In the unit cell of calcium fluorapatite the z co-ordinates of fluoride are 0.25, 0.5 and 0.75

Oral disease control
Predicting caries
Periodontitis Control
Control strategies

False. In the unit cell of calcium fluorapatite the fluoride ions are in the same plane as the calcium triangles and have Z Coordinates of 0.25 and 0.75.

The picture on the right shows an exploded view of a calcium fluorapatite unit cell below a plan view. The unit cell defines the position of 6 column calcium ions which have Z Coordinates of 0, 0.5 and 1.0. The Z Coordinates of 0.0 and 1.0 define the upper and lower limits of the unit cell in the vertical axis (C Axis). Calcium triangles occupy midway positions between these column calcium ions with Z Coordinates of 0.25 and 0.75. Phosphate ions lie in the same plane as the calcium triangles.

The plane of each of the calcium triangles is shown shaded. Note that the unit cell does not include all three of the calcium ions comprising each triangle.

The unit cell of calcium hydroxyapatite is very similar except that a hydroxyl ion is substituted for each of the fluoride ions. Due to their polarity and size, hydroxyl ions are unable to fit within the calcium triangles and are displaced either above or below. This means that the position of hydroxyls along the C axis of the hydroxyapatite unit cell is defined by 2 Z Coordinates for each of the calcium triangles. Thus hydroxyl ions have Z Coordinates of 0.19 or 0.31 and 0.69 or 0.81. If the hydroxyls are incorporated into the unit cell such that one has a Z Coordinate of 0.31 and the other a Z Coordinate of 0.69 then they are brought into close proximity which may result in one of the hydroxyls being excluded from the unit cell. Such "missing hydroxyls" destabilise the hydroxyapatite crystal structure and contribute to a more soluble crystal.



Concepts to Grasp

The mineral phase of tooth enamel and dentine is an impure calcium hydroxyapatite called biological apatite. The contaminating ions further disrupt the crystal structure of the apatite rendering it more soluble. Fluoride stabilises biological apatite by both substituting hydroxyl ions by a process known as ion exchange and by filling gaps left in the crystal structure when hydroxyls are displaced during crystal formation.

Hydroxyapatite or biological apatite into which a significant amount of fluoride has become incorporated is known as fluoridated apatite to distinguish it from the true crystal calcium fluorapatite.

Terms to Learn

Calcium triangles
Column calciums
Missing hydroxyls
Unit cell

Clinical Relevance

Incorporation of fluoride into the apatite of tooth enamel and dentine significantly reduces the incidence of dental caries. The fluoride can be administered both systemically and topically through fluoridation of the water supply. It is also highly effective when topically applied via toothpastes.

Dentists can enhance fluoride uptake by teeth using a technique known as Acidulated Phosphate Fluoride (APF) or by painting teeth with a fluoride-containing varnishes. Fluoride can also be administered in tablet form. In all cases dentists must assess the risk of inducing fluorosis.


Infants should use a dentifrice containing less than 900ppm fluoride

True. Infants are prone to swallow toothpaste and this, in conjunction with other sources of fluoride such as water, may lead to a condition known as fluorosis.

Fluorosis manifests as a mottling of the teeth and while not in itself harmful it is unsightly. There is also concern in the dental profession that infants may accumulate clinically harmful quantities of fluoride especially if the local water contains high natural levels of fluoride.


In order to reduce this risk and the risk of fluorosis, toothpaste manufacturers now produce infant and children's toothpaste with a reduced amount of fluoride, typically 900 ppm.

The fluoride concentration of the enamel surface is higher than at the enamel-dentinal junction

True. The fluoride content of teeth has been measured at various depths from the surface bto the amelo-dentinal junction (ADJ).

In one method, ground sections of teeth were prepared and coated on all surfaces with nail varnish. A small section of varnish was then removed and the section treated with dilute acid to dissolve some of the exposed enamel.

The soluble material was saved. A further section of enamel was then exposed and the section once more treated with dilute acid and the soluble material saved.

The measurements were repeated until the enamel had been dissolved up to the ADJ. The fluoride content of each of the saved solutes was then measured and related to the position of the material relative to the surface of the tooth.

The results show that a gradient of fluoride exists with the concentration at the surface of teeth is very much greater than that at the ADJ.

Fluoridated hydroxyapatite is more soluble than hydroxyapatite

False. The incorporation of fluoride into hydroxyapatite has the effect of neutralising some of the destabilising features of hydroxyapatite. Among these are the so-called "missing hydroxyls" resulting from displaced hydroxyl ions during crystal formation. The resulting gaps can be filled by fluoride ions adding stability to the crystal. Fluoride ions may also replace hydroxyl ions by the process of ion exchange.


Hydroxyapatite which has incorporated fluoride is sometimes known as fluoridated hydroxyapatite and is more stable and therefore less soluble than hydroxyapatite.

Fluoride reduces acid production by Streptococcus mutans by inhibiting lactate dehydrogenase

False. Acid production is inhibited but not via inhibition of lactate dehydrogenase. The inhibition is thought to be due to a combination of


reduction of intracellular pH of the bacteria


inhibition of proton-translocating ATPases which are involved in sugar uptake by the bacterial cell


inhibition of key glycolytic enzymes such as enolase

The data on the right shows the effect of 2 ppm fluoride on the acid production by Streptococcus mutans.

Effect of fluoride on acid production by Streptococcus mutans

Equivalents of acid produced/min
2 ppm calcium fluoride