Increased VEGF Expression in the Epiphyseal Cartilage After Ischemic Necrosis of the Capital Femoral Epiphysis

Abstract

Ischemic injury to the immature femoral head produces epiphyseal cartilage damage and cessation of endochondral ossification. This study suggests that VEGF facilitates the repair of the necrotic epiphyseal cartilage, which is essential for restoration of endochondral ossification and re-establishment of the growth of the immature femoral head after ischemic necrosis. Legg-Calve-Perthes disease (LCPD) is a childhood form of osteonecrosis that produces growth arrest of the secondary center of ossification. The cessation of growth is caused by ischemic damage to the hypertrophic zone of the epiphyseal cartilage where endochondral ossification normally occurs. The role of vascular endothelial growth factor (VEGF) in restoring endochondral ossification in the epiphyseal cartilage after ischemic necrosis was investigated in a piglet model of LCPD because the resumption of normal growth is important for maintaining the spherical shape of the femoral head. Piglet femoral heads were assessed 24 h to 8 weeks after the surgical induction of ischemia. Western blot analysis, ribonuclease protection assay (RPA), immunohistochemistry, and in situ hybridization were performed. Western blot analysis and RPA showed increased VEGF protein and mRNA expression, respectively, in the epiphyseal cartilage of the infarcted heads compared with the contralateral normal heads. In the normal femoral heads, VEGF-immunoreactivity (VEGF-IR) and transcripts were observed in the hypertrophic zone of the epiphyseal cartilage. In the infarcted heads, VEGF-IR and transcripts were no longer observed in the hypertrophic zone because of diffuse cell death in that zone from ischemia. However, VEGF-IR and transcripts were observed in the proliferative zone above the necrotic hypertrophic zone. At 8 weeks, vascular granulation tissue invasion of the necrotic hypertrophic zone was observed with active resorption of the necrotic cartilage. In some areas where the necrotic cartilage was completely resorbed, restoration of endochondral ossification was observed. In these areas, VEGF transcripts were observed in the newly formed hypertrophic zone. VEGF expression was increased, and its spatial expression was altered in the epiphyseal cartilage after ischemic necrosis of the immature femoral head. VEGF upregulation in the proliferative zone after ischemic damage may play a role in stimulating vascular invasion and granulation tissue formation in the necrotic hypertrophic zone of the epiphyseal cartilage. This may be an important step toward facilitating the resorption of the necrotic cartilage and restoration of endochondral ossification leading to further growth and development of the femoral head.