Use of the False-Profile Radiographic View to Measure Pelvic Incidence.

Background: Pelvic incidence (PI) is an important variable in assessing spinopelvic balance that is associated with hip pathology. A lateral radiograph of the pelvis can be used to measure PI, but this view is not routinely performed in the clinical setting during evaluation of hip pain. The false-profile (FP) radiographic view of the hip is commonly obtained to measure acetabular coverage.
Purpose: To evaluate the tolerance of PI measurements to pelvic rotation and assess the feasibility of using an FP radiograph to obtain an accurate measurement of PI.
Study Design: Cohort study (diagnosis); Level of evidence, 2.
Methods: A mathematical model was developed to predict the change in PI with rotation. Fluoroscopic images were obtained of 6 reconstructed cadaveric adult pelvis and femur specimens at varying degrees of rotation, including a perfect lateral and FP image. PI was measured with the midpoint between the centers of the femoral heads as a reference point. The findings were confirmed clinically by retrospectively reviewing FP radiographs and computed tomography (CT) scans of 40 clinical patients. PI was measured on FP radiographs and CT scans by 2 independent reviewers.
Results: With dry cadaveric pelvis specimens, the discrepancy in PI measured between fluoroscopic FP and lateral views was 1.6° (95% CI, 0.7°-2.4°). There was excellent agreement between CT and FP radiographs with regard to measurement of PI (intraclass correlation coefficient = 0.92; 95% CI, 0.78-0.98). Mean discrepancy in PI measured between the 40 clinical FP radiographs and CT scans was 2.8° (range, 0.1°-9.1°).
Conclusion: Increased rotation from a lateral view results in greater error in measuring PI, although relatively nominally with a 2.8° error with the 25° of rotation in clinical true FP views. These data demonstrate that FP radiographs can be used to measure PI with reasonable accuracy.