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Chemical Composition of The Teeth

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The main dental tissues:

Teeth are made of:

Enamel – Dentine – Cementum Enamel and dentine have different

composition Cementum and dentine are very

similar in composition

The relations of the main dental tissues

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Composition of dental tissues

Dental tissues are made of: Organic matter – minerals – water

Different % of constituents depending on calculation of proportions by weight or volume

Enamel: contains very little organic matter (~ 1.3% of dry weight or ~ 1.1% of wet tissues, but ~ 3% of the actual volume) - > 90% inorganic

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Dentine: contains more organic matter (~ 20% of dry weight, or ~ 21% of wet tissues, but ~ 28% of the actual volume), while the inorganic part is ~ 72% of wet weight, or ~ 48% actual volume

Cementum is similar to dentine in composition

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Mineral composition: Most reliable analysis obtained by

heating tissue to 105°C to evaporate water prior to analysis

The most predominant mineral is calcium followed by phosphorus, and finally magnesium

Ca and P are more in enamel Mg and CO2 are more in dentine

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The structure of the inorganic fraction:

i. The main constituent is the crystalline form of calcium phosphate known as apatite with (except probably in enamel) some amorphous calcium phosphate

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ii. Apatites are a crystalline form having the general formula Ca10 (Po4)6 X2, and the most widely distributed type is hydroxy apatite (HA) where x is OH

iii. Apatites belong to the hexagonal system of crystals

The crystal structure of hydroxyapatite

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iv. Calcium in the apatite structure (two types)

A- columnar calcium: forms a series of hexagons

B- hexagonal calcium: lie within the hexagons, and the ions are arranged in triangles placed parallel to each other with adjacent triangles rotated through 60°C, so if viewed along the longitudinal axis, the calcium atoms in the two triangles would appear as a second hexagon

The crystal structure of hydroxyapatite

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v. Phosphate in the apatite structure: phosphates are placed in two tetrahedra

(each consisting of one phosphorus atom with four oxygen atoms) between pairs of calcium ions in the outer hexagon , so that one phosphorus and three oxygen atoms are above the plane of the calcium ions (the fourth oxygen atom being below the plane) and the other phosphate is arranged in the reverse way

The crystal structure of hydroxyapatite

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vi. The hydroxyl ions in the apatite structure:

OH- are placed inside the triangles formed by the calcium ions.

The O is either slightly above, or an equal distance below the plane of the calcium triangles.

There is no room to accommodate two OH group pointing towards each other (-OH--- ---HO-) in adjacent calcium triangles.

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They must either be arranged in an ‘ordered column’ i.e. (OH- OH- OH- ….) along the axis or in ‘disordered column’ with the direction reversed at various places.

The latter is supported by evidence, resulting in voids or vacancies where space prevents an OH group being placed

The crystal structure of hydroxyapatite

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vii. Fluoride in the apatite structure:

Fluoride can enter the vacancies, so that it occupies a central position in the same plane as calcium ions

In addition, it can replace OH ions

The resulting crystal is more stable and less soluble than apatite without fluoride

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viii. Biological apatite are non-stoichiometric: Pure synthetic apatite has Ca:P ratio of

2.15 Ratio is lower in bone and teeth Two properties of apatite explain the

variation in nature

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a. Adsorption: e.g. adsorption of excess phosphate as (HPO4-)

on the crystal surface, and of citrate, CO32-,

HCO3- and magnesium as (MgOH)+ as wellb. Ion exchange: e.g. substitution of Calcium by sodium and

magnesium, or H3O+ for two adjacent calcium, or even absence of some calcium and the addition of one H+ to PO4 3+ to give HPO42- and the absence of OH- to maintain electrical balance

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ix. The more general formula for biologically formed apatite is Ca10-x (HPO4)x (PO4)6-x (OH) 2-

x .XH2O (where x is between o and 2, and normally a fractional number)

x. Another likely component with apatite is Octa Calcium Phosphate (OCP):

Ca8 H2 (PO4)6 .5H2O

i.e. Ca8 (HPO4)2 (PO4)4.5H2O [Ca:P=1.33], thus explaining the lower Ca:P ratio in nature

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The crystallinity of apatite:

Biologically formed crystals are not perfect

Fluoride presence in environment during crystal formation improves crystallinity

Magnesium and carbonate inhibit crystal growth and lead to formation of crystals with poor crystallinity

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The size, shape and orientation of crystals :

The rods or prisms are the anatomical unit of enamel

They are ~ 5μm in diameter and extending through its full thickness

They are shaped like a key hole with a round head or, in some places, a fish tail

The tails of one row fit between the heads of the next, so that the heads are towards the cusp.

