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Variation in salivary access to different intra-oral sites is an important factor in the site-dependence of dental caries. This study explored, theoretically, how access is modified by chewing sugar-free and sugar-containing gums. A finite difference computer model, described elsewhere, was used. This allowed for diffusion and/or reaction of substrate, acid product, salivary buffers, and fixed-acid groups. Site-dependent saliva/plaque exchange was modeled in terms of a 100-μm-thick salivary film covering the plaque (a) flowing directly from the salivary ducts, (b) flowing from the intra-oral salivary pool, or (c) exchanging with the pool. Computed flow-velocities or rates of exchange were based on previous intra-oral measurements. The model was also tested against an in vitro study conducted by two of the authors. In addition, the three proposed models of saliva/plaque interaction were compared, and the effect of salivary film thickness investigate. Results suggested that: (1) although sugar-free gum chewed during a cariogenic challenge causes a rapid rise in plaque pH, sucrose-containing gums cause the pH, after a temporary rise resulting from increased salivary flow, to stay low for an extended period; (2) the computer model reproduced in vitro tests reasonably well; (3) although the three models of the plaque/saliva interaction start from different assumptions, two lead to closely related predictions; and (4) increasing the assumed salivary film thickness by a large amount ( e.g., from 50 to 200 μm) caused no change in modeled Stephan curves, as long as these changes were accompanied by appropriate reductions in film velocity, in accord, theoretically, with the practical clearance data.

The objective of these studies was to simulate the effect of chewing sugar-free and sucrose-containing chewing gums on the return of the pH to neutrality after exposure to sucrose of plaque located on the buccal (BLM) and lingual (LLM) surfaces of the lower molar teeth. In study 1, a 0.5-mm-deep artificial plaque containing Streptococcus oralis cells was exposed to 10% sucrose for one min, and a 0.1-mm-thick film of sucrose-free artificial saliva was then flowed over the plaque surface at the unstimulated salivary film velocities previously found at the BLM and LLM sites. At the time of the pH minimum (pH 4-5), one of three conditions was simulated: (a) a no-gum-chewing control, or chewing for 20 min on either (b) a sugar-free gum or (c) a sucrose-containing gum. The recovery of the plaque pH to resting values was rapid during simulation of chewing a sugar-free gum (SFG), much slower with the no-gum control, and even slower with simulation of chewing a sucrose-containing gum (SCG). The pH recovery was slower with the BLM than the LLM plaque. In study 2, the BLM plaque was exposed to a 2% sucrose solution for 20 min under stimulated salivary conditions, to simulate the consumption of a meal, followed by one of conditions (a), (b), or (c) described above. The pH recovery with simulation of chewing a SCG was faster than with the no-gum control, but much slower than with the SFG simulation. In study 3, when both BLM and LLM plaques were exposed for 20 min to the sucrose levels in chewing-gum-stimulated saliva and then unstimulated conditions were simulated, the pH fall and recovery were similar to those found by others in vivo. From the results of these studies, it would be expected that when the plaque pH has fallen, after exposure to fermentable carbohydrate, it would return to neutrality more readily with the use of sugar-free rather than sucrose-containing gums.

The flow rate of unstimulated whole saliva, the volume of saliva in the mouth before (VMAX) and after (RESID) swallowing, and the surface area of the mouth were measured for determination of the average thickness of the salivary film in five-year-old children with primary dentitions. In 30 subjects of each gender, RESID was calculated from the potassium concentration in unstimulated saliva by a dilution technique. VMAX was calculated as the total of RESID plus the volume normally swallowed (the unstimulated salivary flow rate divided by the swallowing frequency). From these children, ten subjects of each gender who had no missing teeth were selected, and impressions were taken for determination of the surface area of the mouth. Aluminum foil, of known weight per unit area, was adapted to the stone models of different regions of the mouth, and the surface areas were calculated from the weights of foil. The unstimulated salivary flow rate was 0.22 ± 0.14 mL/min, the mean values of RESID and VMAX were 0.38 ± 0.11 mL and 0.50 ± 0.15 mL, respectively, and the mean total surface area of the mouth was 117.6 ± 7.6 cm2. There were no significant differences in these values due to gender. The average thickness of the salivary film in the mouth was calculated to vary between 0.06 and 0.09 mm. Although the values of salivary flow rate, RESID, and VMAX were much lower than those reported for adults, the average thickness of the salivary film was very similar to that in adults.

