Biomedical Engineering Reference
In-Depth Information
the strength and the stability of the interface during the process of the
material replacement by tissue, and the use of materials leading to
metabolically acceptable substances during its degradation [HEN 04].
As a consequence, the composition of resorbable materials is
considerably limited. Most resorbable ceramics are calcium phosphates
(CaP). CaP have a very good biocompatibility due to their chemical
composition containing calcium and phosphorus ions which participate
in normal metabolic process. They have shown no toxicity, no fibrous
tissue around the medical device and no inflammation. Resorption of
CaP proceeds through various pathways: physiochemical dissolution
depending on the CaP solubility and the local pH, physical
disintegration into small particles, and biological degradation which
induces a decrease in the local pH of the environment. Therefore, CaP
that are suitable for biological implications must have a Ca/P ratio
higher than 1 to possess adequate solubility and acidity, and thus
appropriate hydrolysis speed. Moreover, the Ca/P ratio must be kept
below 2. Indeed, tetracalcium phosphate (Ca/P = 2.0) shows too high a
basicity for implantation [GRE 12].
The major CaP ceramics used in biomaterial applications are
hydroxyapatite (HA), tricalcium phosphate (TCP) and a mixture of the
two materials. However, stoichiometric crystalline HA has a very low
solubility and no resorption occurs even after several years of
implantation
in vivo
. Only resorption of poorly crystallized HA with a
high specific surface area is reported. TCP possesses a Ca/P ratio of
1.5 and may crystallize in a rhombohedral structure for β-TCP or in a
monoclinic structure for α-TPC. The dissolution rates of TCP are
higher than that of HA, and therefore the degradation rates increase in
the following order: α-TPC > β-TCP >> HA. On the other hand, TCP
dissolves too fast to allow bone bonding. As a consequence, biphasic
calcium phosphate ceramics composed of HA and TCP are developed.
Their resorption rates are largely determined by the TCP/HA ratio
with a decrease when reducing the content of TCP [THA 04]. These
materials are currently available with TCP/HA ratio ranging from 1.5
to 3 with high porosity (45-70%). Due to their low mechanical
properties (see Table 3.3) and high brittleness, CaP have been
successfully used to replace hard tissue in low load-bearing
applications, mainly as temporary bone fillers. Indeed, HA and TCP
