Total hip replacement is one of the most common surgical procedure in France. In 2010, an estimated 147 513 total hip procedures were performed. This number is constantly increasing.

Although it is common, it can lead to many complications. The intra-prosthetic luxation is the second one after aseptic loosening.

The luxation risk is multifactorial and depends on the relative orientation of femoral et acetabular components. This orientation is influenced by statics and dynamics parameters.

So far, the literature shows that only static parameters are considered by surgeons. Thus, in reference to Lewinneck publication, which states that the luxation risk is lower if the cup is oriented with 15° +/- 10° of anteversion and 40°+/- 10° of inclination with respect to the anterior pelvic plan (APP). Defined by the two anterosuperior iliacs spines and pubic symphysis.

However, this approach is only static and do not take into account the variations of pelvic plan orientation during everyday life.

To consider these variations of pelvic inclination, it is possible to measure the angle between APP and the horizontal (in lying position) or the vertical (in standing position) plan.

Several devices allow the measure of pelvic inclination but they all have important limits (EOS radiography, scanner or navigation). A new device has been developed in Brest to measure this inclination in several positions of the daily life.This software has already been tested in healthy volunteers and results demonstrate an excellent accuracy and reproducibility.

The goal of this study is to described the dynamic of the pelvic bone thanks to this ultrasound based device, and to assess the amount of variation induced by hip replacement procedures.

Inclusion criteria :

• Patient with primary or secondary hip arthritis, at stage II to III of the radiological classification of Tönnis and for whom total hip replacement is indicated by surgeon.
• Signed consent
• Patient beneficiary of a social security plan

Exclusion Criteria:

• Patient Under 18
• Patient Under protective measure (guardianship, curatorship) or unable to consent
• Patient requiring revision THR
• Patient with geographic mobility plan before the end of the follow-up
• Symptomatic contralateral hip
• Symptomatic dorsolumbar rachis
• Pelvis or spine surgery planed during the follow-up
• Pregnant or nursing woman

All collected data that underlie results in a publication.

Data will be available beginning three years and ending fifteen years following the final study report completion.

Data access request will be reviewed by the internal committee of Brest University Hospital. Requestor will be required to sign and complete a data access agreement.