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Why is saliva and a good salivary flow so important?

Saliva is made up of the secretions of the three pairs of salivary glands these are: the parotid, sub-mandibular, sublingual and hundreds of minor salivary glands within the submucosa and some gingival crevicular fluid. These glandular secretions- saliva, constantly bathe the teeth and oral tissues; its presence is vital to the maintenance of healthy teeth and soft oral tissues.

The basic secretory units of salivary glands are clusters of cells called acinar cells, they resembles a many-lobed “berry”, such as a raspberry (acinus Latin for “berry”) hence the name. One of the most important functions of saliva is its buffering ability. This buffering system maintains the acid-base equilibrium by neutralising organic acids present in food, and acids produced by bacteria in caries. This buffering action reduces the H⁺ ion (released by acids) concentration in the oral cavity. Bicarbonate present in saliva is mainly responsible for this.
Bicarbonate is concentrated within acinar cells and released following a secretory stimulus. Carbon dioxide inside cells is converted to bicarbonate (HCO₃-) and H⁺ by carbonic anhydrase. Carbonic anhydrase is an enzyme that assists rapid inter-conversion of carbon dioxide and water into carbonic acid, protons and bicarbonate ions. This chemical reaction is represented below.

CO₂⁺ H₂O <——>H₂CO₃ <—–> HCO₃^– + H⁺

How does the carbonic acid/bicarbonate buffer work?
The pH of the oral cavity is maintained at about 6.3 to ensure the maintenance of the integrity of the tooth structure. Once food is in the mouth, two important events occur:
(1) Drop in pH
(2) Rise in bicarbonate concentration
The drop in pH is caused by increased concentrations of H+ within plaque, due to acid being produced by bacteria when they ferment carbohydrates. The buffering capacity of saliva is due mainly to the presence of bicarbonate ions, secreted within the ducts. The concentration of bicarbonate is determined by the stimulation of saliva and by carbonic anhydrase (secreted by the serous acinar cells in the parotid and sub-mandibular glands). In stimulated saliva, such as when eating, saliva flow rate increases dramatically. More bicarbonate is produced as a by-product of cell metabolism which diffuses into dental plague and helps neutralise the increased amount of acid (H+) produced by oral microbes.

The following equilibrium exists in saliva:

Direction of reaction
<—————————————————
^{Carbonic Anhydrase}
CO₂ + H₂O <——-> H₂CO₃<—–>HCO₃ – + H⁺ {from secreted (lactic acid
bicarbonate) from bacteria}

Carbon dioxide is present in the form of bicarbonate in stimulated saliva. A rise in bicarbonate concentration conveniently removes the increased amounts of H⁺ produced by bacteria, such that the above equilibrium shifts to the left to produce more carbonic acid. However, the concentration of carbonic acid in the mouth is maintained at remarkably constant levels; this means that the excess carbonic acid must be removed. The increase in carbonic acid concentration causes the equilibrium to shift left further such that more CO₂ is produced. The enzyme carbonic anhydrase catalyses this reaction by driving the conversion of carbonic acid to carbon dioxide and water. Because the partial pressure of CO₂ in saliva is now higher than that in the atmosphere and the mouth is an open system, CO₂ diffuses out from the saliva. (Bardow et al., 2008, p.198).

As the concentration of carbonic acid falls, more bicarbonate ions bind to hydrogen ions to form carbonic acid, establishing a new equilibrium. In this way, excess H+ produced within plaque is effectively neutralised and removed, reducing the risk of erosion of teeth. The concentration of HCO₃^– in saliva is not constant, but varies with the flow rate in such a way that unstimulated saliva contains only a few mmole/liter (mM), whereas stimulated saliva contains much higher levels of HCO₃^–, depending on the stimulus intensity.

Saliva’s Protective Function

Saliva limits damages from trauma such as: mechanical, chemical, thermal and biological. Saliva participates in the process of remineralization and slows down demineralization. At pH 6.8 – 7.2 saliva is over saturated with calcium phosphate solution therefore after slight demineralization, lost mineral components can return from saliva to hard tissues of tooth.
The equilibrium between saliva and enamel is represented by the following equation:

Ca₁₀ (PO₄)6 (OH)₂ <——–>10Ca₂+ + 6(PO₄)³– + 2OH^–

The acidifying of environment (e.g. as a result of carbohydrates’ fermentation) increases solubility, and decreases saturation of saliva with calcium phosphates and saliva become an unsaturated solution.
Severe reduction of salivary output not only results in a rapid deterioration in oral health but also has a detrimental impact on the quality of life for the sufferer. Patients suffering from dry mouth can experience difficulty with eating, swallowing, speech, the wearing of dentures, trauma to and ulceration of the oral mucosa, taste alteration, poor oral hygiene, a burning sensation of the mucosa, oral infections including candida and rapidly progressing dental caries.

