Patients wearing complete dentures are generally advised to remove the dentures from the mouth at night, and to store them in water. The use of immersion cleansers is also some times recommended.
Generally patients are told to prevent acrylic dentures from drying out as it is suggested that they will undergo dimensional change.
This study demonstrates that if acrylic dentures are left to dry in air for 8 hours then the density of the Carzdidu colonization is significantly reduced. The dimensional changes in the acrylic resin specimens tested was so small that this would not have any significance clinically.
It is suggested, therefore, that when treating denture stomatitis the regime of allowing the maxillary denture to dry overnight could be used as a simple adjunct to treatment regimes.
INTRODUCTION
In the general instructions given to complete denture wearers it is usually advised that dentures should not be worn at night, and that when out of the mouth they should be stored in water. Most standard textbooks on prosthetic dentistry still offer this advice. The argument is often based upon the premise. that dentures will warp or shrink if allowed to dry out, and that presumably this will affect the tit There are some methods of using denture cleansers that require periods of soaking of the dentures in the cleansers, but these are recommended for the purpose of cleaning rather than preventing dimensional change.
Cahn (1936) was the first to suggest that Candida albicans might be responsible for denture stomatitis.
Subsequent investigations demonstrated that the palatal mucosa, and particularly the denture base, harboured Candidu more often in patients with denture stomatitis than from a control group (Lyon and Chick 1957;
Cawson, 1963; Davenport, 1963; Turrell, 1966; Budtz- Jorgensen et al., 1975). It has been claimed for a long time that poor denture hygiene predisposes to denture stomatitis (Fisher, 1956) and this has been confirmed by Bud&-Jorgensen and Bertram (1970), but no ideal cleaning regime has been established to overcome this problem ( MacCallum et al., 1968; Bud&Jorgensen, 1979; Abelson, 1981; Tarbet, 1982).
In order to control candidal growth it may be useful to use overnight soaking techniques if they are effective. A review of the materials and methods for cleaning dentures by Bud&Jorgensen, (1979) and Walker et al. (1981) concluded that in practise neither mechanical nor chemical denture cleaning adequately maintained dentures plaque free, and Cundidu has been shown to be predominantly embedded within the denture plaque (Budtz- Jorgensen, 1977).
Infection may be a problem for those patients undergoing myelosuppressive cancer chemotherapy, and the hazards of dentures and denturesoaking routines as a source of infection has been indicated (DePaola and Minah, 1983).
Apart from the question of the effectiveness of known cleaning techniques there is also some doubt as to how well patients are instructed in routines for cleaning their dentures. Two surveys in the USA have shown that 65 per cent and 40.8 per cent of patients, respectively, had received no instructions for home care of their dentures (Bauman, 1980).
In 198 1 a survey was carried out in the Department of Restorative Dentistry in the Dental School in Cardiff on 254 consecutive patients presenting for the replacement of complete dentures in order to determine their usual denture-cleaning routine. Approximately half of the participants immersed their dentures in water or a chemical denture cleanser, and 1.6 per cent simply allowed the denture to ‘air-dry’ overnight, usually rinsing the denture before reinserting it in the morning The patients who left their dentures out during the night did not experience any problems in relation to tit on reinsertion.
As there is no satisfactory method of removing Candida from dentures by routine cleaning methods practised by patients, it was considered possible that Candida colonization might be reduced if dentures were allowed to dry. This would only be a viable technique if this did not compromise the denture base. Therefore, it was thought useful to examine the question of change of dimensions of acrylic resin dentures with overnight drying coupled to the hypothesis that dentures that are dry would show less Carzdida colonization than when stored wet.
There is a prolific literature on the dimensional accuracy of denture-base resins. The necessity of having a precise and dimensionally stable replica of the patient’s tissue-bearing surface in the finished denture is axiomatic in denture construction. However, it is acknowledged that none of the present denture-base systems or techniques achieve this objective completely. The linear changes that occur during the construction of denture bases are small and generally the bases are within 1 per cent of the cast upon which they were constructed
The dimensional changes occurring in dentures may be divided into those occurring in fabrication and those occurring during clinical in-service use. Patient cleaning regimes, physical behaviour of the materials and varia tions in the clinical environment, particularly water absorption, are examples of factors that will affect dimensional changes in use. A review of all the aspects involved in the dimensional changes in complete dentures has been published (Woelfel et al., 1962).
