The key signaling pathways, via which GFs guide osteogenic cell differentiation, are the TGF-β/BMP- and Wnt-mediated pathways [19,47]. While the BMP pathway ensures differentiation of MSCs into osteo-chondro-progenitors (OCPs), the Wnt pathway is essential for subsequent osteoblastic commitment, i.e., Wnt acts ‘downstream’ of BMP to ensure that OCPs differentiate into osteoblasts and not chondroblasts [47]. Genes associated with both TGF-β/BMP and Wnt pathway (Wnt receptors) were upregulated at day 7 [34,35] and day 14 [34] on SLA and SLActive surfaces, suggesting the occurrence of osteogenic differentiation at these time points.

GF-regulated signaling pathways exert their effects on differentiating cells via activation of TFs. The TFs Runx2 and Osx are considered as ‘master switches’ and absolute requirements for osteoblast differentiation [21] - while Runx2 is essential for MSC differentiation, Osx acting ‘downstream’ of Runx2 controls osteoblastic fate determination [48,49]. An upregulation of these genes was observed in relation to the TiOBlast, Osseospeed, and SLActive surfaces in the present review. However, at day 7, expression of Osx was significantly greater on Osseospeed than TiOBlast surfaces. This finding is consistent with previous animal [50,51] and human studies [52] where superior in vivo osseointegration (i.e., larger amount of bone-to-implant contact occurring earlier) of Osseospeed versus TiOBlast implants was reported. Thus, it appears that implant surface topography and/or chemistry influence peri-implant bone healing in humans both at the signaling pathway and transcription factor level.

Deposition of new bone on the implant surface involves the secretion of a complex ECM (scaffold) of proteins by osteoblasts, which subsequently undergoes mineralization [9]. Expression of ECM proteins is a reliable indicator of early osteogenic activity [19] and was identified in all four studies at days 7 and 14. All studies reported some evidence of ECM production and/or organization at days 7 and 14. Upregulated genes associated with ECM deposition included various collagens (Col 1 to 11), non-collagen proteins (osteopontin (OPN), osteonectin (ON), osteocalcin (OCN), bone sialoprotein (IBSP), periostin (POSTN), and ECM protein-1), alkaline phosphatase (ALP), and bone-specific adhesion proteins (integrins (ITGB4, ITGB5), laminins (LAMA2, LAMA3), and cadherins (CDH11)). Osteocalcin, the most bone-specific ECM protein and a late marker of osteogenic differentiation [19], was significantly upregulated on Osseospeed (versus TiOBlast) surfaces at day 7 [36]. Osteopontin, an ECM protein essential for mineralization [53], was significantly upregulated on SLActive comparing to SLA surfaces at day 7 [35]. The possibility that implant surface features enhance osteogenic differentiation of MSCs via upregulation of specific genes (e.g., SLActive versus SLA in regard with BMP and Wnt signaling) has been demonstrated in vitro [54].