The traditional way of replacing a missing tooth has long been a denture which can be satisfactory in most respects but suffers from problems relating to damage it may cause in the long term to the natural teeth and gums, since it inevitably exerts pressure on both during use and can tend to cause loss of gum tissue and even loosening of teeth.
The ideal denture design is one that minimizes this risk to the natural teeth and gums, takes up little space in the mouth, yet is held firmly in place. It is difficult to make the perfect denture, but intelligent design helps to provide a denture that is satisfactory in terms of function and aesthetics and doesn’t compromise the oral tissues’ health.
A denture is generally made either as a plastic plate or a metal framework supporting plastic teeth, although porcelain teeth may occasionally be used.
If plastic it rests mostly on the soft tissues of the mouth, and then forces generated when chewing tend to push the plate into the gums to a degree, with the result that the gums can be forced away from the edges of the teeth where they rest up against them. This “gum stripping” is a major concern with plastic dentures and it is therefore necessary to go to a lot of trouble to ensure that the plate rests against as few gingival margins of teeth as possible, although this can cause problems in retention of the plate so a compromise may have to be sought.
A metal denture is better since the design can be a “skeleton” framework of connecting narrow bars which take up much less space and can be made to be supported by the teeth to a large degree. This is achieved by the use of small metal tags or “rests” which fit on to the biting surface of some of the back teeth or the palatal aspect of canines specially prepared to receive them without interfering with the bite. Leaving most of the palate uncovered is healthier for it than having it hidden underneath a plate, and it also helps preserve the sense of taste since some taste buds are situated in the palate.
A partial denture is best made in chrome cobalt alloy which is both light and strong, and doesn’t deteriorate in the mouth due to contact with oral saliva or food or drinks. The denture is actually constructed in a dental laboratory with a custom design for each case which is decided by the dentist who needs to give clear and comprehensive information and instructions to the dental technician.
In designing a partial denture it is customary to use diagrams to illustrate the various features and their position and inter-relationship. This can be done on a paper diagrammatic representation of the dental arches or on a computer.
The design begins with an assessment of the teeth missing and their position. These areas of missing teeth are termed “saddles” and may be bounded by a tooth at either end, or in some cases may be free ended where there is no standing posterior tooth. If there are more than one saddle area then they need to be joined by a connector, and the nature of this will depend on whether it’s an upper or lower denture being provided. Other important parts are occlusal rests to enable the denture to be largely borne by the teeth wherever possible, and clasps plus reciprocals that fit around some of the natural teeth to gain retention.
Clasps are usually placed in the region nearest the saddle areas for maximum benefit, and require an opposite reciprocal support to be effective. The reciprocal element may come from the plate or from a cast “arm” similar to a clasp but wider and not set into an undercut. The clasps are set slightly into undercuts in order to get grip and since chrome cobalt is relatively inflexible the teeth need to be accurately surveyed in order to make best use of undercuts. Undercuts that are too pronounced would not allow a denture to be put in or taken out of the mouth because of inadequate “give” in the clasps. Clasps may be either gingivally or occlusally approaching and are usually placed so that they are out of view in order to make the denture satisfactory aesthetically. A denture that is too obvious is likely not to be worn at all by the patient.
Thus the design of a partial denture can be complex since it has to take into account of several factors, some of which can be conflicting. It is sometimes difficult, for example, to make an upper denture adequately retentive where there are few remaining natural teeth, without clasps showing in the anterior region. However, usually a compromise can be made, and a satisfactory denture provided that takes up the minimum space in the mouth and is supported as far as possible by the hard tissues.
A chrome cobalt partial denture will last for several years, although there may be some shrinkage of gum and bone tissue in the saddle areas due to resorption over time which may necessitate a reline of the plastic parts that fit against the gums, and this is fairly easy to do.