###### Abstract
All of the aforementioned approaches employ external optical tracking systems, which come along with line-of-sight impediment and, consequently, with poor usability. [PERSON] et al. [37] came up with an alternative idea, proposing the registration and tracking for fragment reorientation through inertial measurement units (IMU). One IMU was fixated on the pelvis, the other one on the fragment. A revised version [38] included a small camera in the pelvis IMU facing and tracking a marker attached to the fragment IMU. Sensor fusion combined the two estimates. Both systems were validated w.r.t. an external optical tracking system using bone models and cadavers. The authors could show similar accuracy and high correlations with the tracking system. Nevertheless, in all described approaches visual feedback was provided only through 2D monitors located in the periphery of the operating room (OR). This significantly complicates the process of aligning navigation information with the actual surgical site [39]. Generally, there is consensus that computer assistance is beneficial for PAO in different aspects, also supported by long-term studies involving a high number of patients [40]. However, we believe that usability must be further increased to achieve a broad clinical acceptance [41]. Augmented reality (AR) has opened up new possibilities in orthopedic surgery [42] as described in a recent review paper [43]. In situ visualizations can provide surgeons with an intuitive understanding of the underlying 3D anatomy, resolve the cognitive load of aligning the working space with the one of preoperative data and facilitate surgical guidance. Instruments can be displayed at their target position. Particularly interesting are recently developed off-the shelf optical see-through head-mounted displays (OST-HMD). Their feasibility as a surgical navigation device has not been fully explored yet and is still under discussion [44; 45]. Although preliminary studies showed promising results in various orthopedic interventions, for example, pedicle screw placement [46; 47], percutaneous procedures [48; 49; 50] and hip resurfacing [51], OST-HMD may still suffer from a poor navigation accuracy dependent on the approach taken [44]. In order to render AR content correctly in space, OST-HMD need to know their pose w.r.t. the target object(s). One approach is to employ an external device/camera and estimate the relevant poses from an outside viewpoint (outside-in tracking) using the OST-HMD only as a video see-through device, such as in [50; 52]. The second solution is referred to as inside-out tracking in which the OST-HMD uses its sensors to localize itself either relative to markers (marker-based) or by simultaneous localization and mapping (SLAM) [53] its surrounding (marker-less). In light of the previously mentioned line-of-sight problem, inside-out tracking may be arguably favorable. Marker-based inside-out tracking, as employed in [54; 55], stands out with its simplicity, as the OST-HMD only needs to perform common marker detection, whereas marker-less inside-out tracking often comprises multiple sensor and sophisticated sensor fusion algorithms. This way, however, it enables a world coordinate system where renderings and poses are maintained over time and independent of OST-HMD movements. Furthermore, the surgeon is not disturbed by markers attached to the anatomy. In a proof-of-concept study on synthetic bone models, [PERSON] et al. [56] were the first describing a marker-less inside out approach for the AR-based surgical navigation of PAO osteotomies. AR guided osteotomies performed by novice surgeons were as accurate as when performed in a freehand fashion by an experienced PAO surgeon. However, their approach did not support surgical navigation of fragment reduction, which is an essential step of PAO intervention and thus motivation for our study. To this end, we extended the marker-less inside out approach to the navigation of the fragment reduction as described in our previous case study [57].