BIOMECHANICAL ENHANCEMENT OF IMPACTED BONE GRAFT BY COATING WITH TYPE 1 COLLAGEN AND SKELETAL STEM CELLS

Abstract

Introduction: Impaction bone grafting (IBG) using fresh frozen morsellised allograft is considered by many as the method of choice for replacing lost bone stock encountered during revision hip surgery. Bone marrow contains multipotent skeletal stem cells which have the potential to differentiate down a number of different cell lineages including osteoblasts, chondrocytes and adipocytes. In IBG it is desirable for as many as possible to go on to form bone rather than fibrous tissue to form a solid osseous construct. Whilst it is possible to push cells down the osteogenic lineage in vitro, some of these methods (e.g. the addition of Dexamethasone) are not translatable to clinical practice due to undesirable side effects. In this study we test the hypothesis that by coating the allograft with type 1 Collagen prior to seeding with human bone marrow stromal cells (hBMSC), the cellular adhesion and proliferation down an osteogenic lineage can be increased, leading to improved mechanical and biological properties of the IBG composite.

Methods: A control group of plain allograft and three experimental groups where used to determine the effects that collagen and hBMSC have on IBG (both individually and in combination). The samples where impacted in standardised fashion previously validated to replicate Femoral IBG, and cultured in vitro for 2 weeks. The samples then underwent mechanical shear testing giving a family of stress strain curves for each group, from which a Mohr coulomb failure curve can be plotted. Using the Mohr Coulomb failure equation τ = σ tanΦ + c, the shear strength (τ), Internal friction angle (tanΦ) and inter particulate cohesion (c) can then be calculated. Biochemical analysis was also performed for DNA content and Osteogenic activity.

Results: Mechanical shear testing demonstrated a significant improvement (p=0.002) in the grafts ability to resist shear with the coating of Collagen and seeding with hBMSC (245 vs 299 kPa) as well as improved cohesion between the bone graft particles (46 vs 144 kPa). Regression analysis of the shear strength showed a linear increase with compressive stress (R2 > 0.98) for all groups, indicating that the grafts satisfied the Mohr Coulomb failure law. In the two groups seeded with cells, the collagen coated group also showed increased osteogenic cell activity compared to the plain allograft.

Conclusion: This study has shown a role in the improvement of the mechanical and biological properties of IBG coated with type 1 Collagen and seeded with hBMSC. Collagen coating of IBG is a facile process and translation of the technique into the theatre setting feasible. The improvement in shear strength and cohesion could lead to earlier weight bearing for the patients and allow quicker recovery. The therapeutic implications of such composites auger well for orthopaedic applications. We are currently strengthening the above findings with an in vivo study.