REPARATIVE DENTINE FORMATION - CALCIUM HYDROXIDE MECHANISM

The calcium hydroxide is most commonly used material in dental practice for pulp capping.

The mechanism by which calcium hydroxide initiates the reparative process is unclear. It has been suggested that a rise in pH as a result of the free hydroxyl ions may initiate or favour mineralization (Tronstad et al. 1981).

calcium hydroxide may act as a local buffer against the acidic reactions produced by theinflammatory process (Heithersay 1975). An alkaline pH may also neutralize the lactic acid secreted by osteoclasts, and this may help to prevent further destruction of mineralized tissue.

It has been speculated that the material exerts a mitogenic and osteogenic effect, the high pH combined with the availability of calcium and hydroxyl ions having an effect on enzymatic pathways and hence mineralization (Torneck et al. 1983).

The high pH may also activate alkaline phosphatase activity which is postulated to play an important role in hard tissue formation (Guo & Messer 1976). The optimum pH for alkaline
phosphatase activity is 10.2 (Gordon et al. 1985), a level of alkalinity which is produced
by many calcium hydroxide preparations.

Heithersay (1975) suggested that calcium ions may reduce the permeability of new capillaries, so that less intercellular serum is produced, thus increasing the concentration of calcium ions at the mineralization site.

The presence of a high calcium concentration may also increase the activity of calcium dependent
pyrophosphatase, which represents an important part of the mineralization process.

The reduced capillary permeability following the increase in the number of calcium ions could reduce serum flow within the dental pulp, and consequently the concentration of the inhibitory pyrophosphate ion would be reduced.

This would coincide with an increase in levels of calcium-dependent pyrophosphatase as promulgated by Heithersay (1975), and would result in uncontrolled mineralization of the pulp tissue (Fig. 1). This could possibly explain the high incidence of mineralized canals observed following pulpotomy and direct pulp capping (Langeiand et al. 1971, Seltzer & Bender 1984)

SOME VARIATION in the way in which a dentine bridge is formed, depending on the pH of the material that is used to dress the tooth.

high pH material such as pulpdent
Necrotic zone is formed adjacent to the material, and the dentine bridge then forms between this layer and the underlying vital pulp. The necrotic tissue eventually degenerates and disappears, leaving a void between the capping material and the bridge.

lower pH, such as Dycal
The necrotic zone is similarly formed but is resorbed prior to the formation of the dentine bridge, which then comes to be formed directly against the capping material.

Dentine bridges formed by the high pH materials are histologically identical to those produced by lower pH materials, but are easier to distinguish on a radiograph because of the space between the bridge and the calcium hydroxide.

reference
International Endodontic Journal ,1990,23,283-297

BIOACTIVE GLASSES - NOVAMIN

The bioactive glasses is introduced by HENCH in 1969

Bioactive glass containing calcium sodium phosphosilicate (NovaMinTM) that the glass particles release calcium and phosphate ions intra-orally to promote remineralization.

NovaMin releases fully active calcium and phosphorus ions when in contact with water.This provides a higher concentration of the same ions that are naturally found in saliva. This ensures and enhances the natural self-repair of your tooth surface, crystalline hydroxyl-carbonate apatite (HCA) layer that is chemically and structurally the same as tooth mineral.

The silica containing Ca, PO and Na bind to the tooth surface. The Na buffers the pH above 7, sodium ions (Na+) in the bioactive glass exchanges with H+ ions in body fluids causing pH to increase. (the pH is needed to be above 7 to allow for the precipitation of crystals onto the tooth surface).

ANTIBACTERIAL MECHANISM

The short-term antimicrobial effect of these glasses has been attributed exclusively to their ability to raise pH in an aqueous environment (Allan et al. 2001).

This pH increase results from the release of alkali ions, mainly Na+, and the incorporation of protons (H+) into the corroding material.

(i) the immediate killing effect of glasses on microbiota is related to their sodium content and thus their alkaline capacity.

(ii) the effect with a slow onset after several days is related to apatite precipitation on the bacteria.

(iii) the latter effect is promoted by soluble ionic species rather than the calcium and phosphate ions.

MECHANISM OF ACTION

As bioactive glass is mixed with distilled water rapid dissolution and breakdown of silica network, accompanied by the release of Ca2+, PO4 3- and Si4+ occurs at the glass surface.

Then, sodium ions are leached, leaving behind a silica-rich surface.

Finally, a polycondensated silica-rich gel layer is formed on the glass bulk.

The latter may act as a template for apatite nucleation [11] that grow by assuming more Ca2+ and PO4 3- from the surrounding fluid.

Therefore, the formation of apatite on glass surface is related to the concentration of the effective ions of Ca2+, PO4 3- and OH- released in the reaction medium,as its solution is saturated with calcium and phosphate, which might drive mineral back into the tooth.

ADVANTAGE
Benefits in patients experience reduced calcium, phosphate and fluoride ions caused by hyposalivation resulting from old age, prescription drug use, Sjögren’s Syndrome, diabetes and radiation therapy.

APPLICATION
Increased exposure time yields increased mineralization, at least up to 40 minutes. Exposure time has a generally linear effect on new mineral formation, up to 40 minutes of exposure time, indicating that users of NovaMin dentifrice would be best served to maximize dwell time by refraining from rinsing, drinking, etc. for some time after brushing.

REFERENCE
Australian Dental Journal 2008; 53: 268–273
Acta Biomaterialia , Volume 3 , Issue 6 , Pages 936 - 943
International Endodontic Journal. 41(8):670-678, August 2008
Egypt. J. Solids, Vol. (29), No. (1), (2006) 69
Hench LL, Wilson J, An Introduction to Bioceramics Singapore, World Scientific Publishing, 1993.