Biomedical Engineering Reference
In-Depth Information
of the porosity, i.e. 5, 10, 20, 40 and 80 MPa for 80, 70, 60, 50 and
40% porosity, respectively [197, 198]. The effect of porosity on
the compressive modulus of calcium orthophosphate cements
is available as Fig. 4 in Ref. [411]. Ishikawa and Asaoka showed a
linear relation (R
2
= 0.94) between ln diametral tensile strength and
porosity of a calcium orthophosphate cement where porosity was
controlled by compaction pressure (up to 173 MPa) [141]. Besides,
an empirical relationship between strength, S, and porosity, P, has
been introduced [412]:
S = S
−
b
P
e
,
0
where: S
is the theoretical strength at P = 0 (fully dense) and
b
is an
0
empirical constant.
As the porosity is mainly due to an excess of water used in the
cement compositions, attempts were made to reduce the amount
of water. However, the amount of water determines the rheological
properties of cement pastes: a decrease in water content leads to a
large increase in viscosity, eventually leading to non-flowable pastes.
As calcium orthophosphate cements set at an almost constant volume,
the final porosity can be predicted from the initial composition [197,
198]. A shrinkage degree of ~1% causes no restrictions on clinical
use [193]. Recent studies on the
evaluation of an injectable
macroporous calcium orthophosphate cements revealed a higher
bioresorption rate due to both a higher surface contact with body
fluids (which increases dissolution) and enhancing cellular activity
due to particle degradation [273, 329].
Besides the addition of porogens [296-318], the porosity level
of the self-setting calcium orthophosphate formulations might be
controlled to a certain extent by adjusting particle sizes and the P/L
ratio. When the P/L ratio is high, the porosity of the apatite cement
is low [197, 198]. According to calculations, the tensile strength of
the cements with zero porosity could be as high as 103 MPa [141].
However, a high density and a lack of pores decreases cement
bioresorbability because a newly forming bone appears to be unable
to grow into the implant; it might grow only simultaneously with
dissolution of the cement. Thus, porosity of calcium orthophosphate
cements is a very important factor for the cement biodegradability
[197, 198].
The strength of the cement-prosthesis interface might be studied
by a pullout test. The details are available elsewhere [65].
in vivo
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