Biomedical Engineering Reference
In-Depth Information
system and water during setting and hydration, the presence of Ca-ions and hydroxyl ions,
the hydrogen phosphate ions are neutralised according to
HPO
4
2-
+ H
2
PO
4
-
+ OH
-
PO
4
2-
+ H
2
O,
whereafter the apatite-formation reaction occurs
5Ca
2+
+
3
PO
4
3-
+ OH
-
Ca
5
(PO
4
)
3
OH
This reaction occurs upon the biomaterial surface/periphery towards tissue. The apatite is
precipitated as nano-size crystals (Hermansson et al, 2006). See figure 5.
Apatite crystal
Fig. 5. Nano-size apatite formation in the the contact zone to hard tissue
Katoite is formed as a main phase, and is kept as katoite in the bulk material according to
the mechanism 1 above. However, in long-time contact with body liquid containing
phosphate ions the katoite is transformed at the interface tobody tissue into the at neutral
pH even more stable apatite and gibbsite phases according to
Ca
3
.
(Al(OH)
4
)
2
.
(OH)
4
+ 2 Ca
2+
+ HPO
4
2-
+ 2 H
2
PO
4
-
Ca
5
.
(PO
4
)
3
.
(OH) + 2 Al(OH)
3
+ 5 H
2
O
When apatite is formed at the interface according to any of the reaction mechanisms 2-4
above, at the periphery of the bulk biomaterial, the biological integration may start. Bone
ingrowth towards the apatite allows the new bone structure to come in integrated contact
with the biomaterial. This is an established fact for apatite interfaces. For the CA-system the
ingrowth is discussed below, 4.4. The transition from tissue to the biomaterial is smooth and
intricate.
Fig. 6. Integration of CA in tissue - a model using albino adult New Zealand White rabbits
(Hermansson et al, 2008).
