The purpose of this study is to find out whether placing a pillow between the legs during hip surgery will result in a better aligned hip replacement.

During hip replacement surgery the cup is placed on specific planned angles. Cup placement has an effect on the patients' pain-free range of movement, dislocation rates and satisfaction. However, the angles of the cup orientation (i.e. the tilt and rotation), which are preoperatively planned and postoperatively measured on radiographs, have a high variability even when an experienced surgeon is performing the surgery. One of the reasons this occurs is because the operated leg can move the pelvis to a different position in the time between set-up and actual implantation which may lead to the cup being placed in a non-optimal position. This study aims to see if the difference between planned and achieved cup angles and pelvic movement can be lessened by putting a pillow in between the legs to keep it in a stable horizontal position.

Inclusion Criteria:

• Patient awaiting primary total hip arthroplasty using a posterior approach in a lateral decubitus position
• Patients capable of reading, understanding, and willing to sign the informed consent form

Exclusion Criteria:

• Total hip arthroplasty not using a posterior approach in a lateral decubitus position
• Prior hip osteotomy or fracture osteosynthesis on the index hip