Hydroxyapatites

Blood Calcium Ion Level. In normal adults, the blood Ca " level is estabhshed by an equiUbrium between blood Ca " and the more soluble intercrystalline calcium salts of the bone. Additionally, a subtle and intricate feedback mechanism responsive to the Ca " concentration of the blood that involves the less soluble crystalline hydroxyapatite comes into play. The thyroid and parathyroid glands, the fiver, kidney, and intestine also participate in Ca " control. The salient features of this mechanism are summarized in Figure 2 (29—31). 

Both monocalcium phosphate and dicalcium phosphate dissolve incongmently in water, disproportionating to more basic calcium phosphate and phosphoric acid. The extent of these reactions varies with the temperature and the amount of water. If water is added gradually to anhydrous monocalcium phosphate, equiUbrium conditions first correspond to a mixture of the anhydrous salt and its monohydrate. After conversion to the monohydrate, further reaction affords dicalcium phosphate plus free phosphoric acid. Dicalcium phosphate decomposes in aqueous solution to the more basic hydroxyapatite and phosphoric acid via intermediate octacalcium phosphate. The compHcated stepwise conversion of the acidic mono- and dicalcium phosphates to hydroxyapatite is summarized in equations 6—9. The kinetics are quite complex. 

Tricalcium phosphate, Ca2(P0 2> is formed under high temperatures and is unstable toward reaction with moisture below 100°C. The high temperature mineral whidockite [64418-26-4] although often described as P-tricalcium phosphate, is not pure. Whidockite contains small amounts of iron and magnesium. Commercial tricalcium phosphate prepared by the reaction of phosphoric acid and a hydrated lime slurry consists of amorphous or poody crystalline basic calcium phosphates close to the hydroxyapatite composition and has a Ca/P ratio of approximately 3 2. Because this mole ratio can vary widely (1.3—2.0), free lime, calcium hydroxide, and dicalcium phosphate may be present in variable proportion. The highly insoluble basic calcium phosphates precipitate as fine particles, mosdy less than a few micrometers in diameter. The surface area of precipitated hydroxyapatite is approximately... 

Hydroxyapatite, Ca2Q(PO (OH)2, may be regarded as the parent member of a whole series of stmcturaHy related calcium phosphates that can be represented by the formula M2q(ZO X2, where M is a metal or H O" Z is P, As, Si, Ga, S, or Cr and X is OH, F, Cl, Br, 1/2 CO, etc. The apatite compounds all exhibit the same type of hexagonal crystal stmcture. Included are a series of naturally occurring minerals, synthetic salts, and precipitated hydroxyapatites. Highly substituted apatites such as FrancoHte, Ca2Q(PO (C02) (F,0H)2, are the principal component of phosphate rock used for the production of both wet-process and furnace-process phosphoric acid. 

Tricalcium Phosphate. Commercial tricalcium phosphate (TCP) is actually an amorphous basic calcium phosphate close to hydroxyapatite in composition. Because of its extremely low solubiUty in water, TCP is precipitated almost quantitatively from dilute phosphate solutions with a slurry of hydrated lime. TCP is separated by dmm-, spray-, or flash-drying the TCP slurry, with or without intermediate sedimentation or filtration steps. It is used as an industrial-grade flow conditioner and parting agent. 

Hydroxyapatite (HA) coating on the surface of the hip stem and the acetabular cup is the most recent advancement in artificial hip joint implant technology. This substance is a form of calcium phosphate, which is sprayed onto the hip implant. It is a material found in combination with calcium carbonate in bone tissue, and bones can easily adapt to it. When bone tissue does grow into HA, the tissue then fixes the hip joint implant permanently in position. These HA coatings are only used in press-fit, noncemented implants. 

The head of the femoral component then articulates with an ion-bombarded, HDPE, high walled, acetabular liner which fits iato a screwed ia, machined, titanium, chromium—cobalt—molybdenum or vanadium—aluminum metallic alloy hydroxyapatite-coated acetabular shell/cup. Each of the separate parts of the modular system for total hip arthroplasty is manufactured ia several different sizes. 

