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We have selected heat-treated bone allografts as the graft material since the Tokai Bone Bank, the first regional bone bank in Japan, was established in 1992. In this study, we examined changes in bone mineral density (BMD), and morphology observed by magnetic resonance imaging (MRI), and histological findings of bone grafts in cases followed up for 7-10 years after bone grafting to grasp the remodeling of heat-treated cortical bone allografts for posterior lumber interbody fusion (PLIF). BMD of bone grafts was reduced by half at 10 years after grafting. MRI revealed that bone grafts were indistinguishable initially in only 22.2% of cases, whereas after a lengthy period of 10 years distinguishable in many cases. Histologically, new bone formation at the graft-host interface was observed earlier, at 1 year after grafting, than that at the periphery of canals in the specimens. The laminated structure of the cortical bone eroded over time, and fragmented bone trabeculae were observed in the specimens at 8 years or longer after grafting, though necrotic bone still remained in some sites.

Crude bovine BMP was implanted into the thigh muscle pouch of mice. Tetracycline was injected 1 week and calcein was injected 2 weeks after implant. At 3 weeks after implant, the mice were sacrificed and reviewed histologically. Coexistence of calcified bone and osteoid was observed in Villanueva Goldner stained sections. Toluidine blue staining demonstrated cartilaginous matrix and adjacent locus, and spherical cells varying in shape and size were observed. Calcified lamellar bone was present in the border, and osteoid with osteoblast-like cells was found on the bone marrow side. Calcein labeling appeared as a strong line in the margin and was definitely observed as weak fluorescence in the center under fluorescence microscopy. These results suggest the presence of ossification mode different from the intramembranous and endochondral ossification modes.