A simple rodent subcutaneous assay for identification of new osteoinductive molecules: The key method for screening of novel bone regeneration implants

Treatment of large bone defects and degenerative diseases of the spine is among the most challeng- ing and still unresolved issues in clinical medicine. Therefore, substantial effort has been devoted to the development of novel bone regenerative therapies. Due to their potent osteoinductive properties, Bone Morphogenetic Proteins (BMPs) have been the basis for the development of novel strategies for bone regeneration. The use of animal models is an indispensable part of the preclinical testing of novel therapeutic solutions. The rat subcutaneous assay became the initial screening procedure for the evalu- ation of promising BMP-based osteoinductive devices for bone regeneration because only osteogenic BMPs can induce new bone at any ectopic rodent site. Moreover, this model is used for research on the mechanisms of ectopic bone formation as well as for the evaluation of the inflammatory response to different materials. In this review, we provided an overview of the assay development and previously conducted studies with different methods (flow cytometry, histological and microCT analyses) for the study outcome evaluation. Moreover, we addressed essential issues in the experimental design such as the follow-up period and the sample size. The rat subcutaneous bone induction assay layed the founda- tion for isolation and identification of BMPs followed by testing of new osteogenic devices in higher animal species and humans.