Mechanisms underlying the limited injectability of hydraulic calcium phosphate paste

Calcium phosphate (CaP) cements are being increasingly used for minimally invasive hard tissue implantation. Possible approaches to improve the bad injectability of hydraulic calcium phosphate pastes have been discussed and investigated in a number of recent publications. However, the liquid-phase separation mechanism leading to the limited injectability has not yet been addressed. Liquid-phase separation means that the liquid-to-powder ratio (LPR) of the extruded paste is higher than the LPR of the paste left in the syringe. The goal of this paper was to remedy this situation by looking at the liquid-phase migration occurring during the injection of a paste from a syringe through a cannula. Experimentally, it was seen that the liquid content of both the syringe paste and the extrudate decreased during the paste injection. Moreover, a high extrusion velocity, small syringe size, short cannula and high LPR favored a good injectability. These results could be partly explained in light of rheological measurements performed with the investigated paste.