Growth and dissolution of apatite precipitates formed in vivo on the surface of a bioactive glass coating film and its relevance to bioactivity

Development of bioactive glasses for use as a coating on Ti6Al4V prostheses requires a better understanding of reactions at the bone /bioactive glass interface. Indeed, the bioactive glasses bond to bone through physico-chemical reactions. In vivo, an apatite rich layer is built up on top of a pure silica rich layer at the bioactive glass periphery. In this paper, we have studied Ti6Al4V cylinders coated with a bioactive glass and implanted in sheep femora for two, three and six months. At each time period, the samples were analyzed with scanning transmission electron microscopy coupled with energy dispersive x-ray spectroscopy. In vivo, the bioactive glass dissolution led to the formation on its surface of spherical particles with different sizes. The distributions of Si, Al, Ca, P and Mg concentrations across the particles reveal precipitation of apatite with the incorporation of magnesium. Apatite precipitation is governed by diffusion through an Si layer and occurs under specific supersaturation conditions. Measurements of supersaturation for Ca and P demonstrate that the largest precipitates grow and the smallest dissolve. These results allow us to study the growth and dissolution rate of the apatite precipitates and their relevance to bioactivity. Particles with a radius twice the average radius ( ) grow the fastest and, if the radius increases, the rate of growth decreases. Before three months, the growth of apatite precipitates ( approximately 1 mu m) leads to the growth of a Ca-P interfacial layer. After three months, is of the order of 0.5 mu m, and the majority of the apatite layer dissolves. The effects of aluminum and magnesium on apatite generation are also studied.