Lubrication and Flushing (Saliva Flow Rate)

Providing a constant fluid flow is probably the most important defense function of the salivary glands, because it is the fluid that transports the buffering agents, the antimicrobials, and the mineral content of saliva to help control the equilibrium between the demineralization and remineralization of tooth structure. Also, the fluid output of the glands is essential for diluting acids, flushing food particles embedded around the teeth, clearing refined carbohydrates (acid-producing sugar substrates) and physically removing any displaced bacteria. Oral fluids in contact with food particles results in solubilizing food substances that interact with the taste buds to provide an accurate assessment of taste.

The unstimulated flow rate of the salivary glands is subject to a circadian rhythm, with the highest flow in mid-afternoon and the lowest around 4:00 A.M.

The total amount of saliva secreted varies considerably between and within individuals, depending on the environmental factors. Seasonal variations occur, with flow being lower in warm weather and higher in cold. The act of smoking increases flow rates. Flow is greater while standing than when sitting and greater when recumbent, with these postural changes paralleling changes in systemic blood pressure.

Salivary flow rate is considered as a "key" parameter in caries-risk assessment. Although there is no linear association between salivary flow rate and caries activity, it is important to evaluate whether the secretion is normal or impaired. Saliva flow can be abnormally high a condition termed sialorrhea, (or ptyalism) which is often manifested by drooling. Absence of saliva, xerostomia, or hyposalivation can result in an extremely increased caries risk. A decreased flow rate is a common side effect to a large number of medicines and radiation therapy. For the individual, a regular and longitudinal follow up of the flow rate is of higher clinical value than a single measurement to be able to identify reduction and alterations over time. The salivary flow from both major and minor glands is controlled by parasympathetic (water, electrolytes) and sympathetic (proteins) stimuli. The water fraction is most important for the clearance process while the antimicrobial activity resides mainly in the protein fraction.

Viscosity of Saliva The efficacy of saliva as a lubricant depends on its viscosity and how it changes with shear rate. Increased salivary viscosity may also be associated with an increase in dental caries, although it is difficult to examine flow rate and viscosity independently from each other Until the 1970s most rheological measurements of saliva were made with the Ostwald-type U-type viscometer or its modifications (Waterman et al, 1988).

Buffering Capacity of Saliva Salivary buffering capacity is important in maintaining a pH level in saliva and plaque. The buffer capacity of unstimulated and stimulated whole saliva involves three major buffer systems.

The most important buffering system in saliva is the carbonic acid / bicarbonate system. The dynamics of this system is complicated by the fact that it involves the gas carbon dioxide dissolved in the saliva. The complete simplified equilibrium is as follows:

CO2 + H2 O = H2 CO3 →HCO3¯ + H+

The increased carbonic acid concentration will cause more carbon dioxide to escape from the saliva. The saliva bicarbonate increases the pH and buffer capacity of saliva, especially during stimulation.

The second buffering system is the phosphate system, which contributes to some extent to the buffer capacity at low flow rate. The mechanism for the buffering action of inorganic phosphate is due to the ability of the secondary phosphate ion, HPO4²¯ , to bind a hydrogen ion and form an H2PO4¯ -ion.

The third buffering system is the protein system. In the low range of pH the buffering capacity of saliva is due to the macromolecules (proteins) containing H-binding sites.

The buffering capacity of saliva is important for the maintenance of normal pH levels in saliva and plaque. A low secretion might indicate a low buffering effect and a weak inverse relationship to caries. Nevertheless, unfavorable values of buffer capacity and salivary flow rate should be considered as risk factors for the individual.

The tests commonly used are based on the titration technique with the final pH determined by a dye color change.

Antibacterial (defense) Functions

The most easily understood major antibacterial function is performed by one of the glycoproteins the mucins that trap or aggregate bacteria that are eventually swallowed. The same mucins provide a thin film over the mucous membrane and teeth to serve as lubricants. Four important antimicrobial proteins found in saliva are: lysozyme, lactoferrin, salivary peroxidase and secretory immunoblobulin A (sIgA).

In vitro, lactoferrin strongly inhibited adherence of mutans streptococci to saliva coated hydroxyapatite (HAP) blocks. Lactoferrin combines with iron and copper to deprive bacteria of these essential nutrients.

Salivary peroxidase reacts with saliva to form the antimicrobial compound hypothiocyanate, which in turn inhibits the capability of the bacteria to fully use glucose. Lactoperoxidase strongly adsorbs to hydroxyapatite as a component of the acquired pellicle, and can influence the qualitative and quantitative characteristics of the microbial population of dental plaque.