Dry mouth and Polypharmacy

The sensation of dry mouth is becoming increasingly common in developed countries where adults are living longer. Polypharmacy (the use of a large number of medications- five or more) is very common among the older adult population, and many commonly prescribed drugs cause a reduction in salivary flow. Since polypharmacy is a consequence of having several underlying medical conditions, it is much more common in elderly patients. With increasing age the mix of conventionally prescribed and complementary medicines changes from minimising risk factors (lipid lowering, bone density etc.) to treating disease. With increasing frailty, older people tend to take more medication making polypharmacy a major cause of dry mouth. (Australian Institute of Health and Welfare 2012. Dementia in Australia. Cat. no. AGE 70. Canberra: AIHW). Xerostomic drugs (Xerostomia, subjective sensation of dry mouth), especially those with anticholinergic effects; (anticholinergic drugs inhibit the transmission of nerve impulses by opposing the actions of the neurotransmitter acetylcholine) can cause xerostomia; producing symptoms of dry mouth by reducing saliva volume or by altering the threshold of perceiving a dry mouth. Hyposalivatory drugs reduce salivary flow. Xerostomia also occurs in Sjögren’s syndrome (where immune cells infiltrate and destroy the mucous-producing glands of the body, the specific cause of Sjögren’s is unknown; however, there appears to be a genetic influence occurring more often in families that have other autoimmune diseases. Other factors, as viral infections and stress also appear to be able to trigger the disease.

A 24-hour survey of medicine usage of Australians aged fifty years or older found 43.3% use five(5) or more conventional or complementary medicines while 10.7% use ten (10) or more. Polypharmacy (taking five or more medicines) increases with age with five or more medicines taken by 32.2% of 50-
64 year olds, 49.4% of 65-74 year olds and 66.0% for those over 70 years (Silva M, Hopcraft M, Morgan M. Dental caries in Victorian nursing homes. Aust Dent J. 2014; (59): 321–328.)

In addition to specific diseases of the salivary glands, salivary flow is usually severely impaired following radiotherapy in the head and neck area for cancer treatment in both children and adults of all ages. Clearly oral dryness is a problem which faces an increasingly large proportion of the population. An understanding of saliva and its role in oral health will help to promote awareness among health care workers of the problems arising when the quantity or quality of saliva is decreased.

 

Q. Can you please supply a scientific journal reference on safety implications? Both these topical fluorides (Creighton’s CSDS and SDI’s Riva Star) work, thus have a different fluoride ion release potential?
A. Both CSDS (Creighton Dental) and Riva Star (SDI) have passed the stringent safety assays and have been approved by the Therapeutic Goods Administration.
(Note: The TGA when assessing safety breaks a product down into its individual constituents as shown below.)No detailed information is available to the author on Riva Star but in terms of CSDS the following applies using one drop of AgF (0.03 ml) and one drop of SnF2 (0.03 ml) as the standard treatment dose.
Silver in one drop of AgF = 10.2 mg which is 99.8% lower than an acute oral toxic dose.
• U.S. EPA. (1985). Drinking Water Criteria Document for Silver (Final Draft). Environmental Criteria and Assessment Office, Cincinnati, OH. ECAO-CIN-026, PB86-118288.

Tin in one drop of SnF2 = 2.78 mg which is 95.5-96.75% lower than an acute oral toxic dose for a 10 Kg child and 97.7-98.35% lower than an acute toxic dose for a 20 Kg child.
• (WHO (2004). Inorganic Tin in Drinking-water. Background document for development of WHO Guidelines for Drinking-water Quality. WHO/SDE/WSH/03.04/115.
• Winship KA (1988). Toxicity of tin and its compounds. Adverse Drug React Acute Poisoning Rev.7:19-38.

Fluoride in one drop of AgF plus one drop of SnF2 = 2.53 mg which is 94% lower than a possible acute oral toxic dose for a 10 Kg child and 97.47 lower than a possible acute toxic dose for a 20 Kg child.
• Whitford GM (1992). Acute and chronic fluoride toxicity. J Dent Res. 71:1249-1254.

Given that the recommended technique for using CSDS is to apply AgF with a microbrush leave it on the tooth for a minimum of 1 minute ideally for 3 minutes and then apply SnF2 also with a microbrush; also one standard microbrush holds 0.01 ml of solution. As can be seen from the above the safety margins for CSDS are extremely high. Furthermore, CSDS and Riva Star are not used as topical application liberally applied to all the teeth. They are treatments for carious lesions applied directly on the affected tooth. The silver component is used to attack and destroy bacteria and the fluoride to facilitate remineralisation. They are not applied liberally to all teeth in the oral cavity where some may be ingested and absorbed systemically through the GIT.

