Cyclic silicate active site and stereochemical match for apatite nucleation on pseudowollastonite bioceramic-bone interfaces.

Hydroxyapatite (Ca5(PO4)3(OH)) forms on pseudowollastonite (psW) (alpha-CaSiO3) in vitro in simulated body fluid, human parotid saliva and cell-culture medium, and in vivo in implanted rat tibias. We used crystallographic constraints with ab initio molecular orbital calculations to identify the active site and reaction mechanism for heterogeneous nucleation of the earliest calcium phosphate oligomer/phase. The active site is the planar, cyclic, silicate trimer (Si3O9) on the (001) face of psW. The trimer has three silanol groups (>SiOH) arranged at 60 degrees from each other, providing a stereochemical match for O atoms bonded to Ca2+ on the (001) face of hydroxyapatite. Calcium phosphate nucleation is modeled in steps as hydrolysis of surface Ca-O bonds with leaching of Ca2+ into solution, protonation of the surface Si-O groups to form silanols, calcium sorption as an inner-sphere surface complex and, attachment of HPO4(2-). Our model explains the experimental solution and high resolution transmission electron microscopy data for epitaxial hydroxyapatite growth on psW in vitro and in vivo. We propose that the cyclic silicate trimer is the universal active site for heterogeneous, stereochemically promoted nucleation on silicate-based bioactive ceramics. A critical active site-density and a point of zero charge of the bioceramic less than physiological pH are required for bioactivity.

Bioactivity of diopside ceramic in human parotid saliva.

Diopside ceramic pellets with a nominal composition of 55.5 wt % SiO(2)-25.9 wt % CaO-18.0 wt % MgO were soaked in human parotid saliva (HPS) over different time intervals, to investigate the behavior of the material in a natural medium of high protein content. The results showed the formation of a hydroxyapatite (HA)-like layer on the surface of the ceramic, and suggested that the mechanism of HA-like layer formation in saliva was similar to that showed in vitro test by other silica-based materials. The HA-like layer formed at the interface was found to be compact, continuous, and composed of many small crystallites with ultrastructure similar to that of natural cortical bone and dentine. The study concluded that the high pH conditions (9.8) existing right at the ceramic/human parotid saliva interface promoted HA-like phase precipitation. At this stage of the study, it is possible to suggest that the diopside ceramic could be of interest in specific periodontal applications for bone restorative purposes. Morphology, structure, and composition of the interfacial reaction product were examined by Scanning and Transmission Electron Microscopy techniques (SEM and TEM), combined with Energy Dispersive X-say Spectroscopy (EDS). Changes in ionic concentrations were measured using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), while the pH right at the interface of diopside/PHS were determined with an Ion Sensitive Field Effect Transistor (ISFET-Meter) instruments.

In vitro behavior of osteoblastic cells cultured in the presence of pseudowollastonite ceramic.

Pseudowollastonite ceramic (psW) is a bioactive ceramic that binds to bone when implanted in vivo and may be useful for the treatment of skeletal defects. However, there have been no studies that examined the interaction between psW and osteoblastic cells in vitro. This study investigated the suitability of psW as a substratum for cell attachment and the ability of the material to effect osteoblasts at a distance from the material surface. Fetal rat calvarial cells were plated onto the ceramic and examined by scanning electron microscopy. The findings reported show that cells attached and proliferated on the surface to the ceramic. Attachment by cells to the material can be enhanced by preincubation of psW in serum or media containing fibronectin. The adhesion of cells can be inhibited by addition of GRGDS peptides suggesting that adhesion to psW is mediated by integrin binding to adsorbed proteins. To study the effects of psW at a distance, cells were cultured in the presence but not in direct contact with the material. Subsequent changes in proliferation, alkaline phosphatase expression, and bone nodule formation were assessed. Cells grown in wells containing psW demonstrated an increase in both the rate and total numbers of bone nodules formed, although there were no differences in proliferation or alkaline phosphatase expression. Overall, these results suggest that psW is biocompatible and osteoconductive.

Interactions of silicate ions with zinc(II) and aluminum(III) in alkaline aqueous solution.

We present (29)Si, (27)Al, and (67)Zn NMR evidence to show that silicate ions in alkaline solution form complexes with zinc(II) (present as zincate, Zn(OH)(3)(-) or Zn(OH)(4)(2-)) and, concomitantly, with aluminate (Al(OH)(4)(-)). Zincate reacts with monomeric silicate at pH 14-15 to form [(HO)O(2)Si-O-Zn(OH)(3)](4-) and with dimeric silicate to produce [HO-SiO(2)-O-SiO(2)-O-Zn(OH)(3)](6-). The exchange of Si between these free and Zn-bound sites is immeasurably fast on the (29)Si NMR time scale. The cyclic silicate trimer reacts relatively slowly and incompletely with zincate to form [(HO)(3)Zn{(SiO(3))(3)}](7-). The concentration of the cyclic trimer becomes further depleted because zincate scavenges the silicate monomer and dimer, with which the cyclic trimer is in equilibrium on the time scale of sample preparation. Identification of these zincate-silicate complexes is supported by quantum chemical theoretical calculations. Aluminate and zincate, when present together, compete roughly equally for a deficiency of silicate to form [(HO)(3)ZnOSiO(2)OH](4-) and [(HO)(3)AlOSiO(2)OH](3-) which exchange (29)Si at a fast but measurable rate.