Biphasic calcium phosphate bioceramics: preparation, properties and applications.

Biphasic calcium phosphate (BCP) bioceramics belong to a group of bone substitute biomaterials that consist of an intimate mixture of hydroxyapatite (HA), Ca(10)(PO(4))(6)(OH)(2), and beta-tricalcium phosphate (beta-TCP), Ca(3)(PO(4))(2), of varying HA/beta-TCP ratios. BCP is obtained when a synthetic or biologic calcium-deficient apatite is sintered at temperatures at and above 700 degrees C. Calcium deficiency depends on the method of preparation (precipitation, hydrolysis or mechanical mixture) including reaction pH and temperature. The HA/beta-TCP ratio is determined by the calcium deficiency of the unsintered apatite (the higher the deficiency, the lower the ratio) and the sintering temperature. Properties of BCP bioceramics relating to their medical applications include: macroporosity, microporosity, compressive strength, bioreactivity (associated with formation of carbonate hydroxyapatite on ceramic surfaces in vitro and in vivo), dissolution, and osteoconductivity. Due to the preferential dissolution of the beta-TCP component, the bioreactivity is inversely proportional to the HA/beta-TCP ratio. Hence, the bioreactivity of BCP bioceramics can be controlled by manipulating the composition (HA/beta-TCP ratio) and/or the crystallinity of the BCP. Currently, BCP bioceramics is recommended for use as an alternative or additive to autogeneous bone for orthopedic and dental applications. It is available in the form of particulates, blocks, customized designs for specific applications and as an injectible biomaterial in a polymer carrier. BCP ceramic can be used also as grit-blasting abrasive for grit-blasting to modify implant substrate surfaces. Exploratory studies demonstrate the potential uses of BCP ceramic as scaffold for tissue engineering, drug delivery system and carrier of growth factors.

Three-dimensional finite element analyses of four designs of a high-strength silicon nitride implant.

The effects of implant shape and size on the stress distribution around high-strength silicon nitride implants under vertical and oblique forces were determined using a three-dimensional finite element analysis. Finite element models were designed using as a basis the serial sections of the mandible. Using Auto-CAD software, the model simulated the placement of implants in the molar region of the left mandible. Results of the analyses demonstrated that mainly the implant root shape and the directions of bite forces influence the stress distributions in the supporting bone around each implant. Implant size is a lesser factor. The serrated implants presented a larger surface area to the bone than either the cylindrical or tapered implants, which resulted in lower compressive stress around the serrated implants. With increasing implant diameter and length, compressive stress decreased. The mean compressive stress distribution on the serrated implants was more flat (platykurtic) than on either the cylindrical or tapered implants. Results of studies on two load directions (vertical and oblique) showed that, in either case, the compressive stress in the cortical bone around the neck of the implant was higher than in the cancellous bone along the length of the implant. The most extreme principal compressive stress was found with oblique force. This study provides the first information on the relationship between shape of the silicon nitride implant and stress on the supporting bone.

Highly crystalline MP-1 hydroxylapatite coating. Part I: In vitro characterization and comparison to other plasma-sprayed hydroxylapatite coatings.

A novel pressurized hydrothermal post-plasma-spray process, referred to as MP-1, has been developed to convert the crystalline non-HA and amorphous components of plasma-sprayed hydroxylapatite coating back into crystalline HA. No detrimental effects are observed on the strength of either the base metal or the coating. X-ray diffraction (XRD) and FTIR analysis, surface roughness, shear adhesion strength and calcium solubility testing were conducted on Calcitite coated samples before and after treatment with this process. Other commercially available coatings were also studied using XRD and solubility testing. Quantitative XRD data show that the MP-1 treatment increases the average crystalline HA content of the Calcitite coating from 77% to 96%, while the amorphous content decreases from 21% to 4%. Other commercially available dental implant coatings ranged in crystalline HA content from 45% to 73%, with amorphous phase content ranging from 29% to 62%. FTIR spectra for treated coatings were significantly more well defined, and showed an increase in peak separation and intensity. Surface roughness and shear adhesion strength were not affected by the treatment. In vitro solubility testing revealed that for all coatings there is an initial introduction of calcium into solution over the first 2 h of testing; however, the amount of calcium dissolved was significantly lower for the MP-1 coating. Under a pH and temperature representative of normal physiologic conditions, the rate of calcium dissolution for the MP-1 coating was significantly lower than that of the other commercial HA coatings. In increasingly acidic conditions, the MP-1 coating was compared to the Calcitite coating and was found to have a significantly slower rate of calcium release. The MP-1 treatment enhances typical HA coatings by increasing the crystalline HA content at the expense of the plasma-spray-induced soluble phases without a reduction in the strength of the coating. The resulting coatings exhibit significantly decreased in vitro solubility over a wide range of pH. The results of this solubility testing suggest that the treated coating may show significantly enhanced in vivo stability, even under the extreme conditions encountered during periods of infection or rigorous detoxification procedures. The significant differences between plasma-sprayed HA coatings reported here underscore the need for industry and academic researchers to raise the level of discourse and understanding of HA coatings. By offering consistent and accurate descriptions of coating compositions and methods of analysis, meaningful comparisons between different HA coatings can be made.

