| Summary: | The aims of this thesis were to investigate certain aspects of the regulation of osteogenesis by BMPs and their physiological inhibitors; to determine whether the balance between the two could have an effect on fracture repair; and to suggest strategies for using this information to provide orthopaedic surgeons with better ways to treat recalcitrant fractures . BMP-2 and its inhibitor chordin were produced endogenously during the in vitro osteogenic differentiation of human mesenchymal stem cells (MSCs) in response to dexamethasone. The addition of exogenous BMP-2 increased the rate of osteogenic differentiation of the MSCs. Knockdown of the BMP inhibitor chordin led to an increase in the rate of osteogenic differentiation of human MSCs, secondary to an increase in the bioavailability of BMP-2. These results raise the possibility that the impaired healing of delayed- and non-unions reflects alterations in the ratios of BMPs to their inhibitors. They further suggest that it may be possible to increase the rate of osteogenesis, and thus improve fracture repair in vivo, by down-regulating the endogenous production of BMP inhibitors. To evaluate these possibilities further, the presence of BMP-2 and -14, as well as their inhibitors chordin and noggin, were investigated by semi-quantitative immunohistochemistry in human fracture biopsies. The expression of BMP-2, BMP-14, chordin and noggin during fracture repair was demonstrated in areas of cartilage and bone formation. Levels of expression of these proteins were compared between healing and non-healing human fractures. There was a decreased relative expression of BMP-2 and BMP-14, compared to the BMP inhibitors, in the non-healing fractures v/s the healing fractures. This suggests an imbalance between BMPs and their inhibitors in fractures that do not heal. These data indicate novel ways to reduce rates of non-union by the local application of BMP-2 or BMP-14, or the blocking of BMP antagonists. Fracture healing may also be impaired by drugs used clinically. The antibiotic tobramycin, commonly used to treat or prevent infections of bone, was shown to reduce the osteogenic potential of human MSCs in vitro. An in vivo study was carried out to determine whether a clinically relevant dose of tobramycin would impair fracture repair induced by BMP-2. The biomechanical and radiological properties of the repair tissue, induced by BMP-2 in a rat femoral defect model, were not affected by the presence of tobramycin. Overall, this thesis demonstrates that the balance between BMPs and their inhibitors has a major influence on the rate of fracture repair and that this balance is altered in non-healing fractures. The data suggest novel, clinically relevant, biological strategies for enhancing bone healing.
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