1. Plaque Fluid and the Caries Process
DENT 5302 TOPICS IN DENTAL BIOCHEMISTRY
2 April 2007
Plaque Fluid and the Caries Process
Objectives:
Effect of bacterial acids and plaque fluid on the mineral phase of enamel
The concept of critical pH
Enamel-plaque fluid interaction

2. Outline Plaque fluid composition Stephan curve Enamel substrate
Enamel – plaque fluid interaction
The concept of critical pH
Erosion
Ultrastructure of enamel caries lesion

3. Plaque Composition 80% water 20% solid
Bacterial and salivary protein – 50%
Carbohydrates and lipids – 20-30%
Extra and intracellular polysaccharides
- Synthesized by bacteria
- Bacterial attachment and cohesion
- Reservoir of fermentable substrates
Inorganic components – 25%
Ca, P: several times higher than in saliva
Most Ca is non-ionic. becomes ionized as pH drops
Determine rates of enamel dissolution and remineralization
Other ions: K, Na, Mg, and F.
Critical point: Dental plaque is responsible for the majority of chemical activities
on the tooth surface.

4. Plaque Fluid
Plaque fluid = extracellular aqueous phase of dental plaque
Provide aqueous medium for diffusion and exchange of substances between saliva and tooth surface
Separated from plaque by centrifugation
500 mg wet weight plaque sample 150 nL plaque fluid
Changes in ionic composition of plaque fluid cariogenic conditions
Rested plaque fluid: one to several hours after eating
Starved plaque fluid: following overnight fasting
Total organic acids
(mmol/L) pH
Rested plaque
Starved plaque

5. Lactic acid: the main acid involved in caries formation
Lactic acid concentrations in plaque fluid following a 2-min 10% sucrose rinse
Time (min)
Acid
(mmol/L)
17.5 7
37.5 15
33.4 23
18.6
Lactic
Margolis HC, Moreno EC. Composition and cariogenic potential of dental plaque fluid.
Crit Rev Oral Biol Med 1994;5:1-25

6. Stephan curve Stephan RM. JADA 1940;27:718-723?
Stephan RM. JADA 1940;27:
Changes in hydrogen-ion concentration on tooth surfaces and in carious lesion.
Stephan RM. JADA 1944; 23:
Intra-oral hydrogen-ion concentrations associated with dental caries activity.

7. What contributes to the extent of pH drop after glucose challenge?
Type and amount of CHO available
Bacteria present
Salivary composition and flow
Other food ingested
Thickness and age of dental plaque

8. What contributes to the differences in resting plaque?
Resting plaque pH:
Constant within each individual, but differences among groups.
Caries-inactive – resting pH ~
Caries-prone – lower resting pH
What contributes to the differences in resting plaque?
Bacterial composition affects metabolic properties of plaque
Storage form of CHO energy source when diet is depleted
When the host does not ‘eat’, cariogenic bacteria still produce acids form storage carbohydrates

9. What are the differences in plaque fluid between ‘caries-free’ and caries-positive individuals?
Composition
Na+
Mg2+ K+
Calcium P * pH *
Acid
Lactic
Acetic
Propionic
DS (enamel) *
Margolis HC. Enamel-plaque fluid interaction. Cariology for the Nineties, 1993

10. Hydroxyapatite lattice structure
Enamel substrate
Enamel: 96% by weight or 87% by volume mineral
13 vol % interprismatic space is diffusion channel
Major mineral component (teeth and bone):
Calcium phosphate crystals ~ Hydroxyapatite Ca10(PO4)6(OH)2
Hydroxyapatite lattice structure
Hydroxyl ions form columns of parallelogram
Calcium ions form triangle around hydroxyl ion
Phosphates fill space
Nikiforuk G. Understanding Dental Caries. Karger 1985

11. Current concept: Dental mineral is carbonated HAP
Biological mineral is ‘nonstoichiometric’
Concentration of the chemical components is different from pure HAP
≠ Ca10(PO4)6(OH)2
Substitution of three primary constituents with
- carbonate
- other trace elements (impurities): F, Na, Cl, Mg, K, Zn, Si, Sr
Current concept: Dental mineral is carbonated HAP
Carbonate (CO3)2- substitute (PO4)3- or 2 (OH)-
Carbonate ions disturb the regular array of ions in the crystal lattice
More soluble in acid than pure HAP

12. Discussion (group of 5-6)
When a tooth is just erupted into the oral cavity, it is more susceptible to demineralization.
Why?

