This is very important and quite a time- consuming stage that you don’t really see.
After reading this, you will probably have a new found appreciation for, not just the process, but also the cost of dentures.
This same basic design process is followed in order to come up with the best possible solution given your particular circumstances. The ultimate success of your RPD (removable partial denture
) will depend on many factors- the design is one important aspect.
First the impressions are poured up to give a replica model of your teeth and these are mounted on an articulator. The record of your bite is used to make sure the top and bottom model fit together as they do in your mouth.
The teeth are then surveyed using a special machine, called rather unsurprisingly a 'Surveyor'- this helps to identify the following:
- The best way for the denture to be put in and taken out.
- Areas that can be used for retention (clasps) to help hold the denture in.
- Areas next to teeth which need to be adjusted in the mouth, or blocked out before the denture is made, to allow it to be put in and taken out easily. Elastic impression materials will stretch over a bulbous area of your tooth or ridge, but the hard acrylic or metal of a cobalt chrome denture won’t. If we do not relieve things in this area as you try to put it in, it will scrape over your gums- not very nice!
The denture is then designed by the dentist (ideally), technician or both in communication together.
The following must be planned:
This is a term for the teeth that are to be replaced.
How will the denture resist forces towards your gums during eating? Will it be from teeth... in which case it is known as a ‘tooth borne’ denture
' or from your gums/ridges, known as a ‘mucosal borne
’ denture or a combination of both.
The decision depends on the number and position of the missing teeth
, as well as the quality of the remaining teeth. If all spaces have good teeth either side, then a tooth borne design can usually be used- this is much preferred as teeth are better at taking the forces of your chewing and don’t resorb in the same way that your ridges do.
Tooth-borne dentures must be cobalt-chrome dentures. Mucosa- borne dentures must be made entirely of acrylic. 'Tooth and mucosa borne' will be a cobalt chrome denture with an area that rests just on the gum.
How will your denture stay up or down (if it is a bottom denture)?
Retention stops the denture coming away from your gums. Grip or retention for your denture can be obtained by using metal clasps, guide planes, precision attachments, and, if undercuts are present, sometimes by the shape of your ridge.
All of these technical aspects need to be carefully thought through and considered- it is not just as simple as saying- "Yep let's chuck a clasp on that tooth". For example, the surveying procedure described above will identify if there is enough undercut on the tooth for a clasp and if not, the way the denture is put in may be changed to create this. A bit of white filling material might be used to change the shape of the tooth in the mouth and create one.
How deep is the undercut? If it is too deep, the clasp may grip too much and not come out. Two major types of clasp exist: the clasp might come down from the top of the tooth (occlusally approaching clasp) into the undercut, or possibly up from the bottom upwards (gingivally approaching clasps)- and that’s just the possibilities for one simple clasp… You can see that this is a very involved process! The British Dental Journal books dedicate a thick clinical guide just to this designing process!
How will the denture be prevented from moving sideways and rocking? Bracing will help prevent lateral movement and can be got from:
(i) A metal arm on the other side of the tooth to the clasp.
(ii) The connector of the denture- the part of the denture that holds or connects everything togther e.g. the metal framework.
(iii) The saddle- the bit of the denture on which the missing teeth sit – it can either be entirely acrylic or a metal framework with acrylic on top.
Which type and design of connector will be used? There are many, if not endless variations and the choice will depend on the number and position of the missing teeth; how high your smile is; if there gaps between your teeth where metal may show through; if are you likely to loose more teeth which will need to be added in the near future and more!
Where possible the choice should keep the denture away from the margins of your teeth to help cleaning. On the bottom the decision is generally between a lingual bar and a lingual plate. Bars are generally preferred because they are healthier, but only possible if there is enough room (over 7mm of space).
Plates provide great stability, support and retention- the only downside is they cover the margins of your teeth.
You can have the most beautifully designed denture that will stay in your mouth perfectly and not rock and move but if you have a high smile, visible metal near the front may be unacceptable.
If you have a low smile, you can often have the extra retention and clasps even onto your front incisors and no-one except you and the dentist will ever know.
If you have a high or gummy smile, you are often faced with a choice and it is just a matter of deciding what is most important.
The clasp is there to provide added retention to stop the denture moving and coming down; removing this would certainly improve the look, but is that worth it? It depends on the rest of the design? Is this the only grip? Will losing it be catastrophic? Can a smaller, less visible clasp be used to provide some
support? These are all things which will need to be discussed your dentist during the design phase.