Role of saliva as a modifying factor in dental caries
Indisputably, an adequate secretion rate and saliva of good quality are essential for oral health. Saliva is well known to have specific protective effects against dental caries. The most direct evidence of this is the rampant caries that can occur following the loss of salivary function as a result of irradiation for head and neck tumors. Within a few weeks, tooth surfaces not normally susceptible to caries may be affected, leading to complete coronal destruction. The principal properties of saliva that protect the teeth against caries are:
1. Dilution and clearance of dietary sugars
2. Neutralization and buffering of the acids in plaque
3. Supply of ions for remineralization
4. Both endogenous and exogenous antiplaque and antimicrobial factors
The major functions of different salivary components have been presented in Table 12.
Dilution and clearance of food components and clearance of microorganisms Of the many functions of saliva, the most important is the clearance of oral microorganisms and food components from the mouth to the gut. Therefore, an adequate volume to flush noxious (and also commensal) microorganisms out of the oral cavity is a prerequisite for a healthy balance between host defense and endogenous and exogenous microbial attack in the mouth.
This balance can be disturbed by either extensive growth of bacteria as a consequence, for example, of poor oral hygiene, excessive dietary intake of fermentable carbohydrates, or some systemic diseases¾or reduced SSR (hyposalivation). In the most highly caries-susceptible individuals, a combination of these factors is common. Although caries research has concentrated on salivary clearance of sucrose and fluoride, the principles that apply to sucrose clearance are valid for any substance introduced to the oral cavity. Besides sugars and fluoride, other substances are of relevance to the clinician: chemical plaque control agents (chlorhexidine, etc), chloride in relation to corrosion of amalgam, and citric acid and other acidic products that might be implicated in tooth erosion.
The study of sugar clearance was pioneered by Swenander-Lanke (1957), who found that, following the consumption of solid carbohydrate foods, the concentration of sugar in saliva fell exponentially, at first rapidly and then more slowly. Sreebny et al (1985) noted that sugar solutions were cleared in a two-stage pattern and that the rapid clearance rates over the first 6 minutes and the slower ones thereafter were proportional to the shifts in the SSR at those times. In 1983, Dawes developed a computer model for sugar clearance, based on the following postulate: that the
important factors in clearance were (1) the volume of saliva just before and after swallowing and (2) the unstimulated SSR. The computer predictions based on this postulate were confirmed in studies using an "artificial mouth" system and in human experiments. The computer predicted that the clearance was rapid when both salivary volumes were low and the unstimulated SSR was high.
Figure 97 shows a physiologic model of the oral cavity that includes the most
important properties for understanding the clearance process. The events after intake
of sucrose may be described as follows: In the oral cavity, there is a minimum volume
of saliva after swallowing, the residual volume. Spread out as a thin film, this volume
has been estimated to be, on average, 0.8 mL, but the interindividual variation is
large. Dissolution of a small amount of sucrose in this small volume of saliva will
give rise to a very high sucrose concentration. For example, dissolving one tenth (0.3
g) of a sugar lump in the residual volume will result in sucrose concentrations much
higher than would be found in an ordinary sucrose-containing beverage. The taste of
sucrose, together with optional flavoring agents, stimulates the salivary glands to
respond in a few seconds with an increase in the flow rate. The volume of saliva will
increase until a maximum value is reached. This maximum value is about 1.1 mL (ie,
a normal swallow can be estimated to be 0.3 mL). The swallowing reflex is
stimulated, and some of the sucrose is eliminated. The remaining sucrose is then
progressively diluted by the saliva entering the mouth until the maximum volume is
reached, triggering another swallow, and so on.
After some time, the concentrations of sucrose and the optional flavoring agent reach
such low levels that the stimulation of the glands decreases to an unstimulated state,
which results in a slower clearance process, dependent on the unstimulated SSR. The
time it takes to reach a given detectable low level has been used as a measure of
clearance rate. Several variables are important for the clearance rate, the most
important being the SSR and the volumes of saliva in the mouth before and after
swallowing. A high SSR will result in rapid clearance, compared to the slow
clearance obtained at low SSR. From the great differences in clearance rates, it is
clear that caries risk increases enormously with a low SSR.
The clearance rate is an individual property that is constant over time. However, if
changes in health status cause a decrease in the SSR, a drastic change in clearance rate
will ensue. The clearance rate also differs considerably at different sites, because of
the complicated rheology of the oral cavity. The film overlying the mucous membrane
and the teeth moves at varying rates, from 0.8 to 8.0 mm/min. In sites where the
salivary film may be expected to move rapidly, for example, in proximity to the ductal
orifices, the clearance rate is considerably greater than in sites where the saliva is
stagnant (eg, the buccal areas of the maxillary anterior teeth and mandibular molars),
which may explain in part the pattern of caries on different teeth and tooth surfaces.
Sucrose in saliva and in the salivary film diffuses readily into dental plaque. A few
minutes after sugar intake, the plaque will be overloaded with sucrose, with a greater
sugar concentration than is present in the saliva. Provided that the plaque is not too
thick to impede accessibility of the saliva, the flow of sucrose will be reversed.
Therefore, there is a correlation between pH changes in the plaque and the salivary
clearance of sucrose. In contrast to rapid clearance, slow clearance resulting from
limited salivary accessibility will cause steep Stephan curves (see chapter 2). After
sucrose rinsing, the pH fall in approximal plaque on molars is much more severe in
the center of the surface than it is lingually, because the central area is inaccessible to
saliva for dilution and buffering of the plaque acids.
