A Dual Constant Composition Study of the Kinetics of Mineralization and Demineralization of Bone Substitutes
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SUBSTITUTES B.E. Tucker', G.L. Gorline', D.E. Carney', G.H. Nancollas', and J.A. Bearcroft" "StateUniversity of New York at Buffalo, Department of Chemistry, Buffalo, NY "Smith and Nephew Richards Inc., Memphis TN Abstract: Calcium phosphates are widely used as implantable materials in both dental and orthopaedic applications. Seven different bone substitutes (BS), including Interpore Coralline HAP, Cortical Bio-Oss, Endobon, Cancellous Bio-Oss, Dense HAP (hydroxyapatite), Collagraft granules, and Porous HAP, have been evaluated using the Dual Constant Composition (DCC) kinetics methods. DCC measurements of the kinetics of dissolution and re-mineralization of the BS materials offer considerable advantages for the characterization of minor calcium-containing surface species due to the insensitivity of conventional physical chemical methods such as X-ray diffraction and ESCA. Each sample was pre-conditioned (PC) in 0.15 M NaCl prior to the DCC kinetics procedures. The fastest dissolving material, Collagraft granules, showed a dissolution rate more than two orders of magnitude larger than Dense HAP. Interestingly, the relative supersaturation with respect to HAP (a) following PC was normally negative, whereas with plasma sprayed HAP coated implants, these a values were as high as 20. Surface impurities may play a major role following in vivo implantation, since resorption is probably a major reparative mechanism. It is therefore essential to understand the dissolution behavior. Moreover, in a remineralization mode, trace surface phases may determine the phase and morphology of the newly deposited bone mineral. Introduction: Biological calcium phosphates such as bone are thought to most closely approximate a calcium deficient carbonated apatite1, 2. Hydroxyapatite (HAP, Ca 10(P0 4)6 (OH)2) has been frequently used as a model for bone, despite the fact that bone mineral is normally nonstoichiometric, containing many impurities. Metal implant surfaces may be coated with calcium phosphates prior to implantation in order to improve osseointegration 3' 4 . Although these have been successful to varying degrees, an ideal implantable material should not necessarily utilize metal for structural rigidity and should be completely integrated into the host tissue. Important factors that should be taken into account in an effort to improve the success rate of an implanted material, such as the bone substitutes (BS) include the following: (1) the rate at which the BS de-mineralizes and re-mineralizes after implantation; (2) the most effective calcium phosphate phase(s) for osseointegration; (3) the identification of surface "impurities" that may be helpful in mediating the surface reactions that take place upon implantation; (4) the degree of resorption of the mineral, and the effect that this will have upon bone apposition and adhesion; and (5) the consequences of kinetically stabilizing other calcium phosphate phases such as the more acidic dicalcium phosphate dihydrate (DCPD, CaHPO 4 .2H 2 0) and octacalcium phosphate (OCP, Ca 4
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