Distraction osteogenesis induces endothelial progenitor cell mobilization without inflammatory response in man

Distraction osteogenesis (DO) is a unique postnatal bone formation process, which is characterized by a profuse increase in vascularization. Recently endothelial progenitor cells (EPCs) have been reported to circulate in substantial numbers under physiologic conditions and to contribute to bone regeneration. The authors investigated EPC mobilization in patients undergoing limb lengthening.

Thirteen of 24 consecutive patients who underwent limb-lengthening procedures on weight-bearing long bones (femur and tibia) were included in this study. Peripheral blood samples were taken at four different time points from each patient, that is, before operation (pre-Op), 2 or 3 days after osteotomy (early-PO), before the start of distraction (pre-Dist), and at 7 to 14 days after the start of distraction (during-Dist). Numbers of leukocytes and levels of plasma C-reactive protein (CRP) were determined. After isolating mononuclear cells (MNCs) by centrifugation, we performed FACS analysis on freshly isolated MNCs using antibodies to the cell surface markers; CD34, CD133, vascular endothelial growth factor receptor 2 (VEGFR2), and alkaline phosphatase. MNCs were also cultured in endothelial cell growth medium and numbers of EPC colony-forming units were counted. Plasma levels of EPC-mobilizing cytokines, such as, VEGF, SDF-1, and MCP-1, were determined by ELISA.

Numbers of leukocytes and CRP plasma levels increased significantly during the early-PO period (p<0.01) but were maintained within normal range in the during-Dist period. FACS analysis of freshly isolated MNCs showed that EPC-enriched cell fractions increased after distraction, but that alkaline phosphatase-positive cell numbers were unchanged. Numbers of EPC colony-forming units significantly increased in the during-Dist period (p<0.01). Plasma levels of VEGF and SDF-1 significantly increased in the during-Dist period (p<0.05). In femoral lengthening patients whose healing index was less than 30 days/cm, the number of CFUs was 46.8 in the during-Dist period, whereas it was 12.7 in patients whose healing index was more than 30 days/cm (p=0.088).

This study demonstrates a mobilization of EPC population during distraction osteogenesis in human limb-lengthening patients. Distraction strain provoked increases in the plasma levels of EPC-mobilizing cytokines, such as, VEGF and SDF-1. These findings suggest a possibility that therapeutical approaches which modulate EPC mobilization may speed bone healing by angiogenesis-osteogenesis coupling during distraction osteogenesis.