Responses of bone to titania–hydroxyapatite composite and nacreous implants: a preliminary comparison by in situ hybridization

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The effect of two biomaterials on bone formation in vivo by in situ hybridization, was compared by using RNA probes complementary to collagen α1(I) RNA, osteonectin RNA and osteocalcin RNA. Holes were drilled into the midshafts of rat femurs. Titania–hydroxyapatite composite (THA) or nacre cylinders were implanted and the bone–implant regions collected 14 days after operation. Cuboidal osteoblasts, intensely labelled with the three probes, were seen to be lining the newly formed bone surrounding the THA implant. Between the implant and the new bone, a layer of un-labelled, apparently non-osteogenic cells was observed. By contrast, the nacre implant was bonded to the newly formed bone without any soft tissue interference. Osteoblasts lining the distal surface of the newly formed bone were stained with all three RNA probes, although weaker than in the THA sample. Some of the osteoblasts were flattened. We concluded from the appearance of the osteoblasts that the bone formation in the nacre samples had progressed beyond the phase of maximal synthetic activity. Around the THA implant, the labelling indicated that bone-forming activity was still high. It was concluded that the bioactivity of nacre was higher than that of THA.