Zinc effect on the in vitro formation of calcium phosphates: relevance to clinical inhibition of calculus formation.

PURPOSE: To determine the effect of zinc on the in vitro formation of calcium phosphates and its relevance to calculus inhibition. MATERIALS AND METHODS: Different types of calcium phosphate phases (amorphous calcium phosphate, ACP; dicalcium phosphate dihydrate, DCPD; octacalcium phosphate, OCP; and carbonate hydoxyapatite, CHA) were precipitated from solutions containing increasing concentrations of zinc (Zn) ions. The precipitates were characterized using x-ray diffraction, infrared absorption spectroscopy, and scanning electron microscopy. RESULTS: The presence of Zn ions affected the type and amount of calcium phosphate phases formed. Zn, even at concentrations as low as 0.1 mM/L, inhibited the crystal growth of DCPD, OCP and AP; and, at higher concentrations (0.5 mM to 2 mM/L), promoted the formation of amorphous calcium phosphate, ACP, or Zn-substituted tricalcium phosphate (beta-TCP) depending on the reaction pH and temperature.

Magnesium and carbonate in enamel and synthetic apatites.

This study aimed to: determine the Mg and CO3 distribution in the outer (surface), middle, and inner (closest to the enamel-dentin junction, EDJ) layers of human enamel; and determine the factors affecting the incorporation of Mg into synthetic apatites and the consequence of such incorporation on the properties of the apatites. Results demonstrated that the concentrations of Mg, CO3, and organic components increased from the surface to the inner layers close to the EDJ and a difference in crystallinity from the outer to the inner layers. Initial results indicated that the extent of dissolution of the inner layer enamel is greater than that in the outer or surface enamel. Results on synthetic apatites showed the following: (1) Limited Mg incorporation into apatite was dependent on solution [Mg/Ca] molar ratio, temperature, pH, and the presence of CO3 or fluoride (F); (2) incorporation of Mg causes reduction in crystallinity and an increase in the extent of dissolution of the apatite; (3) the negative effect of Mg on the properties of apatites is synergistic to that of CO3 and antagonistic to that of F; and (4) exposure to acid of Mg-containing apatites causes the dissolution of Mg-rich apatite and precipitation of Mg-poor apatite. The observed decrease in the [Mg/Ca] of enamel and synthetic apatites after acid exposure may explain the observed 'preferential loss' of Mg and CO3 in the initial stages of caries.

Three-dimensional defects in hydroxyapatite of biological interest.

Hydroxyapatite (HA) ceramics are widely used as bone substitutes in the repair of bony defects. These ceramics sometimes differ in their sintering temperatures. High resolution transmission electron microscopy was used to characterize HA ceramics sintered at different temperatures. A new type of defect was observed for the first time for ceramics prepared at 900 degrees C but not on those prepared at 1250 degrees C. This may cause a difference in their in vitro dissolution and in vivo performance.

Synergistic effects of magnesium and carbonate on properties of biological and synthetic apatites.

