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Effect of Silicate substitution level on Bioactivity of silicate substituted apatites

Principal investigator: Karin HING
Co-investigator(s): P.A. Revell

Figure 1	(a) Vascular penetration into macroporosity of PSA02 scaffold (b) association of cells and capillaries with scaffold struts (PSA08), morphology of ingrowth within peripheral region of (c) PSA02 and (d) PSA08 scaffolds at 1 week. Bar = 100 um.Bone mineral has a similar crystallographic structure to hydroxyapatite (HA), a hydrated calcium phosphate with a chemical formula of Ca10(PO4)6(OH)2 and a Ca:P ratio of 5:3 (1.67). However bone mineral differs from HA in that it is characterised by calcium, phosphate and hydroxyl deficiency (reported Ca:P ratios of 1.37-1.87, internal crystal disorder and is heavily substituted with a number of cations and anions, the principal one being carbonate (up to 8wt%), making bone mineral more closely related to an A-B type carbonate substituted apatite. Other ions which readily substitute into the apatitic lattice of bone mineral include sodium (up to 0.8 wt%) and magnesium (up to 0.5 wt%) at the trace level, with elements such as potassium, strontium, zinc, fluorine, chlorine and silicon found at ultra-trace level. All these factors all contribute to an apatite that is insoluble enough for stability, yet sufficiently reactive to allow the in vivo sub-microscopic (5-100nm) crystallites to be constantly resorbed and reformed as required by the body, enabling bone to act as a mineral reservoir and thus a ready source of inorganic nutrients, the metabolic significance of which is becoming increasingly apparent especially given the temporal variation in ultra-trace level elements such as silicon with age and sex and its potential relationship with bone health. The bone healing response to silicon level (0, 0.2, 0.4, 0.8 and 1.5wt% Si) within 5 batches of matched porosity silicate substituted hydroxyapatite (Si-HA) scaffold was assessed by implantation of 4.6 mm diameter cylinders in the femoral intercondylar notch of New Zealand White rabbits for periods of 1, 3, 6 and 12 weeks. Histological evaluation and histomorphometric quantification of bone ingrowth and mineral apposition rate (MAR) demonstrated the benefits to early (<1 week) bone ingrowth and repair through incorporation of silicon, at all levels, in porous hydroxyapatite lattices as compared to stoichiometric (0 wt% Si) hydroxyapatite (HA). The group containing 0.8 wt% Si supported significantly more bone ingrowth than all other groups at 3 and 6 weeks (P<0.05), initially through its elevated MAR between weeks 1-2, which was significantly higher than that of all silicon-containing groups (P<0.05). The level of silicate substitution also influenced the morphology and stability of the repair, with elevated levels of bone resorption and apposition apparent within other silicon-containing groups at timepoints >3 weeks as compared to the 0 and 0.8 wt% Si groups. At 12 weeks the net amount of bone ingrowth continued to rise in the 0, 0.8 and 1.5wt% groups, apparently as a result of adaptive remodelling throughout the scaffold. Ingrowth levels remained highest in the 0.8wt% Si group which was characterised by a dense trabecular morphology in the superficial region graduating to a more open network in the deep zone. These results highlight the sensitivity of healing response to Si level and suggest that an optimal response is obtained when Si-HA is substituted with 0.8wt% Si through its effect on the activity of bone forming and bone resorbing cells.