The role of the body's cellular and immunologic defense systems in moderating the course of the plaque-induced disease needs clarification. The main access that phagocytic cells and their antibacterial products, have to the oral cavity is through the gingival crevice and the tonsils. It is difficult to conceive of the cellular immune system operating in the bacterial plaque, yet about 500 leukocytes per second are estimated as emigrating from the tissues through the gingival crevice into the oral cavity. The majority of these soon disintegrate in the saliva, a phenomenon that may be related to the fact that more intact polymorphonuclear leukocytes occur in caries-free than in caries-susceptible individuals. On a research basis, there is reason to believe that a linkage exists between normal humoral and cellular defenses, and both caries and periodontal disease. How the cells and immunoglobulins exercise this potential is unclear.

The methods of estimation the features of saliva

Saliva Flow Rate

When measuring the flow rate, one can either sample unstimulated or stimulated whole saliva. In addition, saliva from separated secretions, parotid or submandibu lar/sublingual, can be collected. Stimulated whole saliva samples are most often used for routine work. The stimulation can be done by paraffin chewing or by adding droplets of a sour liquid (3% citric acid) on the back of the tongue. The amount of saliva obtained is divided by the collection time and the secretion is expressed as ml/minute or mL/5min. For adult patients, a normal stimulated secretion rate is around 1.0 to 1.5 mL/minute. Values below 0.7 ml/minute should be considered as low and indicate a caries risk. Women generally have somewhat lower stimulated and unstimulated secretion rates than men. In children, the levels highly depend on age and cooperation, but the corresponding levels in preschoolers for stimulated and unstimulated secretions are around 0.5 and 0.3 mL/minute, respectively. Saliva Flow Rate. (Redinova T.L. 1994).

Equipment: the graduated test tube and stop-watch.

Procedure: For collection of unstimulated (resting) saliva, the patient is seated in an upright relaxed position with the head bent forward. The subject lets the saliva passively drip into a graduated tube for 5 to 15 minutes. A patient should incline a head to the chin, set a test tube to the lower lip, slightly open a mouth and should not swallow saliva, and to allow to saliva freely to flow down in the graduated test tube. Register the time of beginning and completion of collection of saliva ( usually 5-15 minutes). Saliva Flow Rate is estimated on a formula:

S= V/t

S – speed of salivation V- value of saliva (ml) t – time of saliva collection (in min)

A gravitation method is advocated. The test tube is weighed before and after sampling and 1 gram corresponds to approximately one milliliter of saliva. Normal (unstimulated) Saliva Flow Rate S= 0,31—0,6 ml/мin, hyposalivation S= 0,03- 0,3 ml/мin, hypersalivation S= 0,61-2,40 ml/мin.

An unstimulated secretion of less than 0.1 mL/minute is considered as a risk value. In cases of hyposalivation, the saliva is often viscous and "foamy" and the secreted volume is difficult to determine.

Determination of mineral potential of saliva ( МPS). (Leus P. And., 1977). is estimated by registration of appearance of crystals at the slow drying out of drop of saliva.

Equipment: pipette, subject glass and microscope.

Procedure: by pipette to collect unstimulated saliva from the bottom of oral cavity

and put on a subject glass. To dry up saliva on air at a room temperature. Examine dried-up drops by a microscope in the reflected light at a small increase

1 point - chaotically located structures of wrong form;

2 point - thin net of lines on all eyeshot

3 point - separate crystals of wrong form against a background a net and lumps;

4 point - treelike (arborescent ) crystals of middle sizes

5point - clear, large, look like a fern or parquet crystalline structure.

To collect 3 drops of saliva Using a foregoing method, estimate each drop of saliva (from three). To expect the average of МPS.

Indexes of МPS from 0 to 1 mean very low, from 1,1 to 2,0 - low, from 2,1 to 3,0 - satisfactory, from 3,1 to 4,0 - high, from 4,1 to 5,0 - very high.

Dentobuff Method for Measurement of Buffer Capacity The bicarbonate concentration is strongly dependent on secretion rate. Since bicarbonate is the chief determinant of the buffer capacity, there is an interrelationship between pH, secretion rate and salivary buffering capacity. A simple method to measure the buffer capacity of saliva, the Dentobuff strip, has been developed by Ericsson and Bratthall. Based on the color change of the indicator paper, the buffering capacity is assessed in comparison with a color chart. A small amount of acid is impregnated on a pH indicator strip. One droplet of stimulated saliva is placed on the testing pad of the strip in a flat position to dissolve the acid. After exactly 5 minutes, the color of the strip is compared with a provided chart, indicating the final pH. The method reflects mainly the bicarbonate buffer system and identifies saliva with low (yellow), intermediate (green), and normal (blue) buffer capacity. It is important that the test is read after exactly 5 minutes as color will change with time and thus give misleading results. The yellow color indicates a final pH of 4 or less, meaning that the saliva was unable to raise the pH. This result should be considered as a risk value.