13. Post-eruptive Maturation
Newly erupted teeth have relatively greater caries susceptibility
During demineralization, carbonate is lost and excluded after remin
Decrease carbonate & increase fluoride in enamel surface
Less susceptible to demineralization
= post-eruptive maturation
Simplified formula of tooth mineral
(Ca)10-x(Na)x(PO4)6-y(CO3)z(OH)2-u(F)u

14. Solubility product (Ksp)
When do teeth dissolve?
Teeth dissolve when pH is lower than a critical pH
Solubility product (Ksp)
Ksp is the ionic activity products of substance at saturation
Ksp = Concentrations of the component ions to the power in saturated solution
e.g., HAP Ca5(PO4)3OH ; Ksp(HAP) = [Ca2+]5[PO43-]3[OH-] = 7.36 x 10-60
Ksp(enamel) = 5.5 x 10-55
Ksp(carbonated-HAP) = 4.57 x 10-49
Ksp is a constant value
Acidic solution:
H+ remove PO43- & OH-
Decrease [PO4] & [OH] in solution
Apatite mineral dissolves
[PO4] & [OH] rise to maintain the saturation level

15. Demineralization Remineralization
Ionic Activity Product (IAP)
Determined the same way as Ksp, but use the ion concentrations
in the solution.
Degree of saturation (DS)
Ratio of the ionic product of a substance in the solution (IAP) to its ionic product at saturation (Ksp )
e.g., for hydroxyapatite (Ca5(PO4)3OH)
DS =
Ksp (ionic activity products at saturation)
IAP (ionic activity products in solution)
1/9
DS > 1 : Solution supersaturated WRT mineral
DS < 1 : Solution undersaturated WRT mineral
DS = 1 : Saturation condition
Demineralization
Remineralization
(WRT = with respect to)
Margolis HC, Moreno EC
Crit Rev Oral Biol Med 1994;5:1-25

16. Critical pH of carious formation in enamel ~ 4.5-5.5
The concept of critical pH
= pH at which a solution is just saturated WRT a particular mineral
If the solution pH > critical pH supersaturated mineral precipitate
If the solution pH < critical pH undersaturated mineral dissolve
Normal condition: Our teeth do not dissolve in saliva or plaque fluid
Saliva and plaque fluid are supersaturated WRT tooth enamel
pH of saliva & plaque fluid > critical pH
Saliva & plaque fluid contain Ca, P, OH IAP > Ksp tooth enamel
The tooth will dissolve when the pH of fluid phase is less than critical pH.
Critical pH of carious formation in enamel ~
Coincide with pH when plaque bacteria ferment carbohydrates
HAP is undersaturated & FAP is supersaturated

17. deposit caries erosionpH
FAP
HAP
deposit
caries
erosion
demineralization
remineralization
Critical pH
Carious lesion forms at pH
Erosion lesion forms when pH < 4.5

18. 'Erosion' ‘acid corrosion'
Loss of dental hard tissue through chemical etching and dissolution by acids of non-bacterial origin
Endogenous acid: gastric acid, gingival crevicular fluid
Exogenous acid: diet, medicine, industry
Gastroesophageal reflux disease, vomiting
Frequent and prolonged ingestion of acidic fruits, fruit juices and acidic beverages
3/4 of a bottle of white wine
Every evening for 34 years
Sipping over a 3 hours after dinner
Wine pH ranges about 3-4.
Dental consumption due to wine consumption. Mandel L. JADA 2005;136:71-75

19. * * * Can acidic food and drinks soften enamel surface?
Enamel samples alternately immersed, 5 sec each, in food or drink and in artificial saliva for 10 cycles. * * * pH
S. Wongkhantee et al., J Dent 2006;34:
Effect of acidic food and drinks on surface hardness of enamel, dentine, and tooth-coloured filling materials.

20. Critical pH is not a fixed value
Solubility isotherm pH
100 10 1
0.1
0.01
0.001
0.0001
HAP
calcium (mol/l)
FAP
oral fluid
Current concepts on the theories of the mechanism of action of fluoride.
ten Cate JM. Acta Odontol Scand 1999;57:325-9.

21. Ultrastructure of enamel caries lesion
Crystal damage from acid:
- Surface etching
- Central defect or hairpin
Crystal core has more dislocations or lattice defects
Higher carbonate content
Dissolving crystals are smaller
Increased intercrystalline space
Larger crystal at prism periphery
from remineralization

22. surface zone and dark zone1.
Surface
zone 2.
Body of
lesion 3.
Dark zone 4.
Translucent
Sound
enamel 1 2 3 4 1 2 3 4
Larger crystals in
surface zone and dark zone
Indication of remineralization
Range of crystal size in each zone of early enamel lesion

23. Recommended references
1. Zero DT. Dental Caries Process. Dent Clin North Am 1999;43(4):
2. Featherstone JD. The science and practice of caries prevention. J Am Dent Assoc 2000;131:
3. Gordon Nikiforuk. Understanding Dental Caries 1. Etiology and Mechanisms, Basic and Clinical Aspects. Basel; New York: Karger Chapters 4 &10.
4. Margolis HC, Moreno EC. Composition and cariogenic potential of dental plaque fluid. Crit Rev Oral Biol Med 1994;5:1-25.
5. Margolis HC. Enamel – plaque fluid interactions. In WH Bowen and LA Tabak (Eds) Cariology for the nineties. University of Rochester Press 1993:

24. Diagram showing effect of increase Ca on degree of saturation of plaque fluid with respect to enamel
Question:
Which line represent individuals with higher tendency for caries formation?