The authors hypothesize that a pelvic kinematic disorder, demonstrated by a significant decrease in sacral slope, is associated with the risk of instability of total hip prosthesis, the sacral slope being measured by an EOS imaging system during the transition to sitting in unstable patients versus patients with no history of instability.

Inclusion Criteria:

• The patient must have given their free and informed consent and signed the consent form
• The patient must be a member or beneficiary of a health insurance plan
• The patient must be at least 18 years old and less than 85 years old
• Patient has a conventional first intention total hip arthroplasty (not dual mobility) by posterior approach
• "Case" patients have at least one previous episode of prosthetic dislocation repaired with surgery
• "Control" patients have no previous episode of prosthetic dislocation with more than 2 year since initial intervention

Exclusion Criteria:

• The subject is participating in a category 1 interventional study, or is in a period of exclusion determined by a previous study
• The subject refuses to sign the consent
• It is impossible to give the subject informed information
• The patient is under safeguard of justice or state guardianship
• Patient is pregnant, parturient or breastfeeding
• Patient with lumbar instrumentation
• Patient with severe dementia (Mini Mental State Examination < 10)
• Patient with American Society of Anesthesiologists score ≥ 4
• Patient experience multiple falls
• Patient requiring early revision surgery (<15 days postoperatively
• Patient with obvious implant positioning error on an X-ray of the pelvis.
• Patient with dislocation of traumatic origin (significant kinetic trauma)