Safe Zone

Suboptimal implant placement is one of the main causes of total hip replacement (THR) failure. It has been reported that a steep cup inclination angle of 50° or more causes edge loading, which leads to increased liner wear and liner fracture rate. Suboptimal cup alignment further causes prosthetic impingement and associated complications, such as dislocation, implant fracture, metallosis, and early loosening [1–4]. A suboptimal cup angle also influences postoperative outcomes after THR. In recent years, with improvements in materials of the bearing parts, the larger diameter of the artificial femoral head has increased the prosthetic range of motion (ROM). If the cup placement angle with regards to stem anteversion is appropriate, dislocation and complications associated with impingement can be avoided without postoperative restriction [5], and the postoperative quality of life of the patient is reported to be high [6].