Standing or supine x-rays after total hip replacement - when is the safe zone not safe?

Area of Science:

• Orthopedic Surgery
• Biomedical Engineering
• Radiology

Background:

• Acetabular prosthesis malorientation is a significant risk factor for complications following total hip replacement (THR).
• Current assessment of acetabular component orientation typically uses supine X-rays, but changes in patient positioning can alter pelvic tilt and thus orientation.
• Standing X-rays have been proposed as a more accurate method to assess acetabular orientation due to positional changes.

Purpose of the Study:

• To determine if acetabular components considered within the 'safe zone' in a supine position can fall outside this zone when the patient is standing.
• To evaluate the impact of body position (supine vs. standing) on pelvic tilt and acetabular component orientation (anteversion and inclination).

Main Methods:

• Thirty patients (12 male, 18 female) underwent pelvic X-rays (lateral and antero-posterior) in both standing and supine positions six weeks post-THR.
• Pelvic tilt, acetabular anteversion, and inclination were measured and compared against established 'safe zone' limits.
• Statistical analysis was performed to compare orientation in supine versus standing positions.

Main Results:

• In the standing position, patients exhibited a significant posterior pelvic tilt, leading to increased acetabular anteversion and inclination (p<0.0001).
• Of the 30 patients, 16 had acetabular components within the 'safe zone' while supine but outside the 'safe zone' when standing.
• Patients were significantly more likely to have their acetabular components outside the 'safe zone' in the standing position compared to the supine position (p<0.0001).

Conclusions:

• Acetabular component orientation assessed in the supine position may not accurately reflect orientation in the standing position.
• A significant proportion of acetabular components considered optimally positioned in supine may be malpositioned when standing, potentially increasing THR complication risks.
• Standing X-rays may provide a more clinically relevant assessment of acetabular orientation in total hip replacement.