###### Abstract
The fragment is then reoriented in space before it is fixed to the remaining pelvis by two screws (shown in gray) at its target position. The area marked in blue is called the iliac crest. Left: lateral view on the pelvis, preoperatively. Right: Medial view on the pelvis, postoperatively. Figure 2: Overview of the preoperative planning workflow. The limitations of the preoperative planning and the challenges of the surgical execution have motivated the development of computer-assisted navigation approaches for PAO. In 1997, [PERSON] et al. [32] presented an approach enabling guided osteotomies through instruments that were tracked optically by an external system and visualized with respect to preoperative computed tomography (CT) data. Correspondence between intraoperative anatomy and the preoperative CT was found by either a landmark- or surface-based matching strategy. A reference marker compensated for anatomy motion. If the contralateral side was chosen for the reference marker, an additional incision became necessary. At a later stage of development, it was also possible to guide the reorientation of the acetabular fragment through a tracked handle. In total, 14 surgeries were conducted, yielding satisfactory results without complications [33; 34]. The approach of [PERSON] et al. [28] not only allowed for implementation of a 3D preoperative plan but included biomechanical considerations, namely the contact pressure distribution on the acetabular cartilage based on discrete element analysis, along with geometrical information. Motion compensation was only limited by using three screws that were digitized using a tracked pointer before starting the osteotomies. If tracking was lost due to motion of the hip anatomy, the surgeon had to re-digitize these screws to restore registration. [PERSON] et al. [35] compared their computer-assisted approach to the conventional approach in a study with a two year follow-up. Intraoperative movement of the anatomy was tracked by a reference marker at the iliac crest (Figure 3, marked in blue) that required an additional incision. After the fragment was freed, it was not tracked anymore and therefore reorientation was not guided. Reduced radiation and operation time in the computer-assisted group were the main findings. No significant differences in operative blood loss, transfusion requirement, correction of deformity and functional improvement were found and there were no complications in both groups. The study of [PERSON] et al. [36] investigated computer-assisted PAO in terms of osteotomy and fragment reorientation accuracy in 3D on five cadaveric ships. Osteotomies were guided by a tracked drill bit guide. Fragment reorientation was achieved by matching landmarks at K-wire locations (for fixation) to the preoperative plan. Postoperative CT evaluation showed that preoperatively planned goals could be realized. Figure 3: Example of lateral center-edge angle (LCEA) and acetabular index (AI) measurement on an AP pelvic radiograph as described in [13]. The LCEA and AI are a measure for the extent of coverage of the femoral head by the pelvic part of the hip joint, the acetabulum. The LCEA is measured between two lines, both starting in the femoral head center, one vertical line and the line drawn between femoral head center and the most lateral point of the acetabular rim (angle in blue). To find the femoral head center, a circle is drawn (cyan). The vertical line of the LCEA is defined as being perpendicular to the line connecting both femoral head centers (pink). The AI is measured between said pink line connecting both femoral head centers and a line running through the medial edge of the sclerotic acetabular zone and through the lateral sourcil (angle in pink). The results of [PERSON] et al. [27], who performed eight surgeries on four cadavers, suggest similar conclusions. However, the final alignment was not measured by postoperative CT, but intraoperatively.