Abstract
Background: Pelvic incidence (PI) is used as a key parameter in surgical correction of adult spinal deformity (ASD) patients. However, there is a limitation to reflecting the exact center or inclination of 3-dimentional anatomical structures in a 2-dimensional (2D) sagittal radiographs, and these can lead to the measurement errors. Therefore, we evaluated whether there is a change in PI measurement according to the actual rotation of the pelvis, and conducted a study on a more accurate method of measuring PI in a 2D sagittal radiograph.Methods: From 2014 to 2015, 30 patients who visited our outpatient clinic were analyzed retrospectively. CT scan images including the lower lumbar spine, pelvis, and both femurs in DICOM format were imported to Mimics Research 17.0 (Materialise NV, Belgium), Solidworks (Dassault systems, France), and AutoCAD 2014 (AUTODESK, US), and the changes in PI according to vertical and horizontal pelvic rotations were evaluated.Results: The average PI according to the horizontal pelvic rotations measured on AutoCAD with 0º, 5º, 10º, 15º, 20º, 25º, 30º, 35º, and 40º was 48.8º, 48.7º, 48.3º, 47.8º, 46.9º, 45.6º, 44.0º, 42.2º, and 39.9º, respectively. The PI of an acceptable error of 6º on radiographs was 35º in the horizontal pelvic rotation. The average PI according to the vertical pelvic rotations measured on AutoCAD with 0º, 5º, 10º, 15º, 20º, 25º, 30º, 35º, and 40º was 48.8º, 49.0º, 49.5º, 50.2º, 51.3º, 52.7º, 54.4º, 56.6º, and 59.4º, respectively. The PI of an acceptable error of 6º on radiographs was 30º in the vertical pelvic rotation. Conclusions: This study revealed that the PI value could differ from the actual anatomical value due to the horizontal and vertical rotation of the pelvis while acquiring the radiograph. In whole-spine lateral radiographs, errors in PI measurement may occur due to rotation of the pelvis or nonvertical projection of X-rays. In the standing pelvic lateral radiographs, placing the overlapping femoral heads at the center and obtaining the straight sacral endplate as much as possible by referring to CT or magnetic resonance imaging would be a more accurate measurement method to define the PI.