Objective To determine the effect of NEL-like type 1 gene (NELL-1) transfection in vivo in the repair of traumatic femoral head necrosis.Methods Twenty-four SD rats were randomly divided into three groups (8 rats per group) according to the lottery method,ie,sham group (served as normal control),NELL-1 treatment group (injected NELL-1 gene by recombinant adenovirus vectors around the hip one week after osteonecrosis model induced surgically) and placebo group (given an equal volume of saline solution at the same time after the induction of osteonecrosis).Femurs were taken from the animals 5 weeks after surgery.Gross observation was performed for morphology changes,X-ray assessment for femoral head height and length ratio (H/L),Micro-CT measure for bone parameters of femoral head including total volume (TV),bone volume (BV),total mineralized content (TMC),trabecular thickness (Tb.Th) and trabecular space (Tb.SP),and histological study for osteocytes,osteoblasts and osteoclasts.Results Preserved femoral head shape was noted in NELL-1 treatment group compared to the obvious flattening of the femoral head in placebo group.No heterotopic osteogenesis was observed in any group.Femoral head H/L ratio for 0.753 2 ± 0.040 2 in NELL-1 treatment group was higher than 0.598 4 ± 0.037 0 in placebo group (P ＜ 0.05),but lower than 0.920 2 ± 0.037 0 in sham group (P＜0.05).TV,BV,TMC and BMD between NELL-1 treatment and sham groups did not differ significantly (P ＞ 0.05),but all were increased compared to placebo group (P ＜ 0.05).There was no significant differences in Tb.Th and Tb.SP among three groups (P ＞ 0.05).Most osteocytes were alive in NELL-1 treatment group.More active osteoblasts and osteoclasts were noted in NELL-1 treatment group than those in placebo group.Conclusion NELL-1 gene transfection can preserve femoral head shape and bone content,promote osteoblast activity and neovascularization and hence is an effective treatment for rat traumatic osteonecrosis.However,the activity of osteoclasts is stimulated simultaneously.