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

An acetabular prosthesis orientated outside the 'safe zone' is one of the key contributing factors in increasing complications after total hip replacement (THR). Although acetabular orientation is routinely assessed using supine x-rays, standing x-rays have been proposed because a change in body position alters pelvic tilt and therefore acetabular orientation. This study aimed to assess whether acetabular components orientated within the 'safe zone' in supine can also be outside the 'safe zone' in standing. Thirty patients (12M, 18F) had lateral and antero-posterior pelvic x-rays taken in standing and supine positions six weeks post THR. Pelvic tilt and acetabular orientation (anteversion and inclination) were measured and compared with respect to the limits of the 'safe zone'. In standing, the pelvis was relatively posteriorly tilted and both acetabular anteversion and inclination increased (p<0.0001). In 16 patients the acetabulum was orientated within the 'safe zone' in supine but outside the 'safe zone' in standing. Patients were significantly more likely to be outside the 'safe zone' in standing than when supine (p<0.0001).