Magnesium (Mg) and carbonate (CO3) are minor elements associated with enamel, dentin and bone apatite. The purpose of this study was to determine the effect of Mg and CO3 on some properties of synthetic apatites to gain insights on their effects on biological apatites. Biological apatites from human enamel and dentin and from bovine bone and synthetic apatites with/without Mg or CO3 were characterized using x-ray diffraction, infrared absorption, thermogravimetry and chemical analyses. Dissolution in acidic buffer was also determined. Results from this study demonstrated: (1) the synergistic effects of Mg and CO3 on reducing the crystallinity and increasing the extent of dissolution of synthetic apatites; (2) dentin and bone, compared to enamel apatite contained higher levels of Mg and CO3; had lower crystallinity and higher extent of dissolution. The lower crystallinity and higher extent of dissolution of dentin and bone compared to enamel apatite may be partly attributed to their higher Mg and CO3 concentrations.

Scanning electron microscopy and electron probe microanalyses of the crystalline components of human and animal dental calculi.

A review of the use of scanning electron microscopy (SEM) and electron probe microanalyses in the study of dental calculus showed that such studies provided confirmatory and supplementary data on the morphological features of human dental calculi but gave only limited information on the identity of the crystalline or inorganic components. This study aimed to explore the potential of combined SEM and microanalyses in the identification of the crystalline components of the human and animal dental calculi. Human and animal calculi were analyzed. Identification of the crystalline components were made based on the combined information of the morphology (SEM) and Ca/P molar ratios of the crystals with the morphology and Ca/P molar ratio of synthetic calcium phosphates (brushite or DCPD; octacalcium phosphate, OCP; Mg-substituted whitlockite, beta-TCMP; CO3-substituted apatite, (CHA); and calcite. SEM showed similarities in morphological features of human and animal dental calculi but differences in the forms of crystals present. Microanalyses and crystal morphology data suggested the presence of CaCO3 (calcite) and CHA in the animal (cat, dog, tiger) and of OCP, beta-TCMP and CHA in human dental calculi. X-ray diffraction and infrared (IR) absorption analyses confirmed these results. This exploratory study demonstrated that by taking into consideration what is known about the crystalline components of human and animal dental calculi, combined SEM and microanalyses can provide qualitative identification.

Formation and transformation of octacalcium phosphate, OCP: a preliminary report.

Octacalcium phosphate, Ca8H2(PO4)6 X 5H2O (OCP), occurs in pathological calcifications frequently as one of the crystalline components of human dental calculi. OCP has also been presumed a necessary precursor of biological apatites in both normal (enamel, dentine, cementum, bones) and pathological (e.g., phosphatic renal stones) calcifications. This study investigated the optimum conditions for the direct in vitro formation of OCP in solutions and in gel systems and the factors affecting its formation and transformation or hydrolysis to apatite. It was observed in both solution and gel systems that the formation of OCP was dependent on definite conditions of pH and temperature (the higher the temperature the lower the pH at which OCP forms, and vice versa), and on the presence of other ions. The presence of pyrophosphate inhibited OCP formation favoring instead the formation of amorphous calcium phosphate while the presence of citrate or carbonate favored the formation of "apatitic" calcium phosphate at the expense of OCP. The presence of oxalate ions caused the formation of mixed OCP/calcium oxalate phases. Hydrolysis of OCP to apatite was suppressed in the presence of magnesium or pyrophosphate, and promoted in the presence of carbonate or fluoride ions. In the presence of oxalate ions, partial hydrolysis of OCP to calcium oxalate and not to apatite was observed. Results from this study give insights on the factors (e.g., pH, temperature, presence of ions besides calcium and phosphate) which influence the formation of OCP and its transformation to apatite and/or calcium oxalate. Ions which demonstrated significant effect on the formation and/or transformation of OCP were magnesium, pyrophosphate, carbonate, citrate, fluoride and oxalate.

Apatite crystallites: effects of carbonate on morphology.

Carbonate is a substituent in the apatite structure; when present, it limits the size of the growing apatite crystals and so influences their shape that they grow more equiaxed than needle-like. The tendency for carbonate apatites to be equiaxed is related to the nature of the chemical bonds formed in the crystal. The interference of carbonate with the good crystallization of apatite, and its weakening effect on the bonds in the structure, increase the dissolution rate and the solubility, thereby presumably contributing to the susceptibility to caries of dental apatites containing carbonate.