## 4 Discussion  
Computer assistance for PAO has been studied for more than two decades with the first study being published in 1997 [32]. Although there is a strong consensus about the benefits in terms of accuracy and safety, computer-assisted surgical navigation of PAO could not be established successfully in clinical practice [70]. In our opinion the main reason is the lack of usability which makes existing approaches still complicated and cumbersome [41]. They depend on an external optical tracking system [32; 33; 34; 26; 28; 35; 36] suffering from line-of-sight issues and provide visualizations on an off-field workstation which complicates the surgeon's task of interpreting and executing the 3D navigation cues correctly [39]. Figure 8: 3D comparisons between preoperative plans and postoperative models for Cadaver \(1\) (**top row**) and Cadaver \(2\) (**bottom row**). Left column shows the fragment orientations, right column the projected osteotomy starting points and their connecting vectors. Green: planned, orange: performed. \begin{table}
\begin{tabular}{l c c c} \hline \hline  & & **Cadaver 1** & **Cadaver 2** \\ \hline \multirow{2}{*}{LCE angle (\({}^{\circ}\))} & preoperative & 34.7 & 43.2 \\  & planned & 54.7 & 59.8 \\  & performed & 48.1 & 57.4 \\ \hline Absolute error (\({}^{\circ}\)) & & 6.6 & 2.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: 2D results of the AR guided fragment reorientation. In this study, we proposed a radiation-free AR-based surgical navigation approach, running locally on an OST-HMD with no need for an external tracking system. Navigation cues are superimposed with the patient anatomy directly at the surgical site. AR provides information regarding perspective and interaction in a way that is easier and more natural to interpret for humans and thus enables intuitive surgical guidance and safe execution. Navigation is further simplified by allowing visualization of instruments at their preoperatively planned position (e.g., chisels), which eliminates the challenges associated with instrument tracking. When tracking is still required (e.g., for registration or fragment reorientation), it is realized using the front-facing built-in cameras of the OST-HMD without interfering with the surgeon's natural hand-eye-coordination. The accuracy of the surgical execution using our navigation approach was evaluated with respect to osteotomies and fragment reorientation by comparing pre- and postoperative CTs of the cadaver surgeries. To this end, a 3D model of the postoperative CT was reconstructed and registered to the preoperative plan using ICP [68; 69]. The planned and performed osteotomy starting points were measured and the 3D distance between them was calculated. [PERSON] et al. [56] followed the same approach in their feasibility study for AR guided PAO osteotomies on synthetic bone models using a landmark-based registration. In their study, the experienced surgeon achieved an average accuracy of 2.4 mm as opposed to 10.8 mm in the herein presented study. Our starting point accuracy was also inferior to the cadaveric study reported by [PERSON] et al. [36]. Their study investigated the performance of computer-assisted PAO using a marker-based outside-in approach and a high-end tracking system. Regarding the osteotomies, only the iliac cut was evaluated yielding an average distance of 1.97 mm compared to 9.7 mm (overall mean) herein presented. However, it has to be taken into account that their cadavers were debrided from soft tissue and ligaments and that postoperative measurements were only performed on a single CT slice, which makes the measurements less sensitive. Nevertheless, the accuracies of navigating the four postoperative starting points with our approach are considered clinically acceptable. This suggests that AR guidance of complex osteotomies is feasible.