SUBSTITUTION OF CALCIUM BY STRONTIUM WITHIN SELECTED CALCIUM PHOSPHATES

Sr incorporation in the molecules of amorphous calcium phosphate, apat itic tricalcium phosphate, hydroxyapatite, octacalcium phosphate and d icalcium phosphate dihydrate was investigated. The concentration of Sr ranged from 225 to 1010 mug/g, i.e. it overlapped with the physiologi cal range of Sr concentrations in human bone. The leading experimental technique was extended X-ray absorption fine structure (EXAFS) at the Sr K edge. Results of these studies demonstrated the following: (1) S r incorporation in the calcium phosphates is compound-dependent. (2) t he coordination of incorporated Sr atoms in the Ca-P molecules is simi lar to that of Ca atoms, but interatomic distances are approximately 0 .015 nm larger, (3) in apatitic tricalcium phosphate, hydroxyapatite a nd octacalcium phosphate lattices Sr atoms may occupy selected Ca site s, which was not the case for dicalcium phosphate dihydrate, (4) in th e apatite lattice Sr atoms are coordinated by 6 PO4 tetrahedrals and ( 5) EXAFS spectra at the K edge of the incorporated Sr may be used to d istinguish the structures of amorphous calcium pbosphate, dicalcium ph osphate dihydrate as well as apatite and its derivatives (apatitic tri calcium phosphate, octacalcium phosphate).