Improving both the biological and mechanical properties of allograft in impaction bone grafting: a role for human bone marrow stromal cells

Background: The use of fresh morsellised allograft in impaction bone grafting for revision hip surgery remains the gold standard. Bone marrow contains osteogenic progenitor cells that arise from multipotent mesenchymal stem cells and we propose that in combination with allograft will produce a living composite with biological and mechanical potential. This study aimed to determine if human bone marrow stromal cells (HBMSC) seeded onto highly washed morsellised allograft could survive the impaction process, differentiate and proliferate along the osteogenic lineage and confer biomechanical advantage in comparison to impacted allograft aloneMethods: HBMSC were isolated and culture expanded in vitro under osteogenic conditions. Cells were seeded onto prepared morsellised allograft and impacted with a force equivalent to a standard femoral impaction (474J/m2). Samples were incubated for either two or four week periods under osteogenic conditions and analysed for cell viability, histology, immunohistochemistry, and biochemical analysis of cell number and osteogenic enzyme activity. Mechanical shear testing, using a Cam shear tester was performed, under three physiological compressive stresses (50N, 150N, 250N) from which the shear strength, internal friction angle and particle interlocking values were derived.Results: Cell viability of HBMSC post impaction, was confirmed with cell tracker green staining, a marker of viable cells, and observed throughout all samples. There was a significant increase in DNA content and specific alkaline phosphatase activity compared to impacted seeded allograft samples. Immunohistochemical staining for type I collagen confirmed cell differentiation along the osteogenic lineage. Mechanical shear testing demonstrated a statistical significant increase in shear strength and interparticulate cohesion in the allograft / hBMSC group over allograft alone at 2 and 4 week intervals (p<0.001).Conclusion: HBMSC seeded onto allograft resulted in the formation of a living composite capable of withstanding the forces equivalent to a standard femoral impaction. HBMSC under osteogenic conditions were observed to differentiate and proliferate along the osteogenic lineage. In addition, an allograft /HBMSC living composite confers a biomechanical advantage over allograft alone These changes resulting in enhancement of biological and mechanical properties of bone graft within impaction bone grafting have implications for translation and future change in orthopaedic practice in an increasing ageing population