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Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
form of
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to i ...

calcium
apatite Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most co ...
with the
formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a chemical formula. The informal use of the term ''formula'' in science refers to the general construct of a relationship between given ...
Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxyapatite is the
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy groups. Both ...
endmember An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity. Minerals often can be described as solid solutions with varying compositions of some chemical elements, rather ...
of the complex apatite group. The OH
ion An ion () is a particle, atom or molecule with a net electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and opposite to charged proton(s) considered positive by convent ...
can be replaced by
fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts typically have dist ...
,
chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts suc ...
or
carbonate In chemistry, a carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of . The name may also refer to a carbonate ester, an organic compound containing the carbonate g ...
, producing
fluorapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, blue ...
or
chlorapatite Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most co ...
. It crystallizes in the
hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon or 6-gon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A ''regular hexagon' ...
crystal system In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals. Informally, two crystals are in the same crystal system if they have simila ...
. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of
dental fluorosis Dental fluorosis is a common disorder, characterized by hypomineralization of tooth enamel caused by ingestion of excessive fluoride during enamel formation. It appears as a range of visual changes in enamel causing degrees of intrinsic tooth disc ...
. Up to 50% by volume and 70% by weight of
human bone The human skeleton is the internal framework of the human body. It is composed of around 270 bones at birth – this total decreases to around 206 bones by adulthood after some bones get fused together. The bone mass in the skeleton reaches max ...
is a modified form of hydroxyapatite, known as
bone mineral Bone mineral (also called inorganic bone phase, bone salt, or bone apatite) is the inorganic component of bone tissue. It gives bones their compressive strength. Bone mineral is formed predominanly from carbonated hydroxyapatite with lower crystall ...
. Carbonated calcium-deficient hydroxyapatite is the main mineral of which
dental enamel Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, cem ...
and
dentin Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by enamel o ...
are composed. Hydroxyapatite crystals are also found in the small calcifications, within the
pineal gland The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycles ...

pineal gland
and other structures, known as
corpora arenacea Corpora arenacea (''singular'': corpus arenaceum, also called brain sand or acervuli or psammoma bodies or pineal concretions) are calcified structures in the pineal gland and other areas of the brain such as the choroid plexus. Older organisms ha ...
or 'brain sand'.


Chemical synthesis

Hydroxyapatite can be synthesized via several methods, such as wet chemical deposition, biomimetic deposition, sol-gel route (wet-chemical precipitation) or electrodeposition. The hydroxyapatite nanocrystal suspension can be prepared by a wet chemical precipitation reaction following the reaction equation below: 10 Ca(OH)2 + 6 H3PO4 → Ca10(PO4)6(OH)2 + 18 H2O The ability to synthetically replicate hydroxyapatite has invaluable clinical implications, especially in dentistry. Each technique yields hydroxyapatite crystals of varied characteristics, such as size and shape. These variations have a marked effect on the biological and mechanical properties of the compound, and therefore these hydroxyapatite products have different clinical uses.


Calcium-deficient hydroxyapatite

Calcium-deficient (non-stochiometric) hydroxyapatite, Ca10−''x''(PO4)6−''x''(HPO4)''x''(OH)2−''x'' (where ''x'' is between 0 and 1) has a Ca/P ratio between 1.67 and 1.5. The Ca/P ratio is often used in the discussion of calcium phosphate phases. Stoichiometric apatite Ca10(PO4)6(OH)2 has a Ca/P ratio of 10:6 normally expressed as 1.67. The non-stoichiometric phases have the hydroxyapatite structure with cation vacancies (Ca2+) and anion (OH) vacancies. The sites occupied solely by phosphate anions in stoichiometric hydroxyapatite, are occupied by phosphate or hydrogen phosphate, HPO42−, anions. Preparation of these calcium-deficient phases can be prepared by precipitation from a mixture of
calcium nitrate Calcium nitrate, also called ''Norgessalpeter'' (Norwegian saltpeter), is an inorganic compound with the formula Ca(NO3)2. This colourless salt absorbs moisture from the air and is commonly found as a tetrahydrate. It is mainly used as a component i ...
and
diammonium phosphate Diammonium phosphate (DAP; IUPAC name diammonium hydrogen phosphate; chemical formula (NH4)2(HPO4) is one of a series of water-soluble ammonium phosphate salts that can be produced when ammonia reacts with phosphoric acid. Solid diammonium phosphate ...

