Nelesh Patel

A comparative study on the in vivo behavior of hydroxyapatite and silicon substituted hydroxyapatite granules

Phase pure hydroxyapatite (HA) and a 0.8 wt % silicon substituted hydroxyapatite (SiHA) were prepared by aqueous precipitation methods. Both HA and SiHA were processed into granules 0.5–1.0 mm in diameter and sintered at 1200 °C for 2 h. The sintered granules underwent full structural characterization, prior to implantation into the femoral condyle of New Zealand White rabbits for a period of 23 days. The results show that both the HA and SiHA granules were well accepted by the host tissue, with no presence of any inflammatory cells. New bone formation was observed directly on the surfaces and in the spaces between both HA and SiHA granular implants. The quantitative histomorphometry results indicate that the percentage of bone ingrowth for SiHA (37.5%±5.9) was significantly greater than that for phase pure HA (22.0%±6.5), in addition the percentage of bone/implant coverage was significantly greater for SiHA (59.8%±7.3) compared to HA (47.1%±3.6). These findings indicate that the early in vivo bioactivity of hydroxyapatite was significantly improved with the incorporation of silicate ions into the HA structure, making SiHA an attractive alternative to conventional HA materials for use as bone substitute ceramics.

Comparison of in vivo dissolution processes in hydroxyapatite and silicon-substituted hydroxyapatite bioceramics

The incorporation of silicate into hydroxyapatite (HA) has been shown to significantly increase the rate of bone apposition to HA bioceramic implants. However, uncertainty remains about the mechanism by which silicate increases the in vivo bioactivity of HA. In this study, high-resolution transmission electron microscopy was used to observe dissolution from HA, 0.8 wt% Si-HA and 1.5 wt% Si-HA implants after 6 and 12 weeks in vivo. Our observations confirmed that defects, in particular those involving grain boundaries, were the starting point of dissolution in vivo. Dissolution was observed to follow the order 1.5 wt% Si-HA>0.8 wt% Si-HA>pure HA and it was found to be particularly prevalent at grain boundaries and triple-junctions. These observations may help to explain the mechanism by which silicate ions increase the in vivo bioactivity of pure HA, and highlight the enhanced potential of these ceramics for biomedical applications.

Carbonate substituted hydroxyapatite: resorption by osteoclasts modifies the osteoblastic response.

Carbonate substitution within the hydroxyapatite (HA) lattice improves osteoconduction, although the mechanism by which this occurs is unclear. Discs of dense, sintered, phase-pure HA and AB-type carbonate substituted hydroxyapatite (CHA) were cultured for 21 days with human CD14+ cells in the presence of macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of nuclear factor (NF)-kappaB (sRANKL), during which time osteoclasts developed and resorbed the ceramic surface. Discs were then seeded with human osteoblasts (HOBs), and proliferation and collagen synthesis were measured. On some discs, the conditioned proteinaceous layer left behind by the osteoclasts was preserved. Proliferation of HOBs was increased on resorbed compared to control (unresorbed) surfaces on both materials, provided this osteoclast-conditioned layer was left intact. Collagen synthesis by HOBs was increased on previously resorbed surfaces compared to unresorbed surfaces. This effect was seen on both materials but was seen at an earlier time point on CHA. The results suggest that osteoclasts can condition synthetic bioceramic surfaces and alter the responses of osteoblasts that subsequently populate them. Carbonate substitution may enhance osteoconduction indirectly via effects on enhanced bioresorption.

Osteoclastogenesis on hydroxyapatite ceramics: The effect of carbonate substitution

Human osteoclasts derived from CD14+ve precursors were cultured on discs of stoichiometric hydroxyapatite (HA) and carbonate-substituted hydroxyapatite (CHA) of varying carbonate contents. The development of osteoclasts was qualitatively different on ceramics compared to dentine, occurring in discrete, confluent subpopulations, which suggests local cell signaling may be important in the process. Resorption was quantified by scanning electron microscopy, surface profilometry, and by calcium release into the culture medium. Cells were characterised by a number of histochemical markers of the osteoclast phenotype. Resorption of the ceramic increased with increasing carbonate content up to 2.35 wt %, when resorption trails and pits characteristic of osteoclast activity were seen. Controlling carbonate content may be one way of controlling the rate of resorption of synthetic HA ceramics.

Comparison of Sintering and Mechanical Properties of Hydroxyapatite and Silicon-Substituted Hydroxyapatite

