Osteoimmunology: Interactions With the Immune System in Spinal Fusion

Spinal bone graft healing. Overview of immune and osteogenic cells through the 3 bone healing phases: inflammation, callus formation (repair), and remodeling. First, inflammatory cells release proinflammatory cytokines as well as anti-inflammatory cytokines. The inactivated macrophage is then polarized into a classic proinflammatory (M1) or an alternatively activated (M2) macrophage. During the callus formation, revascularization occurs with soft then hard callus formation. Finally, mature bone is formed by osteoblasts, osteocytes, and osteoclasts. The balance of M1 and M2 macrophages influences appropriate osteogenesis and bone formation. M2 macrophages promote bone formation and remodeling. Excess activation of M1s can lead to scar formation and pseudarthrosis.