Q. I understand that silver fluoride is less stable compared to fluoride varnish. Both materials are safe if used per recommendations. Both have different clinical indications so it is difficult for me to understand your thoughts why silver fluoride is safer than fluoride varnish. In other words, I would not replace fluoride varnish with silver fluoride because they are used in different clinical circumstances. See link regarding caution with silver fluoride preparations.
https://www.ncbi.nlm.nih.gov/pubmed/9973713
A. The original comparison (CSDS is Safer than Duraphat Varnish) between Duraphat varnish and CSDS was to show the relative safety of the latter when used according to directions. You need to understand that the two products are designed for entirely different purposes. Duraphat is applied topically to prevent caries occurring and CSDS is for arresting or slowing down the progression of established lesions that have reached dentine. As for the stability, you understand that silver fluoride is less stable than fluoride varnish; that is true to some extent, however, this is conditional. Silver fluoride has a shelf life of 30 months at present (tested by a TGA registered laboratory, testing continues and in July we expect it will be 36 months), duraphat varnish is stable for 36 months; most importantly one should read the fine print, under heading 6.4, Special precautions for storage:’ it is recommended not to store above 25 degrees C. The real significance of this is that the 36 month stability of Duraphat varnish is conditional to it being stored at 25C and below. Silver Fluoride has no such precautions for storage. (http://www.mhra.gov.uk/…/spcpil/con1497592027775.pdf)
Furthermore, 6.3 (same reference) duraphat varnish, after opening it is recommended to,” use within 3 months!” Once again no such recommendation is given for silver fluoride after opening. Your understanding; therefore, that Duraphat varnish is more stable than silver fluoride is not the complete picture. CSDS’s silver fluoride, is silver fluoride powder (40%) dissolved in water, Duraphat varnish is: a suspension of sodium fluoride; I ml of which contains 50mg of sodium fluoride; as already stated above, they are two different products designed for different purposes.
The reference you make to the paper by Gotjamanos and Orton (1998) as to the need for caution when using silver fluoride is totally irrelevant. Those who have read and understood the paper would know that the products tested, which are no longer available, were not straight silver fluoride. They contained a mixture of ammonium fluoride, sodium or potassium fluoride, and silver fluoride in addition silver difluoride and hydrofluoric acid, as well as some unidentified additive, which gave the products around 40% more fluoride than the current versions. A subsequent paper by Pai et al., (2007) failed to confirm their findings.

Comparison of CSDS and Duraphat varnish

• The recommended dosage for Duraphat for children of nursery age is 0.25 ml of varnish per child. This gives a child a potential exposure to 5.56 mg of fluoride.
• For the same age group 3 full microbrush applications from one drop of 40% AgF gives a potential exposure of 1.8 mg fluoride (Background: A standard microbrush holds 0.01 ml of solution and one drop of AgF is 0.03 ml).
• If the application of 40% AgF is followed up with an application of one drop of 10% stannous fluoride the additional potential exposure is 0.73 mg fluoride.
• Summary:
• If AgF is used alone using the recommended one drop the potential exposure to fluoride is 68% lower than that from Duraphat varnish.
• If the AgF application is followed up by the application of one drop of stannous fluoride the total fluoride exposure is 54% lower than that from Duraphat varnish.

The Difference between Riva Star and Creighton’s CSDS
From the number of emails and calls I have had it is obvious that there is a great deal of confusion about silver fluoride and its uses. I am often asked if I know where to source potassium iodide, to apply to the tooth/teeth treated with silver fluoride/stannous fluoride to negate the blackening effect”, or “what is the difference between CSDS and Riva Star”?
To answer these questions we need to go back to the basic structure of CSDS’s silver fluoride and that of Riva Star’s silver fluoride. Essentially Riva Star is silver diammine fluoride (silver fluoride powder dissolved in ammonia) and CSDS is a water-based silver fluoride i.e. silver fluoride powder dissolved in water.

CSDS is used to treat open carious lesions in primary molars and root-surface lesions in the elderly. It is water-based and contains no ammonia, has no pungent odour and a pH of 6.3 (water pH 7).
Riva Star is designed to be used on vital asymptomatic permanent posterior teeth where some caries is left to avoid a pulp exposure. It is meant to facilitate the natural repair process in dentine, and must be used in conjunction with glass ionomer cement (GIC) base or restorations. It was not designed for use on open carious lesions; it acts as a continuous source of fluoride helping the repair process.

