Pelvic flexion measurement from lateral projection radiographs is clinically reliable

Pelvic flexion affects orientation of the acetabular cup; however, pelvic position is not static in daily activities. During THA it is difficult to know the degree of pelvic flexion with the patient in the lateral position and that position is static. However, surgeons need to appropriately determine pelvic tilt to properly insert the acetabular component.

We investigated the reliability of pelvic flexion angle that was measured by manually identifying the location of the pubic symphysis and bilateral anterior superior iliac spines using synthesized lateral radiographs.

We synthesized 49 lateral radiographs based on CT data. Each of the 49 radiographs had a unique position: 7° of varying lateral tilt and rotation in each of seven selected pelvic flexion angles. The pelvic flexion angle was measured three times by three independent observers in each position and determined the accuracy (based on the true value from the reconstructions) and reliability of the measures.

The measurement error was 0.1° (range, -4.8° to 4.0°). There was a tendency for errors when the pelvic flexion angle was 0° or ±5°; the errors were less when the pelvic flexion angle was ±10° or ±20°. Lateral tilt was associated with greater error than rotation. The intraclass correlation coefficient (ICC) of the average value was 0.967. For one observer, more than two measurements are necessary for the ICC to be greater than 0.8, and only one measurement was needed for two of the three observers.

Our data suggest measurement of pelvic flexion angle using lateral radiographs is reliable. We recommend the measurement be performed once by two observers for better reliability.