An Ex Vivo Bone Defect Model to Evaluate Bone Substitutes and Associated Bone Regeneration Processes

The increased incidence of bone defects, especially in cases of comminuted fractures or bone tumor resections demands suitable bone grafts and substitutes. The aim of this study was to establish an ex vivo bone defect model to evaluate new bone substitutes and associated repair processes under controlled conditions. Femoral heads derived from patients undergoing total hip replacement were cut into cylinders (20 mm diameter, 7 mm height). A central bone defect (6 mm diameter, 5 mm depth) was inserted centrally. The bone slides were cultured for 28 days and viability was evaluated by lactate dehydrogenase and alkaline phosphatase assay, and Calcein-AM viability staining and DNA quantification. Data revealed the viability of the bone tissue over the tested time period of 28 days, and an increase in cell numbers implicating active cell proliferation processes in the sections. To analyze the bone regeneration potential of this model in combination with a bone replacement material, we injected a collagen-type 1 hydrogel into the central defect. Cellular ingrowth into the gel was evaluated by microscopy and DNA quantification at different time points demonstrating an increase of cells in the defect over time. Finally, gene expression of osteogenic markers indicated an osteoblastic phenotype of the cells in the defect. In summary, the ex vivo bone defect model remains viable and shows active bone repair processes over 28 days. Additional advantages include high reproducibility, manageable costs, and a native bone-implant interface supporting the evaluation of bone substitute materials and associated regeneration processes. Impact statement Testing of new implant materials and bone repair strategies up to date rely mainly on in vivo and in vitro investigation models providing different pros and cons. In this study we established a novel human ex vivo bone defect model with a proven vitality of at least 28 days. The model provides a native bone implant interface and is designed to monitor cell invasion into a critically sized defect filled with the potential implant material. Furthermore, associated repair processes can be documented on the cell and molecular level, including additional advantages such as high reproducibility and manageable costs.