Die Biegesteifigkeit des Osteosynthesematerials bestimmt die Reifung des Kallusgewebes eines Knochenbruches über die Immigration von Makrophagen und die Angiogenese

Abstract

The stability of fracture fixation determines the pattern of fracture healing by different differentiation of the mesenchymal tissue (osteonal versus enchondral ossification). The alteration of the expression of collagen IX-, collagen X-, osteocalcin-, macrosialin-, and VEGF-mRNA in the fracture callus was investigated at different stable fixation devices.Standardized tibial fractures of rats were fixed with a steel nail (mean stiffness 3.2 ± 0.43 N/ mm) or with a polypropylene nail (mean stiffness 0.2 ± 0.04 N/mm). On day 4, 8, 12, 20 and 40 after operation the fracture tissues of 3 rats per nail type were excised. The specimens were examined histologically (Azan staining) or immunhistologically (ED2 as marker for macrophages and v. Willebrand factor for endothelial cells) and the total RNA of each fracture was isolated. Specific cDNA probes for collagen IX (specific marker of immature chondrocytes), collagen X (hypertrophie chondrocytes), osteocalcin (osteoblasts), macrosialin (macrophage specific surface antigen and equivalent of human CD 68) and VEGF (angiogenesis) were synthesized by reverse transcribed-Polymerase Chain Reaction (rt-PCR). After Northern Blotting the membranes were hybridized with the specific cDNA probes and the relative expression quantified by phosphor imaging.Histologically a delay of fracture healing could be observed in the less stable fractures. In the more stable fractures collagen IX-, collagen X-, osteocalcin-, macrosialin- and VEGF-mRNA expression was earlier than in the less stable fractures. In immunohistochemical sections macrophage specific staining was seen earlier and more vessels (v Willebrand factor) were seen from more stable fixed fractures.In our fracture model we can show that the maturation of cartilage, the differentiation of bone, the immigration of macrophages and the formation of new blood vessels depend on the stability of fracture fixation. Because the angiogenetic capability of macrophages is VEGF mediated and endothelial cells promote the terminal differentiation of chondrozytes in co-cultures and in the growth plate the enchondral ossification depends on the presence of VEGF and endothelial cells, we suggest that a less stable fracture fixation impairs the blood vessel formation by increased micro movement and consequently the differentiation of mesenchymal tissues. More stable fracture fixations, however, increase angiogenesis and create a milieu for a fast enchondral ossification.