Introduction There are 1 million cases of major skeletal defects :that occur worldwide each year that lead to significant morbidity and disability and currently require bone grafting as the main mode of treatment. Limitations of bone-grafting include donor site morbidity, reduced osseoinductivity and risk of pathogen transmission to the host. There is considerable interest in finding ways of differentiating mesenchymal stem cells down the osteoblastic lineage to form bone tissue. We hypothesized that there is an optimum strain that promotes differentiation of mesenchymal stem cells into osteoblasts.

Methods: A bioreactor was developed that was capable of applying tensional forces across a culture strip in a graduated manner within a range of 1-4373me. Mesenchymal stem cells were grown on these strips and subjected to cyclical tensile strain at 1Hz. Cell morphology using Scanning Electron Microscopy, mineralization using specialized stains and expression of core binding factor1 (Cbfa1) was studied at various strain levels.

Results: Scanning Electron Microscopy revealed classic osteoblastic cells in the regions subjected to tensile force, especially in the region where average strain was 1312me. X-ray microanalysis revealed calcium deposits on the strip, indicating osteoblastic differentiation. Cbfa1 expression was greatest in the region with an average strain 1312 me followed by a region on the strip subjected to just fluid shear without any tension. Cbfa1 expression was significantly greater in cells subjected to tensile forces than unstrained controls at all levels of strain tested (p< 0.05). Cbfa1 expression was further enhanced significantly by the addition of osteogenic factors (p< 0.05). Significantly greater mineralization (p< 0.05) occurred in the regions subject to tension with the greatest being in the region with an average strain of 1312 me.

Conclusions: Mechanical tensile forces especially in the range of up to 2173me promote differentiation of Mesenchymal Stem Cells into osteoblasts and encourage expression of the Cbfa1 gene. Tensile strain also promotes mineralization. Chemical factors in form of osteogenic media accelerate the differentiation of MSCs and encourages earlier production of osteoblast specific markers. Fluid shear appears to have a beneficial effect in stimulating differentiation into the osteoblast phenotype and, combined with tensile strain, may offer an even greater osteogenic stimulus.