ELUCIDATING THE CHANGES IN VASCULAR STRUCTURES IN A NOVEL SMALL ANIMAL TRAUMA MODEL FOR OSTEONECROSIS OF THE FEMORAL HEAD

Osteonecrosis of the femoral head (ONFH) is a condition of inadequate blood supply that destroys of osteogenic-angiogenic coupling and diminishes femoral perfusion. Insufficient knowledge on molecular background of ONFH brings the need for a small animal trauma model to research and to understand ONFH aetiology for new treatment methods. This study aims to develop a mouse trauma model for ONFH and to show changes in vascular structures. Left femoral head (FH) was dislocated from hip joint, ligamentum teres was cut in 8-week-old male C57BL/6J mice to disrupt the blood supply to FH. Right hip joint was left unoperated as control. Animals ( n = 5 per time point) were sacrificed on 2-3-4-6-8-10 weeks, and ex-vivo μCT was taken to assess bone structural parameters. Haematoxylin/eosin (HE)-staning was done for histology. Comparision of the changes in vascular structures between necrotic and non-necrotic bone were done by immunohistochemical-staining (IHCS) for CD31(endothelial-cell-marker), EMCN (with CD31 labels bone-specific regenerative H-type-vessels), and PDGFRbb (pericyte-marker). μCT-assessment showed increasing trabecular bone loss and decreased BV/TV from 2 to 8 weeks in necrotic-side. HE-staining displayed increased number of empty lacunae in necrotic-side as early as 24hours after operation. By 6 th week, HE-staining showed the presence of bone marrow necrosis and bone-fat accumulation as well as the increased myeloid cell numbers in necrotic-side. These results are supporting the development of small animal trauma model for ONFH is achieved. IHCS staining of vascular structures showed by 4 th week, epiphyseal plate (EP) was invaded by H-type blood vessels in necrotic-side, while EP was observed to be intact in control-side. Also, by 4 th week there was a decrease in PDGFRbb + -vessels in necrotic-side, which may be a sign of decreased stability of the blood vessels in necrotic-side compared to control-side. By 6 th weeks in necrotic-side the H-type blood vessels were shown to be in barbed structure - a sign of degenerated blood vessels-. Trabecular bone loss, increased empty lacunae number, bone-fat imbalance, bone marrow necrosis are reported as signs of osteonecrosis. Our results are coherent with literature and indicated that we were able to effectively generate a trauma model for ONFH in mice for the first time in literature. Based on this model we also observed that ONFH affects the stabilisation of blood vessels as well as causes changes in H-type regenerative blood vessels in which the myeloid cells can also be in interplay of vascularisation and necrotic-bone.