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Calcium carbonate-based bone substitutes derived from natural coral exoskeleton (aragonite) are resorbed and remodeled faster than calcium phosphate-based substitutes. However, coral species with structures appropriate for use as bone substitutes are very limited. Therefore, it is important to evaluate potential of artificial calcium carbonate ceramics as a bone substitute. In this study, calcium carbonate granules with various porosities and pore sizes were prepared by sintering a highly pure (>99.98%) calcium carbonate powder (calcite), and their resorption properties and bone formation abilities were examined in vivo for the first time. The sintered calcium carbonate was resorbed faster than β-tricalcium phosphate, which has a similar structure. However, sintered calcium carbonate did not promote new bone formation during long-term implantation. Furthermore, both resorption and new bone formation were affected by the pore structure. The optimal structures of the artificially sintered calcium carbonate bone substitute were also discussed.

In addition to calcium phosphate-based ceramics, glass-based materials have been utilized as bone substitutes, and silicate in these materials has been suggested to contribute to their ability to stimulate bone repair. In this study, a silicate-containing α-tricalcium phosphate (α-TCP) ceramic was prepared using a wet chemical process. Porous granules composed of silicate-containing α-TCP, for which the starting composition had a molar ratio of 0.05 for Si/(P + Si), and silicate-free α-TCP were prepared and evaluated in vivo. When implanted into bone defects that were created in rat femurs, α-TCP ceramics either with or without silicate were biodegraded, generating a hybrid tissue composed of residual ceramic granules and newly formed bone, which had a tissue architecture similar to physiological trabecular structures, and aided regeneration of the bone defects. Supplementation with silicate significantly promoted osteogenesis and delayed biodegradation of α-TCP. These results suggest that silicate-containing α-TCP is advantageous for initial skeletal fixation and wound regeneration in bone repair.

Tricalcium phosphate (TCP) ceramics are useful biodegradable bone-repairing materials. Silicate-containing TCP ceramics are expected to be useful as biodegradable bone-repairing materials which promote the bone regeneration because it has been reported that the silicate promotes bone formation. In the present study, silicate-containing TCP ceramics were prepared through a wet chemical process at the starting compositions from 0 to 0.05 in the Si/(P+Si) molar ratio. The prepared silicate-containing TCP ceramics were characterized and evaluated in vitro. The crystal phase of the products was α-TCP, and the tendency that the lattice constants linearly shifted from 0 to 0.05 in the starting Si/(P+Si) molar ratio was observed. It is speculated that the added silicate was incorporated in the crystal structure of TCP. The pellets were prepared by a sintering process, and soaked in a simulated body fluid (SBF) to estimate their bone-bonding ability. The addition of silicate to TCP promoted to hydroxyapatite formation on the TCP ceramics in SBF. This result implies the high possibility that the silicate addition would promote the bone-bonding ability of the TCP ceramics.