Vitahium FHS ahoy is a cobalt—chromium—molybdenum ahoy having a high modulus of elasticity. This ahoy is also a preferred material. When combiaed with a properly designed stem, the properties of this ahoy provide protection for the cement mantle by decreasing proximal cement stress. This ahoy also exhibits high yields and tensile strength, is corrosion resistant, and biocompatible. Composites used ia orthopedics include carbon—carbon, carbon—epoxy, hydroxyapatite, ceramics, etc. 

Technetium-99m pyrophosphate is used for bone imaging. The compound appears to have an affinity for the hydroxyapatite crystals within bone, and is formed by addition of up to 7.4 GBq (200 mCi) pertechnetate. 

Dicalcium Phosphate Dihydrate (DPD). Dicalcium phosphate cHhydrate is completely nonreactive at room temperature. At 65—71°C and in the presence of water, it dehydrates and decomposes into hydroxyapatite and acidic monocalcium phosphate, or a free phosphoric acid (18). It is used to some extent in cake mixes in combination with faster acting acid. Its primary function is to provide acidity late in the baking cycle and thus produce a neutral and palatable product. DPD has an NV of 33. It provides sufficient acidity only in products requiring long baking times. 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

Hydroxyapaite, the mineral constituent of bone, is appHed to the surfaces of many dental implants for the purpose of increasing initial bone growth. Some iavestigators beHeve that an added benefit is that the hydroxyapatite shields the bone from the metal. However, titanium and its aHoy, Ti-6A1-4V, are biocompatible and have anchored successfuHy as dental implants without the hydroxyapatite coating. 

There are a number of questions that must be answered ia order to maximize the beaefits of hydroxyapatite, eg, how thick the coatiag should be how should the coatiag be appHed to the implant is the chemical composition oa the implant the same as before the appHcatioa what is the nature of the titanium or titanium aHoy hydroxyapatite interface, how strong is it, and by what means is it bonded and what is the interfacial microstmcture ... 

High matrix rigidity is offered by porous sihca, which can be deriva-tized to enhance its compatibility with proteins, but it is unstable at alkaline pH. Hydroxyapatite particles have high selectivity for a wide range of proteins and nucleic acids. 

The value of for calcium hydroxyapatite can be defined by charge of Ca + and PO ions. From this point of view calcium hydroxyapatite can be used as high-selective adsorbents for high performance liquid chromatography because with increasing of will be rise a selectivity coefficient a. 

The development of novel materials for applications such as catalysis has been a very important area of work, with several classes of materials being developed and applied (for example, zeolites and their mesoporous analogues, hydrotalcites, hydroxyapatites, clays). 

In this work, simple (single-use) biosensors with a layer double stranded (ds) calf thymus DNA attached to the surface of screen-printed carbon electrode assembly have been prepared. The sensor efficiency was significantly improved using nanostructured films like carbon nanotubes, hydroxyapatite and montmorillonite in the polyvinylalcohol matrix. 

A wide variety of precipitates form in cooling water systems carbonates, silicates, sulfates, and phosphates are common. Below and slightly above 212°F (100°C), calcite, aragonite, gypsum, hydroxyapatite, magnesium phosphate, anhydrite, and serpentine are commonly encountered (see Table 4.1). 

Two capabilities of ISS are important in applications to the analysis of ceramics. One of these is its surface sensitivity. Many catalyst systems use ceramics where the surface chemistry of the outer 50 A or less is extremely important to performance. Comparing the ratio of H and O to AI or Si is equally important for many systems involving bonding operations, such as ceramic detectors, thin films, and hydroxyapatite for medical purposes. 

Phosphorus is the eleventh element in order of abundance in crustal rocks of the earth and it occurs there to the extent of 1120 ppm (cf. H 1520 ppm, Mn 1060 ppm). All its known terrestrial minerals are orthophosphates though the reduced phosphide mineral schrieber-site (Fe,Ni)3P occurs in most iron meteorites. Some 200 crystalline phosphate minerals have been described, but by far the major amount of P occurs in a single mineral family, the apatites, and these are the only ones of industrial importance, the others being rare curiosities. Apatites (p. 523) have the idealized general formula 3Ca3(P04)2.CaX2, that is Caio(P04)6X2, and common members are fluorapatite Ca5(P04)3p, chloroapatite Ca5(P04)3Cl, and hydroxyapatite Ca5(P04)3(0H). In addition, there are vast deposits of amorphous phosphate rock, phosphorite, which approximates in composition to fluoroapatite. " These deposits are widely... 

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