Intraoperative stitched fluoroscopic images: effect of parallax on angular measurements of the spine

Background Context: Intraoperative stitched O-arm images are commonplace during spinal deformity correction surgeries; however, the accuracy of stitched images for measuring angular measures is unknown. Purpose: To examine the effect of radiographic parallax effect of stitched O-arm images by assessing the regional curve agreement with measurements from computed tomography (CT). Study Design/Setting: Experimental radiographic study. Patient Sample: Four whole body cadavers (age: 81±14, sex: 2M/2F) and two fabricated spine model phantoms from surgical cases, one with extreme scoliosis and one normal spine, were utilized. Outcome Measures: The limits of agreement for angular measures between CT (gold-standard) and intraoperative stitched fluoroscopic images were calculated. Further, intra- and inter-rater reliability was measured. Methods: A series of adjacent anterior-posterior and lateral images were acquired cranial to caudal using an O-arm in three table configurations (standard position, off-axis in the coronal plane, and reverse Trendelenburg) and stitched manually. Regional angular measures were extracted, and the limits of agreement were calculated between each table position and CT using a Bland-Altman approach. Results: The observers displayed excellent inter-rater reliability across table positions (range: 0.944-0.989) and intra-rater reliability (0.979-0.995). The limits of agreement results showed a similar and better agreement was observed for the Standard and Reverse Trendelenburg than the Off-Axis position. Conclusions: This work shows reliable regional curvature measurements can be calculated with good agreement with CT in common table positions, but care should be taken to ensure the patient is perpendicular to the X-rays, particularly in the lateral view.