Cytocompatibility and Structural Arrangement of the Collagen Fibers: An In Vitro and In Vivo Evaluation of 5% Zinc Containing Hydroxyapatite Granules

Hydroxyapatite (HA) is one of the most employed materials for bone therapy due to its structural similarity with bone, its biocompatibility and physicochemical properties. Additionally, HA performance may be improved by ionic substitution of calcium with divalent bioactive metallic cations such as zinc. In this context, zinc incorporation into HA have been well studied, in spite of conflicting results regarding its biocompatibility: while previous reports on in vitro cytocompatibility have described 5% zinc containing HA (ZnHA) as slightly cytotoxic, this material presented an excellent response on in vivo studies. In order to bring more information on ZnHA biocompatibility, we performed a multiparametric assay evaluating sequentially on the same cells three different viability parameters: mitochondrial activity (XTT), membrane integrity (Neutral Red) and cell density (Crystal Violet Dye Exclusion test). Additionally, we intended to complement the existing data on ZnHA in vivo performance, by assessing its ability to affect the arrangement of collagen fibers on the grafted area, an important indicative of bone maturation. MC3T3-E1 cells were exposed to 24-hours extracts of ZnHA or stoichiometric HA on culture medium (DMEM) and cell viability was assayed. ZnHA was very cytocompatible, since the levels of viable cells on all 3 tests were similar to the HA and polystyrene (negative control) extracts, but significantly higher than cells treated with 4% phenol (positive control). For the in vivo studies, critical size defects in rats calvaria were filled with HA or ZnHA granules. The histological evaluation after 30 and 180 days revealed an increase along time. Event tough ZnHA is cytocompatible the presence of Zn was unable to alter the interaction between collagen fibers and the mineral bone phase, as compared to stoichiometric HA.