Biomedical Engineering Reference
In-Depth Information
regarding hypotension, hazards, local tissue damage and even the burn of
the sciatic nerve caused by bone cements have been reported by different
authors.
100-103
On the other hand, some studies show that the products of wear and
fracture or fragmentation of bone cement, as well as the wear debris of
polyethylene from the articulating surface, may cause a foreign body response
consisting of a macrophage, giant cell foreign body granulomatous reaction.
This tissue can produce a variety of chemical mediators of inflammation
and eventually bone resorption. This process of osteolysis induced by bone
cement fragmentation may be the biological cause for the loosening of the
cemented joint prostheses.
104-105
A wide variety of possible modifications of the formulation of ABCs, aiming
at improving their biological properties, has been investigated. Some examples
are the substitution of initiators, accelerators
106-108
or radiopacifying agents
71,
72, 78
by more biocompatible compounds, or the addition of other monomers
to the liquid phase,
106, 109-113
that improve the biological performance of
abCs.
10.4 Calcium phosphate bone cements
10.4.1 Chemistry of calcium phosphate bone cements
CPCs are formed by a combination of one or more calcium orthophosphates,
which upon mixing with a liquid phase, usually water or an aqueous solution,
form a paste which is able to set and harden after being implanted within the
body. The cement sets as a result of a dissolution and precipitation process,
as represented in Fig. 10.2(b). The entanglement of the precipitated crystals
is responsible for cement hardening.
Calcium orthophosphates are the calcium salts derived from orthophosphoric
acid. Their names, abbreviations, chemical formulae and Ca/P molar ratio
are summarized in Table 4.
114
Some of these calcium orthophosphates can
be obtained by precipitation from an aqueous solution at low temperature,
whilst others can only be obtained at high temperature. All of them can be
used as reactants for CPCs, but only those calcium orthophosphates that
can precipitate at low temperature in aqueous systems can be theoretically
obtained as a result of the CPC setting reaction. However, despite the large
number of possible formulations, the CPCs developed up to now only have
two different end products, precipitated hydroxyapatite (PHA) or brushite
(DCPD). This is a predictable situation since hydroxyapatite is the most
stable calcium phosphate at pH>4.2 and brushite the most stable one at pH<
4.2.
The driving forces controlling dissolution and precipitation reactions are
related to the respective super or under saturation levels defined with regard to
