BACKGROUND:Major complication after total hip replacement was instability in the form of dislocation. The probability of above complications has a great relationship with the mistakes of the angle of acetabular cup prosthesis. OBJECTIVE:To explore the influence of angle of acetabular cup in total hip replacement on the safety of joint flexion and extension. METHODS: A total of 60 patients, who underwent total hip replacement in the Wujin Hospital Affiliated to Jiangsu University from January 2012 to December 2014, were enroled in this study. Three dimensional directional monitoring device, CT scanning and image processing tools were used to preset the angle of the acetabular cup and to set abduction angle and anteversion angle. Test data were recorded and subjected to statistical analysis. RESULTS AND CONCLUSION:Acetabular cup was implanted under five different conditions: abduction angle 45°, anteversion angle 15°; abduction angle 60°, anteversion angle 15°; abduction angle 30°, anteversion angle 15°; abduction angle 45°, anteversion angle 5°; abduction angle 45°, anteversion angle 25°. According to statistics, in the patients with anteversion angle of 15°, when the acetabular cup was placed, the bigger the abduction angle, the bigger the range of abduction angle of the acetabular cup induced by pelvic tilt was. If the anteversion angle increased, the anteversion angle of the acetabular cup was reduced. With the increased pelvic tilt angle, the anteversion angle of the acetabular cup was big. Moreover, in patients with anteversion angle of 15°, the anteversion angle of the acetabular cup should keep consistent. When the abduction angle of the acetabular cup increased, the range of anteversion angle of the acetabular cup induced by pelvic tilt was diminished. These data showed that the abduction angle and anteversion angle of the acetabular cup have mutual restriction. When the acetabular cup was placed, big anteversion angle (15±10)° can wrap the femoral head, reduce the incidence of dislocation, and make flexion and extension safe.