A computer implemented method for determining the sagittal rotation of a patient's pelvis, the method comprising the steps of: - acquiring image data describing a two-dimensional X-ray-image of the patient's pelvis made in an anterior-posterior direction; - determining, based on the image data, position data describing the position of a plurality of pelvic landmarks reproduced in or derivable from the two-dimensional X-ray-image; - acquiring calibration data describing the position of a calibration plane with respect to the patient's pelvis, the calibration plane being perpendicular to the anterior-posterior direction; wherein the method is characterized by the steps of: - determining, based on the position data and the calibration data, calibrated projection data describing a two-dimensional projection representing the plurality of pelvic landmarks of the actual patient's pelvis being projected into the calibration plane in the anterior-posterior direction; - acquiring distance data describing at least one of a medial-lateral distance and a cranial-caudal distance between at least one first pelvic landmark and at least one second pelvic landmark in the calibrated two-dimensional projection; - acquiring, from a database, regression data describing a correspondence, particularly a linear correspondence between at least one of said medial-lateral distances and cranial-caudal distances, and a pelvic sagittal rotation; - determining, based on the distance data and the regression data, the sagittal rotation of the patient's pelvis. - acquiring image data describing a two-dimensional X-ray-image of the patient's pelvis made in an anterior-posterior direction; - determining, based on the image data, position data describing the position of a plurality of pelvic landmarks reproduced in or derivable from the two-dimensional X-ray-image; - acquiring calibration data describing the position of a calibration plane with respect to the patient's pelvis, the calibration plane being perpendicular to the anterior-posterior direction; wherein the method is characterized by the steps of: - determining, based on the position data and the calibration data, calibrated projection data describing a two-dimensional projection representing the plurality of pelvic landmarks of the actual patient's pelvis being projected into the calibration plane in the anterior-posterior direction; - acquiring distance data describing at least one of a medial-lateral distance and a cranial-caudal distance between at least one first pelvic landmark and at least one second pelvic landmark in the calibrated two-dimensional projection; - acquiring, from a database, regression data describing a correspondence, particularly a linear correspondence between at least one of said medial-lateral distances and cranial-caudal distances, and a pelvic sagittal rotation; - determining, based on the distance data and the regression data, the sagittal rotation of the patient's pelvis. - determining, based on the position data and the calibration data, calibrated projection data describing a two-dimensional projection representing the plurality of pelvic landmarks of the actual patient's pelvis being projected into the calibration plane in the anterior-posterior direction; - acquiring distance data describing at least one of a medial-lateral distance and a cranial-caudal distance between at least one first pelvic landmark and at least one second pelvic landmark in the calibrated two-dimensional projection; - acquiring, from a database, regression data describing a correspondence, particularly a linear correspondence between at least one of said medial-lateral distances and cranial-caudal distances, and a pelvic sagittal rotation; - determining, based on the distance data and the regression data, the sagittal rotation of the patient's pelvis.