Purpose: The aim of the study was to compare pelvic incidence (PI) and pelvic tilt (PT) measurements using the bicoxofemoral axis obtained from either femoral heads or acetabular domes on lateral digital radiographs of the spine.
Methods: Standing lateral radiographs of the spine of patients without hip pathologies were analyzed. PI and PT were measured on 50 radiographs using the femoral heads first followed by measurements performed with the acetabular domes to define the bicoxofemoral axis. Agreement between the methods was quantified by intraclass correlation coefficient (ICC) and median error for a single measurement (SEM). Intraobserver reproducibility and interobserver reliability of both methods of identification of bicoxofemoral axis and its impact on PI and PT measurements were tested on 31 radiographs and quantified by ICC and SEM.
Results: There was an excellent agreement in PI as well as in PT between measurements performed using whether the femoral heads or the acetabular domes (ICC: 0.99; SEM: 0.56° for PI and ICC: 0.99; SEM: 0.2° for PT). Excellent intraobserver reproducibility was revealed for both methods (ICC: 0.99 and SEM: ≤0.17° for PI and ICC: 0.99; SEM: ≤0.18° for PT). Both methods presented excellent interobserver reliability (ICC: 0.99 and SEM: ≤0.54° for PI and ICC: ≥0.98; SEM: ≤0.9° for PT).
Conclusions: We suggest that either the femoral heads or the acetabular domes may be used for reliable PI and PT measurements on the lateral standing long-cassette digital radiographs of the spine.