It is well known that abnormal acetabular cup position is an important factor in hip instability, so proper acetabular component orientation is very important in total hip arthroplasty. The accepted safe zone for cup placement is 30°-50° of abduction and 5-25° of anteversion. Danoff et al. from the United States studied the effect of cup orientation on hip dislocation and hypothesized that poorly aligned cups are more likely to cause dislocation. The researchers prospectively followed 1572 patients who had a first total hip replacement for osteoarthritis. After excluding patients with incomplete information, lost to follow-up, and death, a total of 1209 patients were enrolled in the study with a mean follow-up time of 4.8 years. Five highly trained orthopedic surgeons performed all posterior total hip replacements at the same institution. All joints were performed with either a polyethylene lined ceramic femoral head or a metal femoral head. Two independent double-blind reviewers retrospectively measured the position of the prosthesis in these patients on conventional pelvic radiographs using validated computer-assisted methods. The dislocated patients and their orientation relative to the socket cup elements were recorded. The mean cup abduction was 42.8° ± 7.7° and the anteversion was 13.4° ± 7.4°. The cup was placed in the safe zone in 742 (63.7%) cases and not in 423 (36.3%) cases. Forty-one patients with consecutive dislocations of the hip had a mean cup abduction of 43.9° and an anteversion of 11.3°. 21 (52.5%) of these patients had the cup placed safely and 20 (47.5%) did not. Cup position was not an independent risk factor for hip dislocation. In 22 dislocated patients, the anterior tilt of the cup was less than 10°. Adjusting the anterior tilt of the safety zone to 10-25° resulted in 10 patients with hip dislocation (24.4%) having the cup in the new safety zone and 31 patients with hip dislocation having the cup not in the safety zone, at which point the angle of the safety zone was statistically significant. The figure below shows a scatter plot of all patients included in the analysis, with the safety zone boundaries marked by axial lines and dislocation cases circled by circles (the dotted line shows the newly recommended safety zone angle of anteversion based on the use of the posterior approach for total hip arthroplasty in this study). The orientation of the socket cup had little effect on joint dislocation after total hip arthroplasty. The precise placement of the socket cup into the safety zone was not a determining factor in the occurrence of dislocation after arthroplasty. In contrast, changing the anterior tilt of the safety zone to 10-25° may reduce the likelihood of dislocation. The effects of femoral eccentric distance, hip abduction mass, and lower extremity length on hip stability also need to be taken into account.