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Hydroxyapatite

Hydroxylapatite, also called hydroxyapatite, is a mineral. It is a naturally occurring 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. It crystallizes in the hexagonal crystal system. It has a specific gravity of 3.08 and is 5 on the Mohs hardness scale. Pure hydroxylapatite powder is white. Naturally occurring apatites can however also have brown, yellow or green colorations, comparable to the discolorations of dental fluorosis.

Seventy percent of bone is made up of the inorganic mineral hydroxylapatite. Carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enamel and dentin are comprised. Hydroxyapatite crystals are also found in the small calcifications (within the pineal gland and other structures) known as corpora arenacea or 'brain sand'.

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 much differently to them. 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 graft-versus-host disease (GVHD) and rejection.

Many modern implants, e.g. hip replacements and dental implants, are coated with hydroxyapatite. It has been suggested that this may promote osseointegration and there is strong supporting evidence for this.

The mechanism of hydroxylapatite (HA) 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.

Microcrystalline hydroxyapatite (MH) is marketed as a "bone-building" supplement with superior absorption than 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 (BMD).

The importance of calcium on preventing osteoporosis is undeniable. Yet, calcium does not work alone in creating strong, healthy bones. Although calcium is a major constituent of bone, the whole bone matrix is composed of a variety of other minerals including phosphorus and magnesium, trace elements, proteins, collagen, and glycosaminoglycans. Calcium and phosphorus occur in bone in a 2 to 1 ratio predominantly as crystalline structures called hydroxyapatite.

The physiological balance and structure of the hydroxyapatite minerals along with the presence of the organic constituents in bone caused researchers to speculate that it might be appropriate to use an extract of whole bone as a more beneficial form of supplemental calcium. Subsequent research conducted in Britain over a ten-year period showed that was indeed the case. Microcrystalline calcium hydroxyapatite compound (MCHC), and extract of young bovine bone, was found to be an extremely bioavailable form of calcium. The superiority of MCHC compared to traditional forms of calcium was shown in a number of controlled calcium balance and calcium absorption studies. The whole bone extract was exceptionally well-absorbed, producing more prolonged positive balance than soluble calcium salts such as calcium gluconate. MCHC was uniquely found to not only prevent further loss of bone, but to restore its mineral content. It was also surprisingly well-tolerated. Unlike calcium carbonate, the bone extract did not produce carbon dioxide to interfere with digestion.

  • Hydroxyapatite is an essential mineral of normal bone.
  • Crystals of hydroxyapatite deposited in and about the joints cause inflammation.
  • Inflammation and pain are part of hydroxyapatite crystal disease.
  • This disease is distinct and different from gout and pseudogout.
  • Treatment includes measures to reduce inflammation and pain.

 

Hydroxyapatite:

40% (by weight), aqueous ethanol suspension, 60-180 µm

Temperature stability 4 - 121 °C

particle size 60-180 µm

pH range 4 - 13

storage temp. 2-8°C

 

Hydroxyapatite (reagent grade, powder, synthetic)

Synonyms: Calcium hydroxyphosphate, Calcium phosphate tribasic, HAp, Hydroxylapatite

Linear Formula: [Ca5(OH)(PO4)3]x

Molecular Weight: 502.31

Specifications Related Products References Description

Packaging 25, 100 g in glass btl

Properties:

Grade: reagent grade

Form: powder

 

 
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