ULTRASTRUCTURE OF THE BONE-TITANIUM INTERFACE IN RABBITS

The interface zone between cortical bone and threaded non-alloyed titanium implants inserted in the rabbit tibia for 12 months was examined by light and electron microscopy. The implants were removed en bloc with the perfusion-fixed surrounding bone and the undecalcified specimens were, after osmification, dehydrated and embedded in plastic resin (LR White). In ground sections (about 10-mu-m thick) cortical bone appeared to be in direct contact with the implant surface and the implants were thus "osseointegrated". Sections for light microscopy (1-mu-m thick) and electron microscopy (40 nm to 0.5-mu-m) were prepared by using an electropolishing technique by which the bulk part of the metal was electrochemically removed and a fracture technique by which the implant was separated from the embedded tissue before sectioning. In the electropolished specimens an unmineralized zone, 2-10-mu-m wide, was observed at the interface. The interface zone contained osteoid-like tissue (densely packed collagen fibrils, osteocyte canaliculi) but in general no deposits of calcium mineral. This feature of the interface could not be observed in specimens prepared by the fracture technique, indicating that the electropolishing technique had induced serious artefacts, including decalcification of the interface bone. In sections prepared by the fracture technique, mineralized bone was present very close to the implant surface. No gradient of mineral was observed. A thin layer of amorphous material (100-200 nm wide) was present peripheral to the mineralized bone. An electron dense line about 100 nm wide was formed at the border between the mineralized bone and the amorphous layer. The dense layer had the same characteristics as the lamina limitans observed around osteocyte lacunae and canaliculi or the zone between areas of bone with different degree of mineralization. Our observations suggest that mineralized bone reached close to the surface of titanium implants inserted in the rabbit tibia for 12 months but that a direct contact is not established.