• Effect of bacterial acids and plaque fluid on the mineral phase of enamel

• The concept of critical pH

• Enamel-plaque fluid interaction

Objectives:

DENT 5302 TOPICS IN DENTAL BIOCHEMISTRY

2 April 2007

Outline

Plaque fluid composition

Stephan curve

Erosion

Enamel substrate

Ultrastructure of enamel caries lesion

The concept of critical pH

Enamel – plaque fluid interaction

Extra and intracellular polysaccharides

- Synthesized by bacteria

- Bacterial attachment and cohesion

- Reservoir of fermentable substrates

Plaque Composition

80% water

Critical point: Dental plaque is responsible for the majority of chemical activities

on the tooth surface.

20% solid

Bacterial and salivary protein – 50%

Carbohydrates and lipids – 20-30%

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.

• 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 g wet weight plaque sample 150 nL plaque fluid

• Changes in ionic composition of plaque fluid cariogenic conditions

Plaque Fluid

Rested plaque fluid: one to several hours after eating

Starved plaque fluid: following overnight fasting

Rested plaque

Starved plaque

Total organic acids

(mmol/L)

56.3 - 102.1

31.9 - 61.5

pH

5.69 - 6.54

6.78 - 7.08

Lactic

0

17.5

Time (min)

7

37.5

15

33.4

23

18.6

Acid(mmol/L)

Lactic acid concentrations in plaque fluid following a 2-min 10% sucrose rinse

Margolis HC, Moreno EC. Composition and cariogenic potential of dental plaque fluid.

Crit Rev Oral Biol Med 1994;5:1-25

Lactic acid: the main acid involved in caries formation

Stephan curve

Stephan RM. JADA 1940;27:718-723Changes in hydrogen-ion concentration on tooth surfaces and in carious lesion.

Stephan RM. JADA 1944; 23:257-266Intra-oral hydrogen-ion concentrations associated with dental caries activity.

?

?

?

Type and amount of CHO available

Bacteria present

Salivary composition and flow

What contributes to the extent of pH drop after glucose challenge?

Other food ingested

Thickness and age of dental plaque

Resting plaque pH:

Constant within each individual, but

differences among groups.

Caries-inactive – resting pH ~ 6.5 - 7

Caries-prone – lower resting pH

Bacterial composition affects metabolic properties of plaque

Storage form of CHO energy source when diet is depleted

What contributes to the differences in resting plaque?

When the host does not ‘eat’, cariogenic bacteria still produce acids

form storage carbohydrates

Margolis HC. Enamel-plaque fluid interaction. Cariology for the Nineties, 1993

What are the differences in plaque fluid between ‘caries-free’ and caries-positive individuals?

Composition

Na+

Mg2+

K+

Calcium

P

Acid

Lactic

Acetic

Propionic

pH

DS (enamel)

14.2 + 3.5

2.0 + 0.4

59.9 + 4.9

16.2 + 5.2

13.9 + 1.9

‘caries-free’ caries-positive

16.5 + 5.4

2.6 + 0.4

71.4 + 11.3

6.9 + 0.4

15.6 + 3.6

1.8 + 0.7

19.9 + 3.5

5.8 + 1.5

2.6 + 1.2

20.3 + 4.6

5.8 + 1.5

7.02 + 0.05 6.79 + 0.12

7.11 + 0.66 5.42 + 0.68

*

*

*

Major mineral component (teeth and bone):

Calcium phosphate crystals ~ Hydroxyapatite Ca10(PO4)6(OH)2

Hydroxyapatite lattice structure

Phosphates fill space

Nikiforuk G. Understanding Dental Caries. Karger 1985

Hydroxyl ions form

columns of parallelogram

Calcium ions form

triangle around hydroxyl ion

Enamel substrate

Enamel: 96% by weight or 87% by volume mineral

13 vol % interprismatic space is diffusion channel

Biological mineral is ‘nonstoichiometric’

Concentration of the chemical components is different from pure HAP

Substitution of three primary constituents with

- carbonate

- other trace elements (impurities): F, Na, Cl, Mg, K, Zn, Si, Sr

≠ Ca10(PO4)6(OH)2

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

Current concept: Dental mineral is carbonated HAPCurrent concept: Dental mineral is carbonated HAP

Discussion (group of 5-6)

When a tooth is just erupted into the oral cavity, it is

more susceptible to demineralization.

Why?

