## 4 Discussion  
After THR, the ROM is an important factor for patient satisfaction, as it is a major parameter that can restrict activities of daily living, especially in cultures where a higher ROM is demanded [9; 10]. Additionally, impingement limits the ROM and is a common cause of postoperative instability [47; 65]. ROM analyses have raised much attention from the research community using experimental [24; 25; 26; 27; 28; 29] and numerical methods [21; 30; 43; 44; 46; 47; 48; 49; 50]. However, to the best of our knowledge, there is no approach published to assess ROM after THR in surgery planning without applying additional radiation to the patient; for example, by CT scanning. To fill this gap and to provide a tool for computational assessment of ROM, we developed a framework for ROM analysis, based on preoperative MRI data and anterior-posterior radiographs, the latter being usually performed before THR for preoperative planning. Similar to the approach of [PERSON] et al. [66], we used preoperative MRI data to derive the CAD model of the bony structures. In contrast to their study, we manually segmented the MRI data to get our patient-specific volume models. This time-consuming approach needs to be replaced by more efficient and automatic methods like statistical shape modeling [67; 68]. The main advantage of our method is that bone geometries from real patients scheduled for THR and the real implant geometry were used for the analyses. Another advantage is that no patient undergoing computational assessment of ROM with the proposed methodology will be subjected to additional radiation. Hence, our MRI-based framework is capable of estimating postoperative ROM in a realistic manner and, thus, may support clinical decision-making in the future. The positioning of the implant components between the preoperatively planned and postoperatively reconstructed situation showed an acceptable agreement in terms of the acetabular cup inclination and anteversion. However, it should be noted that the MRI scans and radiographs were acquired in the supine position of the patient. In this context, it is important to emphasize that there is a change in pelvic tilt between the supine and standing positions [69]. Further, in the literature, average changes of the pelvic tilt from preoperative supine to the standing position between 2\({}^{\circ}\) and 7\({}^{\circ}\) in posterior direction were reported [70], while with each degree of pelvic tilt, the radiographic cup anteversion changes nearly by 0.7\({}^{\circ}\)[69]. However, neither the pelvic tilt during MRI nor the supine position on the operating table accurately reflects the functional pelvic tilt for all patients, resulting in a difference in functional cup orientation between the supine and standing positions. Comparing the preoperatively planned ROM with the postoperatively reconstructed one, no significant differences were observed for flexion and internal rotation, while a significant difference was detected for external rotation. The detection of the type of impingement agreed well between preoperatively planned and postoperatively reconstructed situations, except in one case for external rotation and two cases for internal rotation. The ROM reached physiological levels during flexion and internal rotation, which is in agreement with previous studies [18; 44; 46; 47; 54]. This supports the reliability of our results, which are based on the previously described CAD methods [18; 30; 38; 53]. Regarding the observed impingement type, our results showed that the ROM in flexion was more likely restricted by bone-to-bone impingement, whereas internal and external rotations were more likely restricted by implant-to-implant impingement. Therefore, restriction of flexion is not only caused by implant-to-implant geometry but mostly by the alignment of the implant components in femoral and pelvic bone [33].