Histological characterization of the early stages of bone morphogenetic protein-induced osteogenesis

On the basis of currently available knowledge, we hypothesize that the initial bone formation, as induced by bone morphogenetic protein (BMP), is influenced by the chemical composition and three-dimensional spatial configuration of the used carrier material. Therefore, in the current study, the osteoinductive properties of porous titanium (Ti) fiber mesh with a calcium phosphate (Ca-P) coating (Ti-CaP), insoluble bone matrix (IBM), fibrous glass membrane (FGM), and porous particles of hydroxy apatite (PPHAP) loaded with rhBMP-2 were compared in a rat ectopic assay model at short implantation periods. Twelve Ti-CaP, 12 IBM, 12 FGM, and 12 PPHAP implants, loaded with rhBMP-2, were subcutaneously placed in 16 Wistar King rats. The rats were sacrificed at 3, 5, 7, and 9 days post-operative, and the implants were retrieved. Histological analysis demonstrated that IBM and Ti-CaP had induced ectopic cartilage and bone formation by 5 and 7 days, respectively. However, in PPHAP, bone formation and cartilage formation were seen together at 7 days. At 9 days, in Ti-CaP, IBM, and PPHAP, cartilage was seen together with trabecular bone. At 9 days, in FGM, only cartilage was observed. Quantitative rating of the tissue response, using a scoring system, demonstrated that the observed differences were statistically significant (Wilcoxon rank sum test, p < 0.05). We conclude that IBM, CaP-coated Ti mesh, FGM, and PPHAP provided with rhBMP-2 can indeed induce ectopic bone formation with a cartilaginous phase in a rat model at short implantation periods. Considering the different chemical composition and three-dimensional spatial configuration of the carrier materials used, these findings even suggest that endochondral ossification is present in rhBMP-2-induced osteogenesis, even though the amount of cartilage may differ.