Furthermore, genes associated with collagen fibril formation/organization (heat-shock protein-47 (HSP-47), pro-collagen C-endopeptidase enhancer (PCOLCE), small leucine-rich proteoglycans (SLRP)) and post-translational modification (pro-collagen lysyl-hydroxylases (PLOD1, PLOD2, PLOD3) and lysyl-oxidase (LOX)) were upregulated on Osseospeed and TiOBlast surfaces [37]. Collagen comprises approximately 90% of the ECM and collagen fibrillogenesis and organization directly determine the biomechanical properties of bone [55,56]. Genes associated with collagen fibril formation, maturation, and post-translational modification expressed by osteoblasts [57,58] were upregulated on TiOBlast and Osseospeed implants, representing early ECM organization at the bone-implant interface. These modifications determine the pattern of collagen cross-linking which in turn influences tissue organization, mineralization, and ultimately mechanical bone strength [56], and in the case of osseointegration, the integrity of the bone-implant interface [37].

While GFs regulate osteogenesis, pro-inflammatory CKs (e.g., IL-1, IL-6, TNF-α) simultaneously regulate the antagonist process of bone resorption via osteoclasts [23]. Moreover, osteoblasts themselves stimulate osteoclastogenesis via macrophage colony stimulating factor (M-CSF) and receptor activator of NF-kB ligand (RANKL) genes but also closely regulate this process via osteoprotegerin (OPG), an inhibitor of RANKL [59].

Two studies reported expression of genes associated with osteoclastic activity and ECM degradation (cathepsin-K (CTSK), tartarate-resistant acid phosphatase (ACP5), and/or matrix metalloproteinases (MMPs)), on Osseospeed and TiOBlast surfaces at day 7 [37], and SLActive surfaces at day 14. However, upregulation of MMP inhibitors (TIMP-2, -3) was also reported on TiOBlast and Osseospeed surfaces suggesting a control of the resorption process. Although no studies reported differential RANKL/OPG expression, a previous in vitro study [60] reported significant downregulation of osteoclastogenic genes on SLActive surfaces. Collectively, these data reaffirm the dynamic nature of bone formation and resorption at the implant-bone interface, even in early healing stages, and suggest the possibility for implant surface technology modulation of bone remodeling.

Angiogenesis is closely related to osteogenesis and occurs simultaneously during bone regeneration [11]. Physiological oxygen tensions in bone are about 12.5% O₂ but fall to 1% O₂ in regeneration sites due to disruption of the local vasculature as a result of injury and/or surgery [61,62]. A key event that stimulates angiogenesis (and osteogenesis) at regeneration sites is hypoxia, via the hypoxia inducible (transcription) factor-1 (HIF-1) that regulates expression of angiogenic genes [63]. The key cells involved in angiogenesis are macrophages, which in response to hypoxia and inflammation release chemotactic and angiogenic growth factors (e.g., VEGF) [40,64], and endothelial progenitor cells (EPCs) which differentiate into endothelial cell lining blood vessels [65]. VEGF is the single most important regulator of EPC differentiation and vessel formation [66]. Moreover, a role for VEGF in osteogenic differentiation has also been suggested mainly via interaction with the BMP signaling pathway [67].