Phase pure hydroxyapatite (HA), 0.8 wt.% silicon-substituted hydroxyapati te (0.8SiHA) and 1.5 wt.% silicon substituted hydroxyapatite (1.5SiHA) were isostatical ly pressed into discs and sintered between 900-1300 °C for 2 hours in order to assess the effect of silicon/carbonate cont nt on the density, microstructure, hardness and Young’s modulus of HA with sintering temperature. C-H-N chemical analysis of the as-prepared SiHA samples showed increa sed c rbonate absorption with silicon content compared to HA samples, but carbonate levels were not d tec ed when the samples were heat-treated. At low sintering temperatures (900-1000 °C) the densification of SiHA samples was inhibited, with this effect being more significant for the hig h level of silicon substitution (1.5SiHA) ceramics which was reflected in the hardness and Young’s modulus values of the ceramics. However, for high sintering temperatures (1200 and 1300 °C), the sintered densities, hardness and Young’s modulus of HA and SiHA were comparable. Furthermore, examina tion of the microstructures by scanning electron microscopy showed that silicon s ubstitution inhibited grain growth at high sintering temperatures. Introduction The similarities in the chemical composition of synthetic hydroxya patite (HA) and bone mineral has led to extensive use of HA ceramics as bone replacement materials. Alt hough bone bonds directly to the surface of HA, the number of medical applications of HA are li mited, primarily due to its relatively poor mechanical properties and slow rate of osseointegra tion compared to some other bioceramics [1]. A possible approach to improve the bioactivity of HA c eramics is to incorporate small levels of physiologically relevant ions into the HA lattic e. It has been shown that carbonate or silicon-substituted hydroxyapatite ceramics, exhibit enhanced in vitro [2,3] and in vivo [4,5] bioactivity compared to phase pure HA. Although these ionic substitutions e nhance the bioactivity of HA, studies have highlighted that the physical properties, such as t he degree of densification and sintered microstructure are affected with both the type and level of ionic substitution into HA. In particular carbonate substitution significantly reduces the sinteri ng temperature, required to achieve near-full density and equivalent grain size of the ceramic compare d to phase pure HA. However carbonate substituted hydroxyapatites are normally sintered in a moi st carbon dioxide environment which promotes densification [6]. Based on previous studies [6] on carbonate substituted HA, it is reasonable to assume that the substitution of silicon or silicate i ons into HA may also influence the densification and grain growth characteristics of the ceramic. T he aims of this study were, therefore, to determine the effect of silicon substitution and sintering temperature on t h mechanical properties of HA. Materials and Methods Phase pure hydroxyapatite (HA), 0.8 wt.% silicon-substituted hydroxyapati te (0.8SiHA) and 1.5 wt.% silicon-substituted hydroxyapatite (1.5SiHA) were prepared by aqueous precipitation reactions according to the methods described elsewhere [7]. The resulting preci pitate was filtered, dried at Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 919-922 doi:10.4028/www.scientific.net/KEM.240-242.919

Nanoscale Morphology of Apatite Precipitated onto Synthetic Hydroxyapatite from Simulated Body Fluid

Dense, polycrystalline, synthetic hydroxyapatite (HA) was incubated for 36 days in modified simulated body fluid (SBF) with increased HCO3 - and reduced Cl- ion concentrations (27 and 120 mM, respectively) closer to actual blood plasma than typical SBF. The resulting precipitated apatite layer was characterized by X-ray photoelectron spectroscopy (XPS) and contact angle measurements and found to be nonstoichiometric, calcium deficient (Ca/P~1.06), non-carbonate containing, and of intermediate hydrophilicity (advancing contact angle, qa=76.5±1.3°). The nanoscale surface topography of the SBF-incubated HA sample was imaged by tapping mode atomic force microscopy (TMAFM), observed to be ≤100 nm in thickness, and composed of three distinct morphologies. These topographically distinct regions were localized within individual grains and facets of the initial HA surface and included: hemispherical, globular structures (maximum lateral dimension, d=44.7±12.7 nm, peak-tovalley height, h=3.6±2.7 nm); elongated, needle-like structures (minimum lateral dimension, w=31.0±8.5 nm, d=104.4±31.1 nm, h=5.0±3.2 nm), and regions of larger, irregularly shaped structures that were relatively smooth (d=504.9±219.1 nm, h=104.0±51.7 nm).

Syntheses of Silicon-Containing Apatite Fibres by a Homogeneous Precipitation Method and Their Characterization

Silicon-containing apatite (Si-HAp) fibres were successfully synthesized by a homogeneous precipitation method. The resulting Si-HAp fibres were composed of carbonate-containing apatite fibres with preferred orientation in the c-axis. The Si contents in the Si-HAp fibres could be controlled by the Si concentration of the starting solutions. TEM observation indicated that the Si-HAp fibres were of single crystal. The Si-HAp fibres have potential as novel materials for high-performance biomedical devices.

Osteoblastic Response to Resorbed Ceramic Surfaces; The Role of the Osteoclast in Osteoconduction

The mechanism by which carbonate substitution within the hydroxyapatite (HA) lattice improves osteoconduction is unclear. Discs of dense, sintered, phase-pure HA and carbonate substituted hydroxyapatite (CHA) were cultured with human CD14+ cells in the presence of macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of nuclear factor (NF)-κB (sRANKL), during which time osteoclasts developed and resorbed the ceramic surface. Discs were then seeded with human osteoblasts (HOBs), and proliferation and collagen synthesis measured. Proliferation was increased on resorbed compared to control (unresorbed) surfaces on both materials. Collagen synthesis was increased on CHA compared to HA, an increase accelerated on a previously resorbed surface. The results suggest that osteoclasts can condition synthetic bioceramic surfaces and alter the responses of osteoblasts which subsequently populate them. Carbonate substitution may enhance osteoconduction via effects on enhanced bioresorption.

Carbonate Substituted Hydroxyapatite; Development and Function of Osteoclasts

Human osteoclasts derived from CD14+ precursors were cultured on discs of stoichiometric hydroxyapatite (HA) and carbonate substituted hydroxyapatite (CHA) of varying carbonate contents. Resorption of the ceramic increased with increasing carbonate content up to 2.35 wt. %. Development of osteoclasts is qualitatively different on ceramics compared to dentine, occurring in discrete, confluent subpopulations, which suggests local cell signalling may be important in the process. Resorption appears to drive further development of osteoclasts. Controlling carbonate content may be one way of controlling the rate of resorption of synthetic HA ceramics.