Biomedical Engineering Reference
In-Depth Information
induce cracks [29]. The following handling requirements (in minutes)
have been formulated for calcium orthophosphate cements, as a
result [134, 254]:
3 ≤
I
< 8
I
-
CT
≥ 1
≤ 15
These parameters are represented schematically in Fig. 5.2. The
second requirement means that the cohesion time (
F
CT
) must be at
least 1 min before
, so that a clinician has at least 1 min to apply
and to mold the material.
I
is the time from which a cement no
longer disintegrates when immersed in Ringer's solution [134]. As
the mixing in a mortar is about 1 min, the shortest
CT
that can be
allowed is about 2 min, so that a clinician has at least 1 min to collect
the paste from the mortar and put it on a pallet knife or into a syringe
with which it is to be transferred to the wound after
CT
CT
and before
I
[134]. For dental applications, time
must be close to 3 min, whereas
for orthopedic applications it must be close to 8 min. However, in no
case it will be tolerable for the clinicians if time
I
F
becomes greater
than 15 min [29, 134].
5.4.2
Mixing
In the clinical situation, self-setting calcium orthophosphate
formulations can be either applied by the fingertips of a surgeon or
injected from a syringe to the defect area of a bone. The first type of
the clinical application requires formulation of high-viscosity cement
pastes and putties, which can be applied manually as dough, while
the second type requires formulation of low-viscosity compositions,
which can be applied by injection from a syringe [134]. Currently,
injection appears to be the preferred method between these two
major options. Thus, a trade-off must be found between a high
viscosity leading to too high injection forces and a low viscosity
increasing the risk of cement extravasations. Viscosity values in the
range of 100-2000 Pa·s are generally considered to be adequate
[255].
In any case, before using a surgeon needs to have a cement powder
and a liquid be mixed properly and thoroughly (to avoid the powder/
liquid encapsulation) within the prescribed time. This process must
Search WWH ::




Custom Search