Dissolution Rate Of Hydroxyapatite Research Articles

The development of non-resorbable bone allografts: Biological background and clinical perspectives.

Bone grafts are typically categorized into four categories: autografts, allografts, xenografts, and synthetic alloplasts. While it was originally thought that all bone grafts should be slowly resorbed and replaced with native bone over time, accumulating evidence has in fact suggested that the use of nonresorbable xenografts is favored for certain clinical indications. Thus, many clinicians take advantage of the nonresorbable properties/features of xenografts for various clinical indications, such as contour augmentation, sinus grafting, and guided bone regeneration, which are often combined with allografts (e.g., human freeze-dried bone allografts [FDBAs] and human demineralized freeze-dried bone allografts [DFDBAs]). Thus, many clinicians have advocated different 50/50 or 70/30 ratios of allograft/xenograft combination approaches for various grafting procedures. Interestingly, many clinicians believe that one of the main reasons for the nonresorbability or low substitution rates of xenografts has to do with their foreign animal origin. Recent research has indicated that the sintering technique and heating conducted during their processing changes the dissolution rate of hydroxyapatite, leading to a state in which osteoclasts are no longer able to resorb (dissolve) the sintered bone. While many clinicians often combine nonresorbable xenografts with the bone-inducing properties of allografts for a variety of bone augmentation procedures, clinicians are forced to use two separate products owing to their origins (the FDA/CE does not allow the mixture of allografts with xenografts within the same dish/bottle). This has led to significant progress in understanding the dissolution rates of xenografts at various sintering temperature changes, which has since led to the breakthrough development of nonresorbable bone allografts sintered at similar temperatures to nonresorbable xenografts. The advantage of the nonresorbable bone allograft is that they can now be combined with standard allografts to create a single mixture combining the advantages of both allografts and xenografts while allowing the purchase and use of a single product. This review article presents the concept with evidence derived from a 52-week monkey study that demonstrated little to no resorption along with invitro data supporting this novel technology as a "next-generation" biomaterial with optimized bone grafting material properties.

Apatitic calcium phosphate/montmorillonite nano-biocomposite: in-situ synthesis, characterization and dissolution properties

Recently, calcium phosphate/montmorillonite composites have received attention as a synthetic bone substitutes. In this study, apatitic calcium phosphate/Montmorillonite nano-biocomposites were in-situ synthesized at 22 °C by reaction between calcium hydroxide and orthophosphoric acid in the presence of different contents of montmorillonite (MNa). Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Brunauer–Emmett–Teller (BET) surface areas were used to characterize the prepared powders. The XRD results show that the composites prepared with 2 and 5 wt% MNa and sintered at 900 °C, show the formation of hydroxyapatite (HAP) structure, whereas that prepared with 10 wt% MNa leads to the formation of β-tricalcium phosphate (β-TCP) structure. The HAP structure decomposes at 1000 °C and leads to the formation of biocomposite containing HAP, β and α-TCP. However, β-TCP composites show thermal stability. FTIR and structural refinement results show the incorporation of clay ions into the apatitic structure causing changes in the crystal structure of the formed calcium phosphate phases. The changes in the composition and structure lead to an increase in the dissolution rate of HAP and a decrease in that of β-TCP.

Rates of hydroxyapatite formation and dissolution in a sandstone aquifer: Implications for understanding dynamic phosphate behaviour within an agricultural catchment

Eutrophication, caused by excessive nutrient concentrations, is a major environmental issue and has significant impacts on both aquatic ecosystems and human health. Phosphorus (P) is a key element that contributes to this eutrophication response. As such, P concentrations are regulated under both the European Union Water Framework Directive (EU WFD) and Habitats Directive (EU HD). While P export to rivers from point sources is well-understood, diffuse sources, particularly that route through connected aquifer systems are less well-characterised. Based on data from a catchment in southern England where an Upper Greensand aquifer controls river baseflow, we hypothesise that dynamic precipitation and dissolution of phosphorus in the form of hydroxyapatite occurs along the groundwater flow path, affecting P storage and export. In addition, P may also occur as the less-reactive mineral fluorapatite or sorb onto, or substitute into, Fe-oxides, which also occur in the aquifer. To investigate hydroxyapatite kinetics in this context, batch experiments were conducted to precipitate the mineral onto sand grains, and then dissolved in a continuous-flow reactor under various pH and concentration conditions. The reactive transport of PO43- along a 1D flow path was modelled to simulate hydroxyapatite precipitation and dissolution. Fe-oxides were modelled as sorbing surfaces for phosphate under chemical conditions representative of the aquifer.Hydroxyapatite rapidly precipitated onto quartz sand grains from a solution that was supersaturated with respect to hydroxyapatite, thus demonstrating that an input of P to Ca-rich, alkaline porewaters can result in the precipitation of secondary hydroxyapatite. The dissolution rate of hydroxyapatite is strongly pH dependent, however within the aquifer porewaters, the solution is often close to equilibrium with respect to hydroxyapatite, which causes the dissolution rate to have a greater dependence on the PO43- concentration. Within the range of measured pH and PO43- concentrations, the dissolution rate ranges between 10-12.25 to 10-9.15 mol kg−1 s−1, contributing P to the catchment headwaters. Over a 1D flow path, there is dynamic precipitation and dissolution of hydroxyapatite, as well as PO43- sorption and desorption, over relatively short timescales. These results indicate that hydroxyapatite is likely to precipitate within the pore space of the aquifer, but can then dissolve and re-precipitate, adsorb and desorb, under expected spatial and temporal variations in pH and water chemistry within the aquifer. Hydroxyapatite may therefore represent a temporary pool of anthropogenic P, sourced from P-fertilisers, which represents a previously unrecognised pathway for anthropogenic P transfer between the soil surface and surface waters.

