Interrogating the osteogenic potential of implant surfaces in vitro; A review of current assays.

Impact statement Titanium implants are already widely used in orthopedics and dentistry, yet, intensive research continues with the aim of modifying and functionalizing implant surfaces to invoke a stronger bone response and to meet current clinical challenges around improving longevity, decreasing morbidity, widening access, and clinical application. A very large number of surface modifications have been studied and the potential for new designs appears to be limitless as new technology grows. This review provides guidance for in vitro assays available to test these technologies, providing a cost-effective means for acquiring robust and physiologically relevant data, before in vivo examination. The success of implantable devices relies heavily on their interaction with the host cells facilitating the osseointegration process. However, with so many new surface modifications, with subtly varying design parameters, in vitro assays can, with proper interpretation, provide valuable information for understanding cellular behavior. This review brings together pertinent in vitro experimental protocols available to researchers and discusses them in relationship to the development of the osteoblast phenotype during bone repair. Consideration is also paid to the influence of endothelial and macrophage cells that can substantially change osteogenic cell activity and thus can provide added value for predicting the osseointegration potential in vivo. Due to the diverse and heterogeneous nature of cell types available for culture use, this review concludes that there is no "gold standard" series of assays. Rather, we present guidance in the experimental design of in vitro assays to better identify those surfaces with promising osteogenic potential.