The development and characterization of biomaterials for bone replacement in case of large defects in preconditioned bone (e.g., osteoporosis) require close cooperation of various disciplines. Of particular interest are effects observed in vitro at the cellular level and their in vivo representation in animal experiments. In the present case, the material-based alteration of the ratio of osteoblasts to osteoclasts in vitro in the context of their co-cultivation was examined and showed equivalence to the material-based stimulation of bone regeneration in a bone defect of osteoporotic rats. Gelatin-modified calcium/strontium phosphates with a Ca:Sr ratio in their precipitation solutions of 5:5 and 3:7 caused a pro-osteogenic reaction on both levels in vitro and in vivo. Stimulation of osteoblasts and inhibition of osteoclast activity were proven during culture on materials with higher strontium content. The same material caused a decrease in osteoclast activity in vitro. In vivo, a positive effect of the material with increased strontium content was observed by immunohistochemistry, e.g., by significantly increased bone volume to tissue volume ratio, increased bone morphogenetic protein-2 (BMP2) expression, and significantly reduced receptor activator of nuclear factor kappa-B ligand (RANKL)/osteoprotegerin (OPG) ratio. In addition, material degradation and bone regeneration were examined after 6 weeks using stage scans with ToF-SIMS and µ-CT imaging. The remaining material in the defects and strontium signals, which originate from areas exceeding the defect area, indicate the incorporation of strontium ions into the surrounding mineralized tissue. Thus, the material inherent properties (release of biologically active ions, solubility and degradability, mechanical strength) directly influenced the cellular reaction in vitro and also bone regeneration in vivo. Based on this, in the future, materials might be synthesized and specifically adapted to patient-specific needs and their bone status.

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The fusion process of mononuclear monocytes into multinuclear osteoclasts in vitro is an essential process for the study of osteoclastic resorption of biomaterials. Thereby biomaterials offer many influencing factors such as sample shape, material composition, and surface topography, which can have a decisive influence on the fusion and thus the entire investigation. For the specific investigation of resorption, it can therefore be advantageous to skip the fusion on samples and use mature, predifferentiated osteoclasts directly. However, most conventional detachment methods (cell scraper, accutase), lead to a poor survival rate of osteoclasts or to a loss of function of the cells after their reseeding. In the present study different conventional and novel methods of detachment in combination with different culture surfaces were investigated to obtain optimal osteoclast differentiation, yield, and vitality rates without loss of function. The innovative method—using thermoresponsive surfaces for cultivation and detachment—was found to be best suited. This is in particular due to its ability to maintain osteoclast activity, as proven by TRAP 5b-, CTSK-activity and resorption pits on dentin discs and decellularized osteoblast-derived matrix plates. In conclusion, it is shown, that osteoclasts can be predifferentiated on cell culture dishes and transferred to a reference biomaterial under preservation of osteoclastic resorption activity, providing biomaterial researchers with a novel tool for material characterization.