Self-Setting Calcium Orthophosphate Formulations: Cements, Concretes, Pastes, and Putties - Calcium Orthophosphates: Applications in Nature, Biology and Medicine - page 487

pressure applied to the cement paste provokes a phase separation
after a certain injection time, is generally referred as filter pressing,
phase separation, or phase migration [125] (see the aforementioned
example for Norian SRS

®

[29], in which a thick mass remained inside

a syringe).
Possible mechanisms underlying the limited injectability of
hydraulic calcium orthophosphate cements have been discussed in
literature [266, 269]. In the case of demixing, the exact composition
of the extruded part of the paste becomes unknown. Moreover, due
to a deviation from the initial P/L ratio, it becomes unclear whether
the setting behavior and the mechanical and histological properties
of the extruded part are still clinically acceptable. Therefore, a good
cohesion of the paste is necessary in order to avoid these problems
[270].
Cohesion (= cohesiveness, “non-decay”) is the ability of a paste
to keep its geometrical integrity in an aqueous solution [125].
For cements, a bad cohesion may prevent setting and may lead to
negative

reactions due to the release of microparticles [271].
Since a high cohesion is the result of strong attractive forces among
the particles, factors enhancing van der Waals forces (attractive)
and decreasing electrostatic forces (repulsive) can be used to
improve cohesion [125]. For example, an appropriate cohesion was
achieved when no disintegration of the cement paste was observed
in the fluid [134, 270]. This can be accomplished by keeping a high
viscosity for the cement paste [22] or using cohesion promoters (e.g.,

in vivo

1% aqueous solution of sodium alginate [176, 272, 273] and other
chemicals [176, 274-276]). Some calcium orthophosphate cements
fulfill both criteria, e.g., Norian SRS , but others fulfill only one or

even none of these requirements. For example, BoneSource

®

TM

[103]

®

and Cementek are not injectable and blood must be kept away

from the implanting site until setting [197, 198]. A poor cohesion
has been associated to a poor biocompatibility that might lead to
inflammatory reactions [271]. Further details on the cohesion
properties of various calcium orthophosphate pastes are available
in literature [270].
Viscosity is a measure of the resistance of a fluid, which is being
deformed by either shear stress or tensile stress. Generally, the
cement viscosity in the range of 100-1000 Pa·s appears to be ideal
[277] and, if possible, a cement should have a constant viscosity