gms | German Medical Science

Questionnaire: Failure of fracture healing with non-union formation is still a major clinical burden. Angiogenesis is closely linked to bone regeneration, but the role of angiogenesis in non-union formation remains unclear. Because established non-unions have been shown to be well vascularized, we hypothesized that lack of VEGF-expression and vascularization during the early time course of fracture healing determine non-union formation.

Methods: In 72 CD-1 mice a femoral ostotomy with a gap size of 1.80mm (non-union group) or a gap size of 0.25mm (union group) were created and stabilized by a pin-clip technique. Healing was analyzed after 3, 7, 14, 21, 28 and 70 days by micro-CT and histomorphometry. Vascularization was determined in different healing zones by immunohistochemical staining of PECAM-1. Additional animals were analyzed after 7, 14 and 21 days for Western Blot analysis of VEGF, BMP-2 and -4 expression.

Results and Conclusions: Micro-CT and histomorphometry showed complete bone bridging in the union group, whereas animals of the non-union group showed atrophic non-union formation. Vascularization increased from day 3 to day 7 in both groups with a subsequent decrease after 14 days. However, overall vascularization did not differ between unions and non-unions over time. Of interest, vascularization within the endosteal healing zone was even higher in non-unions than in unions after 14 days. Expression of VEGF was significantly higher in non-unions, while expression of BMP-2/-4 and PCNA were found significantly reduced compared to unions. Because vascularization during the early time course of fracture healing was not impaired despite the failure of bone healing in non-unions, we have to reject our hypothesis. We conclude that non-union formation is not necessarily due to failure of VEGF-mediated angiogenesis, but rather due to decreased expression of BMP-2/-4 and a disturbed ratio of angiogenic to osteogenic growth factors. Because hypoxia is a strong stimulus for VEGF production, an increased hypoxic environment in the larger segmental defect of the non-union group might have caused the increased expression of VEGF in the non-union group. We hypothesize that in critical size bone defects and situations with impaired vascularization and hypoxia, the principal aim of the injured tissue is to re-establish a vascular network for adequate nutritient and oxygen supply to guarantee cell survival at the site of injury. This may be achieved by an increased VEGF production under those critical conditions. In contrast growth factors for osteoblastic cell differentiation and new bone formation are downregulated, because in this setting general cell survival at the fracture site is of principal importance. Because the intrinsic angiogenic response during non-union formation was sufficient for adequate vascularization, treatment strategies of non-unions should include stimulation of osteogenesis rather than stimulation of angiogenesis.