###### Abstract  
Augmented reality (AR)-based surgical navigation may offer new possibilities for safe and accurate surgical execution of complex osteotomies. In this study we investigated the feasibility of navigating the periacetabular osteotomy of Ganz (PAO), known as one of the most complex orthopedic interventions, on two cadaveric pelvis under realistic operating room conditions. Preoperative planning was conducted on computed tomography (CT)-reconstructed 3D models using an in-house developed software, which allowed creating cutting plane objects for planning of the osteotomies and reorientation of the acetabular fragment. An AR application was developed comprising point-based registration, motion compensation and guidance for osteotomies as well as fragment reorientation. Navigation accuracy was evaluated on CT-reconstructed 3D models, resulting in an error of 10.8 mm for osteotomy starting points and 5.4\({}^{\circ}\) for osteotomy directions. The reorientation errors were 6.7\({}^{\circ}\), 7.0\({}^{\circ}\) and 0.9\({}^{\circ}\) for the x-, y- and z-axis, respectively. Average postoperative error of LCE angle was 4.5\({}^{\circ}\). Our study demonstrated that the AR-based execution of complex osteotomies is feasible. Fragment realignment navigation needs further improvement, although it is more accurate than the state of the art in PAO surgery. periacetabular osteotomy; surgical navigation; augmented reality; osteotomy; computer assisted planning +
Footnote †: journal: Medicalischial, public, supra- and retroacetabular osteotomy (Figure 1, marked as 1, 2, 3 and 4, respectively). For the first osteotomy (ischial), an angled chisel is used (Figure 2, step 6). Immediately adjacent to the acetabulum, the second osteotomy (publs) is performed with a chisel. The third osteotomy (supraacetabular) is performed with an oscillating saw and ends at the crossing point (Figure 2, step 7) with the fourth (retroacetabular) osteotomy. About 15 mm of the fourth osteotomy are performed from inside and outside at an angle of 120\({}^{\circ}\) using the chisel, whereas the remainder towards the ischial spine breaks through a controlled fracture (Figure 1, orange circle), thereby freeing the acetabulum from the pelvic bone. The mobile fragment is then reoriented to the planned target position, where it is fixed to the remaining pelvis with two cortical screws on the iliac crest (Figure 1, screws shown in gray, iliac crest area marked in blue) [1]. For diagnosis and preoperative planning of RHD, conventional two-dimensional (2D) anterior-posterior (AP) radiographs are used to determine two parameters, the lateral center-edge (LCE) angle and the acetabular index (AI) (Figure 3) [13]. A hip is considered dysplastic, if the LCE angle is less than 23\({}^{\circ}\) with an AI of more than 14\({}^{\circ}\), whereas an LCE angle exceeding 33\({}^{\circ}\) with an AI of less or equal than 2\({}^{\circ}\) is considered acetabular overcoverage [14; 15]. Although the limitations of 2D measurements in assessing complex three-dimensional (3D) anatomies are well known, they still represent the gold standard in the preoperative planning of hip surgeries due to the lack of adequate 3D measurement methods. Studies have shown that 2D measurements are biased by the projection of the anatomy in the radiographs [16; 17; 18; 19; 20; 21; 22; 23] and the intra- and inter-reader variability of the measurement methods [20; 24; 25; 26]. This underlines the need for a more robust 3D preoperative planning method in order to accurately determine and implement the optimal correction. However, the existing clinically used preoperative 3D planning approaches [27; 28; 29; 30; 31] are still relying on the 2D parameters LCE and AI and are therefore not true 3D approaches. Figure 1: Surgical technique of periacetabular osteotomy of Ganz (PAO). Four osteotomies, namely the ischial (1), public (2), supra- (3) and retroacetabular (4) are performed, followed by a controlled fracture (area marked in orange), which mobilizes the acetabular fragment (green).