[Effects of nutrient channels on ectopic bone formation in porous apatite blocks impregnated with super fine bone powder].

In this study, we prepared four types of 10-mm-diameter, 10-mm-long, large, columnar, bone powder-impregnated, porous apatite blocks, which did or did not have a single nutrient channel of 1, 2, or 3 mm in diameter in the long-axis direction. We implanted them in the subcutaneous tissue of adult dogs for 4, 8, or 16 weeks, and examined the status of bone formation in each block. As a result, bone formation started with the differentiation of osteoblasts along the wall of macro-pores within an area of dense multinucleated cells, and the area of bone formation consisting of bone along the pore wall and bone marrow in the pores gradually expanded around the block in a ring fashion and inward or outward. In apatite blocks with a 2- or 3-mm nutrient channel, bone formation also proceeded along the channel in the block. At 16 weeks after implantation, even in the middle level, almost the entire porous apatite block with a 3-mm channel became the area of bone formation. However, in other apatite blocks, there remained an area without bone formation; in particular, in blocks without a nutrient channel or with a 1-mm nutrient channel, there remained an area slightly over 4 mm in diameter without bone formation at the middle level. These findings indicate the importance of the nutrient channel for good bone formation in large blocks.