Simplified formula of tooth mineral

(Ca)10-x(Na)x(PO4)6-y(CO3)z(OH)2-u(F)u

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

Post-eruptive MaturationPost-eruptive Maturation

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 is a constant value

Solubility product (Ksp)

Ksp(carbonated-HAP) = 4.57 x 10-49

Ksp(enamel) = 5.5 x 10-55

When do teeth dissolve?

Teeth dissolve when pH is lower than a critical pH

H+ remove PO43- & OH-

Decrease [PO4] & [OH] in solution

Apatite mineral dissolves

[PO4] & [OH] rise to maintain the saturation level

Acidic solution:

Degree of saturation (DS)

Ratio of the ionic product of a substance in the solution (IAP) to its

ionic product at saturation (Ksp )

DS > 1 : Solution supersaturated WRT mineral

DS < 1 : Solution undersaturated WRT mineral

DS = 1 : Saturation condition

DS =Ksp (ionic activity products at saturation)

IAP (ionic activity products in solution) 1/9

Margolis HC, Moreno EC

Crit Rev Oral Biol Med 1994;5:1-25

e.g., for hydroxyapatite (Ca5(PO4)3OH)

Determined the same way as Ksp, but use the ion concentrations

in the solution.

Ionic Activity Product (IAP)

(WRT = with respect to)

The concept of critical pH

Saliva and plaque fluid are supersaturated WRT tooth enamel

= 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

Critical pH of carious formation in enamel ~ 4.5-5.5Critical pH of carious formation in enamel ~ 4.5-5.5

Coincide with pH when plaque bacteria ferment carbohydrates

HAP is undersaturated & FAP is supersaturated

The tooth will dissolve when the pH of fluid phase is less than critical pH.

pH of saliva & plaque fluid > critical pH

Saliva & plaque fluid contain Ca, P, OH IAP > Ksp tooth enamel

pH 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0

FAP

HAP

deposit caries erosion

demineralization

remineralization

pH 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0

Critical pH

Carious lesion forms at pH 4.5 - 5.5

Erosion lesion forms when pH < 4.5

Endogenous acid: gastric acid, gingival crevicular fluid

Exogenous acid: diet, medicine, industry

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.

Loss of dental hard tissue through chemical etching and dissolution

by acids of non-bacterial origin

‘acid corrosion'

Frequent and prolonged ingestion

of acidic fruits, fruit juices and

acidic beverages

Gastroesophageal reflux disease, vomiting

Dental consumption due to wine consumption. Mandel L. JADA 2005;136:71-75

0

50

100

150

200

250

300

Cola Sports drink Orangejuice

Drinkingyogurt

Lemon-grass soup

En

am

el H

ard

ne

ss

Before

After

pH 2.74 3.78 3.75 3.83 4.20

S. Wongkhantee et al., J Dent 2006;34:214-220.

Effect of acidic food and drinks on surface hardness of enamel, dentine, and tooth-coloured filling materials.

Enamel samples alternately immersed, 5 sec each, in food or drink and in artificial saliva for 10 cycles.

*

* *

Can acidic food and drinks soften enamel surface?

Solubility isotherm

Current concepts on the theories of the mechanism of action of fluoride.

ten Cate JM. Acta Odontol Scand 1999;57:325-9.

100

10

1

0.1

0.01

0.001

0.0001

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

HAPHAP

cal

ciu

m (

mo

l/l)

FAPFAP

oral fluid

Critical pH is not a fixed valueCritical pH is not a fixed value

pH

Crystal damage from acid:

- Surface etching

- Central defect or hairpin

Ultrastructure of enamel caries lesion

Larger crystal at prism periphery

from remineralization

• Crystal core has more dislocations or lattice defects

• Higher carbonate content

• Dissolving crystals are smaller

• Increased intercrystalline space

Larger crystals in

surface zone and dark zone

Indication of remineralization

Range of crystal size in each zone of early enamel lesion

1. Surfacezone

2. Body oflesion

3.Dark zone

4.Translucent

zone

Soundenamel

1 2 3 41

2 34

Recommended references

1. Zero DT. Dental Caries Process. Dent Clin North Am 1999;43(4):635-664.

2. Featherstone JD. The science and practice of caries prevention. J Am Dent Assoc 2000;131:887-899.

3. Gordon Nikiforuk. Understanding Dental Caries 1. Etiology and Mechanisms, Basic and Clinical Aspects. Basel; New York: Karger 1985. 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 1993:173-186.

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?