diammonium phosphate
with the desired Ca/P ratio, for example, to make a sample with a Ca/P ratio of 1.6: :9.6 Ca(NO3)2 + 6 (NH4)2HPO4 → Ca9.6(PO4)5.6(HPO4)0.4(OH)1.6 Sintering these non-stoichiometric phases forms a solid phase which is an intimate mixture of tricalcium phosphate and hydroxyapatite, termed biphasic calcium phosphate: :Ca10−''x''(PO4)6−''x''(HPO4)''x''(OH)2−''x'' → (1 − ''x'') Ca10(PO4)6(OH)2 + 3''x'' Ca3(PO4)2


Biological function


Mantis shrimp

The clubbing appendages of the '' Odontodactylus scyllarus'' (peacock mantis shrimp) are made of an extremely dense form of the mineral which has a higher specific strength; this has led to its investigation for potential synthesis and engineering use. Their dactyl appendages have excellent
impact resistance In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.bone A bone is a rigid tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and enable mobili ...
and
teeth A tooth (plural teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth ...

teeth
; bone is made primarily of HA crystals interspersed in a
collagen upright=1.5, Tropocollagen molecule: three left-handed procollagens (red, green, blue) join to form a right-handed triple helical tropocollagen. Collagen () is the main structural protein in the extracellular matrix found in the body's various conn ...
matrix—65 to 70% of the mass of bone is HA. Similarly HA is 70 to 80% of the mass of
dentin Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by enamel o ...
and enamel in teeth. In enamel, the matrix for HA is formed by
amelogenin Amelogenins are a group of protein isoforms produced by alternative splicing or proteolysis from the ''AMELX'' gene, on the X chromosome, and also the ''AMELY'' gene in males, on the Y chromosome. They are involved in amelogenesis, the development ...
s and
enamelin Enamelin is an enamel matrix protein (EMPs), that in humans is encoded by the ''ENAM'' gene. It is part of the non-amelogenins, which comprise 10% of the total enamel matrix proteins. It is one of the key proteins thought to be involved in ameloge ...
s instead of collagen. Hydroxyapatite deposits in tendons around joints results in the medical condition
calcific tendinitis Calcific tendinitis is a form of tendinitis, a disorder characterized by deposits of hydroxyapatite (a crystalline calcium phosphate) in any tendon of the body, but most commonly in the tendons of the rotator cuff (shoulder), causing pain and inf ...
.


Hydroxyapatite in remineralisation of tooth enamel

Remineralisation In biogeochemistry, remineralisation (or remineralization) refers to the breakdown or transformation of organic matter (those molecules derived from a biological source) into its simplest inorganic forms. These transformations form a crucial link w ...
of tooth enamel involves the reintroduction of mineral ions into demineralised enamel. Hydroxyapatite is the main mineral component of enamel in teeth. During demineralisation, calcium and phosphorus ions are drawn out from the hydroxyapatite.The mineral ions introduced during remineralisation restore the structure of the hydroxyapatite crystals.


Common Uses of Hydroxyapatite

At present, the use of hydroxyapatite, or its synthetically manufactured form, nano-hydroxyapatite, is not yet common practice. However, some studies suggest it is useful in counteracting dentine hypersensitivity, preventing sensitivity after teeth bleaching procedures and caries prevention


Dentine Sensitivity

Nano-hydroxyapatite possesses bioactive components which can prompt the mineralisation process of teeth, remedying hypersensitivity. Hypersensitivity of teeth is thought to be regulated by fluid within dentinal tubules. The movement of this fluid as a result of different stimuli is said to excite receptor cells in the pulp and trigger sensations of pain. The physical properties of the nano-hydroxyapatite can penetrate and seal the tubules, stopping the circulation of the fluid and therefore the sensations of pain from stimuli. Nano-hydroxyapatite would be preferred as it parallels the natural process of surface remineralisation. In comparison to alternative treatments for dentine hypersensitivity relief nano-hydroxyapatite containing treatment has been shown to perform better clinically. Nano-hydroxyapatite was proven to be better than other treatments at reducing sensitivity against evaporative stimuli, such as an air blast, and tactile stimuli, such as tapping the tooth with a dental instrument. However, no difference was seen between nano-hydroxyapatite and other treatments for cold stimuli. Hydroxyapatite has shown significant medium and long-term desensitizing effects on dentine hypersensitivity using evaporative stimuli and the visual analogue scale (alongside potassium nitrate, arginine, glutaraldehyde with hydroxyethyl methacrylate, hydroxyapatite, adhesive systems, glass ionomer cements and laser).


Co-Agent for Bleaching

Teeth bleaching agents release reactive oxygen species which can degrade enamel. To prevent this, nano-hydroxyapatite can be added to the bleaching solution to reduce the impact of the bleaching agent by blocking pores within the enamel. This reduces sensitivity after the bleaching process.


