Use of an intramedullary in vivo model to study bone formation and maintenance in ceramic porous domains

Tissue ingrowth, differentiation, and osteogenesis in a porous bioinert bioceramic were studied using an intramedullary model. Pure alumina tubes (1.3 mm outer diameter, 0.6 mm inner diameter, 15 mm length) were implanted in the femoral medullary canal of young female rats for 4, 6, 8, 12, and 16 weeks. Tissues present within each tube were characterized by histology and quantified by histomorphometry. A tissue front consisting of undifferentiated mesenchymal cells, fibrovascular tissue, osteoid, woven bone, and marrow penetrated the tube from both ends. Behind the front, woven bone remodeled to produce a thin layer of lamellar bone that lined the tube walls the entire distance to the tube ends and enclosed a marrow‐filled lumen. The front was considered to represent the differentiation cascade from mesenchymal cells to fibrovascular tissue to osteoid to woven bone and marrow to lamellar bone and marrow. The fronts advanced into the tube with time such that, by 16 weeks, they were close to meeting or had met. In several instances, the tube was completely lined with a thin layer of mature lamellar bone continuous between the two ends and enclosing marrow. This configuration was considered to be the final equilibrium of tissues within the tube. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 68A: 305–313, 2004