may also record the colour of the tooth underneath that is being crowned. This can then be communicated to the lab, to give them a greater idea of what they are working with, as the underlying colour can shine through and affect the colour of the crown needed. Any little details wanted, for example, stains or slight cracks on the tooth, can also be recorded and communicated.
Before the patient leaves, the tooth needs to be temporised. This is where the tooth is covered in order to keep it safe and healthy while the crown is made by the lab. It also helps to prevent movement or further eruption of the tooth that could prevent the crown fitting. The impression taken earlier in the process is used; temporary crown material is put into the impression and the impression is put back over the tooth. A small blob of the material is often put somewhere to give an indication of when the material is set. Once the material has set, the impression and temporary crown is removed. The edges are smoothed off using polishing discs. Temporary cement is then used for cementing the temporary crown in place.
The dental laboratory will then make the crown. This is also a very complex process, but largely falls outside of the clinical practice. The dental technician will fill the impression with dental stone to create a model/die of the tooth to work on. The lab will work to the instruction of the dentist, creating what has been asked for. The process will vary depending on whether it is a full metal, a porcelain fused to metal or a full porcelain crown.
Once the crown returns back from the lab, the dentist will check that it is as they expected. They will check if the crown goes on and off on the model perfectly.

Crown Cementation
The temporary crown will be removed, the temporary cement will be cleaned off the underlying tooth and a cement will be decided upon. The crown will be tried in and out so that the dentist is certain about the placement of the crown. Cement will then be mixed and put into the crown, the tooth will be dried, and the crown will be placed. Excess cement will be cleared away using floss and dental instruments. Pressure will be maintained on the crown so that the cement sets with the crown fully seated/in the correct position.
The patient will then be asked how it feels to bite on. Often it will be slightly ‘high’, so the dentist will mark the teeth using articulating paper, so that the heavy contacts show up. Adjustments will then be made so that the crown is comfortable and doesn't interfere with the patient's bite.
Crowns can be a fabulous restoration, helping to save teeth that would otherwise have to be extracted. They can also be a very aesthetic option, particularly those made of/with porcelain. Crowns can be a very complex and involved procedure, but really demonstrate the important roles that different members of the dental team play in producing an end result that really meets the needs of/pleases the patient.
Summary of stages: