Abstract: Objective　To explore the anatomical positional relationship between the edge of the acetabular cup and the edge of the osseous acetabulum by utilizing the imaging data to reconstruct pelvis 3D model, and to improve the accuracy of intraoperative freehand acetabular cup orientation.    Methods    Based on the CT data of normal acetabulum and Crowe Ⅱ/Ⅲ DDH acetabulum, 3D planning software was used to stimulate the implantation of target  acetabular cup in total hip arthroplasty (orientation: anteversion angle was 20°, abduction angle was 40°, acetabular bony coverage rate was more than 75%). In the 1，4，7，10 o’clock four directions of the right acetabulum, the exposed length of the acetabulum cup, which was the distance between the edge of the acetabular cup and the bony edge of the acetabular, was recorded and statistically analyzed.    Results   The optimal vertical height of the cup implantation for normal acetabulum and Crowe type Ⅱ/Ⅲ DDH acetabulum were (16.26±0.93) mm and (22.09±4.10)mm, respectively. There was statistical differences (P<0.05) between the normal group and the DDH group of exposed length in four directions. The exposed length of the posterior inferior wall of the normal acetabulum (7 o 'clock direction in the right hip, 5 o 'clock direction in the left hip) was (9.71±1.68) mm. The coefficient of variation of the exposed distance in other directions has no obvious difference in different genders and ages.    Conclusions    The exposed length of the posterior inferior acetabular wall is relatively constant to be utilized as an anatomical reference landmark for THA in normal acetabulum, while for Crowe type II/III patients, there is a great variation in the length of the acetabulum edge exposure  in all directions. 

Key words: Developmental dysplasia of the hip, 　Artificial acetabulum, 　Positions of the acetabular cup, Imaging anatomy, 　Computer assisted orthopedic surgery