Caries Prevention

Nano-hydroxyapatite possesses a remineralising effect on teeth and can be used to prevent damage from carious attacks. In the event of an acid attack by cariogenic bacteria, nano-hydroxyapatite particles can infiltrate pores on the tooth surface to form a protective layer. Furthermore, nano-hydroxyapatite may have the capacity to reverse damage from carious assaults by either directly replacing deteriorated surface minerals or acting as a binding agent for lost ions. In the future, there are possibilities for using nano-hydroxyapatite for tissue engineering and repair. The main and most advantageous feature of nano-hydroxyapatite is its biocompatibility. It is chemically similar to naturally occurring hydroxyapatite and can mimic the structure and biological function of the structures found in the resident extracellular matrix. Therefore, it can be used as a scaffold for engineering tissues such as bone and cementum. It may be used to restore cleft lips and palates and refine existing practices such as preservation of alveolar bone after extraction for better implant placement.


As a dental material

Hydroxyapatite is widely used within dentistry and
oral and maxillofacial surgery Oral and maxillofacial surgery is a surgical specialty focusing on reconstructive surgery of the face, facial trauma surgery, the oral cavity, head and neck, mouth, and jaws, as well as facial cosmetic surgery. Specialty An oral and maxillo ...
, due to its chemical similarity to hard tissue. In some toothpaste hydroxyapatite can be found in the form of nanocrystals (as these are easily dissolved). In recent years, hydroxyapatite nanocrystals (nHA) have been used in toothpaste to combat dental hypersensitivity. They aid in the repair and remineralisation of the enamel, thus helping to prevent tooth sensitivity. Tooth enamel can become demineralised due to various factors, including acidic erosion and dental
caries Tooth decay, also known as dental caries or cavities, is the breakdown of teeth due to acids made by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicat ...
. If left untreated this can lead to the exposure of dentin and subsequent exposure of the
dental pulp The pulp is the part in the center of a tooth made up of living connective tissue and cells called odontoblasts. The pulp is a part of the dentin–pulp complex (endodontium). The vitality of the dentin-pulp complex, both during health and after i ...
. In various studies the use of nano hydroxyapatite in toothpaste showed positive results in aiding the remineralisation of dental enamel. Synthetic hydroxyapatite (SHA) is proven to provide successful outcomes in alveolar socket preservation. Socket grafting using synthetic hydroxyapatite can result  in successful bone regeneration.


Chromatography

The mechanism of hydroxyapatite
chromatography Chromatography is a laboratory technique for the separation of a mixture. The mixture is dissolved in a fluid (gas, solvent, water, ...) called the ''mobile phase,'' which carries it through a system (a column, a capillary tube, a plate, or a sh ...

chromatography
is complicated and has been described as "mixed-mode". It involves ionic interactions between positively charged groups on a biomolecule (often a protein) and the phosphate groups in hydroxyapatite, and metal chelation between hydroxyapatite calcium ions and negatively charged phosphate and/or carboxyl groups on the biomolecule. It may be difficult to predict the effectiveness of hydroxyapatite chromatography based on physical and chemical properties of the desired protein to be purified. For elution, a buffer with increasing phosphate and/or neutral salt concentration is typically used.


Use in archaeology

In
archaeology Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. Archaeology is often considered a branch of socio-cultural anthropology, but archaeologists also draw from biological, geological, ...
, hydroxyapatite from human and animal remains can be analysed to reconstruct ancient Diet (nutrition), diets, migrations and paleoclimate. The mineral fractions of bone and teeth act as a reservoir of Micronutrients, trace elements, including carbon, oxygen and strontium. Stable isotope analysis of human and faunal hydroxyapatite can be used to indicate whether a diet was predominantly terrestrial or marine in nature (carbon, strontium); the geographical origin and migratory habits of an animal or human (oxygen, strontium) and to reconstruct past temperatures and climate shifts (oxygen). Post-depositional alteration of bone can contribute to the degradation of bone collagen, the protein required for stable isotope analysis.


Defluoridation

Hydroxyapatite is a potential adsorbent for the defluoridation of drinking water, as it forms
fluorapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, blue ...
in a three step process. Hydroxyapatite removes F from the water to replace OH forming fluorapatite. However, during the defluoridation process the hydroxyapatite dissolves, and increases the pH and phosphate ion concentration which makes the defluoridated water unfit for drinking. Recently, a ″calcium amended-hydroxyapatite″ defluoridation technique was suggested to overcome the phosphate leaching from hydroxyapatite. This technique can also affect fluorosis reversal by providing calcium-enriched alkaline drinking water to fluorosis affected areas.


See also

* Calcium hydroxyphosphate * Calcific tendinitis * Mechanical properties of biomaterials


References


External links

{{Authority control Calcium minerals Phosphate minerals Ceramic materials Biomaterials Hexagonal minerals Minerals in space group 176