Preparation and characterization of fluoridated hydroxyapatite coatings on HT-C/C composites

Monetite-like coatings were deposited from a solution containing Ca and P ions onto H 2O 2 treated Carbon/Carbon composites (HT-C/C) by ultrasonic induction heating (UIH) and subsequently subjected to ammonia hydrothermal treatment to form hydroxyapatite (HA). Subsequently, this HA coating was placed in a NaF solution and hydrothermally treated to produce fluorinated hydroxyapatite (FHA) coatings. The structure, morphology and chemical composition of the HA and FHA coatings were characterized by SEM, XRD, XPS, EDS and FTIR, and the adhesiveness and dissolution rate were examined by a scratch test and an immersion test respectively. The results showed that incorporation of F into HA led to an increase in the adhesiveness but a decrease in the dissolution rate of HA coating on C/C. With increasing hydrothermally fluoridation temperature, the F content in the FHA coatings increased and their dissolution rate decreased. However, the adhesiveness of these FHA coatings on C/C first increased from 42.7 N to 54.2 N as the hydrothermal fluoridation temperature increased from 353 K to 393 K, and then decreased to 18.3 N at 413 K. The reasons for changes in the adhesiveness and dissolution rate of the coatings were discussed.

Effects of pH and acid concentration on erosive dissolution of enamel, dentine, and compressed hydroxyapatite

The aims of this study were to determine the effects of pH and acid concentration on the dissolution of enamel, dentine, and compressed hydroxyapatite (HA) in citric acid solutions (15.6 and 52.1 mmol l(-1) ; pH 2.45, 3.2, and 3.9), using a pH-stat system. After an initial adjustment period, the dissolution rates of enamel and HA were constant, while that of dentine decreased with time. The dissolution rate increased as the pH decreased, and this was most marked for enamel. To compare substrates, the rate of mineral dissolution was normalized to the area occupied by mineral at the specimen surface. For a given acid concentration, the normalized dissolution rate of HA was always less than that for either dentine or enamel. The dissolution rate for dentine mineral was similar to that for enamel at pH 2.45 and greater at pH 3.2 and pH 3.9. The concentration of acid significantly affected the enamel dissolution rate at pH 2.45 and pH 3.2, but not at pH 3.9, and did not significantly affect the dissolution rates of dentine or HA at any pH. The variation in response of the dissolution rate to acid concentration/buffer capacity with respect to pH and tissue type might complicate attempts to predict erosive potential from solution composition.

An investigation of some food‐approved polymers as agents to inhibit hydroxyapatite dissolution

Dental erosion involves dissolution of the hydroxyapatite fraction of enamel and dentine, so agents that reduce the dissolution rate of hydroxyapatite could find application in food products aimed at reducing erosion. This study was performed to test some common food ingredients and additives for their effect on the dissolution rate of hydroxyapatite in a citric acid solution representative of soft drinks. Pyrophosphate, tripolyphosphate and a linear chain polyphosphate (average 25 phosphate units) significantly reduced the hydroxyapatite dissolution rate by 35, 46 and 64%, respectively. Xanthan gum and carboxymethylcellulose significantly reduced the hydroxyapatite dissolution rate by 29 and 16%, respectively. The protective effect may be ascribed to the binding of condensed phosphate or to the formation of an adsorbed layer of gum at the hydroxyapatite surface. Several other common food additives had no statistically significant effect on the hydroxyapatite dissolution rate. Polyphosphate exhibited a considerable persistence of action, causing a reduction in the dissolution rate for 3 h after treatment. Tripolyphosphate was slightly persistent, and pyrophosphate and xanthan gum did not exhibit a substantial persistence of action. A solution containing polyphosphate and xanthan gum reduced the hydroxyapatite dissolution rate by 70% and exhibited a similar persistence of action to the solution containing only polyphosphate. These compounds are suggested to have potential as erosion-reducing agents in soft drinks.

Calcium Fluoride and Fluoridated Hydroxyapatite Formation in Relation to the Acid Dissolution Rate of Enamel Mineral

Hydroxyapatite (HA) discs were treated for 1 min with fluoride solutions of varying concentrations and pH. The calcium fluoride and fluoridated hydroxyapatite (FHA) formed were analysed independently of each other following their separation in molar potassium hydroxide. Only the more intimately associated FHA was found to affect the rate of acid dissolution of HA and its level of formation and the reduction in acid dissolution rate of HA were both found to be linearly related to the logarithm of the ionic fluoride level in the treatment solution following 1 min. pretreatment. The benefit of using acidulated fluoride treatment of enamel is questioned as neutral fluoride treatments of enamel mineral resulted in equivalent protection.

Mechanism for the retardation of the acid dissolution rate of hydroxapatite by strontium.

Dissolution rates and apparent solubilities for synthetic hydroxyapatite in acetate buffers containing phosphate and strontium ions in the range of 10-3 to 10-2 M were determined under various pH and buffer conditions. Critical examination of the role of strontium with a physical model suggests that a calcium-strontium apatite surface complex may govern the driving force of the dissolution reaction.