A histomorphometric and molecular study on stress adaptability of freeze-dried bone allograft

OBJECTIVE: To Investigate stress adaptability of freeze-dried bone allograft. METHODS: Cortical and cancellous allograft were transplanted to each side of the midshaft diaphyseal ulna in two groups of 28 animals. The left transplanted allograft was free from fixation and bore a normal physiological load, while the right transplanted allograft was protected from loading by a simple external fixator and bore less load. Animals were sacrificed at the 2nd, 4th, 8th, 16th week after transplantation and specimens were taken out for bone histomorphometry studies and analysis of collagen gene expression by in situ cDNA-mRNA hybridization. RESULTS: Labeled surface(LS) and bone mineral apposition rate(MAR) of the normally loaded graft-host bone interface were significantly higher than that of the less loaded side at the 4th, 8th, 16th week after transplantation. Parameters reflecting the internal repair process of the allograft, such as LS in cortical and cancellous bone or MAR in cortical bone of the normally loaded side were significantly higher than those of the less loaded side at the 16th week after transplantation. The result of in situ hybridization indicated that more osteoblast-like cells expressing the type I collagen gene were found in the interface or interior of normally loaded grafts. CONCLUSION: The stimulus of physiologic load can accelerate the early union of allograft-host bone interface and later new bone creep substitution to the necrotic allograft.