Is the dentist’s choice of laboratory important?
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The descriptions and general references to various methods of making crowns, using different materials all assume that crowns made are done so by qualified, skilled technicians. Using the latest technology and microscopes ensures that functionally the crown being made has a perfect fit to the model and works well within all movements of the lower jaw in relation to the upper jaw.
A really well hand-made bespoke crown made of porcelain requires phenomenal skills (the best way to describe porcelain is to imagine trying to create a hologram inside a clear marble of a spiral in different colours that is 22% bigger than you want it to be: after firing it at 1000 degrees celsius, it shrinks by 22%. This gives you a shrunken, cracked, rough tooth replica that needs a lot of work recontouring and adding to eventually give you the crown finished and polished.
Where appropriate, a comprehensive diagnostic process is provided, starting with patient liaison, including excellent visual evaluations. This ensures the patient's psychological and aesthetic needs are considered. During the initial consultation, a good laboratory will discuss with the patient their goals in relation to colour. From a ceramists point of view , there are many things to take into consideration when trying to match teeth perfectly:-
A good technician may see a patient but will have have seen quite a few patients in one day and this means that when the time comes to make the tooth, reliance will be upon recorded information. The answer to a good tooth match is therefore not always about seeing the technician but about the use of modern technology in the transmission, storage and retrieval of quality and key information.
The second consideration is :- What is underneath the crowns to be made? This will greatly influence the final crowns in shape and colour. If the remaining tooth structure of the tooth to be made (preparations) are too big, it will mean that there is not always enough room to create the desired shape as the colour might end up opaque and solid looking. If the preparations are too dark, this will influence the material used to make the crowns, as the colour would have to be masked with a material that is very solid (metal) and which will influence the light reflective qualities.
1. Enamel Shells or All Porcelain Crowns (without a sub-structure), get their strength from the underlying support of the dentine and remaining tooth structure by bonding on to them. Light refractive qualities are at their best when a crown is translucent and allows all the light in and out and through a crown. Crowns like these are the most flexible in colour control. If the underlying tooth is a good colour, the technician has control of the shape and colour and the dentist can also influence the colour with different cement colours available on the market . These crowns are able to give the best aesthetics possible. When these crowns are being made and the base tooth colour is dark, it can be bleached and the crowns are then made. All variations of these kinds of crowns are great with colours. The down side is that they are very difficult to cement and can crack upon cementation due to no fault of the dentist or technician (this is the reason that they are not readily offered amongst others). They give the best result, however, if the money is available. Look at case studies - Veneers www.ceramiccentre.com for examples. Underlying colour might need addressing which adds cost. If the occlusion is right (refer Occlusion) and they bond well, they will look great and last a very long time. Some dentists put them on all teeth. The underlying colour becomes part of the new crown.
2. All Porcelain crowns with either metal or ceramic sub-structures get their strength from the underlying support that they are made on. All-ceramic crowns with a harder ceramic substructure (more of a halfway house between the former and metal crowns which I will explain next) are very successful and have good colour matching abilities. This will of course be influenced by the underlying colour of the coping (harder ceramic substructure) which refracts light more like that of a crack in a block of ice. This can cause a less natural appearance in different lights and also when light refracts at certain angles. These crowns are much more colour managed by the ceramist and if he knows the underlying colours he can compensate for many factors, like irregularities in thickness and discolourations. They are more expensive to produce than metal substructure crowns as supply costs and equipment costs are higher generally. These are very strong and very good in compensating for problem areas. Most aesthetic dentistry is built on these sorts of crowns. With the demand for implants increasing, these materials provide the same foundation for all teeth being replaced and so produce more harmony with much more translucency than metal crowns and yet retaining some masking abilities. More tooth structure is saved using these sorts of crowns than the following metal substructure crowns.
3. Metal substructures with porcelain on top stop the light. Light will go in and then shoot back out without having gone through the tooth causing a solid quality. They are believed to be the strongest sort of porcelain crown. They do have an odd look to them even when they are done by the best ceramist in the world. They do look fine in darker areas of the mouth and in fact in the front of the mouth they can also look very good. To achieve this a lot of tooth structure needs to be taken away so that their substructures can be made as discrete as possible. They also stop the light going into the root. This can make the soft tissues look grey. In the front of the mouth, some people do not take to this. These crowns are less expensive than all-ceramic crowns. With long bridge-work, metal substructures still seem to be the strongest and most versatile.
So, in summary it is about a patient communicating their needs and wishes to the dentist who should translate and communicate all the information to the ceramist who would, ideally, have a consultation with the patient and ask the patient if they are bleaching their original teeth as well as;
· explain the pros and cons of colour management for better long-term colour stability of the future restorations
· explain the implications of the treatment plan, with regard to colour and shape and how this affects the chosen colour
· discuss the implications of occlusion on the length and position of occluso-critical teeth
· discuss what the patient had before and listen to their particular goals in relation to shape
· show what is possible by using photographic manipulation and by showing relevant case histories.
A database would allow instant patient portfolio retrieval as all records has to be backed up for seven years. Tooth shade normally changes in this period. With a database like this, a laboratory can consistently review a patient's case history. This ensures repeat crowns are made with absolute consistency.
The dental surgeon needs to know the long-term objectives and desires of the patient and only then can discuss the treatment plan with the laboratory. Scenarios with or without bleaching must be discussed between dentist and laboratory since this can change the planning of the whole treatment.
Complicated cases begin with diagnostic wax-ups and articulations but always impression taking is a central and critical part of the process. Lack of fit of a prosthesis most often results from distortion or inaccuracies in the impression, some of which may be difficult to detect chairside. Very often, a clinician is unaware that his impressions are seriously defective and forwards these to the laboratory with the misguided expectation that a satisfactory restoration will be constructed. The patient therefore has to depend upon good communication between the laboratory and dentist for the best restoration possible. The crowns which suit a given situation should be used to give a particular case the best result. If a patient wants the very best, this needs to be made clear and he/she must be prepared to take on a long term treatment plan as well as being open to hidden costs.
If there is a problem in the future, a patient should return to the dentist and in complicated cases see the technician/ceramist, especially with specific preferences.
All UK laboratories have to be inspected annually by the Medical and Healthcare products Regulatory Agency (Department of Health) and subject to Medical Devices Regulations 2002.
So, yes, the dentist's choice of laboratory is supremely important!
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