The normal concentration of fluoride in saliva is about 1 μmol/L, which is somewhat less than that in plasma, and the salivary concentration is relatively independent of flow rate. Even this low concentration appears to be significant in terms of maintaining the integrity of tooth mineral. After fluoride consumption, the level in plasma peaks within less than an hour, and this produces a corresponding increase in salivary levels, which achieve baseline values usually within a few hours. At low concentrations ( The rate of clearance of exogenous fluoride from saliva is prolonged when initial concentrations are high, due to the deposition of CaF2 on the tooth surface and its gradual dissolution. The clearance rate is also not constant throughout the mouth, but shows considerable site-specificity. In general, clearance is much more rapid lingually than buccally. This appears to be due to the greater lingual exposure to secretions from the major salivary glands, principally the submandibular, whereas buccally, mainly minor mucous gland secretions are present, and these are very viscous and flow at a slow rate.

The hypotheses to be tested were: (i) that chewing sugar-free gum frequently and for long periods would be associated with higher amounts of supragingival calculus, and (ii) that there would be no site-specificity of calculus deposition on the lingual surfaces of the 6 lower anterior teeth. Subjects, 436 in Glasgow and 191 in Winnipeg, were scored for calculus at mesial, lingual, and distal sites on the lingual surface of each of the 6 lower anterior teeth, by the Volpe-Manhold method. They also answered questions on the time since the last prophylaxis, the frequency of gum chewing, the type of gum chewed, and the length of a typical gum-chewing episode. A subset (233) of the Glasgow subjects were scaled and re-scored for calculus 3 months later. When the data for the logarithmic transformations of the initial calculus scores were subjected to stepwise multiple-regression analysis, the only factor which correlated significantly with initial calculus scores in both cities was the time since the last prophylaxis. In the Glasgow subjects scored 3 months after a prophylaxis, there was a negative correlation between chewing sugar-free gum and calculus scores, whereas in the Winnipeg subjects, age and the chewing of sucrose-containing and sugar-free gum were positively correlated with calculus scores. Thus, the results were contradictory with respect to the first-tested hypothesis. The calculus distribution patterns were very similar in the subset of Glasgow subjects and the Winnipeg subjects, with the amounts on the lateral incisors and canines averaging 70.2% and 44.5%, respectively, of those on the central incisors. Thus, the second hypothesis was disproved.

Over a 20-minute period, subjects expectorated 8 samples of whole saliva (EWS) while chewing gum. Flow rates were calculated, and sucrose was analyzed in these samples as well as in saliva collected on filter paper strips from different tooth surfaces. Salivary film velocity (SFV), based on a 0.1-mm-thick film, was estimated from the clearance half-times of KCl in agarose disks positioned in different regions of the mouth. Salivary flow rate peaked at 5.1 mL/min in the first min but fell to about 1.25 mL/ min by the end of the 20 min of gum-chewing. In contrast, flow rate when subjects sucked sour lemon drops averaged about 5.3 mL/min throughout the 20-minute period. The mean salivary sucrose concentration during gum-chewing peaked in the second min at 384 mmol/L (13.1%) but had fallen to 14 mmol/L by the 15-20-minute time interval. The sucrose concentrations on the palatal surfaces of the upper incisors and the facial and lingual surfaces of the lower molars were not significantly different from that in EWS but were much lower on the facial surfaces of the upper incisors and molars, and on the lingual surfaces of the lower incisors. When flow was unstimulated, SFV was 0.8-1.0 mm/min on the facial surfaces of the upper incisors and lower molars but about 5-8 mm/min on the facial surfaces of the upper molars and on the lingual surfaces of the lower incisors and molars. During chewing-gum use, the SFV increased only to 4 and 10 mm/min on the facial surfaces of the upper incisors and lower molars, respectively, but on the facial surfaces of the upper molars and on the lingual aspects of the lower incisors and molars, it became equivalent to that in a well-stirred solution. This study showed that even during gum-chewing, which would be expected to facilitate the mixing of the different salivary secretions, the saliva provided a series of distinctly different fluid environments, some with cariogenic and others with calculogenic potential, for plaque in different regions of the mouth.

