Hydroxylapatite, also called hydroxyapatite, is a naturally occurring mineral form of calcium
apatite with the formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2 to denote
that the crystal unit cell comprises two entities.
Hydroxylapatite is the hydroxyl endmember of the complex apatite group.
The OH- ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite.
It crystallizes in the hexagonal crystal system.
Pure hydroxylapatite powder is white.
Naturally occurring apatites can, however, also have brown, yellow, or green colorations,
comparable to the discolorations of dental fluorosis.
Up to 50% by volume and 7% by weight is a modified form of hydroxylapatite.
Carbonated calcium-deficient hydroxylapatite is the main mineral of which dental enamel
and dentin are composed.
Hydroxylapatite crystals are also found in the small calcifications known as corpora
arenacea or 'brain sand'.
Chemical synthesis of hydroxyapatite Hydroxyapatite can be synthesized via several
methods such as wet chemical deposition, biomimetic deposition, sol-gel route or electrodeposition.
Yagai and Aoki proposed the hydroxyapatite nanocrystal suspension can be prepared by
a wet chemical precipitation reaction following the reaction equation below:
10Ca(OH)2 + 6H3PO4 → Ca10(PO4)6(OH)2 + 18H2O Several studies have shown that hydroxyapatite
synthesis via wet-chemical route can be improved by power ultrasound.
The ultrasonically assisted synthesis of hydroxyapatite is a successful technique to produce nanostructured
hydroxyapatite at high quality standards.
The ultrasonic route allows to produce nano-crystalline hydroxyapatite as well as modified particles,
e.g. core-shell nanospheres, and composites.
Medical uses
Hydroxylapatite can be found in teeth and bones within the human body.
Thus, it is commonly used as a filler to replace amputated bone or as a coating to promote
bone ingrowth into prosthetic implants.
Although many other phases exist with similar or even identical chemical makeup, the body
responds to them very differently.
Coral skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure
allows relatively rapid ingrowth at the expense of initial mechanical strength.
The high temperature also burns away any organic molecules such as proteins, preventing an
immune response and rejection.
Many modern implants, e.g. hip replacements, dental implants and bone conduction implants,
are coated with hydroxylapatite.
It has been suggested that this may promote osseointegration..
Porous hydroxylapatite implants are used for local drug delivery in bone.
It is also being used to repair early lesions in tooth enamel.
Pathology Hydroxylapatite deposits in tendons around
joints resulting in the medical condition calcific tendinitis.
Supplement Microcrystalline hydroxylapatite is marketed
as a "bone-building" supplement with superior absorption in comparison to calcium.
It is a second-generation calcium supplement derived from bovine bone.
In the 1980s, bone meal calcium supplements were found to be contaminated with heavy metals,
and although the manufacturers claim their MH is free from contaminants, people are advised
to avoid it because it has not been well-tested.
However, the limited tests seem to show positive results.
A 1995 randomized placebo-controlled study of 40 people in Europe found that it was more
effective than calcium carbonate in slowing bone loss.
A 2007 randomized double-blind controlled study of an MH supplement called the Bone
Builder found significant positive effects in bone mineral density compared to control.
Hydroxylapatite has been used by Noel Fitzpatrick to facilitate bionic development in animals,
by coating steel rods in hydroxylapatite to encourage natural growth of skin around it.
As a component of nanocomposites, hydroxylapatite is finding uses as a potential new bone replacement
materials.
Chromatography The mechanism of hydroxylapatite chromatography
is complicated and has been described as "mixed-mode" ion exchange.
It involves nonspecific interactions between positively charged calcium ions and negatively
charged phosphate ions on the stationary phase HA resin with protein negatively charged carboxyl
groups and positively charged amino groups.
It may be difficult to predict the effectiveness of HA chromatography based on physical and
chemical properties of the desired protein to be purified.
For elution, a buffer with increasing phosphate concentration is typically used.
Use in archaeology In archaeology, hydroxylapatite from human
and animal remains is analysed in order to reconstruct ancient diets.
The mineral fractions of bone and teeth act as a reservoir of trace elements, including
strontium.
It has been established that the ratio of strontium to calcium in bone hydroxylapatite
broadly reflects an animal's diet during the period before its death when the bone was
being formed, or in the case of dental mineral in childhood.
In either case analysis of Sr/Ca ratios allows an individual's diet to be classified as carnivorous,
herbivorous or omnivorous and either predominantly marine or terrestrially based.
However the difficulty of compensating for post-mortem contamination of archaeological
samples through interaction with groundwater continues to cast doubt on the reliability
of the method.
Stable isotope analysis is considered a more viable alternative, although strontium and
other trace mineral analyses of dental samples are commonly used in situations where this
is impossible because the collagen content of the bone has completely decayed.
Animal structures The clubbing appendages of the Peacock mantis
shrimp are made of an extremely dense form of the mineral, which is being investigated
for potential synthesis and engineering use.
See also Archaeological science
Biomaterials: Mechanical Properties References