Dentures in some form or other have been around since ancient times (probably 2500BC), but were not generally that successful until the material vulcanite was introduced, which was a type of hardened rubber relatively easy to mould to form a plate on which false teeth could be fitted, and these were made from porcelain. Even then, dentures left a lot to be desired in terms of fit and comfort. Freud is said to have had discomfort with his gold plate, and apparently George Washington’s second inaugural address was quite short because his dentures were so uncomfortable and painful.
Then in the early twentieth century acrylic resin became available for the manufacture of both denture bases and the false teeth to put on them, and that has remained the material of choice, sometimes combined with gold or chrome cobalt alloy in making partial dentures.
There are two basic types of dentures: complete and partial. Complete dentures cover the patient’s entire jaw, replacing all the teeth, while partial dentures, preferably using a metal framework, replace one or more missing teeth and are attached to some of the person’s own natural standing teeth.
Complete, or full, dentures are necessary when there are no natural teeth remaining in either jaw. Fortunately this situation is a lot less common now than it was a couple of generations ago when often people would opt for a complete clearance of their mouth at age 21 and full dentures in the mistaken belief that it would save them from dental problems later on in life. This is somewhat understandable since dental decay was rife and restorative dentistry less sophisticated than it is today and also too expensive for many. It has become apparent however, that when teeth are lost the basal bone that held them gradually shrinks away in a process called resorption, so that anyone having all their teeth removed in their twenties can expect considerable loss of alveolar bone by the time they are forty or fifty, often leaving little bony ridge to support a denture, and consequent problems with wearing dentures throughout their later life. We now know that it is in a person’s best interests to keep their own teeth for life and that it can be done by means of good oral hygiene practice and a sensible diet.
There are a few elderly patients who still require full dentures and these are constructed from impressions of the ridges of bone in the mouth and including the palate in the upper jaw. Impressions may be of the soft tissues at rest (muco-static) which can give a good fit but don’t allow for the change of shape in the gums during movements of chewing when there is a certain degree of pull or pressure which can cause displacement. Some dentists prefer to use a muco-displacing technique to get an impression of the tissues under load that more closely resembles the situation during use.
It is also necessary to take into account technical considerations such as the neutral zone when making full dentures. This is the area in the region of the alveolar ridge where the forces of thrust from the tongue are balanced by the forces from the lips and cheeks. Placing false teeth in this area of neutral muscular action gives the denture the best chance of staying in place.
A further consideration with full dentures is that of the freeway space, and this is the slight gap of a few millimetres that is normally present between the teeth of both jaws when they are at rest. If dentures are made too big with consequent reduced freeway space, it makes it difficult for the patient to use them efficiently and causes problems of general discomfort and intolerance.
Partial dentures are used when one or more teeth are missing but there are still some natural teeth remaining. The false teeth still need to sit in the neutral zone and are supported by either a plate or a metal framework (“skeleton”), which often needs to be
attached to several natural teeth by clasps of some kind in order to retain the denture. A plate design is less satisfactory than a skeleton design since it covers taste buds in the roof of the mouth and also presses on the gingival margins around teeth tending to strip away the delicate gum tissue. This gum-stripping effect can be reduced by shaping the plate carefully to avoid gingival margins wherever possible. Even better is to use a thin metal framework, usually made from chrome cobalt alloy which has precision cast clasps to give retention without too much force being applied to the natural teeth, and also incorporates occlusal rests which are small tags of the metal made to fit into the occlusal (biting) surface of some of the back teeth in order to support the denture and prevent it from sinking into the gums.
Partial denture design is a complex subject and involves certain basic principles but also some detailed study of models of the teeth when using an uncompromising material like chrome cobalt alloy. This is ideal in that it is stable in the mouth without tarnishing, and is lightweight and strong. It is quite inflexible however so the provision of clasps around teeth for retention involves careful survey for undercuts which need to be within certain narrow limits for the claps.
Partial dentures may be indicated when there are many teeth missing and too few remaining to adequately support fixed bridgework, although sometimes support can be augmented by placing some strategic implants. The design of the denture is such as to take up minimum space within the oral cavity, minimum pressure on gingival margins, and avoid covering the palate as much as possible. A skeleton design involving a framework of narrow metal bars interconnected can solve the problems and is generally cast as one piece in chrome cobalt alloy.
These kinds of dentures are generally well tolerated, although the lower can be more difficult because of presence of the tongue, and they can give several years’ service provided they are kept clean, and left out of the mouth ideally overnight to give the gums some respite.