## 4 Discussion
Besides the starting points, we compared also the orientations of the three AR guided osteotomies to the preoperative planning, yielding average deviations of 5.3\({}^{\circ}\), 1.0\({}^{\circ}\) and 9.8\({}^{\circ}\) for the supra-, retroacetabular and ischial osteotomy, respectively. For the retroacetabular osteotomy, [PERSON] et al. [56] found a significantly smaller deviation of 7.8\({}^{\circ}\) vs. free-hand 11.7\({}^{\circ}\) for the experienced surgeon when guided by AR. Under realistic OR conditions and performed on cadavers, our result of 1.0\({}^{\circ}\) is clearly superior. In addition, the angle between the supra- and retroacetabular osteotomies, which is essential when performing PAO [1], shows an arguably small mean absolute deviation of 4.6\({}^{\circ}\) when compared to the planned situation. The evaluation of the fragment reorientation revealed that the correction was performed around the correct axes but not with satisfying accuracy. We found an average absolute rotational errors in Euler angles of 6.7\({}^{\circ}\), 7.0\({}^{\circ}\) and 0.9\({}^{\circ}\) for the x-, y- and z-axis, respectively, and a mean absolute difference of 4.5\({}^{\circ}\) in the LCE angle. These findings demonstrate a considerably higher accuracy compared to conventional fluoroscopy-guided PAO where a mean absolute difference in LCE angles of 5.8\({}^{\circ}\) (range 1\({}^{\circ}\)-16\({}^{\circ}\)) was observed [71]. Our method even achieved a higher accuracy compared to high-end, marker-based navigation as described by [36], who reported a mean LCE angle difference of 4.9\({}^{\circ}\) and two cases in which the LCE error was above 10\({}^{\circ}\). Other studies have reported a better navigation performance using AR [47; 54; 55], but on different anatomies and interventions which supports our assumption that not the technical method but the very challenging surgical procedure is mainly responsible for the limited accuracy. Another reason related to the surgical technique may be that our preoperative plan did not account for a lateral translation the mobilized fragment has to undergo to enable reorientation, such that the fragment does not collide with the remaining pelvis. Nevertheless, several technical limitations of our approach and of OST-HMD in general may contribute to the overall inaccuracy. Our proposed motion strategy compensation required the surgeon to actively restore the initial registration w.r.t. the pelvic marker. It is known that HoloLens AR renderings are prone to drift after they have been placed [72]. [PERSON] et al. found a mean displacement of 5.83 mm investigating walking, sudden acceleration, sensor occlusion and object insertion. Consequently, drifts may have also occurred during our experiments. An interesting aspect in this regard is that absolute positional errors, such as the evaluated osteotomy starting points, were rather high, whilst the orientations of performed osteotomies were small. This may imply that the drift is mostly of translational nature. However, continuous motion compensation throughout the procedure may mitigate this issue. Furthermore, other sources of error possibly contributed to the inaccuracy of the registration process, such as errors introduced by ICP [73; 74], tracking [75] or digitizer calibration [76]. For instance, [PERSON] et al. reported that the total registration error using OST-HMD in surgical applications is hardly lower than 5 mm [77]. We will address these issues in our future work, where we will work on more sophisticated visualization to facilitate navigation on the one hand, as well as analyzing potential sources for error throughout the registration process. On the other hand, robotic approaches for performing PAO should also be considered as they allow for higher geometric accuracy. Our study has several limitations. First, only a small number of cadavers were included in the study which limits the generalizability of our results. Nevertheless, we find it important to present the results to the community to motivate further research in this direction.