It has been recognized for some time that the dimen sional changes that do occur in dentures during production when absorbing water and drying are not detectable clinically (Paffenbarger et al., 1962; Goodkind and Schulte, 1970). In studies involving a range of denture base materials including acrylic resin, vulcanite, viny1 acrylic polymer (Luxene 44), epoxy resin (Epoxolon), acrylic resin with glass fibres (Miracle 50 and Miracle loo), polystyrene (Jectron) and pour-type resins (Waefel et al., 1962) it was concluded that dentures do not distort beyond functional acceptability by the drying/wetting cycles that occur in use.
The present dimensional accuracy study is limited to quantifying the effect of an 8-h drying out period on dentures. A simulated overnight storage in air is compared with the popular alternative of maintaining dentures in water.
MATERIALS AND METHODS
Preliminary studies with imprint cultures ( Arendorf and Walker, 1979) showed that the candidal counts on the maxillary denture base are of the same order on both the right and the left side of the fitting surface. Patients attending the Department of Restorative Dentistry for replacement dentures were asked on the day that they received their new dentures to leave their original dentures until their review appointment. The right side of the maxillary denture base was sampled on removal, and the left side after 8 h of storage in air at room temperature. Nineteen patients found to have Candida on the fitting surface of the maxillary denture were used in this study.
Sterile samples of plastic foam (25 X 25 mm) were dipped in Sabouraud’s broth and pressed against the fitting surface of the denture at the appropriate site, and then cultured on Sabouraud’s agar.
Organisms were confirmed as Candida species by standard mycological methods (MacKenzie, 1962; Dolan, 197 1). The density of Candida colonizing either side of the midline of the maxillary denture base was then estimated. To determine the effect of the popular method of overnight storage of dentures in tapwater at room temperature on candidal populations on the denture base a further 10 dentures were similarly examined.
The method of measuring the dimensional changes occurring in denture bases in this investigation is funda mentally the same as that used by Goodkind and Schulte (1970). Five specimen denture bases were processed to simulate the lower ridge form on stone casts prepared from a silicone mould of a brass master die (Fig. I). The casts and specimen plates were measured with a Nikon optical comparator at a magnification of X 10. Measurements were made across the three dimensions shown in Fig 1. As the die was symmetrical, the measurement X to Y and X to Z were anticipated to be similar and this was found to be the case. Consequently, the mean of these two readings are presented according to the formula (XY •l- XZ)/2. This dimension is referred to as the anteroposterior dimension, and YZ the posterior dimension. The specimen plates were constructed following the conventional dental laboratory processing procedures.
The denture-base material used was Trevalon pink (Dentsply Ltd, Weybridge, UK). This is a standard dough-moulded material which has a copolymer powder component consisting of methyl methacrylate and butyl methacrylate in the ratio 96 : 4. The monomer/liquid component consists of 94 per cent methyl methacrylate
and 6 per cent ethylene glycol dimethacrylate plus a trace of dimethyl-ptoluidine. The powder/liquid mixing ratio was 3.5 : 1 v/v. The curing cycle employed was 14 h at
70°C. After devesting and trimming the specimen plates were stored in water at 37°C for three months to become fully water saturated. The plates were then removed from the water and measured, they were then stored in air at room temperature for 8 h and again measured This experimental procedure was repeated on the five plates on two separate occasions.
RESULTS
In the Candida study significant differences were found in both the prevalence (McNemar test P< O-01), and the density (Wilcoxon rank sum test P-C 0.001 ), of the colonization of the contralateral sides of the upper denture base after 8 h of exposure to air at room temperature
( Table I). Only a single colony-forming unit was found on four denture bases after storage in air. To ensure that the low counts obtained after 8 h of exposing the denture to air were not due to the inefficiency of the imprint culture to sample dry acrylic surfaces, five dentures were soaked in
150 ml of tapwater for a further 8 h, and then resampled with the imprint culture. No significant increase in candidal yield on the denture base was obtained, showing that imprint cultures are a sensitive method of isolating Cairdidar even on dry acrylic surfaces.
