Biomedical Engineering Reference
In-Depth Information
of HA and FA consist of individual orthophosphate tetrahedra linked
by calcium cations. The orthophosphate groups forms the skeleton
having two types of calcium sites. The first (Ca(1) site) is occupied
by four calcium atoms that have a nine-fold coordination, while the
second (Ca(2) site) is occupied by six calcium atoms having a seven-
fold coordination. Fluoride (for FA) or hydroxide (for HA) anions
are located within the same channels as the Ca(2) sites and have a
high mobility [1-4, 129, 130]. Thus, the apatite structure contains
Ca-F (for FA), O-H (for HA), Ca-O, and P-O bonds; consideration of
the relative electrostatic strength of these bonds suggests that their
relative destruction rate is consistent with: Ca-F ≈ Ca-OH > Ca(1)-O
> Ca(2)-O > P-O. Considering its structure, once all Ca-F and Ca-O
bonds are broken, apatite is destroyed; while breaking of P-O bonds
is unnecessary for apatite dissolution [87].
7.6 Necessary Assumptions and Limitations
When apatite comes into a contact with an acidic aqueous solution,
dissolution steps 1-5 (see section 7.4) begin. Before giving the
detailed description of the processes involved, the following
assumptions and limitations must be made:
1. The stoichiometric apatite (Ca/P = 1.67) is dissolved and
neither non-stoichiometric layers nor other ions except of
calcium, orthophosphate, hydroxide (for HA) or fluoride
(for FA) are initially present in the crystal lattice, whereas
volume and surface defects (dislocations and dislocation
outlets respectively) might be present and, if present, they are
distributed randomly.
2. Except of the presence of dislocation outlets, the initial surface
of apatite is perfect (molecularly sooth). Otherwise, each
imperfection might act as a possible dissolution nucleus.
3. In spite of the limitations and drawbacks discussed above,
all the afore-discussed dissolution models are correct and
complementary to each other.
4. In some cases, anions of acid might have an influence on
apatite dissolution due to a specific affinity (e.g., citrate) or
by means of formation of insoluble compounds (e.g., sulfate,
fluoride). That is why, anions are specified neither here nor
Search WWH ::
Custom Search