In this study, investigators will demonstrate and prove a ellipse method (Liaw's version) for measurement and detection of acetabulum cup retroversion.

Total hip arthroplasty (THA) is considered to be the best and most reliable treatment of end-stage hip disorders with satisfactory long-term clinical outcomes. However,the incidence of postoperative dislocation ranges from 0.3% to 3% in primary THA. The most important risk factor is malpositioning of implant components. Generally, excessive anteversion and retroversion increase risk of anterior and posterior hip dislocation respectively. Several radiological methods have been proposed to measure anteversion of the acetabular cup on plain radiographs, but most of these methods cannot accurately distinguish cup retroversion from anteversion. In this study, investigators will demonstrate and prove a ellipse method (Liaw's version) for measurement and detection of acetabulum cup retroversion.

Inclusion Criteria:

• Patients undergone total hip arthroplasty
• Patients undergone total hip arthroplasty with more than two postoperative anteroposterior radiographs of pelvis
• Patients undergone total hip arthroplasty with postoperative pelvic CT

Exclusion Criteria:

• Patients with congenital pelvis abnormality or pelvic fracture history
• Patients with postoperative periprosthetic fracture(s)
• Patients with postoperative infection or implants loosening
• Patients undergone total hip arthroplasty without more than two postoperative anteroposterior radiographs of pelvis
• Patients undergone total hip arthroplasty without postoperative pelvic CT

Patients from Shuang Ho Hospital