Influence of lower limbs discrepancy and pelvic coronal rotation on pelvic incidence, pelvic tilt and sacral slope.

Purpose: Pelvic incidence (PI), pelvic tilt (PT) and sacral slope (SS) are important parameters in sagittal spine alignment evaluation. The measurements are a projection of the three-dimensional pelvis onto a two-dimensional radiograph and they may be influenced by orientation of the pelvis. The aim of this study was to assess the influence of pelvic rotation in the coronal plane (CPR) on radiographic accuracy of PI, PT, and SS measurements.

Methods: Radiological evaluation of the CPR angel was performed on 1 radiological phantom. The radiographs were taken in 5° CPR increments over a range of 0°-45° (evaluated with a digital protractor). On each of the lateral radiograph, PI, PT, and SS were measured three times by three independent researchers. The lowest CPR that changed PI, PT, or SS by ≥6° (the highest reported error of measurement of these parameters) was considered as unacceptable. Next, CPR was calculated based on the distance between femoral heads (FHD). The agreement of the calculated and measured CPR was quantified by the intraclass correlation coefficient (ICC) and the median error for a single measurement (SEM), with value 0.75 considered as excellent agreement.

Results: PI, PT and SS could be measured with an acceptable error of 6° on radiographs with up to 20° pelvic rotation. From 20° CPR onwards the S1 endplate was distorted, that makes the measurements of PI, PT and SS questionable. There was an excellent agreement between CPR measured with a protractor and calculated based on FHD with ICC of 0.99 and SEM of 1.1°.

Conclusions: Rotation of the pelvis in the coronal plane during acquisition of radiographs influences PI, PT and SS measurements. Substantial error of PI, PT and SS measurements occurs with CPR of more than 20° which is equivalent to a lower limb discrepancy of 5.2 cm. CPR may be calculated while acquiring the radiograph. Further evaluation of the influence of CPR on spinopelvic parameters with a larger sample would be valuable.