The mean values of the dimensional accuracy results are shown in Table II. The contraction exhibited in each plate across the dimensions recorded was extremely small. An overall mean contraction of 0.0267 mm (O-05 per cent) with a range from O-003 to 0.055 mm (0*005-0.1 per cent).
DISCUSSION
The results suggest that prolonged exposure of dentures to air is an efficient method of reducing the candidal populations on the fitting surface of the denture. This might be a useful prophylactic measure against denture stomatitis, as a threshold density appears necessary for epithelial invasion (Wain et al, 1976). Similarly, the increased candidal density on the fitting surface of those dentures immersed in water suggests that this procedure alone, theoretically, could increase the oral candidal populations to levels seen in oral candidosis (Arendorf and Walker, 1979).
The amount of expansion which takes place in heat- cured denture-base materials as a consequence of water uptake from the-as-processed’ state to the’fully saturated state is well documented, and varies between 0.02 and 0.35 per cent (Bates et al., 1980; Stafford et al., 1980). It is noteworthy that a change in dimension of O-5 per cent on a denture of 50-mm molar to molar span is
0.25 mm.
The diffusion coefficient in water absorption is an important parameter as it governs the time necessary for a polymer to take up or lose water (Braden, 1964). The diffusion coefficient is a function of the polymer, and also the thickness of the section being considered A thicker section will take much longer to equilibrate than a thin section, and proportionately, in a given period of time, will lose or gain less quickly. In addition there is a temperature effect upon sorption. In a cooler environment a denture will lose water more slowly than if warmer, and so if removed from the mouth lose water more slowly if the ambient temperature of the room is low. If then restored to the mouth the denture absorbs water at a faster rate than when it desorbed
It is important to recognize the polymer used in the denture in question when giving advice to the patient, as some polymers lose water more quickly than others ( Stafford and Braden, 1968) Trevalon has a low diffusion coefficient (Stafford et al., 1980) and thus can be predicted to lose and gain water very slowly. In a period of eight hours this small water loss would not be expected to lead to any marked dimensional change, and this is supported by this experimental work
Some workers (Woelfel and Paffenbarger , 1959) have induced warpage in dentures and shown that shrinkages up to 0.5 mm (- 1 per cent) across the posterior section of a complete denture does not cause serious misfit nor discomfort. It is not until the shrinkage is increased to O-9 mm (-2 per cent) that the denture bases do not fit properly. The average amounts of contraction (of 0.05 per cent) we have found by leaving bases in air for 8 h is so small that it can be reasoned that the effect upon the fit of the dentures is of no clinical significance.
The properties of standard denture-base resin when thermocycled were studied by Hargreaves (1983), using serviceably realistic parameters. It was shown that thermocycling between water at 37°C for 16 h to air at 20°C for 8 h and back again had little effect, in the short term, on the surface of the resin, nor on the mechanical properties as measured by transverse bend tests, tensile and hardness tests.
Thus the treatment of a patient with denture stomatitis should include, alongside anti-fungal therapy, a period of time when the dentures are left out of the mouth at night in a dry environment. This period of dry storage should continue for at least a month after this period in order to prevent a recurrence.
CONCLUSIONS
This work shows that the density of Candida colonization on the maxillary denture base is significantly reduced when dentures are left in air to dry for 8 h, whilst the density increases if stored in water for a similar period. It is proposed, therefore, that this routine should be recommended for periods of time for those patients who are susceptible to candidal infection. Furthermore, patients should be dissuaded from soaking their dentures overnight in tapwater as the resultant significant increase in candidal populations on the maxillary denture-fitting surface could contribute to local candidosis. It is advisable for patients to use the’air-drying’ routine for up to a month after active candidal therapy in order to reduce the possibility of a recurrence. A routine such as this would not lead to any marked loss of physical properties ofthe acrylic resin. The study also shows that dimensional changes are so small that they do not affect the fit of the denture.
SOURCE: Department of Restorative Dentistry and Department of Oral Medicine, Dental School, University of Wales, College of Medicine, Cardiff
