Fibrin Gel-Immobilized Primary Osteoblasts in Calcium Phosphate Bone Cement: In vivo Evaluation with Regard to Application as Injectable Biological Bone Substitute

Osteogenic injectable bone substitutes may be useful for many applications. We developed a novel injectable bone substitute based on osteoblast-fibrin glue suspension and calcium phosphate bone cement (BC). Human osteoblasts were isolated from trabecular bone samples and cultured under standard conditions. Osteoblasts were suspended in fibrinogen solution (FS). BC was cured with thrombin solution. 8 × 4 mm injectable bone discs were prepared using silicon molds and a custom-made applicator device. Discs containing BC, BC/FS, or BC/FS/osteoblasts were implanted subcutaneously into athymic nude mice. After 3, 9 and 24 weeks, specimens were explanted and subjected to morphologic and biomechanical evaluation. In vitro fibrin gel-embedded osteoblasts displayed a differentiated phenotype as evidenced by alkaline phosphatase, collagen type 1 and von Kossa stains. A proportion of osteoblasts appeared morphologically intact over a 3-day in vitro period following application into the BC. BC/FS and BC/FS/osteoblast discs were sparsely infiltrated with vascularized connective tissue. There was no bone formation in implants from all groups. However, positive von Kossa staining only in BC/FS/osteoblast groups suggests engraftment of at least some of the transplanted cells. Biomechanical evaluation demonstrated initial stability of the composites. Young’s modulus and maximal load did not differ significantly in the BC/FS and BC/FS/osteoblast groups. The practicability of osteoblast-containing injectable bone could be demonstrated. The dense microstructure and the suboptimal initial vascularization of the composites may explain the lack of bone formation. Modifications with regard to enhanced osteoblast survival are mandatory for a possible application as injectable osteogenic bone replacement system.