Biomedical Engineering Reference
In-Depth Information
an interpretation of the collected data, particularly in complex
cement formulations [207].
A way to assess the rate of a cement hardening is to measure
its setting time, which means the time required to reach a
certain compressive strength, generally close to 1 MPa. The most
straightforward approach is to prepare cement samples with a well-
controlled geometry (e.g.,
cylinders), incubating these samples for
various times in the right environment (temperature, humidity) and
assessing the composition and mechanical properties of the samples
as a function of time [207]. One should stress that setting time for
calcium orthophosphate cements often corresponds to an earlier
stage in the overall setting reaction, typically 5-15% of the overall
reaction, while the end of the cement setting is typically reached
after several days [106, 172]. Gillmore needles have been used
with success to measure the initial (
) setting times
of calcium orthophosphate cements [99]. Namely, a light and thick
needle is used to measure the initial setting time
I
) and final (
F
I
, while a heavy and
thin needle for the final setting time
[134]. The clinical meaning is
that the cement paste should be implanted before time
F
I
and that the
wound can be closed after time
F
(Fig. 5.2).
Implantation
notallowed
Implantation
interval
Woundclosure
SETTING
0
CT
3
I
8
F
15
Time(min)
Figure 5.2
A diagram of the setting parameters relevant for a calcium
orthophosphate cement:
CT
: cohesion time;
I
: initial setting
time;
: final setting time. Adapted from Ref. [29] with
permission.
F
The cement should not be deformed between times
I
and
F
because in that stage of the setting process any deformation could
Search WWH ::
Custom Search