The technique for using Riva Star differs from that of CSDS in the following way:
1) The initial stage is the same with both products; namely isolation of the area to be treated with cotton rolls.
2) However, after that if Riva Star is to be used near gingival tissues these tissues need to be protected with a gingival barrier (due to the product having a very high pH)
3) Silver fluoride is applied with the silver brush provided taking care to apply the solution to the treatment site only.
4) Immediately after, use the green brush to apply a generous amount potassium Iodide to the treatment site. The treatment surface initially appears creamy white; more potassium iodide needs to be applied until it turns clear.
5) The gingival barrier is removed, and the cavity is blot dried. A Glass ionomer (GIC) base or restoration is placed.

The technique for using CSDS is as follows:
1) The teeth to be treated are not cleaned, if there is gingival or occlusal food impaction it is removed. Soft caries is not removed.
2) The teeth to be treated are isolated with cotton rolls.
3) CSDS is water-based with a pH near that of water; therefore, the adjoining soft tissues are unlikely to suffer chemical burns; thus, no gingival barrier is needed.
4) Using a microbrush, silver fluoride is applied to the lesion for a minimum of one minute, ideally for three minutes.
5) Stannous fluoride solution is then applied with a new microbrush. The stannous fluoride precipitates elemental silver, and turns the lesion black.
6) The treated area is covered with Orabase Protective Paste (Convatec).
7) Ideally, the patient is examined two to three weeks after this application. If the lesion remains black it means caries has not progressed, and it can either be left open or restored with GIC. If the black colour of the treated lesion is starting to fade, this is a sign that caries is still progressing and another application of CSDS is warranted. This black staining of a treated carious lesion is an important property of CSDS, as it serves as a visual indicator of the status of the carious lesion

The Science:

Silver fluoride in solution contains silver ions, which actively destroy bacteria in the following ways:
1) Silver ions attach themselves to the bacterial cell membrane which:
a) Alters the membrane structure and permeability
b) Causes leakage of the cytoplasm
c) Impairs the cells transport activity
2) Penetrates inside the cell and nucleus to:
a) Cause mitochondrial dysfunction
b) Destabilise and denature proteins
c) Destabilise ribosomes
d) Interact with DNA
3) Induces cellular toxicity and oxidative stress by the generation of reactive oxygen species (ROS) and free radicals the result is:
a) Proteins and liquids are oxidised
b) DNA base is oxidised
4) Modulates cell signalling, altering the phosphotyrosine profile. Tyrosine phosphorylation is considered to be one of the key steps in signal transduction and regulation of enzymatic activity.

Stannous Fluoride (SnF2).

Metals, like tin (Sn), actually sterilize themselves after a certain period of time. This is called the oligodynamic effect; defined as “a toxic effect of metal ions on living cells” (including bacteria), even in relatively low concentrations. The exact mechanism of this action is still unknown, but some data suggests that the metal ions denature protein of the target cells by binding to reactive groups (proteins contain functional groups from each amino acid these groups are reactive and contribute to protein function) resulting in their precipitation and inactivation.
CSDS not only has the silver ion that disrupts the cell wall by binding with the sulfhydryl groups in the protein of the cell wall forming silver sulphides destroying cell membranes, and inhibits enzyme activity, but it has the added benefit of the stannous (Sn 2+) ion, also denaturing bacterial protein by binding to reactive groups in the same way silver ions do, to kill bacterial. CSDS with the added advantage of the Sn2+ ion is able to launch a two pronged attack on bacteria resulting in the more efficient and quicker demise of bacteria. In addition the stannous 2+ ion is oxidised by surrendering two electrons to form the stannic ion Sn 4+ in so doing it reduces silver ions to metallic silver. The stannic ion (Sn4+) exerts a powerful electrostatic force acting as a coagulant; the strong positive charge of the stannic ion neutralised the negative charge of the denatured proteins, from the dead bacterial cells, collagen from broken down dentine, the other remnants of the carious lesion, elemental silver, silver oxide, calcium and phosphates from saliva; that is able to bind them together into a hard black callous. This black callous forms a diffusion barrier over the treated carious lesion depriving any remaining organisms, if any should survive this vicious attack by silver and stannous ions, of any substrate to reactivate  caries.

Fluoride in AgF and SnF2

The prime function of fluoride is to re-mineralise carious dentine. To understand how this works one has to look at the anatomy of a carious lesion. It consists of several zones; the first zone is infected with bacteria and other break-down products. The next layer contains denatured collagen fibres, minerals including calcium, phosphates and all the by-product of the carious process. The next layer is demineralised dentine with the collagen fibres still intact. We believe these collagen fibres acts as a template for remineralisation, and the fluoride ion creates an alkaline environment that facilitates the binding of calcium, phosphates and other minerals in this remaining intact collagen fibre template.