Using an App based algorithm to determine axial rotation and pelvic tilt at the time of an AP radiograph: a sawbone study

INTRODUCTION: Axial rotation of the pelvis impacts inclination and anteversion measurements of the acetabular component on plain radiographs. Currently the axial rotation is not taken into account when providing acetabular component measurements on plain radiographs. The current study evaluates a novel App based algorithm to determine pelvic axial rotation at the time of an AP pelvis radiograph. MATERIALS AND METHODS: AP and lateral radiographs were taken of the sawbone pelvis to calibrate the App. The sawbone was then rotated 11 times to the left (average 5.3°, range 0.2°-10.5°) and 10 times to the right (average 5.2°, range 1.2°-11.3°). The sawbone was then tilted forward 14 times (average 5.6° range 0.1°-11.6°) and 13 times backward (average 5.9°, range 0.1°-11.2°). The amount of axial rotation and tilt of the sawbone pelvis was determined using a digital caliper. AP pelvis radiographs were taken for each position. Using a novel App based calculation algorithm the amount of axial rotation and pelvic tilt was determined from the AP Pelvis radiograph. RESULTS: The algorithm determined the axial rotation of the pelvis at the time of the AP pelvis radiograph with an average difference of 0.6° (range 0°-1.8°) compared to the digital caliper measurement. For measuring pelvic tilt, the difference was 2.3° (range 0.4°-4.5°) compared to the digital caliper measurement. CONCLUSIONS: The algorithm used to determine axial rotation of the pelvis at the time of an AP pelvis radiograph is highly accurate. The technology can help to more accurately determine acetabular component position on standard radiographs and intraoperative C-arm images.