Table 1 Biological processes and immune-related events in the different periods of DO

Latency period

Formation of hematoma

^{26,27,29,31,124,125}

Various immune cells infiltrate and release pro-inflammatory cytokines such as IL-1 and IL-6 to debride the osteotomy site and promote the initial migration, proliferation, and differentiation of MSCs.

^{26,27,28,29,31,124,125,126}

Inflammatory response

Formation of the outer cartilaginous callus adjacent to the periosteum and the soft callus in the gap

Inflammatory cells and MSCs release growth factors such as TGF-β, BMP, IGF, and VEGF to promote preliminary soft callus formation.

BMP2 and BMP4 are secreted by immature chondrocytes, and their expression can significantly decline because chondrocytes mature and secretion is ceased.

Distraction period

Absorption of the cartilage callus

^37,38,39

Immunosuppressive response plays an important role in the distraction period.

^46,62

A surprising amount of neovascularization and spread toward the center of the distraction gap

The expression of IL-6 appears a second lower peak in response to mechanical stretch to stimulate intramembranous osteogenesis by promoting recruitment and osteogenic differentiation of osteogenic precursor cells.

^28,46

Formation of fibrous interzone

TGF-β1 is continually highly expressed and is consistently distributed with type II receptors, thereby contributing to the proliferation of osteoblast precursor cells and the secretion of extracellular matrix.

^{29,65,68,69,70,71,72}

The expression of BMP and Smad, a downstream signaling pathway molecule, increases, thereby taking over the role of TGF-β and allowing a large number of osteogenic precursor cells to successfully differentiate into osteocytes, which play a role in both intramembranous and endochondral ossification.

^{71,91,126,127,128,129,130,131,132}

Multipotent stem cells infiltrate, proliferate, and differentiate with intramembranous ossification to produce immature woven bones

The RANK-to-OPG ratio continually increases, regulates the activity of osteoclasts, and promotes the absorption of cartilaginous callus that forms during the latency period.

^73,74,75,76

Mineralization and remodeling of parts of the bone at the ends of the distraction gap

Numerous inflammation and immune-related signal pathways, including FAK, MAPK, P38, ERK, Smad, TAK1, PIK3/AKT, Wnt, NF-κB, and mTOR respond to mechanical stimulation or cytokine signal transmission, thereby participating in angiogenesis and osteogenesis.

^{78,80,81,82,84,85,86,90,93,94,98,99,100,101,102,103,104,105,106,109,110,111,112}

Macrophages are widely present in the distraction gap, and M2 phenotype polarization occurs, which promotes the osteogenic differentiation of osteogenic precursor cells.

^47,52

Consolidation period

Fusion of the central unmineralized zone

^27,83

Osteogenic-related cytokines and signal pathway factors that were highly expressed in the distraction period are rapidly downregulated.

^27,83

Complete mineralization of the new bone

The expression of TGF-β1 is maintained at a certain level in the early consolidation period, and TGF-β1 participates in bone maturation and mineralization.

^29,115

Remodeling

The Wnt signaling pathway is maintained at a certain level in the early consolidation period, and it plays an important role in the process of bone mineralization.

^72,118

The RANKL-to-OPG ratio continually increases and peaks in the late consolidation period. Hence, osteoclasts, which are essential for bone remodeling, become extremely active in the mid- and late consolidation periods.

^97,98,120

The expression of IL-1β and TNF-α increases, and they participate in osteoclastogenesis in conjunction with RANKL.

^75,76