## 2 Materials and Methods
### Preoperative Planning  
Each cadaver was CT scanned using a Somatom Edge CT(r) device (Siemens, Erlangen, Germany). The slice thickness was 1.0 mm and the in-plane resolution (x-y) was 0.4 \(\times\) 0.4 mm (Figure 2, step 1). 3D models of pelves and femora were extracted using the global thresholding and region growing functionalities of a commercial segmentation software (Mimics Medical, Materialise NV, Leuven, Belgium) [58; 59; 60] (Figure 2, step 2).  
An experienced orthopedic surgeon conducted the 3D preoperative planning. An in-house developed software was used (CASPA, Balgrist University Hospital, Zurich, Switzerland). For navigation and outcome evaluation, we defined an anatomical coordinate system with its origin in the formal head center, where the y- and z-axes are parallel to the anterior pelvic plane (APP) and the z-axis pointing to the contralateral femoral head center (Figure 2, step 3). The APP is spanned by 4 landmarks: the right and left ASIS (yellow circles in Figure 2, step 3) as well as the right and left cubic tubercle (gray circles in Figure 2, step 3) [61]. The choice of this coordinate system is motivated by the conventional, X-ray based PAO planning, because the APP corresponds with the perspective of an anterior X-ray. In CASPA, three anatomical landmarks were defined at the idiopathic eminence (Figure 2, step 4: L1), the anterior inferior iliac spine (AIIS) (Figure 2, step 4: L2) and at the deepest point of convexity between anterior superior (ASIS) and inferior iliac spine (AIIS) (Figure 2, step 4: L3), required later for intraoperative registration initialisation of the preoperative plan to the patient anatomy. The software allows to define 3D osteotomy plane objects which can be translated and rotated freely in 3D space. For each of the four osteectomies, such a plane object was created and placed according to [PERSON]' original description [1] (Figure 2, step 5). Thanks to the AR technology, all plane objects can be later in-situ visualized on the real anatomy to navigate the cuts. For the first osteotomy (ischial), we additionally planned the poses of the chisel such that the surgeon could align the physical instrument intraoperatively (Figure 2, step 6). The idea for guiding the direction of the supracetabular osteotomy was to visualize a plane and a crossing point which marks the end point of the supracetabular osteotomy and the beginning of the retroacetabular osteotomy. According to [PERSON] et al., said crossing point should be at an angle of 120\({}^{\circ}\) and was AR-displayed as a cross in addition to the planes (Figure 2, step 7). After the crossing point, only a small part of the retroacetabular cut can performed with the use of a chisel, while the remaining osteotomy has to be performed as a controlled fracture toward the ischial spine [1]. Therefore, adequate guidance of the retroacetabular osteotomy by visualizing the crossing point along with the osteotomy plane was crucial to prevent fracture of the posterior column, which is considered a severe surgical error [1].  
Separating the acetabular fragment from the remaining pelvis was realized by Boolean operations [62] between the pelvic 3D model and the cutting plane objects (Figure 2, step 8). As both cadavers had a physiological hip anatomy, the acetabulum was planned to be rotated laterally and anteriorly in order to achieve a fragment transformation similar to the correction of developmental hip dysplasia through PAO. To this end, the surgeon manipulated the fragment in space via mouse interaction towards the desired target pose (Figure 2, step 9). Equal amount of correction was planned for both cadavers, whereas for cadaver 1 we planned the correction on the left hip joint and on cadaver 2, the right side underwent surgery.