The replacement of missing posterior teeth is not always successful but may contribute to improved chewing ability. This case will build on the experiences gained in Case Guide 1 with extended guidance on assessing the challenges of replacing missing posterior teeth. The aim of this case is to review the assessment of the patient’s needs and the advice given on the risks and benefits, which apply to wearing a removable partial denture (RPD) that replaces posterior teeth. The anatomy of the edentulous alveolar ridge is reviewed with reference to its limitations in providing support for chewing forces transmitted to it via the denture base. The benefits of using a compound impression material in the preliminary stages are suggested. The use of occlusal rims to make a jaw registration record is explained and the importance is stressed on avoiding any interference caused by the denture teeth with the occlusal contacts of the natural teeth. The techniques are described for adjusting the finished denture to accommodate the dynamics of occlusion and the activities of the buccal and alveolingual sulcus.
Review the assessment described for the patient interested in having a removable partial denture (RPD) to replace missing anterior teeth. We now have the added or different request to replace some missing posterior teeth, so more information about their perceived disability is needed. In addition to those questions set out in Case Guide 1, we need to ask the patient the following questions for further information.
These and other questions should lead to an understanding of the patients’ level of motivation. They may be surprised that you want to know all this. You have the opportunity to explain that many patients do not get on well with dentures that replace back teeth. The reasons are that chewing involves biting down onto the teeth. If the denture teeth are supported on the residual ridge, that is by a layer of mucosa over bone, the mucosa is likely to be pinched between the denture and the bone. It may then become bruised and tender. Teeth also come together when the patient swallows and when he or she wants to reseat a denture if it should rise off the ridge. By the end of the day, a partial denture that contributes to chewing may become sore and irritating. Some patients are more comfortable leaving the denture out for as long as they can and using it just to eat.
In addition to the examination performed in Case Guide 1, there is a need to establish the following:
Advanced resorption of the alveolar ridge coupled with high attached muscle or soft-tissue attachment would have a negative effect on the stability and resistance to loading of the denture base (see Appendix B.2 Anatomy of the residual alveolar ridge). An opposing denture is less likely to deliver high chewing forces than natural teeth. Overerupted natural teeth leave little space and cause an uneven occlusal plane, which may have functional or aesthetic consequences.
You should have enough information to be able to offer the patient a summary of the risks and benefits of having a partial denture fabricated, which will replace some or all of the missing posterior teeth. In addition to the risks that were set out for the partial denture replacing some anterior teeth to restore a natural appearance, the partial denture replacing posterior teeth comes with additional challenges. Several studies have shown that a significant percentage of patients whose denture replaced posterior teeth did not wear the denture. The benefits to chewing performance are variable (see Appendix B.1 Impact of removable partial dentures on chewing function). Once again, the patient has to be informed that wearing a partial denture carries the increased risk of dental caries and periodontal disease and that meticulous oral hygiene and denture cleaning are necessary to prevent adverse effects on the remaining teeth and soft tissue. Remind the patient that there is a risk when a denture replaces posterior teeth, that it may not be worn because the bite forces transmitted to the mucosa may cause bruising and pain.
Stock impression trays are either designed for the dentate arch or the edentulous ridge but not both. In order to capture the full extent of the dentate areas of an arch, we have to use a dentate tray. This will not provide support for the impression material over the edentulous sections. One solution is to make the impression in two stages.
Firstly, make an impression of the edentulous area using a firm impression material such as impression compound. The purpose is to provide some support for the less viscous impression, which will be made subsequently over the entire arch. Compound is a mixture of waxes and fillers, which when warmed up in hot water, becomes soft but not fluid. The manufacturers may advise a water temperature of about 70°C, and this is best obtained using a thermostatically controlled water bath. However, pouring boiling water into a dish over the compound will also achieve the desired result, as the water temperature soon cools down as the compound heats up. The dish should be lined with gauze to prevent the compound sticking. When the compound can be molded like a soft dough, it can be placed in the stock tray over the area of the edentulous ridge. It should not be placed over dentate areas as it will not withdraw out of undercut. It is useful to be able to soften just the surface of the compound with a gas flame to make it even softer than the main bulk of the compound. Use a safe propane torch that shuts off the gas supply as soon as you let go of it to put it down. The heat of the surface compound, which has just been flamed, should be quenched in the hot water before placing the tray and compound in the patient’s mouth. As soon as the tray is seated, the patient should be asked to pout the lips, protrude the tongue, and move the jaw from side to side. The tray and compound can be removed after about 30 seconds, as it will soon cool down and become firm (▶Fig. 2.1) When the compound is cold, it may be trimmed with a sharp knife. The blade must be scalpel—sharp, so a craft knife with a new blade is ideal. The compound that has spread around the natural teeth should be removed, as space is needed for the second material, which should be elastic. Compound has a high viscosity, especially, if it is cooler than the recommended working temperature. If the compound impression looks overextended in the edentulous part of the impression, it may be reheated and returned to the mouth for refinement. It may also be trimmed back short of the sulcus so that it allows at least 2 mm of space for the second impression material.