Chewing-gum stimulation of salivary flow (at the time of the pH minimum following exposure of plaque to carbohydrate) has been shown to cause a rapid increase in plaque pH. The objective of this study was to determine whether the rise in plaque pH is primarily due to the increased buffering capacity of stimulated saliva, or to the fact that an increased flow rate increases the concentration gradient for acid to diffuse from the plaque into the overlying salivary film, which will be moving at a higher velocity. This was investigated with an in vitro technique in which artificial plaque (0.5 or 1.5 mm deep) containing S. oralis cells was exposed to 10% sucrose for one min. The pH values at the proximal and distal undersurfaces of the plaque were then monitored during the passage of a 0.1-mm-thick film of a sucrose-free artificial saliva over the surface, at a range of film velocities (0.8-8 mm/min) that have been estimated to occur in vivo. When a minimum plaque pH had been achieved, the salivary film velocity was either (a) kept the same, with or without 15 mmol/L HC03 (the concentration measured in chewing-gum-stimulated saliva), (b) increased to 86.2 mm/min, or (c) increased to 86.2 mm/min with 15 mmol/L HC03 added to the artificial saliva. The findings suggest that after sucrose ingestion, the rapid rise from minimum plaque pH values, which can occur with gum-chewing stimulation of salivary flow, is due to the combined effects of the increase in salivary film velocity, and of a greater availability of bicarbonate. The former appeared to be more important in the thinner plaque (0.5 mm deep) than in the thicker one (1.5 mm deep).

Results from a computer model suggest that following exposure of dental plaque to sucrose, the rate of clearance of acids from plaque into the overlying salivary film will be greatly retarded at low film velocities. This was investigated with an in vitro technique in which artificial plaque containing S. oralis cells was exposed to 10% sucrose for one min. The pH at the proximal (P) and distal (D) undersurfaces of the plaque (0.5 or 1.5 mm thick) was then monitored during the passage of a 0.1-mm-thick film of a sucrose-free solution over the surface. Over the range of salivary film velocities that have been estimated to occur in vivo (0.8-8 mm/min), lower minimum pH values and increased times for the pH to recover toward neutrality occurred at the lower salivary film velocity. Lower pH values were also reached with the 0.5- than with the 1.5-mm-thick plaque. P/D pH gradients, with a lower pH distally, developed at film velocities of 0.8 and 8 mm/min, and the gradients were much more pronounced at the lower velocity. No P/D pH gradients developed when the film velocity was 86.2 mm/min. Incorporation of dead S. oralis cells into the plaque at percentages up to 57% reduced the extent of the pH fall and prolonged the recovery of the pH toward neutrality. The results support the prediction that, other factors being equal, plaque located in regions of the mouth with low salivary film velocity will achieve pH values lower than those of plaque of identical dimensions and microbial composition located in areas where salivary film velocity is high.

We recently developed a computer model of oral sugar clearance (Dawes, 1983), and an artificial mouth has now been constructed which will allow variation in the salivary parameters identified in the model and the study of their effects on bacterial acid production. Using a thin layer of S. mitior, strain 572, over a miniature Sb electrode to measure bacterial pH changes during sugar clearance from the "mouth", we found that three consecutive sets of "Stephan curves" could be obtained with the same bacteria during one day. In each of several experimental series, one salivary parameter was varied, while other parameters were held constant. The maximum pH decrease and the surface area of the registered Stephan curve (Δ pH.min) were used as measures of acid production. The results indicate that the volume of "saliva" in the mouth before and after swallowing, the unstimulated salivary flow rate, and the buffer capacity had significant influences on the extent of the pH changes. Other factors, such as the maximum flow rate, the delay between start of stimulation and maximum flow, the volume of saliva in the mouth at time zero, and the taste threshold for sugar, were of lesser importance, confirming some predictions from the computer model.