Crystallites within rods are oriented in a cuspal-cervical direction in the tail end, but perpendicular to this direction in the head end

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Each row of prisms is inclined to its neighbors by 2°

In the outer third the rows of prisms are parallel and roughly perpendicular to the enamel surface

The outer surface of enamel frequently lacks the normal arrangement of rods (or prisms) but is arranged either in continuous layers parallel to the surface or as onion like curves

This prism-less layer is usually 20-30μm thick, and present in deciduous truths and 70% of permanent teeth, although it did not cover the whole of the surface in most teeth, probably because it was worn a way by abrasion

The apatite crystals in this layer are arranged almost at right angles to enamel surface in contrast to those within the prisms

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Note:

These changes in direction produce the optical phenomenon known as the Hunter Schreger bands

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The crystals in enamel are ~ 10x larger than those of bone or dentine i.e. smaller surface area/unit weight

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Minor inorganic constituents of enamel and dentine

a. Higher concentration on the surface of enamel than within (F, Pb, Zn, Fe, Sb, Mn, Cl, Se)

b. Lower concentration on surface than within (Na, Mg, CO32-)

c. Distribution approximately uniform (K, Sr, Cu, Al)

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Concentrations range from a few ppm to <0.01 ppm.

Only strontium, F and Zn reach or exceed conc. Of 100 ppm through out the teeth

Ions that attach readily to apatite crystals tend to increase in parts of teeth which are exposed mostly to body fluid i.e. outer enamel, outer cementum and inner dentine

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Ions that dissolve out from crystals easily, will tend to decrease in the above parts

Sodium concentration of enamel is higher than that of any other tissue in the body

Magnesium rises in concentration from about 0.45% in outer enamel to 2% in inner dentine

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Factors affecting the composition of enamel and dentine

1. Position in tooth: already discussed

2. Type of tooth: e.g. F on surface of enamel is higher in incisors than in molars-opposite for proteins

3. Effect of age: increase in F and Sr with age. Some may decrease or increase due to decreased permeability

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Organic Matter of Dentine

1. Collagen:

a. Higher in the outer third

b. Contains chondroitin sulphate

c. OH-lysine higher than in skin

d. Is linked to a phospho protein through an oligo saccharide

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2. Non collagen matrix:

a. Approx. 20 components

b. 2 large molecules: a glycoprotein containing sialic acid, and a proteoglycan containing CS. Both have phosphoserine

c. Serum albumin and immunoglobulins are also present

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3. Lipids:

a. Some is bound to, or trapped by, the mineral matter

b. F.a., MAG, DAG, lecithin and cardiolipin are not bound

c. Cholesterol, its esters and TAG are partially bound

4. Citrate

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Organic Matter of Enamel

A. Protein

1. Outer and inner enamel2. Consist of peptides of MW <35003. High in ser, pro, and gly4. Standing in acid becomes insoluble5. Contains bound carbohydrates (hexoses, fucose and xylolose)6. High content leads to reduction in spread of caries

Soluble Insoluble

- Inner enamel- Larger- High in gly & leu

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B. Lipids: similar to dentine. Give strong staining reaction in early caries due to release from minerals

C. Citrate: higher on the surface and near the amelo-dentinal junction than in the middle

D. Lactate: similar distribution, but lower concentration

The relations of the main dental tissues

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10 -Cementum

Primary )Cell-free(

Secondary)Cell-containing(

- Contains cells & Lacunae with canaliculi & is Lamellated also- Covers the apical two- thirds of the root

-A series of lamellae parallel to direction of root-present on the coronal third of the root

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Both contain collagen fibers of the periodontal membrane embedded

Similar composition to dentine but lower ash content (Ca & P)

Formed intermittently by cementoblasts, lying between the edge of the periodontal membrane, and a thin layer of uncalcified ‘pre cementum’

Continued formation through out life Amount & arrangement is influenced by

occlusal stress