Analysis of the Bony Interface with Various Types of Hydroxyapatite In Vitro

Rat bone marrow cells, capable of forming bonelike tissue, were cultured on three types of plasma sprayed hydroxyapatite that differed in degree of crystallinity from 15%, 43% to 69%. The interface between the mineralized extracellular matrix and the hydroxyapatite was studied with scanning and transmission electron microscopy. At the onset of bone formation, calcium and phosphorous-rich afibrillar globules, with a diameter varying from 0.2 to 0.8 μm, were produced and deposited onto the different substrata. These globules subsequently fused to form a homogeneous layer to which collagen fibres became anchored. Individual globules could be distinguished on the non-degrading 69 % crystalline hydroxyapatite, but were partially fused with the degrading surfaces of 15 % and 43 % crystalline materials. Transmission electron microscopy revealed distinctly different interfacial structures with the various types of hydroxyapatite. A 20 to 60 nm thick, glycosaminoglycan containing electron dense layer was clearly visible on 69 % crystalline hydroxyapatite, but was less well defined on the more amorphous materials. On the latter, a 0.4 to 1 μm wide amorphous zone was regularly seen to which both parallel and oblique aligned collagen fibres were attached . The results of this study show that the crystallinity of plasma sprayed hydroxyapatite is an important parameter which influeces the establishment of the bony interface and may, as a result, have an effect on the bone formation rate and bonding strength between hydroxyapatite and bone tissue