Objective:To determine the influences of structural changes after valgus impacted femoral neck fracture on hip range of motion (ROM) so as to provide evidence for clinical judgment of whether reduction is necessary or not in the internal fixation of such fractures.Methods:1. 3D reconstructions of the CT hip scans were performed for the 73 patients who had been treated at Department of Orthopaedic Surgery, Shanghai Sixth People’s Hospital Affiliated to Shanghai Jiao Tong University for valgus impacted femoral neck fractures from January 2019 to April 2019.The femoral neck-shaft angle, anteversion angle, femoral offset, axial alpha angle, lateral center edge angle (LCEA), anterior center edge angle (ACEA) and center displacement were measured and compared between the affected and healthy sides to determine the influences of the fracture on the above indexes. 2. Hip motions (flexion and MIR-90°) were simulated on bilateral sides to determine the influences of structural changes after fracture on hip ROM using stepwise regression and Logistic regression. 3. The distribution of femoral-acetabular contact points on the femoral side was observed in simulation of hip flexion to detect the potential area for femoracetabular impingement (FAI) induced by the fracture displacement.Results:1. The valgus impacted femoral neck fractures had significant influences on femoral neck-shaft angle, anteversion angle, femoral offset and axial alpha angle. Compared with the healthy side, on average, the femoral neck-shaft angle increased by 5.1°, anteversion angle decreased by 6.5°, femoral offset decreased by 8.2 mm and axial alpha angle increased by 9.7° on the affected side, showing significant differences ( P<0.05).The displacements of the femoral head center averaged 9.2 mm. There was no significant difference in LCEA or ACEA between the affected and healthy sides ( P>0.05). 2. Compared with the healthy side, on average, the simulated hip flexion decreased significantly by 27.0° and the hip MIR-90° decreased significantly by 20.3° on the affected side after fracture ( P<0.05). Regression analysis showed that femoral anteversion angle, ACEA and displacement of the femoral head center had a significant influence on hip ROM, especially the anteversion angle. When the anteversion angle decreased by more than 7.1°, the hip flexion would decrease by at least 20%. 3. The points of FAI distributed more widely on the fracture side. Compared with the healthy side, the impact points extended outward and upward in hip flexion and extended inwardly in hip MIR-90° on the affected side. Conclusions:After a valgus impacted femoral neck fracture, if the femoral anteversion angle has been decreased by more than 7.1°, the hip ROM can be greatly influenced and the points of FAI can be distributed more widely. Therefore, reduction should be recommended before internal fixation of the fracture.