ULTRASTRUCTURAL STUDY OF INTERACTION BETWEEN Ca-P CERAMIC AND POLYMER IN A NEW INJECTABLE BONE SUBSTITUTE (IBS)

A composite, composed of biphasic calcium phosphate, BCP (60% hydroxyapatite, HA + 40% β-tricalcium phosphate, β-TCP) granules and a hydrophilic polymer (hydroxy-propyl-methyl cellulose, HPMC) was developed as an injectable bone substitute (IBS). The composite was prepared by incubating for 6 and 12 months a mixture of 60% BCP powder (w/w) and 40% HPMC. The composite was then steam sterilized and non-sterilized composite served as control. The interaction between ceramic crystals and polymer and the effect of sterilization were investigated using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HrTEM). Results: SEM demonstrated the degradation of ceramic in the composite after incubation. HrTEM demonstrated the presence of a dissolution zone, about 13 nm, associted with the HA crystal surfaces. Precipitated globular microcrystals (2-3 nm in diameter) represented the first zone of interaction between HA crystals and the polymer. Under this zone, the inter-reticular distances of HA lattice planes appeared enlarged by 1.2% [from 0.817 to 0.827 nm for (100) lattice planes]. The enlargement of lattice planes could be due to HPO₄ incorporation in the newly formed apatite crystal lattice. In β-TCP crystals, dissolution took place on several nanometers. Conclusion: This study demonstrated that interaction between hydrophilic polymer and calcium phosphate ceramic occurs only on very thin layer (several nanometers).